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Sedimentology of the Beck Spring Dolomite, eastern Mojave Desert, California

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Sedimentology of the Beck Spring Dolomite, eastern Mojave Desert, California

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Content SEDIMENTOLOGY OF THE BECK SPRING DOLOMITE, EASTERN MOJAVE DESERT, CALIFORNIA by Melinda Lee Marian A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geological Sciences) January 1979 UMI Number: EP58659 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMI Dissertation Publishing UMI EP58659 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProOuest ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 U N IV E R S IT Y O F S O U T H E R N C A L IF O R N IA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES. CALIFORNIA 90007 This thesis, written by Melinda Lee Marian under the direction of h^.T....Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of Master of Science (Geological Sciences) Dean Date THESIS COMMITTEE Chairm an ACKNOWLEDGMENTS This study was supported in part by a grant from the Geological Society of America, Penrose Fund, to the author. Special thanks go to Dr. Robert Osborne for his patience and encouragment during the past two years. Thanks are also extended to his wife, Sally, for her understanding of all the interruptions of their family life in the name of graduate student education. I wish to thank Dr. Stan Awramik for his enthusiastic introduction to stromatolites and the mysteries of life in the Precambrian and for the extended use of much of his valuable literature. I wish to express my thanks to Dr. Lawford Anderson with whom I have exchanged much good-natured abuse during my career at U.S.C. I wish also to thank him for use of his laboratory for my petrochemical studies and for the excellent instruction I enjoyed in all of his classes. Finally, I wish to thank my husband, Ron. This thesis would never have been completed if it were not for his encouragement, patience, and many skills. Ron endured snakes, scorpions, sleet storms, and 120 degree heat to help with the field work. He also drafted all the figures in this thesis and developed and printed all the photographs. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS ..... V LIST OF TABLES............. viii ABSTRACT ....... 1 INTRODUCTION............................. 4 Rationale and Previous Work. ................... 4 Geologic Setting................ 12 STRATIGRAPHY AND PRIMARY SEDIMENTARY AND BIOSEDIMENTARY STRUCTURES....... ........ 17 Type Locality............. 17 Other Localities ..... 17 Beck Spring. . ............... 19 Acme Talc Mine. ......... 30 Snow White Mine ...... 36 Saratoga Spring. ........ 40 Silurian Hills ........ 47 Summary and Discussion. ........ 50 Stratigraphic Correlations ................ 50 Nature of the Upper and Lower Contacts.... 55 Primary Sedimentary and Biosedimentary Structures and Environmental Interpre tations „ ................. 57 PETROGRAPHY................... 71 Procedures ............. 71 R 0 s ui .11 s ........................................ 7 2 Mineral Composition....................... 72 Page Allochemical Grains........................... 91 Textures and Fabrics......................... 96 Summary and Discussion............................. 99 PETROCHEMISTRY............................................ 104 Atomic Absorption Spectroscopy (AAS) Methods.... 104 X-Ray Diffraction (XRD) Methods................... 110 Results and Discussion............................. Ill Summary............................................... 145 STROMATOLITES............................................. 147 Classification and Biostratigraphy............... 147 Systematic Descriptions............................ 150 Summary and Discussion............................. 190 CONCLUSIONS................................................ 195 REFERENCES................................................. 202 LIST OF ILLUSTRATIONS Figure Page 1. Generalized columnar section of Precambrian to Lower Cambrian strata, Death Valley and eastern Mojave Desert region................... 6 2. Index map showing location of study locales, major mountain ranges, prominent structural features, major access routes, and major physiographic features of the Death Valley and eastern Mojave Desert regions............. 10 3. Map of the outcrops of the Pahrump Group (cross- hatched regions) in the greater Death Valley region and eastern Mojave Desert regions 13 4. Generalized columnar section of the Beck Spring Dolomite, Beck Spring, Kingston Range........ 20 5. Legend for generalized and detailed columnar sections of the Beck Spring Dolomite......... 22 6. Photographs of sedimentary structures and litho- types of the Beck Spring Dolomite............. 24 7. Photographs of small-scale primary structures in the Beck Spring Dolomite....................... 27 8. Generalized columnar section of the Beck Spring Dolomite, Acme Talc Mine, Alexander Hills.... 31 9. Detail of stromatoltic intervals, Acme Talc Mine section..................................... 34 10. Generalized columnar section, Beck Spring Dolo mite, Snow White Mine, eastern Kingston Range............................................. 37 11. Generalized columnar section, Beck Spring Dolo mite, Saratoga Spring, southern Ibex Hills... 41 12. Detail of shoaling upward cycles, lower cherty member, Beck Spring Dolomite, Saratoga Spring section ...................„................ 44 13. Generalized columnar section, Beck Spring Dolo mite, Silurian Hills section, Silurian Hills. 48 v Figure Page 14. Stratigraphic correlation diagram showing the proposed lithostratigraphic correlations of the informal members of the Beck Spring Dolomite.......................................... 51 15. Schematic diagram showing emergent and submergent episodes during deposition of the Beck Spring Dolomite.......................................... 68 16. Photomicrographs of detrital and replacement minerals and fabrics in rocks of the Beck Spring Dolomite. ............................ 83 17. Photomicrographs of evaporitic mineral pseudo- morphs and associated accessory minerals 89 18. Allochemical grains in the Beck Spring Dolomite. 92 19. Plot of Mn ppm versus Fe ppm levels in samples of the Beck Spring Dolomite and other formations........................................ 126 20. Plot of Mn ppm versus Ba ppm levels in samples of the Beck Spring Dolomite and other formations........................................ 128 21. Plot of mole % FeO versus mole % MnO for samples of the Beck Spring Dolomite and other formations........................................ 132 22. Plot of Na ppm versus weight % acid-insoluble residue of samples of the.Beck Spring Dolo mite and other formations..................... 138 23. Proposed time ranges of important stromatolite groups............................................ 151 24. Photographs of column structure and mode of occurrence of Conophyton c.f. qarqanicum 156 25. Reconstruction of Conophyton stromatolite forms. 159 26. Photographs of detailed laminar structure of Conophyton c.f. qarqaicum...................... 162 27. Reconstructions of the Baicalia-like stromato lite forms........................................ 168 vi Figure Page 28. Photographs of Baicalia-like stromatolite forms. 170 29. Photographs of macro- and microstructure of the nodular stromatolites..................... 176 30. Reconstructions of nodular stromatolite forms... 178 31. Photographs of macro- and microstructure of the stratiform stromatolites................. 187 vi LIST OF TABLES Table Page 1. Primary sedimentary and biosedimentary structures and lithotypes of the Beck Spring Dolomite... 58 2. Petrologic characteristics of the Beck Spring Dolomite, carbonate facies................. 73 3. Petrologic characteristics of the Beck Spring Dolomite, mixed carbonate-clastic facies 76 4. Petrologic characteristics of the Beck Spring Dolomite, terrigenous elastics.............. 79 5. Mineral composition of the Beck Spring Dolomite. 81 6. Ca-Mg interference in Mn, Fe, I<, Na, Ba, and Sr measurements...................... 107 7. Percent relative standard deviation of elemental analyses by AAS................................ 109 8. Informal members of the Beck Spring Dolomite-- major, minor, and trace element analyses, weight percent insoluble residue, and petrologic characters.......... 112 9. Selected samples from the Johnnie, Crystal Spring, and Seventeenmile Point Formations-- major, minor, and trace element analyses, weight percent insoluble residue, and petrologic characters ............ 116 10. Mean trace element composition of the informal members of the Beck Spring Dolomite.......... 121 11. Mean trace element composition— total Beck Spring versus total Johnnie-Seventeenmile Point Formations ..... 122 12. XRD minerology and structual ordering of samples of the Beck Spring Dolomite and other forma tions ............ 140 viii ABSTRACT The Beck Spring Dolomite is a presumed continental margin deposit of late Precambrian age which is exten sively exposed in the greater Death Valley and eastern Mojave Desert regions of southern California. The Beck Spring Dolomite is the middle member of the Pahrump Group, a collection of strata deposited in an elongate trough, the Amargosa Basin, which has been interpreted by various authors as an aulacogen, an epicontinental embayment, or a fault-bounded, shallow water cratonal shelf. The Beck Spring Dolomite shows unusually uniform stratal thickness and lithologies throughout the region of this study. The Beck Spring Dolomite strata are thickest in the eastern portions of the basin (330-480 m) and thin progressively to less than 30 m in the western regions where the strata interfinger with terrigenous clastic sediments of probable tidal channel or fluvial origin. The Beck Spring Dolomite includes three informal members, the lower laminated, oolitic-pisolitic, and upper cherty members, first proposed by Gutstadt (1968) from the type locality. These members are persistent in the eastern portion of the study area. A fourth "lower cherty" member, defined in this study, dominates in the western portion of the basin. The lower and upper cherty members are characterized by fine, mm laminated dolomites, intraclasts, abundant black chert nodules and interbeds. Included in the upper member are columnar Baicalia-like and nodular stromatolites. The mixed clastic-carbonate lower cherty member strata in the western portions of the basin have well-developed lenticular bedding and planar, low angle cross strata. The oolitic-pisolitic member contains abundant ooliths, pisoliths, peloids, and intraclasts. Grapestone aggregates are prominent at the Beck Spring locale. The lower laminated member consists of macrolaminated (3-5 mm) dolomite with abundant flat pebble conglomerate, eroded and draped laminae, and columnar stromatolites (Conophyton). The Beck Spring Dolomite is composed of well-ordered replacement dolomite with minor quantities of chert and quartz in the upper and lower cherty members and traces of iron oxides and evaporitic mineral pseudomorphs throughout the formation. Petrochemical trace element analyses indicate signi ficantly higher iron and manganese levels in the laminated members than in the oolitic-pisolitic member. This may reflect primary differences in microenvironments of deposition, selective post-depositional alteration due to differences in porosity, or selective accumulation of 2 iron and manganese by the algal mat communities of the laminated members. The Beck Spring Dolomite contains a stromatolite assemblage characterized by Conophyton c.f. qarganicum, a Baicalia-like columnar form, two types of nodular stromatolites, and several varieties of Stratifera. This assemblage indicates a probable Middle to late Middle Riphean age for the Beck Spring Dolomite. The uniform thickness, lithologies, and associated structures throughout the formation and the region of study suggest a stable tectonic regime during deposition of the Beck Spring Dolomite. Combined evidence from petrology, stratigraphy, primary sedimentary and biosedi mentary structures, petrochemistry, and stromatolites indicates that the Beck Spring Dolomite was deposited on a shallow, stable platform. This inferred facies, which probably included offshore shoals, restricted lagoons, and broad tidal flats with ponds, channels, and levees, bears many similarities to Holocene depositional environ ments of the Bahamas, the Antilles, Western Australia, and the Persian Gulf. 3 INTRODUCTION Rationale and Previous Work The Beck Spring Dolomite is the middle member of the Pahrump Group, a continental margin deposit of late Pre- cambrian (Riphean) age, which is extensively exposed in the eastern Mojave Desert and Death Valley regions. Together with the overlying late Precambrian and Cambrian strata they form a deposit of essentially unmetamorphosed strata nearly 7000 m thick. The late Precambrian-Cambrian tectonic history of part of the western North American continental margin is recorded within these strata. The tectonic history includes possible rifting and initiation of the Cordilleran miogeocline. The late Precambrian history of western North America is dominated by one or possibly two rifting events (Burchfiel and Davis, 1975). The first rifting event is thought to be pre-Belt in age (850-1450 m.y.) and the second within the interval from post-Belt to pre-Winder- mere (<850 m.y.). Evidence for the pre-Belt rift event is the preservation of Belt/Purcell Supergroup strata in elongate basins, possibly aulacogens,in the Belt trough of Montana, Idaho, British Columbia and Alberta, the Uinta Mountains region of Utah and Colorado, and selected regions of Vancouver Island (Gabrielse, 1967; King, 1976). The Windermere Group equivalents unconformably overlie the "Beltian" strata. Evidence from orogenic episodes, intrusives, and fold belts recorded in the Windermere strata indicate the existence of the post-Belt, pre- Windermere rift event. The Pahrump Group is generally considered correlative with strata assigned to the Belt/Purcell Supergroup (King,1976; Stewart and Suczek, 1977), but the age of the Pahrump Group has not been accurately determined (Wright et aJL., 1976). K-Ar dating of the crystalline basement underlying the Pahrump Group and sills of the Apache Group, which are presumed correlative to those which intrude the Pahrump, bracket the initiation of Pahrump Group sedimentation between 1.7 and 1.4 b.y. (Wright et. a_l. , 1976) . The overlying late Precambrian to Cambrian strata assigned to the Noonday Dolomite and the Johnnie, Wood Canyon and Stirling Formations (Fig. 1) are thought to be time equivalents of the Windermere Group (King, 1976; Wright et al., 1976). Initiation of Cordilleran miogeo- clinal sedimentation has been placed at the base of the Kingston Peak Formation (Stewart, 1972), the Wood Canyon Formation (Diehl, 1976), and the Noonday Dolomite (Hazzard, 1937). The Pahrump Group apparently was deposited in a narrow north-northwest trending trough approximately 50 km wide and 100 km long. This depositional basin, termed the Amargosa Basin by Wright et: aJL. , (1976) , 5 FIGURE 1: Generalized columnar section of Precambrian to Lower Cambrian strata, Death Valley and eastern Mojave Desert region. Modified from Wilson (1978). GE N E RALIZE D COLUMNAR SECTION OF PR E CAM BRIAN TO LOWER C A M B R IA N STRATA DEATH VALLEY AND EASTERN MOJAVE DESERT REGIONS Top 2 z « a 8* §2 u i E ubOy ' t l f ' - J O o L i H ' O- c c c e 5 Is x 2 O X "• o !i . >j .n:tn f -^_i j - o " * 3° 5 2§$ I ^ v^ V -> ^ .v \A r h /y v w w v v y v /’ - • -r. • ar. —z. V A A A A / V N A A A / s / Quortzite, siltstone and dolomite Quortzite, gcnorolly feldspnthic, locally conglom eratic,orkose in lower port, Qlenellid. trilobite fragments "Dolomite, siltstone and sandstone, . SiLOji] b o *, Cruzionq Quartzite Mixed sedimenrory rocks; quanzite, clastic dolomite ord quiirtziilc dolomite, pebbly conglomerate, siltstone dolomite, I'tr.estone, volconir ash, Boxonlo Q uartzite, abundantly cros-ibedded, minor dolomite n o e tic dolomite an d quartzite obundcmtlv crossbedded Algo! dolomite with suverol "percent insoluHes -Clastic quartzih'c dolomite -Algal dolomite w ith less th a n o n e p e rc e n t in s c lu b le 3 .Clastic, limestone,th in ly b e d d e d U J Sii as 3 <r> O ' O “* V j wp _t«»<- I «t < i , O « » ? 2I f t E a ( X ^ — f \ R r S ? I Arkoslc sondstone and siltsione p ' f > o a O . o Conglomerate (diamictite) and sandstone, ond subordinate siltstone and shale) graded beds and gigantic closts common: may be os much os 1800m thick Conglomerate (dlomictlte) and sandstone, generally poorly bedded Sondstone and siltstone well laminated to massive thark gray dolomite, well laminated to massive, commonly stromatolitie, &esJL^.rxn5 L 0 , IfififlRig. oatitlc lenses and beds Mixed sedimentory rocks; sandstone siltstone, shale, dolomite,minor conglomerate, volconic osh Diobase (omphibolite) Chert Limestone, stromotolitlc, r ^r -ri B . o L t Q L i g , dacuiojibyJaa Dolomite,siliceous ^ 17 / 1 - T 'T Diabase Muds tone ^f el dsput hi c sandstone ond siltstone Arkosic conglomerate sondstone and siltstone Metasedlinents, medium green schls* facies, mostly granitic gneiss commonly plygmatlc, Dlabose and granitic Intrusives METERS 1 5 0 0 - fe? Bot tom M o d ified from Wright et at. (1 9 7 6 ) FfET - 5 0 0 0 - 4 0 0 0 - 3 0 0 0 -2000 - 1 0 0 0 O has been interpreted as an aulacogen (Wright et: aJL. , 1976), an epicontinental embayment (Harrison and Reynolds, 1976), and as a fault-bounded shallow water cratonal shelf (Stewart and Suczek, 1977) . Wright et a_l. (1976) proposed the initiation of vertical tectonics and develop ment of the Amargosa aulacogen during Crystal Spring and Beck Spring time and this presumably was associated with the pre-Belt rift event. This assertion has been questioned by others, notably Stewart and Suczek (1977), who cite the apparent uniformity in thickness and lithology and associated environments of deposition as evidence against the vertical tectonic model. Determin ation of the depositional environments and time strati- graphic relationships of the formations assigned to the Pahrump Group and exposed along the proposed basin margins could be crucial in resolving the structure and tectonic regime of the Amargosa Basin and its associated contin ental margins. Extensive stratigraphic mapping and some petrologic studies of the Pahrump Group have been carried out by L.A. Wright, B.W. Troxel, and their students (Diehl, 1976; Roberts, 1976; Wright et aJL., 1976, 1978), however, much of their data has not yet been published. Although the Beck Spring Dolomite has been recognized and mapped for many years, only one report describing the petrology and depositional environments of the type Beck Spring Dolomite has been nublished (Gudstadt, 1968). Numerous reports have described the abundant microbiota of the black cherts of the upper portion of the Beck Spring Dolomite. These cherts contain some of the earliest fossils of eukaryotic organisms (Cloud et. a^L. , 1969; Licari, 1971, 1978). Columnar and stratiform stromatolites have been described by Licari (1971) from exposures of the Beck Spring Dolomite in the Alexander Hills and Kingston Range. No attempt was made to classify the forms in detail or to utilize the stromatolites as either environmental or biostratigraphic markers. The Beck Spring Dolomite is of considerable impor tance both for reconstructing the regional Precambrian tectonics and increasing our knowledge of Precambrian biogeology. The main emphasis of this study is to define the detailed stratigraphy, petrography,petrochemistry, and stromatolite paleoecology and biostratigraphy for 5 selected areas where the Beck Spring Dolomite is exposed (Fig. 2). These areas include 3 localities presumed to be dominantly carbonate facies, Beck Spring (BS), Snow White Mine (SWM),and Acme Talc Mine (ATM); one mixed clastic-carbonate facies, Saratoga Spring (SS); and one metamorphosed, possibly mixed clastic-carbonate facies equivalent, Silurian Hills (SH). Stratigraphic studies are used to document proposed uniformity in thickness, petrology and depositional 9 FIGURE 2: Index map showing location of study locales, major mountain ranges, prominent structural features, major access routes, and major physiographic features of the Death Valley and eastern Mojave Desert regions. Study locales are Virgin Spring Wash (VSW), Saratoga Spring (SS), Acme Talc Mine (ATM), Beck Spring (BS), Snow White Mine (SWM), Winter's Pass (WP), Silurian Hills (SH), Seventeenmile Point (SMP) and Rainy Day Mine (RDM). Major mountain ranges are the Black Mountains (BM), Ibex Hills (IH), Nopah Range (NR), Alexander Hills (AH) , Kingston Range (I<R) , Clark Mountains (CM), Halloran Hills (HH), Silurian Hills (SH), and Avawatz Mountains (AM). 1 0 BM Shoshone VSW NR Tecopa BS AH *"»'9 SWM ATM KR WP g a r l o c k f a u l t z o n e SH / f ! l " " V SH CM HH 2 0 mi Baker RDM 20 SMP 3 0 km 116 environments for the various Beck Spring Dolomite locales. Detailed petrologic study of hand samples and thin sections provides evidence for interpreting original depositional environments and subsequent diagenetic alteration. Petro chemical investigations are used to define geochemical profiles of the Beck Spring Dolomite which may be of value in regional correlation and also may be used to test geochemical facies and diagenetic models in conjunction with textural and petrographic data. Stromatolite assemblages are tentatively identified and comparisons made with distinctive biostratigraphic ranges of similar assemblages outside the Death Valley region. Such bio stratigraphic studies may add critical data concerning the age of the Beck Spring Dolomite and the Pahrump Group. Stromatolite studies also may provide additional informa tion to test environmental reconstructions. Geologic Setting The Pahrump Group was originally mapped by Noble (1934) and named by Hewett (1956) and Kupfer (1960). The Pahrump Group consists of three formations, which are in ascending order, the Crystal Spring Formation, the Beck Spring Dolomite, and the Kingston Peak Formation (Fig. 1). These formations were named for prominent geographic land marks in the eastern Mojave Desert. The Crystal Spring Formation rests with profound unconformity on a crystalline basement from which K-Ar dates of 1.7 b.y. have been 1 2 FIGURE 3: Map of the outcrops of the Pahrump Group (cross hatched regions) in the Death Valley and eastern Mojave Desert regions. Major mountain ranges are the Funeral Mountains (FM), the Panamint Range (PR), the Black Mountains (BM), the Owlshead Mountains (OH), the Ibex Hills (IH), the Saddle Peak Hills (SP), the Alexander Hills (AH), the Kingston Range (KR), the Silurian Hills (SH), and the Avawatz Mountains (AM). Modified from Wright et a_l. (1976) . 13 O m O I , ’ I . v 'v<W \ I \ I / ? ; v , - w « ■ % . , V, - v ' - ?/ ; V v y/ K \jb - « y , y y a n I ® ,. ^ / i s 5U o „ w t /' / . ^ o> / I t ^ J ‘ JD ^.? , . ^ ^ j ~D. y/ 1 ^^S5$fiT/f > y . . . c i € ' / ( ? ) y pmy\ " " / C ' s , V . ""WV/ ^ ./v / _ r'v'- / ^ ? .............. • ' m • ;=*/ £■ £. ,f? r ^ . V / 2 , , n * . s 7 - “ ' " i / / h w h CD is CO f 'V V - f i /Y I i ^ J F l y O I f c n obtained (Silver et. al,. , 1962) and is intruded by diabase sills thought to be correlative with similar 1.2 b.y. old diabases which intrude strata of the Apache Group in southern and central Arizona (Shride, 1967; Wrucke, 1972). The age of initiation of Pahrump sedimentation appears to be bracketed between 1.7 and 1.2 b.y. Wright et al. (1976) believe initiation of sedimentation occurred at approximately 1.4 b.y. The Pahrump Group and the Beck Spring Dolomite crop out extensively in the Death Valley region and adjacent areas to the south (Fig. 3). The region is bounded on the west by the Panamint Range and on the east by the Kingston Range. Strata extend northward to the Funeral Mountains and at least to Silurian Hills and the Halloran Summit complex in the south (Stewart, 1970; Wright et al., 1976; E. Dewitt, pers. comm., 1978). The eastern Mojave Desert is a structurally complex region but all of the studied areas, with the possible exception of Silurian Hills, are believed to retain their original paleogeo- graphic relations to each other (G.A. Davis, pers. comm., 1977). The sites of this study are presumed to lie within the allochthonous plate of the Winter's Pass thrust, whereas Silurian Hills may represent a window through the allochthon (G.A. Davis, pers. comm., 1977). The region is bounded on the northwest and west by branches of the Garlock and Death Valley fault systems and on the east by the Clark Mountain thrust complex. 16 STRATIGRAPHY AND PRIMARY SEDIMENTARY STRUCTURES Type Locality The type locality for the Beck Spring Dolomite lies in a narrow east-trending canyon in the Kingston Range near Horse Thief Springs. The published stratigraphic section as reported by Gutstadt (1968) consists of 3 informal members; the lower laminated, middle oolitic, and upper cherty members. The lower laminated, member is composed of 150-200 m of mosaic to pelletal dolomite with "bird's eye" structure and stromatolites. The oolitic middle member contains 100-150 m of mosaic dolomite with abundant oolite, pisolite, peloid, and grapestone inter vals. The upper member (100-150 m) has abundant nodules and interbeds of chert, stromatolites, oolite, pisolite, peloids, and grapestone. The upper member is dominantly mosaic dolomite with evidence of partial silicification and dedolomitization. Other Localities Stratigraphic sections were measured at 5 localities in the eastern Mojave region. Measurements were made using a Jacob's staff and Brunton compass or Abney level following Compton (1960). Samples were collected from each distinctive lithologic interval. A bed is defined in this study as a sedimentation unit formed under essentially constant physical conditions and constant delivery of the same material during deposi 17 tion (Reineck and Singh, 1975). A lamina is defined as the smallest megascopic layer in a sedimentary sequence (Campbell, 1967). Although disagreement exists in the definition of thickness limits for beds and laminae (McKee and Weir, 1953; Ingram, 1954; Campbell, 1967) this study limits laminae to layers less than 1 cm in thick ness. Thin bedding (or thick laminae) in the disputed 1-10 cm interval is not common in the Beck Spring Dolomite where beds are usually several meters in thickness and laminae are limited to l-5mm in thickness. More than 80% of the observed laminations are bio- sedimentary laminations produced by algal mat activity (cryptalgalaminites). Cryptalgalaminites may be distin guished from inorganic sedimentary lamination by the following criteria: (1) the lamination is not readily explainable as a product of a sedimentary process such as quiet water settling, current deposition or chemical precipitation, (2) the laminae do not pinch and swell to compensate for underlying topographic relief but often encrust the surface, (3) the laminae show a distinctive intralamina microstructure which consists of a variety of clotted, banded, or striated structures, (4) traces of algal filaments or microfossils may sometimes be present in the laminae, (5) small-scale disconformities, mm size domes and bubbles, sparry laminoid fenestrae, and dessi- cation cracks are common, (6) intraformational algal chip 18 conglomerate composed of clasts of cryptalgal sediment are common, as are (7) columnar, nodular and domal stromatolites (Aitken, 1967; Monty, 1976). Using the above criteria, dolomites lacking cryptalgal lamination are rare. As a consequence, the term "lamination" as used in this report refers almost exclusively to crypt algal ami nation . Beck Spring.