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Petrography and statistical analysis of the Tapeats Sandstone (late Precambrian-Cambrian), southeastern California

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Petrography and statistical analysis of the Tapeats Sandstone (late Precambrian-Cambrian), southeastern California

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Content PETROGRAPHY AND STATISTICAL ANALYSIS OP U THE TAPEATS SANDSTONE (LATE PRECAMBRIAN- CAMBRIAN), SOUTHEASTERN CALIPORNIA by Cyril Francis Loho (H 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) August 1972 UMI Number: EP58594 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 EP58594 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 ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90 0 0 7 '73 L7c ?7 T h is thesis, w ritte n by .......Craili.Z^NCIS.LOBO.............. un d e r the d ire ction o f h.X&....Thesis C o m m ittee , and a pp ro ve d by a ll its members, has been p re sented to and accepted by the D e a n o f T he G ra dua te School, in p a r tia l fu lfillm e n t o f the requirem ents f o r the degree o f Ma ster of S_c ie nce_ (Gep l_g gi c a 1_ S c.ie n ces) 7 ^ Dean D a te Sep tember 1972 THESIS COMMITTEE Chairman CONTENTS Page ILLUSTRATIONS .................................. iv TABLES........................................... Vi ABSTRACT........................................ vii IMTRODUCTIQH .................................... 1 Location................... • • • 1 Previous w o r k .............................. 1 Purpose............................. 10 Acknowledgments.............. 11 PROCEDURES...................................... 12 Field and sampling procedure.............. 12 Laboratory procedure ....................... 13 Petrographic procedure ..................... 14 Statistical procedures ..................... 18 RESULTS......................... 20 Megascopic characteristics ............. « • 20 Microscopic characteristics .......... 21 Mineralogy ................ 21 Texture..................... 2? Statistical analysis ....................... 45 Dendrograph-cluster analysis ........... 45 Time-trend analysis ................... 53 ii Page Cross-association ..................... 68 Frequency distribution and analysis of variance.............................. 69 Radiographic analysis ..................... 77 DISCUSSIOR OF RESULTS .......................... 92 Classification .............................. 92 Provenance.................................. 93 Depositional environment ................... 96 CONCLUSIONS .................................. 103 REFERENCES...................................... 106 iii II&USTRATIONS Figure Page 1. Index map showing location of sections . . 2 2. Photomicrograph of basal sample in the Marble Mountain section • 29 3. Photomicrograph of a typical subarhose from the Marble Mountain section ..... 31 4. Photomicrograph of a typical ortho- quartzite from the Marble Mountain section 33 5. Photomicrograph of quartz arenite showing quartz with good overgrowths, phosphorite and glauconite ..................... 35 6. Dendrograph depicting mutual relationships between and within twelve variables measured in the Marble Mountain section . 46 7. Time-trend curves for five variables measured in the Marble Mountain section . 55 8. Time-trend curves for six variables measured in the Marble Mountain section . 57 9. Time-trend curves for six prominent vari ables in the Mountain Pass section .... 64 10. Time-trend curves for six prominent vari ables in the Mesquite Pass section .... 66 11. Histograms for "Total Quartz" and "Total Quartz plus Quartzose Rock Fragments" and chi square goodness of fit for binomial frequency distribution . ............... 70 12. X-radiography contact print of sample collected 3 m above the basal Precambrian gneiss in the Mountain Pass section . . . 79 13. X-radiography contact print showing parallel lamination in subarkose from the Marble Mountain section ......... 81 iv I Figure Page 14. X-radiography contact print showing small- scale planar cross-stratification with tangential foresets ..................... 83 15* X-radiography contact print showing bio- turbation in a cross-stratified quartz arenite . . . . ......................... 85 16. Paleocurrent analysis of cross-bed data, with maximum foreset dip azimuths determin ed from samples collected in the Mesquite Pass, Mountain Pass, and Marble Mountain sections................. 87 v Table Page 1. Location and description of sections . . . 4 2. Mean values, standard deviation and range in composition determined from modal analysis of 86 samples in the Marble Mountain section..................... 22 3. Mean values, standard deviation and range in composition determined from modal analysis of 25 samples in the Mountain Pass section ...... ................. 25 4. Mean values, standard deviation and range in composition determined from modal analysis of 29 samples in the Mesquite Pass section ....................... 27 5* Textural analysis of quartz from samples . selected randomly across the Marble Mountain section ..... ............... 38 6. Textural analysis of quartz from samples selected randomly across the Mountain Pass section................................ . 40 7. Textural analysis of quartz from samples selected randomly across the Mesquite Pass section................. 41 8. One-way analysis of variance and Duncan’s New Multiple Range test for “Total Quartz” 74 9. One-way analysis of variance and Duncan’s New Multiple Range test for ’ ’Total Quartz plus Quartzose Rock Fragments” ........... 75 10. One-way analysis of variance and Duncan’s New Multiple Range test for ’ ’Total Quartz” using five sample groups................. 76 vi ABSTRACT Oriented samples from three stratigraphic sections randomly selected from five sections of Tapeats Sandstone were subjected to detailed petrographic and radiographic analyses. Two of the sections are exposed in the Clark Mountains, one near Mountain Pass, one near Mesquite Pass and the other section is exposed in the Marble Mountains. Twelve variables were measured in thin section for each of 140 samples of dominantly feldspathic quartzites. Although polynomial time-trend analysis shows con siderable variation in mineralogy and texture with thick ness in each stratigraphic section, no parastratigraphic units could be recognized within the three sections. R-mode dendrographs provided the following clusters for the twelve variables measured in the Marble Mountain section, which was selected because of relatively large sample size and a lack of post-depositional deformation. Cluster I, microcline-plagioelase-orthoclase; Cluster II, chert and accessory minerals; Cluster III, undulatory quartz and quartzose rock fragments; Cluster IV, mica and matrix; Cluster V, hematitic and limonitic cements; and Cluster VI, normal quartz and silica cement. The clusters are apparently related to stability of detrital mineral components and partly to post-depositional diagenesis. The sample size for the variables “total quartz*' and "total quartz plus quartzose rock fragments" was suf ficiently large to have an 80 percent probability of detecting a 10 percent difference between two means at the 5 percent level of significance. A one-way analysis of variance showed statistical homogeneity between the Marble Mountain and Mesquite Pass sections with respect to these two variables. The major source of variation, determined by the Mew Duncan. Multiple Range test, was ascribed to the samples from the Mountain Pass section. A combination of petrographic, radiographic and field data suggests a shallow marine or beach environment for these sediments with the source rocks being chiefly granitic rocks and their metamorphic equivalents, as well as older sedimentary rocks. vii INTRODUCTION Location Excellent outcrops of sedimentary quartzites of late Precambrian-Cambrian age occur in a number of strati graphic sections in southeastern California, where they rest nonconformably over Precambrian granite and granite- gneiss. These quartzites form part of the thinner sequences of platform or cratonal facies associated with the Cordilleran geosyncllne. The age of these quartzites is debatable and the nomenclature used varies with dif ferent authors. The sections in the present study were measured in the Ivanpah, Clark and Marble Mountains (Pig. 1) in San Bernardino County, California and the units are assigned to Tapeats Sandstone. The location of each section is described in Table 1. Previous Work Previous petrographic work on these quartzites has been meager. Little systematic data has been recorded on the mineral composition of these rocks and little detailed petrographic analysis attempted. As a result, problems relating to provenance and depositional history of these sedimentary quartzites have yet to be resolved. 1 Figure 1. Index Map showing location of sections. Dark circles represent location of sections used in the present study. 2 3 3* 3 5°3 O1 - 4 0 L O C A T IO N OF S E C T I O N S Mesquite Pass S e c tio n T a y l o r Mine S e c t io n Mountain Pass S e c t i o n 35 - Oro Wash S e c t i o n M ar b le Mountain S e c t io n SCALE 16 km 115° 1 5 ' II5°45' TABLE 1 LOCATION AND DESCRIPTION OP SECTIONS 0 Section Location Thickness of Tapeats Sand stone measured from basal con tact with Pre cambrian eneiss Overlying unit 1. Mesquite Pass 2.5 km east of Mesquite Pass; Sec. 29, T. IJi N., R. 13 E. 43 m (29 samples) Bright Angel Shale (Cambrian) 2. Taylor Mine 1 km north of Taylor mine; Sec. 9, T. 17 N., R. 13 E. 43 m (29 samples) Bright Angel Shale (Cambrian) 3. Mountain Pass 6 km northwest of Mountain Pass; Sec. 35, T. 17 N., R. 13 E. 37 m (25 samples) Parautochthonous slice of Pre cambrian gneiss TABLE 1 (Continued) LOCATION AND DESCRIPTION OE SECTIONS Section Location Thickness of Tapeats Sand stone measured from basal con tact with Pre cambrian gneiss Overlying unit 4. Oro Wash 0.6 km east of Allured Mine; 93 m Bright Ansel Shale (Cambrian; Sec. 22, T. 15 I., E. 14 E. (62 samples) 5. Marble Mountain 3 km northeast of Cadiz; • Sec. 12, T. 5 E., R. 14 E. 132 m (86 samples) Latham Shale (Cambrian) <J1 6 Prospect Mountain Quartzite was the name given "by Hague (1883) to basal sedimentary rocks of Cambrian age exposed in Prospect Ridge near Eureka, Nevada. The origin of the name Tapeats Sandstone dates back to Noble (1914) for exposures of sandstone along Tapeats Creek in Arizona. Schenk and Wheeler (1942) reported that Prospect Mountain Quartzite and Tapeats Sandstone are lithogenetically the same, but employ the former name because of priority of definition. Wheeler (1944) claimed that the Prospect Mountain Formation of late Precambrian and early Cambrian age was found throughout the entire Great Basin area and locally designated as Tapeats Sandstone, Tintic and Brig ham Quartzites. McNair (1951) noted that the Tapeats Sandstone extends further than other Cambrian formations and the Lower and Middle Cambrian rocks of the Great Basin and Colorado Plateau are lithofacies becoming younger east ward. Hewett (1956) recognized two facies on the basis of thickness and lithology in the Cambrian sedimentary rocks older than the Cambro-Devonian Goodsprings Dolomite. The eastern facies is represented by the Tapeats Sandstone and overlying Bright Angel Shale, whereas the western facies is represented by Noonday Dolomite, Prospect Mountain Quartzite and Pioche Shale in ascending order. The boundary between the two facies in the Ivanpah Quadrangle is the Mesquite thrust. Hewett also suggested that the Prospect Mountain Quartzite is equivalent to three units 7 (Wood Canyon Formation, Stirling Quartzite and Johnnie Formation) recognized by Nolan (1929) near Johnnie, and by Hazzard (1937) in the Nopah Range. Nolan, Merriam and Williams (1956) observed that the base of the Prospect Mountain Quartzite was not exposed in the stratigraphic section measured at Prospect Ridge (type area). Burchfiel (1964) assigned a Precambrian age to the Johnnie Formation and Stirling Quartzite because of a lack of fossils and a Precambrian-Cambrian age to the Wood Canyon Formation. Lithologic studies on the rocks under present study have been carried out by Hazzard (1933) and Stewart (1970). Hazzard (1933) described Cambrian rocks of the eastern Mojave Desert based on work in the Marble, Ship and Providence Mountains. The basal quartzite in the Marble Mountains was assigned to the Lower Cambrian. The quartz ite grades into greenish-gray fossiliferous shale of Cambrian age. From a study of a few