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Seafloor precipitates and carbon-isotope stratigraphy from the neoproterozoic Scout Mountain member of the Pocatello Formation, Southeast Idaho: Implications for neoproterozoic Earth history

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Seafloor precipitates and carbon-isotope stratigraphy from the neoproterozoic Scout Mountain member of the Pocatello Formation, Southeast Idaho: Implications for neoproterozoic Earth history

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Content SEAFLOOR PRECIPITATES AND C-ISOTOPE STRATIGRAPHY FROM THE NEOPROTEROZOIC SCOUT MOUNTAIN MEMBER OF THE POCATELLO FORMATION, SOUTHEAST IDAHO: IMPLICATIONS FOR NEOPROTEROZOIC EARTH HISTORY by Nathaniel James Lorentz 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 2003 Copyright 2003 Nathaniel J. Lorentz Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1417930 Copyright 2003 by Lorentz, Nathaniel James All rights reserved. INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. ® UMI UMI Microform 1417930 Copyright 2004 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90089-1695 This thesis, written by N a th a n ie l James L orentz under the direction of h i s thesis committee, and approved by all its members, has been presented to and accepted by the Director of Graduate and Professional Programs, in partial fulfillment of the requirements for the degree of M aster o f S c ie n c e in th e G e o lo g ic a l S c ie n c e s Director D ate August 1 2 . 2003 Thesis Committee Chair Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS I am grateful to Dr. Frank Corsetti, Dr. Paul Link, and Dr. David Bottjer for overseeing this project and thesis; Jay Kaufman for isotopic analyses and discussion; Dave Rodgers for map support; Chad Wittkop for field assistance and helpful discussions; Matthew Clapham for insightful comments regarding this thesis; Nicole Bonuso, Pedro Marenco, Alison Olcott, and Sara Pruss for helpful discussions regarding portions of this thesis; and the Geological Society of America, the Evolving Earth Foundation, and the USC Earth Sciences Department for research funds. Also, thanks to Dennis Johnson and Cathy Summa for great beginnings. Last, but not least, thanks and love to all of my family and friends for their support. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Acknowledgments.....................................................................................................ii List of Tables..............................................................................................................v List of Figures.......................................................................................................... vi Abstract...................................................................................................................viii Chapter 1. The purpose of this report..................................................................... 1 Chapter 2. The geologic history of the Pocatello Formation, Pocatello and northern Bannock Range, southeastern Idaho........................................................ 7 2.1. Introduction........................................................................................................ 7 2.2. Deposition of the Pocatello Formation.............................................................8 2.3. Geologic history...............................................................................................10 2.3.1. Geologic history of the Pocatello and northern Bannock Ranges........... 10 2.3.2. Cordilleran events not recorded in the Pocatello area...............................11 Chapter 3. Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic Earth history......................................... 13 3.1. Introduction to the glacial record in southeastern Idaho..............................13 3.2. Stratigraphic setting of the Pocatello Formation...........................................18 3.3. Methods.............................................................................................................29 3.4. Observations..................................................................................................... 31 3.4.1. Unusual lithofacies....................................................................................... 31 3.4.2. Isotopes.......................................................................................................... 47 3.5. Discussion........................................................................................................ 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.5.1. Seafloor precipitates.....................................................................................55 3.5.2. Isotopic interpretations.................................................................................56 3.5.3. Is the ‘Carbonate and Marble’ unit a cap carbonate?................................57 3.5.4. Addressing arguments against a post-glacial origin for the ‘Carbonate and Marble’ from previous glacial perspectives..................................................59 3.5.5. Does the ‘Carbonate and Marble’ unit represent a cap-like carbonate deposited independent of post-glacial processes?................................................60 3.5.6. Implications for global 61 3 C curves............................................................ 61 3.6. Conclusions...................................................................................................... 62 Chapter 4. Radiometric constraints on “snowball Earth” strata........................ 64 4.1. Introduction and methods................................................................................64 4.2. Results...............................................................................................................64 4.3. Discussion and implications........................................................................... 66 Chapter 5. Concluding remarks: the climate of Neoproterozoic climate.........72 References................................................................................................................73 Appendix 1...............................................................................................................88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table 1. Isotopic values from the pink cap dolostone........................................ 48 Table 2. Isotopic values from three along-strike sections of the ‘carbonate and marble’..................................................................................................................... 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure 1. Illustration of the Kaufman et al. (1997) vs. Kennedy et al. (1998) debate......................................................................................................................... 2 Figure 2. Typical “snowball earth” sedimentary and geochemical association..................................................................................................................3 Figure 3. The location of the study sites near Pocatello, Idaho.........................6 Figure 4. Stratigraphic column of the Pocatello Formation, southeastern Idaho. ................................................................................................................................... 15 Figure 5. Cobble conglomerate of the Scout Mountain Member..................... 19 Figure 6. Upper glaciogenic diamictite of the Scout Mountain Member........20 Figure 7. A striated clast from the upper diamictite of the Scout Mountain Member....................................................................................................................21 Figure 8. Erosive basal contact of the cobble conglomerate of the Scout Mountain Member...................................................................................................24 Figure 9. An iron-rich turbidite from below the upper diamictite of the Scout Mountain Member...................................................................................................25 Figure 10. The pink, finely laminated, dolostone “cap” of the Scout Mountain Member.....................................................................................................................26 Figure 11. A polished hand sample of the pink dolostone..................................27 Figure 12. Pink dolostone-chip breccia................................................................30 Figure 13. Possible sheetcrack cements...............................................................32 Figure 14. Pseudo-tepee structures....................................................................... 33 Figure 15. Seafloor fans suggesting three-dimensional radiation.....................35 Figure 16. Seafloor fans recognized by textural contrasts.................................36 Figure 17. Laterally continuous fan horizons...................................................... 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 18. A polished hand sample horizontally through a fan horizon showing vii undulose laminae between fan clusters.................................................................39 Figure 19. Seafloor fan casts intercalated with siliciclastics............................. 40 Figure 20. Vertical cross-section photomicrograph of a seafloor fan............... 42 Figure 21. Plan view photomicrograph of seafloor fans with flat abutments...44 Figure 22. Plan view photomicrograph of seafloor fans with hexagonal growth habit.......................................................................................................................... 46 Figure 23. Isotopic values through the pink dolostone unit.............................. 49 Figure 24. Carbon isotope values through multiple along-strike sections (1, 2, and 3) of the carbonate and marble unit of the Scout Mountain Member.........51 Figure 25. Cross-plots of carbon and oxygen isotopes from sections 1, 2, and 3 of the carbonate and marble unit of the Scout Mountain Member.....................52 Figure 26. A composite section of 6,3C values through the Neoproterozoic succession near Pocatello, ID.................................................................................53 Figure 27. Global distribution of Neoproterozoic glacial deposits...................65 Figure 28. Correlations proposed between the Neoproterozoic successions in Pocatello, ID and Death Valley, CA...................................................................... 67 Figure 29. Radiometric constraints on Neoproterozoic glacial strata.............. 70 Figure 30. Locations of Neoproterozoic glacial deposits along the Cordillera..................................................................................................................89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT viii A ‘carbonate and marble’ unit tops the Scout Mountain Member of the Neoproterozoic Pocatello Formation. It contains unusual carbonate textures and records negative 81 3 C values similar to Neoproterozoic “cap carbonates”, but no glacial strata have been recognized directly underlying this unit, and it does not conform to generally accepted glacial intervals. It may be an actual cap carbonate in the absence of an underlying glaciogenic diamictite, thus representing the aftermath of an unrecognized glaciation. Alternatively, it may be a cap-like carbonate produced by non-glacial processes. Analysis of radiometric constraints on Neoproterozoic glacial successions worldwide shows that Neoproterozoic glaciations happened more than twice and were not synchronous. As only two glaciations are currently recognized in the Pocatello succession, it is plausible that the carbonate and marble represents additional glaciation, and previous correlations along the Cordillera may need to be reconsidered. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 1. The purpose of this report It is generally accepted that low-latitude, global-scale glaciations occurred in Neoproterozoic time (e.g., Sohl et al., 1999), though the nature of such glaciations, including how many times glacial ice approached equatorial latitudes at sea level, is debated (Fig. 1) (Kaufman et al., 1997; Kennedy et al., 1998). A stratigraphic pattern is associated with low-latitude glacial events in most Neoproterozoic basins around the world (Kaufman et al., 1997; Hoffman et al., 1998a; Kennedy et al., 1998): glaciogenic deposits, which may be rich in ferric iron, are overlain by unusual carbonates termed “cap carbonates” (Fig. 2). Cap carbonates have unusual qualities that suggest they are the direct and subsequent byproducts of aberrant ocean chemistries imposed by these massive glaciations. They most commonly consist of finely laminated, allodapic dolostone (Hoffman and Schrag, 2002). They may display large crystal arrays (seafloor precipitates), subaqueous tepee structures, rollup structures, and other unusual carbonate textures (Kennedy, 1996; James et al., 2001; Hoffman and Schrag, 2002); often record negative 81 3 C values (Kaufman et al., 1997; Kennedy et al., 1998); may record positive 83 4 S values (Hurtgen et al., 2002); and are interpreted to have been deposited during post-glacial transgression (Kennedy, 1996; Hoffman and Schrag, 2002) as a transgressive systems tract. Given the apparent global distribution of the aforementioned stratigraphic patterns, interpreting Neoproterozoic Earth history is extremely challenging. Biostratigraphic indicators are rare and ambiguous (Knoll, 2000). Many chemostratigraphic markers, such as 51 3 C and 8 7 Sr/8 6 Sr, are altered and/or equivocal (Kaufman et al., 1997; Kennedy et al., 1998; Knoll, 2000) and others Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1. Illustration of the Kaufman et al. (1997) vs. Kennedy et al. (1998) debate. Kennedy et al.’s interpretation is significant because it allows for only two glaciations to be recognized in any particular Neoproterozoic succession. Alternatively, Kaufman et al. interpret more than two Neoproterozoic glaciations by analysis of geochemical trends, interpreted to represent the aftermath of glaciation, independent of whether they are associated with glaciogenic deposits. Note that both assume Neoproterozoic glaciations were global and thus synchronous. Figure 1. after Kennedy et al., 1998 Global 813C VPDB after Kaufman et al., 1997 Global 8 l3 c PDB -8 -4 0 +4 +8 ■8 -4 0 +4 +8 M a ■ 1 ■ * | 1 1 1 * 500 550 600 700 650 * I I I I I * t « I I I I I 750 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 2. Typical “snowball earth” sedimentary and geochemical association (-750 Ma) showing an iron-rich glaciogenic deposit immediately overlain by a carbonate with negative 51 3 C and positive 63 4 S values, with a subsequent return to previous ocean chemistries (compiled from many sources listed in the text). Figure 2. post-glacial units cap carbonate glaciomarine sediments pre-glacial units .......... Xl3r ^carbonate -10 0 +10 ^34S(sulfate) 10 20 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (e.g., 53 4 S) are not yet well developed. Major erosional surfaces (sequence boundaries) can be useful for interregional correlation, but they can have local tectonic overprinting nullifying their implications for intercontinental correlation. In combination, the integration of different stratigraphic techniques can be more successful in solving Neoproterozoic correlation problems, but without radiometric constraints their usefulness is ultimately limited. Unfortunately, radiometric constraints in Neoproterozoic successions are rare (Link et al., 1993), though new data are accumulating {Colpron, 2002 #28;Lund, 2003 #35}. Nonetheless, correlations have been suggested and interpretations have been made. This has led to a number of unresolved debates. With regard to the nature of Neoproterozoic glaciation, the mechanism(s), magnitude, timing and duration, and number of events (almost everything!) remains debated (e.g., Kaufman et al., 1997; Kennedy et al., 1998; Evans, 2000; Hoffman and Schrag, 2002). The aim of this thesis is to assess the validity of two popular points regarding Neoproterozoic glaciation: that such glaciations happened only twice (“Sturtian” - ca. 750-700 Ma and “Marinoan” - ca. 600 Ma) (Kennedy et al., 1998) and were synchronous (Kirschvink, 1992; Hoffman et al., 1998b). As discussed above, stratigraphic techniques employed without radiometric tie points are of limited utility, thus it seems fair to reassess these issues from a simple, objective viewpoint: from existing lithologic evidence and age constraints of these glaciogenic units, what conclusions about these topics can we reasonably and rigorously derive? To address this topic, I have 1) analyzed Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. new lithologic and chemical evidence of glaciation from the Neoproterozoic Pocatello Formation in southeastern Idaho and 2) analyzed existing radiometric constraints on Neoproterozoic glacial strata worldwide. Specifically, this study examines a ~26 meter thick carbonate unit within the Scout Mountain Member (SMM) of the Pocatello Formation north of the Portneuf Narrows near Pocatello, Idaho described by Link (1987) as the ‘carbonate and marble’ unit of the SMM (found at stop 3 of Link, 1987; Fig. 4). It is stratigraphically above the glacial units in the SMM but far below the incised valleys in the Caddy Canyon Quartzite. Fanning and Link (in prep) report a U-Pb SHRIMP zircon age of 667 ± 5 Ma from a fallout tuff immediately below the carbonate unit. The carbonate is unusual because it displays lithofacies and isotopic values similar to the aforementioned cap carbonates, but does not rest directly on known glacial strata, nor does it fall within the temporal window of the older Sturtian glaciation(s) and the younger Marinoan glaciation(s). As seafloor precipitates are rare in post- Paleoproterozoic time (Sumner and Grotzinger, 1996) but more common in the unusual carbonates that cap certain Neoproterozoic glacial strata (Hoffman et al., 1998a; Kennedy et al., 1998), their discovery in the Pocatello Formation is significant for our understanding of Neoproterozoic Earth History. Chapter 2 provides an overview of the deposition and geologic history of the Pocatello Formation in the study area. Chapter 3 addresses new evidence of glaciation in the Pocatello Formation. Chapter 4 analyzes radiometric constraints of Neoproterozoic glaciations worldwide. Last, Chapter 5 contains a summary and concluding remarks. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 3. The location of the study sites, stops 2 and 3 from Link (1987). Base map is the Inkom 7.5-minute quadrangle near Pocatello, ID. Figure adapted from Link (1987). Figure 3. 1 km 112°2r 30" Stop 3 of Link, 1987— A Blackrock W Canyon Road BLM : Road Old Highway 91 Portneuf Road 42°47' 30' Fort Hall Mine Road —Stop 2 of Link, 1987 Pocatello, Idaho Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 2. The geologic history of the Pocatello Formation, Pocatello and northern Bannock Range, southeastern Idaho 2.1. Introduction The following is an overview of the geologic history of the Neoproterozoic Pocatello Formation in the Pocatello and northern Bannock Ranges, southeast Idaho, in order to place the units described in detail later in this thesis in geologic context. The Pocatello area is of wide geological interest because it straddles the hingeline between miogeocline and craton as well as thrust zones and hinterland of the Sevier Orogeny (Link, 1983; LeFebre, 1984; Link et al., 1985; Burgel et al., 1987; Kellogg, 1992). The Pocatello area is also interesting for this study because the Pocatello Formation contains a record of glaciation (Ludlum, 1942; Crittenden et al., 1971; Trimble, 1976; Crittenden et al., 1983; Link, 1983; Link et al., 1994) from a time when ice ages are thought to have been of a global scale (e.g., Kirschvink, 1992; Evans, 2000; Hoffman and Schrag, 2002). Such glaciation may have caused or played a role in the evolutionary development of animals after billions of years of exclusively “simple” organisms existing on the planet (Hoffman et al., 1998b; Corsetti et al., 2003). This is a major area of interest for those who study the Pocatello Formation and correlative units along the Cordillera, from northern Canada to Caborca, Mexico (Stewart, 1972; Stewart, 1991; Link et al., 1993). The Pocatello Formation crops out in the Pocatello and Bannock Ranges, south and east of Pocatello, ID and west of the Portneuf Range. These are approximately north-south trending ranges that lie in the Paris-Putnam-Willard Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thrust sheet of the Idaho-Wyoming thrust belt and in the northernmost Basin and Range, bounded to the north by the Snake River Plain (Trimble, 1976). The Pocatello Range includes a local high, Chinese Peak (formerly Chinks peak). 2.2. Deposition of the Pocatello Formation Neoproterozoic rocks of western North America are typically divided into a lower diamictite and volcanic succession and an upper terrigenous detrital succession (Crittenden et al., 1972; Stewart, 1972; Stewart and Poole, 1974; Stewart and Suczek, 1977; Stewart, 1991; Link et al., 1993). In this context, the Pocatello Formation represents the diamictite and volcanic succession in southeastern Idaho. Of the Pocatello Formation, the Bannock Volcanic Member constitutes the “volcanic” and the Scout Mountain Member constitutes the “diamictite”. Rocks from this time are thought to record the rifting of Rodinia (Link et al., 1993), a Proterozoic supercontinent resulting from the Grenville orogenesis event. Chemistry of the Bannock volcanics is consistent with intraplate rifting and these rocks are thought to be correlative along the Cordillera with other presumed syn-rift volcanics such as the Irene and Leola volcanics in NE Washington and the Golden Cup volcanics in central Idaho (Stewart, 1972; Stewart and Suczek, 1977; Harper and Link, 1986; Lund et al., 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Timing and progression of rifting remains uncertain. Volcanism and abrupt facies changes have been interpreted to suggest Rodinian rifting initiated ca. 750-700 Ma (Stewart, 1972; Stewart and Poole, 1974; Stewart and Suczek, 1977; Ross, 1991; Stewart, 1991; Heaman et al., 1992; Karlstrom et al., 2000; Timmons et al., 2001). These data are in apparent conflict with tectonic subsidence models that predict the development of a passive margin ca. 600- 550 Ma (Armin and Mayer, 1983; Bond, 1984; Bond et al., 1984; Bond et al., 1985; Devlin and Bond, 1988; Levy and Christie-Blick, 1991). This conflict was reconciled by the general acceptance of a two-phase rift scenario (e.g., Prave, 1999). However, recent work by Colpron et al. (2002) and Lund et al. (2003), for example, suggests a reevaluation of two-phase rifting by providing support for multiple and/or stepwise rifting events, casting doubt on the synchroneity of rifting along the Cordillera. For example, rift-related volcanics of the Edwardsburg Formation of central Idaho are dated at ca. 685 Ma (Lund et al., 2003) fitting neither the early or late rifting phase suggested above. Sedimentary rocks of the Scout Mountain Member are interpreted to record a variety of shallow to deep marine environments. The diamictites of the Scout Mountain Member themselves are interpreted as subaqueous lodgement tillite or flow tillite (massive diamictite), or as deep-water mass flows (interbedded diamictite and graded sandstone) (Link, 1983). They are also thought to be correlative along the Cordillera with, for example, lower strata of the Canadian Windermere Supergroup and the Surprise Diamictite of the Death Valley Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. region’s Kingston Peak Formation (Lund et al., 2003). The Scout Mountain Member is thought to have been deposited coevally with the above-mentioned volcanics during the apparent incipient breakup of the Rodinia. By ca. 600 Ma rifting had led to the formation of the proto-Pacific ocean and an Atlantic-style passive margin allowing for deposition of thickening- westward sediment packages collectively termed the Cordilleran Miogeocline. The miogeocline saw continued deposition and remained relatively undeformed until Late Devonian time (Burchfiel et al., 1992; Poole et al., 1992). 