-- The section was measured on both north and south facing slopes of the ridge due north of the workings of the Vulcan Mine in the Kingston Range, approximately 27 km east of Tecopa, California (Sec. 32, T. 20 N., R. 10 E., Horse Thief Spring quadrangle, Figs. 4,5). The Beck Spring Dolomite forms prominent cliffs and the main ridge crest in this area. The blue-grey strata (N3-N6) are steeply dipping to slightly overturned. The section is 485 m thick. The lower 222 m is dominated by fine 1-2 mm to coarse 1-2 cm,subparallel to undulose, medium grey (N4-N5),laminated and thin-bedded dolomite. Irregular to crescent-shaped spar and chert-filled vugs and fenestrae (Gustadt's "bird's eye structures") occur throughout the interval. They are especially prominent in the interval 30 m below the first occurrence of the pisolite-oolite beds. Large, broadly domal and ridge shaped laminated structures are probably pseudodomal and stratiform stromatolites. Areas of eroded and draped laminae are also common (Fig. 6D,H). The large eroded FIGURE 4: Generalized columnar section of the Beck Spring Dolomite, Beck Spring, Kingston Range. s Joia'.u BECK SPRING SECTION 400 3 5 C 3 OC 0oO° 1 OC Red-grey shale, Kingston Peak Fm. Contact talus covered Lenses of dark brown shale, 0.5-1.5 m thick Chert nodules to 0.5 rr, Laminae in d is tin c t, disrupted and contorted around nodules Abundant in tra c la s tic s Chert nodules and interbeds to C.5 m thick S i l i c i f i e d , nodular stromatolites Well laminated with chert nodules 0.2 m to 0.6 m, interbeds to 0.3 m . thick Red-brown granule sandstone Chert nodules and interbeds, 0.1 to 0.2 m thick P is o lite lenses, 0.5 to 1.0 m thick Contorted and brecciated laminae Ooliths, l-2mm, i r. lenses and interbeds, 0.3 to 1.0 m thick Well laminated, (l-3mm), laminae forming hummocks and ridges to G. 7 rr in heigh: Well laminated Abundant spar and c h e rt-b ille d fenestras, highly '-ecrystall i zee Laminae 2-3mm, planar to moderately steeply eroded and oraped Poorly laminated (Partial talus cover) Sparry vugs and channel: Laminae, 2-3mm, disrupted and draped Poorly laminated, slump breccia Laminae l-3mm, scattered s p a r - fille d fenestrae Thir, l-2cm beds Laminae l-2mr. C-racational contact, red-brown shale, Crystal Sprino Fm. ! . 0 km BECK , SPRING \ 2 1 FIGURE 5 Legend for generalized and detailed columnar sections of the Beck Spring Dolomite. 2 2 STROMATOLITIC STRUCTURES 2; UJ C_5 o oc co < C LlJ h- DC ^ :s> LU h- ' S L CJ> 1 —1 n> Q DC UJ r- CO CO DC < < S' —I 2_ CD II1 O O O L u DC 1 — CO © 6 <3 X* A a >C fs i W S ; • ' DC | ^ O- M Pi soli ths Ooli ths Grapestone Intraclasts Brecci a Macrointraclastic beds (clasts 5cm - Microintraclastic beds (clasts 5mm - Mud cracks Chert nodules Fenestrae Sparry vugs and veins Baicalia-11ke forms Conophyton Nodular Wavy, wrinkled laminae Undulose laminae Planar laminae Eroded and gently draped Eroded and steeply draped >1 m) 5cm) 23 FIGURE 6: Photographs of sedimentary structures and lithotypes of the Beck Spring Dolomite. (A) Lenticular bedding in a sandy dolomite, lower cherty member, Sara toga Spring section. Exposed portion of scale is approximately 10 cm long. (B) Low angle planar cross strata in a sandy dolomite, lower cherty member, Saratoga Spring section. Exposed portion of scale is approximately 7 cm long. (C) Shoaling upward cycle in lower cherty member strata, Saratoga Spring section: (a) shows flaggy, basal shale, (b) massive dolomite, and (c) wavy, intraclastic laminated dolomite. Scale is 15 cm long. (D) Eroded and steeply draped lami nation, lower laminated member, Beck Spring section. Lens cap is 52.5 mm in diameter. (E) Cherty intraclastic packstone- wackestone , lower laminated member, Snow White Mine section. Lens cap is 52.5 mm in diameter. (F) Large scale carbonate intraclasts in a subarkosic sandstone, lower cherty member, Saratoga Spring sec tion. Bar is 5 cm long. (G) Laminated, bedded pisolite unit, oolitic-pisolitic member, Saratoga Spring section. Exposed portion of scale is approximately 12 cm long. (H) Eroded and gently draped lami nation, lower laminated member, Snow White Mine section. Bar is 3 cm long. (I) Rounded, intraformational conglom erate, lower laminated member, Acme Talc Mine section. Lens cap is 52.5 mm in diameter. 24 25 and draped structures appear to be the result of erosion of cm to m size blocks of semi-lithified algal laminated sediment. Blocks and small fragments have been ripped up and redeposited in the surrounding mud at various angles. Subsequent overgrowth by algal mats bridged many of these hummocks and troughs but did not conform precisely to the shape of the irregular surfaces,indicating at least some rigidity to the new mat growth. The smaller mm and cm size eroded and draped structures (Fig. 7C) bear a super ficial resemblance to cross bedding with numerous uncon formities and reactivation surfaces. Laminated, domal algal structures several m in diameter have been truncated by erosion. These structures may represent incipient domal stromatolitic growths which were thwarted by repeated erosion. Erosion of these larger structures could have provided much of the large cobble to boulder size laminated intraclastic debris which is common in the lower laminated member. The lamination in the lower member is predominantly coarse, several mm or more in thickness. Laminated intraclasts show angular, stair step edges and little flexure suggesting semi-rigid or lithified mats (Figs. 61, 7G). The rectangular, pris matic shape of the clasts plus their angular edges and laminated structure is similar to high energy, flat pebble conglomerate, channel bank slump and beach washover ridge deposits (Shinn et al., 1969; Bardie, 1977). Much of the FIGURE 7: Photographs of small-scale primary structures in the Beck Spring Dolomite. (A) Planar macrolaminae, lower lami nated member, Acme Talc Mine section. Bar is 1 cm long. (B) Macrolaminated dolomite with microintraclastic (int) layer, lower laminated member, Snow White Mine section. Bar is 1 cm long. (C) Eroded and gently draped laminae with small diastems (d), lower lami nated member, Snow White Mine sec tion. Bar is 1 cm long. (D) Disrupted, wavy lamination, lower laminated member, Snow White Mine section. Bar is 1 cm long. (E) Thin cm bedding with large sheet cracks (sc). Beds are slightly warped and bend upwards at the ends, upper cherty member, Snow White Mine section. Bar is 2 cm long. (F) Detail of eroded and draped lamination showing refolded, curled, lamina edge, lower laminated member, Acme Talc Mine section. Bar is 0.5 cm long. (G) Intraformational breccia, lower laminated member, Acme Talc Mine section. Note jagged intraclast margins and poor sorting. Inter spaces are filled with coarsely crystalline dolospar. Bar is 1 cm long. 27 28 lower portion of the section is highly fractured, eroded, and covered by surficial slump breccia. Distinct beds are difficult to define but probably would average 1-2 m in thickness. The laminated zone is succeeded by a light to medium grey (N5-N6), 60-70 m thick pisolitic-oolitic-intraclastic interval. The allochemical grains are generally 1-2 mm to more than 1 cm in diameter and moderately sorted and rounded. Ooliths and pisoliths are often selectively silicified. Allochems occur in thin beds and lenses no more than 5 m thick which are stratified with broadly undulose, coarsely laminated dolomite and irregular, contorted, and brecciated dolomite. The coarsely laminated dolomite gradually changes to finely laminated, wavy to wrinkled dolomite in the upper part of the zone. The top of the interval is a gradational change with the introduction of spar and chert-filled fenestrae and red- brown weathering chert nodules. The upper 150-180 m is dominantly thin 0.5 to 1 m beds of medium grey (N4-N5) to tan (5 YR 6/4) dolomite with abundant chert nodules and interbeds. The dolomite is massive to sandy with contorted laminae prominent around chert nodules and lenses. Rare red-brown weather ing granule sandstone beds from 0.4 to 0.6 m thick and chocolate brown shale lenses from 0.5 to 1 m thick are the only terrigenous clastic units present. Nodular 29 stromatolites are present in the upper cherty intervals. Non-silicified forms also are present but are much less conspicuous. Acme Talc Mine.— The area studied lies approximately 18 km southeast of Tecopa, California, in the Alexander Hills, a southern extension of the Nopah Range. The stratigraphic section (Fig. 8) is a composite of two sections measured (1) on the west facing slope of the ridge between the workings of the Acme and Western Talc Mines and (2) in a dry stream bed which cuts the ridge (Sec. 4, T. 19 N., R. 7 E., Tecopa quadrangle). The Beck Spring Dolomite in this area forms shallow slopes with a few prominent ridges near the top of the section. Expo sures are excellent in the stream bed due to smoothly eroded surfaces but the section is disrupted by surficial slump deposits and minor faults. The composite section totals 405 m. The same three informal members appear to be present here as at the type locality. The lower portion of the section is light to medium grey (N4-N5), well laminated and cherty in its lowermost parts. It lies in gradational contact with the red-brown to orange shale and dolomite of the upper Crystal Spring Formation. Well-developed beds of intraformational conglomerate with intraclasts of mm and cm size (micro- intraclastic deposits, Fig. 7B) to tens of cm's size (macrointraclastic deposits, Fig. 6 1) are present. The 30 FIGURE 8: Generalized columnar section of the Beck Spring Dolomite, Acme Talc Mine, Alexander Hills r n e 1 o r s ACME TALC MINE SECTION 4 0 0 350 300 250 20 0 - I 50 ICO 50 V ^ ! l » Ci^ar? / » 'V # / < » q = d ' 3 j % g? / Grayish-red shale, Kingston Peak Fm. Sharp contact Tan to med. gray; laminae contorted and disrupted Brown shale, laterally discominuaus Poorly laminated to massive; abundant chert nodules and interbeds to 0.3 m Interbeds of intact and fragmented stramatol ites ( Q o i c d i o - like) , flat pebble conglomerate, and scattered small chert nodules (See detailed section) B a i c alia - I i ke stromatolites oo o o oo o ® © ® © Ooliths and pisoliths in lenses and thin beds, 10 cm to I m Light gray, poarly laminated to massive-, abundant fenestrae Well laminated (1-3mm), eroded and gently draped-, scattered mud cracks and curls Well laminated-, eroded, disrupted and steeply draped L Cobble to boulder sized angular in+raclasts Planar, undisrupted laminae AiZiA / k \ / a\ Intraformationai rubble C o n o p h y t o n. often frcctured and overturned Boulder sized angular intraclasts Abundant nodules and lenses of chert 5 x 20 cm Tan to med. gray, laminae highly contorted; abundant fenestrae Very dark gray, intraclastic Dark gray, intraclastic Gradational contact Light brown dolomite with lenticular sncle interbeds, Crystal Spring Fm. 1.0 km W EST ER lALC MIN ACME TALC MINE 32 intraformational conglomerate occurs in distinct beds and admixed with coarse, 2-3 mm, laminated dolomite and columnar stromatolites. Eroded and draped laminae as well as spar-filled fenestrae and vugs to several cm in length also are common. The pisolitic-oolitic interval is relatively thin (15 m) and shows no preferential weathering of allochems. Ooliths and pisoliths as much as 6 mm in diameter were observed but no grapestone was found. Beds are thin, 15-30 cm in maximum thickness. Above this zone is a series of beds of columnar- layered and columnar stromatolites similar to members of the group Baicalia Krylov. Abundant micro- and macro- intraclastics material also is present. Well developed transitions from stratiform to columnar-layered to disrupted columnar stromatolites are observed (Fig. 9). The disrupted columns often are surrounded and overlain by deposits of laminated intraclasts. These sequences suggest repeated erosional events of moderate to high energy and low environmental stability. Sheet cracks (straight-sided in cross section), curled stromatolitic laminae, eroded and draped laminae and wrinkled, wavy laminations are also present throughout the interval (Figs. 7 D,E,F). The upper 220 m is irregularly to indistinctly laminated and contains irregular red-brown weathering 33 FIGURE 9: Detail of stromatolitic intervals. Acme Talc Mine section. (A) Interval from approximately 260 to 375 m in the section showing detail of vertical successions of stromatolite morphologies and associated sedimentary structures. (B) Detail of several cycles of Stratifera and Baicalia-like form growth and disruption. ers 1 meter chert nodules, seams and interbeds as much as 0.3 m thick. There is also one bed of red weathering shale approxi mately 6 m thick. The upper contact with the tan-grey shale of the Kingston Peak Formation is distinct and sharp suggesting a possibly disconformable contact. Snow White Mine.— The Snow White Mine section (Fig. 10) was measured on the west and southeast facing slopes of a small ridge approximately 1 km south of the workings of the Snow White Mine (Sec. 6, T. 20 N., R. 11 E., Horse Thief Springs quadrangle). Snow White Mine lies at the eastern edge of the Kingston Range approximately 14 km east of Beck Spring. The strata in this area form the crest of the ridge as well as the low lying slopes on either side. The section is incomplete in this area. The base of the section is covered by alluvium, and the upper portions of the section are offset by several large faults. However, lithostratigraphic sequences similar to those observed at other Beck Spring Dolomite localities are exposed here. The preserved, measurable section at this locality totals approximately 296 m. The basal strata are medium to dark grey (N3-N4) with fine mm laminae and scattered chert nodules. Above this interval are well laminated strata with laminae from 1 to 5 mm thick, pseudodomal and stratiform stromatolites, irregular spar-filled fenestrae, vugs and channels, abundant micro- and macro- 36 FIGURE 10: Generalized columnar section, Beck Spring Dolomite, Snow White Mine section, eastern Kingston Range. SNOW WHITE MINE SECTION A 300 — 250 — 200 §1 oqQ i&ST @ @@ ‘ / X x 50 C 5 C3 1:3-=7 7- Ot= i < £ ? ■ j c ^aS_Z_ 00 — A c^.A 50 — 0 — Breccia of well laminated dolomite Massive Cherty laminae, l-3mm Talus slopes Beds of massive dolomite, scattered chert nodules, a lte rn a tin g with beds o f wavy laminated, oatmeal texture in tr a c la s tic s and s i l i c i f i e d nodular stro m tolite s Fault breccia Thin 0 .1 - 0 .2m beds o f o o lith s , p i s o l it h s , and in tra c la s ts Eroded and draped, 2-4mm laminae Eroded and draped, th ic k laminae (to 1 cm) I. C km 1ntraformational rubble Talus cover Very dark grey (N2-N3), wavy, in d is tin c t laminae Quartz-rich sandstone. Crystal Sprang Fm. EXCELSIOR MINE./, / S N O W WHITE ’ / mineV \ 3800 38 intraclastics, and eroded and draped laminae. The pisolitic-oolitic interval is only about 15 m thick. Allochems are quite distinct and weather promi nently due to partial silicification. Well-preserved intraclasts as much as several cm in length and ooliths and pisoliths to 6 mm in diameter are the dominant allochemical grains. The allochems are well rounded and show moderate sorting. The upper cherty portion of the section is fault offset at two stratigraphic levels making accurate reconstruction impossible. However, lithologic changes across the faults are not drastic and the large segment between the faults preserves many structures and textures. Fault breccias dominate the top and bottom of this inter val. The intervening dolomite is massive to highly con voluted, medium grey (N4-N5) to tan-orange (10 YR 7/4) with thin sandy and shaly lenses less than 0.3 m thick. Red-brown weathering chert nodules and seams are abundant. Silicified isolated nodular stromatolites are well pre served and abundant in distinctive "oatmeal texture" cherty intraclastic intervals (Fig. 6E). Well-developed thin beds from 2-3 cm thick contain prominent sheet cracks (Fig. 7 E ) . Laminations are wavy and wrinkled in the lower portion and irregular, indistinct,or absent in the upper portion of the upper cherty member. The basal Kingston Peak strata is red-brown weathering shale but 39 the contact is obscured by fault debris and talus. Saratoga Spring.— This study area is located on the east facing ridges above Saratoga Spring at the southern tip of the Ibex Hills (Sec. 2, T. 18 N., R. 5 E., Avawatz Pass quadrangle). The area lies within the southern boundary of Death Valley National Monument approximately 12 km from Highway 127. The measured section consists of 331 m of interbedded carbonate and clastic strata (Fig. 11). The strata weather to form a prominent slope of small ridges and saddles. The Saratoga Spring section differs from the previous sections in the presence of frequent sandy and shaly intervals, the distribution of chert throughout the section,and the distinctive variations in weathering colors. The coarse-grained, terrigenous elastics are sand to granule size grains ranging in composition from quartz- arenite to subarkose (McBride, 1963) and often contain an average of 25% carbonate intraclasts. These coarse grained elastics occur as thin beds and lenses within both carbonate and shale intervals. The shale occurs as flaggy, medium orange (10 R 5/4 or 10 R 3/4) to medium grey (N4-N5) beds usually less than 0.5 m in thickness. Terrigenous clastic strata, mostly shale, comprise 32 m (9.8%) of the 331 m of strata measured. The majority of these clastic intervals occur within the lower 150 m 40 FIGURE 11: Generalized columnar section, Beck Spring Dolomite, Saratoga Spring section, southern Ibex Hills. SARATOGA SPRING SECTION ©@j§> 300 250 200- 0 0 50 Red-brown shale, Kingston Peak Fm. P is o lith s to 5mm in diam eter,occasionally interbedded with laminated dolomite Black (N2) dolomite, highly r e c ry s ta lliz e d Q uartz-rich sandstone Dark grey (N3-N4) dolomite Red-brown sandy dolomite Quartz-rich sandstone Q uartz-rich sandstone lenses Chert seams and contorted nodules Dark grey (N3) dolomite Orange shale Granular sandstone Interbeds of massive dolomite, undulose, wavy laminated dolomite with chert nodules and carbonate i n tr a c la s ts , and shales and sa ndstones Dark grey (N3) dolomite Sandy granule dolomite with lenses o f qu artz-rich sandstone Shale lenses Quartz-rich sandstone lenses Quartz-rich sandstone, Crysta1 Spring Fm. 1.0 km 42 of the section. The base of the section lies conformably on a granule quartzarenite of the upper Crystal Spring Formation. The strata are thin-bedded, usually from 0.3 to 0.6 m thick. The dolomite varies in weathering color from dark grey (N3) to medium orangish brown (5 YR 5/6) with the majority probably medium grey (N4-N5) to light brown (5 YR 6/4). Chert nodules, wavy and wrinkled laminae and sandy dolo mite are distributed throughout the lower 275 m of strata. Moderately sorted and rounded carbonate intraclastic material is frequently mixed with clastic grains, mostly quartz and potassium feldspar, in many of the carbonate intervals. Intraclasts as much as several cm long occur in the lower member (Fig. 6F). The dominant clast size is usually less than 1 cm. A crude cyclical pattern is observable in some portions of the lower 150 m of this section (Figs. 6C,12). Each cycle consists of an interbedded carbonate and shale sequence. The base of the sequence is usually a thin orange or grey cryptographic dolomite with good conchoidal fracture. Above the massive dolomite is a transition from flat to undulatory to highly wrinkled laminae often associated with chert and/or intraclast's at the top of the sequence. This transition can occur in a stratigraphic thickness of 3 m or so but usually occurs in less than 1 m. These sequences may represent 43 FIGURE 12: Detail of shoaling upward cycles, lower cherty member, Beck Spring Dolomite, Saratoga Spring section. 44 Orange to purple shale Massive li g h t tan to orange dolomite Medium grey (N4-N5), fin e m m laminae Medium grey (N4-N5) to very dark grey (N2-N3) Orange-brown dolomite with broad 0.5cm laminae Orange shale Sandy, medium grey (N4-N5) Orange-purple shale Very dark grey (N2- N3) Fine m m laminae Red-orange sandy shale Medium grey (N4-N5) Fine m m laminae Fine m m laminae Red-orange dolomite with coarse sand grains and chert nodule In tra c la s ts 0 .3 -2 .0cm Broad 0.5cm laminae Fine m m wavy laminae Massive medium grey (N4-N5) Medium grey (N^-N5) Orange-red shale 45 small amplitude, shoaling-upward cycles. The shaly- massive dolomite intervals may be quiet water, suspension deposits. Subsequent emergence then is accompanied by the development of planar then wavy, wrinkled algal mat deposits. The fine intraclastic material is similar to deposits of curled, dessicated algal mat chips seen in the Bahamas (Shinn et a_l. , 1969; Ginsburg and Hardie, 1975; Hardie, 1977). The chert layers and nodules may reflect early diagenetic infilling of primary laminoid fenestrae also common in algal mat deposits (Shinn et aJL. , 1969; Davies, 1970a; Hailey, 1975; Zamarreno, 1975; Monty, 1976; Hardie, 1977). Sandy dolomite intervals show planar to low angle cross stratification (Fig. 6B) and possible laminated, lenticular bedding (Fig. 6A). Similar low angle cross strata and lenticular bedding has been described from many "tidalite" sequences (Klein, 1971, 1975; Barnes and Klein, 1975). The pisolitic interval at Saratoga Spring occurs near the top of the section. At the base of the interval is a small notch in the ridge containing a small fault. Pisoliths show distinctive sheared and deformed fabrics. The dolomite immediately adjacent to the fault zone is coarsely crystalline and jet black. The beds change dip from 20 to 30 degrees but substantial offset or loss of strata is not indicated. Pisoliths as much as 5 mm in diameter are the only 46 apparent allochems in this interval. The pisoliths and interlaminated dolomite (Fig. 6G) form a uniform bed approximately 6 m thick. Numerous pisoliths show partial silicification and weather differentially to produce a distinctive red-brown, knobby surface. The top of the section is covered by talus and/or alluvium but reconnaisance of the area indicated that the covered section comprised only an additional 9-15 m of interbedded pisolite and laminated dolomite. The pisolite beds were traced to within 1.5 m of the base of the over- lying Kingston Peak Formation. The contact itself is also covered by talus. Silurian Hills.— The Silurian Hills section (Fig. 13)lies at the northwest corner of the Silurian Hills approximately 0.4 km northeast of the end of the road and 6 km from High way 127 (Sec. 26, T. 16 N., R. 8 E., Silurian Hills quadrangle). This area has been studied in detail by Kupfer (1954, 1960). The measured section is composed of low lying, nearly vertical to overturned strata. Only 40 m of strata is apparent at this locality. The rocks are mostly blotchy, blue-grey (5 PB 3/2) to orangish grey (10 YR 7/4) dolomite with dark red-brown weathering chert seams and nodules. The blue-grey carbonate shows a faint wavy lamination and minor brecciation. There also is some highly weathered and altered pink to orange weathering,1.5 to 1.8 m thick, FIGURE 13: Generalized columnar section, Beck Spring Dolomite, Silurian Hills section, Silurian Hills. SILURIAN HILLS SECTION meters Pink-red sandstone, Kingston Peak Fm . Yellow-brown sandy dolomite Blue-grey laminated dolomite Pale orange quartz-rich sandstone Blotchy blue-grey dolomite Orangish-tan to grey dolomite, laminae wavy and crinkled Granular quartz-rich sandstone Blotchy blue-grey dolomite Chert seams, 10 to 15 cm thick Yellow-orange dolomitic sandstone Pink sandstone, Crystal Spring Fm . 1.0 km granule size, quartz-rich sandstone and shale. Chert seams 7 to 10 cm thick occur. The basal quartz-rich carbonate rock rests on a red-brown weathering sandstone. The upper contact occurs between a sandy, yellow-brown carbonate with chert nodules and a poorly sorted pink to red weathering sandstone. Summary and Discussion Straticrraphic Correlations.— Proposed stratigraphic correlations for the 5 studied areas are shown in Fig. 14. This lithostratigraphic correlation is based on the apparent persistance of the informal members recognized by Gutstadt (1968). Similarities among the three dominantly carbonate locales are particularly striking. All three sections show similar planar to wavy laminated structures of presumed algal origin with minor basal chert in the lower member; similar allochemical types and distributions in the oolitic-pisolitic member; and a well- developed, cherty and stromatolitic upper member. A proposed lower cherty member also is included in the correlation diagram. The designation of this informal member is based on distinct stratal differences in the lower and upper portions of Gutstadt's (1968) lower laminated member. The proposed lower cherty member is especially well-developed at the Acme Talc Mine section. The base of the section has abundant chert nodules and thin interbeds and fine, wavy,and undulose laminated 50 FIGURE 14: Stratigraphic correlation diagram showing the proposed lithostrati- graphic correlations of the informal members of the Beck Spring Dolomite. Abbreviations used in the figure are: L.C. = lower cherty member, L.L. = lower laminated member, O.P. = oolitic-pisolitic member, and U.C. = upper cherty member. 51 Beck Spring BS SWM 450 ATM SS Acme Talc Mine 400 400 Snow White Mine 10 km 300 350 350 Saratoga Spring @ @ @/ X 250 300 300 300 200 °® @ @ 7 «°oo/. o .P - <5 ® 7 — - " ¥ZZ 250 250 17 200 200 00 150 50 50 00 V.. 50- 50 Thickness in meters dolomite. Above this zone is a distinct change to broad, semi-planar laminated dolomite with eroded and draped structures and abundant large- and small-scale intra- formational conglomerate. The distinct change from finely laminated cherty beds to broadly laminated, eroded beds also occurs at Snow White Mine though it is less well developed. Most of the lower portion of the Snow White Mine section is covered, but the lowest observable outcrops are finely laminated and cherty. The lower cherty unit is moderately well developed at Beck Spring. It consists of minor basal chert in a few thin , finely- laminated dolomite beds. The bulk of the section at Saratoga Spring does not correlate well with Gutstadt's type Beck Spring section. The Saratoga Spring section may, however, show positive affinities to the proposed lower cherty member inasmuch as it contains only finely laminated and cherty beds. The lower 2 30 m of the Saratoga Spring section therefore is tentatively assigned to the lower cherty member. All of the strata at Silurian Hills appear to belong to the lower cherty unit based on analogy to the Saratoga Spring section. Most of the strata at Saratoga Spring are presumed lower member equivalents due to (1) their position below the pisolitic interval, (2) the lack of well developed nodular or columnar stromatolites, and (3) the possible nature of the Beck Spring~I<ingston Peak contact at 53 Saratoga Spring. Although the contact was not observed directly, the proximity of the pisolitic bed to the over- lying shale indicates no substantial change in lithology beneath this contact. Within the 1.5 m interval between the stratigraphically highest exposure of the pisolite and the lowest occurrence of shale assigned to the Kingston Peak Formation, a transition occurs from deposi tion in a presumed agitated, shallow water carbonate shoal environment to deposition in a deeper water sub marine fan and channel complex (Stewart, 1972; Basse, 1978). Due to the drastic nature of the change in deposi- tional environment, one is inclined to view this contact as unconformable. Very little Beck Spring Dolomite is preserved in the Silurian Hills section. The preserved strata show characteristics of both the upper cherty and lower cherty members. The Silurian Hills strata have chert veins, nodules and interbeds. Faint, recrystallized lamination is present in the carbonate. Weathered sandstone and shale are interbedded with the carbonate. Kupfer (1960) cites no evidence for the presence of unconformable upper or lower contacts for the strata assigned to the Beck Spring Dolomite at Silurian Hills. However, the extremely thin deposit at Silurian Hills must be the result of nondeposition, deposition in a very different facies, or loss by erosion. The Beck Spring Dolomite at Silurian Hills is associated with coarse-grained clastic strata in the Crystal Spring and Kingston Peak Formations which have been interpreted as nearshore deposits along the southern margin of the Amargosa Basin (Wright et al., 1976). The lack of thick strata of the Beck Spring Dolomite at Silurian Hills may reflect thinning towards the paleotopographic high (Mojave Upland) along the proposed southern margin. The closest apparent petrologic and stratigraphic analogue of the Beck Spring Dolomite strata at Silurian Hills is the lower part of the section at Saratoga Spring. The Silurian Hills section is there fore lithostratigraphically assigned to the lower cherty member based on this similarity. Nature of the Upper and Lower Contacts.