Sedimentary Geology 141±142 2001) 465±499 www.elsevier.nl/locate/sedgeo

Neoproterozoic Chuar Group ,800±742 Ma), Grand Canyon: a record of cyclic marine deposition during global cooling and supercontinent rifting

Carol M. Dehler*, Maya Elrick, Karl E. Karlstrom, Gary A. Smith, Laura J. Crossey, J. Michael Timmons

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-1116, USA Accepted 19 January 2001

Abstract Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic Ð a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic ¯uctuations during the climatic transition into the Sturtian Ice Age ,750±700 Ma), as well as extension related to the dispersal of Rodinia. The mid-Neoproterozoic Chuar Group 1600 m thick) is predominantly composed of mudrock with subordinate meter-scale beds of dolomite and sandstone. Facies analysis leads to the interpretation of a wave- and tidal-in¯uenced marine depositional system. Diagnostic marine features include: 1) marine fossils and high local pyrite content in the mudrock facies; 2) mudcracked mud-draped symmetric ripples and reverse ¯ow indicators in the sandstone facies; 3) facies associations between all facies; and 4) no unequivocal terrestrial deposits. Chuar facies stack into ,320 dolomite- and sandstone-capped meter-scale cycles1±20 m thick) and non-cyclic intervalsof uniform mudrock facies20±150 m thick). Nearly all cycleshave mudrock bases indicating subtidal water depths. Dolomite-capped cycles shallow to peritidal environments peritidal cycles), some indicating subaerial exposure of varying degrees exposure cycles). Sandstone-capped cycles shallow to peritidal environments with no evidence of prolonged exposure peritidal cycles). Non-cyclic intervals represent the deepest water depths ,10sof meters). Peritidal cycles and non-cyclic intervals dominate the lower and middle Chuar Group. In the upper Chuar Group, the percentage of exposure cycles increases, as does the thickness of individual cycles and non-cyclic intervals, and the degree of subaerial exposure in the cycle caps. Correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness. Some thickness changes are coincident with local and regional extensional structures. These cycle trends re¯ect a resolvable mixture of tectonic and climatic controls. Cycles are interpreted to be high frequency and glacioeustatically-controlled based on comparison of cycle character and thickness to Phanerozoic examples, and lateral continuity of many cycles, respectively. Cycles in the lower and middle Chuar Group show characteristics suggestive of low- amplitude sea-level changes similar to meter-scale cycles from global greenhouse climates in the Phanerozoic. Upper Chuar Group cycles show characteristics suggestive of moderate-amplitude sea-level changes similar to Phanerozoic greenhouse± icehouse transition climates, indicating an increase in continental ice volume. Variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence, evidenced by thickening trends coincident with

* Corresponding author. Present address: Department of Geology, Utah State University, Logan Utah, 84322-4505, USA. Tel.: 11-91-435- 7972001. E-mail address: [email protected] C.M. Dehler).

0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0037-073801)00087-2 466 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 active rift-related structures, and also by variability in sediment supply. The presence of dolomite caps, in this otherwise siliciclastic succession, is interpreted to be due largely to rapid changes in climate related to short-term glacioeustatically- controlled sea-level changes. Four crude lithostratigraphic sequences 150±775 m thick) are de®ned based on dolomite-poor to dolomite-rich stratigraphic intervals. Long-term changes in cycle cap-lithology may be driven by long-term changes in climate, similar to short-term glacioeustatic controls on meter-scale cycle cap lithology. Large-scale thickness variability in these sequences is due to local differential tectonic subsidence. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Neoproterozoic; Chuar Group; Grand Canyon; Glaciation; Rodinia; Cyclicity

1. Introduction Laurentian margin ascomponentsof a developing miogeocline. The Neoproterozoic era 1000±543 Ma) isan important time in Earth history in terms of tectonic, atmospheric, oceanic and biologic evolution. It 2. Geologic setting, age control, and stratigraphy included the breakup of the supercontinent Rodinia Dalziel, 1997), dramatic climatic changes of possibly The Chuar Group is exposed exclusively in several global scale Hoffman et al., 1998), and the related right-bank tributariesto the Colorado River in eastern biological evolution leading up to the advent of multi- Grand Canyon, Arizona, USA Fig. 1). Thisexposure cellular life Knoll, 1994). A re®ned understanding of isbounded on the eastby the Butte fault zone, and on these phenomena and how they are interrelated all other sides by the Great Unconformity and requires detailed multidisciplinary studies of Neopro- Cambrian TapeatsSandstone,or locally by the over- terozoic sedimentary successions. lying Neoproterozoic Sixtymile Formation or under- The Chuar Group providesan important and well- lying Nankoweap Formation Figs. 1 and 2). The dated reference succession in southwestern North Chuar Group isgently folded, the main structure America Link et al., 1993; Karlstrom et al., 2000) being the north-trending Chuar syncline in the eastern that haspotential global importance for understanding part of the exposure belt Fig. 1). The Chuar Group is the middle Neoproterozoic. A new U±Pb date of also present in the subsurface along the Arizona±Utah 742 ^ 6 Ma on an ash at the top of the Chuar Group border where three wellspenetrated asmuch as700 m Karlstrom et al., 2000), combined with new evidence of Chuaria-bearing mudrocks, interbedded with dolo- for a marine origin thispaper), allowsusto useChuar mite and sandstone Rauzzi, 1990; Wiley et al., 1998). strata to understand mid-Neoproterozoic oceans and Structural and stratigraphic interpretations of the to correlate it with other successions in the western Chuar Group suggest that it was deposited in a rift US and globally. Thisisone of severalpapersthat link basin related to the development of the Cordilleran the Chuar Group to global Neoproterozoic systems margin Sears, 1990; Timmons et al., 2001). Chuar Karlstrom et al., 2000; Porter and Knoll, 2000; Group sediments were deposited synchronously with Timmonset al., 2001), and will be followed by movement on a series of N±S striking normal faults as another paper on the C-isotope stratigraphy of the shown by thinning of some strata towards the adjacent Chuar Group see Karlstrom et al., 2000 for prelimin- Butte fault zone Fig. 1), subtle thickening of some ary data). strata into the hinge region of the Chuar syncline, and This paper synthesizes new and published data on thickness changes across subsidiary intraformational the sedimentology and stratigraphy of the Chuar faultsTimmonset al., 2001). Although the Chuar Group, and presents new interpretations of deposi- Group is preserved today only on the west side of tional environments, depositional models, and the Butte fault zone, the persistent ®ne-grained char- controls on sedimentation and stratigraphy. We inte- acter suggests that the Chuar Group overlapped this grate these results with information from correlative fault zone throughout the Chuar deposition, and that sequences in western North America and Australia in the Butte fault did not become emergent until after an attempt to view these scattered outcrops of mid- Chuar deposition. Neoproterozoic marine successions bordering the The age of the basal Chuar Group is constrained by C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 467

Fig. 1. Geologic map of the Chuar Group, eastern Grand Canyon modi®ed from Timmons et al. 2001)) and location of the measured sections in thisstudy. 468 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 469

Table 1 General characteristics of the Chuar Group and overlying sixtymile Formation

Member/Formation Thickness m) Main rock types Fossilsa Lower contact

Sixtymile Fm. , 40±60 Siltstone, sandstone, intraclastic None found Unconformity? conglomerate and breccia Walcott Mbr , 250 Mudrock and dolomite Acritarchs, vase-shaped microfossils, Gradational bacteria, eukaryotic ®laments? Awatubi Mbr 200±344 Mudrock, sandstone, and dolomite Acritarchs, vase-shaped microfossils, Gradational bacteria, eukaryotic ®laments? Carbon Butte Mbr 45±70 Mudrock and sandstone None found Gradational Duppa Mbr 30±180b Mudrock, sandstone, and dolomite Acritarchs Gradational Carbon Canyon , 350 Mudrock, sandstone, and dolomite Acritarchs, bacteria Gradational Jupiter Mbr 400 Mudrock, sandstone Acritarchs Gradational Tanner Mbr 185 Mudrock, sandstone, and dolomite Acritarchs Angular unconformity

a See Table 2 for references. b Speci®c thickness measured by Ford and Breed 1973). a Rb±Sr whole-rock date of 1070 ^ 70 Ma derived 1973) Fig. 2). The GalerosFormation isfurther from the underlying Cardenas Basalt and associated divided into the Tanner, Jupiter, Carbon Canyon and diabase dikes and sills McKee and Noble, 1974; Duppa Members; the Kwagunt Formation is divided Elston and McKee, 1982). A direct age for the top into the Carbon Butte, Awatubi, and Walcott of the Chuar Group comesfrom a U±Pb zircon date MembersFord and Breed, 1973). We found these on an ash bed of 742 ^ 6 Ma Karlstrom et al., 2000). member subdivisions more useful for stratigraphic Paleomagnetic correlationsof the Grand Canyon analysis and summarized these in Table 1. Supergroup with other Meso- and Neoproterozoic Some previousworkershave interpreted the Chuar successions from North America indicate that Chuar Group to represent shallow marine paleoenviron- deposition occurred between ,850 and 740 Ma, at ments, and others have interpreted it to represent paleolatitudesof 5±20 8 north of the equator Weil et coastal plain, alluvial plain, and lacustrine deposi- al., 1999). Stromatolites and microfossil assemblages tional settings Ford and Breed, 1973; Vidal and found throughout the succession also indicate a Knoll, 1983; Reynoldsand Elston,1986; Reynolds Neoproterozoic age Vidal and Knoll, 1983; Vidal et al., 1988; Cook, 1991). Until thisstudy,no facies and Ford, 1985; Porter and Knoll, 2000). or stratigraphic analyses had been conducted in terms The Chuar Group isa 1600 m-thick, apparently of basin evolution, nor were depositional models conformable, fossiliferrous, unmetamorphosed suc- suggested for the majority of the Chuar Group. cession that is composed of dominantly mudrock Attempts to understand Chuar basin evolution are with subordinate, but recurring, sandstone and limited by the small areal extent of the Chuar outcrop carbonate beds. This succession is .85% mudrock, exclusively exposed in a 150 km2 area in eastern and the interbedded sandstone and carbonate mostly Grand Canyon; Fig. 1), which makes some aspects dolomitic) beds are typically cm-scale to m-scale in of traditional basin analysis and reconstruction impos- thickness Fig. 2). It is subdivided into the Galeros sible.Thislimitation iscountered by the excellent Formation lower) and the Kwagunt Formation vertical exposure of the succession, which provides upper) at the base of the prominent, thick sandstone ample information for interpreting the depositional unit of the Carbon Butte Member Ford and Breed, history of the Chuar basin.