2.3. Geologic History 2.3.1. Geologic history o f the Pocatello and northern Bannock Ranges LeFebre (1984) presents a five-part deformational history for the Pocatello and northern Bannock Ranges in the Pocatello area. 1) (Potential) middle Jurassic greenschist facies metamorphism of a regional extent: responsible for the ubiquitous and consistent grade of metamorphic rocks of the area. This metamorphism likely predates the earliest thrusting in the area, and thus is at least latest Jurassic in age. 2) Folding and faulting from the Sevier Orogeny next affected the area (see Armstrong and Oriel, 1965; Armstrong, 1968; Armstrong, 1972; Speed, 1983; Allmendinger and Jordan, 1984; Speed et al., 1988; Allmendinger, 1992; Burchfiel et al., 1992; Saleeby et al., 1992). Third, east-trending, high-angle faulting: absolute age of these faults is unknown, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. but they are relatively younger than subsequent Cenozoic Basin and Range extension. 4) North-trending Basin and Range faulting: the ridge containing Chinese Peak is a Basin and Range horst. 5) Low-angle gravity sliding: likely post-dates Basin and Range deformation, transporting sedimentary blocks down slope in many areas. The ridge containing Chinese Peak north of the Portneuf Narrows contains overturned strata juxtaposed against upright strata to the immediate south (this area is the primary focus of this investigation, figure 4). It is thought that folding and thrusting along the Putnam thrust associated with the Sevier Orogeny 1) created an recumbent antiform 2) which was planed off by a later thrust fault (Blackrock Canyon thrust) and 3) subsequently cut by east-west- striking tear faults placing overturned strata next to upright strata (LeFebre, 1984; Link et al., 1985; Burgel et al., 1987). See Kellogg (1992) for an alternate interpretation. 2.3.2. Cordilleran events not recorded in the Pocatello area The Cordillera underwent numerous major tectonic events that are not recognized in the Pocatello area. In late Devonian time, offshelf strata of the Antler assemblage were thrust ~200 km eastward on the Roberts Mountain thrust during the Antler Orogeny, signifying contraction and an end to western North American passive margin deposition (Roberts et al., 1958; Poole et al., 1992). This was followed in Mississippian time by deformation that produced Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. East-West trending troughs that by Pennsylvanian time displayed up to 6 km of displacement, called the Ancestral Rocky Mountains (Burchfiel, 1979; Kluth and Coney, 1981; Burchfiel et al., 1992). In the Permian, the Sonoma Orogeny thrust rocks of the Havallah Basin eastward on the Golconda thrust, but not to the eastward extent of the previous Antler assemblage (Speed, 1977; Gabrielse et al., 1983; Brueckner and Snyder, 1985). In Triassic time, numerous offshore terranes began to accrete along the western margin of the Cordillera (Monger, 1977; Davis et al., 1978; Monger et al., 1982; Saleeby, 1983; Miller, 1987; Armstrong, 1988; Gabrielse and Yorath, 1989). In late Cretaceous time shallowing of the subducting Farallon Plate shut off magmatism in the western Cordillera and caused deformation and magmatism to migrate eastward, reactivated zones of weakness from the Ancestral Rockies, forming the Laramide Rocky Mountains (Laramide Orogeny) (Armstrong, 1974; Burchfiel and Davis, 1975; Coney and Reynolds, 1977; Keith and Wilt, 1986; Burchfiel et al., 1992; Cowan and Bruhn, 1992). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 3. Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic Earth history 3.1. Introduction to the glacial record in southeastern Idaho The southeastern Idaho Neoproterozoic succession is generally interpreted to record two episodes of glaciation. Evidence for the first is found in two diamictites of the SMM (Fig 3). New SHRIMP U-Pb zircon ages suggest that the SMM contains rocks deposited during a late phase of the Sturtian glaciation (less than 715 Ma) in line with previous correlations (Crittenden et ah, 1983; Link and LeFebre, 1983; Link and Smith, 1992; Link et ah, 1993). Fanning and Link (in prep.) dated an epiclastic crystal tuff bed in the Bannock Volcanic Member at 711 ± 4 Ma and a rhyolite clast within the upper SMM diamictite at 717 + 4 Ma. Interestingly, Lund et al. (2003) present 685 + 7 and 684 + 4 Ma SHRIMP U-Pb zircon ages from felsic volcanic rocks intercalated with mafic volcanics and diamictites of the Edwardsburg Formation in central Idaho that have been correlated with the Bannock volcanics and SMM diamictites; given the new age constraints on the Pocatello Formation, this correlation is questionable. The uppermost SMM diamictite unit is overlain by a thin, pink, finely laminated dolostone unit that records negative 61 3 C values (Smith et al., 1994) and thus fits the previously described pattern common in Neoproterozoic stratigraphic successions. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Evidence for a second glaciation is inferred from the incised valleys of the Caddy Canyon Quartzite ~2000 meters above the Pocatello Formation, interpreted as representing sea level drawdown associated with “Marinoan” glaciation (commonly considered to be ca. 600 Ma) (Fig. 4) (Fevy et al., 1994). The Browns Hole Formation, stratigraphically 500 to 1000 m above the Caddy Canyon Quartzite, contains an extrusive unit dated at 580 ± 7 Ma (Ar4 0 -Ar3 9 date recalculated by Christie-Blick and Fevy (1989), using updated decay constants, hence constraining the incised valleys to be older than ~580 Ma). Of these events, only the Pocatello Formation contains direct evidence for glaciogenic deposition with an associated cap carbonate. A potential third, older glaciation may be indicated through correlation. Two ice ages are reported from the Formation of Perry Canyon, Fremont Island, Utah, the younger of which is correlated with diamictites of the SMM of the Pocatello Formation; the older is thus inferred to correlate with strata not exposed in the Pocatello Formation (Crittenden et al., 1983). This study examines a ~26 meter thick carbonate unit within the SMM of the Pocatello Formation north of the Portneuf Narrows near Pocatello, Idaho described by Fink (1987) as the ‘carbonate and marble’ unit of the SMM (found at stop 3 of Fink (1987); Fig. 3). It is stratigraphically above the glacial units in the SMM but far below the incised valleys in the Caddy Canyon Quartzite (Fig. 4). Fanning and Fink (in prep.) report a U-Pb SHRIMP zircon age of 667 ± 5 Ma from a fallout tuff immediately below the carbonate unit. The carbonate is unusual because it displays lithofacies and isotopic values similar to the aforementioned cap carbonates, but does not rest directly on Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4. Stratigraphic column of the Pocatello Formation, southeastern Idaho (see figure 4 for locality information). Previous workers suggested that the succession directly records two episodes of glaciation represented by 1) the diamictites plus cap carbonate of the Pocatello Formation (Sturtian; Smith et al., 1994), and 2) the sequence boundary within the Caddy Canyon Quartzite (putative Marinoan; Levy et al., 1994). A third, older glaciation, presumably not exposed in the Pocatello Formation, is inferred through correlation with the formation of Perry Canyon (not shown here) (Crittenden et al., 1983). Generalized columns adapted from Link et al. (1993). Radiometric dates from Fanning and Link (in prep.) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fisure 4. Pocatello, ID Sedgw ick PeakQ tZ / W indy Pass Argillite Caraelback Mountain Qtz Mutual Fm 580 Ma Q lnkom Fm F a- a'Mannoan Caddy Canyon Papoose Creek Fm 661+1-5 Ma Blackrock Canyon Lms Sturtian 717+/-4 Ma (Clast) 711+/-4 M a Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4 continued. Limestone/marble Limestone/silty Dolostone Conglomerate Sandstone Siltstone/shale Shale Diamictite Volcanic I I •• • • • • •• • » < Z=Z J —L — * o. ^ jo, c> ^ o •. o . • n^V rri Sequence Boundary Fe Iron-rich Strata ▲ Glaciogenic Deposit 8 C 1 O Negative 5 C Values Seafloor Precipitates Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. known glacial strata. As seafloor precipitates are rare in post-Paleoproterozoic time (Sumner and Grotzinger, 1996) but more common in the unusual carbonates that cap certain Neoproterozoic glacial strata (Hoffman et al., 1998a; Kennedy et al., 1998), their discovery in the Pocatello Formation is significant for our understanding of Neoproterozoic Earth History. 3.2. Stratigraphic Setting of the Pocatello Formation The Pocatello Formation is divided into three members: the Bannock Volcanic Member, the SMM, and the (informal) upper member (Link, 1983) (Fig. 4). The basal Bannock Volcanic Member consists of greenschist metabasalts and volcanic breccias including angular metabasalt clasts recrystallized to chlorite and albite. It exists as a lenticular body within the overlying SMM, reaching a thickness of 400 meters north of the Portneuf Narrows (Crittenden et al., 1983). Chemistry of the Bannock volcanics is consistent with intra-plate, rift- related volcanism (Harper and Link, 1986; Link and Smith, 1992). The medial SMM consists of two glaciogenic diamictites separated by sandstones, siltstones, and a massive cobble conglomerate (Fig. 5). The diamictites have been considered stade deposits within a single glaciation (Crittenden et al., 1983), the lower interpreted as subaqueous sediment gravity flows and the upper interpreted as subaqueous lodgement tillite or flow tillite (Figs. 6&7) (Link, 1983; Link et al., 1994). There is lateral facies variation of the conglomerate, and depending on the section, Link (1983) attributes the conglomerate between the diamictites to deep-water channel-fill or shallow Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5. Cobble conglomerate of the Scout Mountain Member at stop 2 19 Figure 5. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 6. Upper glaciogenic diamictite of the Scout Mountain Member at stop 3. Figure 6. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 7. A striated clast from the upper diamictite of the Scout Mountain Member at stop 3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 Figure 7. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. water/fluvial deposition. A disconformity is observed locally below the conglomerate south of the Portneuf Narrows (Fig. 8B). Iron-rich turbidites occur in the interval immediately below the uppermost diamictite south of the Portneuf Narrows, described as Stop 2 by Link (1987) (Figs. 4&9). At most localities, a thin, finely laminated pink dolostone with negative 61 3 C values lies in depositional contact above the uppermost diamictite and is interpreted as the post-glacial cap carbonate for the section (Figs. 10&11). A sequence boundary incised into the dolostone is overlain by a transgressive sequence of sandstone, siltstone, and carbonate (studied here). The (informal) upper member of the Pocatello Formation is composed of more than 600 meters of laminated argillite/shale, siltstone, and quartzite (Crittenden et al., 1971; Trimble, 1976; Crittenden et al., 1983; Link, 1983). The section north of the Portneuf Narrows is overturned, which led to some confusion in terminology (Trimble, 1976); I use the terminology of Link (1983). The upper member has been interpreted to represent a global sea level rise event (Link and LeFebre, 1983; Link and Smith, 1992). This study considers outcrops of the SMM’s uppermost carbonate and marble unit at stop 3 of Link (1987) north of the Portneuf Narrows, three miles southeast of Pocatello, Idaho on the Inkom 1:24,000 Topographic Quadrangle (Fig. 3). At this locality, the upper diamictite is overlain by pink dolostone chip breccia interpreted to represent clasts of the aforementioned cap carbonate Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 8. A) Cobble conglomerate of the Scout Mountain Member and B) 24 erosive basal contact of the conglomerate at stop 2. Figure 8. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 9. An iron-rich turbidite from below the upper diamictite of the Scout 25 Mountain Member at stop 2. Figure 9. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 10. The pink, finely laminated, dolostone “cap” of the Scout Mountain 26 Member at stop 2. Figure 10. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 11. A polished hand sample of the pink dolostone from stop 2. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 11. l i t I cm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. brecciated during cutting of the overlying erosional sequence boundary (Fig. 12). Overlying the sequence boundary is ~50 meters of poorly sorted, medium-to-coarse-grained arkosic sandstone, brown/burgundy in color with a steel grey weathered tint, and containing abundant planar and trough cross- stratification, dish structures, and flute casts. The sandstone fines upward into ~40 meters of grey siltstone containing asymmetric and climbing ripples, load casts, and rare intercalated grey limestone. The 667 Ma fallout tuff bed occurs near the middle of this succession (Fanning and Link, in prep). The carbonate and marble unit is the next prominent cliff-forming unit and is up to 26 meters thick. The lowermost exposures of the carbonate and marble unit consist of pink, finely crystalline calcite marble with wavy, silty thin beds and laminae spaced centimeters apart through the unit. The unit’s color shifts from pink within the first six meters to creamy white and grey up-section. The silty laminae weather tan to buff throughout, though they are not pronounced on fresh surfaces. Petrographic analyses of the unit reveal that the carbonate is chiefly moderately recrystallized, laminated micrite. Further description of this unit is included below in the ‘ U n u su a l L ith o fa cies’ section. The upper member overlies the carbonate and marble with a gradational contact and consists of phyllitic shale in the study area (Link, 1987). 3.3. Methods The uppermost carbonate and marble unit of the SMM was sampled at one- meter intervals at three sections along strike. Also, the pink dolostone unit atop the Scout Mountain Member diamictite was sampled at the cm scale Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 12. Pink dolostone-chip breccia from stop 3. 30 Figure 12. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. at one section at stop 2. Samples were cut to thin-section dimensions, wet polished with 120 and 320 grit paper, and dried with compressed air until all visible moisture had evaporated. After air-drying at least an additional 24 hours at room temperature, the samples were drilled with one-millimeter carbide or diamond drill bits and the rock powders were collected. The samples were carefully microdrilled to avoid secondary phases. For these samples, lighter color corresponded to increased recrystallization; therefore, darker areas were preferentially drilled over lighter and cracks and fissures in samples were avoided as they were subject to preferential fluid flow and likely to enhance chemical alteration. Isotopic (61 3 C, 61 8 0 ) analysis of the microdrilled powders was performed at the University of Maryland in the lab of Dr. A. J. Kaufman on a Micromass IsoPrime dual-inlet gas-source stable-isotope mass-spectrometer with a peripheral MultiPrep system for on-line carbonate reactions. Uncertainties using this method of analysis are better than 0.05 % o for both carbon and oxygen isotopes. Additionally, a 8 7 Sr/8 6 Sr value was acquired from the base of the carbonate and marble unit. 3.4. Observations 3 .4 .1 . U n u su a l lith o fa c ie s Unusual lithofacies in this carbonate and marble unit include ubiquitous seafloor-precipitated carbonate cements, possible sheet crack cements (Fig. 13), and pseudo-tepee structures (Fig. 14) (Lorentz et al., 2002). Sheet crack cements average 2-4 cm in length and several mm in thickness and are spaced on average 2-4 cm apart. Pseudo-tepee structures are approximately 3 cm Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 13. Possible sheetcrack cements at stop 3 with a hammer handle for scale. Figure 13. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 14. Pseudo-tepee structures at stop 3. 33 Figure 14. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in height with a 10 cm wavelength. They are not likely true tepee structures indicative of subaerial exposure as no other signs of desiccation are observed throughout the unit and it occurs within an upward deepening succession. Probable seafloor precipitate horizons vary in thickness from several mm up to two cm. They are most pronounced in the pink carbonate and occur as contrasting yellow crystal needles in an upward oriented, fan-like radiating arrangement (Fig. 15). Such fans can also be recognized higher in the unit, where color contrasts are not prominent, by the same textural contrasts (Fig. 16). Figure 17 shows fan layers that can be traced for the length of the outcrop and have three-dimensional aspect, consistent with precipitation on the seafloor. Figure 18 shows two sedimentary horizons following the undulose topography between fan clusters, implying that crystal growth was typically faster than ambient sediment rain, although examples are also found where silty beds and laminae appear to blanket the fans and episodically terminate fan growth (Fig. 19). In microscopic view, individual fan crystals measure 0.5-1.0 mm in width and are surrounded by micrite (Figs. 20-22). Crystals commonly appear in clusters and less frequently are in contact with each other creating flat abutments (Fig. 21). The boundaries between the crystals and the surrounding micrite are sharp and distinct. Crystals have a hexagonal plan view cross-section and are filled with a coarse, sparry calcite mosaic (Fig. 22). The spar crystals range from 0.2-0.4 mm in diameter; in general, each crystal is 1/2 to 1/3 the width of a fan. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 15. A) Seafloor fans from stop 3 suggesting three-dimensional radiation 35 and B) a thin section of similar clustered fans from stop 3 scanned with plain, non-polarized light. Figure 15. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 16. Seafloor fans recognized from stop 3 by textural contrasts. 36 Figure 16. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 17. Laterally continuous fan horizons at stop 3 along the length of an outcrop including prominent intercalated siliciclastics consistent with seafloor deposition and intermittent seafloor fan smothering by siliciclastics; the oval in A is the comer in B. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 18. A polished hand sample horizontally through a fan horizon from stop 3 showing undulose laminae between fan clusters, suggesting fan growth was faster than sediment rain. Figure 18. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 19. A) From stop 3, seafloor fan casts intercalated with siliciclastics 40 and B) seafloor fans above a coarse sand base. Figure 19. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 19 continued. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 20. Vertical cross-section photomicrograph of a seafloor fan from stop 3 with plain polarized light. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 20. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission Figure 21. Plan view photomicrograph of seafloor fans from stop 3 with crossed nicols. These fans seem to have grown into each other, creating flat abutments, though each maintaining a hexagonal growth habit. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 21. r’ x r-y £5 ilgB illl iiaai h h h Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 22. Plan view photomicrograph of seafloor fans from stop 3 with hexagonal growth habit: A) plain polarized light and B) crossed nicols. Figure 22. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.4.2. Isotopes Carbon isotopic values through the meter-thick dolostone overlying the diamictites of the SMM are unvarying, giving 51 3 C values between -3.6%o to -3.8%o VPDB, corroborating values reported by Smith et al. (1994) (Table 1; Fig. 23). A cross-plot with oxygen values shows a weak correlation but is opposite of an alteration trend between depletion of 61 3 C and 51 8 0 values. Table 2 and figure 24 show carbon isotopic analyses of samples collected from three stratigraphic sections of the carbonate and marble unit. Section 1 produced 61 3 C values ranging from -2.9%o to -6.9 % o VPDB, defining an overall declining upward trend. Sections 2 and 3 show a more unvarying trend with 61 3 C values ranging from -4.3 to -5.3%o VPDB and -4.3 to -6%o VPDB, respectively. These data corroborate negative 61 3 C values reported by Smith et al. (1994) with improved sampling resolution. A cross-plot of 5I3 C and 81 8 0 shows a definite trend for section 1, a definite but less spaced trend for section 2, and no trend for section 3 (Fig. 25). A single 8 7 Sr/8 6 Sr value obtained from a carbonate fan from within the first meter of column 1 yielded a value of 0.7094267. This value is likely altered (A. J. Kaufman, pers. com., 2003), though to what extent is unclear. Smith et al. (1994) report a value of 0.70811 for the interval but propose to substitute values of 0.7073-0.7076 through correlation with a less altered succession. The next carbonates are found in the Blackrock Canyon Limestone. They record 51 3 C values between -0.9% o and l.l%c VPDB through 20 m (Fig. 26) (Smith et al., 1994). Last is the Middle Carbonate Member of the Caddy Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1. Isotopic values through the pink cap dolostone of the SMM. l m O m from sample 13C 180 POC-22 -3.57 -13.802 POC-23 -3.677 -13.613 POC-24 -3.61 -14.108 POC-25 -3.761 -14.995 POC-26 -3.619 -13.989 POC-27 -3.593 -14.408 base Figure 23. Isotopic values through the pink dolostone unit from stop 2. Figure 23. 1 -6 0 0 m z z z z J L J L - 3.55 - 3.6 u C O - 3.65 - 3.7 - 3,75 - 14.6 - 14.2 13.4 18 8 O Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2. Isotpoic values fro m three along-strike sections o f th e 'carbonate a n d marble' unit. ro 0 > c l E ro ro c o u 0 ) to ro C O v O it V O V O 0 0 ID ro N " L O c n in L O r H IN t H i in ro ro ro C O rd L O L O L O L O in i— 1 i id \t" t H I 1 — 4 1 i— 4 i i — i j i— 4 1 1 — 1 > H 1 iH 1 t H I r - 4 i t H I iH 1 T — 1 1 i— l 1 in G ] L O C O V O N ; C O in [N C O L O V D C O 1 — 1 v q i i L O i N " i N - i i N " i i n- ■ I 1 N " i id i i L O o i— l f M C O L O V D o i— l C M C O Tt L O V O n 0 0 c n i— 4 i— 4 i— l ^ 1 i— 1 i— 4 - j -J _1 -J -J _1_1 _i — I _i -J _l _1 _ J -J Z z z Z z Z Z Z Z Z Z z z Z Z z to to to to to to to to in to to in to to to to E L O E L O E L O E L O E L O £ L O E L O E E £ E E E E E X) in C O rsi t - H o L O L O L O L O in L O L O L O E 1 — 1 i— i i — i 1 — 1 i— i i— 1 t H c n 0 0 IN v d in C O i— l o in to G ) C O 0 [N c n 0 0 in L O L O rf C O in id L O in v d it i— i t i— l j 1 — 1 J 1 — 1 1 i — l 1 T — i 1 1 — 1 1 1 — 1 1 i— 4 t i— l 1 C O ro L O tT L O in C O IN r - 4 v q N - 1 L O i 1 i N " i ■ 1 N " 1 L O i i O i— i o <r i— i <r C N <r C O <r L O <r io c IN < t * C O <r i— I < C i— i <r 3 Z 2 z 3 z 3 Z 3 z z 3 z 3 z 3 z 2 z to in to to to in to to to in 50 0 ) w ro _Q O L _ fM C o o E E f- (u t h o b c C O c n c o E E E E 0 in ro c u to ro X ) E E o H O i t ro vq in N ; cn C O Tfr tH C O i— i rs j C N cn ro i— 4 rN c n (N d VD c d i— l i VD cn o C O IN N - o i — i i IN IN C O i— i i r s i i C N i T — 1 J i — 1 1 i— i i i— i i rs i i 1 — 1 1 1 — 1 t i— i i (N i i— i i i— l 1 i— i • LO cn V O n i 00 cn cn 00 1 — 1 ^1; ro i— 4 IN l-H N " VD ' i t LO rs i ro in LO LO vd LO N" LO 0 ) C L £ ro in i— l LO VD IN C O c n O i— l iH 1 — 1 C N i— i ro i— i 1 — 1 LO i— 4 in i— 4 in t — 4 0 0 1 — 1 c n l— l < o IN u U U u u U u u U U u u u u U U u o O o o O O o o o o o o o o o o O o_ Q_ Q_ CL o _ CL Q. a . CL Q. CL 0 . CL CL CL Q. CL c o 4 - < u E E E E E E E E E E E E c u m i— l o C T i 0 0 IN IO L O N " ro C N i— l in IN IN IN r - 4 1 — 1 1 — 1 i— l i— 4 r — 4 i— 4 i— 4 i— 4 < u in ro X ) E E E E o c n c o iv io Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 24. Carbon isotope values through multiple along-strike sections (1, 2, and 3) of the carbonate and marble unit of the Scout Mountain Member at stop 3. Figure 24. 8 I3C (% 0VPDB) S C (%oVPDB) -6 -3 0 S L iC (%oVPDB) ( ) i n Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 25. Cross-plots of carbon and oxygen isotopes from sections 1, 2, and 3 of the carbonate and marble unit of the Scout Mountain Member at stop 3. Section 1 shows a strong alteration trend (coincident lightening of C and O isotopes), section 2 shows a moderate alteration trend, and section 3 shows no alteration trend. Figure 25. O Section 1 ■ 3 “ □ Section 2 A Section 3 - 4 - O A c - 6 - -15 -20 -25 -20 -15 -10 -18 -16 -14 -12 ...................| ..................................... | ' -18 -16 -14 Section 1 "2 ‘ Section 2 .4 . Section 3 -4- -3 - 4 ♦ -4 - * ♦ V ♦ ♦ A 5 - ♦ , * * -6 - ♦ . ♦ = • - 7 - ♦ -6 - ♦ Strong trend Moderate trend No trend Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 26. A composite section of S1 3 C values through the Neoproterozoic succession near Pocatello, ID from the Scout Mountain Member from this study and Smith et al. (1994). The isotope trend’s positive swing has an inflection point in the Blackrock Canyon Limestone, later than it should be if compared to other post-Sturtian successions. That negative SI3 C values persist past ca. 667 Ma suggests that there is more isotopic variation from 700 Ma to 600 Ma than previously recognized. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Pocatello, ID Sedgwick Peak Qtz , Windy Pass Argillite Camelback Mountain Qtz 'PreC Mutual Fm 580 Ma Q * 3 O 0 Inkom Fm s 33 $ 0 * tn pq Caddy Canyon Qtz Papoose Creek Fm Blackrock Canyon Lms 667+/-5M a Sturtian 711+/-4 Ma ( J C L Marinoan Smith e ta L 1994 Smith e ta u 1994 This stud> This studv -3 0 € € “ 1 ! 1....... " 1 1 1 i I t i l l 3 1 o q ‘ c to O s Canyon Quartzite, with 61 3 C values from the same stratigraphic level of 3.9% o to 8.8%o VPDB (Fig. 26) (Smith et al., 1994). 3.5. Discussion 3 .5 .1 . S e a flo o r p recip ita tes The observed carbonate crystal fans are likely primary seafloor cements (e.g., Grotzinger and Knoll, 1995; Sumner and Grotzinger, 1996; Woods et al., 1999), and the hexagonal crystal outline with a coarse spar mosaic is consistent with primary aragonite replaced by calcite (Sandberg, 1985). Several sedimentary features, such as sand/silt-draped fans and three-dimensional fan beds, indicate that the fans formed on the sea floor and produced subtle sea floor topography. It is likely that siliciclastic influxes intermittently smothered and terminated fan growth, as opposed to fan growth stopping before siliciclastic covering, as the unique chemical conditions associated with fan growth persist throughout the entire carbonate and marble unit, and if left unrestricted, such fans are capable of meters of growth (e.g., Hoffman et al., 1998a). Secondary precipitation does not satisfactorily explain the form of the carbonate fans. If the fans were secondary features one would expect the crystals to display diffuse margins and/or contain relict micrite. Additionally, where fanning in the carbonate needles is recognizable, it is toward stratigraphic top. Precipitation in a secondary joint or void, for example, would create crystal orientations symmetric about the axis of the joint or void. These have not been observed. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 .5 .2 . Iso to p ic in te rp re ta tio n s For the purposes of this paper, negative 51 3 C values are significant in terms of their qualitative similarity with many Neoproterozoic post-glacial carbonates. The negative 81 3 C values of the pink dolostone are likely primary. The uppermost carbonate of the SMM has likely undergone some alteration, as apparent by the cross-plot of 81 3 C and S1 8 0 values for section 1, variation between section 1 and sections 2 and 3, and as discussed in Smith et al. (1994). This is insignificant for the 51 3 C values as they are relatively resistant to alteration by secondary fluids (Banner and Hanson, 1990). Also, the similarity of absolute values between sections 2 and 3, the lack of a diagenetic trend in the cross-plot of 81 3 C and 51 8 0 for section 3, as well as the presence of key correlative excursions in all three profiles suggests that the values are still a reasonable approximation for original 51 3 C values of seawater at the time of deposition, with section 3 being the most accurate. Alteration would be expected to have a more significant impact on Sr isotopic values. The 8 7 Sr/8 6 Sr value of 0.7094267 would typically be thought indicative of post-Marinoan seawater (Smith et al., 1994; Kennedy et al., 1998). Smith et al. (1994) circumvent this quandary by suggesting through correlation that values of 0.7073-0.7076 are more representative for the interval, therefore advocating the interval to be post-Sturtian. Given the radiometric constraints above, this is a reasonable reconciliation of the data, and at this time I don’t feel further analysis is needed. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 .5 .3 . I s th e ‘ C a rb o n a te a n d M arble’ u n it a ca p c a rb o n a te? The carbonate and marble unit lies within a transgressive succession and contains features consistent with post-glacial cap carbonates found in other Neoproterozoic succession such as seafloor precipitates, negative carbon isotopes, subaqueous teepee structures, and possible sheetcrack cements, but does not overlie a known glacial unit. It is reasonable to assume that post glacial transgression often flooded areas without underlying glaciogenic deposits, depositing a cap carbonate on non-glacial strata. In a sequence stratigraphic framework, glaciogenic deposits should dominate deposition in the lowstand systems tract in deeper basinal settings that record little deposition except during glaciations. Cap carbonates, on the other hand, should be transgressive systems tract deposits generally cratonward of lowstand systems tract deposits. The magnitude of transgression subsequent to global-scale glaciation implies that many Neoproterozoic post-glacial carbonates could rest upon an underlying sequence boundary in lieu of a glaciogenic deposit. This is known from the Otavi Group, Namibia where the Maieberg cap carbonate rests on glacial deposits in basinward settings and on underlying pre-glacial units in cratonward settings (Hoffman et al., 1998a). T h e carbonate and marble unit of the SMM has a closely underlying sequence boundary that locally removes the pink dolomite cap on the upper SMM diamictite. The carbonate and marble unit also displays unusual lithofacies and records negative 61 3 C values similar to other post-glacial Neoproterozoic carbonates. T h erefo re, it is possible that the unit represents a post-glacial cap carbonate, possibly a ‘last gasp’ of Sturtian glaciation, pushing the minimum limit for Sturtian glaciation worldwide to ca. 670 Ma. Alternatively, if the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. presence of the underlying pink dolostone represents the end of an ice age with subsequent change to warm water conditions (e.g., Kaufman et al., 1997; Kennedy et al., 2001; Hoffman and Schrag, 2002) the unit could represent an intermediate glaciation between the Sturtian and Marinoan intervals. I prefer this interpretation for the following: Bowring et al. (2003) demonstrate that a discrete, bona fide “snowball” glaciation (glaciogenic deposit with cap carbonate) recorded by the Gaskiers deposits in Newfoundland lasted less than 1 m.y., and approximately 35 Gaskiers-duration events could fit within the minimum geochronologic window (707 Ma - 672 Ma) between the carbonate and marble unit and the underlying diamictites. Also, if the glaciation were to extend the Sturtian interval from 750 Ma to 670 Ma, it would be an ice age of ~70 m.y. An ice age longer than the Cenozoic Era has no precedent or current model that comes close to explaining it. Regardless, either interpretation can support a scenario where the sandstone below the carbonate and marble represents outwash from a glaciation that left diamictite only in more basinal locations. Up-section, the sandstone fines into overlying siltstone that could be due to continued post-glacial transgression with very high sedimentation (indicated by climbing ripples) from ongoing glacial outwash. In time, sea level would have transgressed enough with pronounced post-glacial ocean chemistries to deposit carbonate lenses in the siltstone despite the high sedimentation rate. Finally, when sea level was high enough and/or sedimentation had slowed, a “true” cap carbonate (the carbonate and marble unit) was deposited, though it is very silty. Thus, the carbonate and marble may be a true cap carbonate with underlying glaciogenic units that were not recognized because they are not diamictites. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Following cap carbonate deposition, strong positive shifts in 61 3 C are observed in both post-Sturtian and post-Marinoan deposits worldwide (e.g., Kaufman and Knoll, 1995). Following Sturtian glaciation specifically should be ~100 m.y. of 61 3 C values from 8% o to 12% o VPDB or greater (e.g., Jacobsen and Kaufman, 1999). As radiometric constraints support an interpretation that the Scout Mountain Member diamictites are Sturtian, the overlying carbonate and marble unit should display these positive 61 3 C values. However, neither the carbonate and marble unit nor the Blackrock Canyon Limestone demonstrates this (Fig. 26); the Blackrock Canyon Limestone hovers around 0%c VPDB and the carbonate and marble is distinctly negative. Excluding the possibility that the isotopic signature of the carbonate and marble is a holdover from the previous glaciation is the 180 k.y. residence time of carbon (Broecker and Peng, 1982); 35 m.y. (707 Ma - 672 Ma) is roughly 194 times carbon’s residence time in the present ocean. Thus, there was more carbon isotopic variation between 700 Ma and 600 Ma than previously recognized. If an inferred positive 61 3 C plateau between Sturtian and Marinoan glaciations is the only argument against glaciation during the interval, the possibility that the carbonate and marble is a true post-glacial cap carbonate of ca. 670 Ma glaciation here gains strength. 3 .5 .4 . A d d ressin g a rg u m e n ts a g a in st a post-glacial o rig in for th e ‘ C a rb o n a te a n d M a rb le ’ fro m p revio u s g la cia l persp ectives Kennedy et al. (1998) hypothesize that only two global-scale glaciations occurred in Neoproterozoic time (e.g., Sturtian and Marinoan) based on a cladistic analysis of 12 post-glacial carbonates in six Neoproterozoic successions. Lithologic criteria for identifying Sturtian deposits include cap Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5I3 C values greater than 0%c; dark, organic-rich carbonate; and anastomosing and rhythmic laminae in the cap. Criteria proposed for identifying Marinoan deposits are 8 7 Sr/8 6 Sr greater than 0.7079, 61 3 C values becoming more depleted up-section, pseudotepees and sheet cracks in the cap, tubestones in the cap, and aragonite or barite crystal fans in the cap. Under these criteria, the carbonate and marble unit could be classified as a Marinoan cap carbonate (if it is a cap carbonate). However, the thin, pink dolostone atop the diamictite of the SMM w o u ld a lso be classified as a Marinoan cap carbonate! The geochronology presented above would preclude this interpretation, suggesting that it may not be prudent to use lithostratigraphic descriptions of cap carbonate to imply the age or to infer the absolute number of glaciations. That the carbonate and marble is in a transgressive succession is only generally consistent with Neoproterozoic cap carbonates. Two popular models for cap carbonate formation (Hoffman et al., 1998b; Kennedy et al., 2001) require the base of a cap carbonate to be a transgressive surface. The carbonate and marble unit was not deposited on a transgressive surface and thus does not constitute a “genuine” cap carbonate according to these models. However, this reasoning need not strictly apply if relative transgressions and regressions were mediated tectonically, and depositional models for the area (e.g., Link, 1983) suggest this to be reasonable. 3 .5 .5 . D o es th e ‘ C a rb o n a te a n d M arble’ u n it rep resen t a ca p -lik e ca rb o n a te deposited independent o f post-glacial processes? One can speculate that the overturn of an anoxic stratified ocean could bring bicarbonate-rich, S1 3 C-depleted bottom waters generated through microbial Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sulfate reduction onto the shelf independent of glaciation (Grotzinger and Knoll, 1995). Such a scenario may have produced the upper carbonate of the SMM. Similar seafloor precipitated fans and depleted 61 3 C values, in the absence of a recognized underlying glacial deposit, have been reported from the Neoproterozoic Johnnie Formation (Pruss and Corsetti, 2002) and are known in Lower Triassic strata associated with global anoxic events (Woods et al., 1999). Therefore, post-glacial processes may not be a requirement for the formation of unusual carbonate lithofacies and negative 61 3 C anomalies. The finely laminated dolostone cap likely indicates a return to warm water conditions, probably beyond the scope of interstade warming, and therefore a detachment from the previous ice age (Kaufman et al., 1997; Kennedy et al., 2001; Hoffman and Schrag, 2002), and there are no u n e q u ivo ca l signs of glaciation between that dolostone’s overlying sequence boundary and the carbonate and marble unit despite intense investigation. Therefore, until new evidence suggests otherwise, it is reasonable to interpret the carbonate and marble unit as a cap-like carbonate precipitated independent of glacial processes. 3 .5 .6 . Im p lica tio n s for global b I 3 C c u rv e s Radiometric constraints on the Pocatello Formation demonstrate that 61 3 C values were negative at 667 Ma, conflicting with previous 81 3 C compilations showing highly positive values at this time (e.g., Kaufman et al., 1997; Kennedy et al., 1998; Kaufman et al., 1999). An inflection point from negative to positive values isn’t observed until the Blackrock Canyon Limestone, and values don’t become highly positive until the Middle Carbonate Member Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the Caddy Canyon Quartzite, immediately below proposed Marinoan drawdown (where they should be swinging negative again, not positive). The isotopic record of the better-constrained Pocatello Formation suggests that the interval from 700 Ma to -650 Ma in previous 51 3 C compilations need to be reconsidered. If there is enough confidence in the ca. 600 Ma portion of previously mentioned S1 3 C curves, these highly positive values below the proposed Marinoan drawdown may constrain the Caddy Canyon incised valleys between 630 Ma and the previously mentioned 580 Ma (compare with Canadian values between 8% o and 10%c VPDB from Jacobsen and Kaufman (1999), p. 48). This is speculative, and as none of the data included in this speculation are mine, I’m at this point willing to leave this topic for others to explore further. 3.6. Conclusions The uppermost carbonate and marble unit of the SMM contains seafloor precipitates, records negative S1 3 C values, and falls within a transgressive succession, all consistent with other carbonates that cap Neoproterozoic glacial successions worldwide. It does not, however, rest upon a glaciogenic diamictite. As seafloor precipitates are rare in post-Paleoproterozoic time (Sumner and Grotzinger, 1996) but more common in the unusual carbonates that cap certain Neoproterozoic glacial strata (Hoffman et al., 1998a; Kennedy et al., 1998), their discovery in the Pocatello Formation is significant for our understanding of Neoproterozoic Earth history. The carbonate and marble unit may be an actual cap carbonate in the absence of an underlying glaciogenic diamictite, thus representing the aftermath of glaciation recorded here only Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in sequence stratigraphic relationships. Alternatively, the possibility exists 63 that not all Neoproterozoic units with unusual lithofacies and negative carbon isotopes need be true cap carbonates indicative of post-glacial processes. Either scenario presents challenges for interpreting Neoproterozoic successions with regard to global-scale glaciation. Inferring the number and age of ice ages from lithostratigraphic descriptions alone is problematic. Previously suggested criteria are unsuccessful when applied to the Pocatello Formation. Thus, caution must be exercised when interpreting the age and number of ice ages in Neoproterozoic time in the absence of unambiguous geochronologic data. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 4. Radiometric constraints on “snowball Earth” strata 4.1. Introduction and Methods The following is a review of radiometric constraints on Neoproterozoic glaciations; it does not consider evidence from advanced stratigraphic techniques to infer absolute ages without radiometric constraints since nearly all of these are equivocal on some level. Thus, I use the term “unconstrained” to mean lacking radiometric constraints. Previous workers have assembled exhaustive compilations of the Neoproterozoic glacial record (Fig. 27) (Hambrey and Harland, 1981; Harland, 1983; Evans, 2000) that I here aim to synthesize and update where relevant. Units have been chosen (somewhat subjectively) on the basis of robust evidence of their glaciogenic origins and credibility of age constraints. Lone unusual carbonates, without underlying glaciogenic units, are not considered as evidence for previous glaciation though, ultimately, they could be (e.g., Kaufman et al., 1997). A minimum age of 543 Ma is assigned if none is available or as a minimum cutoff, and dates are U-Pb zircon unless otherwise noted. 4.2. Results Results are summarized in appendix 1. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 27. Global distribution of Neoproterozoic glacial deposits (from Evans, 2000). Note that such deposits are recognized on all continents. The Pocatello succession is #7. Figure 27. ,gl,22, > 5,66, 2,53. [42-44, Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.3. Discussion and Implications Interpreting and correlating Neoproterozoic successions is extremely challenging without radiometric constraints. An example of this is found in the Death Valley succession of the southwestern United States. Ages of units within the succession are constrained to between 1.087 Ga and 543 Ma, 544 m.y., the approximate duration of the Phanerozoic! With so much leeway, deciphering events recorded within the succession as well as correlating the succession across the continent can lead to multiple interpretations. Figure 28 illustrates the confusion generated by simply attempting to correlate the Death Valley and Pocatello successions. With this intracontinental correlation so problematic, two main questions arise: are other current intracontinental correlations correct, and if not, are not intercontinental correlations even more problematic? Bowring et al. (2003) demonstrated that the Neoproterozoic Gaskiers glaciogenic deposits in Newfoundland, formed during a discrete “snowball” episode, were deposited in less than 1 m.y. This differs from the Hoffman et al. (1998b) 10 m.y. estimate of “snowball” glacial duration based on the amount of time needed to outgas enough C 0 2 to produce a severe greenhouse to end glaciation. It does falls within the Sohl et al. (1999) estimate of several 105 to several 106 years based on magnetic reversals recorded in the Elatina Formation. This is significant in that it seems to refute the snowball Earth hypothesis (Kirschvink, 1992; Hoffman et al., 1998b) in a strict sense. Also, it requires that if glaciations are to have been synchronous, all “Marinoan” Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 28. Different correlations proposed between the Neoproterozoic successions in Pocatello, ID and Death Valley, CA. Despite difficulties in recognizing and dating glaciations in Death Valley, correlations have been drawn to other succession in the Cordillera, such as the Pocatello Formation, and are used to imply that only two glaciations are recorded in those successions. General columns and correlations based on: Christie-Blick and Levy (1989), 1 Link et al. (1993), Levy et al. (1994), 2Prave (1999), and Abolins et al. (2000). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 28. Pocatello, ID S ed g w ick Peak Q tz/ W indy Pass A rgillite C am elback M ountain Q tz Death Valley, CA M utual Fm 580 Ma Stirling Q tz Inkom Fm a'cFI Marinoan Marinoan Johnnie Fm D O Caddy C anyon Qtz N oonday D lm t P ap oose Creek Fm K ingston B laekrock C anyon Lms Peak Fm u. B eck Spring D lm t Sturtian Crystal Spring Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. glaciogenic deposits should fall within 1 m.y. of 580 Ma, and that “Sturtian” deposits should correlate to within 1 m.y. of some age within 750-700 Ma. A 1 m.y. interval does not allow for all “Marinoan” successions to correlate (Fig. 29). Specifically, the Jbe'liat diamictite is older than 595 Ma Rb-Sr, and the Caddy Canyon drawdown is pre-580 Ma Ar4 0 /Ar3 5 by some duration that allowed for deposition of ~1000 meters of sediment and erosion of two recognized sequence boundaries. Only successions with loose constraints of many tens of millions of years can be correlated. Neither a 1 m.y. nor a 10 m.y. interval allow for all “Sturtian” successions to correlate. Notably, the Kaigas and Rapitan deposits do not overlap. Barring this, comparing the Kaigas and Ghubrah deposits seem promising in that the overlap in their age-constraints could fit the “Sturtian” interval neatly between 739 Ma and 735 Ma. Unfortunately, that time window predates glaciogenic strata of the Pocatello Formation by 20 m.y. and strata of the Edwardsburg Formation by ca. 50 m.y.! This argues that Neoproterozoic glaciations could not have been synchronous, even by using a generous 10 m.y. time window. It also argues that there had to have been more than two such glaciations. However, if glaciations were not globally synchronous, some continents could have undergone more glaciations than others, further nullifying their significance for global correlation and interpretation. Caution should be used in basing interpretations of Neoproterozoic history and processes on the number of glaciations recorded in a succession. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 29. Some radiometric constraints on Neoproterozoic glacial strata summarized in Appendix 1. Vertical bars show the constraints on glaciations; horzontal blocks show generally accepted glacial intervals. Neither a 1 m.y. or 10 m.y. interval allow for all successions to correlate within the generally accepted Sturtian and Marinoan intervals. This argues that there were more than two such glaciations and that glaciations were not synchronous. *Neither the formation of Perry Canyon or the Edwardsburg Formation are shown in this figure. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 29 ■ a JD ■ a ■ a C /3 543 550 560 c/3 600 650 700 I £ 750 800 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter 5. Concluding remarks: the climate of Neoproterozoic climate This thesis aimed to address the debates over the number and synchroneity of Neoproterozoic glaciation by analyzing new lithologic and chemical evidence of glaciation from the Neoproterozoic Pocatello Formation in southeastern Idaho and existing radiometric constraints on Neoproterozoic glacial strata worldwide. From these efforts, up to six main conclusions can be drawn: 1) lithostratigraphic descriptions without robust age constraints are not adequate to correlate Neoproterozoic glacial successions, 2) glaciations may not be preserved in the rock record solely by diamictites, 3) alternatively, “cap carbonates” may not always represent post-glacial processes, 4) there were more than two Neoproterozoic glaciations, 5) such glaciations were not globally synchronous, and 6) the 61 3 C compilations that show positive values ca. 665 are incorrect because the SMM records negative values and robust age constrains. 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Wilkinson (Editors), Frontiers in geology and ore deposits of Arizona and the Southwest. Arizona Geological Society Digest, 16: 502-554. Kellogg, K.S., 1992. Cretaceous thrusting and Neogene block rotation in the northern Portneuf Range region, southeastern Idaho. In: P.K. Link, M.A. Kuntz and L.B. Platt (Editors), Regional Geology of Eastern Idaho and Western Wyoming, Geological Society of America Memoir 179, pp. 95-124. Kennedy, M.J., 1996. Stratigraphy, sedimentology, and isotopic geochemistry of Australian Neoproterozoic postglacial cap dolostones: deglaciation, delta (sub 13) C excursions, and carbonate precipitation. Journal of Sedimentary Research, 66(6): 1050-1064. Kennedy, M.J., Christie-Blick, N. and Sohl, L.E., 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals? Geology, 29(5): 443-446. Kennedy, M.J., Runnegar, B., Prave, A.R., Hoffmann, K.H. and Arthur, M.A., 1998. Two or four Neoproterozoic glaciations? Geology, 26(12): 1059- 1063. Kirschvink, J.L., 1992. Late Proterozoic low-latitude global glaciation: The snowball Earth. In: J.W. Schopf and C.C. Klein (Editors), The Proterozoic Biosphere: A Multisisciplinary Study. Cambridge University Press, Cambridge, pp. 51-52. Kluth, C.F. and Coney, P.J., 1981. Plate tectonics of the Ancestral Rocky Mountains. Geology, 9: 10-15. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Knoll, A.H., 2000. Learning to tell Neoproterozoic time. Precambrian Research, 100: 3-20. 82 LeFebre, G.B., 1984. Geology of the Chinks Peak Area, Pocatello Range, Bannock County, Idaho, M.S. thesis, Idaho State University, Pocatello. Levy, M. and Christie-Blick, N., 1991. Tectonic subsidence of the early Paleozoic continental margin of eastern California and southern Nevada. Geological Society of America Bulletin, 10: 1590-1606. Levy, M., Christie-Blick, N. and Link, P.K., 1994. Neoproterozoic incised valleys of the eastern Great Basin, Utah and Idaho: fluvial response to changes in depositional base level. In: R.W. Dalrymple, Boyd, R., and Zaitlin, B.A. (Editor), Incised-valley Systems: Origin and Sedimentary Sequences SEPM Special Publication No. 51, Tulsa, pp. 369-382. Li, Z.X., 2000. New paleomagnetic results from the ‘cap dolomite’ of the Neoproterozoic Walsh tillite, northwestern Australia. Precambrian Research, 100: 359-370. Lindsay, J.F., Braiser, M.D., Shields, G., Khomentovsky, V.V. and Bat-Ireedui, Y.A., 1996. Glacial facies associations in a Neoproterozoic back-arc setting, Zavkhan Basin, western Mongolia. Geological Magazine, 133: 391-402. Link, P.K., 1983. Glacial and tectonically influenced sedimentation in the Upper Proterozoic Pocatello Formation, southeastern Idaho. In: D.M. Miller, V.R. Todd and K.A. Howard (Editors), Tectonic and Stratigraphic Studies in the Eastern Great Basin, Geological Society of America Memoir 157, pp. 165-181. Link, P.K., 1987. The Late Proterozoic Pocatello Formation; A record of continental rifting and glacial marine sedimentation, Portneuf Narrows, southeastern Idaho. In: S.S. Beus (Editor), Centennial Field Guide Volume 2, Rocky Mountain Section of the Geological Society of America, pp. 139-142. Link, P.K. et al., 1993. Middle and Late Proterozoic stratified rocks of the western United States Cordillera, Colorado Plateau, and Basin and Range Province. In: J.C. Reed, Jr. et al. (Editors), Precambrian: Conterminous U.S. The Geology of North America. Geological Society of America Decade of North American Geology Series, Boulder, Colorado, pp. 463-595. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Link, P.K. and LeFebre, G.B., 1983. Upper Proterozoic diamictites and 83 volcanic rocks of the Pocatello Formation and correlative units, southeastern Idaho and northern Utah. Utah Geological and Mineral Survey Special Studies 60: 1-32. Link, P.K., LeFebre, G.B., Pogue, K.R. and Burgel, W.D., 1985. Structural geology between the Putnam Thrust and the Snake River Plain, southeastern Idaho. In: G.L. Kerns, Kerns, R.L. (Editor), Orogenic patterns and stratigraphy of north-central Utah and southeastern Idaho, Utah Geological Association Publication 14, pp. 97-117. Link, P.K., Miller, J.M.G. and Christie-Blick, N., 1994. Glacial-marine facies in a continental rift environment: Neoproterozoic rocks of the western United States Cordillera. In: M. Deynoux et al. (Editors), International Geological Correlation Project 260: Earth’s Glacial Record. Cambridge University Press, Cambridge, U.K., pp. 29-59. Link, P.K. and Smith, L.H., 1992. Late Proterozoic and Early Cambrian stratigraphy, paleobiology, and tectonics: northern Utah and southeastern Idaho. In: J.R. Wilson (Editor), Field Guide to Geologic Excursions in Utah and Adjacent Areas of Nevada, Idaho, and Wyoming. Utah Geological Survey, Salt Lake City, pp. 461-481. Lorentz, N.J., Corsetti, F.A. and Link, P.K., 2002. Seafloor precipitates and negative dC values from the Scout Mountain Member of the Pocatello Formation, southeast Idaho. In: F.A. Corsetti (Editor), Proterozoic- Cambrian of the Great Basin and beyond: Pacific Section SEPM Volume and Guidebook 93, pp. 43-49. Ludlum, J.C., 1942. Pre-Cambrian formations at Pocatello, Idaho. Journal of Geology, 50: 85-95. Lund, K., Aleinikoff, J.N., Evans, K.V. and Fanning, C.M., 2003. SHRIMP U- Pb geochronology of Neoproterozoic Windermere Supergroup, central Idaho: Implications for rifting of western Laurentia and synchroneity of Sturtian glacial deposits. Geological Society of America Bulletin, 115(3): 349-372. Ma, G., Lee, H. and Zhang, Z., 1984. An investigation of the age limits of the Sinian System in South China. Chinese Academy of Geological Sciences, Bulletin of the Yichang Institute of Geology and Mineral Resources(8): 1-26. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. McDonough, M.R. and Parrish, R.R., 1991. Proterozoic gneisses of the Malton Complex, near Valemount, British Columbia: U-Pb ages and Nd isotopic signatures. Canadian Journal of Earth Sciences, 28: 1202-1216. Miller, M.M., 1987. Dispersed remnants of a northeast Pacific fringing arc; Upper Paleozoic terranes of permian McCloud faunal affinity, western U.S. Tectonics, 6: 807-830. Miller, N.R. et al., 2003. Significance of the Tambien Group (Tigrai, N. Ethiopia) for snowball Earth events in the Arabian-Nubian Shield. Precambrian Research, 121: 263-283. Monger, J.W.H., 1977. Upper Paleozoic rocks of the western Canadian Cordillera and their bearing on Cordilleran evolution. Canadian Journal of Earth Sciences, 14: 1832-1859. Monger, J.W.H., Price, R.A. and Templeman-Kluit, D.J., 1982. Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, 10: 70-75. Narbonne, G.M. and Aitken, J.D., 1995. Neoproterozoic of the Mackenzie Mountains, northwset Canada. Precambrian Research, 73: 101-121. Poole, F.G. et al., 1992. Latest Precambrian to latest Devonian time: Development of a continental margin. In: B.C. Burchfiel, P.W. Lipman and M.L. Zoback (Editors), The Cordilleran Orogen: Conterminous U.S. The Geology of North America. Geological Society of America, Boulder, pp. 9-56. Postel’nikov, Y.S., 1981. Late Precambrian Chivida tilloids of the Yenisey Ridge, Middle Siberia, U.S.S.R. In: M.J. Hambrey and W.B. Harland (Editors), Earth’s Pre-Pleistocene Glacial Record. Cambridge University Press, Cambridge, pp. 375-379. Prave, A.R., 1999. Two diamictites, two cap carbonates, two d1 3 C excursions, two rifts: The Neoproterozoic Kingston Peak Formation, Death Valley, California. Geology, 27: 339-342. Pruss, S.B. and Corsetti, F.A., 2002. Unusual aragonite precipitates in the Neoproterozoic Rainstorm Member of the Johnnie Formation. In: F.A. Corsetti (Editor), Proterozoic-Cambrian of the Great Basin and beyond: Pacific Section SEPM Volume and Guidebook 93, pp. 51-59. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Roberts, R.J., Hotz, RE., Gilluly, J. and Furguson, H.G., 1958. Paleozoic rocks of north-central Nevada. American Association of Petroleum Geologists Bulletin, 42(12): 2813-2857. Ross, G.M., 1991. Tectonic setting of the Windermere Supergroup revisited. Geology, 19: 1125-1128. Saleeby, J., 1983. Accretionary tectonics of the North American Cordillera. Annual Review of Earth and Planetary Sciences, 15: 45-73. Saleeby, J. et al, 1992. Early Mesozoic tectonic evolution of the western U.S. Cordillera. In: B.C. Burchfiel, P.W. Lipman and M.L. Zoback (Editors), The Cordilleran Orogen: Conterminous U.S. The Geological Society of America, pp. 107-168. Sandberg, P., 1985. Aragonite cements in ancient limestones. In: N. Schneiderman and P.M. Harris (Editors), Carbonate Cements: SEPM Special Publication 36, pp. 33-57. Shackleton, R.M., Ries, A.C., Coward, M.P. and Cobbold, PR., 1979. Structure, metamorphism and geochronology of the Arequipa Massif of coastal Peru. Journal of the Geological Society of London, 136: 195- 214. Smith, L.H., Kaufman, A.J., Knoll, A.H. and Link, P.K., 1994. Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions: a case study of the Pocatello Formation and lower Brigham Group, Idaho, USA. Geological Magazine, 131(3): 301-314. Sohl, L.E., Christie-Blick, N. and Kent, D.V., 1999. Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: implications for the duration of low-latitude glaciation in Neoproterozoic time. Geological Society of America Bulletin, 111(8): 1120-1139. Speed, R.C., 1977. Island-arc and other paleogeographic terranes of late Paleozoic age in the western Great Basin. In: J.H. Stewart, C.H. Stevens and A.E. Fritsche (Editors), Paleozoic paleogeography of the western United States. Society of Economic Paleontologists and Mineralogista Pacific Coast Paleogeography Symposium 1, Los Angeles, pp. 349-362. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Speed, R.C., 1983. Evolution of the sialic margin in central-western United 86 States. In: J.S. Watkins and C.L. Drake (Editors), Studies in continental margin geology. American Association of Petrolium Geologists Memoir 34, pp. 457-468. Speed, R.C., Elison, M.W. and Heck, F.R., 1988. Phanerozoic tectonic evolution of the Great Basin. In: W.G. Ernst (Editor), Metamorphic and crustal evolution of the western United States; Rubey Volume 7. Prentice-Hall, Englewood Cliffs, New Jersey, pp. 572-605. Stewart, J.H., 1972. Initial deposits in the Cordilleran geosyncline: evidence of a late Precambrian (<850 m.y.) continental separation. Geological Society of America Bulletin, 83: 1345-1360. Stewart, J.H., 1991. Latest Proterozoic and Cambrian rocks of the western United States - an overview. In: J.D. Cooper, and Stevens, C.H. (Editor), Paleozoic Paleogeography of the Western United States - II. Pacific Section SEPM, pp. 13-37. Stewart, J.H. and Poole, F.G., 1974. Lower Paleozoic and uppermost Precambrian Cordilleran miogeocline, Great Basin, western United States. In: W.R. Dickenson (Editor), Tectonics and Sedimentation. SEPM Special Publication 22, pp. 28-57. Stewart, J.H. and Suczek, C.A., 1977. Cambrian and latest Precambrian paleogeography and tectonics in the western United States. In: J.H. Stewart, C.H. Stevens and A.E. Fritsche (Editors), Paleozoic Paleogeography of the Western United States. Pacific Section SEPM, pp. 1-18. Summa, C.L., 1993. Sedimentologic, Stratigraphic, and Tectonic Controls of a Mixed Carbonate-Siliciclastic Succession; Neoproterozoic Johnnie Formation, Southeast California, Ph.D. Dissertation thesis, Massachusetts Institute of Technology, Cambridge. Sumner, D.Y. and Grotzinger, J.P., 1996. Were kinetics of Archean calcium carbonate precipitation related to oxygen concentration? Geology, 24(2): 119-122. Thompson, M.D. and Bowring, S.A., 2000. Age of the Squantum ‘Tillite’, Boston basin, Massachusettes: U-Pb zircon constraints on terminal Neoproterozoic glaciation. American Journal of Science, 300: 630-655. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Timmons, J.M., Karlstrom, K.E., Dehler, C.M., Geissman, J.W. and Heizler, 87 M.T., 2001. Proterozoic multistage (ca. 1.01 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending grains in the southwestern United States. Geological Society of America Bulletin, 113: 163-180. Tollo, R.P. and Aleinikoff, J.N., 1996. Petrology and U-Pb geochronology of the Robertson River Igneous Suite, Blue Ridge province, Virginia: Evidence for multistage magmatism associated with an early episode of Laurentian rifting. American Journal of Science, 296: 1045-1090. Trimble, D.E., 1976. Geology of the Michaud and Pocatello Quadrangles, Bannock and Power Counties, Idaho, United States Geological Survey Bulletin. Valdiya, K.S., 1995. Proterozoic sedimentation and Pan-African geodynamic development in the Himalaya. Precambrian Research, 74: 35-55. Van Kooten, G.K. et al., 1997. Geological investigations of the Kandik area, Alaska and adjacent Yukon Territory, Canada. State of Alaska Division of Geological and Geophysical Surveys, Report of Investigations 96- 6A. Vidal, G. and Moczydlowska, M., 1995. The Neoproterozoic of Baltica: Stratigraphy, palaeobiology and geological evolution. Precambrian Research, 73: 197-216. Walter, M.R., Veevers, J.J., Calver, C.R., Gorjan, P. and Hill, A.C., 2000. Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretive models. Precambrian Research, 100: 371-433. Woods, A.D., Bottjer, D.J., Mutti, M. and Morrison, J., 1999. Lower Triassic large sea-floor carbonate cements: their origin and a mechanism for the prolonged biotic recovery from the end-Permian mass extinction. Geology, 27(7): 645-648. Zheng, Z., Li, Y., Lu, S. and Li, H., 1994. Lithology, sedimentology, and genesis of the Zhengmuguan Formation of Ningxia, China. In: M. Deynoux et al. (Editors), Earth’s Glacial Record. Cambridge University Press, Cambridge, pp. 101-108. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 1 N o rth A m eric a n C o rd illera - Numerous glaciogenic deposits are preserved along the Canadian and U.S. Cordillera generally correlated as the ‘diamictite and volcanic succession’ (Fig. 30) (Stewart, 1972; Stewart and Suczek, 1977; Link et al., 1993). In Canada the Windermere Supergroup records the Rapitan Group dated at 755 + 18 Ma and younger Ice Brook glaciogenic deposit (Narbonne and Aitken, 1995). The upper Tindir Group is similar to and possible correlative with the Rapitan Group, but is constrained between 644 ± 18 Ma K-Ar and 532 ± 11 Ma K-Ar (Van Kooten et al., 1997)! In British Columbia and Washington the Toby Formation has a maximum age constraint of 736 +23/ -17 Ma (McDonough and Parrish, 1991) and a minimum age constraint of 569.6 + 5.3 Ma (Colpron et al., 2002). Farther south in the Cordillera of central Idaho the Wind River Meadows Member of the Edwardsburg Formation is dated at 685 + 7 Ma and may correlate with the Toby Formation (Lund et al., 2003) and the Placer Creek Member of the Edwardsburg Formation is dated at 684 + 4 Ma (Lund et al., 2003). It is unclear as to whether the Wind River Meadows and Placer Creek Members represent stades within a single ice age or separate ice ages. The formation of Perry Canyon, Fremont Island, Utah records two glaciogenic deposits interpreted as from discrete glacial intervals, the uppermost of which is also correlated to the Scout Mountain Member of the Pocatello Formation (Crittenden et al., 1983). If the lower glaciogenic deposit of the formation of Perry Canyon correlates with the Wind River Meadows Member, it would suggest the Edwardsburg Formation records two discrete ice ages. The Scout Mountain Member diamictites are constrained between 711 ± 4 Ma and 667 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 30. Locations of Neoproterozoic glacial deposits along the Cordillera (modified from Link et al., 1993). Arrow denotes the Pocatello succession (the focus of this study). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1000 m | 500 km Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. + 5 Ma (Fig. 3) (Fanning and Link, in prep). Stratigraphically up-section from the Pocatello Formation, a major incised valley system in the Caddy Canyon Quartzite is interpreted to record sea level drawdown associated with glaciation (Levy et al., 1994) and is constrained to be older than 580 Ma Ar4 0 / Ar3 9 (Christie-Blick and Levy, 1989). Still farther south in the Cordillera of the SW United States the Kingston Peak Formation records at least two glaciations, the older Surprise Diamictite and the younger Wildrose Diamictite (Prave, 1999). At the stratigraphic level between the Upper and Lower Noonday Dolomite, an additional ice age may be recorded in the Ibex Formation (Corsetti et al., 2002). Last, an erosional sequence boundary in the Johnnie Formation is interpreted as correlative to the Caddy Canyon Quartzite drawdown and thus may also record glaciation (Christie-Blick and Levy, 1989). Tectonic eustacy, however, is a viable alternate interpretation for the Johnnie incision (Summa, 1993). None of these events are directly dated and are constrained as younger than 1.087 Ga (Heaman et al., 1992). A p p a la ch ia n s - The Konnarock glaciogenic deposit is younger than 758±12 Ma (Aleinikoff et al., 1995). The possibly correlative Grandfather Mountain Formation is younger than 742 + 2 Ma (Fetter and Goldberg, 1995). The possibly correlative Metchum River Formation in Virginia is older than rifting dated at 730-700 Ma (Tollo and Aleinikoff, 1996). If these units are correlative, they represent a glaciation between 770 Ma and 700 Ma. Fauquier Formation may be glaciogenic and is constrained between 730-700 Ma event and 564 + 9 Ma (Aleinikoff et al., 1995). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N ew E n g la n d - The Squantum ‘Tillite’ is constrained between 595 ± 2 Ma and 570 Ma (Thompson and Bowring, 2000). N ew fo u n d la n d - The Gaskiers glaciogenic deposit is dated at 580 Ma a n d constrained to a 1 m.y. interval (Bowring et al., 2003). G reen la n d an d a d ja cen t - The Tillite Group records the Ulveso and Storeelv Formations loosely bracketed between 1000 Ma Rb-Sr and 543 Ma (Hambrey and Spencer, 1987). Also, the Moraeneso Formation records glaciation as post-1230 ± 20 Ma Rb-Sr (Kalsbeek and Jepson, 1983). NE Svalbard records the older Elbobreen Formation as post-939 + 8 Ma and younger Wilsonbreen Formation (Gee et al., 1995). N o rw a y a n d a d ja cen t - A general section includes the Smalfjord glaciogenic deposit as younger than 630 Ma Rb-Sr and the younger Mortensnes glaciogenic deposit (Gorokhov et al., 2001). In Finmark the Smalfjord and Mortensnes Formations of the Vestertana Group record two intervals of glaciation; though they are certainly Neoproterozoic, exact radiometric constraints are so far considered too unreliable to use with any certainty (Evans, 2000). C a led o n id es - The Sito, Vakkejokk, Langmarkberg, Lillfjallet, and Moelv Formations are considered correlative glaciogenic deposits (Evans, 2000). The Lillfjallet Formation is cut by dikes dated at 665 + 10 Ma K-Ar (Claesson and Roddick, 1983). The Moelv Formation is overlain by shale dated at 612 ± 18 Ma (Vidal and Moczydlowska, 1995). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Irela n d a n d S co d a n d - The Port Askaig Tillite is constrained between 806 ± 3 Ma and 601+4 Ma and the Loch na Cille boulder bed as post-601 + 4 Ma (Evans, 2000; Dempster et al., 2002). Condon and Prave (2001) identified the Inishowen ice-rafted debris deposits as ca. 595 + 4 Ma through correlation, though it is unclear if these deposits correlate with the Loch na Cille boulder bed or represent an additional glaciation. F ra n c e - The putative glaciogenic Granville Formation in Normandy is constrained between 584 ± 4 Ma and the Cambrian (Dupret et al., 1990; Guerrot and Peucat, 1990). E a ste rn E u ro p e - The “Laplandian Horizon”, known mostly from bore holes, seems to record two distinct glacial intervals with a minimum age of 551 + 4 Ma (Compston et al., 1995). Three glaciogenic deposits are recorded in the Urals, the older, correlative Tany and Vil’va Formations, the medial Koyva Formation, and the younger, correlative Churochnaya, Staryye Pechi, and Kurgashlya Formations (Chumakov, 1981a; Chumakov, 1981c; Chumakov, 1981b). These formations are Neoproterozoic, though no radiometric constraints exist. A sia - In Siberia the undated Chivida Formation of the Chingasan Group is a possible glaciogenic deposit (Postel’nikov, 1981). Also, the Teptorgo and Patom Groups may contain undated glaciogenic strata. In Mongolia the Maikhan Uul member of the Tsagaan Oloom Formation records two possibly glaciogenic deposits as post-777-732 Ma (Lindsay et al., 1996). In south China the older Nantuo and younger Jiankou Formations are constrained as Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. post-748 ± 12 Ma (Ma et al., 1984). North China records the unconstrained Luoquan Formation (Zheng et al., 1994). Within the Tarim block of China the older Beiyixi, medial Tareeken or Altungol, and younger Hangelchaok Formations record unconstrained glaciation (Gao and Qian, 1985). S o u th A m erica - Peru records the Faro Member of the Marcano Formation that is constrained between underlying gneisses that are likely 1200-970 Ma (Evans, 2000) and granites at ca. 440 Ma (Shackleton et al., 1979). In Brazil the Jequitaf Formation and correlative units record a single episode of glaciation loosely constrained as Neoproterozoic (Karfunkel and Hoppe, 1988). A fric a - The following units essentially represent Neoproterozoic glaciations recorded in Africa. SW Namibia records the Kaigas glaciogenic deposit constrained between 772 + 5 Ma and 741+6 Ma, and the younger Numees glaciogenic deposit (Frimmel et al., 2002). In NW Namibia the Otavi Group records the older Chuos and younger Ghaub glaciogenic deposits, both younger than 746 ± 2 Ma (Hoffman et al., 1996). In Mauritania the Jbe'liat diamictite is constrained between 630 Ma Rb-Sr and 595 Ma Rb-Sr (Clauer, 1987). In Zaire/Zambia glaciogenic deposits are the Great Conglomerat and the younger Petit Conglomerat (Cahen and Lepersonne, 1981b; Cahen and Lepersonne, 1981a). In some localities the Great Conglomerate and the Petit Conglomerat are part of a three-diamictite complex; age constraints and correlations are poor. In Ethiopia the Tambien Group records a single glacial succession possibly constrained between ca. 800-750 Ma and ca. 613 Ma (Miller et al, 2003). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. O m a n - The Huqf Supergroup records the Ghubrah glaciogenic deposit dated at 723 +16/ -10 Ma (Braiser et al., 2001), the Fiq glaciogenic deposit dated at 654+ 12 Ma K-Ar (Gorin et al., 1982) and 562 + 42 Ma Rb-Sr (Dubreuilh et al., 1992), and the younger Shuram Formation (Hoffman and Schrag, 2002). H im a la ya - In India the Blaini Formation is tentatively thought to record Neoproterozoic glaciation, though age constraints are poor (Brookfield, 1987; Valdiya, 1995). The Manjir Formation (Frank et al., 1995) and Tanakki diamictite (Brookfield, 1994) are possible correlatives. A u stra lia - All but one of Australia’s glaciogenic deposits (the Egan Formation) are correlated to either the “Sturtian” or “Marinoan” glacial intervals represented by the Sturt Tillite and younger Elatina Formation, respectively, in south Australia. These deposits are post-777 ± 7 Ma (Walter et al., 2000), though the aforementioned regional correlations are not based on radiochronometric constraints. The Kimberley region of NW Australia records the older Walsh, medial Landrigan, and younger Egan glaciogenic deposits, all unconstrained (Grey and Corkeron, 1998; Li, 2000; Corkeron and George, 2001). A n ta rctica - The Goldie Formation may record Neoproterozoic glaciation, but at present the age constraints are in conflict (Evans, 2000). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Lower Cambrian trace fossils of the White-Inyo Mountains, eastern California: Engineering an ecological revolution

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Late Holocene depositional history of western shelf margin, Gulf of California, Mexico

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Creator Lorentz, Nathaniel James (author)

Core Title Seafloor precipitates and carbon-isotope stratigraphy from the neoproterozoic Scout Mountain member of the Pocatello Formation, Southeast Idaho: Implications for neoproterozoic Earth history

Degree Master of Science

Degree Program Geological Sciences

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

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-308738

Unique identifier UC11327947

Identifier 1417930.pdf (filename),usctheses-c16-308738 (legacy record id)

Legacy Identifier 1417930.pdf

Dmrecord 308738

Document Type Thesis

Rights Lorentz, Nathaniel James

Type texts

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

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

Repository Name University of Southern California Digital Library

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