-- The contact between the Beck Spring Dolomite and Crystal Spring Formation is well exposed at the section localities. The contact appears gradational and therefore conformable in the areas observed. The gradational nature of the contact is especially well developed at the Acme Talc Mine and Beck Spring localities. In other areas the Beck Spring- Crystal Spring contact commonly is a thrust fault (Wright, 1968). However, Wright (1968) also suggests the Beck Spring-Crystal Spring contact may be disconformable in some areas because the diabases which intrude the Crystal Spring Formation do not intrude the Beck Spring Dolomite. The contact between the Beck Spring Dolomite and the 55 Kingston Peak Formation is difficult to interpret. In all the localities studied, the basal unit of the Kingston Peak Formation is a tan to grey, red-brown weathering shale. The only clean exposure of the upper contact is at the Acme Talc Mine section. There the contact occurs between a ridge-forming, disrupted laminated dolomite and the shale. The contact is sharp and planar. No rip up clasts or truncation of underlying structures which might suggest an erosional surface were observed. However, lack of these structures may be due to the limited erosional capability of the fluid in which the overlying sediment was entrained. The presence of a disconformity between the Kingston Peak Formation and the Beck Spring Dolomite at Acme Talc Mine can neither be demonstrated nor eliminated as a possibility. Angular unconformities between the Kingston Peak Formation and underlying Beck Spring strata have been reported in the southeastern and central Panamint Range (Wright, 1968; Labotka and Albee, 1977) and in the eastern Kingston Range (Wright, 1968). Wright (1968) states that the Kingston Peak-Beck Spring contact is believed to be conformable within the central region of the Amargosa Basin, an area which includes all of the study locales except Snow White Mine. However, the inclusion of large amounts of Beck Spring Dolomite clasts in the conglomer ates of the Kingston Peak Formation suggests a more wide 56 spread occurrence of the Beck Spring Dolomite than is indicated by the areas now exposed. Therefore, both the paleogeographic extent of the Beck Spring and the nature of the Beck Spring-Kingston Peak contact cannot be resolved from this or previous studies. Primary Sedimentary and Biosedimentary Structures and Environmental Interpretations.-- The primary structures of the Beck Spring Dolomite show the effects of algal domination of the environment in the formation of distinctive algal structures and modification and exclusion of other sedimentary structures. The omni present algal mats agglutinate otherwise free-moving carbonate and clastic grains, whereas the erect and columnar stromatolites most likely precipitated carbonate directly and acted as sediment baffles. Primary structures similar to those observed in shallow marine clastic deposits (i.e. low angle planar cross strata and lenticular bedding) are observed only in clastic-rich dolomite where algal control of laminae morphology is reduced or eliminated. The primary structures and litho- types observed are listed in Table 1. The table is intended as a comparison of the overall morphological similarities of sedimentary structures and does not intend to imply necessary similarity in genetic processes responsible for the structures. The wavy to wrinkled macrolaminae, microintraclastic beds, and chert nodules suggest deposition of the lower 5 7 TABLE 1: Primary sedimentary and biosedimentary structures and lithotypes of the Beck Spring Dolomite. Abbreviations are: LC = lower cherty member? LL = lower laminated member; O-P = oolitic-pisolitic member; UC = upper cherty member. Asterisk indicates Saratoga Spring section only. 58 Structures and Lithotypes Beck Spring Informal Members LC LL 0-P UC Mi croenvironments References planar, macro laminae (3-5mm) with sheet cracks (Stratifera)? X Recent: Supratidal pool and channel Intertidal High algal marsh, levee,pond Ancient: Supratidal Intertidal Shinn et ad. , 1969 Kinsman and Park, 1976 Hardie, 1977; Monty and Hardie, 1976 Matter, 1976; Roehl, 1967 Gill, 1977 convolute, microlaminae (l-3mm) (Stratifera)? X X X Recent: High algal marsh Intertidal - supratidal Ancient: Middle to upper intertidal Tidal pond, supratidal marsh Ginsburg and Hardie, 1975; Hardie, 1977 Deffeyes et al., 1965; Davies, 1970a Read, 1973, 1975 Hoffman, 1975, 1976 smooth to undulose microlaminae (1-3mm) (Stratifera)? Ul < r > X X X X Recent: Low intertidal Upper intertidal Ancient: Upper intertidal Lower intertidal, levee Woods and Brown, 19 75 Logan et al,. , 1974 Hoffman, 1975 Read, 1973, 1975; Wanless, 1975 TABLE 1 (continued) Structures and Lithotypes Beck Spring Informal Members LC LL 0-P UC Microenvironments References eroded and draped macrolaminae X Recent: Levee banks, channel bars Ancient: Upper levee back slope Hardie, 1977 Wanless, 1975 planar, low- angle cross strata X Recent: Channel lags Ancient: Tidal channels Hardie, 1977 Wanless, 1975; Lucia, 1972; Klein, 1971, 1975 lenticular- bedded, lamin ated dolomite X X Recent: Beach ridge, washover crest Ancient: Tidal flat Hardie, 1977 Klein, 1971, 1975; Barnes and Klein, 1975 chert laminae and nodules O ' * o X X Recent: Ephemeral supratidal lakes Ancient: Intertidal flats, supratidal ponds Von der Borch, 1976 (amorphous silica only) Hoffman, 1976; Eriksson et al., 1976 TABLE 1 (continued) Structures and Lithotypes Beck Spring Informal Members LC LL 0-P UC Microenvironments References mixed arkosic - carbonate intraclastic granule sands X X Recent: Channel, beach Ancient: Channel lags Hardie, 1977 Klein, 1975 shale lenses and interbeds X X Recent: Tidal flat, lagoonal Ancient: Tidal flat, subtidal Klein, 1971; Reineck and Singh, 1975 West et al., 1968; Hoffman, 1975; Thompson, 1975 laminoid fenestral fabric X (X) X Recent: Lower to upper intertidal Supratidal Ancient: Intertidal - supratidal Lower levee backslopes Logan et al., 1974; Hagan and Logan, 1975 Shinn et_ _al. , 1969 Grover and Read, 1978; Read, 1973, 1975; Hailey, 1975 Wanless, 1975 O' TABLE 1 (continued) Structures and Lithotypes Beck Spring Informal Members LC LL 0-P UC Microenvironments References nodular stromatolites X X Recent: Beach terrace Intertidal - deep water Ancient: Low intertidal, shallow subtidal Intertidal - supratidal Hardie, 1977 Monty, 1971, 1973 Hoffman, 1975; Rezak, 1957 Donaldson, 1976 columnar stromatolites X? X? X Recent: Intertidal - subtidal Subtidal Ancient: Intertidal - subtidal Subtidal Logan et. al. , 1974 Playford and Cockbain, 1976 Donaldson, 1976; Playford et al., 1976 Serebryakov, 1976 macrointra- clastics (pebble, cobble intra- formational conglomerates) cn CO X Recent: Inner intertidal - outer supratidal Low algal marsh, levee, pond Ancient: Intertidal - shallow subtidal Supratidal Davies, 1970b; Shinn et al. 1969 Hardie, 1977 Hoffman, 1975; Matter, 1967; Gill, 1977 Roehl, 1967 TABLE 1 (continued) Structures and lithotypes Beck Spring Informal Members LC LL 0-P UC Microenvironments References microintra- clastics (algal chip and flake intraformational conglomerates) X X X X Recent: High algal marsh, levee, pond Ancient: Upper levee back- slopes Hardie, 1977; Ginsburg and Hardie, 1975 Wanless, 1975; Hailey, 1975 massive dolomite X X X Recent: Subtidal Ancient: Subtidal Hardie, 1977 Read, 197 3; Hoffman, 1975; Thompson, 1975 oolite, pisolite X Recent: Of f shore banks, shoals Subtidal Shinn et al., 1969; Illing, 1954; Heckel, 1972 Schneider, 1975 grapestone X Recent: Offshore banks, shallow subtidal Ancient: Subtidal Purdy, 1963 Zamarreno, 1975 cn Go cherty member in a low to moderate mechanical energy environment with occasional emergence. The laminated dolomite was probably deposited in association with various morphologies of algal mats. Strata assigned to the lower cherty member at Saratoga Spring also show low angle planar cross strata and lenticular bedding characteristic of low to moderate energy environments with alternating currents. Lower cherty member structures and stratigraphy at Saratoga Spring are quite similar to structures and strata in shoaling upward cycles described by Hoffman (1975) for the Precambrian Rocknest Formation in Canada. Hoffman interprets the shale and intraclastic packstone intervals as sublittoral shelf deposits and the light grey stromatolitic, cherty and intraclastic dolo mite as exposed tidal flat and supratidal algal marsh deposits. By analogy, this would suggest small amplitude shoaling cycles throughout the lower cherty portion of the formation at Saratoga Spring. Wright et aJL. (1976) suggest that the increase in shale and sandstone content in the lower cherty member strata at Saratoga Spring is due to influx of clastic material into the area from uplift and erosion of the Mojave Upland along the proposed southern margin of the Amargosa Basin in Beck Spring time. However, lateral migration of associated nearshore clastic deposits from an intraplatform source also might produce the observed 6 4 clastic strata in the Beck Spring deposits at Saratoga Spring. The structures in the lower laminated member suggest a shallow, alternately emergent, agitated environment with possible large crustose macrolaminated Stratifera and Conophyton columnar stromatolite bioherms succeeded by finely laminated, low lying mats and Baicalia-like columnar stromatolites in the upper portions of the interval. The Baicalia-like forms and associated flexible finely laminated mats probably provided the material of the microintraclastic conglomerate from chips and flakes of the edges of curled, exposed mats. The coarsely laminated, stratiform mats may be the source of the large macrointraclastic conglomerate clasts. Both types of algal structures probably were directly or indirectly producing carbonate and lithifying rather rapidly. Induration in an agitated environment such as this would have been necessary for growth stability. The oolitic-pisolitic member has characteristics associated with well washed carbonate shoals and associated nearby deeper water environments. Large size, good rounding and moderate sorting of allochemical grains suggest shallow, moderate to strongly agitated water. Although intraclasts and peloids can be produced in a wide range of water depths, normal ooliths and pisoliths usually require at least some degree of agitation for formation (Bathurst, 1975). The grapestone aggregates probably formed in a manner similar to modern grapestone by algal boring and cementation in 5-10 m of water with only occasional agitation (Purdy, 1963). The lower portion of the upper cherty member also shows the effects of frequent moderate energy current action by the presence of broken columnar stromatolites, nodular stromatolites, and winnowed intraclast deposits. The primary structures of the upper portion of the upper cherty member are very similar to those of the lower member, i.e. smooth to wavy microlaminae, minor fine algal chip conglomerate, abundant fenestral fabrics, and chert nodules and interbeds. The usually lenticular clastic intervals of the upper cherty unit might represent isolated tidal channel, pond, or beach deposits reworked into the carbonates or quiet water settling of wind blown dust. Evidence for extensive exposure and dessi- cation is not indicated. Possible shrinkage cracks in thin beds ( <2 cm) may indicate occasional exposure in isolated ponds or subtidal dewatering due to sediment compaction. The upper cherty member is, therefore, likely the product of a low to moderate mechanical energy environment with periodic exposure and deposition. If these strata are compared with Holocene sedi- mentological models, microenvironments associated with each of the informal members might be represented as 6 6 shown in Figure 15. This succession of microenvironments might be roughly interpreted as two long cycles of emergence as depicted in Figure 15. The modern microfacies analogs suggested for the Beck Spring Dolomite often occur as lateral associations. The microenvironmental changes seen in the Beck Spring Dolomite could have resulted from minor changes in sea level and associated redistribution of adjacent litho- topes. The facies changes more likely resulted from development of offshore bars or shoals, perhaps related to minor regional subsidence. The lack of abrupt litho- logic changes and the persistence of presumed equivalent microenvironmental lithotypes through hundreds of meters of strata may indicate only slow, gradual subsidence. Rapid subsidence and/or marked facies changes resulting from major reorganization of syndepositional tectonic elements is not apparent within the Beck Spring Dolomite. The depositional environments represented by the informal members of the Beck Spring Dolomite, especially in the Saratoga Spring area, are quite similar to the mixed fluvial-tidal facies proposed for the underlying Crystal Spring Formation (Roberts, 1976). The Beck Spring Dolomite lacks the extensive mudstones, chert, and diabase of the Crystal Spring Formation but contains similar algal-laminated and columnar stromatolitic carbonates, feldspathic sandstone, and minor silty and 67 FIGURE 15: A schematic diagram showing emergent and submergent episodes during deposition of the Beck Spring Dolomite. 68 Upper Cherty Member Oolitic- Pisolitic Member Middle to high intertidal flat with channels and ponds Fine, wavy laminae, intraclasts, nodular stromatolites, chert nodules and seams, sheet cracks, evaporitic mineral pseudomorphs EMERGENCE Offshore barrier-bar complex with associated lagoon Ooliths, pisoliths, grapestone SUBMERGENCE Lower Laminated Member Lower Cherty Member High intertidal to supratidal with minor ponds and levees Planar macrolaminae, intraclastic breccias and conglomerates, Baicalia- like stromatolites, mud curls and sheet cracks, evaporitic mineral pseudomorphs EMERGENCE Conophyton, flat pebble conglomerate, eroded and draped laminae, planar macrolaminae Middle to high intertidal flat with channels and ponds Fine, wavy laminae, shales, sandstone, chert nodules and seams, intraclasts 69 sandy shale. The gradational nature of the Crystal Spring- Beck Spring contact in many of the locales also argues for closely associated depositional environments at least between the upper Crystal Spring and lower Beck Spring members. In areas where the Crystal Spring-Beck Spring contact may be disconformable (i.e. the major tale bearing regions of the Crystal Spring Formation, Wright, 1968)the lithologies and structures of the upper Crystal Spring Formation and the lower Beck Spring Dolomite are still similar. It appears, therefore, that no substantial readjustments in depositional environments occurred from deposition of the middle or upper portions of the Crystal Springs Formation through deposition of the upper member of the Beck Spring Dolomite. 70 PETROGRAPHY Procedures Mineralogic and textural data were obtained from examination of 71 thin sections and 141 slabbed hand samples. Thin sections were point-counted using the Gagolev-Chayes method (Galehouse, 1971). All samples containing clastic material were stained with sodium cobaltonitrate for detection of potassium feldspar (Friedman, 1971). Approximately half the carbonate slabs and thin sections were stained with Alizarin Red S to aid in distinguishing calcite and dolomite. However, the Alizarin stain did not always work well due to the dark color and fine grain size of the samples. More reliable calcite-dolomite data was obtained from x-ray diffraction and atomic absorption spectroscopy studies (see Petro chemistry section). Volume percentages for minerals and allochemical constituents were determined by petrographic modal analysis. Fabric and porosity types were defined and classified according to Choquette and Pray (1970). Clastic rocks were classified using the system of McBride (1963). Carbonate rock classification was according to Folk (1962) and Dunham (19.62). Paragenetic sequences were attempted for each sample. Key small- scale sedimentary structures were noted. Carbonate grain size was described using the nomenclature of Folk (1965). Micrite was defined as carbonate grains l-4yam, microspar 71 4-30yum, and spar >30yum in maximum observable dimension. The size boundary between ooliths and pisoliths was 2 mm. Microintraclastic deposits are defined as those in which component allochems were usually less than 5 mm in maximum dimension. Macrointraclastic deposits contained allochems from 5mm to 10 cm+ in maximum dimension. Results Summaries of the principal petrologic character istics for the informal members in the carbonate, mixed carbonate-clastic and Silurian Hills facies, and the terrigenous elastics are presented in Tables 2, 3, and 4. A separate tabulation for the proposed lower cherty member was not attempted due to the extremely small number of available samples. Samples from the lower cherty member are included in the grouping for the lower laminated member. A listing of observed minerals and their approx imate volume percentages and ranges for the overall study is presented in Table 5. Mineral Composition.— Dolomite dominates the car bonate mineralogies. Micrite and microspar are predom inant in the lower members and the upper cherty member while microspar and spar dominate in the oolitic-pisolitic interval. All samples show dolomite replacement coupled with lesser or greater degrees of aggrading neomorphism. It is extremely difficult in many cases to decide if the present dolospar is primary spar replacement or pseudospar. 72 TABLE 2 Petrologic characteristics of the Beck Spring Dolomite, carbonate facies. Abbreviations are: fe = fenestrae; fr = fracture; vug = vug; be = intercrystalline; mo = moldic; bp = interparticle; wp = intra particle; me = micro «0.06 mm) ; sms = small meso (>0.06-0.5 mm); lms = large meso (>0.50-4.0 mm); smg = small mega (>4.0-32.0 mm); sp = dolospar; ch = chert; q - quartz; o = Fe oxides; op = open; micINT = microintraclastic deposits; xstal = crystalline;_n = number of individual samples; X = arithmetic mean; and s = standard deviation. Porosity types and pore fillings are listed in decreasing order of frequency of occurrence. 73 Petrologic Characteristics Allochems (%) ooliths pisoliths peloids intraclasts grapestone Allochem size (mm) Sorting Dolomicrite (%) Dolomicrospar {%) Dolospar {%) Quartz-chert (%) Fe oxides (%) Support (%) Porosity types and sizes Pore fillings Lower Laminated- Cherty Members n=22_ Range X s 00 1 o o 10.3 4.6 0.0- 0.6 0.1 0.1 0 0 0 0.0-30.6 3.2 3.3 0.0-50.7 7.0 1.4 0 0 0 0.01-5.5 poor 0.0-94.4 23.9 5.5 0.0-79.6 36.8 3.7 0.6-67.4 22.7 7.7 0.0-11.4 2.5 2.1 0.0- 1.1 0.2 0.1 matrix/xstal/grain 68.2 18.2 13.6 lms-smgfe, mc-smsfr, lms-smgvuq, mcbc, mc-smsmo, mcbp sp,ch,q,o <i 4^ Oolitic-Pisolitic Member n=13_ Range X s Upper Cherty Member n=12 _ Range X s 0.0-83.0 33.8 10.2 0.0-43.7 6.1 8.6 0.0-17.8 2.1 1.9 0 0 0 0.0-51.9 10.6 5.8 0.0- 1.9 0.4 0.2 0.0-39.8 10.5 7.4 0.0- 1.7 0.6 0.7 0.0-33.8 7.9 6.3 0.0-43.0 5.5 7.6 0.0-45.4 0 9.0 0 0 0 o 0 4*. 1 t —' O O 0.06-10.0 fair-good poor 0.0-42.3 10.2 2.9 0.0-58.1 25.7 21.4 0.2-57.3 22.6 11. 2 2.2-62.7 30.9 9.5 0.6-95.6 33.7 10.6 1.4-95.6 24.9 27.6 0.0- 8.2 1.8 1.2 0.4-41.8 10.9 9.9 0.0- 1.1 0.2 0.2 0.0- 1.1 0.4 0.5 matrix/'xstal/grain matrix/xstal/grain 30.8 15.4 53.8 77.8 22.2 00.0 mc-lmsfr\ sms-smqbp, mc-lmsfe, mc-smsmo, s-lmswp, lms-smgvuq, mc-smqfr, lms-smqvuq memo, mcbc me-smsbe, smswp ch,sp,q,o ch,q,sp,o, op TABLE 2 (continued) Petrologic Characteristics Lower Laminated- Cherty Members Oolitic-Pisolitic Member Upper Cherty Member Sedimentary micro structures micINT rare - common common common parallel laminae abundant rare rare - common convlute laminae abundant common abundant eroded and draped laminae common absent rare other small sheet cracks, evaporite pseudo morphs mud curls small sheet cracks, evaporie pseudo morphs Nomenclature Folk (1962) dolointramicrudite, dolomicrosparite, dolosparite dolointra-, oo-, pelsparrudite, dolosparite dolomicrosparite dolosparite, dolo intramicrudite Dunham (1962) mudstone, wacke- stone, crystalline carbonate packstone, grain- stone, crystalline carbonate mudstone, wacke- stone, crystalline carbonate CJl TABLE 3: Petrologic characteristics of the Beck Spring Dolomite, mixed carbonate-clastic facies. Abbreviations are: fe = fenestrae; fr — fracture; vug = vug; be = intercrystalline; mo = moldic; bp = interparticle; wp = intra particle; me = micro (<0.06 mm) ; sms = small meso (>0.06-0.5 mm); lms = large meso (>0.50-4.0 mm); smg = small mega (>4.0-32.0 mm); sp = dolospar; ch = chert; q = quartz; o = Fe oxides; op = open; tr = trace ( < 0.01%) ; micINT = microintraclastic deposits; xstal= crystalline; n = number of individual samples; X = arithmetic mean; s = standard deviation. Porosity types and pore fillings are listed in decreasing order of frequency of occurrence. 76 Petrologic Characteristics Allochems {%) ooliths pisoliths peloids intraclasts Allochem size (mm) Sorting Dolomicrite (%) Dolomicrosparite (%) Dolospar (%) Quartz-chert (%) Fe oxides (%) Support (%) Porosity types and sizes Pore fillings <1 <1 Saratoga Spring Lower Laminated- Cherty Members n=7 Range X s 0.0-29.6 6.8 11.8 0 0 0 0 0 0 0.0-16.8 4.0 6.8 0.0-19.4 2.8 7.3 0.04-10.0 good 2.7-91.7 34. 3 34.9 6.9-55.9 28.2 21.6 0.0-37.1 9.4 13.4 0.9-46.6 19.4 18.7 tr-2.6 0.9 1.0 matrix/xsta1/grain 42.9 28.6 28.6 mc-smsfr, me-lmsfe, mcbc, me-smgbp, memo, smswp s, sp Saratoga Spring Oolitic-Pisolitic Member n=3 __ Range X s 27.5-54.0 40.8 18.8 0.0- 0.2 0.1 0.1 0.0-27.5 13.8 19.4 0.0-41.5 20.8 29.3 0.0-12.3 6.2 8.7 0.18-6.0 good 35.6-46.5 41.0 7.7 6.2-25.0 15.6 13.3 0 0 0 0.0- 1.0 0.7 0.5 tr- 0.2 0.1 0.1 grain 100.0 mc-smgfr, mc-lmsbp, mc-smswp, mc-smsmo, me-lmsvug q, ch,sp,op Silurian Hills Lower Cherty Member n=2 Range X s 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 1 —1 1 o o 7.4 10.4 0.0-51.4 25. 7 36. 3 19.5-98.1 58.5 55.6 1.0- 1.5 1.2 0.4 0.9- 9.2 5.0 5.9 xstal 100.0 mc-lmsbc, mc-smsfr, s-lmsvug q,ch,sp,s,op TABLE 3 (continued) Petrologic Characteristics Saratoga Spring Lower Laminated- Cherty Members Saratoga Spring Oolitic-Pisolitic Member Silurian Hills Lower Cherty Member Sedimentary micro structures micINT rare absent absent parallel laminae common rare absent convolute laminae abundant common foliation? eroded and draped laminae absent absent absent other evaporite pseudo morphs — - Nomenclature Folk (1962) dolomicrite, dolo microsparite , dolopelmicrosparite, dolointrasparrudite dolo-, oo-( pel- micrudite dolomicrosparite dolosparite Dunham (1962) mudstone, wackestone wackestone crystalline carbonate oo TABLE 4 Petrologic characteristics of the Beck Spring Dolomite, terrigenous elastics. Abbreviations are: bp = inter particle; fr = fracture; o = Fe oxides; sp = spar; tr = trace (<0.01%);_n = number of individual samples; X = arithmetic mean, s= standard deviation. Porosity types and pore fillings are listed in decreasing order of frequency of occurrence. 79 Petrologi c Characteri sties Saratoga Spring Terrigenous elastics 50% Detrital Silicates n=3 _ Range X s Quartz (%) 48.1-71.9 59.5 11. 9 Feldspar (%) 2.9-11.5 6.7 4.4 Matrix {%) 6.0-44.3 24.1 19. 2 Authigenic quartz-chert (%) tr Fe oxides (%) 1.9- 8.3 4.7 3.3 Acid plutonic rock fragments i> o i —i C M 1 O • o 1.2 Sorting moderate-good Support grain Porosity types and size mcbp, mefr Pore fillings o, sp Sedimentary Microstructures lenticular bedding common low angle cross strata common mixed carbonate intraclasts common Nomenclature McBride (1963) subarkose 80 TABLE 5: Mineral composition of the Beck Spring Dolomite. (n=56) Minerals Observed Ranae (%) X s Dolomite 0- 100. 0 85.9 25.9 Calcite 0- 98.1 3.4 17.2 Quartz 0- 71.5 5.4 14. 4 Chert 0- 41.5 3.6 8.9 Potassium feldspar 0- 11.5 0.5 1.8 Plagioclase 0- 0.9 0.0 1.5 Magnetite 0- 3.0 0.2 0.5 Hematite 0- 5.3 0. 3 0.9 Limonite 0- 8.8 0.2 1.2 Pyrite 0- 0.5 0.0 0.1 Acid plutonic rock fragments 0- 2.1 0.2 1.2 Biotite 0- 2.0 0.1 0.3 Halite 0- 0.8 0.0 0.1 Anhydrite 0- 0.9 0.0 0.1 Gypsum tr* - - Abbreviations are: n = number of individual samples, X = arithmetic mean, s = standard deviation, tr = trace (<0.01%), and * detected in only one sample. The only fairly certain cases of primary or void-filling spar are seen in large-scale late-stage fractures and veins (Fig.16D,F,H) and in large fenestrae and vugs. The fenestrae and vugs are often initially lined with bladed, euhedral quartz (Fig.l6F,H) and the spar may have been a secondary replacement of initial solution void-filling chert (Walker, 1962). Laminated samples usually show well developed alternation of dark, dense micritic laminae and clear microspar and spar interlaminae. The microstructure of these laminations is described in detail in the Stroma tolite section of this thesis. No evidence for primary dolomite is observed. The presence of well preserved ooliths, pisoliths, and grapestone indicates initial deposition as calcite or aragonite. Primary dolomite usually precipitates as fine-grained muds and crusts. Primary dolomitic ooliths and pisoliths have not been reported in any of the modern dolomite-producing environments (Shinn, et al., 1969; Illing et al., 1965; Friedman and Sanders, 1967). The quartz and chert observed in the samples are probably of eo-mesogenetic origin in all cases except the clastic-intraclastic rocks at Saratoga Spring. Evidence for eo-mesogenetic origin for the chert includes a high degree of fabric selectivity in the chert laminae, uniformity of chert crystal size and shape (i.e. no 82 FIGURE 16: Photomicrographs of detrital and re placement minerals and fabrics in rocks of the Beck Spring Dolomite. (A) Rounded polycrystalline quartz grain (pq) from granule sandstone lenses, lower cherty member, Saratoga Spring section. Note bimodal grain size distribution. Cross-polarized light. (B) Detrital silicate grains in carbonate intraclast sample, lower cherty member, Saratoga Spring section. Me = microcline, q = quartz, and mi = micrite. Most of the detrital grains have been cemented by dolospar. Cross polarized light. (C) Granule sandstone showing large quartz grain (q) and granitic rock fragment with K-feldspar (kf). Lower cherty member, Saratoga Spring section. Note bimodal grain size distribution. Cross-polarized light. (D) Dominant porosity types, lower laminated member, Snow White Mine section. Interlaminae fenestrae are filled with chert (ch) and quartz (q). Laminae are dense, dark micrite (mi). Dolospar (s) fills late-stage fracture. Cross-polarized light. (E) Radial fibrous chalcedony (cd) and dolospar (s) filling inter laminae space. Lower laminated mem ber, Snow White Mine. Cross-polar ized light. (F) Dolospar (s) and quartz (q) filling a vug in a microspar (ms) matrix. Quartz shows syntaxial overgrowths into microspar. Vugs are lined by a single layer of mosaic quartz crystals, then filled by equant dolospar. Cross-polarized light 83 FIGURE 16: (continued) (G) Oolith core being replaced by dolomicrospar (ms) which is in turn being replaced by euhedral mosaic quartz (q). Oolith is surrounded by bladed and equant dolospar (s). Cross-polarized light. (H) Large megavug lined by bladed quartz (q) and filled with coarsely crystalline dolospar (s). Cross polarized light. All scale bars are 200yum long. 84 bladed crusts), preservation of finely detailed micro- biotal structure within the chert (Licari, 1978), micro- spar-spar overgrowths of chert laminae, and parallelism of breaks and distortions in chert and carbonate laminae. The equant, uniform character of the chert crystals suggests a replacement origin rather than a void-filling origin. The cherty laminae may represent total replace ment of micrite or some type of organic material. Origin of the chert, especially the large interbeds, is unresolved. Theories for the origin of sedimentary chert usually rely on the presence of abundant siliceous organisms (Dapples, 1959; Lowenstam, 1942),or detrital silica which has been converted to clays (Swett, 1968) or directly replaced by carbonate (Walker, 1960). There is no evidence for the presence of a large siliceous fossil assemblage or for substantial detrital quartz deposits,with the possible exception of the Saratoga Spring strata. Undetected, now replaced, shale may have existed. Dolospar does replace chert in some of the samples. The diagenetic quartz and chert most frequently fill interlaminae fenestrae, solution-enlarged fenestrae and vugs, dolomolds, and fractures(Fig. 16D). The chert is fine-grained (0.01-0.02 mm), subhedral and equant. The quartz occurs as interlocking mosaics of equant, euhedral crystals up to 2.0 mm in length. Radial fibrous and bladed quartz also occurs lining megapores in probable solution-enlarged fenestrae (Fig. 16F,H). Colloform chalcedonic-opaline crusts line interlaminar spaces in samples from the lower members at the Snow White Mine and Acme Talc Mine sections (Fig. 16E). These length-slow chalcedony varities may be additional indica tors of evaporitic or semi-arid depositional conditions (Folk and Pittman, 1971; Siedlecka, 1972a). Slightly undulose monocrystalline quartz also fills dolomolds, late stage veins, and allochem cores (Fig. 16G). Detrital quartz is observed only in subarkoses from Saratoga Spring and intraclastic rocks. The quartz in the thin clastic- intraclastic interbeds at Saratoga Spring is well sorted and rounded, undulose, and mono- and polycrystalline (Fig. 16A,B,C). Plagioclase, microcline, and potassium feldspar (K-spar) form up to 11.5% of the mineral content of the clastic rocks from Saratoga Spring. Several of the arkosic beds at this locality show a marked bimodality in grain size (Fig. 16A,C). The large grains (0.5-2.3 mm) are mostly well rounded mono- and polycrystalline quartz, microcline, and K-spar, whereas the small grains (0.02- 0.30 mm) are dominantly subangular monocrystalline quartz. A few scattered acid plutonic rock fragments occur. The grain size bimodality may indicate a mixing of nearshore beach or tidal channel grains with finer offshore, pond., or lagoonal sediment. 