Fig. 2. Stratigraphic column of the Grand Canyon Supergroup and composite measured section of the Chuar Group from this study. The facies distributions throughout the Chuar Group are shown on the right with general environmental interpretations. Facies abbreviations are linked to Table 2. 470 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 Shallow subtidal marine, high-energy shoreface/peritidal subaqueous tidal and longshore dunes in migrating tidaltidal/longshore channels/ sand waves in subtidalbarrier area), bar complex? Subtidal to intertidal marine, low energyhigher-energy with eventsrecorded by coarser grainsbelow fair-weather wavebase.Thicker intervalsare deepestwater faciesin Chuar Group Subtidal to intertidal marine, lowwave energy and tidal in¯uenceswith rareenergy higher eventsstormsorhigh tides) Shallow subtidal to intertidal marine, moderate to high-energy tidal ¯atto high-energy to above swash) fair-weather wavebase) to low energy below fair-weather wavebase) Interpretation Stratifera ± a Inzeria Interbedded with variegated mudrock facies. All faciesexcept large-scale crossbedded sandstone facies All faciesexcept subfacies, coarse dolomite facies, and all Walcott carbonate facies Laminated and massive dolomite faciesand both mudrock facies b 8 wt% Cook, biomarkers , a ), reactivation a Ford and Breed, 1973 and a algal ®laments, acritarchs, a and large-scale trough crossbeds. Ripple laminae, b 50 cm thick cosets, locally bi-polar ), asymmetrical ripple laminae, planar crossbeds and trough a # , rare hummocky crossbeds and associated groove casts, ¯aser, a 0.5 cm deep) in heterolithologic intervals, abundant local pyrite # soft-sediment deformation, and medium andcrossbeds thinly found within bedded larger-scale trough sedimentary structures.to Beds wedge-shaped. tabular Porter and Knoll, 2000 and1991; referencestherein), Karlstrom TOC et al., 2000), meter-scale to 10 meter-scale intervals crossbeds  referencestherein) wavy, and lenticular bedding, rip-up clasts ofmudcrack mudrock casts and rare common, dolomite, soft-sediment deformationconvolute load bedding, casts, ¯uid-escape structures), bedlenticular geometry and is bedsare tabular cm- to to meter thick surfaces Schopf et al., 1973; Fordal., and 1988; Breed, Porter 1973; and Bloeser, Knoll, 1985;Mud-shale/mudstone/silt-shale/siltstone/local 2000) Summons clay-shale, et clay is illitic/ mixed layer Dehler, unpublished data),quartz. silt Planar-horizontal and and sand ripple are laminaemassive, composed 2-7 thinly-bedded lenses of mm of thick siltstone sets) andcm sandstone, to deep), mudcrack casts symmetrical and interferenceintraclast ripplemarks impressions in common, silt-shale, rare saltnodulesin pseudomorphs, the marcasite lower Awatubi Memberscale Ford to and 10-meter scale Breed, intervals. 1973); Acritarchs meter- Vase-shaped microfossils, Quartz arenite/subarkose. Commonly massive, parallelplanar to laminations, low-angle symmetrical ripplemarks somemud with drapes mudcracked Quartz arenite/subarkose. Large-scale 1±2 mtangential crossbeds thick) sets of low-angle Clay-shale/mud-shale/silt-shale, clay is illitic/mixed layerunpublished Dehler, data). Planar-horizontal laminae tolenses massive, of siltstone, thin-bedded sandstone, and silici®ed ooids and pisoids, mudcracks Table 2 Summary of sedimentary facies and interpretations, Chuar Group,Facies Grand Canyon Common sedimentary features and fossils Facies associations Variegated mudrock facies mr-var) Laminated and crossbedded sandstone facies ss-lam) Large-scale crossbedded sandstone facies ss-cross) Dark mudrock facies mr-dark) C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 471 Intertidal with modi®cation in thezone supratidal Cook, 1991; thisstudy) Interpretation Shallow subtidal to intertidal marine, dominantly lower energy current withenergy high- events. Current was multidirectional based on the equant shape ofto domes. intertidal' `Littoral Ford and Breed, 1973) Shallow subtidal to supratidal marine,moderate low energy to wave and tidalwith rare in¯uences higher energy high eventsstormsor tides) Shallow subtidal to intertidal marine, dominantly moderate energy current with high-energy events. `Littoral to intertidal' Ford and Breed, 1973) Shallow subtidal to intertidal marine,energy low current with higher energywave current scour, tidal runoff). `Littoralintertidal' to Ford and Breed, 1973) a Coarse dolomite facies, dark mudrock facies, and laminated dolomite facies Interbedded with laminated dolomite and dark mudrocks All faciesexcept large-scale crossbedded sandstone facies Interbedded with laminated dolomite and variegated to dark mudrocks Interbedded with laminated dolomite and variegated to dark mudrocks 15 cm , , a 2 m thick. , and cm-scale of Cloud 1988) Stratifera of Ford and Ford and Breed Baicalia 0.6 m in dia; intraclasts of heads # Baicalia Ford and Breed, 1973) Inzeria 0.6 cm thick. # Schopf, 1975; Horodyski, 1993; Porter and Knoll, 2000; this a of Ford and Breed 1973b); ) study) Dolosiltite/dolarenite. Dominantly massive with rarelaminae, ghost vugs, horizontal intraclastic breccia, fractures, and dissolution pits acritarchs Peloidal dolomicrite/dolosiltite boundstone. Crinkly to0.25 smooth mm laminae ave. thickness) informs, compound domes stratiform, range domal, in and dia columnar intraclasts from abundant 1 commonly cm ¯at to pebblecommon, 2 calcite and m pseudomorphs clast-supported), of wide vugs gypsum and crystals 1and near top cm Breed, of to 1973). unit 1 Ford Tabular m thin tall, to medium beds Peloidal dolomicrite/dolosiltite/dolowackestone/rare peloidal boundstone. Smooth to crinkly laminaemm 1 to mm±1 cm-scale cm relief, thick) locallycross- with contorted and local laminae, wavy asymmetrical laminae, ripple massivepartings with common wispy between laminae, laminae, dark symmetrical,interference mudrock asymmetrical ripplemarks, and thin beds ofmudcracks ¯at-pebble cm scale, conglomerate, and 2 horizons withcasts, 60 rare cm-deep lenses cracks), mudcrack of ooidsstructures and and pisoids, ¯uid-escape locally structures; fetid, tabularrarely rare beds lenticular, `molar-tooth' 1 gypsum, cm±1 calcite, m andhalite?), thick, silica primary pseudomorphs gypsum after crystals Wide con¯uent domesare domesof m-scale Filamentous and spheroidal bacteria, vase-shaped microfossils con¯uent columnsare Peloidal doloboundstone. Crinkly to smooth non-isopachous laminae5 1± mm ave.) in columnar,upward single and and rarely clustered taper bulbous upward heads of that widening) widen 2±8 cm wide, tall), clustered in equant mounds common, one to three generationsoftwo bioherm, tabular local beds black chert, one to 1973) Boxonia Peloidal doloboundstone. Crinkly to smooth0.01±0.03 poorly-de®ned m laminae thick) in compound stratiform,columnsare columnar to straight and branching domal forms, 0.04 mare dia equant and to up elongate to 0.3 andmudcracksin top tall), can of domes be unit Ford compound and cmsto Breed, 2 1973); tabular m bed dia); local continued / ) ) and ) Inzeria Baic Ba-Box Facies Common sedimentary features and fossils Facies associations Table 2  Massive dolomite facies dol-mas) Stromatolite buildup facies Subfacies: Inzeria Stratifera  Laminated dolomite facies dol-lam) Stromatolite- buildup facies Subfacies: Baicalia  Stromatolite build-up facies Subfacies: Baicalia Boxonia  472 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 Interpretation Intertidal marine with supratidal modi®cations, moderate to high energy Cook, 1991; Dominantly thisstudy). lower energy current with high-energy events Shallow subtidal marine, high- energythicker beach beds) and correlative offshoredeposits storm and onshore peritidal thinner lenticular beds) Cook, 1991; this study) Shallow subtidal marine, moderate toenergy, high- near wavebase subaqueous dunesmigrating in tidal channels/tidal sand waves, storm swept, and associated laminites forming in intertidal area) a Interbedded with laminated dolomite and dark mudrocks Interbedded with laminated dark mudrocks, locally associated with laminated dolomites Overlain by interbedded calcareous dark mudrocksand interbedded laminated and massive dolomite beds ) Dolomicrite/dolosiltite. Elongate domal forms 0.3±0.9wide m and long, 0.15 m 0.3 m tall)dolomite with with crinkly alternating to laminae smooth oftepee laminae chert, structures commonly 0.8±1.5 and intraclasts highly mm) common, contorted, of headsthick) are in tabular beds 0.9 m Silica-cemented and -replaced ooids andcrossbedded, pisoids. trough Massive crossbeds, to shale ghost intraclasts,tabular 1 to cm±1 lenticular meter beds. thick FilamentousSchopf cyanobacteria et within al., grains 1973) Peloidal?) dolowackestone/grainstone. Planar horizontal andlaminae low 0.5 angle cm ave. thickness)high-angle and planar very crossbeds thin 20 cm bedsin thick to Lava-Chuar foreset), massive, Canyon hummocky local 1.0±3.0 crossbeds mthick in set dia., of 0.5 low-angle m thick) foresets,dissolution and intraclasts features single and associated 1.0 secondary with m monomict chert,to breccia, local massive medium beds to thick ) continued See Fig. 2 for stratigraphicDenotesdiagnosticmarine distribution feature. of facies. a b ¯aky dol. Facies Common sedimentary features and fossils Facies associations Table 2  Stromatolite build-up facies Subfacies: ¯aky dolomite  Oolitic± pisolitic grainstone faciesdol- oo/pis) Coarse dolomite facies dol-crs) C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 473 3. Facies analysis and paleoenvironmental marginal-marine settings and rare in lakes Berner and interpretation Raiswell, 1984; Can®eld and Raiswell, 1991).