thin sections from various horizons of the quartzites Hazzard found these rocks to display a constant mineral composition, poor sort ing and a similar degree of rounding. No formational names were suggested for the different rock units. Stewart (1970) suggests that the Upper Precambrian and Lower Cambrian rocks of the southern Great Basin are initial deposits of the Cordilleran geosyncline, which formed a wedge of sediment ranging in thickness from 400 feet in his eastern region to 21,000 feet in his western 8 region. The Tapeats Sandstone, as employed by Stewart (1970), refers to a thin shelf deposit in the eastern region equivalent to the thicker sequences of strata to the west. Stewart (1970, p. 13) suggests a Late Pre- cambrian-Early Cambrian age for the Tapeats Sandstone in its westernmost exposures. He recognizes three units of the Tapeats Sandstone in the Marble Mountains in a section measured two and a half miles northeast of Chambless. The two lower units (97.5 and 27 m thick, respectively) are correlated with the Middle and Upper Wood Canyon Formation and the uppermost unit with the Zabriskie Quartzite. In the Clark Mountains, the Tapeats Sandstone is composed of gray to grayish purple, medium to coarse-grained quartz ites (52 m) overlain by yellowish gray, fine- to medium- grained quartzite (8 m). Lithologically, the lower unit is correlated with the middle Wood Canyon and the upper unit with the Zabriskie Quartzite. In the Marble Mountains, strata correlative with Bright Angel Shale is named Latham Shale and Chambless Limestone of early Cam brian age. The base of the Cambrian is placed by Stewart (1970) in the upper Wood Canyon Formation on the basis of first appearance of olenellid trilobites and archeocya- thids. More recently, Stewart (1972) proposed initiation of the Cordilleran geosyncline by rifting at less than 850 m.y. on the basis of diamictite and diabase in the Kingston Peak Formation in the southern Great Basin and 9 other equivalent formations. He suggests a change in the tectonic framework of Worth America after deposition of the Belt Supergroup (1250 to 850 m.y.). However, the deposi- tional environment of the Belt sediments and equivalents including the Crystal Spring Formation and Beckspring Dolomite must be better established and more radiometric data is needed before Stewartfs conclusion can be justified The timing seems adequate to account for thick clastic sedimentation in the upper Precambrian and lower Cambrian on the basis of plate tectonic concepts for the origin of miogeosynclines or miogeoclines. Detailed mapping and radiometric dating in the Clark Mountain thrust complex (Burchfiel and Davis, 1971* 1972) has added greatly to an understanding of problems in the sedimentary reconstructions in the southern Great Basin. Three major thrust plates are recognized in the Cordilleran frontal thrust belt. Prom east to west they are the Keystone, Mesquite Pass (Hewettfs Mesquite thrust) and Winter*s Pass plates. The eastward directed Mesozoic thrusting is progressively younger eastwards. The Mes quite Pass, Taylor Mine, Mountain Pass and Orowash strati graphic sections (Pig. 1) were sampled east of the Key stone basal thrust and are therefore cratonal and auto chthonous. However, Precambrian gneiss*Tapeats Sandstone and Bright Angel Shale are locally involved in minor parautochthonous slices (Burchfiel and Davis, 1971) below 10 the Keystone thrust. Recognition of the southward continu ation of the thrust belt is hindered by emplacement of the Teutonia Quartz Monzonite, but recent evidence (Davis, 1972, personal communication) suggests that south of Oro- wash the thrust belt has a southeast trend compared to a northeast trend north of the Clark Mountain thrust com plex. The southward continuation also shows extensive involvement of basement in contrast to dominantly decol- lement thrusting north of the Clark Mountain area. The Marble Mountain section is cratonal but allochthonous, with the thrust belt situated to the east and the geo synclinal hinge zone to the northwest of it. Purpose The main objective of this pilot study was the ap plication of petrographic, radiographic and statistical techniques in interpreting the depositional history of cratonal quartzites assigned to the Tapeats Sandstone. Included was the assessment of several statistical pro cedures that may be applicable to a broader study which includes the clastic arenites from correlative miogeo- clinal units. 11 ACKNOWLEDGMENTS The author is indebted to Dr. Robert H. Osborne for suggesting the topic and for advice on various phases of this research, and to Drs. Gregory A. Davis, Donn S. Gors- line and Donald P. Palmer for critically reading the manuscript and providing helpful suggestions in the study. The research was supported by grant #PRF 3942-12 of the Petroleum Research Fund, administered by the American Chemical Society. Gratitude is also expressed to my col leagues James Gibson and Stephen Pavlak for their assist ance in the field work. PROCEDURES Field and Sampling Procedure Seven stratigraphic sections were investigated in southeastern California and southern Nevada during the summer of 1969. Reconnaissance surveys were conducted to ascertain the nature of the rocks under study- No samples were collected from sections where the quartzites showed considerable metamorphism and recrystallization, where the rocks were excessively friable or where the section con sisted dominantly of float or scree which prevented the collection of oriented samples- A 5-foot Jacob's staff and Abney level were used to measure and sample the sections. Oriented samples were collected from strati graphic sections exposed at each locality in Figure 1. The sampling method was a random spot sampling technique considering the quartzites as massive homogeneous units. A predetermined sampling interval of 5 feet (1-52 m) was employed. Sampling was initiated at the contact with the Precambrian gneiss and was continuous except for a few locations where collection was hampered by friable samples or float in a covered interval. The distance from the base of the sections was recorded for each sample. 12 A randomization procedure was introduced to select three sections from the five stratigraphic sections measur ed and collected.in the field. The purpose of the random ized design was to initiate a detailed petrographic analy sis and assess the procedures before undertaking a wider study which would include other cratonal and miogeosyn- clinal sections. Samples collected from the Marble Mount ain section, Mountain Pass Section and Mesquite Pass Section comprise the basis for this study. Laboratory Procedure Slabs (approximately 3 mm thick) were sawed from each sample parallel to the strike and dip directions and perpendicular to the stratification, and then radiographed using a Penetrex 50 KV Industrial unit. The following factors, among others, affect the quality of the resultant radiographs: the excitation potential (40 KV), the cur rent (8 milliamps), film (Kodak industrial type M), the distance from the source set at 80 cms and the exposure time (100 seconds) which varied almost linearly with thickness. Plates were then processed using three and a half minutes for developing and fixing. The contrasts observed in the radiographs are due to differential absorption, the lighter areas are more opaque to X-rays and the darker portions are more easily penetrated by the X-rays. Thus 14 more obscure internal structures and textures may be re vealed by density differences of the material comprising the rock. The rock samples and slabs were usually studied alongside, and utilized for confirming certain character istics noted in the radiographs. Analysis of each radio graph included the general description of texture, measure ment of the apparent maximum grain diameter which may indicate the mechanical energy in the environment, the ap parent long axis (a) and the axis (b) at right angles to it, the ratio of which related to the sphericity of the clasts. Sedimentary structures were also sought and ap propriate data recorded to infer paleocurrent directions and depositional history. Petrographic Procedure One oriented thin section was prepared for each sample normal to stratification but chosen either parallel to the field strike or dip direction. A point counting procedure was employed to estimate the volume of the constituents comprising the arenites. Prom an initial count it was determined that 300 counts per thin section were sufficient to infer that the cor rect volume of each variable measured lies within about 5 percent on either side of the obtained value with a 95 per cent confidence (Van der Plas and Tobi, 1965)• A 1 mm point distance was used so that the interval was nearly always greater than the largest particle diameter in the thin section. Furthermore, to reduce the hias in the point-counting procedure, the starting point in the grid was determined by using a random numbers table (Dixon and Massey, 1969). Each primary variable counted was uniquely defined to form a basis for interpretations in the analysis of sedimentation patterns, provenance and depositional environment. The variables counted were subdivided on the basis of operationally sound criteria governed by internal and external discontinuity (Griffiths, 1967)- These variables formed mutually exclusive classes which are generally easy to identify. They are as follows: normal (non-undulatory) quartz, undulatory quartz, orthoclase, microcline, plagioclase, mica, accessory minerals, chert, rock fragments, silica cement, other cement (dominantly iron oxides) and matrix. Derived variables include total quartz, total feldspar, quartz/feIdspar ratio, grain/ matrix ratio. Elements internally and externally continuous in clude quartz, feldspar, mica and accessory minerals. Quartz was subdivided into normal and undulatory quartz on the basis of strain extinction on a rotation of a few degrees in the case of the latter. Quartz occurs as clear, colorless grains easily identified by its low relief 16 (little higher than Canada balsam), low birefringence and positive uniaxial figure. Feldspars were counted separately as orthoclase, microcline and plagioclase with the distinction being made on the basis of twinning and extinction. Untwinned plagio clase could have been counted as orthoclase to constitute a minor error. Generally, microcline and plagioclase are easily recognized by their twinning, while orthoclase may pose a more difficult problem. It is distinguished from quartz by its cleavage, lower relief than Canada balsam (Becke test), lower birefringence, negative biaxial figure and its turbidity and clouding as a consequence of altera tion. Rock fragments and chert comprise elements which are externally continuous but internally heterogeneous. Quartzose rock fragments were predominant, being composed of interlocking or silica cemented quartz grains having a distinct outline of their own. This class includes poly crystalline composite quartz, orthoquartzite, and meta- quartzite. Granitic rock fragments with aggregates of quartz and feldspar were noted separately. Other rock fragments included argillite and volcanics. Chert is re cognized by its microgranular appearance. Its varieties included jasper, agate and chalcedony. Other variables include cement and matrix. Silica cement refers to overgrowths of quartz generally in opti- 17 cal continuity with, the contiguous quartz grains. Often there is a distinct boundary between the quartz grain and the cement, the margins being outlined by clay, iron oxides or matrix **dust rims.” Other cement was almost exclusively iron oxides (hematite and limonite), and very rarely cal- cite. Matrix consisted of a very fine-grained admixture composed chiefly of sericite-illite and silt-sized quartz. X-ray diffraction analysis of feldspar clasts and con glomerate pebbles was employed to supplement petrographic studies. Petrographic analysis also included a textural investigation in which grain size, roundness and sphericity were estimated to better understand the depositional fabric. Samples were randomly selected from each section such that consecutive samples were at least 15 feet (4.5 m) apart. In traverses with a 1 mm interval, the apparent maximum grain diameter (a) and another at right angles to it (b) were measured for quartz, feldspar and in some thin sections also quartzose rock fragments. The apparent maximum grain diameter was used in the size analysis and the b/a axial ratio was also computed. Sphericity and roundness were also measured using the visual chart pre pared by Krumbein (1941). The measurements for quartz and feldspar were separately tabulated for intersample and intra-sample comparisons. Grain size measurement in thin section is a function of size, shape, orientation and pack 18 ing (Griffiths, 1967). Statistical Procedures The main geological objectives of the study are related to a better understanding of the sedimentary history of these quartzites and the possibility of cor relation across the craton. Statistical applications were greatly aided by facilities available at the University Computing Center (IBM model 360/65 later replaced by IBM model 370/155). Simple statistics of each variable measured in the modal analysis was carried out using systems library programs (BMD01D, Dixon, 1970). Correlation matrices were computed for each stratigraphic section and the lower half of the matrix was used as computer input to plot dendro- graphs (McCammon and Wenninger, 1970), which provided mutual relationships between and within the twelve measured variables. Variation in composition and texture with thickness in each section was investigated using poly nomial time-trend analysis (Pox, 1964)* Both raw data and weighted data (percent of maximum) were plotted for each variable in each section at a fixed smoothing interval of 5 feet (1.52 m). Time-trend plots were used for correla tive purposes as well as understanding variation in sedi mentation patterns and provenance with time. The strati graphic sequences were matched using cross-association co- efficients (Sackin, Sneath and Merriam, 1965). The fre quency distributions of mineral components were compared with theoretical models of constant probability applicable to point-counting methods (Ondrick and Griffiths, 1969)* The sample size needed to detect a given difference be tween means was determined using procedures suggested by Sokal and Rohlf (1969). A one-way analysis of variance (BMD07V, Dixon, 1970) was designed to determine variation between sections for each variable with sufficient sample size. The New Duncan/Multiple Range test (Dixon, 1970) was applied to pinpoint the source of variation. RESULTS Megascopic Characteristics In the Marble Mountain section, the quartzites in the basal 75 are dominantly pale red (5R 6/2) and minor medium gray (H5)« The next 30 m (75 to 105 m. from the base of the section) comprises light gray (H6) and medium dark gray (N5) quartzites and the uppermost 27 m in the measured area consist of fine- to medium-grained pinkish gray (5YR 8/l) to light red (5B. 6/6) quartzites. The color designations refer to fresh cut samples. The rocks are compact and largely fresh with little alteration along joints. Porosity is more evident in coarser varieties. The composition is predominantly quartz ranging in size from fine-grained sand to pebbles having a diameter up to 27 mm. Pebbles occurring in the local conglomerate beds about 8 m above the base of the section are well rounded, and composed of quartz, chert, agate and jasper. Feldspar clasts are prominent in the basal part of the section showing pink and white rectangular to subrounded cross-sections. They tend to be less abundant higher in the section and are almost absent in the upper 27 m. Other clasts recognizable in the hand specimens include quartzbse rock fragments and argillite; some of the latter are weathered away leaving large cavities. The chemical 20 21 cement is dominantly silica, causing the rock to sometimes break with a conchoidal fracture. Ferruginous cement is more prominent in some samples and.in one it obscures the fine-grained quartz. Rocks in the Mountain Pass section are predominantly light brownish gray (5YR 6/1). Grain size varies from medium- to very coarse-grained with some conglomeratic horizons. The pebbles are composed of quartz, feldspar, quartzose rock fragments, chert, banded agate, and jasper. Feldspars are dominant in the basal 26 m. Large clasps of pink orthoclase are particularly characteristic of the basal samples. The rocks are generally fresh, some samples show ing considerable weathering and leaching, especially of feldspar. Quartzites in the Mesquite Pass section range in color from light brownish gray (5YR 6/1) to moderate red (5R 5/4). Grain size is generally medium- to coarse grained sand with few conglomeratic units. Pebbles in these include red jasper, quartz, chert, and agate. Microscopic Characteristics Mineralogy Petrographic modal analysis shows a variation in mineralogy with thickness. The mean percentage composi tion, standard deviation and range for each measured variable is summarized in Tables 2, 3» and 4 for each stratigraphlc section. 22 TABLE 2 MEAN VALUES, STANDARD DEVIATION AND RANGE IN COMPOSITION DETERMINED PROM MODAL ANALYSIS OP 86 SAMPLES IN THE MARBLE MOUNTAIN SECTION Standard Mean Deviation Maximum Minimum ____________________2_________ t Normal Quartz 65.15 11.33 89.33 27.15 Undulatory Quartz 9.97 4.50 28.43 1.66 Total Quartz* 75.12 9.42 92.66 44.66 Orthoclase 3.35 2.39 7.60 o • o Microcline 1.27 1.10 3.96 0.0 Plagioclase 1.06 0.92 3.33 o • o Total Peldspar* 5.68 4.01 14,00 0.0 Mica 0.48 0.69 3.66 o • o Accessories 0.58 0.73 5.62 o • o Chert 0.84 0.75 2.66 0.0 Q.R.P. 2.79 3.93 23.64 0.0 Silica Cement 4.23 2.44 11.66 0.0 Other Cement 4.47 6.48 51.66 0.0 Matrix 5.33 4.35 24.91 0.33 Others** 0.68 2.17 20.0 0.0 * Derived variables. ** Includes voids, glauconite, cellophane, unstable rock fragments. 23 In the Marble Mountain section, quartz was pre dominantly unstrained with a much smaller proportion show ing undulose extinction. This is more conspicuous in the upper 2? m where the arenites are more mature. Quartz grains often have inclusions of zircon, mica and feld spars. Feldspar clasts are more prominent in the basal part of the section. The upper 27 m are totally devoid of this mineral. The proportion of the feldspars reflects the availability in the source area rather than differential stability, inasmuch as microcline is more stable to weathering than orthoclase but is usually second in con tent. Feldspar clasts may be fresh or highly altered to kaolin or sericite. Occasionally, both varieties may appear in the same sample, which may suggest a dual source (altered sedimentary and fresh igneous) or it could be related to differential post-depositional alteration. Mica is predominantly muscovite. The flakes are often parallel to the stratification and are sometimes bent around the quartz and feldspar grains. Heavy mineral segregations are observed in a few thin sections from samples collected 38 and 76 m above the base. Chert is commonly well- rounded and hence detrital. Some jasper both red (hema- titic) and green (chloritic) and banded agate occur and are considered as varieties of chert. Quartzose rock frag ments are dominant in the lower part of the section, with the basal sample containing nearly 25 percent of the esti 24 mated volume* The clasts are largely well-rounded to sub- rounded orthoquartzite or polycrystalline aggregates of quartz grains. Some rock fragments are definitely of granitic derivation made up of aggregates of quartz, feld spar and mica. Argillaceous clasts are occasionally seen, whereas volcanic and metamorphic rock fragments are ex tremely rare. The amount of secondary quartz generally increases with detrital quartz content. Few overgrowths were noted on feldspars. Other cement was almost exclusively fer ruginous hematite and its weathered product limonite. One exotic sample contained 51 percent of hematite cement which tended to obscure the fine-grained quartz and posed problems in the point count. Other authigenic minerals included glauconite (7 percent) in a sample collected 104 m above base, indicating mildly oxidizing to mildly reduc ing conditions. The same sample also contained phosphorite (11 percent) occurring as brown, isotropic cellophane. Both glauconite and phosphorite may be regarded as characteristic of shallow marine shelf environments, but can be worked into intertidal and supratidal environments. Matrix is partly detrital occurring as fine laminae or filling interstices, or it may be derived from post- depositional alteration of mica, feldspars. Microscopic studies of rocks from the Mountain Pass section show a lower quartz content but greater volume of 25 TABLE 3 MEAN VALUES, STANDARD DEVIATION AND RANGE IN COMPOSITION DETERMINED PROM A MODAL ANALYSIS OP 25 SAMPLES IN THE MOUNTAIN PASS SECTION Mean Normal Quartz 48.26 Undulatory Quartz 15*51 Total Quartz* 63*78 Orthoclase 4.99 Microcline 0.90 Plagioclase 0.66 Total Feldspar* 6.54 Mica 0.25 Accessories 0.46 Chert 0.75 Q.R.P. 8.72 Silica Cement 5.98 Other Cement 1.38 Matrix 9.82 Others** 2.29 Standard Deviation Maximum Minimum _______________i_________ % 12.49 64.66 7.66 5.71 27.33 3.66 12.32 79.94 23.00 3.51 15.33 0.0 0.9 6 3* 66 0.0 0.58 1.66 0.0 4.64 20.00 0.0 0.38 1.33 0.0 0.51 1.33 0.0 0.86 4.00 0.0 10.09 44.00 1.31 2.82 12.66 1.33 0.92 3.33 0.0 4.09 24.00 2.66 1.31 5.66 0.0 * Derived variables. ** Includes voids, unstable rock fragments. 26 feldspars, quartzose rock fragments and matrix than the other two sections. Undulatory quartz is more conspicuous which may be a function of post-depositional deformation. Quartz sometimes show mylonitized borders especially near the base and top of the section. Orthoclase is very dominant among the feldspars, with microcline next and plagioclase negligible. Feldspars near the base of the section are fragmented and highly altered. Hock fragments are again predominantly quartzose, scattered throughout the section and ranging up to 44 percent. In the basal sample metaquartzite predominates along with strained elongated quartz. Orthoquartzite, composite quartz and granitic rock fragments are less abundant. The large quartzose rock fragments sometimes posed problems in point counting, be ing larger than the microscopic field of view. Quartzose rock fragments frequently showed sutured borders. Second ary quartz ranged up to 13 percent. Interlocking grains resulting from pressure solution were frequently observed. Silica cementation is little affected by close proximity of the Keystone Basal thrust but granulation and re crystallization along veins appears to be a consequence of post-depositional deformation. Shearing is evident near the top of the section possibly related to the emplacement of the overlying parauthochthonous thrust slice. Some shearing is also suggested near the base of the section. In the Mesquite Pass section total quartz increases TABLE 4 MEAN VALUES, STANDARD DEVIATION AND COMPOSITION DETERMINED PROM A MODAL OP 29 SAMPLES IN THE MESQUITE PASS RANGE IN ANALYSIS SECTION 27 Mean t o Standard Deviation Maximum _ _ . _ _ % Minimum % Normal Quartz 67.97 6.90 84.00 46.66 Undulatory Quartz 6.79 2.72 12.33 2.66 Total Quartz* 74.7 6 5.72 88.33 59.00 Orthoclase 4.16 1.77 8.00 0.33 Microcline 1.01 1.67 7.00 0.33 Plagioelase 1.24 0.77 3.66 0.0 Total Feldspar* 6.42 2.45 11.66 1.00 Mica 0.09 0.19 0.66 0.0 Accessories 0.45 0.53 1.66 0.0 Chert 1.26 1.66 7.66 0.0 Q.R.P. 3.13 2 • 21 10.00 0.66 Silica Cement 7.25 2.78 14.00 2.66 Other Cement 1.91 1.85 7.00 0.0 Matrix 3.79 2.26 9.00 0.66 Others** 1.09 1.06 4.00 0.0 * Derived variables. ** Includes voids, unstable rock fragments. upward in the section in the same manner as in the other two sections. Orthoclase is most abundant, plagioclase next and microcline last. Muscovite mica is rare and chert more prominant than in the sections discussed above. Secondary silica cement is more abundant and overgrowths on feldspars are also recorded. Ferruginous cement is less conspicuous. The quartzites in this section appear to be mineralogieally more mature and the composition more constant with thickness than the other two sections. Texture Mean grain size is a function of (1) the size range of available materials and (2) amount of energy imparted to the sediment which in turn depends on current velocity and turbulence of the transporting medium (Folk, 1968). Sort ing is a measure of the spread of distribution and depends on (1) size range of material supplied, (2) type of deposition, (3) current characteristics, and (4) time; that is, the relationship between the rate of supply and efficiency of the sorting agent (Folk, 1968). Roundness refers to sharpness of edges and corners and is dependent on physical and chemical properties of the particle, its size and history of abrasion. Sphericity is a function of (l) the internal anisotropism of the directional hardness and (2) the original shape of the particle. Prolonged abrasion tends to increase the sphericity. Figure 2. Photomicrograph of basal sample in the Marble Mountain section showing granule sized quartzose rock fragments which comprise 25 percent of this sample. (Between crossed polarizers.) 29 u J U J z * o Figure 3. Photomicrograph of typical sub- arkose (McBride, 1963; from the Marble Mountains containing total quartz (71 percent), total feldspar (14 percent;, mica (2 percent), matrix (9 percent). (Between crossed polarizers.) 31 32 0.2 m m Figure 4, Photomicrograph of a typical ortho- quartzite TPetti;john, 1957) or quartz arenite (McBride, 1963) showing well-sorted mosaic, sac- charoidal quartz. Normal quartz (86 percent), undulatory quartz (3 percent), silica cement (7 percent), matrix (3 percent). (Between cross ed polarizers.) u ju j gp a Figure 5. Photomicrograph of quartz arenite (McBride, 1963) collected 103 m above base of Marble Mountain section, showing quartz with good overgrowths, Phosphorite (dark) 11 percent, glauconite (lighter) 7 percent, in plane polarized light. 35 9£ 37 Grain size, sphericity and roundness data are sum marized in Tables 5> 6, and 7. Microscopic size deter mination yielded results consistent with work done on other quartzite suites (Griffiths, 1967)- In the Marble Mountain section mean size of quartz ranged from -0.262 phi (very coarse sand) in the basal sample to 2.40 phi (fine sand) in a sample collected 114 m above the base. The mean size from 78 samples across the section is 1.16 phi falling in the medium sand range. Sorting varied from poor to well-sorted. Using Folk and Ward scale of sorting (in Folk, 1966) to compare results, five samples were poorly sorted, nine were moderately sorted, three were moderately well sorted, and one well sorted. Using Fried man's scale (in Folk, 1966), nine are moderately sorted, eight moderately well sorted, and one well sorted. Eleven samples showed positive skewness and seven negative skew ness . In the Mountain Pass section mean size varied from 0.69 phi (coarse sand) to 1.33 phi (medium sand). The mean size from five random samples across the section was 1.14 phi. Sorting is fairly consistent applying Folk and Ward’s scale (Folk, 1966). Four samples are moderately well sorted and one moderately sorted, whereas all are moderately well sorted according to Friedman’s scale. Less grains per sample were counted in this section, being hampered by extensive cementation and granulation along TABLE 5 TEXTURAL ANALYSIS OP QUARTZ PROM SAMPLES SELECTED RANDOMLY ACROSS THE MARBLE MOUNTAIN SECTION Sample distance Grain Size above base Mean Standard Deviation Skewness Kurtosls Axial Ratio Spheri city Round ness meters xphi °Bhi Sknhi Kchl b/a Visual Visual 122.5 2.08 0.656 -0.148 -0.043 0.67 0.73 0.59 113.5 2.40 0.486 -0.294 1.044 0.67 0.73 0.59 105.5 1.52 0.565 -0.108 1.259 0.67 0.72 0.52 96.0 1.24 1.288 0.171 -0.358 0.71 0.74 0.52 90.0 1.58 0.588 0.187 0.470 0.68 0.77 0.51 84.0 0.847 1.101 -0.014 -0.272 0.65 0.71 0.51 76.0 0.896 1.104 0.229 -0.324 0.67 0.74 0.49 67.0 1.58 0.983 -0.122 0.237 0.63 0.73 0.52 61.0 1.08 0.858 0.050 -0.058 0.66 0.74 0.54 50.0 1.12 1.197 0.060 -0.383 O.65 0.75 0.54 u 00 TABLE 5 (Continued) TEXTURAL ANALYSIS OP QUARTZ PROM SAMPLES SELECTED RANDOMLY ACROSS THE MARBLE MOUNTAIN SECTION .— . ' I 1 , ■ ■ 1 ■■■■■■,,_ ■■■■■. , iL »■■■-■■?«_ M- 1 . . . — « ■!■■■ mm — ...... ■ __J i ■ HIBJL---■■ ■■ ■■■ — ilm ■— ujm.. Sample distance Grain Size_______________ above base meters Mean xnhi Standard Deviation °Dhi Skewness Skuhl Kurtosis Knhi Axial Ratio b/a Spheri city Visual Round ness Visual 44.0 0.600 0.789 0.154 0.215 0.67 0.78 0.57 41.0 1.33 0.890 0.223 -0.021 0.66 0.69 0.50 36.5 1.04 0.797 0.112 -0.254 O.65 0.69 0.52 30.5 1.70 0.896 -0.291 0.190 0.64 0.73 0.52 20.0 0.767 0.736 0.017 0.610 0.68 0.70 0.53 13.5 -0.023 0.753 0.272 0.108 0.72 0.79 0.59 9.0 0.913 0.732 -0.104 0.072 0.67 0.73 0.55 0.0 -0.262 1.082 0.275 -0.140 0.66 0.74 0.58 Whole Section 1.161 1.078 -0.064 -0.163 0.67 0.73 0.54 TABLE 6 TEXTURAL ANALYSIS OE QUARTZ PROM SAMPLES SELECTED RANDOMLY ACROSS THE MOUNTAIN PASS SECTION Sample distance Grain Size above base Mean Standard Deviation Skewness Kurtosls Axial Ratio Spheri city- Round- ness meters Xnhi °Dhl slShi KDlii b/a Visual Visual 35.0 0.693 0.680 -0.004 -0.200 0.67 0.69 0.51 27.5 1.327 0.671 0.042 0.515 0.64 0.68 0.51 20.0 1.233 0.689 -0.163 0.760 0.65 0.68 0.51 13.5 1.187 0.655 -0.142 0.038 0.63 0.65 0.51 1.5 1.30 0.735 0.318 0.027 0.67 0.67 0.49 Whole Section 1.136 0.723 -0.014 0.224 0.65 0.67 0.50 O TABLE 7 TEXTURAL ANALYSIS OF QUARTZ FROM SAMPLES SELECTED RANDOMLY ACROSS THE MESQUITE PASS SECTION Sample distance Grain Size above base Mean Standard Deviation Skewness Kurtosis Axial Ratio Spheri city- Round ness meters xnhi °Dlli s^nhi Knhi b/a Visual Visual 39.5 0.483 0.866 0.016 -0.121 0.68 0.71 0.52 30.5 1.29 0.778 -0.126 -0.155 0.65 0.69 0.52 20.0 0.600 1.011 -0.173 0.130 0.66 0.70 0.56 12.0 1.18 0.760 0.076 -0.061 0.65 0.67 0.54 6.0 0.767 0.499 0.356 0.066 0.67 0.70 0.53 Whole Section 0.900 0.833 -0.099 0.333 0.66 0.69 0.53 - i ^ H 42 borders. Three samples were negatively skewed and two positively skewed. In the Mesquite Pass section mean size varied from 0.48 phi to 1.29 phi, the mean size for the whole section (five samples) 0.9 phi (coarse sand range). Using the same scales mentioned above, one sample is well sorted, three moderately sorted and one poorly sorted (Polk and Ward scale). Three samples are moderately well sorted and two moderately sorted according to Priedmanfs scale. Three samples are positively skewed and two negatively skewed. Skewness and kurtosis have little significance if any in thin section examination of size (Polk, 1966). Environmental classification on the basis of textural parameters (Mason and Polk, 1958; Priedman, 1962; Miola and Weiser, 1968) did not seem appropriate in the present study to warrant meaningful interpretations because of limited sample size, restricted measurement of the size range, and post-depositional diagenesis and deformation. An interesting approach relating grain size dis tribution to depositional processes was proposed by Visher (1969) using log-probability curves. The analyses are based on recognizing subpopulations within the individual grain size distributions. Each subpopulation may be related to a different mode of sediment transport and deposition. He observed consistency of curve shapes from sample to sample produced by similar processes and also between ancient and modern sand bodies. Recognition of ancient environments no doubt poses problems due to post- depositional mixing, diagenetic changes, etc., which may modify the grain size curve. Moreover, thin section size analysis only provides a window of the size range (0.1 to 2 mm). But this approach to textural analysis may be useful in conjunction with other criteria. Samples in the present study very roughly approximate probability curves corresponding to beach or shallow marine environments (Visher, 1969, Figs. 7, 8, 18, 19). The mean roundness for quartz in the Marble Mountain section varies from 0.49 to 0.59 with a mean value for the whole section of 0.54. Quartz grains in the basal 14 m and top 27 m are better rounded (0.55 to 0.59). Feldspar clasts being less durable to abrasion are angular to sub rounded (mean 0.40). Microcline has better rounding among the three feldspars because of higher stability. Round ness of quartz in the Mountain Pass section varies from 0.49 to 0.51 with a mean of 0.507, and in the Mesquite Pass section roundness ranges from 0.52 to O.56 with a mean of O.53. These values are higher than average round ness of quartz reported by Griffiths (1967) for quartzites, and could be related to mature sediments or an operator bias. All the samples fall within the rounded category (0.4 to 0.6) of Pettijohn (1957). Sphericity was measured by visual comparison and 44 | b/a axial ratio. The MbM and “a" axes plotted on a scatter diagram shows increasing variability (heteroscedasity) with an increase in mm values of "a" and 1 1 b” axes. In the Marble Mountain section ( 2651 quartz grains) values range from O.63 to 0.72 for b/a with a mean of 0.667. These values fall within the range of axial ratios reported by Griffiths (1967) for measurements in thin sections. Oriskany Quartzite has a mean value of 0.677 from measure ments of nearly 10,000 grains in 86 samples. The ratio in the Mesquite Pass section (580 quartz grains) has a narrower range of O.65 to 0.68 and a similar mean value of 0.663, and the Mountain Pass section (700 quartz grains) has values varying from O.63 to O.67 and a mean sphericity of 0.651. The lower axial ratios could infer that the quartz grains were initially more elongated or suffered less abrasion. However, the main advantage of size and sphericity measurements is their use as a descriptive and petrographic rather than an analytic and petrologic tool (Griffiths, 1967). Main drawbacks in visual comparison techniques for roundness and sphericity are the high operator variation involving psychophysical procedures and personal bias. Griffiths (1967) showed that sources of variation due to differences among grains, operators and days are all significant. Statistical Analysis Dendrogranhs-Cluster Analysis 45 The relationship between each pair of variables measured in the modal analysis was expressed as a product- moment similarity coefficient. The lower half of the cor relation matrix was used to generate R-mode dendrographs for each section and for all the samples combined. The dendrograph is a two-dimensional diagram depicting rela tionships between and within the groups of variables and thus provides a more graphical representation of the total relationship (McCammon, 1968). The hierarchical grouping imparts a pyramidal structure to the dendrograph. The significant number of clusters and related independence of the variables can be inferred from spacings between hier archical levels and wider spacings between clusters. Perhaps the most meaningful set of relationships may be observed in the Marble Mountain section (Pig. 6) which is probably a function of sample size (N = 86) and lack of post-depositional deformation. Cluster I comprises micro- cline, plagioclase and orthoclase, all highly correlated (microcline and plagioclase 0.710; plagioclase and ortho clase 0.704). Their presence is largely dependent on the availability in the source area, and may be indicative of moderate energy (being less stable than detrital silica), and rapid deposition. Feldspars are prominent in the Figure 6. Dendrograph depicting mutual rela tionships between and within (cor relation coefficient +1.0 to -1.0) the twelve variables measured in the Marble Mountain section. 46 4? o 4 ' - o % o o o * . V s y?. \ V ' S ' V *o 'S ' ' S ' a *o. % \ V V ■3. % > / > V . % V I o I 0> o I CD o h- o I CD o I i n d I o ro o CVJ o I o d ro- 48 basal part of the section and in decreasing order of dominance are orthoclase, microcline and plagioclase. Microcline is suggestive of plutonic igneous or gneissic source, but like the other two could also be derived from older sediments. Polk (1968) suggests that nearly all feldspar is derived from primary igneous or metamorphic source, an inference based on the poor durability of the mineral. Thus the high correlation of the feldspars is linked partially to their stability; all members occur to gether or are all absent depending on the availability in the source area, mineralogical maturity of the sediment and possible diagenetic alteration to matrix. Cluster II consists of chert and accessory minerals along with the feldspars. Both occur in minor quantities (less than 1 percent mean value) and are relatively more stable than the feldspars and better rounded, being derived largely from older sediments. The presence of heavy minerals is related to lithology of the source area, stability and durability, hydraulic factors and diagenetic changes. Chert is regarded as a chemically precipitated sedimentary rock (Polk, 1968) frequently occurring as a cavity filling as chalcedony, agate or as a chemical cement. In the present study chert is detrital, rounded and most probably derived from older sediments. Chert is less stable than quartz and will not be found in highly mature sediments while some ultrastable heavy minerals (tourmaline, 49 j zircon) are characteristic of supermature arenites. How- ever, both chert and accessories occur in small amounts and their relationship may be fortuitous. Cluster III shows a moderate correlation between undulatory quartz and quartzose rock fragments (0.55)* This substantiates views regarding the decreasing stability of detrital silica minerals in the following order: normal quartz-undulatory quartz-polycrystalline quartz (quartzose rock fragments)-chert. Blatt (1963) concluded that supermature orthoquartzites have high con tent of normal quartz as a result of selective destruction of undulatory quartz versus unstrained quartz. Thus, persistence of undulatory quartz and quartzose fragments infer a shorter history of abrasion and/or a lower energy environment than suggested by normal quartz. An increase in normal quartz upwards in the section is accompanied by a corresponding decrease in undulatory quartz and quartzose rock fragments. Strained grains have a higher dislocation density which increases their free energy and thermo dynamic instability (Cortrell, 1961). Thus, percentage of undulatory and polycrystalline quartz may be used as a rough index of amount of transport or abrasion. Cluster IV which consists of mica and matrix also shows a moderately significant correlation (0.46). Matrix refers to an admixture of fine micas (sericite), clays (illite) and silt-sized quartz. Matrix could be detrital 50 which washed in and filled interstices in the clastic sediment, and its presence probably infers a low to moderate mechanical energy inasmuch as this fine material would not be washed out under these conditions. The matrix could also be derived diagenetically from altera tion of mica and feldspars. Hawkins and Whetten (1969) showed experimentally that graywacke matrix may form diagenetically by hydration and recrystallization of un stable clasts. Mica could also have a dlchotomous deriva tion. Muscovite flakes wrapped around quartz grains sug gest a detrital origin. However, secondary diagenetic mica is possible depending on the amount of clay in the inter stices and post-depositional modification in the sedi ments. In cratonic shelf sandstones, diagenesis may be terminated at the locomorphic stage partly due to small amounts of interstitial clay but similar strata suffering tectonic deformation may pass through the phyllomorphic stage with the formation of micas bordering the quartz grains (Dapples, 1967). There is little or no evidence for deformation in this section. The correlation between mica and matrix is largely related to breakdown or weathering of the mica and incorporation into the matrix, but it could also be related to the hydraulic equivalence of the two. There is evidence in several samples of textural inversions with associations of poorly sorted quartz, mica, matrix and argillaceous rock fragments probably related to 51 change in paleocurrent direction and source. Cluster V comprises other cement, which is almost exclusively hematite and limonite. The ferruginous cement is precipitated diagenetically and is independent of other variables. Oxidizing conditions are necessary for the formation and survival of hematite, while limonite is often a weathering product of other iron minerals. Normal quartz has an inverse relationship with all other variables except silica cement with which it has a moderately significant similarity (0.44) to form Cluster VI. High content of normal quartz in the variable total quartz may be indicative of higher energy or a longer history of abrasion. Moreover, higher content of quartz with arenite framework will favor pressure solution and silica cementation with quartz overgrowths in optical con tinuity with the detrital grains. Siever (1959) reports that most extensive quartz cementation occurs in clay-free sandstones, but samples with 5 to 10 percent clay may also show large amounts of secondary quartz. In an experimental study of quartz overgrowths and synthetic quartzites, Ernst and Blatt (1964) observed that unstrained grains (normal quartz) exhibit more numerous and fully developed overgrowths than strained (undulatory) quartz. The greater the strain in a grain, the more difficult it is to enlarge itself. Thus, normal quartz and silica cement are compat ible on thermodynamic grounds. 52 The clusters for the Mountain Pass section and Mes quite Pass section are slightly different and may be as cribed to their smaller sample size or post-depositional modification. In the Mountain Pass section, where little plagioclase is recorded, orthoclase and microcline are highly correlated (0.87) with plagioclase showing moderate ly significant similarity (0.57)* Mica is correlated with feldspars, and quartzose rock: fragments with matrix, the latter suggesting that the matrix may be detrital and as sociated with lower energy QRF. Undulatory quartz forms a cluster with the cements (silica and ferruginous). This section shows signs of post-lithification tectonic deforma tion, probably related to emplacement of the Keystone thrust plate. This deformation has affected the mineral ogy as well as structures. Evidence includes increase in undulatory quartz upwards in the section, marginal granu lation of quartz grains, and some recrystallization along veins. The clusters may not be meaningful. The Mesquite Pass section also shows clusters which are more difficult to interpret. The highest correlation is between microcline and mica (0.55)* Orthoclase and plagioclase form a cluster with other cement. Matrix and undulatory quartz have the next highest correlation (0.5^) which could be related to lower energy and detrital matrix or be purely fortuitous. Other clusters include acces sories combined with matrix and undulatory quartz; quartzose 53 | rock fragments and chert have a moderately significant cor relation (0.43); silica cement is correlated with clusters comprising feldspars, mica and other cement; and normal quartz is independent having a negative correlation with the other variables combined. The dendrograph for all the samples (140) combined closely matches the one described in detail (Marble Mountain section, Fig. 6). The essential differences in clude mica being correlated with feldspars and the matrix combines with less stable undulatory quartz and quartzose rock fragments to form a single cluster. The relationships among the variables are summarized as follows: Cluster I, orthoclase, microcline and plagioclase; Cluster II, mica with variables of cluster I; Cluster III, accessories and chert with variables of clusters I and II; Cluster IV, quartzose rock fragments, undulatory quartz and matrix; Cluster V, other cement (hematite-limonite); and Cluster VI, normal quartz and silica cement. The combination of samples from different locations may be unwarranted, parti cularly if sedimentary processes and post-depositional modifications are different. Time-trend Analysis Time-trend analysis is finding increased use in the study of sedimentary sequences (Vistelius, 1961; Fox and Brown, 1965; Osborne, 1970). It is a moving averages 54 technique which facilitates smoothing of noisy data, and the undulations in data obtained are useful in interpret ing fluctuations in mechanical energy and sedimentation patterns In the depositional environment. They may also be useful for stratigraphic correlations. The five feet (1.5 ©) sample interval afforded a good measure of short period fluctuations. Except for one sample collected at 12 feet (3*6 m) because of interbedded siltstones, no gaps or covered intervals occur in the stratigraphic sections being described. Polynomial time-trend curves were plotted for each petrographic variable. Plots of variables useful for interpretative purposes are depicted in Pigures 7 and 8 (Marble Mountain section), Figure 9 (Mountain Pass section) and Figure 10 (Mesquite Pass section). Smoothing equa tions 4, 5, 6, and 7 (Fox, 1964) using 9» 11, 13 and 15 terms, respectively, seemed suitable to explain variation in the stratigraphic sections and retain significant variations in the curves. There is substantial variation in mineralogy and texture with thickness in the Marble Mountain section. The basal 29 m is characterized by a gradual decrease in normal quartz upward. An increase in undulatory quartz accentuated by a marked minima at 20 m. The total quartz content is almost constant with a very slight decrease up ward. Total feldspar shows a decrease upward and so does Figure 7. Time-trend curves for five variables measured in the Marble Mountain section. Straight-line plot: weighted data (percent of maximum point-count) smoothed once. Circled plot: smoothed data with percent of the total sum of squares indicated (1). 55 NORMAL QUARTZ METERS \ 3 d I20i 105- 90H 75H 60H 45' 30i 40 80 P eg UNDULATORY QUARTZ 25 % 40 80 % TOTAL FELDSPAR 40 80 % QUARTZOSE SILICA ROCK FRAGMENTS CEMENT 40 80 % 23 % 40 80 % K J \ o\ Figure 8. Time-trend plots for six variables measured in the Marble Mountain section. Straight line plot: weighted data (percent of maximum point-count or textural value smoothed once). Circled plot: smoothed data with percent of the total sum of squares indicated (l). 57 MICA MATRIX 132 46 % 120- 105 90- 75 60- 45-, 30- 80 % 40 Peg 43 % 80 % 40 QUARTZ SORTING 40 80 % QUARTZ I . ROUNDNESS (XR) 2. SIZE (Xmm) (2) 12% 12% (I) 40 80 % MAXIMUM GRAIN DIAMETER 33 % V J I CD quartzose rock fragments and silica cement. Matrix shows a few minor fluctuations hut has a gradual decrease up ward. Mica shows a close relationship with matrix, both showing a local minima at 11 m. This distance from the base corresponds with a conglomeratic quartzite unit and the high maxima for maximum grain diameter. Thus, local high energy may be responsible for the removal of mica and matrix and for the influx of pebbles. The next 90 feet (27.5 i) is marked by slight in crease in normal quartz with a corresponding decrease in undulatory quartz while total quartz changes little. Total feldspar also is consistent with minor fluctuations. Quartzose rock fragments show a very slight increase and silica cement a marked decrease upward. Mica and matrix show corresponding fluctuations and an increase upward. In the same direction, mean quartz size increases and the sorting becomes poorer. The source area for the basal 55 m remained essentially the same. The variation in composi tion possibly infers amount of reworking and rapidity of deposition. From 55 to 99 m above the base, normal quartz shows little variation with local minima at 76 m and maxima at 90 m. Undulatory quartz shows more significant variation and decreases upward. Total feldspar also shows a gradual decline upward with local maxima at 84 m. Quartzose rock fragments show little variation and so does silica cement, 60 the latter having a maxima at 90 m. Matrix and mica show considerable variation with both showing sharp changes in trend at 75 which also corresponds to an influx of argillaceous rock fragments and poor sorting of quartz, and is indicative of lower energy in the environment. Sorting perhaps shows the most interesting relationship, closely matching plots for mica and matrix with a maxima at 76 m (poor sorting) and a minima at 70 and 90 m (good sorting). Bimodality of quartz is evident in several samples with coarse sand grains better rounded than the finer ones. Variation in trend components may be related to changes in paleocurrent directions. Several foreset dips in this part of the section suggest northeasterly and easterly currents, specifically samples at 61, 76 and 84 m above the base. And therefore it is probably significant that bimodality (poor sorting), higher content of detrital matrix and mica, argillaceous rock fragments are co incident with these easterly and northeasterly currents in contrast to westerly and northerly currents in most of the section. It could also be related to transgressive epi sodes accompanied by textural inversions. At 99 m hema- titic arenite is present with associated sedimentary structures such as ripple marks, groove molds. It may indicate emergent conditions of at least strong redoxo- morphic diagenesis probably during early burial. Between 99 and 104 m the quartzites are interbedded 61 with units of thin-bedded sandstone and siltstone, above which is a massive medium- to fine-grained massive cliff- forming quartzite. Quartzite samples between the siltstone beds contains phosphorite (collophane), glauconite, some iron-stained unrecognizable fragments (?) and churned bed ding (bioturbation). The sample also displays excellent planar tangential cross-stratification with easterly fore set dip. The uppermost unit (104 to 132 m) is marked by sharp increase in normal quartz, total quartz, silica cement accompanied by better sorting and roundness and inferring a supermature sediment. This trend corresponds with a decrease in undulatory quartz, quartzose rock frag ments, matrix, maximum grain diameter, and mean quartz size. Feldspar is totally absent in this portion (upper 28 m) . As a summary, considering the Marble Mountain section as a whole, normal quartz shows an increase upward while undulatory quartz has an inverse relationship. Total feldspar shows a gradual decrease from 14 percent near the base and is absent in the top 28 m. Quartzose rock frag ments are more prominent in the basal portion of the section and are occasionally seen higher up in the section. Silica cement inflections show a marked similarity with those for normal quartz. Figure 8 shows remarkable similarity in plots for mica and matrix. Quartz sorting is 62 i poorer with increase in mica and matrix. Quartz roundness shows little change, although samples near the base and top of the section appear to be better rounded. Quartz mean size measured in thin section and the maximum grain diameter estimated from radiographs show a decrease upward in the section. Opposing inflections are observed at some points and may be related to diversity of composition (quartz, quartzose rock fragment, chert, feldspar) of maximum grain diameter measured in the radiographs contrast ed with quartz mean size measured in thin section. In the Mountain Pass section, time-trend plots for normal quartz show considerable variation with thickness with a maxima at 7-5 m followed by a decrease up to 28 m and then increases again to the measured top (37 m). Un dulatory quartz on the other hand shows a continuous in crease upward in the section. This may be a function of strain induced by post-depositional deformation probably associated with the Keystone thrusting event (about 92 m.y.b.p., Burchfiel and Davis, 1971)> rather than primary derivation. This is accompanied by marginal granulation of quartz and contortion of strata upward. Total quartz shows minor fluctuations but increases continuously up wards. Total feldspar has an inverse relationship, de creasing upward from 20 percent near the base to zero per cent about 100 m above base. Quartzose rock fragments are abundant at the base and appear throughout the section. 63 Silica cement increases upwards. Matrix material is abundant at the base and also increases upward. It is derived from the alteration of feldspars and mica but could also be detrital. The source rocks in this section are dominantly plutonic igneous and their metamorphic equiva lents. Rectangular clasts of feldspar and large rock frag ments at the base suggests little or no transport with the clasts probably derived from the basement Precambrian gneisses. In the Mesquite Pass section, consistency of com position is more pronounced than variation with thickness. Normal quartz shows little variation with thickness and gradually increases upward above 30 m. Undulatory quartz again shows an inverse relationship with minor fluctua tions. Total quartz is consistent for most of the section. Feldspar shows very sharp fluctuations and is found throughout the section, and the same is the case with quartzose rock fragments which show minor variation. A rough break in the facies can be drawn between 23 and 25 m, with the lower 23 m showing continuous increase in normal quartz, total feldspar and silica cement, and a gradual decrease in undulatory quartz and matrix. Above 25 m variables displaying pronounced increases include normal quartz, quartzose rock fragments, while decrease is seen in total feldspars and matrix. The source area and deposi- tional energy are apparently consistent throughout the r Figure 9- Time-trend curves for six prominent variables in the Mountain Pass section. Straight line plot: weighted data (percent of the maxi mum point-count) smoothed once. Circled plot: smoothed data with percent of the total sum of squares indicated (l). 64 NORMAL QUARTZ METERS 37t 26 30 80 40 PCg METERS 37 2 0% 30 40 QUARTZOSE % 80 ROCK FRAGMENTS UNDULATORY QUARTZ TO TA L FELDSPAR 14% % 80 40 20% % 80 40 3 6 % 40 80 % SILICA CEMENT 41% 40 80 MATRIX o\ s j i Figure 10. Time-trend curves for six prominent variables in the Mesquite Pass section. Line plot: weighted data (percent of the maximum point-count) smoothed once. Circled plot: smoothed data with percent of the total sum of squares indicated (l). 66 NORMAL QUARTZ METERS 43t 301 80 40 P€g 43t 2 9 % 301 40 80 P€g % QUARTZOSE ROCK FRAGMENTS UNDULATORY QUARTZ 55 % % 80 40 TO TAL FELDSPAR 72 % % 80 40 7 5 % 40 % 80 SILICA CEMENT 22% 40 % 80 MATRIX o\ 68 i section. Cross-Association An attempt to match the three stratigraphic sections from time-trend plots was not very rewarding. Although several systematic fluctuations were observed as well as general fining of the sediment upwards, no significant marker horizons such as heavy mineral segregations, fossil- iferous beds, could be found to define parastratigraphic units. The qualitative matching sequences of strata has been attempted by Sackin, Sneath and Merriam (1965) by use of cross-association coefficients to compute the similarity measure between the sections. The variables were assigned classes (nominal scale) from point count data for each sample. The sequences in each pair of sections are made to slide past one another to determine the association co efficient based on the number of matches and comparisons. The similarity index (SL) between two sections measures the proportion of the values that can be paired off as match ing sequences. For total quartz the similarity index for the three pair of sections are as follows: Marble Mountain and Mountain Pass, 0.45; Marble Mountain and Mesquite Pass, 0.75; Mountain Pass and Mesquite Pass, 0.89. These values represent the average match. Thus, qualitative similarity is highly significant with respect to total quartz between the Mesquite Pass and Mountain Pass sections, both of which 69 are in close proximity and have nearly similar thicknesses. Strong similarity is also seen between the Marble Mountain and Mesquite Pass sections. Frequency Distributions and Analysis of Variance Frequency distributions of the mineral components determined in the modal analysis were compared with theoretical models of constant probability applicable to point counting methods. The following conditions must be fulfilled before the model is accepted. The chance of a point falling on a quartz grain, for example, is the same for (l) each point, (2) each quartz grain, and (3) the fact that the point has fallen on a quartz grain does not affect the chance of points falling on other quartz grains. If the probability is between 5 and 95 percent, the bi nomial distribution is accepted, if less than 5 percent or greater than 95 percent, the Poisson model is a better fit. Occasionally a negative binomial distribution may be ex pected if the chance of a point falling on an item in creases the chance of other points falling on that item. This non-constant probability model indicates a patchy or layered occurrence, while a constant probability model im plies homogeneity of distribution. The observed distribution and the theoretical model is compared by a chi-square goodness of fit (Fig. 11). The Figure 11. Histograms for “Total Quartz1 1 and “Total Quartz plus Quartzose Rock Fragments." Chi square goodness of fit for binomial frequency distribu tion accepted for each variable in each section at the 0.05 level of significance. 70 T O T A L QUARTZ I 5 i 1 0 - X = 3 .0 3(3 d f) 5 - 177 207 237 Points 267 OBSERVED VALUE 15-1 1 0 - X -3.87 ( 3 df) T I 69 125 181 Points 30i 237 2 5- 20- 15- 1 0 - 5- X -5.12 (4 df) 134 176 218 Points 260 TOTAL QUARTZ + Q.R.F. (SEC. I) N *2 9 (SEC.3) N -2 5 (SEC. 5) N * 86 15*1 1 0 - X * 5.8 0(3df) 200 220 240 Points 260 THEORETICAL VALUE (BINOMIAL) I 5 l 1 0 - X -0 .6 7 (3 df) 180 30- 200 220 Points 240 25- 20- 1 5 - 1 0 - X « 0.35 (5 df) 130 170 210 Poin t s 250 290 72 positive binomial was accepted at the 0.05 significance level for "Total Quartz" and "Total Quartz + Quartzose rock fragments" in all three sections. In the Marble Mountain section these are the only two variables showing homo geneity of distribution. Accessory minerals exhibit a Poisson frequency fit but this may not be acceptable be cause this component is usually segregated in layers In samples having a significant quantity. The Mountain Pass section shows homogeneity of distribution of detrital elements except quartzose rock fragments and accessories. The constant probability model Is accepted for all detrital elements in the Mesquite Pass section. A one-way analysis of variance model was set up to study variation of mineral composition between strati- graphic sections. The New Duncan Multiple Range Test was used to determine the source of variation (Dixon, 1970). The sample size (N) was estimated on the basis of an 80 percent probability of detecting a 10 percent difference between two means at the 5 percent level of significance (Sokal and Rolf, 1969* p. 246). However, only Total Quartz (N = 25) and Total Quartz plus Quartzose Rock Fragments (N = 18) fell within the range of samples available for the three sections. The null hypothesis that there is no dif ference in the means among the three sections (samples are drawn from populations with the same mean) is rejected for both variables even at the 1 percent significance level 73 (Tables 8 and 9). The F-test on rejecting the homogeneity hypothesis does not ascertain the source of variation. The New Duncan Multiple Test ( = 0.05) was used to com pute least significant differences. Each difference is significant if it exceeds the corresponding shortest sig nificant range. The source of variation is ascribed to the Mountain Pass section for both variables. The Mesquite Pass and Marble Mountain sections are statistically homogeneous with respect to these two variables and both differ significantly from the Mountain Pass section for each variable. This variation could be ascribed to lower mineralogical and textural maturity of the sediment in the last named section implying a possible local depositional environment with lower energy or rapid deposition of sediment with less transport. The latter is obvious in the basal portions of the Mountain Pass section with rectangular clasts of feldspar and large fragments of gneiss suggesting a gradational contact. Furthermore, this section is overriden by a parautochto- nous slice of Precambrian gneiss causing brecciation and contortions in the upper part of the section which could alter the point count data to some degree. The basal sample also shows some evidence of shearing. One-way analysis of variance design using five groups (Table 10) comprising the Mountain Pass and Mes quite Pass sections and three arbitrary units of the Marble 74 TABLE 8 ONE-WAY ANALYSIS OP VARIANCE FOR ” TOTAL QUARTZ1 ' Source of Sum of Mean Variation____________Squares______ DP______Square____P Ratio Among Sections 23489.2 2 11744.6 14.83** Within Sections 108443.2 137 791.5 Total 131932.4 139 DUNCAN'S NEW MULTIPLE RANGE TEST (oc= .05) Sample Rank_________ Section________ Size_____Mean__________________ 1. Mountain Pass 25 191.36 2. Mesquite Pass 29 224.69 ^ Homogeneous subset 3. Marble Mountain 86 225.33 V 75 TABLE 9 ONE-WAY ANALYSIS OP VARIANCE FOR "TOTAL QUARTZ + Q.R.F." Source of Variation Sum of Squares JDP Mean Square P Ratio Among Sections 5379.4 2 2689.7 5.23** Within Sections 70417.8 137 513.9 Total 75797.2 139 JDUNCAN1 S NEW MULTIPLE RANGE TEST (OC = .05) Rank Section Sample Size Mean 1. Mountain Pass 25 217.16 2. Marble Mountain 86 233.02 A Homogeneous subset 3. Mesquite Pass 29 234.14 n|/ 76 TABLE 10 ONE-WAY ANALYSIS OF VARIANCE FOR TOTAL QUARTZ USING FIVE SAMPLE GROUPS s b s b s b , m l1 z a f B s a c g s g s — Lim .u-SS S B S S S S Z E E E S S S S E S S B S S S T - ” " " ■ I'l a s a B s g s s s a g c a s B a i . B T i m . . ■;m ^ s s s a s s m tm s s s s s s S B A ^ .i r 1 1 E S s s s a m m m c s s s m u s E m B S S S Source of Sum of Mean Variation_____________ Squares______ DF______Square___F Ratio Among Groups 34097.8 4 8524.4 11.76 Within Groups 9783.8 135 724.7 Total 131932.6 139 DUNCANfS NEW MULTIPLE RANGE TEST (PC - 0.05) Rank Group Sample Size Mean 1. 2. 3. 4. 5. Mountain Pass Section 25 191.36 Marble Mountain Section (Middle) 29 214.31 Marble Mountain Section (Lower) 29 221.48 Mesquite Pass Section 29 224.69 i Marble Mountain Section (Upper) 28 240*75 Homogeneous subset 77 Mountain section (lower, middle and upper) also rejects the null hypothesis for Total Quartz with the source of varia tion being due to the Mountain Pass section (immature sedi ments) and the upper Marble Mountain unit (supermature). The Mesquite Pass section forms a homogeneous subset with the lower and middle members of the Marble Mountain section. The analysis of variance model applied to other variables is summarized below, but inferences are question able because of insignificant sample size. Means for chert (P = 2.0) and accessories (P = 0.92) showed no variation between sections. Normal quartz (P = 29*4**), Quartzose rock fragments (P = 15*26**), matrix (P = 16.8**), plagioclase (P = 4.04*), maximum grain diameter (P = 8.3**) showed no significant variation between means for the Marble Mountain and Mesquite Pass sections which form homogeneous subsets in each case and the source of varia tion was due to the Mountain Pass section. Radiographic Analysis The rocks often appear to be homogeneous in hand specimens but display sharp contrasts in the radiographs due to variations in mineralogy, texture, compaction and stratification. Quartz permits easy penetration of X-rays and appears dark, feldspars are more dense to X-rays and appear lighter. The composition of the matrix and nature 78 of the cement can display sharp contrasts in the radio graphs. Alteration of minerals and presence of hetero genous clasts may he discernible in the radiographs. One of the most meaningful aspects of this portion of the study are the internal sedimentary structures. In the Marble Mountain section uniform stratification appears to be dominant, and the thickness of layers is generally about one centimeter. The contrasts between strata are frequently accentuated by varying amounts of matrix or cement. Quartz-rich laminae appear darker when cemented by silica and lighter with more matrix or cement. In some cases elongate clasts parallel the bedding and help to define it further. Some samples exhibit massive bedding. Two display graded bedding ranging from coarse to fine grained quartz within a few centimeters. Although strati fication is obscure in some samples as a consequence of alteration, the upper 27 m show faint lamination as a result of uniformity of composition and silica cementation. Cross-stratification is well displayed by 17 samples and is obscure in a few more. Foreset dip and azimuth were measured for strike and dip slabs radiographed, and the maximum foreset dip and azimuth computed. McKee and Weir (1953) defined a cross-stratified unit as one with layers deposited at an angle to the original dip of the formation. Allen (1963a) elaborated on their terminology and proposed a classification based on six criteria. Applying Allen's r Figure 12. X-radiography contact print showing poorly sorted and angular clasts of feldspar (dark, rectangular), coarse quartzose rock fragments and quartz in a sample collected 3 m above the basal Precambrian gneiss in the Mountain Pass section. Scale bar = 1 cm. T- top of bed. 79 30 Figure 13. X-radiography contact print showing parallel lamination in subarkose collected 32 m above the base of the section in the Marble Mountains. Scale bar = 1 cm. T - top of bed. 81 CM CO ; ♦ ;*;> ' - S j , / • * jL r f - . *> - t ' - > • • ' (& i j t i p ’ f y f ' & £ ■ < "' *•* • . ' ■ ■' \4 V- : -v *$•• • i ‘ .'■ n * S < ' S : /**'*’ ’ ' 7 * • * ' : r < ■ ' * . . V Figure 14. X-radiography contact print showing small-scale planar cross-stratifica tion with tangential foresets. Also shows parallel stratification and diastems. Sample collected 65 m above base in the Marble Mountain section. Scale bar = 1 cm. T - top of bed. 83 84 Figure 15. X-radiography contact print showing bioturbation in a cross-stratified quartz arenite sampled 103 m above the base of the section in the Marble Mountains. Lamination in this sample accentuated by phosphorite glauconite and iron stained minerals. Scale bar = 1 cm. T - top of bed. 85 Figure 16. Paleocurrent analysis of cross-bed data, with maximum foreset dip azimuths determined from samples collected in the Mesquite Pass (l), Mountain Pass (3) and Marble Mountain (5) sections. Vector mean is indi cated for each section. R - vector magnitude and L - con sistency ratio. 87 88 ( R * 4 .1 ; L * 5 0. 6 %) 2 56 292 ( R*2. 6; L * 6 4. 6 %) 349 (R* 11.3 ; L * 59.7 %) 89 classification to the present study, the grouping of the cross-stratified units is commonly solitary, the magnitude is small to medium scale, the character of the lower bound ing surface is generally nonerosional (planar), the cross bedding is tabular and the lithology of the set is usually homogeneous. Maximum foreset dip azimuths may be classified into three main trends for the whole section, six directions suggestive of westerly currents, six easterly, and seven northerly to northwesterly (Fig. 12). The vector mean is 349°. The observed trimodality may suggest currents of tidal origin with cross-stratification developed during flood and ebb tides and associated longshore currents may account for the third trend. An alternative explanation could imply possible changes in paleocurrent direction with time. Northwesterly currents predominate whereas evidence of easterly currents is occasionally observed, particularly between 55 and 104 m above the base of the section. In the Mountain Pass section four samples displaying small-scale cross-stratification have a mean foreset azimuth of 292° (westerly currents) and eight samples exhibiting good tabu lar cross-bedding in the Mesquite Pass section, six indi cate westerly trends and two easterly with a mean foreset dip azimuth of 256°. More data are needed to verify the forementioned trends. The paleocurrent data is in general agreement with Stewart's (1970) predominantly westerly 90 trends to account for dispersion of the upper Precambrian and lower Cambrian sediment of the southern Great Basin. Other sedimentary structures observed in the Marble Mountains include few instances of ripple marks, groove molds on the sole of samples found between 99 and 104 m, churned bedding (bioturbation) related to organic activity in samples collected between siltstone beds. Numerous diastems are characterized by irregular boundaries formed during breaks in deposition. Mottling was noticed in some samples as a result of alteration in patches or concentra tion of porous areas in the rock, possibly resulting from leaching out of material. Stratification in the Mountain Pass section is also generally uniform but often obscure and subjected to con tortions, minor faulting and other deformational features probably related to the Keystone thrusting event. The basal units are high in feldspars exhibiting good contrasts in the radiographs but poor stratification. Pebbles and granules are found throughout the section, and associated measurements of imbrication also suggest westerly currents. Radiographs from the Mesquite Pass section show uniform stratification and numerous diastems and occasional graded bedding and small ripple marks. The maximum grain diameter, which may be a good indicator of mechanical energy in the depositional environ ment, was measured from radiographs in both strike and dip 91 slabs and the larger apparent maximum diameter was consider ed in the study. In the Marble Mountain section the size ranged from 0.5 to 21 mm and varied from fine-grained units at the top to conglomeratic units near the base. The mean size is 3.8 mm. Samples from the Mountain Pass section showed apparent maximum grain diameter from 1.0 to 19*0 mm with a mean diameter of 7.5 mm, and samples from the Mes quite Pass section had a mean value of 4.2 mm and a range of 1.0 to 16.0 mm. DISCUSSION OP RESULTS Classification On the basis of the volume of constituents deter mined by the petrographic modal analysis, the rocks ranged from supermature orthoquartzites (Pettijohn, 1957) or pure quartz arenites (Polk, 1968; McBride, 1963) to feldspathic quartzites or subarkose with the latter being more dominant. The choice of end members is based on the classification proposed by McBride (1963)* The Q-pole comprises total quartz, quartzose rock fragments (polycrystalline quartz, orthoquartzite, metaquartzite) and chert. The P-pole is essentially total feldspar and also granitic rock frag ments (aggregates of feldspar and quartz). The R-pole includes all other (unstable) rock fragments (shale, slate, volcanics, etc.). Thus, the common stable frame work components (detrital silica) are grouped in one pole indicating end products of weathering and abrasion and two poles of unstable components (feldspar and other rock fragments)• In the Marble Mountain section, the basal ?6 m com prises subarkose, 76 to 104 m is a transitional zone con sisting of subarkose, sublitharenite and quartz arenites, while the uppermost 28 m are exclusively quartz arenites. 92 93 Basal 28 m in the Mountain Pass section are subarkose over- lain by quartz arenite. In the Mesquite Pass section, sub arkose dominates the basal 25 m whereas quartz arenite is more characteristic of the upper 18 m. Provenance Quartz is the most abundant mineral constituent but has only minor importance in determining the provenance. Presence of distinct mineral inclusions of zircon, biotite, apatite in quartz are characteristic of granitic igneous rocks but could also be present in quartz grains derived from gneisses and schists. Although Polk (1968) still classifies quartz on the basis of extinction, this property is probably more useful in understanding sediment history rather than provenance. Blatt (1963) showed that there is no way of distinguishing between quartz grains from plutonic igneous rocks, schists and gneisses. He reported only 14.9 percent of quartz was non-undulatory in 101 rocks with no significant difference among the three rock groups. Quartz in extrusive rocks was predominantly (91 percent) non-undulatory; however, these rocks supply little quartz to the sedimentary cycle and the presence of associated rock fragments and zoned plagioclase is required to confirm a volcanic source. In the present study there is no evi dence to suggest a volcanic source. Thus, origin of quartz considered on the basis of undulatory extinction is 94 | not warranted. Furthermore, strain in quartz may be induced by- deformation after lithification (Bailey, Bell and Peng, 1958) as is the case in the Mountain Pass section with the accompanying fracturing and marginal granulation. Barring post-depositional deformation, relative amounts of detrital silica could throw light on the maturity of the sediment. The stability among these minerals in the decreasing order is normal quartz-undulatory quartz-polycrystalline quartz (quartzose rock fragments)-chert (Blatt, 1963). R-mode dendrograph clusters agree with this relationship. Thus the increasing proportion of non-undulatory quartz as is seen in the supermature orthoquartzites is probably a good inference of sedimentary history of abrasion rather than provenance. The relative abundance of feldspar in a sediment is related to availability in the source area, maturity of the sediment and diagenetic processes at the site of deposition. Two feldspar types (highly altered and fresh) are sometimes observed in the same sample suggesting a dual source, fresh-igneous and altered sedimentary or it could infer selective post-depositional alteration. Stewart (1970) reports feldspar tends to break easily along cleavages and is concentrated in finer grained sedi ments. The present study notes feldspar accumulation in the coarser basal sediments and total absence in the mature 95 finer grained arenites. Also, the apparent grain size difference over the Marble Mountain section is not pro nounced (Quartz X = 1.16 0 and Feldspar X = 1.48 0). Microcline and dominantly sodic plagioclase suggests a plutonic acid igneous or gneissic source. Heavy minerals, predominantly detrital zircon, tourmaline, magnetite and monazite, confirm a granitic or gneissic source, but could also be derived from older sediments. Chert is detrital (rounded) and characteristic of reworked sediments. It is gradually eliminated with maturity. Presence of jasper (hematitic) and agate also suggests an easterly source. Anderson and Creasy (1958) report veins and pods of jasper in Arizona, overlain by Tapeats Sandstone inferring a Precambrian or Cambrian age for these veins and probable source for clasts in the quartzites under study. Conglomeratic quartzites contain pebbles of quartz, quartzite, chert which are possibly inherited from older sedimentary deposits and swept into the environment during local episodes of transgression. Rock fragments are predominantly quartzose. Granitic fragments are recognized by aggregates of quartz, feldspar and mica, orthoquartzites as cemented quartz grains derived from older sediments, and metaquartzites are derived from gneisses and schists. The source area for the Tapeats Sandstone (Late Precambrian-Cambrian) in the present study appears to be 96 the older Precambrian granitic rocks and their meta morphosed equivalents as well as older sedimentary rocks. There has been little change in source material with time, and the sediments were derived predominantly from the east. Variation in mineralogical and textural maturity is large ly a function of the source area, history of abrasion, rate of deposition and the depositional environment. The deposition appears to be more rapid in the basal portions and the maturity increases upward, which could possibly be related to tectonism, (?) changing from an unstable region with high influx of terrigenous material to a stable shelf environment with excellent reworking. The decrease in feldspars upward in each section may be related to in creased distance from the sedimentary source in the case of a transgressive sequence. Depositional Environment Reconstruction of depositional environments for sedimentary quartzites poses problems, the chief one be ing the scarcity of fossils and lack of paleontological control. Great reliance has to be placed on lithology, sedimentary structures and stratigraphy. The arenites collected from the Tapeats Sandstone in the Marble Mountain section are overlain by the Latham Shale, dominantly an olive-gray siltstone with minor lime stone and sandstone and containing excellent trilobite 97 fossils. Overlying this unit is the Chambless Limestone characterized by the algae Girvenella. The quartzite- shale-limestone sequence is characteristic of a marine environment. In the Mountain Pass section the quartzites show tectonic contact with a parautochtonous slice of Pre cambrian gneiss and the Mesquite Pass section are also in close proximity to the thrust. In the general area the Tapeats Sandstone is overlain by the Bright Angel Shale. These vertical stratigraphic associations may be suggestive of a marine transgressive sequence. Petrographic analysis showed the quartzites as mineralogically submature to supermature. The feldspar content (average 6 percent) may also reflect mineral maturity on grounds that the source area was capable of supplying over 50 percent feldspar. Lanphere (1963) re ports a modal analysis of granodiorite (1*4 b.y.b.p.) with feldspar content 51 percent. The mineral maturity based on quartz/feldspar ratio is not applicable to sands derived from feldspar-poor rocks. Moreover, the decline of feld spar upwards along with increasing maturity is probably related to efficiency of the environment. The basal quartzites were probably rapidly deposited with little ef fects of the shoreline processes, while the supermature sediments (e.g., the upper 28 m in the Marble Mountains) experienced the full effects of the surf filter of the high energy beach environment before deposition offshore. 98 | Presence of glauconite and phosphorite may also be characteristic of marine environments. Glauconite forms mainly between 30 and 880 m, while optimum conditions for the precipitation of phosphorite is shallow warm water be tween 30 and 300 m (Heckel, 1972). In recent sediments, glauconite is largely confined to the central and outer parts of the continental shelf regions, which are character ized by slow deposition or non-sedimentation (Kukal, 1971)• Textural maturity ranges from submature to super- mature on the basis of roundness and sorting. Increased sorting is related to fining of the sediment upwards while poor sorting is associated with increase in matrix, mica and argillaceous rock fragments and may be related to textural inversion coincident with changes in paleocurrent directions. The coarse grain size and local conglomeratic units together with the absence of turbidite sedimentary structures favors a shallow marine environment. The dominant sedimentary structures observed were uniformity of stratification. A fifth of the samples con tain small to medium scale, generally low angle, planar cross-stratification. Regular and nearly parallel lamina tion has been associated with beach foreshore (Thompson, 1937). More recently Allen (1963a and b) associated laminations in sandstones to upper flow regime which could also be attained in the swash backwash region of the beach environment while small-scale cross-stratification is ex 99 plained by migrating assymetrical ripples in a lower flow regime. Clifton, Hunter and Phillips (1971) found strik ing similarity in depositional structures around the near shore environment with those described by Allen (1963a and b). They recognized planar bed forms within the swash zone, shoreward inclined ripple cross-lamination and gently inclined cross-stratification in the assymetric (offshore) facies and medium scale foresets that dip directly or obliquely to sea in the inner rough facies. Migration of facies resulting from changes in waves or tides could provide useful criteria for paleoenvironmental interpre tations . The shallow marine or beach environment most reason ably explains the dominantly coarse- to medium-sized detritus from studies of modern shelf environments. Turbidity currents may carry coarse material into the open ocean environment but there is no evidence in the present study of structures typical of the Bouma sequence. Sedimentary models for the Late Precambrian- Oambrian quartzites also involves tidal conditions at the time of deposition. Two opposing views may be recognized, one proposing that the epeiric seas were tide dominated (Klein, 1970b, 1971a) while the other favors tideless epeiric seas (Shaw, 1964; Irwin, 1965) with wind processes being dominant. More probably it is a combination of these processes, with transport of coarse detritus across the 100 shelf possibly involving high tidal currents as well. These may account for the bimodality of cross-stratifica tion. Houboult (1968) reported internal cross stratification in the sand ridges and considerable dis persal of coarse sediment in the tide-dominated North Sea, where the flood current does not follow path of the ebb current. Stewart (1970) reports predominantly westerly currents in his study of Upper Precambrian and Lower Cambrian sediments in the southern Great Basin. He sug gests currents were of tidal origin at right angles to the isopach line and explains unimodality by citing the ex ample of the Bay of Fundy, where cross-strata develop al most entirely during ebb tides. The shorter-than-present earth-moon distance and extremely high tidal velocities (Merrifield and Lamar, 1968; Olson, 1970) required to transport coarse material to the open ocean and produce large-scale cross-stratification are not justified from evidence available. Paleontologic evidence is restricted to Scolithus tubes and organic burrowing (bioturbation). More fossil evidence might be found in the interbedded siltstones which were not sampled. Most authors agree that few sedi mentary structures are confined to a particular environ ment, and for a significant interpretation a number of associated sedimentary structures must be considered along with gross lithology, mineral content and fossils. Thus 101 in summary, the uniformity of stratification, tabular cross-stratification, numerous diastems, together with largely mature sediments suggests a marine environment probably beach or shallow marine (dominantly subtidal). Furthermore, the supermature quartzites were reworked in the beach environment and laid down in this high energy zone, whereas the submature sediments were more rapidly and loosely deposited seaward from the surf zone. During quiescent periods (stillstands) the interstices in these loose sediments were occupied by finer matrix or fine laminae of silt-clay may be deposited which accentuates the stratification observed in the radiographs. Stewart (1970) considers the Tapeats Sandstone in the areas under present study to be equivalent to the Wood Canyon Formation and Zabriskie Quartzite. Klein (1971a, b) suggests an intertidal environment for these two forma tions in California and Nevada. However, structures characteristic of intertidal environments, such as emergence run-off structures, mud cracks, herring-bone cross-stratification, tidal and flaser bedding (Klein, 1970a; Reineck, 1972) were not observed in the sections under investigation. A fluvial origin for these quartzites also seems improbable, because fluvial sandstones are characterized by thick sets of trough cross-stratification, unimodal cross-stratification, climbing ripples and re stricted lateral continuity of sediments (Allen, 1965; Visher, 1972). Thus negative evidence also favors shallow marine shelf or beach environment. CONCLUSIONS Variation in mineralogy and texture with thickness is accentuated in the polynomial time-trend analysis of weighted data (percent of maximum) for each variable. Matching the three stratigraphic sections from time-trend plots was not rewarding. Although several systematic fluctuations are observed, no parastratigraphic units could be recognized. R-mode dendrographs provided meaningful clusters for the twelve variables measured in the Marble Mountain section. The clusters are related to the stability of the detrital mineral components and post-depositional diagene sis. They are as follows: Cluster I, microcline- plagioclase-orthoclase; Cluster II, chert and accessory minerals and minerals of cluster I; Cluster III, undula- tory quartz and quartzose rock fragments; Cluster IV, mica and matrix; Cluster V, hematitic and limonitic cements; and Cluster VI, normal quartz and silica cement. "Total Quartz1 1 and "Total Quartz plus Quartzose Rock Fragments" were the only variables with sufficient sample size for an 80 percent probability of detecting a 10 percent difference between two means at the 5 percent significance level. Both these variables also fitted con- 103 stant probability model (positive binomial) at the 5 per cent level of significance in all the three sections, indicating homogeneity of distribution in each strati- graphic section. An analysis of variance design showed statistical homogeneity between the Marble Mountain and Mesquite Pass sections with respect to these two variables. The source of variation (determined by the New Duncan Multiple Range test) was ascribed to the Mountain Pass section and was probably due largely to mineralogical and textural immaturity of the sedimentary deposits and also the post-lithification deformation probably related to the Keystone thrusting event. Radiographic analysis shows dominantly uniform stratification, cross-stratification in about a fifth of the samples collected, and numerous diastems. Foreset azimuth dips indicate dominantly westerly currents but easterly and northerly current directions are also observed suggesting possible bimodal tidal current directions in a shallow shelf environment and long-shore (?) currents to account for the third mode. Data was not sufficient for significant conclusions. 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D. and Weir, G* W., 1953* Terminology for stratification and cross-stratification in sedimentary rocks: Geol. Soc. America Bull., v. 64, p. 381-390. McNair, A. H., 1951* Paleozoic stratigraphy of part of northwestern Arizona: Am. Assoc. Petroleum Geologists Bull., v. 35, P* 503-541. Merrifield, P. M. and Lamar, D. L., 1968, Sand waves and early earth-moon history: Jour. Geophys. Research, v. 73, P* 4767-4774. Miola, R. J. and Weiser, D., 1968, Textural parameters: an evaluation: Jour. Sed. Petrology, v. 38, p. 45- 53* Nolan, T. B., 1929* Notes on stratigraphy and structure of the northwestern portion of Spring Mountains, Nevada: Am. Jour. Sci., v. 17, p. 461-472. Nolan, T. B., Merriam, C. W., and Williams, J. S., 1956, The stratigraphic section in the vicinity of Eureka, Nevada: U. S. Geol. Survey Prof. Paper 276, p. 6-7. Noble, L. F., 1914, The Shinumo quadrangle, Grand Canyon district, Arizona: U. S. Geol. Survey Bull., v. 549, 100 p • Olson, W. 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Creator Lobo, Cyril Francis (author)

Core Title Petrography and statistical analysis of the Tapeats Sandstone (late Precambrian-Cambrian), southeastern California

Degree Master of Science

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

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Language English

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Permanent Link (DOI) https://doi.org/10.25549/usctheses-c30-114350

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