87 Iron (Fe) oxides and sulfides are observed as late stage minerals in nearly all samples. The most frequently occurring iron oxides are hematite, magnetite, and limon- ite. The magnetite occurs as very finely dessiminated, euhedral grains and clusters usually 0.01-0.15 mm in diameter. Magnetite is also common in some eo-meso- genetic chert interlaminae (Fig. 17B). Hematite alter ation frequently rims and/or replaces magnetite euhedra. Many of the late-stage fractures are lined with hematite and limonite. Hematite and limonite also occur as irregular stains across many slides. The sulfides usually form less than 3% of the total observed volume. Excep tions to this occur in the subarkose at Saratoga Spring in which the hematite and magnetite often form 5-8% of the total. Biotite is seen rarely in the carbonates of the lower laminated member, in the subarkosic sands at Saratoga Spring and in the altered samples from Silurian Hills. Possible dolomite pseudomorphs of halite, gypsum, and anhydrite occur in cherty and micritic laminae in samples from the lower laminated and upper cherty members at Saratoga Spring, Beck Spring, Acme Talc Mine and Snow White Mine (Fig. 17A,B,C). The pseudomorphs occur as isolated, euhedral crystals in the chert laminae. The pseudomorphs in the cherty laminae may represent late 88 FIGURE 17: Photomicrographs of evaporitic mineral pseudomorphs and associated accessory minerals. (A) Anhydrite pseudomorphs (a) in a chert (ch) matrix. Outline of the central rectangular prism has been enhanced to show its three- dimensional form. Bar is 20/Am long. (B) Gypsum pseudomorphs (g) and magnetite (m) grains in a chert (ch) matrix. Bar is 50/tm long. (C) Halite pseudomorphs (h) in a chert (ch) matrix. Cubic shape of crystals is clearly visible near the top center of the photograph. Bar is 20/xm long. 89 90 stage replacements of early diagenetic chert or relict crystals protected from initial chert replacment of the carbonate. The pseudomorphs contained within the micritic laminae are probably of eogenetic origin. Aliochemical Grains.-- Intraclasts, peloids, ooliths, pisoliths, and grapestone occur in the Beck Spring Dolo mite (Fig. 18A-G). Intraclasts and peloids are the dominant forms comprising up to 50% of some samples, especially in the lower laminated member. Intraclasts often show peloidal or laminated internal structure and range in size from 0.15-5.5 mm (Fig 18B). Peloids are ovoid to ellipsoidal in shape and micritic with either well-defined or obscure borders. They range in size from 0.01-0.45 mm with the dominant size approximately 0.15-0.20 mm (Fig. 18E,F,G). Peloidal deposits usually show moderate to fair sorting while mixed intraclast-peloid deposits are usually poorly sorted. The peloids and intraclasts of the microintra- clastic interbeds of the lower laminated member are frequently leached,leaving only micritic rims and sparry core s. Intraclasts are mostly derived from associated algal- laminated sediment. The origin of the peloids is less clear. The peloids with obscure borders may represent clotted, micritic remnants produced by aggrading 91 FIGURE 18: Allochemical grains in the Beck Spring Dolomite. (A) Ooliths (o) and pisoliths (ps). Negative print of a thin section, oolitic-pisolitic member, Acme Talc Mine section. Bar is 3 mm long. (B) Intraclast (i) packstone. Nega tive print of a thin section, oolitic-pisolitic member, Snow White Mine section. Bar is 3 mm long. (C) Segment of a grapestone aggre gate, plane-polarized light, oolitic- pisolitic member, Beck Spring section. Bar is 200jum long. (D) Grapestone. Negative print of a thin section, oolitic-pisolitic member, Beck Spring section. Bar is 3 mm long. (E) Peloids (pi), plane-polarized light, lower cherty member, Saratoga Spring section. Bar is 200jum long. (F) Peloids replaced by chert, cross polarized light, upper cherty member, Beck Spring section. Bar is 50yWm long. (G) Peloids (pi), lower cherty mem ber, Saratoga Spring section. Bar is 200/tm long. 92 9 3 neomorphism (Bathurst, 1975; Beales, 1965). The distinct, compact forms, however, were probably deposited as peloids. The peloids originally might have been produced by some Precambrian interstitial dwelling infauna (S.M. Awramik, pers. comm., 1978) which has left no fossilized trace, or by comminution and degradation of algal mat particles (Wolf, 1965). Well-preserved ooliths and pisoliths are seen in all the studied areas. Details of radial fibrous and con centric microstructure are still preserved in many samples (Fig. 18A,B). Core replacement by chert, quartz, and spar is common. Complete silicification of ooids and peloids is observed in samples from the Beck Spring locality (Fig. 18F). Ooliths average 1.5 mm in diameter while pisoliths occur up to 6 mm in diameter. The ooliths and pisoliths are interpreted as normal, moderate to high energy marine allochems which formed in agitated water with little subaerial exposure. The ooliths and pisoliths do not show the characteristic protruding nuclei, asymmetry in shape, or scalloped edges of quiet water ooliths (Freeman, 1962). Dessication cracks similar to those observed by Adeleye (1975) are not present. The pisoliths are interpreted as large marine ooliths and not vadose in origin because (1) they occur in lamin ated deposits with other carbonate allochems (intraclasts 94 and peloids), (2) they show no inverse size grading, (3) they show no fitted polygonal structures with downward elongation, (4) they have no perched inclusions, and (5) they show good spheroidal to ovoid shapes and concentric lamination (Dunham, 1969). The pisoliths observed in this study lack the irregular, diffuse, sometimes dis continuous internal laminae of most oncolites (Blatt, et al., 1972). However, algal microfossils have been reported from pisolitic structures in the upper cherty member of Beck Spring (Licari, 1971, 1978). Licari used pisolith and oncolite interchangably and it is difficult to determine from the reported data if non-algal pisoliths were also observed. Aggregate grains of grapestone (Illing, 1954) are clearly preserved only at Beck Spring (Fig. 18C,D). Samples usually show from 2 to 7 component particles and moderately preserved concentric laminations. Possible relict, leached grapestone also is observed in samples of microintraclastic rock within the laminated member at Snow White Mine. Grapestone has been interpreted as a shallow (<10 m) deposit produced by algal filament boring and cementation of biogenic skeletal grains and crypto crystalline grains washed into subtidal waters from nearby shoals (Illing, 1964; Beales, 1958; Bathurst, 1975). The peloids and intraclasts observed in the Beck Spring Dolomite samples appear to have been initially 95 micritic in composition. The ooliths and pisoliths may have had initially micritic or microspar compositions. Most allochems now show at least some degree of replace ment by coarser crystalline spar or silica. Textures and Fabrics.— Determination of textures and fabrics is fundamentally a problem of sorting out diagenetic and primary processes and effects. Many of the samples of Beck Spring Dolomite show remarkable preservation of detail in view of their age. However, the overprint of replacement and aggrading neomorphism dominates many other samples. The lower laminated member is dominantly matrix- supported with minor crystalline and grain-supported fabrics. The grain-supported examples are usually intra- clastic and peloidal rocks. The crystalline fabrics are extensively replaced. Well-preserved laminae from 1 to several mm in thickness are generally composed of alternating layers of micrite and microspar or microspar and spar. Interlaminae also contain minor colloform chalcedonic quartz and chert. Small-scale dislocation structures resembling micro-sheet cracks are also common. Porosity in the lower laminated member is mostly fabric-selective laminoid fenestrae (Choquette and Pray, 1970; Grover and Read, 1978). Frequent fracture-channel porosity is also common. Some solution enlargement of the fenestrae has occurred and produced dramatic "vuggy" or 96 "bird's eye" rocks. The "vugs" are dominantly sub parallel to bedding and relict laminations,and should properly be called megafenestrae. The megafenestrae are infilled by coarsely crystalline spar and quartz and offset many of the spar- and chert-filled fractures. Minor moldic, intercrystalline, and interparticle porosity is also observed. Moldic pores are usually roughly rhombic in outline and filled by quartz. They may represent possible solution of dolomite rhombs and infilling by silica. Intercrystalline porosity is usually observed in microsparry "sucrose" dolomite. Interparticle porosity is dominant in peloidal and intra- clastic samples. 0.02-0.10 mm fractures occur in most samples and are filled with a variety of minerals inclu ding spar, quartz, chert, hematite and limonite. The hematite and limonite usually line the fractures and may argue for the presence of oxidizing conditions during their deposition. This would make the fractures of telo- genetic origin. A few rare open mesopores are also observed. The oolitic-pisolitic member shows both grain and matrix support but was probably originally chiefly grain- supported. Apparent matrix support may be due to degrad ation and extensive replacement of small allochems between the larger grains giving the illusion of matrix support. Some samples show clear indications of pseudo 97 spar filling between grains. Other samples show bladed spar crusts and equant cores suggestive of pore filling. However, the bladed to equant fabric alone is insufficient evidence for primary porosity (Beales, 1965; Bathurst, 1975). The persistance of patchy areas of micritic material scattered between the allochems could then be either relicts of original micritic matrix or micritized allochems. Additional evidence for pseudospar origin of the interstital material is the preservation of micritic matrix between particles in slides showing little evidence of aggrading neomorphism. Dominant porosity types include fracture, inter particle, and intraparticle. Pores are usually of meso- pore size and are infilled with chert, spar, quartz, and iron oxides. Minor chert and quartz-filied micro molds and mesovugs also occur. The upper cherty member is dominantly matrix- supported with poor sorting of the scattered peloids and intraclasts which occur in this unit. The porosity is primarily micro-mesolaminoid fenestrae, micro-mesomolds, and micro-small megafractures. The fenestrae are filled with chert, some spar, and late stage quartz. Minor pore types include vugs, intercrystalline and intra particle pores. Most pores are infilled with chert or quartz but a few open fenestrae, vugs, and fractures are seen. Late stage iron oxides commonly line the 98 fractures. The grain-supported clastic rocks of Saratoga Spring show a dominant interparticle porosity with micrite and hematite infillings. Summary and Piscussion The carbonate of the Beck Spring Dolomite was probably initially deposited as aragonite and/or calcite. No evidence for primary dolomite is observed. Possible mechanisms for dolomite formation in the Beck Spring include (1) refluxing of dense supratidal brines (Deffeyes et al., 1965), (2) capillary pumping- upsucking of high magnesium content solutions through evaporation (Shinn et al. , 1965), (3) solution- canni balization (Goodell and Garman, 1969), (4) episodic freshwater flushing of shallow hypersaline sediments (Siedlecka, 1972b; Folk and Land, 1975), (5) incongruent dissolution of magnesium calcites (Land and Epstein, 1970 Folk and Siedlecka, 1974), and (6) mixing of meteoric and normal marine waters (Hanshaw et aJL.,1971; Land, 1973a,b; Badiozamani, 1973). Dolomite formation does not always require solutions with high magnesium/calcium (Mg/Ca) ratios (>3:1) (Badiozamani, 1973), nor does it require exclusively hypersaline environments (Folk and Land, 1975). High salinities often inhibit dolomite formation due to competition from other ions, notably sodium and potassium The easiest way to form dolomite is to reduce the salinity, a process usually resulting from dilution of marine sediments by meteoric or near surface freshwater (processes 4,5, and 6 above). Incongruent or selective dissolution relies heavily upon the presence of shelly matter and is probably locally restricted and would not produce large volumes of dolomite. Refluxing requires hydrologically improbable conditions and requires the precipitation of vast quantites of gypsum (Land et .al., 1975). Capillary pumping is adequate to produce caliche soil horizons but clearly cannot produce large volumes of dolomite (Land et. al.. , 1975). Solution-cannibalization is an operable process but could not supply the necessary magnesium to dolomitize large sections of carbonate rocks without dissolving away adjacent continents (Land et al., 1975). Meteoric-marine water mixing is probably the dominant process operating in the production of large dolomite deposits, especially those which show little evidence of arid-evaporitic climate. Episodic freshwater flushing may be the dominant mechanism for dolomitization in deposits which definitely can be identified as hyper saline and arid. The paleoclimate of the environment of deposition of the Beck Spring Dolomite is not uniquely defined by its petrology. The fenestral fabrics, ooliths, pisoliths, peloids, grapestone, sheet cracks, laminated dolomicrites, 100 and microintraclastics are indicative of shallow, moder ately agitated marine conditions with occasional emergence. The lack of substantial quantities of evaporitic minerals plus the uncertainty as to their primary or diagenetic origin may be interpreted to indicate an environment that was dominantly not hypersaline. However, portions of the strata may be of a hypersaline facies due to lateral shifts of associated shallow marine facies. Therefore, either or both of the freshwater mixing or flushing processes may have operated depending on the temporal sequence of microenvironments and climate. The diagenetic history of the Beck Spring Dolomite has been long and complex. A simplified, generalized, post-depositional, paragenetic sequence for the Beck Spring Dolomite may include: (1) eo-mesogenetic develop ment of fenestral textures due to decay of organic mater ial, dessication of algal mats, gas evolution, etc.; (2) replacement of calcite or organic material by chert perhaps due to fresh water influx, dissolution of car bonate and resulting elevation of pH, subsequent evapor ation and/or concentration of pore solutions, reduction in pH and precipitation of silica; (3) replacement of carbonate and/or silica by dolomite occurring either penecontemporaneously with evaporation which aided silica precipitation or slightly after silica deposition; (4) development of evaporitic minerals from concentrated 101 pore solution; (5) continued replacement of carbonate and silica minerals with a general increase in grain size (aggrading neomorphism); (6) solution enlargement of fenestrae to produce large channel-vug systems which were then infilled with quartz and spar, solution leaching of allochem cores and selective silicification of cores; (7) dedolomitization of some strata to form dolomolds and intercrystalline pores which were infilled with chert, deposition of calcitic veins; (8) continued replacement of chert by spar; and (9) fracturing and development of extensive telogenetic fracture-channel systems, infiling of those fractures with quartz, iron oxides and spar. The textures and mineralogies observed in the laminated and cherty members of the Beck Spring Dolomite indicate dominantly low to moderate energy, matrix- supported mudstone to wackestone facies and may represent deposits from moderate to high energy shoals and/or associated lower energy subtidal environments. Laminoid fenestral porosity, characteristic of tidal deposits, dominates in the laminated and cherty members. Inter- and intra-particle porosity is the most abundant tyoe in the oolitic-pisolitic facies. The mineralogy is uniformly dolomite of probable replacement origin. Early diagenetic chert is common and most frequently fills fabric selective fenestral pores. Iron oxides and sulfides are distributed through- 102 I out the formation and are especially concentrated in the ! clastic-rich carbonate strata at Sartoga Spring. 103 PETROCHEMISTRY Information required for environmental reconstruction and facies analysis of ancient formations is limited by the lack of macrofossils and in addition, in the case of the Beck Spring Dolomite, by the essentially monominer- allic nature of the strata. However, supplemental infor mation relating to depositional facies possibly may be derived from trace element composition. Elemental analyses were performed in an attempt to (1) construct a geochemical profile for the Beck Spring Dolomite which might be of use in regional correlation, (2) test the applicability of geochemical facies analyses as proposed by Friedman (1969) ,Eriksson et a_l. (1976) , Kinsman (1969) and Land and Hoops (1973), and (3) aid in identification and correlation of some possible Precambrian-Cambrian strata at Seventeenmile Point near Baker, California. X-ray diffraction analyses were performed primarily for (1) identification of dolomite and calcite, (2) crystallographic structure analysis of the dolomite,and (3) identification of mineralogy of insoluble residues. Atomic Absorption Spectroscopy (AAS) Methods Whole rock samples, free of obvious weathering features, were crushed in a jaw crusher lined with tung sten carbide plates to prevent iron contamination. Crushed samples were pulverized by shaking for 20 minutes in a 104 Spex Ball Mill also lined with tungsten carbide. Sample powders were placed in glass petri dishes and dried at least 24 hours at 120 degrees C. An initial 1:100 dilution was made by adding 1 g sample powder, 3 mis concentrated HC1 (AR, Mallinckrodt), 1 ml concentrated HNO^ (AR, Mallinckrodt), and 5 mis distilled, deionized water to a pre-weighed 120 ml linear polyethylene (Nalgene) bottle. After sample effervescence ceased, an ionization suppressant solution (approximately 1500 ppm Na from NaCl in 3% HC1, 1100 ppm K from KCl in 3% HCL, or 1100 ppm Li from LiCO^ in 3% HC1) was added to a total solution weight of 100 g. Additional dilutions and synthetic standards made with 500-1000 ppm stock solutions were matched in terms of ionization suppressant and acid content (Dean and Rains, 1971). All dilutions and standard concentrations were determined by gravimetric measurement. Calcium(Ca) and magnesium (Mg) concentra tions were determined in a 1:150,000 dilution. Dilutions for other elements ranged from 1:100 to 1:5000. Samples utilizing initial 1:100 dissolutions were centrifuged (1400 x g, 30 minutes) or very carefully decanted before analysis to remove fine particulate insoluble residue which can clog the intake capillary of the spectrometer and cause physical interferences in the flame (Dean and Rains, 1971). These concentrated sample dilutions (1:100 or 1:200) also required special care in 105 matching sample and standard matrices due to the extremely high levels of Ca and Mg (100-200,000 ppm) and substantial acid content (7%). A short study was performed to determine the presence of physical and/or chemical interferences in these high acid, high Ca and Mg background samples. Iron (Fe) , man ganese (Mn), strontium (Sr), barium (Ba), sodium (Na), and potassium (K) standards were made in 1100 ppm K or 500 ppm Na solution with 4% added HCl-HNO^ (3:1; v:v) and 5% added deionized, distilled water. A second set of standards for each element was made identical to the first except for the addition of 0.2 g CaCO^ (AR, Mallinc krodt) and 0.1 g MgSO^ (AR, Mallinckrodt). Blanks with added Ca and Mg were used because results indicate there are substantial amounts of contaminating Sr, Na, K and Fe in the dry reagent grade chemicals. Trace element concentrations relative to the unloaded standards were measured and ppm’s computed. Substantial (10-27%) interferences were observed for Mn and Fe (Table 6). In order to overcome this problem, standards for Mn analysis were matched to samples in Ca, Mg and acid content. Fe, in samples with an original 2000 ppm or less total concentration, was determined by the method of standard additions (Dean and Rains, 1971). It would be the recommendation of the writer that future studies of carbonate petrochemistry utilize the standard additions 106 107 TABLE 6: Ca-Mg interference in Mn, Fe, K, Na, Ba, and Sr measurements. Element PPM-Gravimetric calculation PPM-Observed from absorbance % Error Mn 0.5675 0.3988 -29.6 0.6979 0.5220 -25.2 Fe 2.0707 1.8927 - 8.6 2.0707 1.8374 -11.3 K 1.0700 1.0771 + 0.7 Na 1.0262 1.1290 0 0 ■ —1 1 1.0262 1.0366 0 • 1 —1 + Ba 1.9499 119337 - 0.8 Sr 1.0108 0.9682 - 4.9 1.0108 0.9399 - 7.0 * strong interference indicated • k * possible mild interference indicated X = arithmetic mean X Error % -27.4* -10.0* + 0.7 - 5.0** - 0.8 - 6.0** method where possible. Initial sample dissolutions could be split and one sample loaded with a multi-element standard, e.g. Sr-Na-Ba or Fe-Mn. This would prove efficient, however, only if trace element levels of the samples being measured are all of similar magnitude. The author has observed extremely wide variations in Fe and Mn contents and trial and error sampling to group unknowns of similar levels may be the only recourse. Sample absorbances were measured using a Perkin- Elmer (P-E) model 370 Double Beam Atomic Absorption Spectrometer equipped with appropriate hollow cathode element lamps and a P-E Model 165 chart recorded. Ca, Mg, Sr, and Ba were analyzed using N^O as the oxidant and acetylene as the fuel. Fe, Mn, K, and Na utlized an air-acetylene flame. Triplet sets of 5 absorbance readings were recorded for each sample. All unknown samples were bracketed by appropriate standards. Reported ppm values represent the mean of the 3 separate determin ations and were calculated using linear regressions along drift corrected standard curve segments. Percent relative standard deviation- for measurements of Ca,Mn, Na and K was less than 1.0% (Table 7). Mg, Fe, and Sr analyses had standard deviations of less than 2.5%. Ba measurement precision varied widely from 3.6% to 71.0% with a mean relative standard deviation of approximately 23.4%. 108 TABLE 7: Percent relative standard deviation of elemental analyses by AAS. Element Range % s Mean % s Ca 0.08-3.35 0.82 Mg 0.14-6.46 1.15 Fe 0.06-5.36 1. 78 Mn 0.07-4.58 0.88 Ba 3.59-70.9 23.4 Sr 0.12-14.2 2.54 Na 0.16-1.60 0. 61 K 0.11-13.8 0. 89 s = standard deviation 109 X-Ray Piffraction (XRD) Methods Sample powders prepared for AAS were used for XRD studies. Insoluble residues from samples yielding greater than 10 weight percent insoluble residue in the AAS studies were collected by dissolution of large volumes of sample powder in a 3:1:5; v:v:v solution of concen trated HC1, concentrated and deionized, distilled water. Residues were allowed to settle and excess solution was decanted. The residues were washed twice in distilled, deionized water to remove remaining acid then dried at 40 degrees C. Fluorite was added as an internal standard to sample and insoluble residue powders. Sample powders were ground in a porcelain crucible with a 15-20% sample volume equivalent of fluorite powder. Powders were mixed with technical grade acetone to form a slurry, then spread on glass slides and allowed to dry at room temperature. Analyses were performed using a Philips (Norelco) x-ray diffraction unit equipped with a Norelco goniometer, a Norelco scintillation detector with a focusing crystal monochromator, a Hewlett-Packard data control and pro cessor unit and a Honeywell chart recorder. The radiation was copper (Cu) K oc . The detector system was equipped with a 1 degree divergence slit, 1 degree scatter slit, and a 0.006 inch receiving slit. Time constant was 2.0, chart speed 1 inch/minute and scan spped 1 degree 26 / 110 minute for all scans. Whole rock powders were scanned from 20 to 60 degrees 2 8 . Insoluble residues were scanned from 5 to 80 degrees 2 8 . Results and Discussion To test the presence of microenvironmental trends (i.e. lagoonal versus open marine, etc.) within the Beck Spring Dolomite, samples representing the three informal members were analyzed. The geochemical charac terization of the Beck Spring Dolomite was also compared with other shallow water dolomites of similar age and areal extent (Johnnie Formation and Crystal Spring For mation) to determine if distinct patterns unique to the Beck Spring Dolomite were present. Samples of possible Pahrump Group correlative strata from Seventeenmile Point, California, were also analyzed and compared with known Pahrump Group samples. Trace and major element ppm concentrations, insoluble residue weight percent and petrologic characters of the Beck Spring Dolomite and other similar carbonate rocks are presented in Tables 8 and 9. Ca and Mg levels and ratios are essentially the same for all samples except those which have been calcitized. Fe/Mn ratios are all similar although there may be some real differences in absolute levels of Fe and Mn both within the Beck Spring Dolomite and between the Beck Spring Dolomite and Johnnie-Seventeenmile Point samples. Ill TABLE 8: Informal members of the Beck Spring Dolomite— major, minor, and trace element analyses, weight percent insoluble residue, and petrologic characters. The letters of the sample codes indicate sample locale (see Fig. 3). The numbers following the locale code indicate stratigraphic position in feet in the measured sections. Abbreviations are: LAM = laminated, CH = cherty, micINT = microintra- clastic, OO = oolitic, PISO = pisolitic, X = arithmetic mean, s = standard deviation, and * indicates calcitized samples. 