The Chuar Group hasbeen subdivided into nine 3.2. Variegated mudrock facies facies and these are classi®ed using siliciclastic The variegated mudrock facies ,25% of Chuar .50% siliciclastics), carbonate .50% carbonate) Group) is also interpreted to represent deposition in and mudrock nomenclature of McBride 1963), marine subtidal to intertidal environments. These Dunham 1962), and Potter et al. 1980), respectively environments are similar to those interpreted for the Table 2). Salient points, from Ford and Breed's dark mudrock facies, however, this facies is higher in pioneer work on the Chuar Group 1973 and refer- silt content. This facies also contains greater iron encestherein) and from a detailed study of the oxide concentrationsand lessorganic matter than Walcott Member by Cook 1991), are integrated the dark mudrock faciesTable 2). The variegation into the descriptions and interpretations Table 2). is interpreted to be due to variable post-depositional Many facies appear throughout the succession, and oxidation processes permitted by the permeability of othersappear only once. Therefore, faciesare listed thissiltyfacies.The environmental interpretation of in decreasing order of abundance between and within this facies is also based on mudcracks and mudcrack each faciescategory. Detailson the distribution of casts, facies associations, and the presence of marine faciesare shownin Fig. 2 and Table 2. fossils Table 2).

3.3. Laminated and crossbedded sandstone facies 3.1. Dark mudrock facies The laminated sandstone facies cms to ,4m The dark mudrock facies ,60% of Chuar Group) thick) is interpreted to represent deposition by tidal indicatesdepositionof organic-rich mudsin low- and wave processes in the shoreface and intertidal energy, marine subtidal to intertidal environments. areas. This interpretation is based on wavy to ¯aser This interpretation is supported by mudcrack casts bedding, an array of bedformsand crossstrati®ca- and mudcracks, facies associations, marine fossils tion types, associated paleocurrent data, and facies and pyrite content. Dark mudrock intervalsthat do associations. not exhibit mudcracks or mudcrack casts were depos- Wavy to ¯aser bedding, including mudcracked ited below fair-weather wave base and represent the mud-draped symmetric ripplemarks, is common in deepest water facies in the Chuar Group. The presence tidal environmentsand isthe result of ¯uctuating of mudcrack casts and mudcracks indicates the peri- current strengths during tidal or storm deposition on odic submergence, emergence and desiccation of a the tidal ¯at Fig. 3a; Reineck and Wunderlich, 1968; mud¯at. The association of the dark mudrock facies Klein, 1971; Reineck and Singh, 1980). Paleocurrent with other facies, interpreted to represent marine analysis of symmetric±ripple±crest trends, domi- environments, strongly suggests related environments nantly from the Carbon Butte Member n ˆ 24†; and of deposition for the dark mudrock facies Table 2). lesser from the Carbon Canyon Member n ˆ 11† and Fossil types in the dark mudrock facies have been Awatubi Member n ˆ 3†; shows a broad distribution, observed in other Proterozoic marine successions yet an overall north-northwest to south-southeast Hofmann, 1977; Vidal and Knoll, 1983; Link et al., trend Fig. 4a). Thismay indicate a shoreline trend 1993; Knoll et al., 1989, 1991) and also suggest a that was roughly north-northwest during deposition of marine environment. The diversity of the fossil the upper Galerosand lower Kwagunt Formations. assemblage suggests shallow to moderately shallow The thin- to medium-bedded trough and high-angle marine conditionsVidal and Knoll, 1983). Locally crossbed sets indicate high-energy multidirectional, high pyrite abundanceshave been reported from the bidirectional, and unidirectional traction currents. Chuar Group Ford and Breed, 1973; Porter and Local reactivation surfaces indicate reversing ¯ow, Knoll, 2000; Shen and Can®eld, unpublished data). likely in a tidal regime Fig. 3b). Some of the cross- These pyrite abundances are common in marine and bedscould indicate a high-energy shoreface with 474 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 dunes and bars, or could represent tidal bedforms. The shoreline was intermittently affected by storms. low-angle and horizontal parallel laminae were likely Paleocurrent analysis of foresets show a radial distri- to be produced in the lower intertidal to subtidal zone bution of maximum dip directions n ˆ 38†; with a by traction ¯ow or suspension settling. The rare dominant ¯ow towardsthe southeastand bipolar hummocky cross strati®cation indicates that the ¯ow towards the northeast and southwest Fig. 4b). C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 475 These data are consistent with the hypothesis of a undulatory, and show internal ripple laminae. This north-northwest trending shoreline and the sandstone type of compound strati®cation is observed in tidally facies recording complex wave and tidal processes. produced epsilon foresets e.g. Allen, 1963; Clifton, Petrographic comparisons between the Chuar Group 1983). However, the lack of mud along individual laminated sandstone facies and Big Cottonwood cross-bed surfaces is problematic for a tidal setting. Formation sandstones interpreted to represent tidal Most descriptions of tidal channel epsilon crossbeds bundling Ehlers and Chan, 1999) show striking simi- include changes in grain size within the foresets, such larities. Further work needs to be done on Chuar sand- astidal bundlesor heterolithic bedding Allen, 1963; stones to demonstrate tidal bundling. Reineck and Singh, 1980; Clifton, 1983). However, This facies is associated with all other facies and mud-poor tidally produced epsilon crossbeds from the hasoverlapping sedimentary featureswith the Cambrian Waterfowl Formation are attributed to mudrock facieswavy to lenticular bedding) and the predominance of storm-enhanced tidal currents large-scale crossbedded sandstone facies trough Cloyd et al., 1990). Mud-poor high-energy tidal crossbedding). The tabular to lenticular geometry of deposits also occur in the Lower Proterozoic Mount this sandstone facies indicates that the sand was likely Guide Quartzite Simpson and Erikkson, 1991). The to be distributed in isolated to broad sheets that occu- lack of mud could also be due to local high tidal pied lower intertidal and shallow subtidal areas. current velocity in the lower tidal ¯at/shallow subtidal area, the presence of an ebb-dominated current, or the 3.4. Large-scale crossbedded sandstone facies scarcity of mud in this part of the system. The medium-to-large-scale trough cross strati®ca- Thisfacies4±11 m thick) isonly presentin the tion and its intimate association with the low-angle basal red sandstone of the Carbon Butte Member large-scale crossbeds could indicate the migration of Fig. 2) and isinterpreted to representa combination dunes within a tidal channel. Other possibilities are of tidal- and wave-produced bedformsand tidally that trough crossbeds re¯ect dunes within the shore- produced epsilon crossbeds on a shallow marine face of an associated barrier bar, or that all crossbeds shelf. This interpretation is based on the large-scale, are a complex of sandwaves that formed from tidal low-angle-inclined strata, superimposed smaller-scale and/or wave in¯uences in the shallow subtidal and sedimentary features within individual cross strata, intertidal area. association with small- to large-scale trough cross- Limited paleocurrent data of the low-angle large- beds, paleocurrent data, and facies associations. scale crossbeds and associated trough crossbeds show The dominant sedimentary feature in the basal red multi-directional current ¯ow that isinterpreted to sandstone is the sharply bounded, thick-bedded, indicate complex wave and tidal currents. Maximum wedge-shaped sets of low-angle cross strati®cation dip directionsof the low-angle large crossbedstrend that are very similar to tidally-produced epsilon cross- dominantly to the southeast n ˆ 16† Fig. 4c) and bedding Allen, 1963; Clifton, 1983) Fig. 3c). Indi- indicate either that the edgesof migrating channels vidual inclined layersare thinly bedded, commonly had trendsroughly perpendicular to the ¯ow direction

Fig. 3. Photos of sedimentary features in selected facies in the Chuar Group: a) mudcracked, mud-draped symmetric ripples of the variegated mudrock facies. Similar features are found in the dark mudrock facies and the laminated sandstone facies. This suite of features is interpreted to have formed from tidal processes in the intertidal zone; b) planar tabular crossbeds showing transport to the right white arrow) and smaller- scale climbing bedform within these foresets showing transport to the left other white arrow). This suite of features is interpreted to indicate reversing ¯ow direction, likely in a tidal regime; c) large-scale low angle cross strati®cation in the basal red sandstone of the Carbon Butte Member. These sedimentary features are interpreted as tidal point bar deposits or possibly tidal dunes or bars; d) laminated dolomite facies showing crinkly lamination indicating microbial in¯uences on deposition of micrite; e) large-scale mudcracks in the laminated dolomite facies are an indication of prolonged subaerial exposure and contortion of dolomicrite ®ll is interpreted as due to compaction and possibly a component of seismic shaking; f) bedding-plane view of dolomicrite-®lled polygonal cracks in the polygonal marker bed of 3e); g) meter-scale sandstone-and dolomite-capped peritidal cycles in the middle Chuar Group; h) two meter-scale exposure cycles in the middle Chuar Group. Hammer marks the top of the lower of the two exposure cycles. Meter-scale peritidal cycles stratigraphically above in background. 476 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

Fig. 3. continued) C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 477

Fig. 3. continued) 478 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

Fig. 3. continued) C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 479

Fig. 4. Equal area rose diagrams showing paleocurrent data from the sandstone facies: a) ripple-crest trends in the Carbon Canyon through Awatubi Members; b) maximum dip directions of the foresets in the Carbon Canyon through Awatubi Members; c) maximum dip directions of large-scale low-angle cross strati®cation in the basal red sandstone of the Carbon Butte Member; d) maximum dip directions of foresets in the basal red sandstone of the Carbon Butte Member. See text for interpretation of paleocurrent data.