1 1 2 Samples Petrologic Characters Fresh Color Ca Wt.(%) Mg Wt. (%) SWM 5 0 LAM,CH N 3 22.4 12.8 c n c p SWM 85 LAM,CH N3-N4 19.0 10.0 0 ( 1 > S i ^ B -P 0 SS 222 LAM N3-N4 21.9 11.5 P S 0 P SS 780 LAM N3-N4 24. 3 13.2 U 0 -P P 0 ATM 140 LAM,CH N4-N5 20.4 11. 3 0 c £ -H 0 g P I 0 BS 165 LAM N4-N5 23. 3 13.5 P I SS 10 45* LAM N2-N3 29.5 5.71 X 21.9 12.0 s 1.92 1.40 SWM 2 70 micINT N4 21. 2 13.2 0 ■H -p SWM 591 OO-PISO N5-N6 21. 5 12.4 ■H i —1 o ATM 720 PISO N5-N6 22.8 13.4 W P •H 0 f t P 1 £ BS 680 grapestone N4-N5 22.2 12.7 1 { - H 0 0 ■H S -p BS 780 OO-INT N4-N5 23.0 13. 3 - r - j l —1 O SS 1055* PISO N4-N5 34. 5 3.43 o SS 1085 PISO N5-N6 5.30 2.70 X 19. 3 11.3 s 6.90 4.20 >i SWM 6 70 LAM,CH N6 22.2 12.5 ■p p 0 ATM 1110 LAM,CH 5Y2/1--N4 17. 3 9.0 3 ,C P U 0 S i P B BS 910 LAM,CH N5 23.1 13.3 0 0 ft s ft X 20.9 11.6 £> s 3.10 2. 31 SH BS-SH LAM? 5PB 3/2 37. 8 0.20 113 Ca/Mg TABLE 8 (continued) Fe Mn ^ Fe/Mn ppm ppm Ba ppm Sr ppm 1.75 488 59.3 8.23 ND 90. 1 C / ) p p C 0) rd X2 e >1 < D -P S 1.90 1260 458 2.75 70. 2 256 1.90 11,400 401 28.4 ND 128 P 0) P ,-C 0) 1.84 3630 395 9. 19 ND 77.4 O -P rd P c 0 *H > e o rd p i p i 1.81 1560 86. 0 18 . 1 77. 7 114 1.73 467 57.9 8.06 51.0 59.8 5. 18 1920 66 . 2 29.0 87.7 329 X 1.82 3130 243 12.4 33.2 121 s 0.07 4210 193 9.20 37.3 70.6 1.61 880 54 . 0 16.3 62.8 52.3 u •H -p 1.73 325 20 . 8 15.6 48.5 92.8 •—1 o 1. 70 294 97.6 3.01 61. 1 57.0 W P ■H C L ) C U P | £ 1.75 447 133 3 . 36 104 57. 7 o c u •H S 4 - > 1.73 456 63. 3 7 . 20 85 . 2 77.6 *H rH o 10.0 526 40 . 8 12.9 ND 111 o 1.95 553 28. 7 19. 3 75 . 6 89.6 X 1.74 502 66 . 2 10. 8 72.9 71. 2 s 0.11 236 42.6 7. 10 19 . 8 17.8 1.78 1600 55 . 1 29.0 7 3 . 8 93.5 +J P (U 1.92 1240 151 8.21 80. 6 79 . 1 pper Ch Member 1. 74 300 98.7 3.04 94.6 68. 1 X 1.81 1047 102 13.4 83.0 80. 2 P> s 0.09 672 48.0 13.7 10.6 12.7 SH 151 1510 862 1.75 ND 1074 114 1 1 5 SH Upper Cherty Member Oolitic-Pisolitic Member co XI w XI to o o o o o o o h - 4 o o o o o o • t • 9 • 4 • • • 9 • • 4 • • 00 o co to d d cn cn O' 00 00 to to d to d> CO cO CO cn to -O d CO to d O' cn oo cn M i —1 to cn co h - 4 i —1 h - 4 to to 00 to M to I-4 to Cn 00 to d 00 d 00 CO O' 00 O' to 00 <1 00 • 4 d • • • 4 « » • • • 00 o O d CO oo CO to -o d O' o o h - 4 h - 4 I —1 I —1 M to to I-4 to 00 CO co to d 00 00 M CO h - 4 00 to h - 4 o d 00 Cn co t —* <1 00 to tO M co <1 CO d oo cn 00 cn co 00 d to I —1 ( —* M H-4 t-4 t-4 o cn js. o o o cn o o h - 4 d CO to oo h-4 9 4 9 9 4 • • 0 * I • 9 • • <1 cn o cn -J CO to 00 to cn M oo oo CO CO cn co d o I —1 to to cn to cn cn -J cn i —1 to 00 M <1 O' to 00 i —1 to M O 00 cn cn to I —1 o to o 9 • 4 9 • » • • 4 • 4 4 • • • I —1 d cn d CO cn to cn oo 00 t —* I-4 cn -J o O 00 O o o o o o o o Lower Cherty and Laminated Members co x o o • • d s * U1 O 00 K) h - 4 o to O 4 5 » O O ' * O CO cn cn • • Cn 00 o o <1 cn o o t o <i cO co to to CO o o o o o • 4 • • to cn oo cn cn cn to 00 I-4 d I-4 00 cn o oo -J • • • • d cn I-4 CO to i —4 cn d 00 cn co Cn d o d d H- 4 cn oo O' o 4 • • • d cn d cn - J d - J oo o 00 o 00 4 • • • t-4 d o -o o o o o CO cn cn to co to 00 cn o o o o o cn <n co o O o to ■d to < 1 CO X C O 1 - 5 O O cop 2 P \ X /0 H 0 0 CO CO H- 0 Qj | —1 c c (D t y s! r t 0 H > Cd tc H 00 o 0 D r t H* D d 0 CL TABLE 9: Selected samples from the Johnnie, Crystal Spring, and Seventeenmile Point Formations— major, minor, and trace element analyses, weight percent insoluble residue, and petrologic characters. The first several letters of the sample codes indicate sample locale (see Fig. 3). The letters following the locale code indicate formation and lithologic types. Abbreviations are: LAM = laminated, OO = oolitic, JO = Johnnie Fm. oolite, 25-0 = Section 25, Seventeenmile Point oolite, 26-0 = Section 26, Seventeenmile Point oolite, Jdol = laminated dolomite, Johnnie Fm., 26aM = Section 26, Seventeenmile Point Fm., middle member, lower portion, 26bM = Section 26, Seventeenmile Point Fm., middle member, upper portion, CS = Crystal Spring Fm., 25L = Section 25, Seventeenmile Point Fm., lower member, 26L = Section 26, Seventeen mile Point Fm., lower member, and * = calcitized samples. 116 (D -p w -H -P r —I £ O C D O i —I rd C D > • H -r-j p p £ tr1 x: w o Sample Petrologic Character Fresh Color Ca Wt. (%) Mg Wt. (%) WP-JO OO 10YR 6/6 22.4 11.6 VSW-JO OO 10YR 6/6 21.8 12.1 SMP-JO* OO 10YR 6/6 27.6 2. 39 RDM-O OO 5Y 7/2 20. 4 10.8 SMP 25-0 OO 10YR 7/4 21. 4 10.6 SMP 26-0 OO 10YR 7/4 14. 3 7.90 X 20.1 10.6 s 3 . 30 1.63 VSW-Jdol LAM N3-N4 21.6 11.9 AH-Jdol LAM N3-N4 22.2 12.9 SMP 26aM LAM N3-N4 22.8 12.5 SMP 26bM LAM N3-N4 21. 9 12. 3 SMP 25M LAM N3-N4 22.3 12.3 X 22.2 12.4 s 4.50 3.60 SS-CS MASSIVE N5-N6 21.5 12.6 BS-CS LAM N3-N4 20. 8 12.2 SMP 25L MASSIVE N5-N6 20.8 10. 8 SMP 26L MASSIVE N5-N6 21.0 11.7 C D •P •H £ c o O -P i —I c o cd Q i —I rd C D > •H -rH £ P £ ty rP W o *o o* £ W •H -P U £ a c u in -H rd i —I rd *h -p P U ) ty > 1 W u o X 21,0 s 3, 30 11.8 7.80 117 TABLE 9 (continued) Ca/Mg Fe ppm Mn ppm Fe/Mn Ba ppm Sr ppm 1.93 17,600 1350 13.0 130 121 0 ) 1.80 10,700 1910 5. 60 670 116 -p t o - H -P ' —1 d O C D 11.5 6240 3160 1.96 151 125 O <H rd <D > 1.89 4620 1020 4.52 ND 86. 2 * H -r-1 /H H G d g er 2.02 20,000 1480 13.5 64.8 157 w o 1.81 12,100 1300 9. 31 10.0 119 X 1.89 s 0.09 13,000 6050 1410 325 9. 19 4. 12 175 282 120 25. 1 <D -P -H £ co 0 4-> 1 I g O (D Q rH fd ( D > ■pH *H g d g g 1 rC w o 1.82 1. 72 1.82 1.78 1.81 X 1.79 s 0.04 6080 7670 7045 3160 8050 334 483 875 756 1100 18.2 15.9 8 . 05 4.18 7.32 6400 1960 710 306 10.7 6.00 ND 10.7 ND ND ND 2. 14 4. 78 135 188 97.0 79. 2 60.8 112 50.6 td G o - H CO P -P Ci C U 2 < D I 1 i —i rd fd > -P *H cd d >1 p w u 1. 71 1. 70 1.93 1.79 X 1.78 s 0.11 1960 3620 4330 2180 400 410 970 780 4.90 8.83 4.46 2. 79 3020 1140 640 282 5.24 2.56 5. 30 15.0 25. 6 13.9 15.0 8.32 152 229 121 60.0 140 70.4 118 Crystal Spring Equivalents? Johnnie Dolomite Equivalents w X | c n X | o o O o I-1 o o o O o o o o oo on NO on M no on NO 00 4^ 00 ON 4* <1 VO 00 o M 00 o <1 ON NO on NO M M i —* I-1 On 00 on <1 4i» on on m -vj on 00 ON <1 00 00 00 on 00 on 4* 1 — 1 4^ VO M NO VO on cr> NO on ON 00 NO 4*. 4* M M M NO NO h-* NO NO M M M O NO vo 4* 00 00 -vj on 4* ON M 00 00 NO M 4^ O NO on <i vO on VO NO VO 4^ NO oo NO M O o O on o o m O O M o 4* on on NO M M on oo o NO 00 4^ VO O 0O on on 00 NO NO O -vj 4* o 4s. 4* 4*. o NO NO 00 <1 M oo on on o on M O NO 00 NO NO t —1 00 • i • • • • 9 9 • • • • • ON 00 i-1 00 00 on 00 ON o 00 o on ON O O o o O O O o o o o o Johnnie Oolite Equivalents i n X o o o o o o O o » • • ■ • • • • I-1 ON 00 <1 4* 4* on on -vj h-> 00 4* NO on oo 4* i-1 M 4a» -vj 00 4» 00 <1 4* o -vj 4 . VO -vj ON NO 00 ON • > • • • • ON VO <1 NO VO on I-1 4 a - 00 M on o i-1 M 00 -vj 00 o NO M 00 M NO 4 . ON o O 00 4* -vj 00 M O o o O -vj O • • • • • • • • 00 -vj NO O o ON O n O -vj VO VO 00 o on VO ON 00 4. on M NO oo oo NO 4. VO NO o M • • • • • • • • 00 -vj o -vj VO 4* 00 ON o o o o o o o X! W o o Oop S! 0 \ X W H C D 1 3 i n c n H- 0 Qj H C C C D O ' H — C D s; r t TABLE 9 (continued) Fe and Mn levels are higher in the lower laminated and upper cherty member samples than in the oolitic-pisolitic member samples (Table 10). This may be due to original differences in nearshore brackish deposition of the laminated and cherty members versus open marine deposition of the oolitic-pisolitic member, greater diagenetic alteration of the oolitic-pisolitic member as a result of greater primary fabric porosity or, perhaps, selective concentration of Mn and Fe in the algally-laminated sediments of the laminated and cherty members. Recent blue-green algae are known to highly concentrate Fe and Mn (Jones et al., 1978) in laboratory culture. Similar accumulation of Fe and Mn may have occurred in the Beck Spring algal mats and have been preserved as relict, albeit redistributed,Fe- and Mn-containing minerals. Fe and Mn content of the overall Beck Spring Dolomite (1660+ 2830 ppm Fe; 144+ 147 ppm Mn) versus the Johnnie-Seventeenmile Point samples (9700+ 5490 ppm Fe , * 1060+ 475 ppm Mn) shows considerably higher levels in the latter,even allowing for the large standard deviations (Table 11). This may also indicate closer correlation of the geochemistry of the Seventeenmile Point Formation samples with the Johnnie Formation than with the Beck Spring Dolomite. The Crystal Spring Forma tion carbonate shows Fe and Mn levels intermediate 120 TABLE 10: Mean trace element composition of the informal members of the Beck Spring Dolomite Element Lower Cherty- Laminated Members (n=7) X ppm s Oolitic-Pisolitic Member (n=7) X ppm s Upper Cherty Member (n=3) X ppm s Ca 219,000 19,300 193,000 69,000 209,000 31,000 Mg 120,000 13,500 113,000 42,000 116,000 22,900 Fe 3,130 4, 210 502 236 1, 050 672 Mn 243 194 66.2 42.6 102 48.0 Ba 33.2 37.3 72.9 19.8 83.0 10.6 Sr 121 70.6 71.2 17.8 80.2 12.7 Na 163 100 191 112 121 33.3 K 124 200 29.9 28.4 92.0 68.8 All values to 3 significant digits. X = arithmetic mean, s = standard deviation, n = number of individual samples. TABLE 11: Mean trace element composition— total Beck Spring versus total Johnnie-Seventeenmile Point Formations. Element Beck Spring X ppm s (n=15) Johnnie- X ppm (n=10) SMP s Ca 207,000 46,000 211,000 24,800 Mg 116,000 28,000 115,000 14,500 Fe 1, 660 2, 830 9, 700 5, 500 Mn 144 147 1, 060 475 Ba 59.0 33.9 88.6 209 Sr 92.9 49.9 116 37.9 Na 166 94.4 92.8 52.1 K 79.8 131 275 311 All values to 3 significant digits. X = arithmetic mean, s = standard deviation, SMP = Seventeenmile Point Formation, n = number of individual samples. 1 2 2 between the Beck Spring and Johnnie-Seventeenmile Point sets . Ba values are highly variable and have large standard deviations. No group trends are apparent in the Ba data. Sr values for all the samples are quite low and very similar. No trends are apparent for the average Na and K values among the Beck Spring informal members. However, slight differences in overall Na and I< content of the Beck Spring Dolomite versus the Johnnie-Seventeenmile Point Formations samples are observed. The former show slightly higher Na levels and slightly lower K levels. However, the standard deviations are large. Na and I< are quite mobile elements and their diagenetic pathways are poorly known. The observed differences are probably not of any environmental significance. In ancient carbonates trace element composition is a product of diagenetic modification of primary elemental compositions and/or introduction of additional trace elements. The observed values therefore may represent either true relicts of original depositional environment chemistry or the composition of epi-telo- genetic solutions and cements. The diagenetic, post- depositional changes which take place in carbonate sediments are usually a sequential series of dissolution- precipitation reactions which tend to enhance the stability 123 of the crystal lattice and decrease the porosity. Relict depositional versus diagenetic origin for trace element composition can not be positively deter mined. However, by investigating trace element levels in various Recent carbonate constituents and environments and comparing those results with ancient sediments, similar trends and element ratios of possible environ mental or correlation significance may be observed. Geochemical facies models for Recent environments utilizing Mn-Fe-Ba concentrations have been reported by Pilkey and Goodell (1964) and Friedman (1969). A similar Mn-Fe model has been described by Eriksson et al. (1975, 1976) for Proterozoic sequences in South Africa. All of the Mn-Fe-Ba models utilize the principle of differential concentration of Mn, Fe, and Ba in fresh and marine waters. Mn, Fe, and Ba in solution average 0.003-0.050 ppm in marine waters and 0.058-0.560 ppm in fresh waters (Friedman, 1969). Fresh water is slightly acidic and contains more soluble Mn, Fe, and Ba than weakly alkaline marine waters. Mn-, Fe-, and Ba-bearing waters entering lagoons, bays, and estuaries precipitate Mn and Fe oxides and BaSO^ in the weakly alkaline, brackish water (Friedman, 1969). This results in enhance ment of Mn, Fe, and Ba contents in the sediments and greater availability of soluble Mn, Fe, and Ba to carbonate-producing organisms living in these waters. 124 The above values refer to the three elements only as they occur in dissolved form. However, the principle form of migration of Mn, Fe, and Ba in fresh water may be in suspension absorbed to clay particles (Leutwein and Weise, 1962). Plots of Mn versus Fe and Mn versus Ba concentrations of the samples from the present study are presented in Figures 19 and 20. The bulk of the Beck Spring Dolomite samples plot within the lagoonal domain. The oolitic- pisolitic Beck Spring member samples tend to cluster closer to the open marine field than the laminated dolomites. The Johnnie and Seventeenmile Point Formation samples all plot with distinctly higher Mn and Fe contents suggesting more brackish conditions than those of the Beck Spring Dolomite samples. The Johnnie and Seven teenmile Point samples also show high clay contents which were not detected in any of the Beck Spring samples. Perhaps the high Mn and Fe levels in the Johnnie and Seventeenmile Point samples reflect deposition in a more restricted, clastic-rich coastal or marginal marine environment than the Beck Spring. Problems may exist, however, in explaining the source of the measured Fe and Mn in the samples. It is not known whether the measured Fe and Mn occur in carbonate min erals or other accessory minerals. Some of the Fe and Mn may have leached from accessory minerals, notably 125 FIGURE 19: Plot of Mn ppm versus Fe ppm levels in samples of the Beck Spring Dolomite and other formations. Modified from Friedman (1969). No error bars are indicated because the relative error is too small to be visible at the scale of the graph. Fields outlined by stippled patterns indicate subenviron- mental groupings of samples from the Friedman (1969) study. Narrow solid lines encircle cluster of Beck Spring Dolomite, Johnnie Formation, and Seventeenmile Point Formation samples. 1 2 6 127 10' 10 10 ' Mn ppm I01 O BS oo • BS doi A J oo ▲ J do I □ SMP oo ■ SMP dol 0 SMP lower dol 0 CS do! a % MARINE LAGOONAL (BRACKISH) 10 o « « I « 4 I - 1juL » » ■ i i i ■ 1 1 -I I 1 L X l i . 0 I I , . , ,1 l-J_ULU_______I , ■■ t I __‘ » « « » 1 10 o 10 10 10' Fe ppm 10 10 FIGURE 20; Plot of Mn ppm versus Ba ppm levels in samples of the Beck Spring Dolomite and other formations. Modified from Friedman (1969). Relative % standard deviation for Ba measurements is indicated by the horizontal bars. Mn error is too small to be visible at the scale of the graph. Stippled patterns outline groups of samples from proposed sub environments of Friedman (1969). 1 Mn ppm I04 : i A ' 0 □- m 3 _K i --- -Ah »▲ a — 23- A ~ i O 2 _ FRESHWATER i o- h LAGOONAL ■° BRACKiSH -T-O- vN -o— MARINE ! 0 -t __________i_______j_______i_ _ —i_____i____i_ 10 I0‘ h A - 1 O BS oo tt BS dol A J oo ▲ J do! □ S M P oo K SM P dol S3 S M P lower do! 0 CS dol ml . > , i„. . l - . J . 10 Ba ppm 129 magnetite, during sample preparation. The highest observed Beck Spring Dolomite Fe contents are from samples from Saratoga Spring. These samples also contain the highest Fe accessory mineral concentrations (Table 3). A 1% magnetite concentration could generate 7236 ppm Fe jLf totally dissolved but oxide minerals do not gener ally dis solve substantially in HC1. The average magnetite concentration for the carbonates from the Saratoga Spring section is 0.8 weight percent,which could presumably result in Fe levels of 5789 ppm. Fe levels for the Saratoga Spring samples range from 536-11,400 ppm. The Fe/Mn ratios may, however, provide evidence that the observed Fe and Mn levels do not reflect magnetite concentrations. The Fe/Mn ratio in magnetite averages about 400 (range =98-750) (Deer et al., 1966). Fe/Mn ratios of the Beck Spring samples are 2.75-29.0 indicating Mn values well in excess of the usual levels incorporated in magnetite and hematite. The graph of Mn versus Ba (Fig. 20) discloses facies trends similar to the Mn versus Fe plot but is complicated by the unreliability of the Ba measurements (average relative standard deviation = 23.4%). However, the plot does seem to suggest a distinction between the carbonates of the Beck Spring Dolomite and the Johnnie-Seventeenmile Point Formation. The Beck Spring samples cluster near 130 the boundary of the lagoonal-brackish and freshwater zones. The Seventeenmile Point-Johnnie samples lie within the freshwater zone. All of the samples are believed to be marine due to preserved structures and lithologies. However, the Johnnie-Seventeenmile Point samples might be considered more brackish than the Beck Spring samples. It is likely that the true Ba concen trations are somewhat less than the reported values due to the instrument "noise" encountered when measuring samples near the detection limit. All of the samples would therefore plot closer to the Mn axis and more "marine" in character. The Eriksson et a.1. (1976) model is plotted in Figure 21. The facies model for this study assumes atmospheric character enr^chment> depletion) peculiar to the Proterozoic (Cloud, 1972). The concentration of Fe and Mn in sea water was presumably much greater in the early Precambrian due to the reduced environment. Measurements of samples from subenvironments characterized on the basis of primary structures, etc., reveal the tidal-subtidal- lagoonal zones of the graph. The authors stress the increased Mn-Fe levels of their samples are due to the Precambrian atmospheric conditions but the model may have applicability to the late Precambrian Beck Spring environ ments as well. If one assumes a reduction in absolute Fe and Mn levels due to increased atmospheric oxidation, 131 FIGURE 21: Plot of mole % FeO versus mole % MnO for samples of the Beck Spring Dolomite and other formations. Modified from Eriksson et al. (1976) Relative % standard deviation is too small to be indicated at the scale of the graph. Stippled patterns outline clusters of samples from the proposed sub environments of Eriksson et al. (1976). Narrow, solid lines mark clusters of Beck Spring Dolomite, Johnnie, Crystal Spring, and Seven teenmile Point Formation samples. FeO wt % 2.5 2.0 RESTRICTED LAGOON - CLASTIC INPUT (Fe - rich dolomites ) • BS dol □ SMP oo m SMP do! S 3 SMP lower do! 0.5 SUBT/DAL TIDAL 0 0.5 1.0 5 M n 0 wt % 133 differences in the relative Fe and Mn concentrations of the Beck Spring and Johnnie-Seventeenmile Point samples can still be discerned. The high Fe and Mn content Beck Spring samples are again from Saratoga Spring, the mixed carbonate-clastic facies. The samples occupying the "tidal" zone in the Eriksson et aJL. (1976) study are recrystallized dolomites and cherts which form a proposed shallow basin shelf deposit distant from major clastic input. This may be similar to the environment of depo sition of the carbonate Beck Spring facies samples. Of all the carbonate trace elements, strontium has been the most extensively studied. The literature is voluminous and somewhat inconclusive. Good reviews can be found in Kinsman (1969) , Wolf et a_l. (1967), and Bathurst (1975). The concentration of Sr in a carbonate rock is primarily dependent on Sr, Ca, and Na concentration in the surrounding water and the temperature of the depositional environment. High temperatures, Ca contents, and salinities produce the highest Sr content in the resulting minerals. Recent calcitic sediments show Sr values of about 1000-1200 ppm. Aragonitic skeletal materials often show values up to 10,000 ppm (Badiozamani, 1973). In ancient limestones these values often drop to an average of 400- 700 ppm (Kinsman, 1969). Loss of the Sr is attributed 134 to diagenesis and neomorphism of the original sediment in the presence of meteoric fresh water with reduced Sr content or precipitation of calcite from highly super saturated solutions such as exist in supratidal evaporitic facies (Kinsman, 1969; Badiozamani, 1973; Veizer and Demovic, 1974). As a general rule earlier diagenetic calcite tends to contain more Sr than later diagentic calcite due to refluxing and re-equilibration with meteoric water during solution and reprecipitation. However, meteoric ground waters can have exceedingly high Sr contents if, for example, they drain high Sr content limestone terrains. Cements precipitated from such solutions at any time could have very high Sr contents. Such Sr-rich ground water solutions may be responsible for the elevated Sr values observed in the Silurian Hills sample. This sample shows evidence of extensive replace ment and calcitization. It is difficult to interpret which of the processes of Sr level reduction has been operative within the Beck Spring Dolomite. The sedimentary structures of the Beck Spring are suggestive of the classic supratidal-evapo- ritic facies. However, lack of substantial quantities of relict evaporitic minerals might indicate a more humid environment. Re-equilibration with low Sr content meteoric ground waters might be the best explanation for the reduced Sr levels observed. This would undoubtedly 135 have been aided by the apparent extensive epigenetic fabric porosity in the Beck Spring Dolomite samples. Sr/Ca ratios in continental subsurface and surface _ 3 waters average 0.32-3.2 x 10 . Limestone ratios average _ 3 0.6 x 10 . Veizer and Demovic (1974) defined a bimodal Sr distribution in which high Sr/Ca ratio samples (1.0- _ 3 5.0 x 10 ) were hypersaline and/or deep sea and low _ 3 Sr/Ca ratio samples (0.1-0.8 x 10 ) were littoral, neritic, shallow bathyal organogenic or organodetrital limestones. The low Sr/Ca groups were predominantly high Mg calcite. The high Sr/Ca groups were aragonite and low Mg calcite. The majority of the samples from the Beck Spring Dolomite, Johnnie,and Seventeenmile Point Formations fall within the low Sr, non-hypersaline group. Interpretation of Na and K concentrations is hindered by the lack of data on partition coefficients for calcite, dolomite, and aqueous solutions. Consequently diagenetic pathways cannot be proposed with any certainty. Mean Na concentrations of the Beck Spring and Johnnie-Seven- teenmile Point samples are 165.5 and 92.8 ppm, respec tively. K samples levels average 79.8 and 274.4 ppm, respectively. Na sample ranges of 5000-8000 ppm have been found in Recent biogenic carbonate (Pilkey and Goodell, 1964). The low levels of the samples of this study may indicate considerable solution and loss of Na and K through diagenetic interaction with meteoric waters. 136 Veizer et aJL. (1977) discuss a geochemical facies model using Na and insoluble residue contents to approx imate paleosalinity of the depositional environment pore solutions. The model correlates high Na content and high insoluble residue content with hypersaline facies. The authors define an approximate 230 ppm Na level boundary between hypersaline and marine facies (Fig. 22). This model is based on the premise that higher insoluble residue, usually clay, contents occur in the hypersaline, nearshore facies than offshore. The Beck Spring, Johnnie, Crystal Spring, and Seventeenmile Point samples all plot within the "marine" field of this model. The lack of any linear relationship between Na content and insoluble residue content also suggests the Na is not associated with the insoluble residue and may be trapped interstitially in the carbonate lattice or as solid inclusions in the carbonate crystals (Veizer et aJL. , 1977). All of the samples except those from Silurian Hills show dolomite with good ordering based on the presence and sharpness of the primary dolomite superstructure peaks (Goldsmith and Graf, 1958) (Table 12). Other samples containing calcite also show loss or reduction in reflec tion intensity from the ordering peaks indicating possible breakdown in the stoichiometric ordering due to late secondary calcitization.. Reflections from (101) are 137 FIGURE 22: Plot of Na ppm versus weight % acid-insoluble residue of samples of the Beck Spring Dolomite and other formations. Modified from Veizer et aj_. (1977) . Relative % standard deviations are too small to be indicated at the scale of the graph. Stippled patterns outline sub- environmental fields proposed in Veizer ejt a_l. (1977) . Blank regions are undefined environ mental zones because no samples of those elemental concentrations occurred in the original Veizer et al. (1977) study. 138 10' fc r CL CL O I 0 2 1 0 9 O A 0 D □ Ov D O O BS oo • BS dol A J oo A J do) □ SMP oo ■ SMP dol 0 SMP lower do! 0 CS dol HYPERSALINE m 0 0 MARINE m A A - f I I - I t J 10 o 10 10 % Insoluble Residue 139 TABLE 12: XRD mineralogy and structural ordering of samples of the Beck Spring Dolomite and other forma tions . Abbreviations are : "+" = present in substantial quantities, " = not detected, "tr" = detected in trace quantities ( < 5%) . Mole % Ca-Mg are calculated from AAS data (Tables 8 and 9) for whole rock composition. The values approach mineral composition in only the single carbonate rocks. Sample codes are locale code followed by stratigraphic position or lithotype (see Tables 8 and 9). BS-SH is a Beck Spring Dolomite sample from the Silurian Hills section. 140 SS 1085 Caj-QMg.^ + -(101), - + o l -quartz, magnetite 58 42 (021) Beck Spring Beck Spring Lower Oolitic-Pisolitic Laminated-Cherty Member Members co Od 03 > CO CO 03 CO > CO CO CO CO CO CO CO • “3 CO CO CO CO 2 2 2 § i —1 <i co I —1 I —1 -J to o 00 00 <i CO ro CO O M 00 to CO CO CO o o NO CO co 4^ 4* O to CO o CO O M o CO O O o n O O O Q Q Q o Q O Q PJ PJ pj pj PJ PJ PJ PJ PJ PJ PJ PJ pj 00 CO CO CO CO CO CO <1 CO CO CO CO CO 00 CO CO CO CO 4^ CO CD CO 00 CO <o 2 2 2 2 2 2 2 2 2 2 2 2 2 P IQ iQ iQ IQ cQ cQ P p P p P P 1 —1 4i> 4* 4* 4* to 4* 4=. 4^ 4^ 4^> tO CO CO 4* CO CO M1 4* CO to to 4* + + + + + + + + + + + + + I ! 1 ! i i s i ! 1 M h~* O 1 —1 1 —1 M h-1o "P h-1 • v > o o O to o O O O CO o O 1 —1 i — 1 I —1 i —1 1 —1 i—1 I —' I —' M i— 1 M I —1 >o 1 i 1 1 I ! + I I s s f t 1 rt i ft + 1 (t + ! ft + + g g g g i P C PJ g f t N Sample Ca-Mg Dolomite Ordering Calcite Quartz Insoluble Residue TABLE 12 (continued) Sample Mole % Ca-Mg Dolomite Ordering Calcite Quartz Insoluble Residue Beck Spring Upper Cherty Member SWM 670 Ca56Mg44 + -(101) - - ATM 1110 BS 910 Ca58Mg42 Ca56Mg44 + + -(101) -(101) - + a -quartz SH BS-SH Ca92Mg08 - missing all but (006) + tr WP-JO Ca58Mg42 + -(101) _ — e Oolite valent VSW-JO SMP-JO Ca56Mg44 Ca87Mg13 + + -(101) -(006) , (021), (101) + — o c -quartz # -quartz, talc •H *H C 0 c tJ1 £ w 0 b RDM-0 SMP 25-0 Ca57Mg43 Ca60Mg40 + + -(101) _ + + SMP 26-0 Ca57Mg43 + -(101) - - £ NJ Sample Mole % Ca-Mg TABLE 12 (continued) Dolomite Ordering Calcite Insoluble Quartz Residue c o < D 4 - > d ) -p £ •H ‘ H d) C E H £ 0 n3 rd H > 0 0 - H b Q £ tf1 w C O ■ p H £ r u C n C D -P £ tH C O -H fO L > L Oi-H U W £ O' W VSW-Jdol Ca57Mg43 + -(101) - - AH-Jdol Ca55Mg45 + - tr SMP-26aM Ca57Mg43 + -(101) - - SMP-26bM Ca56Mg44 + -(101) - - SMP-25M Ca57Mg43 + -(101), (102) - - SS-CS Ca55Mg45 + - BS-CS Ca55Mg45 + - + SMP-25L Ca58Mg42 + tr + SMP-26L Ca56Mg44 + tr tr ^-quartz, magnetite a-quartz, leuten- bergite missing from many of the samples but this is probably due to the weak intensity of the peak,not its absence. The (101) peak did appear on diffractograms in which the major peaks were allowed to go off scale. The dominant residues from the Beck Spring Dolomite samples are quartz and traces of magnetite. The Johnnie-Seventeenmile Point samples also contain quartz and a Fe-Mn variety of clinochlore, talc, and an uniden tified clay mineral. The Beck Spring residues are medium dark grey to jet black in color and are texturally similar to powdered charcoal when dry. The Johnnie-Seven- teenmile Point sample group residues are similar to the Beck Spring residues in color and texture. In contrast, the oolite-pisolite residue from the Johnnie-Seventeen- mile Point samples is brilliant orange in color. This orange residue may well be responsible for the character istic tan to yellow color of the Johnnie oolite. When observed with the petrographic microscope, the residues usually show large amounts of rounded quartz grains and a felty black or orange matrix. The clinochlore leutenbergite may be the black felty material but was detected in only 1 sample. The presence of graphite cannot be totally discounted due to the overlap of the main characteristic peaks of graphite and those of quartz. The overwhelming volumetric contribution by quartz makes direct detection of the black and orange matrix materials 144 extremely difficult. The only solution appears to be physical separation of the matrix and quartz using heavy liquids or preparation of grain thin sections of residues for direct petrographic studies. The only minerals identified other than quartz occur in residues from Seventeenmile Point and Virgin Spring Wash samples, both areas of high fault density. The talc and clinochlore detected in these samples might be directly related to low grade metamorphic events associated with faulting and subsequent igneous intrusion along the faults. Summary Few distinctive differences in geochemistry occur among the Beck Spring sample group and other groups or among the members of the Beck. Spring Dolomite. This may be a result of only minor differences in depositional environments, diagenetic homogenization of trace element compositions, small numbers of samples, or some combin ation of these factors. Significant differences do appear to exist in the Fe and Mn levels of the Beck Spring and Johnnie-Seven- teenmile Point sample groups (Table 11). The oolitic-pisolitic member of the Beck Spring Dolomite appears to have lower Mn and Fe levels than the other members (Table 10). This may reflect deposition of the oolitic-pisolitic member under more open marine 145 conditions than the laminated and lower cherty or upper cherty members or selective accumulation of the Fe and Mn by the algal mats of the laminated and cherty members. Fitting of the data of this study to published geochemical facies models suggests lagoonal to tidal flat deposition of the Beck Spring samples and restricted lagoonal or brackish water deposition of the carbonate of the Johnnie and Seventeenmile Point Formations. 146 STROMATOLITES Classification and Biostratiqraphy Classification of stromatolites is complicated by the presence of at least 12 independent, parallel classification schemes (Krylov, 1976) which all use essentially different criteria or sets of criteria. The types of criteria used depend to a great extent upon the size of the samples relative to the size of the stromatolites and the philosophical approach and purposes of the investigator. Such classifications are far from objective and are therefore somewhat unsatisfactory. This study employs a classification system based on observable characters (i.e. mode of occurrence, column shape, lamina shape and microstructure, etc.). The descriptions follow the format of Walter(1972). The macromorphological nomenclature of Hofmann (1969) and Walter (1976) is used. The microstructure descriptions follow Komar (1966, 1977). Microstructures represented in the 1977 classification of Komar are identified by the appropriate type number. Group and form binomial nomenclature is used to identify stromatolites. In all cases, reference is made to the source of the group descriptions and diagnoses. These references are placed in parentheses beneath the heading for each group descrip tion. In particular, the analyses follow the descrip tions and classifications of Komar (1966), Kyrlov(1967), 147 Komar et al. (1965), and Walter (1972). The use of stromatolites for biostratigraphy depends upon the uniqueness and constancy of micro- and/or macromorphology of time-restricted forms of assemblages. The plasticity of macromorphology has been documented by Krylov (1967) and Serevryakov(1976b). A combination of biotic (algal-bacterial inflora) and abiotic (current, sediment supply, degree of exposure, etc.) appears to control the resulting macromorphology (Logan et. aJL. , 1964; Cloud and Semikhatov, 1969; Awramik, 1976; Serebryakov, 1976b; Horodyski, 1977). In addition, no clear consensus exists concerning the degree of algal microbiota control of stromatolite microstructure (Gebelein,1974; Awramik, 1977). It is likely that the dominant morphological control at least of microstructure is biologic but varies both with type of growth, depositional environment,and time spent within that environment (i.e. seasonal changes in algal growth versus seasonal climatic changes, etc.). The lack of constancy in macromorphology and the extended time ranges of many stromatolites (Semikhatov, 1976) makes the use of individual forms as "index fossils" questionable or at least quite limited. The trend in recent studies is towards the use of assemblages of stromatolite forms to define time-restricted intervals (Preiss, 1976; Semikhatov, 1976). For example, individual forms of the stromatolite group Conophyton Maslov may 148 occur in deposits of greater than 2.0 b.y. to 600 m.y. in age but deposits containing both Conophyton Maslov and Baicalia Krylov appear to be restricted to a range of approximately 1400-1000 m.y. (Preiss, 1976; Semik hatov, 1976). The use of the concept of stromatolite assemblages also allows for the gradual changes in group and form morphologies in response to biologic evolution and abiotic modifications of the environment. The persistance of distinct time-restricted forms and assemblages has been well documented in the U.S.S.R. (Krylov, 1963, 1967; Semikhatov and Komar, 1965; Semikhatov, 1976). Many of the Russian taxa have been described from other continents, notably Australia (Preiss,1976; Walter, 1972) and North Africa (Bertrand- Sarfati, 1972). Confirmation of the Russian correlation worldwide awaits many further descriptions of stromato lites from other continents in combination with additional geochronology. The Russian biostratigraphic studies of changes in complexes of certain groups and forms of stromatolites have produced a four-fold division of Precambrian time younger than 1700 m.y. The four intervals are Early Riphean, 1700+100 to 1350+50 m.y.; Middle Riphean, 1350+ 50 to 950+50 m.y.; Late Riphean, 950+50 to 675+25 m.y.; and Terminal Riphean or Vendian, 675+25 to 570+25 m.y. These dates are based on isotope geochronology of the 149 stromatolites and stratigraphically related deposits. Time ranges of key stromatolite groups are presented in Figure 23. Where assemblages of these forms are found, tentative correlations, at least, can be made to the type sequences of the Siberian platform. Systematic Descriptions Columnar stromatolites are defined in this study as forms in which the dimension the in direction of growth is usually greater than at least one of the transverse dimensions (Walter, 1976). Columns are often cylindrical to subcylindrical in shape and distinctly segregated from the surrounding rock. Columns may be branched or unbranched and display many different growth attitudes, branching styles, surface ornamentations, etc. Columnar forms often occur in large numbers in close lateral association. Bridging or interconnecting laminae between adjacent columns may occur but if the columns are entirely laterally linked they form columnar-layered forms. Nodular and bulbous forms are types in which the dimension in the direction of growth in usually less than the transverse dimensions. The nodular and bulbous forms are frequently isolated or discrete with lateral linkage of basal laminae only. The nodular forms are generally hemispherical to irregularly shaped in vertical cross section. The marginal laminae of the nodular forms drape over the sides of the nodule then extend laterally 150 FIGURE 23: Proposed time ranges of important stromatolite groups. Modified from Semikhatov (1976). 151 ISOTOPIC AGE (m.y.) O o LO C\J I o o r- . o LO oo o LO or i LO O 0"- r > - LO LO i i Aphebian Riphean o cu cr -s GEOLOGICAL AGE Early Middle Late fD "5 3 z s QJ C0N0PHYT0N JACUTOPHYTON COLONNELLA KUSSIELLA BAICALIA TUNGUSSIA PANISCOLLENIA COLLENIELLA STRATIFERA IRREGULARIA GONGYLINA NUCLEELLA BOXONIA LINELLA Ul CO into surrounding material giving the forms a flattened base. In bulbous stromatolites the marginal laminae tuck in towards the center of the base of the stroma tolite producing a narrow, pinched or constricted base. Stratiform stromatolites are non-columnar forms with essentially flat, wavy, or irregular laminae which are laterally continuous within the entire outcrop or deposit. Stratiform stromatolites are essentially synonymous with cryptalgalaminate sediments. Column "walls" are structures at the margins of the columns formed by one or more laminae from within the column bending down and coating the column margin (Walter, 1976). Walls may be continous or discontinuous and patchy. The term laminae as used in the microstructure descriptions refers to the usually dense, dark micrite or microspar layers which preserve fine clotted, striated, etc. details of structure. Interlaminae are the semi transparent layers between the micrite-microspar laminae and are usually composed of subhedral to euhedral microspar, sparite, or chert. Columnar Stromatolites Group Conophyton Maslov (Maslov, 19 38; Komar et al.,19 65; Komar, 1966; Krylov, 1967) Group diagnosis: The group is characterized by non 153 branching columns formed by laminae with a high conical or subconical arch, the top of which is always facing upwards and is generally pointed. The laminae are usually abruptly thickened in the apical region and form an axial zone. A few specimens in a particular outcrop may lack such axial zones. Outside the axial zone the laminae have a relatively constant thickness which produces a regular, concentric arragement in cross section. Axial ribs or flattenings occur in some forms. The micro structure is usually ribbon or striated form. The columns are usually subcylindrical with a straight axis and vertical or subvertical orientation. Column margins are smooth, bumpy, or fimbrate with connecting bridges. A column wall is sometimes present. Columns vary from a few centimeters to a meter or more in diameter. The height is usually many times the diameter and can reach tens of meters. Age: Dominantly Early to Middle Riphean with a few specimens from the Aphebian and Late Riphean into the Vendian. Distribution: Forms have been identified in Europe, Asia, Africa, North America, and Australia. Conophyton c.f. qarganicus Description Mode of occurrence: These stromatolites occur as laterally linked, overturned columns in a 8-10 m thick 154 interval in a dolomite sequence associated with large intraformational breccias and broadly planar laminated dolomicrite and dolomicrosparite (Fig. 24A). In the area of best exposure, fractured, meter-size blocks of the deposit appear to have been shed to form large rubble deposits. The axes of the stromatolite columns within the blocks now lie generally parallel to the bedding surfaces of the formation. The axial zones and cracks have been infilled with the same laminated, fine-grained dolomite which comprises the matrix of the stata imme diately above the stromatolites. Directly overlying the blocks is a layer of intraformational rubble containing broken column tips and flat pebble conglomerate. The columns in this outcrop,therefore, are not in growth position though they maintain the original relation to immediately adjacent columns within a given block. Column shape and arrangement: The columns are circular to subcircular in cross section and up to 0.4 m in dia meter (Fig. 24A,C). Axes of the columns appear to have been erect, straight, subparallel, and spaced 15-45 cm apart (Fig. 24B). Laminae from the edges of the columns interconnect laterally with adjacent columns forming a nearly continuous growth surface (Fig. 25A). Rare linear ridges connecting column crests are observed (Fig. 25B,C). Horizontal cross sections through adjacent columns show distinctive figure eight patterns (Fig. 24E) . Basal FIGURE 24: Photographs of column structure and mode of occurrence of Conophyton c.f. qarqani cum. (A) Horizontal cross sections of Conophyton columns showing close lateral column associations and linkages to adjacent columns. Note prominent infilled cores. Lines are added to emphasize laminae profiles. Lens cap is 52.5 mm in diameter. (B) Vertical section through an exposure showing elongate, replaced central core of one column. Column is continuous from base to top of boulder. Note steeply curved laminae between adjacent columns. Lens cap is 52.5 mm in diameter. (C) Large, circular column cross section. Column of near maximal diameter. Hammer is 30 cm long. (D) Vertical slab through column detailing laminated, replaced core (c), broad column laminae, and valley with dolospar vugs between adjacent columns. Bar is 1.5 cm long. (E) Horizontal section of sample of photograph (D) showing cross section through replaced cores (c) and mid section of figure eight pattern between adjacent columns. Bar is 2 cm long. 156 157 interconnected laminae bend upwards steeply 50 to 60 degrees to form the sides of the cones. The column surfaces are very smooth and even. Branching: No branching was observed. Margin structure: Margins are without walls and laminae interconnect from one column to the next adjacent column. Lamina shape: Laminae are conical, extremely regular in thickness (0.8-1.0 mm), straight and parallel with little variation in angle of slope (Fig. 26). Laminae flatten smoothly into parabolic valleys between columns (Figs. 24D, 26C). There is also a slight tendency for the laminae to bend inwards as they approach the central column cores. Axial zone: The axial zones in nearly all the samples have been eroded to form subcylindrical tubes filled with layered dolomite and dolospar. The layered dolomite was probably deposited in the hollow, eroded axial zones whi. 1 the blocks of columns were lying on their sides. The lower half of some cores is filled with the layered dolomite whereas the upper half is filled with coarsely crystalline dolospar. Wide cracks through the columns show similar layered infillings with the same planar orientation as the material in the cores (Fig. 26D). The filled cores are ovoid to circular in cross section and 2-8 cm in diameter. Large section of infilled cores have been followed longtitudinally for nearly .1 m (Fig. FIGURE 25: Reconstructions of Conophyton stromatolite forms. (A) Generalized reconstructed synoptic growth surface. Subsequent erosion and fracturing of columns and axial zones is not shown. (B) Detail of horizontal cross section through a ridge connecting two adjacent columns. (C) Three-dimensional reconstruction of the interconnecting ridge struc ture. Plane indicates approximate position of the detail of drawing (B) . (D) Detail of axial zone laminae. Type II or Type III laminae. 159 i cm 1 cm 160 24B). Column laminae truncate abruptly in the axial zone with irregular, stepped edges (Fig. 26A,B). The cylindrical shape of the replaced cores may suggest an inherent structural weakness or susceptibility to solution. This would be expected if the axial zone laminae are thickened are their number reduced. Partial replacement of Conophyton axial zones by barite has been reported by Donaldson(1976) who cites possible high ini tial porosity due to gas entrapment as a cause of the selective diagenesis of the axial zones. Walter (1972) describes C. basalticum samples which, have suffered extreme axial zone alteration where in most cases nothing remains but an open tube lined with quartz cystals or filled with a solid core of quartz. It is probable, therefore, that the structures observed in the present study do represent selectively replaced axial zones. Two samples may show possible relict axial zone laminae. One sample is the broken tip of a small column which shows faint, irregular pointed traces. The other sample is a vertical section through a possible axial rib projecting from an infilled core. Cross sections through this structure reveal a faint laminae pattern similar to a combination of Types II and III of Komar et al. (1965) (Fig.25D). Microstructure and texture: Microstructure appears to be fragmented ribbon-striated to linearly striated form 161 FIGURE 26: Photographs of detailed laminar structure of Conophyton c.f. qarqanicum. (A) Detail of column laminae and replaced central cores (c). Bar is 1 cm long. (B) Detail of column laminae and replaced core showing a fragment of column laminae in core matrix. Bar is 1 cm long. (C) Enlarged section of laminae in intercolumn saddle. Note apparent thickening of the laminae and crescent-shaped vugs filled with coarsely crystalline dolospar (sp). Bar is 1 cm long. (D) Detail of large-scale fracture (fr) through column which has been infilled by layered,fine-grained carbonate similar to the core fillings. Orientation of layered carbonate in fractures is subparallel to layering in core fillings sug gesting penecontemporaneous deposi tion in fractures and cores. Bar is 1 cm long. (E) Striated ribbon microstructure. Note lenticular dolospar areas between striae. Bar is 200yum long. (F) Striated ribbon microstructure showing striated dolomicrite laminae and clotted dolomicrosparite inter laminae. Bar is 200>(m long. 162 nj* H 163 (Komar Type 10) (Fig. 26E,F). Dark laminae are composed of brown micrite with fine, very dark grey striae extending in an interwoven fashion through the laminae. Laminae average 0.06-0.09 mm in thickness. Laminae are often continuous for several cm. Some lenticular micro spar inclusions also occur in the laminae giving them a pinching and swelling character. Interlaminae are composed of equant, subhedral spar and microspar and average 0.06 mm in thickness. Interspaces: Columns are all laterally linked with interspaces filled with the connecting laminae (Figs. 24D,26C). Laminae and interlaminae often thicken to 2-4 mm in the intercolumn spaces. The valleys between the columns are often filled by crescent-shaped spar- filled fenestrae. A few pockets of microintraclastic debris, mostly solution leached intraclasts 1-2 mm long, also occur. Secondary alteration: The entire deposit is dolomite which is probably an early diagenetic replacement of primary aragonite or calcite. The most prominent alter ation feature is the total solution and infilling of the axial zones. Intensity of the replacement decreases outward from the cores and fractures. Laminae in parts of the columns distal to the cores and fractures show only minor microspar development and retain, fine details of laminae structure. Therefore, it appears likely that the main route of movement of diagenetic solutions was through the axial zones and between the laminae in the intercolumn valleys. This may reflect initial high porosity in these regions due to trapped gas, fluids, organic material, etc. Comparisons: The high conical laminae, circular cross sections, and striated microstructure with lenticular inclusions are convincing evidence for placing these samples in the group Conophyton. The presence of probable axial zones is also supportive of this assignment but the lack of well preserved axial zone laminae is proble matic. Further field studies will be needed to locate samples which may preserve the fine details of the axial zones and provide definitive evidence of the affin ities with the group Conophyton. The Conophyton forms which show the greatest similarities to the samples from the Beck Spring Dolomite are C. garganicum Korolyuk, C. cylindricum Maslov, and C. basalticum Preiss. All of these forms possess macrolaminae (1-2 mm) of very constant thickness and orientation. Axial zones of C. basalticum are 4-20 mm wide and show Type I axial zone laminae (Komar et a_l. , 1965) . C. cylindricum has ribbon micro structure. C. garganicum has a fine, linearly striated microstructure. C . cylindricum, however, occurs as tightly spaced tall columns resembling organ pipe clusters. C- gargani cum occurs as tightly to moderately spaced 165 columns often connected by transition bridges. Inter connected C. qarqanicum columns show smooth surfaces and figure eight patterns in cross section. They are 0.2-1.5 m in diameter and form distinct concentric circles, ovals or ellipses in cross section. Axial ribs are often present. The angle of the axial zone laminae is sharp, apex angles averaging 80-90 degrees. Diameters of the axial zones are 2.4-7.9 mm with Type III lamina shapes. The diameter of the axial cores in the Beck Spring Dolomite samples is greatly in excess of the reported values for Conophyton axial zones. This is probably due to solution enlargment and does not reflect the original diameter. Similarities in bridge structure, spacing, macrolaminae, and microstructure, angle of laminae, and possible axial zone lamina type suggest assignment of the observed forms to C. c.f.qarqanicum. Distribution: Conophyton is restricted to the lower laminated member of the Beck Spring Dolomite outcrop at Acme Talc Mine, Alexander Hills. Group Baicalia Krylov (Preiss, 1972,1976; Krylov, 1967) Group diagnosis: The stromatolites have tuberous, tor tuous subcylindrical columns with infrequent to frequent parallel to markedly divergent branching often with a distinctive constriction at the base of the new branch. The columns are naked or have patchy walls. In some 166 forms peaks and bridges occur frequently. Age: Middle to Late Riphean Distribution: Widely distributed in Europe, Asia, North Africa, North America, and Australia. Baicalia-like forms Description Mode of occurrence: The stromatolites occur as inter- tonguing biostromes in a dolomite deposit associated with abundant macrointraclastic breccia and disrupted, lami nated dolomicrites and dolomicrosparites. Column shape and arrangement: Columns are broad, tuber ous to bumpy, subcylindrical, irregular to club-shaped with round to ovoid cross sections (Fig. 27). The columns are frequently broken, restacked, and coalesced with intraclastic debris filling the interspaces (Fig. 28B,F,G). Intercolumn debris is generally matrix- supported with moderate to poor sorting indicating limited winnowing of debris (Fig. 28D). Orientation is usually very irregular due to frequent erosion. Columns are generally 2-8 cm in diameter. Vertical transitions from Stratifera to the Baicalia-like form, erosion, and resumption of stratiform growth are often observed (Fig. 9). Branching: Most of the apparent branching observed was due to the eroded nature of the stromatolites. The irregular disposition of the column sections gives the 167 FIGURE 27: Reconstructions of the Baicalia- like stromatolite forms. (A) Growth form reconstructed by serial slab method. Note large erosional cavities and apparent column coalescence in lower left corner. (B) Reverse side view of column section in drawing (A). (C) Generalized sketch of outcrop surface from which sample in drawings (A) and (B) were obtained. Bar is 10 cm in length. 168 169 FIGURE 28: Photographs of Baicalia-like stromatolite forms. (A) Outcrop surface showing erosion- induced pseudobranching and possible true branching. Lens cap is 5 2.5 mm in diameter. (B) Column detail showing inter connecting laminae, ragged and eroded margins and abundant fine intraclastic material in column interspaces. Irregular white vugs are filled with coarsely crystalline dolospar. Bar is 2 cm long. (C) Club-shaped forms showing only mild disruption of growth pattern. Typical flat Stratifera growth is apparent at the base of the photo graph.. Bar is 3 cm long. (D) Detail of photograph (B) showing curled lamina tip and fine intra clastic material in dark micritic matrix. Bar is 1 cm long. (E) Ribbon microstructure composed of alternating dark micritic laminae and clotted dolomicrospar. Bar is 200/tm long. (F) Large, irregular club-shaped column showing eroded, ragged margins and reorientation of laminae over erosion surfaces. Bar is 2 cm long. (G) Detail of photograph (F). Note large dolospar-filied and layered vugs in intercolumn spaces. Bar is 1 cm long. 170 171 appearance of branching but is due to erosion and sub sequent regrowth (Fig. 28A/B/C). The two samples which were sectioned for reconstructions show only this erosion- induced reorientation (Fig. 27A/B,C). Field and photo graphic observations of outcrops (Figs, 27C, 28A,B) suggest branched forms in many exposures but further sampling and sectioning will be necessary to confirm the presence of true branching. Margin structure: The margins are naled to ragged with abundant peaks, cornices and bridges. The columns lack wall structures. Laminae drape 1 to several cm into the interspaces. Some laminae appear to interconnect with adjacent columns in steeply concave parabolas giving the appearance of columnar-layered stromatolites. Margins of the columns are frequently sharp, straight, erosional surfaces. Overhanging laminae are frequently refolded or curled at their ends (Fig. 28D) suggesting a spongy, flexible consistency to the growing surfaces. Lamina shape: Lamina shape is generally gently to steeply convex. Microunconformities are common and sharp with angular relationships up to 90 degrees. Typical, lamina shapes are illustrated in Figure 28B,F,and G, Microstructure and texture: Laminae are quite uniform in thickness, generally 1 mm. or less in width, light grey to pale cream in color, and very well preserved in the field and slabs. The microstructure is composed 172 of alternating streaky to distinctly banded dark brown to grey micrite layers, 0.2-0.3 mm thick, with a few localized peloids (0.06-0.15 mm) with diffuse margins and thicker, 0.5-0.6 mm layers of microspar containing thin, 0.02-0.05 mm clotted to streaky narrow micritic stripes (Fig. 28.E) . The microstructure is slightly diffuse ribbon type (Komar Type 1 or 3). Similar micro structure also has been described as "film microstructure" by Bertrand-Sarfati (1976). Interspaces: The interspaces are filled with dark grey (N2-N3) intramicrudites. Abundant clear, euhedral, spar- filled vugs and fenestrae occur between overhanging laminae and irregularly scattered among the algal laminated intraclastic debris (Fig. 28F,G). Secondary alteration: The stromatolites are composed entirely of dolomite probably of early diagenetic origin. Probable primary laminoid fenestrae have been solution enlarged and filled with spar and quartz. Late stage spar, quartz, and iron oxide veins cross cut the laminae. The micrite laminae and interlaminae have undergone some aggrading neomorphism producing microspar and pseudospar patches with relict micrite inclusions. Pseudomorphs of anhydrite and halite as well as euhedral monocrystalline quartz occur in the micrite laminae. Comparisons: Although the columns are severely broken, they do preserve some characteristics of the group Baicalia Krylov: (1) the irregular, tuberous and bumpy columns with some indications of pinching and swelling, (2) the presence of laminar microunconformities, (3) naked column margins, and (4) banded microstructure. The greatest difficulty in assigning these forms to the group Baicalia Krylov is the lack of positive confirmation of true branching. The outcrop surfaces are suggestive of divergent or £*-parallel branching but this cannot be proven without additional samples. These stromatolites were originally generally described by Licari (1971) as members of the groups Kussiella Krylov or Colonella Komar.They lack Colonella1s straight columns, smooth margins and regular subcylin- drical column cross sections,and Kussiella' s straight, cross-ribbed, subcylindrical columns and parallel branching style (Komar, 1966). The forms show the greatest similarity to eroded samples of B. burra Preiss (Preiss, 19 76) and B. ingilensis Nuzhnov. B. burra has a distinct banded sparry and peloidal microstructure, numerous erosional unconformities, recumbent folds of the edges of the laminae and occasional bridging laminae. burra does not appear, however, to have extensive overhanging peaks and bridges. B. ingelensis reportedly has long overhanging peaks,but a detailed description of this form was not available to the author. Distribution: The Baicalia-like form is apparently restricted to the lower portion of the upper cherty member of the Beck Spring Dolomite at Acme Talc Mine, Alexander Hills. Unassigned Nodular and Bulbous Stromatolites (Maslov, 19 37; Rezak, 195 7; Komar, 19 66) Type A Description Mode of occurrence: This type occurs in fine grained, medium to dark grey dolomite. The nodules are usually silicified and associated with large black chert nodules and interbeds and finely laminated dolomite. A few non- silicified nodules were observed. Nodules appear in tightly packed clusters spaced at wide intervals in a thin (<1 m) bed. The stromatolites appear laterally discrete but stack vertically in a loosely alternating fashion (Fig. 29B). Each vertical layer is separated from the underlying layer by an erosional surface and 1-3 cm of wrinkly laminated dolomite and finely dissem inated chert. Nodule macromorphology: The forms are nodular to elongate nodular and unlinked (Fig. 30A). They are ovoid to ellipsoidal in cross section. The probable nodule dimensions of 6-8 cm in height have now eroded to 4-6 cm in height. None of the specimens is complete but length is presumed to be 20-24 cm assuming symmetry in overall shape. Nodules are closely spaced and 175 FIGURE 29: Photographs of macro- and micro structure of the nodular stroma tolites . (A) Diffuse, clotted ribbon microstructure of nodular type A, plane-polarized light. Bar is 200/<m long. (B) Nodular type A. Note fine, uniform laminae and possible wall structure. Bar is 2 cm long. (C) Nodular type B, irregular nodule shape. Scale is 15 cm long. (D) Nodular type B, hemispherical nodule shape. Lens cap is 5 2.5 mm in diameter. (E) Nodular type B, irregular nodule shape. Bar is 2 cm long. (F) Diffuse, clotted banded micro structure, nodular type B, plane- polarized light. Bar is 200yum long. 1 7 6 177 FIGURE 30: Reconstructions of nodular stromatolite forms. (A) Reconstruction of nodular type A showing smooth, regular laminae and elongate, tubular shape. (B) Reconstruction of nodular type B showing irregular, bumpy shape with overhangs and small nodules within the confines of the larger structure. (C) Reverse side view of nodule in drawing (B). 