N±NE along a shoreline trending N±NW), or that inlet area Reinson, 1992). This facies is intimately tidally/longshore-drift-produced sandwaves were associated with the variegated mudrock facies and the moving parallel to the shoreline. Maximum dip laminated sandstone facies, which are interpreted as directions of associated trough-crossbeds n ˆ 23† wave- and tidal-in¯uenced mixed sand±mud tidal ¯at show a radial distribution, and imply a complexity and offshore equivalents Table 2). of current typesthat are likely a combination of tidal and wave in¯uencesFig. 4d). Additionally, 3.5. Laminated dolomite facies crosslaminae within symmetric ripple formsin the lower ,2 m of the basal red sandstone show dominantly The laminated dolomite faciescmsto ,3 m-thick) eastward and lesser westward transport, indicating represents marine subtidal and intertidal deposition. reverse-¯ow conditions and ¯ow perpendicular to the The laminated dolomitesin the Walcott Member interpreted shoreline orientation. were previously interpreted as shallow subtidal to Based on the combined data, the large-scale, low- intertidal marine environmentsCook, 1991). We angle crossbeds are likely epsilon crossbeds, and agree with thisinterpretation and extend it to apply indicate the migration of tidal channelsthat were to all laminated dolomitesin the Chuar Group. This in¯uenced by long-shore drift e.g. Reinson, 1992). interpretation is based on the similarity of a suite of The associated trough crossbeds likely represent features to those seen in modern and ancient shallow dunes, some which were migrating within the inter- subtidal and intertidal marine environments e.g. preted tidal channels, and others that experienced Hardie and Ginsburg, 1977; Esteban and Klappa, complex currents either in the shoreface or simply 1993; Shinn, 1993; Demmico and Hardie, 1994; within sinuous tidal channels. This combination of Table 2). The most diagnostic features are the diver- wave- and tide-in¯uenced featureshave been sity and types of lamina, mudcracks, internal upward- observed in ancient and modern barrier island systems shallowing trends, and facies associations. and are most similar to features seen in the barrier Laminae ,1±10 mm) range from crinkly to wavy 480 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 to planar and form from suspension settling of ®ne litesin the Chuar Group were interpreted to represent grains, microbial agglutination or baf¯ing Demmico dominantly `littoral' and rare intertidal conditions and Hardie, 1994, and traction ¯ow Fig. 3d). The Ford and Breed, 1973; Cook, 1991). Our extension crinkly laminationscommonly form isolated build- of these interpretations is based on the presence of upsmm to cmstall) and can be recumbent or wildly vugs, facies associations, internal upward-shallowing contorted. Where laminae are associated with ripple- trends, and the occurrence of stromatolite morpho- marks and within ripple cross-laminated sets, they typesidenti®ed in the Chuar Group Table 2) in likely formed from tractional ¯ow produced by tidal other marine successions. Inzeria and Stratifera are or storm processes in the shallow subtidal and inter- reported from Precambrian marine successions such tidal zone. asthe Uchur-Maya region of Siberia Serebryakov, Within thicker .60 cm) bedsof thisfacies,an 1976), the Beck SpringsDolomite in Death Valley internal upward-shallowing trend is interpreted California Marian and Osborne, 1992), and in the based on the presence of mudcracks and or) a upper Mackenzie Mountain Supergroup and lower massive dolomite layer at the top. The mudcracks Windermere Group in Canada Narbonne and Aitken, are diverse in width mms to 15 cm) and depth 1995). Baicalia hasbeen reported from Precambrian mmsto 60 cm). The larger mudcracksare found in marine successions such as the Atar Group of Maur- two beds, at least one can be traced across the ®eld itania Bertrand-Sarfati and Trompette, 1976), the area polygonal marker bed), and are considered to Murky Formation in Canada Hoffman, 1976), the indicate prolonged subaerial exposure Fig. 3e and Uchur-Maya Region of Siberia Serebryakov, 1976), f). Facies associations with demonstrably marine the Beck SpringsDolomite Marian and Osborne, faciesaid in the environmental interpretation of the 1992), the Australian Supersequence 1 Hill et al., laminated dolomite faciesTable 2). 2000), the Mackenzie Mountain Supergroup Narbonne and Aitken, 1995), and Shaler Group 3.6. Massive dolomite facies Young, 1981). Boxonia hasbeen reported from marine successions such as the upper Little Dal Thisfacies20 cm to ,11 m thick) represents Group, Canada, the Polarisbreen Group, Spitsbergen, subaerial exposure and modi®cation of other dolomite and the Bitter SpringsFormation, Australia,aswell as facies. Birds-eye vugs, intraclastic breccia, and modi- from marine Neoproterozoic successions in the ®ed sedimentary structures collectively indicate Uchar-Maya region, the Olenek Uplift, and the subaerial exposure. Birds-eye vugs are common in Anabar Massif Preiss, 1976; Semikhatov, 1976). the massive dolomite facies and typically form in the upper intertidal to supratidal region Shinn, 3.8. Oolitic/pisolitic grainstone facies 1968). Intraclastic breccia, found irregularly in®lling cavities within the massive dolomite facies, are likely Thisfacies0.15±2.0 m thick) isinterpreted to due to solution collapse. Associated with the massive record shallow subtidal high-energy deposition and dolomite facies are soft-sediment-deformed laminae coeval offshore and onshore intertidal) storm and wispy laminae) interpreted to represent ¯uid escape tidal deposits. This facies was previously interpreted and differential compaction on the upper intertidal and as a high-energy environment, speci®cally shoal and supratidal ¯at. Massive dolomite facies with a high tidal channel deposition Cook, 1991). Inter- concentration of exposure features are interpreted to pretationsare basedon grain character, rare trough represent zones of prolonged subaerial exposure crossbedding, facies architecture, facies associations, Figs. 2 and 6). and modern and ancient analoguesCook, 1991; Table 2). 3.7. Stromatolite build-up facies 3.9. Coarse dolomite facies Thisfacies30 cm±13 m thick) representsmarine subtidal and intertidal environments, with some The coarse dolomite facies 6±24 m thick) repre- subfacies experiencing subsequent long-term subaer- sents high-energy conditions on a wave-dominated ial exposure Fig. 2, Table 2). Previously, stromato- shelf. This interpretation is based on 3-D hummocks, C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 481 a diversity of cross strati®cation types, and a shallow- no unequivocal evidence of terrestrial deposition in ing-upward trend. The hummocksand associated any facies e.g. lacustrine and alluvial deposits, paleo- hummocky cross strati®cation indicate combined- sols) Table 2). Carbon isotope data from the Chuar ¯ow traction currentsgenerated by stormsnear fair- Group show values consistent with Proterozoic weather wave base Harms et al., 1975). The associated seawater Karlstrom et al., 2000 and high local pyrite high-angle crossbeds are interpreted as the result of concentrations are suggestive of a marine setting e.g. migration of shoreface dunes. The low-angle and hori- Berner and Raiswell, 1984). zontal planar laminationswere likely produced by The Chuar Group issimilar to other Proterozoic swash or generated by suspension settling and or) marine successions representing similar depositional traction ¯ow. systems. For example, the `carbonate-terrigenous' member of the Crystal Springs Formation and 3.10. Chuar sea vs. Chuar lake `transition beds' of the Pahrump Group Link et al., 1993), the Thule Group of Canada Hoffman and Previousworkershave suggestedthat the Chuar Jackson, 1996), and the Burra Group Walter et al., Group represents lacustrine deposition Reynolds 2000) are all thick 1000s of meters), mixed siliciclas- and Elston, 1986; Reynolds et al., 1988), although tic±carbonate shallow-marine successions that are unequivocal data diagnostic of a lacustrine setting) stromatolite and acritarch bearing. There is no single were never reported. The depositional environments modern depositional system that can be used as an of Precambrian sedimentary successions are dif®cult analog for the Chuar Group. However, it contains to interpret due to the lack of body fossils and biotur- carbonate faciessimilarto thoseforming today in bation. We use microfossils, sedimentary structures, the Persian Gulf and Shark Bay, and has features facies associations, and geochemical trends to within the siliciclastic facies that are similar to discount the lacustrine hypothesis. For example, many modern tidal ¯ats such as those in the North vase-shaped microfossils and acritarchs similar to Sea and the Sea of Cortez. those in the Chuar Group are found in the Draken Conglomerate Formation, Spitsbergen, which is inter- 3.11. Sediment source preted to be marine based on the dimensions of the unit 2000 m thick and 650 km wide), sedimentary 3.11.1. Terrigenous sand and mud facies consistent with a marine platform setting, no Based on facies analysis, terrigenous sand and mud evidence of alluvial deposits, and carbon and sulfur were likely brought into the Chuar basin by a combi- isotopic values that are suggestive of Proterozoic nation of geostrophic, longshore, tidal, and storm seawater Knoll et al., 1991 and references therein). currents, local continental runoff and aeolian trans- Additionally, the Chuar Group microfossil assem- port. There isno evidence for a proximal delta within blage has been found in Proterozoic marine succes- the study area, yet it is likely there was one or more, sions in East Greenland and Svalbard Vidal and farther a®eld, that supplied much of the siliciclastic Knoll, 1983; Knoll et al., 1986), aswell asin the detritusto the basin. The overall ®ne-grained nature of marine Red Pine Shale of Utah Hofmann, 1977; the siliciclastic sediments in the Chuar Group Nyberg, 1982a; Link et al., 1993; Crossey et al., suggests low regional relief. 2000). Vase-shaped microfossils have also been The source of the sand is hard to determine based found in the marine upper Pahrump Group Link et on the scarcity of paleocurrent indicators and sand- al., 1993 and referencestherein), and the marine lower stone maturity quartz arenite to subarkose; Table 2). Little Dal Group of Canada contains Chuaria circu- The most likely source for the sand, based on avail- laris Aitken, 1981; Narbonne and Aitken, 1995). able paleocurrent data and the patchy distribution of Sedimentary structures and features in the two sand- some sand bodies, is longshore currents from distal stone facies of the Chuar Group are indicative of tidal deltaic sources and local derivation off the continent processes, and these two facies together are intimately by wind and local run off. The frosted texture of some interbedded with all other faciesTable 2). All Chuar sand grains within the mudrock and sandstone facies are consistent with a marine setting and there is facies suggests a component of aeolian transport. 482 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 The ¯uctuations in siliciclastic grain size and overall 4. Chuar Group cycles and stratigraphic trends siliciclastic ¯ux throughout Chuar deposition could be due to changesin climate and or) in the proximity to The traditional subdivisions of the Chuar Group distant deltas. membersand formations;Ford and Breed, 1973 Preliminary results from XRD analyses of the and referencestherein) are extremely useful for Chuar mudrocksindicate predominantly illite-rich mapping and descriptive purposes, but do not capture claysand quartz silt,with intervalsof mixed potential scales of genetically related strata. We layer clayswithin the Tanner, Carbon Canyon looked at stacking patterns cycles and lithostrati- and Walcott MembersTable 2; Dehler, unpub- graphic sequences) towards understanding the genesis lished data). The mixed-layer clays likely represent of the stratigraphic framework, and many of these input of volcanic ash into the Chuar basin. This is patterns crosscut formation and member divisions. consistent with reported ash layers from the Chuar Cyclicity in the Chuar Group isrecognized at the Group Elston, 1979; Link et al., 1993; Karlstrom meter-scale ,1to,20 m), and aslithostratigraphic et al., 2000). sequences 100-m-scale) Figs. 5 and 6), and as non- cyclic intervals20±150 m-thick intervals).Lateral correlation of cycles and lithostratigraphic sequences 3.11.2. Organic component was possible in intervals of the Chuar Group where Planktonic microbes, such as Chuaria circularis, lateral exposure and marker units are present mostly were the likely source of organic material supplied middle and upper Chuar Group). Describing and inter- to the basin throughout Chuar deposition ave. preting Chuar cyclicity hasshedlight on the controls weight% TOC ˆ 0:83; Dehler, unpublished data, on sedimentation and stratal patterns, and will aid in Table 2). Organic carbon concentrationsin interpretation of large-amplitude C-isotope variability mudrocksare controlled by a combination of carbon recognized in the Chuar Group e.g. Karlstrom et al., in¯ux from organic productivity, post-depositional 2000). preservation potential, and dilution by other sedi- ment typesPedersonand Calvert, 1990. The dark 4.1. Meter-scale cycles mudrock facieshad a greater organic carbon in¯ux, better post-burial preservation, and/or lower silici- Meter-scale cycles ,320 total) are present in all clastic sediment supply than the variegated mudrock membersof the Chuar Group Fig. 3g and h). Two facies. types are observed: sandstone- and dolomite-capped cycles with mudrock bases. Mudrocks are the domi- nant rock type in all cycles80±90% of cycle thick- 3.11.3. Carbonate ness), and sandstone-capped cycles are the most Sourcesfor Chuar carbonate grainsinclude preci- common cycle type ,75%). Sandstone-capped pitation induced by microbial activity, abrasion of cyclesare dominant in the Tanner Member through pre-existing carbonate rocks detrital mud and the lower Awatubi Member Figs. 3g and 6). Dolo- peloids), and direct precipitation from seawater as mite-capped cyclesare common in the Carbon ooids/pisoids or carbonate mud. The main source of Canyon and lower Duppa Membersand dominate carbonate waslikely precipitation induced by benthic the Walcott Member Figs. 3h and 6). Many cycle microbial communitiesliving in the peritidal realm. capscan be traced laterally where exposure permits The clotted texture observed in the lamina of some e.g. Fig. 7a and b). stromatolite complexes peloidal doloboundstones; Table 2) suggests in situ carbonate production Fair- 4.1.1. Sandstone-capped cycles child, 1980). Filamentous and spheroidal plant fossils The sandstone-capped cycles 0.15±19 m-thick, in the outer rindsof coated grainsin the ooid/pisoid 3.6 m ave.) consist of dominantly variegated or dark grainstone facies indicate that micro-organisms mudrock facies2.9 m-thick ave.), overlain by the played a role in the precipitation of these grains laminated sandstone facies 0.7 m-thick ave.). The Schopf et al., 1973). contactsbetween faciesare gradational or sharp C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 483

Fig. 5. Examplesof meter-scale cyclesin the lower, middle, and upper Chuar Group.

Figs. 5 and 6). Commonly, the mudstone grades 4.1.2. Laminated to massive dolomite-capped cycles upward into a siltstone and is capped sharply or The laminated to massive dolomite-capped cycle gradationally) by the laminated sandstone facies. 0.2±19.1 m-thick, 2.7 m ave.) ispresentin all dolo- The sandstone cap, typically the laminated sandstone mite-bearing members. The variegated to dark facies, is always overlain by one of the two mudrock mudrock faciesdominatesthe cycle 2.1 m-thick faciesand thisupper contact with the overlying cycle ave.) and is capped by laminated to massive dolomite is gradational to sharp. The sandstone cap typically facies0.6 m-thick ave.). Thiscycle type doublesits hasparallel horizontal laminae, and exhibitsmud thickness in the Walcott Member Figs. 5 and 6). The partingsand mudcrack castsin ,40% of the cycles. contact between the mudrock and dolomite faciesis The sandstone-capped cycles represent shallowing gradational or sharp. The mudrock facies commonly from the relatively deeper water mudrock facies grades upward into a siltstone and is overlain by the offshore) to higher-energy, shallow subtidal to inter- dolomite cap. The contact between the dolomite cap tidal environmentsshoreface/peritidal area) and are and the mudrock of the overlying cycle isgradational referred to asperitidal cycles.The gradational or or sharp. The dolomite cap is commonly mudcracked sharp contact between the mudrock and overlying and gradesupwardsinto ,10s of cms of massive to sandstone cap indicates a gradual or abrupt regression, wispy-laminated dolomite. It is also common for the respectively. The gradational or sharp contact cap to be either entirely massive or entirely laminated. between the sandstone cap and overlying mudrock Several dolomite capsdisplay evidence of prolonged of the next cycle indicatesgradual deepening trans- subaerial exposure, particularly in the Walcott gressive-prone cycle) and abrupt deepening fully Member. The polygonal marker bed in the upper regressive cycle), respectively. Carbon Canyon Member containslarge-scaleand 484 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

Fig. 6. Meter-scale cycles and lithostratigraphic sequences in the Chuar Group. Intrabasinal correlation shows that many marker bed cycle caps are traceable across the ®eld area. The bundling of dolomite-rich and dolomite-poor intervals de®ne four lithostratigraphic sequences. *South- ern localities section is a composite section: Tanner Member measured in Basalt Canyon, Jupiter through Duppa Members measured in Lava Chuar Canyon, Carbon Butte Member measured in Kwagunt Canyon, Awatubi and Walcott Members measured in Sixtymile Canyon, as was the Sixtymile Formation Timmons et al., 2001). See Fig. 1 for distances between localities. C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 485 deep prism cracks ,0.3±0.5 m deep) that form 30± water into the peritidal zone peritidal cycles). The 50 cm in diameter polygonsFig. 3e and f). Several upper contact with the Baicalia cap issharply overlain massive dolomite caps are vuggy or have local solu- by dark mudrocksof the overlying cycle and isa fully tion collapse breccias Cook, 1991). regressive cycle Fig. 7b). The top of the Bacalia/ This cycle type is interpreted to represent shallow- Boxonia cap isgradational with mudstone of the over- ing from relatively deeper water mudrock facies) to lying cycle and is transgressive prone. peritidal environmentsdolomite facies;i.e. peritidal cycles) with some caps experiencing subaerial expo- 4.1.5. Non-cyclic intervals sure exposure cycles) or prolonged subaerial expo- Non-cyclic intervalsare recognized in all members sure exposure cycles E1±E7 in Fig. 6). This cycle ,40% of succession) and are de®ned by uniform type can be transgressive or regressive prone. mudrock intervals20±150 m-thick intervals)that lack meter-scale cycles Fig. 6). The thickness of 4.1.3. Pisolite±oolite-capped cycles uniform mudrock intervalsisgreater in the upper Pisolite±oolite capped cycles ,2% of all cycles) Chuar Group. The lack of meter-scale cycles may are present only in the Walcott Member and are indicate deposition in water that was too deep to notably thicker than other cycle types1.5±10.5 m- feel the effects of sea-level change subtidal missed thick, 6.4 m-thick ave.). The dark mudrock facies beats, e.g. Elrick, 1996). Relatively deeper water dominatesthe cycle 5.7 m-thick ave.) and issharply conditionswere likely the result of either differential or gradationally overlain by a cap of silici®ed oolite± tectonic subsidence, which kept local depositional pisolite 0.4 m-thick ave., Fig. 5). No exposure centersin relatively deeper water mud deposition), features have been observed in the caps. This cycle or long-term sea-level rise. type is interpreted to represent shallowing from below wave base dark mudrock facies) into the shoreface± 4.1.6. Controls on meter-scale cyclicity foreshore/peritidal area peritidal cycle). This is a Possible controls on the development of meter- fully regressive cycle type. scale cycles in carbonate and siliciclastic settings include delta avulsion, tidal-channel migration, carbo- 4.1.4. Stromatolite 0build-up)-capped cycles nate tidal-¯at progradation, episodic tectonism, Four unique stromatolite build-up)-capped cycles climate-controlled changesin terrigenoussediment 1.5±12 m-thick, 6.5 m-thick ave.) have been identi- supply and glacioeustasy. Several of these controls ®ed Figs. 2 and 5). The variegated mudrock or dark can be ruled out when considering Chuar cycles. mudrock facies1.75 m-thick ave.) issharply or Since there isno evidence for a proximal delta, gradationally overlain by laminated dolomite 5 m- delta avulsion is not a viable control for the sand- thick ave.) and stromatolitic subfacies of variable stone-capped cycles. Tidal-channel migration e.g. morphotypese.g. Figs.5 and 7a,b). Thin interbeds Cloyd et al., 1990) isa viable control on sandstone- of mudrock faciesand/or laminated dolomite facies and dolomite-capped cycles, however, there are few sharply to gradationally overlie the stromatolite cap tidal channelsin the Chuar Group and thismechanism Figs. 5 and 7a,b). cannot explain the majority of cycles. Ginsburg's The Inzeria/Stratifera complex and the `¯aky dolo- 1971) carbonate tidal-¯at-progradation model was mite' cycle capsexhibit featuresindicating subaerial developed to explain upward-shallowing cycles on exposure of signi®cant duration Fig. 2). These two the Bahama platform. It cannot be used to explain cycles are interpreted as shallowing from offshore into Chuar dolomite-capped cyclesbecausethe Chuar the peritidal zone, followed by prolonged subaerial Group is a dominantly siliciclastic depositional exposure exposure cycles). Both of these cycles are system that lacks a substantial subtidal carbonate sharply overlain by peritidal facies and are therefore factory and also lacks a mechanism to cause cessation transgressive prone. of seaward progradation of the carbonate tidal ¯at The `Baicalia' and `Baicalia/Boxonia' capsshow e.g. edge of steep-sided platform). The remaining no evidence for long-term exposure and are inter- potential cycle controlsare episodictectonism, preted to represent shallowing from relatively deeper climate-controlled changesin terrigenoussediment 486 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

Fig. 7. a) Meter-scale cycle correlation in the marker bed interval of the Carbon Canyon Member. See Fig. 1 for location of measured sections and marker bed interval, and see Fig. 5 for explanation of symbols. Lateral discontinuity of some cycles is attributed to changes in paleo- bathymetry due to local differential tectonic subsidence and or) sediment supply. Note location of Butte fault relative to measured sections and thinning trend towards fault from site 9b to site 9a. b) West-to-east correlation of Baicalia marker bed across the Chuar syncline. See Fig. 1 for location of measured sections and see Fig. 5 for explanation of symbols. Note that the Baicalia bed is thickest in the axis of the syncline, suggesting differential tectonic subsidence during deposition. Also note lateral continuity of the Baicalia marker bed. supply, and glacioeustasy, of which the latter is deepening regressive-prone cycle, e.g. Elrick, favored see below). Other factors, including local 1996). Although ,80% of cyclesin the Chuar effectsfrom short-term climate change, variable Group are regressive prone, ,20% are transgressive tectonic subsidence, and sediment supply are consid- prone. Therefore, if episodic tectonism was the ered in¯uenceson cycle character i.e. could control control on regressive-prone cycles, it would be neces- uniformity of the cycle thickness, lateral continuity of sary to call upon another control to produce the trans- the cycle, or cycle cap lithology), yet cannot explain gressive-prone cycles, which are in many cases, overall cycle development. otherwise identical to the regressive-prone cycles. It Episodic fault-controlled subsidence Cisne, 1986) would be easier to call upon a mechanism that could isan unlikely driver for generating many of the Chuar produce both transgressive- and regressive-prone cycles. For episodic tectonism to be the driver, we cycles. Additionally, Cisne's 1986) model predicts would expect cycles to consistently show abrupt the thinning of cyclesaway from the controlling C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 487

Fig. 7. continued) structure. We observe many laterally continuous on meter-scale cyclicity, glacioeustasy is the only cycles that are uniform in thickness, and also observe viable control to produce this consistent pattern for cyclesthat thin in different directionsrelative to one all Chuar cycle types. another and relative to active structures i.e. not all The Chuar cycle typesperitidal and exposure cyclesthin in the samedirection; Fig. 7a and b). cycles) are both transgressive-prone and regressive- Climatically controlled changesin terrigenoussedi- prone, which is readily explained by glacioeustasy ment supply, combined with basin subsidence, have e.g. Elrick, 1995). Peritidal cycles show deposition been invoked to explain siliciclastic parasequences or in the offshore area during transgression mudrock cycles) in Cretaceous nearshore deposits of the facies) followed by deposition in the shoreface±fore- Western Interior seaway e.g. Elder et al., 1994). shore/peritidal area laminated sandstone facies, all Although thisisa likely mechanismto explain the dolomite facies) during regression. Exposure cycles origin of purely siliciclastic cycles in the Chuar require a drop in sea level, because marine carbo- Group, it cannot explain the origin of dolomite- nate-sediment production cannot ®ll above sea level. capped cycles especially exposure cycles) or the The lateral persistence of many cycles supports occurrence of interbedded dolomite-capped cycles a glacioeustatic control e.g. Grotzinger, 1986), with sandstone capped cycles. however, the distance of correlation in this study is Evidence supporting glacioeustasy as the control on limited by the areal extent of Chuar outcrop belt Chuar cycle development includesthe predictability maximum intrabasinal correlation of 10 km). For of vertical facies patterns, the nature of cycle types, example, all stromatolite-capped and many laminated and the lateral persistence of many cycles. Regardless dolomite-capped cyclesare traceable acrossthe®eld of the cycle cap rock type, .60% of the Chuar Group area. The sandstone caps are generally not distinctive consists of stacked meter-scale cycles, which are and therefore make poor marker beds, yet the basal consistently composed of basal mudrock facies red sandstone and white sandstone marker beds are offshore environment) overlain by peritidal facies within cyclesthat are laterally traceable acrossthe which may be modi®ed by subaerial exposure). ®eld area Figs. 2 and 6). Thiscyclic pattern requiresa short-term cyclic Although there isno time control on meter-scale control. After ruling out all other potential controls Chuar cycles, the interpreted corresponding ¯uctuations 488 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 in sea level could be of high frequency 104 to 105 trendsof cycle and cycle-cap thicknessvariability are years). This is likely because the range of thicknesses not alwayspredictable: all cycle capsdo not thin or of Chuar cyclesapproximatesthoseof meter-scale pinch out towardsthe Butte fault, nor do cycles cyclesthat are interpreted to be glacioeustatically consistently thicken into the axis of the Chuar controlled, and attributed to orbital forcing e.g. syncline Fig. 7a). This is likely due to the generation Beach and Ginsburg 1978; Heckel, 1986; Goldham- of differential fault-generated subsidence along strike mer et al., 1987). Orbitally forced upward-shallowing of the Butte fault e.g. `porpoising' of the Chuar cyclestypically are generated in ,20, 40, 100 and syncline axis, Timmons et al., 2001). Many of the 400 ky frequencies. cycle and cycle cap thickness changes can be explained by variations in sediment supply. This 4.1.7. Local and climatic in¯uences on Chuar cycle factor can cause variations in paleobathymetry character that result in laterally discontinuous, lenticular Although glacioeustasy is interpreted to have bodies of sediment e.g. `island model' of James controlled Chuar cycle development, other factors 1984), Pratt and James1986), Kozar et al. in¯uenced the variability in thickness and laterally 1990)). Thisfactor isa likely in¯uence on cycles continuity of cycles, as well as the variability in where there isno demonstrablerelationshipwith rock type of the cycle cap. These factors are variabil- structural features Fig. 7a). ity in local paleobathymetry controlled by variation The variability in cycle-cap rock type speci®cally in tectonic subsidence and sediment supply) and where dolomite faciesintermittently cap terrigenous short-term climate change. mudrock facies in a succession otherwise dominated From Chuar Group facies, cycle analyses and intra- by sandstone-capped cycles) can be explained by basinal correlation, it is apparent that some Chuar short-term climate change associated with glacial- cycleschange laterally in thicknessandthe cycle interglacial phases and less so by Walther's Law. cap isnot alwayslaterally traceable Figs.5±7a). The application of Walther'sLaw to the scaleof indi- These observations can be explained by variations in vidual cyclesimpliesthat carbonate, sandstone, and paleobathymetry, controlled by variability in tectonic mudrock faciesco-existedon the shelf.If thiswere the subsidence and sediment supply. Differential tectonic case, we should see signi®cant inter®ngering and subsidence is interpreted as an in¯uence on paleo- mixing of these rock types on a cm-scale. This is bathymetric variability, and hence, cycle character, observed locally in some of the more heterolithic because some cycle thickness changes are coincident intervalsin the Chuar Group basalJupiter, Carbon with intraformational faults, and some cycle caps Canyon, Duppa, and basal Awatubi Members), where thicken or thin coincident with tectonic structures. the dolomites are silty to locally sandy or have mudrock For example, the Baicalia cycle cap shows multige- partings. However, many Chuar dolomites are low in nerational bioherm growth at localitiesthat are coin- siliciclastic content, do not have mudrock partings, and cident with the Chuar synclinal axis, which has been exhibit sharp contacts with siliciclastic facies. demonstrated to be controlled by syndepositional movement on the adjacent north-south-trending 4.2. Model for generation of dolomite-capped cycles Butte fault Fig. 7b; Timmonset al., 2001). Another example isthe three prominent dolomite bedsin the From the Quaternary record, it isapparent that it uppermost Chuar Group. These beds pinch out later- wasdrier at tropical latitudesduring glacial intervals ally to the east towards the Butte fault), suggesting than during interglacial intervalsClapperton, 1993; that deposition during upper Walcott time preceded Broecker, 1996). Soreghan 1997) recognized how and/or wasconcurrent with activity along the Butte these Quaternary low-latitude wet-to-dry cycles fault Elston, 1979; Cook, 1991). These examples are might work in concert with glacioeustatic sea-level best explained by the generation of differential changes to generate mixed siliciclastic±carbonate tectonic subsidence during development of the upward-shallowing cycles. She suggested that for Chuar syncline, the Butte fault, and the greater Cordil- the late Paleozoic of the southwestern US, low- leran rift basin Timmons et al., 2001). However, the latitude climatic conditionsduring high-frequency C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 489

Fig. 8. Depositional model for the generation of short-term) meter-scale cycles in the Chuar Group. During short-term transgressions, high terrigenous sedimentary input suggests a wet climate phase linked to short-term interglacial phase in low-latitude regions). During short-term regressions, terrigenous input is diminished due to a change to more arid conditions linked to glacial phase in low-latitude regions). However, this short-term arid climate phase can be over-run by long-term climate change and or) tectonism, and can result in high terrigenous input during short-term regressions e.g. Jupiter Member sandstone-capped cycles). See Fig. 6 for the link to lithostratigraphic sequences. sea-level rise and highstand were relatively wet and dominated Chuar depositional system. During short- siliciclastic sediment supply increased resulting in term transgressions interglacial phase), the climate in siliciclastic-dominated deposition in nearshore and the low-latitude study area was relatively wet, result- offshore environments. During the following sea- ing in deposition of up to sand-sized siliciclastic level fall and lowstand, the climate was drier and grains in the nearshore and offshore regions Fig. 8). siliciclastic sediment supply was decreased, During the ensuing regression glacial phase), the permitting an increase in carbonate productivity and climate was dry, resulting in a decrease in the silici- carbonate cycle cap deposition. Soreghan's 1997) clastic sediment supply particularly sand-sized frac- model impliesthat Walther'sLaw cannot be applied tion), which permitted an increase in carbonate to successions deposited during rapid climate productivity in peritidal environmentsFig. 8). changes. When lowstands were reached, peritidal environments Using Soreghan's 1997) model, we suggest the were subaerially exposed. The occurrence of some following conditionsto explain the deposition of terrigenous sand and silt in dolomite caps could be dolomite cycle caps in the otherwise siliciclastic- due to: 1) reworking of relict grainsdeposited during 490 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 wetter phases; 2) eolian input; and or) 3) input from and Read, 1992; Balog et al., 1997; Elrick, 1995). All geostrophic or longshore currents. of these Phanerozoic successions were deposited In thismodel, we interpret that during all short-term during greenhouse climate modes when the volume transgressions and highstands wet climate), silici- of continental ice was low to absent e.g. Fischer, clastic muds were being deposited offshore Fig. 8) 1982) therefore, the amplitude of the short-term with no carbonate production in peritidal environ- glacioeustatic sea-level oscillations were low. As a ments. It is only during regressions that siliciclastic result of relatively low-amplitude oscillations less sediment supply decreased enough to permit carbo- than 10 m), short-term sea-level fall rates are slow nate production and during lowstands the carbonates such that tidal ¯at progradation rates can keep pace were subsequently exposed). In the lower and middle with or exceed sea-level fall rates generating common parts of the Chuar Group, many successive cycles are tidal-¯at capped cycles. With the slower rise rates, capped by sandstone. This suggests that over extended deeper water conditionsare rarely attained suchthat intervals of time, some other mechanism in¯uenced cycle bases tend to be composed of relatively shallow siliciclastic sediment supply during sea-level fall/ water subtidal facies and deposition during the initial lowstand. Soreghan 1997) suggests that increased rise is common transgressive-prone cycles). tectonic activity could overprint the effectsof In the upper two membersof the Chuar Group, the glacial±interglacial climatic in¯uencessuchthat the character of the cycleschanges.Here, the cycles entire upward-shallowing cycle is siliciclastic in double their average thickness from ,3to,6 m), composition. Tectonic activity could overprint the basal cycle facies are consistently composed of climate change during Chuar time during generation rocks representing the deepest water facies dark of hinterland relief due to faulting. We additionally mudrock facies interpreted as sub-storm wave-base suggest long-term climate change as a control on deposits), there is a decrease in the number of trans- cycle-cap rock type, as discussed below. gressive prone cycles ,10%), and prolonged subaer- The Soreghan model was developed using icehouse ial exposure features within exposure cycle caps are climate conditionsPennsylvanianand Quaternary) more common ,40% of caps; Fig. 6). These features when glacioeustatic ¯uctuations were large 50± are more characteristic of Middle to Upper Ordovi- 100 m). This model is also relevant during less cian and Mississippian cycles that have been inter- extreme climate modes. For example, even during a preted to form in response to moderate amplitude greenhouse mode, orbitally induced changes in low- glacio-eustatic sea-level oscillations Elrick and latitude rainfall patternshasbeen invoked to explain Read, 1991; Read et al., 1995; Pope and Read, meter-scale carbonate-siliciclastic cycles in the 1998): moderate amplitude oscillations signal an Cretaceous of the Western Interior seaway Barron increase in continental ice volume and a transition et al., 1985; Elder et al., 1994; Sageman et al., 1997). into an icehouse climate mode. This interpretation of low-amplitude oscillations for the lower and 4.2.1. Implications of glacioeustasy and rapid climate middle Chuar Group followed by moderate amplitude change oscillations in the upper Chuar Group is consistent Almost all of the cycles in the lower and middle with the age of the uppermost Chuar Group of Chuar Group are capped by peritidal faciesperitidal 742 Ma, which iscloseto the age of the widespread cycles), less than 2% of the cycle caps display Sturtian glaciation estimated to have occurred evidence of prolonged subaerial exposure, and 20% between about 750±700 Ma e.g. Knoll, 2000; Walter of all cycles are transgressive prone. In addition, non- et al., 2000). cyclic deeper subtidal mudrock intervals, which are If thisinterpretation of increasingamplitudesof interpreted as intervals of subtidal `missed beats', are short-term glacioeustatic oscillations in the mid- present throughout the succession. These four key Neoproterozoic iscorrect, it hasinterestingimpli- features are characteristic of cyclic successions of cationsfor paleoclimate and paleogeographic Cambro-Ordovician, Devonian, Upper Triassic, reconstructions of this time interval. Several paleo- and Cretaceous age e.g. Strasser, 1988; Koerscher geographic reconstructions for the Meso- and Neopro- and Read, 1989; Osleger and Read, 1991; Montanez terozoic suggest that the majority of continents lay at C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 491 equatorial to low latitudesPowell et al., 1993; Li et warmer. Regardless of continental reconstruction, al., 1995; Karlstrom et al., 1999; Evans, 2001), though the interpretation of Chuar Group cyclesrecordsa the lack of paleomagnetic control on the paleolatitude transition from low amplitude to moderate amplitude of some continents also permits continental posi- glacioeustatic sea-level change implying an increase tionsat higher latitudes.If the mid-Neoproterozoic in continental ice volume starting before 742 Ma. world wascharacterized by dominantly low-latitude continents, then the interpreted low- to moderate- 4.3. Lithostratigraphic sequences amplitude glacioeustatic oscillations imply not greenhouse/greenhouse±icehouse transition modes, Four crude lithostratigraphic sequences ,150± but rather a global climate cold enough to permit 775 m thick) were identi®ed in the Chuar Group and small to moderate volumes of continental ice to accu- are de®ned by dolomite-poor to dolomite-rich strati- mulate on low-latitude continents. The ensuing wide- graphic intervals Fig. 6). The base of sequence 1 sits spread Sturtian Ice Age then suggests very large above a long-term exposure cycle, E1 Tanner dolo- continental ice volumes. In fact, the Sturtian Ice mite, Figs. 2 and 6). Above E1 is ,180 m of sand- Age is the ®rst of at least two widespread glacial stone-capped peritidal cycles interbedded with three eventsinterpreted by Hoffman et al. 1998) to repre- non-cyclic intervals. The top of this sequence consists sent a `snowball Earth'; a climate mode much more of several dolomite-capped peritidal and exposure severe than the well known icehouse climate modes of cycles, including the Inzeria/Stratifera complex E2, the Phanerozoic Era. Assuming a low-latitude disper- Figs. 2 and 6). Sequence 2 consists of ,450 m of sing supercontinent, the Chuar Group records the tran- peritidal cycles sandstone- and dolomite-capped), sition from a very cold Earth cold enough to and associated non-cyclic intervals. Above this is accumulate small to moderate volumes of ice on ,325 m of peritidal dolomite- and sandstone- low latitude continents) to a widely glaciated Earth. capped) and exposure cycles. Sequence 3 is de®ned However, the Chuar Group hasno record of tillitesor by 150 m of stacked peritidal cycles sandstone caps) associated incised valleys, and combined with the and non-cyclic intervals, and capped by ,5 m of the ,742 Ma date at the top of the succession, this Boxonia/Baicalia complex. Sequence 4 consists of a suggests that widespread glaciation post-dates lower ,200 m dominantly non-cyclic interval over- 742 Ma. lain by 250 m of peritidal and exposure cycles all Paleomagnetic constraints on continental positions dolomite caps), and non-cyclic intervals. This in the Meso- and Neoproterozoic are uncertain sequence contains several long-term exposure cycles enough Meert, 1999) such that an alternative inter- E4±E7) Fig. 6). pretation can explain the interpreted low- to moderate These four sequences can be correlated across the amplitude glacioeustatic oscillations. If at least one ®eld area Fig. 6) suggesting at least a regional continent lay at relatively high latitudes, then small control. The limited age control and limited areal to moderate amountsof continental ice could accumu- extent of the Chuar Group outcrop belt doesnot late and cold global climatic conditionsneed not be permit further correlation; however, the thickness of invoked. In fact, the global climate could be relatively the sequences argues for long-term ,106 years) warm and the one or more higher-latitude continents) controls. accumulating ice could account for the interpreted For meter-scale cycles, we interpret the presence of low to moderate amplitude glacioeustatic oscillations; dolomite caps in this siliciclastic-dominated succes- more similar to Phanerozoic analogues. sion as the result of a dry climate associated with Thus, depending on mid-Neoproterozoic paleogeo- lowstands of short-term sea level glacial phase). If graphic reconstructions, the interpreted low to moder- thismeter-scale cycle interpretation iscorrect, then ate amplitude glacioeustatic oscillations support thick stratigraphic intervals that are rich in dolomite either a globally very cold climate assumes all conti- caps might suggest long-term dry climate phases, and nentsare equatorial to low latitude) leading into the thick intervalsof sandstone-capped cyclesmight be Sturtian Ice Age, or if at least one continent lay at a signaling long-term wet climate phases. These long- higher latitude, the global climate could have been term wet-to-dry climate changesmight be related to 492 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

Fig. 9. Regional correlation of the Chuar Group with other known mid-Neoproterozoic successions in western North America and a repre- sentative best-dated) reference section from eastern Australia the Adelaide basin). Inset shows location of mid-Neoproterozoic successions relative to plate reconstruction proposed by Brook®eld 1993), Karlstrom et al. 1999) for the inception of Rodinia breakup. Abbreviations and citations for each succession follows: ASB ˆ Australian Superbasin Walter et al., 2000); P ˆ Pahrump Group Marian and Osborne, 1992; Link et al., 1993); CG ˆ Chuar Group Ford and Breed, 1973; Cloud, 1988; thisstudy);BC ˆ Big Cottonwood Formation Link et al., 1993), Note: We cannot ®nd an original reference that reports the presence of vase-shaped microfossils Melanocyrillium)orChuaria in the Big Cottonwood Formation as was suggested by Link et al. 1993)and Ehlers and Chan 1999) and references therein; UM ˆ Uinta Mountain Group Nyberg, 1982a,b; Vidal and Ford, 1985; Link et al., 1993); MM/W ˆ Mackenzie Mountain and Windermere Supergroup Narbonne and Aitken, 1995); S ˆ Shaler Group Young, 1981; Asmerom, 1991. long-term changesin continental ice volume Read et sequences 2 and 3; however, correlation was not al., 1995; Read, 1998) Fig. 6). possible for the entire succession Fig. 6). These Intrabasinal correlations between lithostratigraphic variationsin thicknessarelikely attributable to differ- sequences show thickness variations across the basin ential tectonic subsidence related to regional exten- Fig. 6). The greatest thickness variations are seen in sional structures Timmons et al., 2001; this study). C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 493 5. Regional Correlation of Chuar Group break up of the Rodinia supercontinent about 750 Ma Link et al., 1993; Karlstrom et al., 2000). The Chuar Group hasbeen correlated with other Many sedimentary successions can be correlated successions in North America based on lithostratigra- with the Chuar Group based on isotopic age control phy, biostratigraphy, and geochronology Link et al., Fig. 9). The lower Little Dal Group, Canada isolder 1993). The 742 Ma date from the Chuar Group Karl- than the 779 Ma ma®c dike that intrudesthe lower strom et al., 2000), when combined with re®ned part Heaman et al., 1992). The CoatsLake Group paleontologic and stratigraphic information from the unconformably overliesthe Little Dal Group and is Chuar Group Porter and Knoll, 2000; thisstudy)and subsequently overlain by the Rapitan Group which is new paleomagnetic data from North America and younger than the 755 ^ 18 Ma granite clast in the Australia Karlstrom et al., 1999), allow for an tillite G.M. Ross in Klein and Beukes, 1993). The updated synthesis of mid-Neoproterozoic strati- Shaler Group, Victoria Island is older than the graphic correlationsFig. 9). 723 Ma Franklin dikesand younger than the 1.2 Ga Poorly sorted diamictites, interpreted to be tillites, basalt Wanless and Loveridge, 1972; Heaman and cap many of the mid-Neoproterozoic successions and Rainbird, 1990). In Australia, the upper Callana are correlated with Sturtian glaciogenic rocksof Group isyounger than the 827 Ma Gairdner dikes Australia ,750±700 Ma; Knoll, 2000; Walter et and containsthe 802 Ma on Rook tuff. The overlying al., 2000). Although precise dates are lacking for Burra Group hasa basalrhyolite of 777 Ma and is many of these glaciogenic rocks, it is often assumed capped by the Sturtian tillite of Australia Walter et that they may be synchronous and hence the base of al., 2000) Fig. 9). these deposits is often used as a datum for correlations Despite the long stratigraphic ranges of Precam- Fig. 9; e.g. Knoll, 2000). Sturtian tillitesare not brian fossils and stromatolites, biostratigraphy can present in the Chuar Group, the Uinta Mountain still be a useful correlation tool. The Chuar Group Group or the Shaler Group. The Sixtymile Formation, contains numerous stromatolites, the acritarch, which conformably?) overliesthe Chuar Group, does Chuaria circularis, and the vase-shaped microfossil not resemble a tillite, yet does exhibit paleo-channels Melanocyrillium, all of which are found in other Mid- tensof metersdeep that may indicate effectsof base- Neoproterozoic deposits Fig. 9). Perhaps the stron- level change caused by a glacially induced marine gest evidence for correlation of the Chuar Group with regression. A 12½ negative shift in C-isotope compo- the Red Pine Shale of the Uinta Mountain Group lies sition in the upper Chuar Group supports this latter in the commonality of a Chuaria/vase-shaped micro- interpretation Dehler et al., 1999; Karlstrom et al., fossil assemblage Ford and Breed, 1973; Hofmann, 2000), because this shift is similar in magnitude to 1977; Bloeser, 1985; Vidal and Ford, 1985; Nyberg C-isotopic shifts worldwide that precede the Sturtian 1982a,b; Link et al., 1993). The Little Dal Group of glaciogenic deposits Kaufman and Knoll, 1995; Canada and the Burra Group of Australia also contain Kaufman et al., 1997; Hoffman et al., 1998). Chuaria Narbonne and Aitken, 1995), asdo many Most of these mid-Neoproterozoic successions are pre-Sturtian successions in the North Atlantic region dominantly shallow-marine deposits that are stroma- Hofmann, 1977; Vidal and Knoll, 1983; Vidal and tolite-bearing and or) acritarch-bearing and contain a Ford, 1985). Vase-shaped microfossils are also known mixture of siliciclastic and carbonate rock types Fig. from the Beck Spring Dolomite and the overlying 9). Similar marine rock types and fossil assemblages `transition beds' of the Pahrump Group Horodyski, lead to the concept that all of these successions were 1987), the `lower Uinta Mountain Group' Nyberg, connected to a common ocean. Emerging carbon 1982b), and other localitiesworldwide Porter and isotope data also imply this connection e.g. Knoll, 2000). Asmerom, 1991; Narbonne and Aitken, 1995; Dehler Formally named stromatolites in the Chuar Group et al., 1999; Prave, 1999; Karlstrom et al., 2000; include Baicalia, Inzeria, Stratifera, and Boxonia Walter et al., 2000). Additionally, all successions in Ford and Breed, 1973; Cloud, 1988), and some or Fig. 9 contain evidence of synextensional deposition, all of these stromatolites are present in other mid- suggesting that basin formation was related to the Neoproterozoic successions Fig. 9). Baicalia isthe 494 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 most cosmopolitan stromatolite, is found in all stro- that it was hundreds of kilometers east of the site of matolite-bearing mid-Neoproterozoic successions eventual rift separation of Laurentia from western included in Fig. 9, and is a potential index fossil for continentsFig. 9 inset). the lower?-mid-Neoproterozoic. Inzeria ispresent Cratonic basins commonly overlie pre-existing within all of the North American successions, Strati- Precambrian structures Bally and Snelson, 1980), fera is present throughout the Canadian successions, which is also the case for the Chuar Group. The and Boxonia ispresentin the upper Little Dal Group Chuar Group overlies ,3 km of Mesoproterozoic Fig. 9). The Beck Spring Dolomite and `transition sedimentary and volcanic rock Unkar Group, Fig. beds', the upper Chuar Group, and the Red Pine 2), interpreted to record pre-Chuar deposition 1.2± Shale have the most fossils and or) stromatolites in 1.1 Ga) in an extensional basin that formed at high common: these successions likely shared the same anglesto the Grenville collisionalorogen during the sub-basin Fig. 9). assembly of Rodinia Timmons et al., 2001). The Michigan basin is underlain by part of the Keweena- wan rift complex 1.1±1.2 Ga) Fowler and Kuenzi, 6. Basin interpretation 1978) and the Williston basin overlies a Precambrian suture zone Thomas et al., 1987). Likewise, the The Chuar Group isinterpreted to represent a Adelaidian basins overlie a series of older sedimen- cratonic basin a basin of extensional origin ¯oored tary basins Preiss, 2000). Klein 1995) hypothesizes by continental crust inboard of the plate margin that the formation of cratonic basins is coeval with the Miall, 1984; pp. 505, 596)) that experienced exten- breakup of supercontinents, which we postulate was sion related to the rifting of Rodinia. This basin inter- the fate of Rodinia during Chuar deposition: the Chuar pretation is based on: the internal stratigraphy of the basin likely formed as a downwarped part of the crust Chuar Group; N-trending syn-depositional structural corresponding to weaknesses inherited from underly- data, the position of the Chuar Group relative to the Sr ing Precambrian structure extensional Unkar basin) 0.706 line, and the relationship between the Chuar and responded to extensional stresses applied by the Group and the underlying Unkar Group. One possible distant rifting of Rodinia. shape of the basin Fig. 9 inset) follows the Rodinia The mid-Neoproterozoic deposits distributed across reconstruction of Brook®eld 1993), Karlstrom et al. western North America and Australia were all 1999). likely sub-basins within a larger basin that predated The Chuar Group stratigraphy is similar to that in the Cordilleran miogeocline e.g Ross, 1991a; Fig. 9). other cratonic basins, which are characterized by These sub-basins likely formed in response to exten- successions that are typically kms-thick, can be sion and were drowned by marine waters as Rodinia strongly affected by regional and global cyclicity, broke apart. The relative lack of Neoproterozoic ma®c and bedscan be traced laterally for kmsMiall, intrusive rocks ,770±720 Ma) associated with mid- 1984, p. 599). These basins are most commonly ®lled Neoproterozoic successions in the south and central with shallow marine sediments, as is the Chuar Group. western US. Death Valley, northern Arizona, north- The basins are long-lived features, representing 10s± ern Utah), as opposed to the coeval successions in 100sof millionsof yearse.g. Klein, 1995), which isa Canada and eastern Australia, indicate that succes- reasonable duration for the Chuar basin e.g. Karl- sions were likely deposited in distinctive tectonic strom et al., 2000). sub-basins within the greater intracratonic rift basin North-trending syn-depositional normal faults are Fig. 9). present in the Chuar Group and suggest that the active structural grain during Chuar deposition was parallel to the Cordilleran rift margin Timmonset al., 2001). 7. Conclusions These data suggest that Chuar Group deposition was concurrent with the rifting of Rodinia and was 1. Detailed sedimentologic and stratigraphic analyses affected by E±W extension. The position of the indicate that the Chuar Group ismarine not lacus- Chuar Group relative to the Sr 0.706 line indicates trine). Diagnostic marine features include: i) C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499 495 marine fossils and high local pyrite content in the dates, and C-isotopes) to mid-Neoproterozoic mudrock facies; ii) mudcracked mud-draped sequences in western North America and Austra- symmetric ripples and reverse ¯ow indicators in lia, suggesting the possibility of a greater marine the sandstone facies; iii) facies associations cratonic basin with distinct sub-basins. between all facies; and iv) no unequivocal terres- trial deposits. Fine-grained siliciclastic facies and lesser dolomite facies) dominate the Chuar Group Acknowledgements and imply a wave- and tide-affected marine deposi- tional system. Local basin shape is unknown due to This research was supported by National Science the limited areal extent of the Chuar Group and Foundation Grant EAR-9706541 to K. Karlstrom, M. limited paleocurrent data, yet the shoreline likely Elrick, J. Geissman, S. Bowring, A. Knoll), Conoco had a N-NW trend at least during middle and late Oil and Ben Donegan Consulting. It was also Chuar deposition) and was part of a low-energy, supported by the Geological Society of America, the low-relief shelf. Colorado Scienti®c Society, the Of®ce of Graduate 2. Meter-scale cyclicity was likely controlled by Studies Scholarship and Wanek Awards University high-frequency glacioeustasy. The nature of cycles of New Mexico), and the Society of Sedimentary in lower and middle Chuar Group dominantly Geology to Dehler). We also thank the National peritidal, thin non-cyclic intervals, lower percen- Park Service at Grand Canyon for their cooperation. tage of exposure cycles showing prolonged subaer- Invaluable ®eld assistance was provided by Daniel ial exposure, more transgressive-prone cycles) Koning, Claudia Borchert, Anna Snider, Kate Duke, suggests low-amplitude sea-level changes. The Mike Gaud, and Elizabeth Langenburg. Thispaper nature of upper Chuar Group cyclesfewer periti- evolved through discussions with Arlo Weil, Susan- dal cycles, thicker non-cyclic intervals, greater nah Porter, Nick Christie Blick, Andrew Knoll, Dave percentage of prolonged subaerial exposure cycles, DesMarais, Peter Scholle, and Linda Kah. We thank fewer transgressive-prone cycles) suggests moder- reviewersPeter Scholle, Mike Pope, and Paul Link for ate-amplitude sea-level changes. This change helping to improve thispaper. suggests an increase in continental ice volume between middle and upper Chuar time, and combined with the 742 Ma age of the top of the References Chuar Group, suggests that the period ,742 Ma recordsa global climate change leading to the Aitken, J.D., 1981. Generalizationsabout grand cycles.In: Taylor, M.E. Ed.), Short Papersfor the Second International Sympo- Sturtian Ice Age. Differential tectonic subsidence, sium on the Cambrian System. 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Shallow marine record of orbitally-forced cyclicity in a Late Triassic carbonate variations suggest that differential tectonic subsi- platform, Hungary. J. Sedim. Res. 67 4), 661±675. dence wasa control on local accommodation Barron, E.J., Arthur, M.A., Kauffman, E.G., 1985. Cretaceous variability. rhythmic bedding sequences: a plausible link between orbital 4. The Chuar basin was a cratonic basin that variationsand climate. Earth Planet. Sci. Lett. 72, 327±340. responded to distributed extension during Beach, D.K., Ginsburg, R.N., 1978. Facies succession of Pliocene± Pleistocene carbonates northwestern Great Bahama Bank. breakup of Rodinia. It correlates microfossils, AAPG Bull. 64, 1634±1642. stromatolites, facies interpretations, isotopic Berner, R.A., Raiswell, R., 1984. C/S method for distinguishing 496 C.M. Dehler et al. / Sedimentary Geology 141±142 02001) 465±499

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