178 10 cm 10 cm t s w 179 vertically separated by thin 1-3 cm layers of wrinkled laminated dolomite. Erosion and truncation of outer margins is common. Nodules appear to originally have had a smooth hemispherical shape with no pronounced constriction of laminae or overall shape at the base. Margin structure: Nodule margins are smooth with a few small bumps. Surface erosion commonly leaves the margins ragged and uneven. Edges of nodules show what may be fragments of a wall structure. Lamina shape: Laminae are often sharply outlined by silicified interlaminae. Laminae are fine, smooth to slightly wavy and gently convex across the central areas of the nodules. The laminae bend steeply downwards at the nodule margins. Some marginal laminae drape beyond the base giving the nodule a distinctly hemispherical shape. Laminae are fairly regularly spaced and average 0.30-0.50 mm in thickness. Laminae are thicker towards the centers of the nodules and thin at the margins of the nodule. Microstructure and texture: Laminae are slightly wavy to wrinkled and 0.21-2.7 mm in thickness with the mode at 0.60-0.90 mm. They are composed of dark brown micrite with patches of equant, subhedral microspar (Fig. 29A). The microstructure is fragmented, clotted ribbon form (Komar Type 3?). The micritic laminae have a clotted, grumose appearance due to microspar replacement. The interlaminae are layers of subhedral chert and quartz with replacement dolospar rhombs scattered mostly along the margins. Interlaminae are 0.12-1.50 mm in thickness. Interspaces: Materials between the nodules are stratiform to wrinkled laminated dolomite and finely disseminated chert. Secondary alteration: The stromatlites are composed of uniform replacement dolomite. The dolomite replacement has been accompanied by a moderate amount of aggrading neomorphism resulting in microspar and small pseudospar patches within the laminae. The grumose texture also may be a replacement relict. Interlaminae are filled with early diagenetic chert and later quartz and dolospar. Comparisons: Ovoid, compact nodular form is suggestive of members of the groups Collenia Walcott, Colleniella Korolyuk, Paniscollenia Korolyuk, Nucleela Komar, and Gonqylina Komar. The studied sample shows no evidence of extensive lateral linkage such as observed in Paniscollenia, Nucleela, and Gonqylina. The nodules are smooth and compact and do not possess bud or tubercles characteristic of Colleniella. The form is grossly similar to a stromatolite described as Collenia ex.gr. ferrata Grout and Broderick in Maslov (1937) and Grout and Broderick (1919). No complete or more recent descriptions of Collenia ex.gr. ferrata could be found. Distribution: The form is restricted to the upper cherty member of the Beck Spring Dolomite section at the Beck Spring locality in the Kingston Range. Only one specimen was studied so distribution is uncertain. Type B Description Mode of occurrence: These types occur as widely ( >30 cm) and closely ( < 5 cm) spaced nodules in cherty and intra- clastic beds. Nearly all of the nodules are silicified, weather red-brown and protrude prominently from the surrounding dolomites (Fig. 29C,D,E). The nodules are scattered throughout the interval but do form some heavy local accumulations in thin, discontinuous beds as much as 15 cm thick. Nodule macromorphology: The nodules are hemispherical, irregularly bulbous or inverted trapezoidal in shape (Fig. 30B,C). Many forms show distinct overhangs and ''mushroom" shapes. All forms show some degree of constriction at the base. Nodule bases are generally flat. There are no interconnecting laminae. The surfaces of the nodules are smooth to bumpy. Nodules average 5- 15 cm in width and 5-10 cm in height. Sections parallel to bedding show ovoid to ellipsoidal shapes. Longitu dinal sections are mushroom, bulbous, or trapezoidal in form. Margin structure: Nodule margins are bumpy to slightly ragged. The laminae from the upper portions of the 182 nodules drape over the sides of the nodule and tuck under the overhangs usually clinging closely to the sides of the form. Occasionally a few laminae will extend down from the overhang and disappear into the surrounding matrix. There are indications of a patchy wall formed by 1 to 3 wavy laminae. Lamina shape: Laminae are wavy to gently convex. Laminae are thickened in the central parts of the nodules. Laminae either bend steeply and tuck in at the nodule margins or are truncated, apparently, by erosion. Lamina thickness is usually 1 mm or less but is variable. However, individual lamina maintain constant thickness along their lengths. Microstructure and texture: The microstructure is a clotted, indistinctly banded type (Komar Type 1 or 3). The laminae are composed of diffuse, dark clotted micrite and microspar (Fig. 29F). The laminae are 0.21-2.7 mm thick (mode 0.60-0.90 mm) and are separated by chert and quartz interlaminae(0.21-1.50 mm) thick. The chert is extensively replaced by dolomite rhombs giving it a patchy, clotted appearance. Cherty interlaminae also contain rare pseudomorphs of halite. Interspaces: Material between the nodules consists of moderate to well sorted intraclastic debris, black chert chips and rounded quartz grains. Wrinkled laminae, subparallel to the nodules also occur. Secondary alteration: Samples have been replaced by dolomite. Interlaminae spaces and fenestrae have been filled with early diagenetic chert. Comparisons: Type B differs from Type A in its more irregular, bumpy and bulbous shape, irregular and bumpy margins, the presence of small buds or nodules within the confines of the larger nodule, the greater thickness and variability of the laminae, and in its simple, grumose banded microstructure. Type B stromatolites are similar in overall form to members of the groups Collenia Walcott, Paniscollenia Korolyuk, and Colleniella Korolyuk. They are similar to Colleniella in the presence of numerous small nodules or buds within the confines of the larger structure but lack extensive lateral linkage to adjacent nodules. The overall morphology of some of the more regular hemispheroids is guite close to that of Paniscollenia. However, the hemispheroids also lack extensive lateral linkage. The microstructure of Colleniella and Panis- collenia is composed of dense pelitomorphic laminae and sparry interlaminae with scattered peloids or clots. This is grossly similar to the microstructure of the observed samples. The Beck Spring Dolomite samples bear striking gross morphologic resemblance to photographs of Collenia symmetrica Walcott in Rezak (1957) but no details of laminae or microstructure were presented in that paper. Distribution: The nodules occur widely in the upper cherty members at the Snow White Mine and Beck Spring localities, both of which occur in the Kingston Range. Group Stratifera Korolyuk (Komar, 19 66) Group diagnosis: Stratifera is a laminar structure which forms alternating rounded hummocks and depressions. Laminae are smooth to slightly wavy. Lamination shows moderate to high inheritance. Microstructure is simple, grumose form. Lamination is continuous throughout the rock. Age: Widely distributed throughout the Riphean. Distribution: Forms have been described from Europe, Asia, Africa, North America, and Australia. Strati fera f. indet. Mode of occurrence: The stromatolites occur as regular, continuous strata extending laterally for tens of meters. Continuous beds of less than 0.8 m to nearly 30 m in thickness occur. Beds are usually disrupted by inter- tonguing or tabular intraclastic and flat pebble con glomerate deposits or sandstones and shales. Transitions from finely laminated Stratifera to columnar-layered and columnar stromatolites are observed at Acme Talc Mine (Fig. 9). Planar to broadly undulose forms are dominant in the lower members at Beck Spring, Snow White Mine and Acme Talc Mine localities. Highly wrinkled and crennu- 185 lated morphologies (Fig. 310,0) occur at Saratoga Spring and in the upper cherty members of Snow White Mine, Beck Spring and Acme Talc Mine sections. A few thin, sandy, dark laminated beds also occur at Saratoga Spring. Macromorphology: The forms are usually broadly undulose forming ridges and domes up to 0.5 m in height and 1-2 m in length (Fig. 31D,E). The broadly arching forms are usually composed of 1-3 mm macrolaminae. The laminae are continuous for meters in either direction with only small diastems and sheet cracks present. Forms with extremely wrinkled and wavy,smooth laminae are also observed (Fig. 31C). These forms are usually confined to thin beds less than 1 m in thickness. The laminae of the wrinkled, wavy forms are often silicified and weather differentially producing distinctive convoluted surfaces. Lamina shape: Laminae in the broad undulose to domal forms are light grey to cream in color, 1-5 mm thick, smooth and continuous. Inheritance is high to moderate with considerable variation in the thickness of the laminae. Single laminae, however, maintain the same thickness throughout their lengths. The wrinkled, wavy forms have fine, wavy laminae with moderate inheritance. Dark laminae are usually a fairly constant 1 mm in thickness. Light laminae vary from <1 mm to >15 mm in thickness (Fig. 31C). FIGURE 31: Photographs of macro- and micro structure of the stratiform stromatolites. (A) Distinct, banded microstructure from a planar laminated form, plane- polarized light. Bar is 200jum long. (B) Distinct, peloidal, banded microstructure from a wavy laminated form, plane-polarized light. Bar is 200jum long. (C) Highly wrinkled and wavy laminae and thin bedding. Scale is 15 cm long. (D) Highly wavy laminated form. Scale is 15 cm long. (E) Cherty bedding surface of wavy laminated forms. Hammer is 3 3 cm long. 187 -py 188 Microstructure and texture: Microstructure of the fine laminae appears peloidal (Fig. 31A,B) (Komar Type 3 or 5a). Thicker laminae show distinct banded or ribbon microstructure which may be composed of peloids (Fig. 31B) (Komar Type 1 or 3). The laminae are 0.06-0.90 mm in thickness and are composed of dark grey to brown micrite with minor microspar patches. Interlaminae are 0.15- 1.2 mm thick and are dominantly equant, subhedral micro spar and spar. Occasional interlaminae have a vermiform microspar-micrite texture and fibrous chalcedonic fillings. Non-laminated portions: There are occasional small pockets of fine-grained unlaminated micrite or very fine intraclastic debris between domal or nodular expressions and between thick laminae. In sandy layers these resemble lenticular silt-filled flasers. Secondary alteration: All forms are replaced by dolomite and show some evidence of aggrading neomorphism. The neomorphic processes have caused extensive destruction of laminar microstructure in some samples. In these highly altered samples the only laminar structures pre served are scattered micritic clots. Abundant fenestrae are filled with dolospar, chert, and quartz. Late stage fractures are filled with iron oxides, quartz, and spar. Comparisons: The distinctive stratiform morphology plus smooth laminations and high to moderate inheritance indicate these forms most likely belong to the Group Stratifera Korolyuk. They differ from Irreqularia Korol yuk in the lack of corrugated laminae and the presence of high inheritance. They lack Gonqylina Komar's lumpy microstructure. Of the forms for which detailed descrip tions were available,S. undata Komar and S. rara Korolyuk are the most similar. S. undata and S. rara lack the extremely tight subvertical nodules and pseudo columns of the other forms and possess peloidal micro structures. Variability in thickness of light-colored laminae was also reported for _S. undata. Convex threads on the upper surface of the laminae, a key character for S. rara,were not observed. Distribution: Widely distributed in both upper and lower members of the Beck Spring Dolomite in all the areas of study. Summary and Discussion Four types of stromatolite morphologies occur in the Beck Spring Dolomite: (1) the irregular, broken Baicalia- like forms, (2) the large, symmetrical Conophyton form, (3) the various bun-shaped, cherty nodules (Collenia?, Colleniella?), and (4) the stratiform types (Stratifera). Three different microstructures were observed including: (1) the striated, ribbon microstructure (Komar Type 1.0) of the Conophyton form, (2) the diffuse, streaky ribbon microstructure of the Ba i ca1 i a -1. i ke form (Komar Type 1 or 3), and (3) the diffuse to distinct peloidal 190 banded types of nodular type A, nodular type B and the Stratifera forms (Komar Type 1 or 3) . The biostratigraphic age ranges of the observed stromatolites of the Beck Spring Dolomite assemblage indicate Middle Riphean age or younger (Fig. 23). If identification of the Collenia or Colleniella forms and the Baicalia-like form can be substantiated, the age could be Late Riphean to very Late Riphean. Collen iella forms have only been reported from Vendian and Cambrian deposits (Krylov and Semikhatov, 1976). The Baicalia-Conophyton assemblage is, however, a very typical Middle Riphean assemblage (Semikhatov, 1976). A Baicalia or Jacutophyton-like form, also of presumed Middle Riphean age, has been reported from the underlying Crystal Spring Formation (Howell, 1971). Roberts (1976) reported Baicalia and Conophyton forms in his study of the lower member of the Crystal Spring Formation. How ever neither of these studies provided detailed descrip tions of macro- or micromorphologies. No comparisons with other forms or groups were attempted. Roberts (1976) contains no photographs, drawings, or reconstructions of the stromatolites. Lacking detailed morphologic descriptions, these stromatolite identifications can only be considered tentative. The environmental-ecolog.ic importance of the assem blages cannot be empirically defined because few Recent 191 analogues of the columnar stromatolites exist (Logan et al., 1974; Donaldson, 1976; Playford et ad., 1976). Based on the few Recent examples (Donaldson, 1976; Horodyski, 1977) and analysis of associated sedimentolog- ical structures and facies in ancient deposits (Rezak, 1957; Awramik, 19 71; Hoffman, 1976) Conophyton has been interpreted as primarily a subtidal form. The irregular, eroded edges of the laminae and cracked columns of the Conophyton columns in the Beck Spring Dolomite suggest moderate energy disruption. Associated planar macro laminated sediments containing evaporitic mineral pseudo- morphs and large scale flat pebble conglomerate partially composed of broken Conophyton tips suggest emergence and moderate to high energy disruption. However, this may not be the environment of stromatolite growth although it is the environment of final deposition. The Cono phyton columns themselves show evidence of a low-energy growth environment because they are smooth, show no curls or erosion of growth surfaces, have limited fenestral porosity,and have extremely regular, uniform laminae. The group Collenia Walcott was originally a rather broad group including nearly all columnar, columnar- layered, and nodular forms. The literature is difficult to interpret/as authors have not consistently conformed to the reassignment of many members of this group and 192 detailed descriptions are rare. The samples from Snow White Mine bear a striking resemblance to the Collenia symmetrica Walcott samples from the Belt Supergroup strata of Montana (Rezak, 1957). Rezak (1957) and Horodyski (1976) have interpreted the environment of deposition of these forms as tidal flat to shallow subtidal due to the presence of extensive algally- derived, winnowed intraclastic material, shrinkage cracks, and wavy bedded dolarenite surrounding the nodules. The depositional setting for the nodules at Snow White Mine is essentially the same. Baicalia forms have generally been reported from environments presumed to have low to moderate energy, emergent to subtidal conditions (Preiss, 1972; Donaldson, 1976; Hoffman, 1976). The broken, eroded columns of the Baicalia-like forms in the Beck Spring Dolomite suggest exposure to variable moderate to high energy current regimes. Refolded laminae tips and associated intraclastic materials may also suggest occasional emergent conditions. The Stratifera forms are widely dispersed in time and microenvironments. The wrinkled and wavy to broadly undulose stromatolitic forms in the Beck Spring Dolomite are similar in gross morphology to algal mats of Recent tidal flat environments (Davies, 1970a; Kinsman and Park, 1976; Hardie, 1977). 193 | No oncolites were observed in this study. However, | Licari (1971) reported obtaining microfossils from | "oncolites and pisolites." Licari (1971) used the ! two terms interchangably and the photographs and descriptions in that report do not clearly define the | "oncolitic" structures. It would be quite reasonable | to assume, however, that oncolites do occur in the ! deposits of the Beck Spring Dolomite. Further field | investigations will be needed to determine the presence and distribution of oncolites. 194 CONCLUSIONS The Beck Spring Dolomite is a tabular body of predominantly algal laminated dolomite of surprisingly constant thickness and lithology. The formation consists of three informal members (Gustadt, 1968) which are recognizable and. mappable throughout the studied region. All three informal members occur at the Beck Spring, Acme Talc Mine and Snow White Mine localities. The upper cherty member does not occur at Sartoga Spring and the lower laminated and upper cherty member do not occur at Silurian Hills. A fourth informal member, the lower cherty member, is defined and described in this study. The lower cherty member occurs in all the study localities and is best developed at the Acme Talc Mine section in the Alexander Hills. The formation, maintains a thickness of 335-480 m for nearly 55 km. The thickest exposures (480 m) occur at the apparent eastern margin of the Amargosa Basin. The strata thin progressively to the west and interfinger with clastic rocks. The most westerly exposures of the Beck Spring Dolomite occur in the Telescope Peak area of the Panamint Range where the Beck Spring Dolomite thins to 30 m then disappears near a proposed paleogeographic high (Labotka and Albee, 1977). Throughout the region of the present study, the Beck Spring Dolomite maintains its characteristic shallow marine platform facies. 195 There is no evidence to suggest rapid or substantial subsidence during deposition of the Beck Spring Dolomite. Deposition appears to have taken place on a stable plat form in shallow subtidal to slightly emergent environ ments. No definite support for vertical tectonic movement associated with aulacogen formation can be discerned from studies of the Beck Spring Dolomite. However, large scale tectonic readjustments could have occurred prior to Crystal Spring and/or after Beck Spring deposition. The geometry of the Amargosa Basin, appears to be more complex than, described by Wright et. aJL. (1976) . This conclusion is supported by the examination of the strata extending along a west trending line near the presumed northern margin of the basin. Nearly pure carbonate strata of relatively constant thickness occurs east of the Ibex Hills region. Beck Spring Dolomite strata become increasingly clastic-rich as one goes west from the Ibex Hills to the Panamint Range (Labotka and Albee, .1977) . The change in lithologies and stratal thickness along this west trending line suggests a more complex depositional environment than the uniform car bonate platform environment proposed by Wright et al. (1976) . Definition of the source of the clastic rocks of the Beck Spring Dolomite in the western regions of the Amargosa Basin is essential for definition of both the 196 geometry and tectonic framework of the basin. Wright et al. (19 76) propose uplift and erosion of the Mojave Upland as the source of clastic sediment in the carbonate platform rocks occupying the northern regions of the basin during Beck Spring time. However, the fine-grained sandstones and shales of the Beck Spring Dolomite also might have been eroded from local paleotopographic highs within the platform similar to the World Beater Island complex (Labotka and Albee, 1977). Alternatively, one could also interpret the shale, silt, and sandstone as deeper water basinal facies rather than nearshore facies. The clastic deposits would then form the central portions of a basin surrounded by marginal carbonate platforms. The bulk of the clastic material could bypass the carbonate platforms via submarine channels leaving relatively pure carbonate margins with minor tidal channel or fluvial clastic input. It is apparent that large scale tectonic uplift of the basin margins is not neces sarily the only process which could supply elastics to strata assigned to the Beck Spring Dolomite. The contact between the Beck Spring Dolomite and the underlying Crystal Spring Formation appears to range from gradational to perhaps disconformable in the study area. The lack of presumed 1.2 b.y. old diabase int.ru- sives in the Beck Spring strata and the possible dis- conformable nature of the Beck-Spring Crystal Spring 197 contact suggests an age of 1.2 b.y. or less for the Beck Spring Dolomite. The contact between the Beck Spring Dolomite and the overlying Kingston Peak Formation is probably discon- formable in the areas in which it was observed in this study (Acme Talc Mine and Saratoga Spring). However, the contact is reported to vary from essentially conformable to angularly unconformable in other regions (Wright, 1968; Lafootka and Albee, 1977). The Beck Spring Dolomite is composed of well-ordered replacement dolomite with significant amounts of early diagenetic chert in the lower and upper members. The rocks of all but the oolitic-pisolitic member are pri marily matrix-supported, laminated, peloidal, and intra clastic dolomicrites with abundant laminoid fenestral fabrics. They show great similarities to the tidal flat deposits of Holocene environments. Samples from the oolitic-pisolitic member are both grain- and matrix- supported. The samples may, therefore, represent depo sition both on shoals and/or in nearby shallow subtidal tide-dominated water. The presence of pseudomorphs of anhydrite, halite, and gypsum indicates high salinity pore solutions most likely were present during very early diagenesis. Evaporitic mineral relicts occur in only trace amounts. The "hypersaline" facies may have been restricted to isolated supratidal ponds and marshes and apparently was not widespread. The dominant biosedimentary structures of the Beck Spring Dolomite are various morphologies of cryptalgal- aminites. Algally produced or bound carbonate may account for more than 80% of the strata assigned to the Beck Spring Dolomite. Indeed, the formation might be viewed as a giant, tabular algal "megamat" covering tens of kilometers of shallow marine platform. Wavy, wrinkled, and smoothly undulose cryptalgal- aminated as well as columnar stromatolites occur. Small and large scale intraformational conglomerate, peloidal- intraclastic beds, chert nodules and interbeds are common. Sheet cracks and curled and warped laminae and thin beds are frequent in portions of the lower laminated and upper cherty members. Occasional thin granule sandstone or shale lenses occur in the upper and lower cherty members. The minor clastic beds may represent migration of associ ated nearshore bars, channels, or ponds. The succession of microenvironments in the dominantly carbonate Beck Spring deposits suggests two slow cycles of emergence and submergence. The mixed carbonate and clastic Beck Spring strata at Saratoga Spring may reflect increased regional tectonic activity at the western extremity of the Amargosa Basin (Labotka and Albee, 1977). Petrochemical analyses also suggest subtidal, restricted lagoonal, and/or tidal flat depositional envi- 199 ronments for the lower and upper laminated members and open marine to lagoonal environments for the oolitic- pisolitic member. This correlates well with observed primary structures, stromatolites, and lithologies. The stromatolite assemblage recognized in the Beck Spring Dolomite consists of Conophyton c „ f. garganicum, a Baicalia-like columnar form, two types of nodular stro matolites, and several morphologies of Stratifera. This assemblage is interpreted as a deposit of shallow subtidal to supratidal environments. The stromatolites present indicate a probable late Middle Riphean age for the Beck Spring Dolomite. Confirmation of the late Middle Riphean age depends, however, on substantiation and confirmation of the stromatolite identifications. A late Middle Riphean age (approximately 1100-900 m.y.) is 200-400 m.y. younger than the age previously assigned to the Beck Spring and Crystal Spring Formations (Wright et al., 1976; Licari, 1978). The younger age may be highly significant in the evaluation of the age of eukary otic microfossils preserved in the Beck Spring Dolomite (Licari, 1978). The proposed late Middle Riphean age would place approximately 99% of the confirmed eukaryotic microfossils at a time 200-400 m.y. younger than is generally accepted. 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Depositional systems of the mid-Tertiary Gene Canyon and Copper Basin Formations, eastern Whipple Mountains, California

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Carbonate microfacies of the Upper Monte Cristo Limestone and the Lower Bird Spring Group at Mountain Springs, Clark County, Nevada

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Physicochemical characterization of sediment facies and paleoclimatic inferences, California Continental Borderland

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Stratigraphy, sedimentology and vertebrate ichnology of the Copper Canyon Formation (Neogene), Death Valley National Monument

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Status of the elementary school principal in California

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Sedimentary structures in vibra-cores from the Oxnard Shelf, California

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Stratigraphy and sedimentary petrology of the Moss Back Member of the Late Triassic Chinle Formation, north Temple Wash - San Rafael Desert area, Emery County, Utah

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The Drummond Mine Limestone: Mississippian basin plain carbonate turbidites and hemipelagites in the Pioneer Mountains, Blaine County, south-central Idaho.

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Stratigraphy and sedimentation patterns Upper Miocene Puente formation (Mohnian-Delmontian) northwestern Puente Hills, Los Angeles County, California

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Structural geology of the eastern Whipple Mountains, San Bernandino County, California

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Holocene sedimentological parameters of the outer California Continental Borderland

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Paleoenvironmental analysis of the Upper Cretaceous (Santonian/Campanian) Forbes Formation, Sacramento Valley, California

Asset Metadata

Creator Marian, Melinda Lee (author)

Core Title Sedimentology of the Beck Spring Dolomite, eastern Mojave Desert, California

Degree Master of Science

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,Sedimentary Geology

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c30-87368

Unique identifier UC11225502

Identifier usctheses-c30-87368 (legacy record id)

Legacy Identifier EP58659.pdf

Dmrecord 87368

Document Type Thesis

Rights Marian, Melinda Lee

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA