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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon

Carol M. Dehler, Susannah M. Porter and J. Michael Timmons

Geological Society of America Special Papers 2012;489;49-72 doi: 10.1130/2012.2489(03)

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Notes

The Geological Society of America Special Paper 489 2012

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon

Carol M. Dehler* Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84333, USA

Susannah M. Porter* Department of Earth Science, University of California at Santa Barbara, Santa Barbara, California 93106, USA

J. Michael Timmons New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801, USA

INTRODUCTION roborate data from rock units around the world, suggesting that at this time the supercontinent Rodinia was breaking up, Earth’s cli- The Chuar Group is known today for its beautiful patterns mate was undergoing glacial and interglacial cycles, there were of Martian-like colors, most commonly seen from the air or the massive perturbations to the global carbon cycle, and single- canyon rims. Now far inboard from the ocean and framed within celled protists were diversifying. Its rich, well-preserved record the eastern Grand Canyon, it is hard to imagine that these strata is one of the few well-dated successions of this time period, mak- represent part of a calm ocean inlet near the equator during Neo- ing the Chuar Group a world-class “type section” for this pivotal proterozoic time. Field geologic data indicate that the Chuar sea interval in Earth system history (Karlstrom et al., 2000). Here we was affected by tides and waves and was in a seismically active present an overview of current knowledge on the environmental, basin. The repetitions in stratigraphic patterns indicate that sea climatic, biological, and tectonic context of the Chuar landscape level slowly rose and fell in tempo with global changes in cli- as it was ~800–740 m.y. ago and discuss how the Chuar Group mate. Although animals would not appear for another ~200 mil- rock record has implications for global change during this time. lion years (m.y.) and land plants for another ~300 m.y., fossil data show that these shallow ocean waters were teeming with GEOLOGIC BACKGROUND single-celled life, most of it microscopic. This is the scenario, ca. 750 Ma (mega-annum, or millions of years ago), that is The Chuar Group is exposed exclusively in several right- revealed from recent research on the striking strata of the Chuar bank, east-ﬂ owing tributaries to the Colorado River in eastern Group in eastern Grand Canyon. Grand Canyon, Arizona, USA (Fig. 1; Sheet 1, map on inserts Ongoing and recent research on Chuar Group rocks not only accompanying this volume1). This exposure is bounded on the provides insight about the Chuar basin, but it also contributes east by the Butte fault zone (East Kaibab monocline system), to our understanding of the greater Earth system during mid- and on all other sides by the “Great Unconformity” marked by Neoproterozoic time. Tectonic, stratigraphic, sedimentologic, the overlying Cambrian Tapeats Sandstone (Fig. 2). Locally, geochemical, and paleontologic studies of the Chuar Group cor- the Chuar Group is overlain by the Neoproterozoic Sixtymile

*E-mails: [email protected]; [email protected]. 1Geologic Map of Eastern Grand Canyon, Arizona is also available as GSA Data Repository Item 2012287, online at www.geosociety.org/pubs/ft2012. htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

Dehler, C.M., Porter, S.M., and Timmons, J.M., 2012, The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth’s History: Geological Society of America Special Paper 489, p. 49–72, doi:10.1130/2012.2489(03). For permission to copy, contact [email protected]. © 2012 The Geological Society of America. All rights reserved.

49 Downloaded from specialpapers.gsapubs.org on January 24, 2013

50 Dehler et al. 35° 58'N 36° 25'N

es d a s

i 111° 47'W 111° 47'W l N a P

Butte fault

e y n

n a

i C

c d

o e R n l o m ibab

st Ka

Bright Ange Bright Ea Angel Bright Vishnu Canyon Vishnu 10 km Buried fault Proterozoic monocline of Laramide trace Approximate Bass Canyon Proterozoic normal fault, ball on the normalProterozoic fault, side downthrown Laramide monocline Laramide Proterozoic monocline Proterozoic K7-115-3 Chuar Group Cardenas Basalt and Diabase Unkar Group Gorge MetamorphicGranite Suite 112° 35'W 112° 35'W Figure 1. Location map showing outcrop extent of the Grand Canyon Supergroup and major tectonic elements in eastern Grand Canyon (modiﬁ ed from Timmons et al., 2001). Timmons ed from (modiﬁ and major tectonic elements in eastern Grand Canyon Supergroup of the Grand Canyon outcrop extent Figure 1. Location map showing 35° 58'N 36° 25'N Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 51

mi. 54 1b 1a Pz ek 2 4b Cre mi. 55 p 3 6 ea Y w X o 4a k Chuar syncline n a N mi. 56 5 7 B Pz 9 8 100 mi. 57 6 111 Pz 12 7 mi. 58 ek re Colorado River t C un ag Kw EXPLANATION mi. 59 Pz Paleozoic rocks undifferentiated Malgosa 8 Awatubi Z Zs Sixtymile Formation mi. 60 Pz Zkw Walcott Member Sixtymile Zka Awatubi Member

tl u a f mi.e 61 tt u B Carbon Butte Member Kwagunt Zkc Zgd Duppa Member mi. 62 Little Colorado Line of Chuar syncline Zgc Carbon Canyon Member 5 E River cross section a s Chuar Group Zgj Jupiter Member 1 t

Neoproterozoic A A’

Galeros

Zgt Tanner Member K 2 a mi. 63 Pz 4 i

Zn Nankoweap Formation 3 b a

Lava Chuar Creek b

Yc Cardenas Basalt 9a M mi. 64 Yd Dox Formation A o Unkar Meso- 9b n Proterozoic normal fault (ball on o c 9c l downthrown side) in Basalt Canyon e mi. 65 Proterozoic syncline Palisades fault Laramide monocline Pz Laramide syncline 10 mi. 66 Measured section (see text), number in square 2 3a linked to section line from Dehler et al. (2001)

11 mi. 67 Pz

Tanner 0 1 2 kilometers graben mi. 68 mi. 69

mi. 70

Figure 2. Geologic map of the Chuar Group with measured section and cross-section locations (modiﬁ ed from Timmons et al., 2001). Downloaded from specialpapers.gsapubs.org on January 24, 2013

52 Dehler et al. C (Basal) Tapeats Sandstone variegated and black mudrocks Neo Sixtymile Fm black mudrocks ? karsted dolomite breccia x x x 742±6 Ma sandstone basalt dolomite crystalline upper dolomite couplet mr-dark lower dolomite couplet stromatolites basement dol-mas dol-lam pis/ooid interbedded sandstone and (or) siltstone beds <1 m thick dolomite and (or) silty dolomite flaky beds typically <1 m thick Member Walcott flaky dolomite dol.

vase-shaped microfossil mr-dark

acritarch mr-var ss-1 x x x ash bed dol-mas Awatubi Member dol-lam Kwagunt Formation Kwagunt mafic dikes and sills Ba-Box Baicalia-Boxonia prolonged subaerial exposure mr-var white sandstone ss-1 Butte Carbon unconformity Mbr basal red sandstone* * ss-1&2 paleochannel mr-var mr-dark large cross-beds ss-1 large mudcrack dol-mas Duppa Member dol-lam denotes diagnostic tidal feature

* Baicalia Baic Tonto polygonal marker bed Gp. Fm. Six. mr-var mr-dark ss-lam * dol-mas dol-lam Middle Neoproterozoic Middle CHUAR GROUP * Carbon Canyon Member Canyon Carbon CHUAR GROUP

mr-var mr-dark GALEROS FORMATION ss-lam Nko. Fm Jupiter Member Jupiter

200 GRAND CANYON SUPERGROUP * Stratifera/Inzeria Inzeria

meters mr-var 100 mr-dark ss-lam UNKAR GROUP Tanner Member 0 Tanner dolomite d-crse NANKOWEAP ? FORMATION intertidal supratidal distal subtidal distal FACIES proximal subtidal proximal UNKAR GROUP BASALT 400 m (UPPER) Mesoproterozoic basement crystalline PALEO- CARDENAS 1070±70 Ma ENVIRONMENT

Figure 3. Generalized stratigraphic column (on left) of the Chuar Group, showing relationships with underlying and overlying units. To right, composite measured section of the Chuar Group, including facies interpretations from Dehler et al. (2001). Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 53

Formation and rests upon the Nankoweap Formation (Figs. 2 and and 1890s. He described the Chuar “terrane,” discovered and 3). These strata are gently folded by the north-trending Chuar named the microfossil Chuaria circularis, and noted the pres- syncline, which parallels the trace of the Butte fault (Figs. 1 ence of stromatolites (Walcott, 1894, 1899). A more recent com- and 2). The Chuar Group is also present in the subsurface along prehensive study of the Chuar Group strata was conducted in the the Arizona-Utah border where three wells have penetrated as late 1960s and early 1970s (Ford and Breed, 1973b and refer- much as 700 m of Chuaria -bearing mudrock interbedded with ences therein), yielding information on the general stratigraphic, dolomite and sandstone (Rauzi, 1990; Wiley et al., 1998). Chuar sedimentologic, and paleontologic characteristics. These workers strata equivalents are found regionally in northern Utah (Uinta and most others have interpreted all or part of the Chuar Group Mountain Group and Big Cottonwood Formation), in Death Val- to represent a nearshore protected marine setting (Cook, 1991; ley, California (middle Pahrump Group), and in northern Canada Dehler et al., 2001; Ford and Dehler, 2003; Vogel et al., 2005). (Little Dal and Coates Lake Groups; Link et al., 1993; Dehler Sedimentary structures are the foremost features for inter- et al., 2001, 2010; Dehler, 2008). Strata of Chuar age are found preting Chuar paleoenvironments. The best preserved and most on most continents; the Australian equivalents may be closely abundant sedimentary structures are in the middle part of the related to the Chuar Group, as these two continents may have Chuar Group: the upper Carbon Canyon Member, the Carbon been juxtaposed during deposition (Karlstrom et al., 1999). Butte Member, and the lower Awatubi Member (Fig. 3). One The Chuar Group is a 1600-m-thick, apparently conform- of the most informative and common sedimentary features are able, fossiliferous, unmetamorphosed succession composed of ripple marks that are symmetrical in cross section (Fig. 4). These ~85% mudrock, with interbedded meter-thick sandstone and are visible on bedding planes of dipping sandstone beds along the dolomite beds (Fig. 3). It makes up most of the upper Grand Can- yon Supergroup and is subdivided into the Galeros Formation (lower) and the Kwagunt Formation (upper), with the contact at the base of the prominent, thick sandstone unit of the Carbon Butte Member (Ford and Breed, 1973a) (Fig. 3). The Galeros Formation is further divided into the Tanner, Jupiter, Carbon Canyon, and Duppa Members; the Kwagunt Formation is divided into the Carbon Butte, Awatubi, and Walcott Members (Ford and Breed, 1973a). Multiple data sets collectively indicate that the Chuar Group is mid-Neoproterozoic in age (ca. 800–742 Ma). The age of the basal Chuar Group is constrained by preliminary U-Pb analyses of diagenetic monazite in the Tanner Member that indicate that the base of the Chuar Group is ca. 800 Ma (Williams et al., 2003). A direct age for the top of the Chuar Group comes from a U-Pb zir- A con date from an ash bed of 742 ± 6 Ma (Karlstrom et al., 2000). Chuar Group paleomagnetic data, which record the original mag- netization of the rocks acquired during their deposition and thus their latitude at that time, correlate with other Neoproterozoic successions from North America, indicating that Chuar deposition occurred between ca. 850 and ca. 740 Ma at paleolatitudes of 5°–20° north of the equator (Weil et al., 2004). These constraints are consistent with Chuar stromatolites and microfossils, which correlate with middle Neoproterozoic fossiliferous successions elsewhere (Vidal and Knoll, 1983; Vidal and Ford, 1985; Porter and Knoll, 2000; Nagy et al., 2009). Microfossils and carbon- isotope composition suggest robust correlation with the Red Pine Shale, Uinta Mountain Group, Utah, which is known to be no older than 770 Ma on the basis of detrital zircons in an informal formation lower in the group (Dehler et al., 2007, 2010). B

THE CHUAR SEA Figure 4. Photographs of symmetrical ripple marks from sandstone units in the middle Chuar Group. The shape of the ripple marks requires oscillatory ﬂ ow and suggests deposition along a shoreline. Research on the Chuar Group strata started in the late 1800s (A) Mud cracks and ripple marks on the sole of a bed from the Jupiter and continues today. Charles D. Walcott was the ﬁ rst to study the Member. These associated features most likely formed in a tidal set- stratigraphy and paleontology of the Chuar Group, in the 1880s ting. (B) Ripple marks in the Carbon Butte Sandstone. Downloaded from specialpapers.gsapubs.org on January 24, 2013

54 Dehler et al.

Butte fault, especially between Carbon and Lava Chuar Canyons. and oft-visited single “brain” at the top of the Carbon Canyon slot The symmetrical shape of these ripple marks requires a back- canyon was washed away in a recent fl ash fl ood; however, smaller and-forth fl ow of water. Therefore, we know these sediments blocks from the same unit are still scattered in the streambed.) (silt and sand) were subjected to oscillatory currents within the Another spectacular and prominent stromatolite, Baicalia, wave zone along a shoreline. Waves can form in lakes, oceans, appears in the Carbon Canyon Member (Ford and Breed, 1973a). and even rivers, so these features alone do not help to pinpoint a Although typically <0.5 m in all dimensions, this stromatolite specifi c paleoenvironment, yet they do indicate a shoreline set- is not only important for paleoenvironmental information (dis- ting of some sort. However, Chuar ripple marks are commonly cussed below), it is a useful marker bed for mapping and inter- draped with a thin veneer of mudstone with mud cracks (Fig. 4). preting tectonically controlled thickness changes in Chuar strata Mud cracks require a wet environment to deposit the mud and a (Figs. 6B and 7). When viewed from the top (on rare bedding- dry environment to dry out and crack the mud. This combination plane exposures), Baicalia buildups look like a forest of large of sedimentary features—ripple marks with mud-cracked mud broccoli heads. In many places the Baicalia heads are broken up drapes—requires a continuum of changing physical conditions. and in chaotic orientations, indicating reworking by storm waves. The rippled sands require a fast oscillatory current (e.g., fl ood A more subtle but unnoticed stromatolite interval is present fl ow) that then must be followed by a slow current (e.g., slack at the base of the Jupiter Member and is prominently exposed as water) to deposit the mud. The entire mud-armored ripple mark a white cliff (or a waterfall on rainy days) along the Lava Chuar– must then dry out for mud cracks to form (e.g., low tide). By far, Carbon Canyon loop hike in Lava-Chuar Canyon. This stromato- the easiest place for this combination of sedimentary features to lite interval has a complex assemblage of stromatolites, ranging form, and be preserved in the rock record, is along a tidally infl u- from marble-sized (centimeters in diameter) to river-raft–sized enced (marine) shoreline. (meters in diameter). Two species have been identifi ed in this The Carbon Butte Member, marked at the base by a big red interval, Inzeria and Stratifera (Ford and Breed, 1973a). Typi- cliff of sandstone, preserves other sedimentary features that also cally, the forms are arranged within one another—domes within suggest a tide- and wave-infl uenced environment (Fig. 5; Dehler domes within domes. There are also pockets of broken stromato- et al., 2001). Cross-bedding in these sandstones shows opposing lites within this interval, indicating again the occurrence of infre- directions of paleofl ow, indicating that underwater dunes were quent large storms. moving in opposite directions, again a diagnostic tidal feature These microbial reefs or mounds are useful indicators of (Fig. 5A). Associated with these decimeter-scale cross-beds are water depth—and thus paleoenvironment—because they only several meter-scale, very low angle cross-bed sets (Fig. 5B); these grow when submerged. Thus we can constrain minimum water are likely the sides of shifting tidal channels. The Carbon Butte depth to be at least as deep as the height of a mound (if the micro- Member also hosts soft-sediment deformation features, indica- bial laminae continue to the base of the mound). For example, tive of either very fast deposition of sediment or seismic shak- because the “brain” stromatolites can be 2 m (6.56 ft) tall, we can ing. The best place to see these features is in Kwagunt Canyon, infer that water depth had to be at least 2 m. Another environmen- either right along the Butte fault or west of the Butte fault where tal condition required by stromatolites is that they receive enough Kwagunt Creek fl ows over the lower cliff-forming red sandstone light for microbes to photosynthesize. Thus water depth could (Fig. 5C). not have been too great—less than ~100 m at an absolute maxi- Stromatolites are a persistent sedimentary feature in the mum (328 ft), the lower boundary of the photic zone today, or Chuar dolomites and are found in most members (Figs. 3 and 6). even shallower in Proterozoic time owing to lower solar luminos- The laminations refl ect the episodic growth of these structures, ity (Sagan and Mullen, 1972). The water also must not have been and their overall shapes are strongly infl uenced by the physical too cloudy with sediment, because photosynthetic organisms conditions of the environment in which they formed. Stromato- need relatively clear water to receive sunlight. This latter factor lites are much rarer today than they were during Chuar time, per- has implications for the climatic conditions present when these haps because of destructive grazing by animals (Garrett, 1970; microbial communities were living (i.e., low clastic sedimenta- Awramik, 1971), but modern examples can be found in Shark tion rates, and hence more arid during deposition of carbonates). Bay, Western Australia, and in the Gulf of California, off the The fact that Chuar stromatolite taxa have been found in marine Baja Peninsula. deposits of Proterozoic age worldwide suggests a marine origin There are at least six different types of stromatolites in the for Chuar Group stromatolites (Dehler et al., 2001). Chuar Group (see Ford and Breed, 1973a, and Cook, 1991; In addition to stromatolites, there is evidence for other Dehler et al., 2001), some more recognizable and accessible than microbially infl uenced carbonate precipitation throughout the others (Fig. 6). Most Chuar stromatolites appear only once in Chuar Group. These sedimentary structures are common in the the 1600-m-thick succession, making them unique stratigraphic Chuar dolomite beds and show horizontal laminations that are markers (Fig. 3). Probably the best known of the Chuar Group crinkly, not smooth, in appearance (Fig. 6C). The fl at, crinkly stromatolites is Boxonia, a specimen that looks like a giant brain laminations are interpreted to represent broad, fl at microbial (Fig. 6A) and can most easily be seen on a stroll up Kwagunt mats that formed in the shallow subtidal, intertidal, and supra- Canyon in the low hillsides past the Butte fault. (A spectacular tidal areas (Dehler et al., 2001). Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 55

A A

B B

C C Figure 6. Photographs of various microbial marker beds in the Chuar Figure 5. Photographs of sedimentary structures in the Carbon Butte Group. (A) The stromatolite Boxonia, aka the “brains,” Kwagunt Can- Sandstone, including (A) bipolar cross-bedding in the white sandstone yon. If you had been snorkeling in this Neoproterozoic sea, you might marker bed; pencil points in main direction of fl ow, and climbing dune have scraped your belly on these microbial mounds (now dolomite). below pencil shows fl ow in the opposite direction; (B) low-angle cross- (B) This example of the stromatolite Baicalia shows that the microbial beds (cliff ~5 m tall); and (C) soft sediment deformation. These latter buildup was onlapped and eventually covered by fi ne-grained sedi- features suggest deposition in a tidal setting concurrent with sediment ment. (C) Crinkly laminated dolomite, which formed in a microbe-rich loading (caused by seismic activity or rapid sedimentation). peritidal environment. All of these microbial deposits (photos A–C) likely formed in shallow, clear water, perhaps in a setting similar to the Persian Gulf coast today. Downloaded from specialpapers.gsapubs.org on January 24, 2013

56 Dehler et al. 20 cm Baicalia marker bed

SITE 3

SITE 5 SITE 2

WEST LIMB SITE 1A EAST LIMB BUTTE CHUAR SYNCLINE AXIS CHUAR SYNCLINE CHUAR SYNCLINE FAULT 3 km Figure 7. Stromatolite cycles from the Carbon Canyon Member, showing sedimentary response to movement associated with the Chuar syncline (modiﬁ ed from Dehler et al., 2001). Note that the carbonate interval is thickest in the axis of the syncline.

Another special feature in the Carbon Canyon Member, which The extensive and colorful shale in the Chuar Group does occurs tens of meters below the Baicalia bed, is a dolomite bed that not reveal much information about Proterozoic Earth conditions contains deep and contorted mud cracks (Fig. 8). These strange when studied in the fi eld. All shale, because of its fi ne-grained cracks, arranged in polygonal shapes in bedding-plane view, likely nature, does indicate a low-energy depositional environment, represent a time when sea level was lowered, and this part of the but many of the beautiful Martian colors apparent on the Chuar land was exposed for a long time. These cracks are similar to car- hillsides were created by post-depositional processes. Although bonate mud cracks forming on supratidal fl ats in the Persian Gulf much of this shale weathers to a red or green color, it is com- today. This marker bed and the Baicalia marker bed can be seen monly gray to black on a freshly exposed surface, indicating that along the north face of the north fork of Kwagunt Canyon. most of the Chuar shale was originally organic rich. Geochemical studies of the Chuar shale reveal high total-organic-carbon per- centages (up to 9.39 wt%; Dehler, 2001; Nagy et al., 2009). The organic content found throughout the Chuar Group requires high biologic productivity in combination with oxygen-poor waters near or below the ocean fl oor. Carbon-isotope composition and related clay composition from these shales have implications for climate change and the carbon cycle of the Chuar Group (Fig. 9; see below). In summary, the sedimentary rock types, or facies, in the Chuar Group indicate a variety of depositional environments (Fig. 10). The facies indicate deposition in an overall low-energy marine embayment that was infl uenced by tidal and wave processes, infrequent large storms, microbial activity and carbonate precipitation, and quiet water deposition of mud and organic matter. All of these facies suggest relatively shallow water (tens of meters or less) or times of intermittent exposure on a tidal fl at. Not all of these environments were present at the same time: Figure 8. Photograph of a large contorted mud crack in a dolomite bed (the “polygonal marker bed,” Carbon Canyon Member, Kwagunt there were sand-dominated (siliciclastic) intervals and dolomite- Canyon). This feature is similar to modern mud cracks found in the dominated intervals. Organic-rich mud was deposited throughout supratidal zones of the Persian Gulf. Chuar time, dominantly offshore and also in lagoonal and tidal Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 57 (cool)

dry

(warm) wet Climate interpretation 100 m p 75 e kaolinite 50 illit illite no data no data no data no data 25 quartz quartz Mineralogy (cumulative wt %) (cumulative Mineralogy 0 90 80 CIA no data no data no data

no data

Mbr.

Member Member

Member Member Member

60 Member Jupiter

Butte

Carbon Canyon Carbon Awatubi Duppa Tanner Walcott Carbon C B E A 13 C D δ features 10 8 6 PDB) 4 00 / 0 2 ( carb C 13 t to the carbonate carbon curve (shown as circles). (shown t to the carbonate carbon curve δ Composite curve Composite -6 -4 -2 0 dolomite abundance 3 1 4 2 sequences Lithostratigraphic karsted dolomite Tanner dolomite Baicalia /Box. flaky dolomite polygonal bed red sandstone dolomite couplet Baicalia Stratifera / Inzeria ca. 770 Ma ca. 760 Ma 1070±70 Ma 742±6 Ma

xxxx

Mbr. Member Basalt

Member Member Member Member

Jupiter Member Jupiter

Butte

Carbon Canyon Carbon Cardenas Awatubi Duppa Tanner

Walcott

Carbon

(upper)

Galeros Formation Galeros

Kwagunt Formation Kwagunt

Group

Chuar Group Chuar

Sixtymile Unkar Formation Formation

Nankoweap

Tapeats Sandstone Middle Neoproterozoic Middle Neo Mesoprot. ? C

200 100 0 meters Figure 9. Lithostratigraphic sequences, carbon-isotope curve, weathering indices (CIA, chemical index of alteration), and shale mineralogy of the Chuar Group. The high-magnitude mineralogy of the Chuar Group. of alteration), and shale weathering indices (CIA, chemical index Figure 9. Lithostratigraphic sequences, carbon-isotope curve, this is not the case with with ancient glacial deposits, but are commonly found associated These excursions is characteristic of Neoproterozoic strata. in carbon-isotope values variability for discussion (from Dehler et al., 2005). See text volume. suggest changes in local climate and global ice data sets do, however, and correlative excursions This group’s the Chuar Group. carbon that are best ﬁ from organic Squares indicate isotope values Downloaded from specialpapers.gsapubs.org on January 24, 2013

58 Dehler et al.

capped cycle dolomite-

sandstone- capped cycle

re re

sho

dal

resho re

fo fo

eritid

iti per

ce refa

sho

e hor

shor off o shoreline shoreline deposition of dolomite, slower weathering rates, arid

deposition of sand faster weathering rates, relatively humid REGRESSION (significant sea-level fall) sea-level REGRESSION (significant REGRESSION (subtle sea-level fall) sea-level REGRESSION (subtle

foreshore

eritidal

p e

site of cycle generation stromatolite shorefac tidal channel

fault

e or

h offs KEY: shoreline organic-rich mud dolomite mudcrack dune sand hummock faster weathering rates, humid TRANSGRESSION deposition of organic-rich mud, Figure 10. Depositional models for Chuar Group cycles, showing organic-rich mud deposition during sea-level rise (transgression), dolomite-rich deposition during major sea-level dolomite-rich deposition during major sea-level rise (transgression), mud deposition during sea-level organic-rich showing Figure 10. Depositional models for Chuar Group cycles, (lithostratigraphic and large-scale (meter-scale) small-scale These models can be used to explain (regression). fall and sand deposition during more subtle sea-level (regression), fall some dolomite) is controlled by local deposits (i.e., some sandstone, water The change in lithology of shallow in the Chuar Group. observed sequences—100 m scale) cyclicity for discussion. to global changes in climate. See text changes in climate that are linked Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 59

fl at areas. Understanding the relationship among these paleoen- cycles. We view these sequences as larger-scale versions of the vironments involves looking closely at how the different sedi- Chuar meter-scale cycles and hypothesize that they represent mentary facies are vertically and laterally related to one another similar (but longer duration) sea-level fl uctuations whereby in the rock record (stratigraphy). This will be addressed in the the sandstone-rich intervals indicate higher sea level and the next section. dolomite-rich intervals indicate lower sea level (Fig. 9). These sequences track the carbon-isotope signatures and shale compo- CHUAR SEA LEVEL AND CLIMATE sition remarkably well and will be discussed below. Chuar strata also provide information about the carbon The stacking of different facies, or stratigraphy, in the Chuar cycle, which in turn can be used to infer both climatic and biotic Group reveals information about how the different deposi- changes (e.g., glaciations and extinction events). The carbon tional environments were related, and how sea level and climate cycle is approximated by measuring the ratio between the two changed through Chuar time. The Chuar strata are markedly stable isotopes of carbon (12C, the lighter and most abundant cyclic, meaning that there are many repetitions of stacked facies, one, and 13C, the heavier one) preserved in carbonate rocks and and therefore repetitions of environmental change (Figs. 3, 9, organic matter in shale. A carbon-cycle curve is constructed by 11A). Most Chuar cycles consist of shale overlain by dolomite or plotting the measured ratios through time. The curve expresses sandstone, and are meters to tens of meters thick (Fig. 11B). The changes in the rate of organic-carbon burial, which refl ects rela- shale was originally deposited offshore as mud, and the sand- tive rates of primary productivity, sedimentation, and organic- stone or dolomite was originally deposited near the shoreline as matter decomposition (e.g., DesMarais, 1997). Where the curve sand or precipitated as carbonate, respectively. The changes in shows more positive values (expressed in parts per thousand, ‰), facies indicate changes in water depth; because Chuar strata are it indicates an increase in primary productivity and/or organic- laterally continuous across the region (in the subsurface), these carbon burial; more negative values can indicate lower rates of water-depth changes likely refl ect sea-level oscillations (Figs. 3 primary productivity and/or organic carbon burial. The Chuar and 9). More specifi cally, the sandstone cycle tops represent carbon-isotope curve exhibits four major excursions (i.e., rises more subtle sea-level changes, whereas some dolomite cycle and falls), one of which is among the largest ever recorded in caps indicate times of signifi cant exposure (Figs. 6 and 11B) and Earth history (cf. Melezhik et al., 1999). This positive excursion therefore greater drops in sea level. (15‰; Fig. 9), recorded in the lower Awatubi Member, has been These cycles are similar to those in younger strata (Pha- interpreted to indicate high rates of organic-carbon burial, refl ect- nerozoic, less than 543 m.y. ago) that are caused by Milankov- ing high primary productivity and high rates of sedimentation ich cycles (e.g., Beach and Ginsburg, 1978; Goldhammer et al., (Dehler et al., 2005). This interpretation is supported by the shale 1987; Sageman et al., 1997). Milankovich orbital cycles include mineralogy (see below). the variations in the shape of the Earth’s orbit (eccentricity) and The mineral composition of well-preserved shale provides the tilt and wobble of the Earth’s axis (obliquity and preces- a view of relative weathering rates in the source area through sion, respectively), each complete orbital cycle taking between Chuar time. Shale rich in the mineral kaolinite (the weathering 10 k.y. and 100 k.y. (Milanković, 1941). The interactions among product of other unstable minerals, such as feldspar) indicates these orbital parameters cause changes in the amount of solar relatively high weathering rates, whereas shale rich in the min- radiation received by the Earth, hence infl uencing Earth’s cli- eral feldspar indicate relatively low weathering rates, because mate and, more specifi cally, causing changes in the ice volume feldspar is unstable at Earth’s surface conditions. A weathering in polar regions. There are >300 m-scale cycles (hypothesized “index” or chemical index of alteration (CIA) can be calculated to refl ect orbital changes) in the Chuar Group, each cycle repre- for shale on the basis of the elements it contains (see discussion senting durations on the order of 40,000–100,000 yr (Dehler et in Chapter 2 [Timmons et al., this volume], and in Nesbitt and al., 2001). This would suggest that the Chuar Group represents Young, 1982). Shale with a high CIA (>80%) indicates increased ~30 m.y. of geologic time, consistent with other age estimates for weathering rates, and shale with a lower CIA indicates decreased the Chuar Group (see Geologic Background, above). There are weathering rates (Fig. 9). Because rates of weathering are fewer thicker cycles in the upper Chuar Group, and these cycles strongly determined by rainfall, these indices can be used to infer are all capped with dolomite; these features indicate relatively times of relative aridity and humidity in the area where the Chuar higher magnitude sea-level change (Fig. 11C). Greater sea-level sediment was weathered from its parent rock (the “source area”). change corresponds to melting and freezing of more glacial Shale mineral compositions indicate that weathering rates, and ice. Therefore, the Chuar Group cycles indicate the presence of thus relative humidity, varied signifi cantly during Chuar time, global ice throughout Chuar time, and an increase in global ice with less weathering during dolomite-rich times (lower sea level) volume and a lowering of global temperatures in late Chuar time and more intense weathering during sandstone-rich times (higher (Kwagunt Formation). sea level; Fig. 9; Dehler et al., 2005). The Chuar Group cycles can be grouped into four strati- Combining the stratigraphic data with the carbon-isotope graphic sequences (Fig. 9). Each sequence shows a bundling of curve and the shale mineralogy has resulted in a compelling cli- sandstone-rich cycles, followed by a bundling of dolomite-rich mate story (Fig. 9). Sandstone-rich intervals correlate with higher Downloaded from specialpapers.gsapubs.org on January 24, 2013

60 Dehler et al.

Figure 11 (Continued on facing page). (A) Photograph of meter-scale cycles A in the Carbon Canyon Member of the Chuar Group. Geologist for scale (circled in white). (B) Photograph of meter-scale cycles in the Carbon Can- yon Member, along the north fork of Kwagunt Creek. White arrows denote single shallowing-upward cycles. E— exposure interval at top of one of the dolomite-capped cycles.

B Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 61 (offshore) exposure peritidal shallow subtidal E DOLOMITE-CAPPED CYCLES (upper Awatubi/Walcott Members) UPPER CHUAR GROUP peritidal peritidal/shoreface shallow subtidal (offshore) shallow subtidal (offshore) m m ps ps MIDDLE CHUAR GROUP (Carbon Canyon Member) SANDSTONE-CAPPED AND DOLOMITE-CAPPED CYCLES shallow subtidal (offshore) peritidal/shoreface

ps 10 m 10 (Jupiter Member) LOWER CHUAR GROUP SANDSTONE-CAPPED CYCLES laminated dolomite facies ooid-pisoid grainstone facies laminated sandstone facies variegated mudrock facies dark mudrock facies regressive cycle transgressive-prone cycle exposure zone massive poorly sorted intraclast crinkly lamina smooth planar lamina stromatolitic build-up mudcrack, small and large planar tabular foresets trough crossbeds fluid-escape structure interference ripplemark symmetric ripplemark contorted bedding tepee structure Explanation E m ps C ). (C) Measured sections of meter-scale cycles in the lower, middle, and upper Chuar Group. Note how the cycles are thicker and become more carbonate domi- and are thicker the cycles middle, and upper Chuar Group. Note how in the lower, cycles meter-scale Figure 11 ( Continued ). (C) Measured sections of increase in ice controlled by an overall time, likely change through Chuar character indicates an increase in magnitude of sea-level The change in cycle nated in the upper Chuar Group. on the planet. volume Downloaded from specialpapers.gsapubs.org on January 24, 2013

62 Dehler et al. TABLE 1. SUMMARY OF FOSSILS FOUND IN CHUAR GROUP STRATA Formation Member Fossil(s) LocygolohtiL/noita secnerefeR Shales just overlying basal dolomite Walcott (1899), Ford and Breed Chuaria unit (1973a, 1973b), Nagy et al. (2009)

Ford and Breed (1969)§, Vidal and Acritarchs* Lower shales? # Ford (1985) , Horodyski (1993)**

Bacterial filaments Pisolites, shales, chert nodules in Schopf et al. (1973), Horodyski and unicells carbonate (1993), Link et al. (1993) Dolomite nodules in shales ~15 m Walcott below the top of member; chert Bloeser et al. (1977), Bloeser, † nodules in thin dolomite beds above (1985), Vidal and Ford (1985), VSMs basal dolomite; shales above basal Horodyski (1993), Porter and Knoll dolomite; shales associated with (2000), Porter et al. (2003) pisolite Hydrocarbon Argillaceous dolostones 5–6 m above Summons et al. (1988), Vogel et al. biomarkers base of member on Nankoweap Butte (2005) Sphaerocongregus Shales ~3 m above flaky dolomite Nagy et al. (2009) variabilis

Kwagunt Ford and Breed (1973a, 1973b), Chuaria Shales throughout member Vidal and Ford (1985), Nagy et al. (2009)

Downie (1969)§, Vidal and Ford Acritarchs Shales throughout member (1985), Horodyski (1993), Nagy et al. (2009)

† Vidal and Ford (1985), Horodyski VSMs Shales throughout member Awatubi (1993) Filamentous bacteria Shales throughout member Horodyski (1993) Possible eukaryotic Mudstone 50 m below contact with Horodyski and Bloeser (1983) algal filaments Walcott Sphaerocongregus selahS ygaN te .la )9002( variabilis Hydrocarbon †† esaB fo rebmem legoV te .la )5002( biomarkers Carbon detroper slissof oN slissof detroper Butte Duppa Acritarchs Shales throughout member Nagy et al. (2009) Unicells and Silicified microbial laminated Schopf et al. (1974), abstract cited Carbon filaments carbonate in Schopf (1975) Canyon Acritarchs Shales throughout member Nagy et al. (2009) Chuaria selahS ladiV dna droF )5891( Galeros Jupiter Vidal and Ford (1985), Nagy et al. Acritarchs Shales throughout member (2009) Ford and Breed (1973b), Vidal and Chuaria Shales throughout member Ford (1985), Nagy et al. (2009) Tanner Vidal and Ford (1985), Nagy et al. Acritarchs Shales throughout member (2009) *There are no confirmed reports of ornamented acritarchs from the Walcott Member. See footnotes below. †VSMs—vase-shaped microfossils. §In an appendix to Ford and Breed (1969), Downie (1969) describes small round bodies with surface textures that are “sometimes smooth or shagrinate [sic] or wrinkled and ‘spongy’” from a single sample collected from the “Chuaria horizon.” According to Ford et al. (1969), this interval is “20 feet below the ‘Corn Flake Bed’ to just above silicified oolite,” i.e., the upper Awatubi through lower Walcott Members. The exact position of this sample within the interval was not recorded. #Vidal and Ford (1985) report Chuaria, VSMs, and ‘cf. Stictosphaeridium sp.’, a “‘wastebasket’ grouping” comprising “single or clustered thin-walled spheromorphs without any distinctive primary ornamentation” (p. 377) from a single sample at an unspecified zone in the Walcott Member. **Horodyski (1993) reported scarce to common acritarchs from several samples collected from the lower ~80 m of the Walcott Member, but did not provide taxonomic lists. †† Vogel et al. (2005) report gammacerane and extreme enrichment of ααα C27 steranes. Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 63

(more positive) carbon-isotope values and kaolinite-rich shale, Chemical byproducts of photosynthesis encouraged precipita- and dolomite-rich intervals correlate with lower (more negative) tion of carbonate coatings on the grains; as a result, the grains carbon-isotope values and feldspar-rich shale. We hypothesize, are made up of concentric rings of carbonate (known as ooids or therefore, that the sandstone-rich intervals indicate deposition pisoids, depending on their size). They are preserved today, along during locally wetter times, when more clastic sediment was with the carbonate grains in cherty oolite and pisolite units, dis- delivered to the Chuar basin at faster rates, and during globally tributed at a number of intervals in the Walcott Member (Schopf warmer times, when sea level was higher and there was less gla- et al., 1973; Cook, 1991). cial ice. The dolomite-rich intervals indicate increased carbonate In shallow, low- to high-energy waters in the Chuar basin precipitation during locally drier times, when less sediment was grew stromatolites; these organo-sedimentary structures are delivered to the basin (as silt and sand), and during globally cooler described in the previous section. Living within—and prob- times, when sea level was lower and there was more glacial ice. ably dining upon—the bacterial mats growing on the surface In the past decade, much attention has been paid to Neopro- of these stromatolites were amoebae, single-celled organisms terozoic glacial deposits, some of which are about the same age that move about on pseudopods, i.e., fi nger-like extensions of as the Chuar Group (see Hoffman and Li, 2009, for a summary of the cell. Many of these lived in tests, protective “houses” they age constraints). These deposits are anomalous for many reasons, built themselves, with only their pseudopods protruding outside most notably the fact that many were deposited at sea level in (Fig. 12). Although the cells decayed long ago, the tests are abun- equatorial regions and are associated with extreme variability in dantly preserved in association with bacterial mats in dolostones the carbon-isotope curve (Hoffman et al., 1998a; Evans, 2000). (the “fl aky dolomite”; Cook, 1991) near the base of the Walcott There is an array of hypotheses to explain these relationships (see Member (Porter and Knoll, 2000) and in upper Awatubi mud- Hoffman and Schrag, 2002, for a review of these hypotheses); stones (Bloeser et al., 1977; Bloeser, 1985). However, they are the best known is the “snowball Earth” hypothesis (originally most abundant (preserved by the billions!) in carbonate concre- suggested by Harland, 1964, extended by Kirschvink, 1992, and tions ~15 m below the top of the Walcott Member on Nankoweap revived by Hoffman et al., 1998b), which suggests that Earth’s Butte (Fig. 13A). These concretions formed in situ after the sur- oceans were completely frozen over for at least 10 m.y. at a time rounding shale had been deposited; it is likely that the decay and that these special conditions explain the large variability of the amoebae themselves promoted carbonate precipitation, in the carbon cycle. It has been hypothesized that all extreme essentially entombing their tests in carbonate rock (Porter and carbon-isotope excursions in the Neoproterozoic—even those Knoll, 2000). Their remains are distinctive; the tests look like not stratigraphically tied to glacial deposits—may indicate gla- tiny vases or bags with an opening at one end. They are known in ciation (Kaufman et al., 1997). The Chuar Group has such excur- the literature as vase-shaped microfossils or VSMs, and are often sions, and although it lacks glacial deposits, its stratigraphic referred to informally as Bonnie Bags, after Bonnie Bloeser, the data sets (Figs. 9 and 11C) independently suggest that glaciers woman who fi rst discovered them in the Chuar Group (Bloeser were on the planet (Dehler et al., 2001). Importantly, the Chuar et al., 1977; Bloeser, 1985). At least 11 species are known from Group indicates that there was ice on the planet between 800 and these concretions, and an additional species has been found in 742 Ma (at least at the poles), yet not in lower latitudes as sug- Awatubi mudstones (Bloeser, 1985; Porter et al., 2003). Spe- gested by the “snowball Earth” hypothesis. These fi ndings not cies differ in the shape of the test itself (Fig. 13B versus 13G only help to constrain the timing of a “snowball Earth” (if this versus 13E), the shape of the test opening (Figs. 13B and 13C), hypothesis holds), they also support the idea that there is more than one way to get large-scale changes in the Proterozoic carbon cycle. The variability in the Chuar carbon cycle appears to be ultimately controlled not by “snowball Earth” conditions, but, A B rather, by concomitant changes in local humidity and global ice volume (Dehler et al., 2005).

LIFE IN CHUAR WATERS

The Chuar fossil record (Table 1) indicates not only that there was a diversity of life in the Chuar sea but also that organisms lived in a variety of different habitats, from high-energy shallow waters to low-energy offshore waters, attached to sand grains, grazing on microbial mats, or fl oating in the water column. Although primarily single celled, life by this time was Figure 12. Representative sketches of (A) arcellid testate amoebae, and (B) euglyphid testate amoebae. Note the difference in pseudopod highly diverse, both taxonomically and ecologically. shape. Both arcellid and euglyphid testate amoebae may have mineral- In the shallow, high-energy, warm-water environments of ized scales embedded in their tests; these are shown here only in image the Chuar sea lived cyanobacteria attached to carbonate grains. B, however. Modifi ed from fi gure 2 in Ogden and Hedley (1980). Downloaded from specialpapers.gsapubs.org on January 24, 2013

64 Dehler et al.

ABCD

15 µm 15 µm 15 µm EFG H I J

15 µm 15 µm 10 µm 15 µm 25 µm 15 µm K LM N

15 µm 20 µm 10 µm 20 µm OP QSR

10 µm 30 µm 30 µm 300 µm 5 µm

Figure 13. Representative body fossils from the Chuar Group. (A) Dolomite concretion bearing billions of vase-shaped microfossils (VSMs). Upper Walcott Member, Nankoweap Butte. Geologic hammer for scale. B–E, G, I, K, VSMs, all from upper Walcott Member dolomite concretions. F, H, J, modern testate amoebae. L–Q, acritarchs from the Jupiter Member (L, Q) and the Tanner Member (M–P). Note circular excystment structure in P and medial split in Q. R, Chuaria circularis. Specimen from the Awatubi Member. S, Sphaerocongregus variabilis, from the Awatubi Member. Image in A is courtesy of A. Knoll.

and the presence of indentations (Fig. 13D) or scales (Fig. 13I). similar to these ancient forms live not in the ocean but on moss, Some of the tests look identical in form to those of lobose amoe- in leaf litter, or in lakes, suggesting that several ancient lineages bae today (Fig. 13H), a group closely related to slime molds of testate amoebae may have moved onto land, perhaps following (Baldauf, 2003). Others are similar to those of euglyphid amoe- the food when land plants evolved, some 300 m.y. later (Porter et bae (Fig. 13J), close relatives of the Foraminifera and Radiolaria al., 2003). By analogy with their modern counterparts, Neopro- (Nikolaev et al., 2004). Interestingly, the modern groups most terozoic testate amoebae probably ate any or all of the organisms Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 65 in Chuar waters. They may have eaten each other, as well, as their abundance in certain beds suggests that protists were impor- some amoebae do today. Evidence that some testate amoebae tant components of the Chuar ecosystem. may have been preyed upon comes from semicircular holes in One organism living in the Chuar sea appears to have been the test walls of many specimens (Fig. 13K), although it is not a giant relative to the rest: the ~1–3-mm-diameter Chuaria cir- clear who may have made these or how (Porter et al., 2003). In cularis (Fig. 13R). Although its remains are now found as fl at any case, VSMs provide some of the earliest evidence for preda- discs on shale bedding planes, Chuaria was originally a smooth, tors in the fossil record, indicating that relatively complex food featureless, probably planktonic sphere. The discovery in early webs were in place ~740 m.y. ago. Neoproterozoic strata from India of probable Chuaria speci- Farther offshore lived a diversity of planktonic microor- mens attached to the end of another cm-sized ribbon-like fos- ganisms. These likely included ciliates, voracious single-celled sil, Tawuia, suggests that Chuaria may represent a reproductive micropredators covered in hair-like projections called cilia. stage in the life cycle of a seaweed (Kumar, 2001). Holdfast-like These particular species likely lived in low-oxygen waters or at structures on some Tawuia specimens suggest that the seaweed the boundary between oxygen-rich and oxygen-depleted waters, lived attached to the sediment; once mature, Chuaria would have dining on bacteria that thrived in the low-oxygen environment. become detached from its parent seaweed and dispersed plank- Evidence for this comes from the presence of gammacerane, a tonically (Kumar, 2001). This may explain why Chuaria is much hydrocarbon molecule preserved in lower Walcott dolostones more widespread in rocks of this age than is Tawuia (Kumar, (Summons et al., 1988; Vogel et al., 2005). Gammacerane is the 2001). Although Chuaria occurs in both shallow and deeper geologically stable form of tetrahymenol, a lipid produced pri- water environments preserved in the Chuar Group, Tawuia has marily by ciliates that consume signifi cant quantities of bacteria never been found in the Chuar Group. and live in or close to anoxic environments (Sinninghe Damsté Upper Chuar shales preserve ~5–20 µm aggregates of even et al., 1995). Dinofl agellates, a group comprising both photosyn- tinier (<1 µm) organic-walled spheres, sometimes surrounded thetic and heterotrophic protists, may also have been present in by a membrane (Fig. 13S; Moorman, 1974; Cloud et al., 1975; the Chuar sea, as suggested by the presence of dinosterane bio- Nagy et al., 2009). These structures, similar to those described markers in Walcott samples (Vogel et al., 2005). (Dinosteranes under the names Sphaerocongregus variabilis and Bavlinella are the geologically stable form of dinosterols, which today are faveolata, are thought to be the remains of bacteria, although an almost exclusively formed by dinofl agellates [Volkman et al., origin from framboidal pyrite cannot be ruled out (Nagy et al., 1993].) Additional hydrocarbon molecules preserved in both 2009). Sphaerocongregus variabilis (= Bavlinella faveolata) basal Walcott and basal Awatubi strata are consistent with the has an unusual distribution: It is extremely rare in diverse acri- presence of chemoautotrophic bacteria (i.e., bacteria that fi x their tarch assemblages, but it occurs sporadically in high concentra- own carbon and get energy from chemical reactions rather than tions commonly by itself in rocks characteristic of low-oxygen from the sun) and of abundant red and/or golden-brown algae environments and/or interbedded with glacial diamictites (Summons et al., 1988; Vogel et al., 2005). (Knoll et al., 1981; Vidal and Nystuen, 1990). Its occurrence Additional microorganisms are represented in Chuar strata in the Chuar Group is consistent with this pattern; S. variabilis by beautifully ornamented organic-walled microfossils known as is absent from the high-diversity acritarch assemblages found acritarchs. Acritarchs form the bulk of the Proterozoic fossil record in lower Chuar strata, but it is recovered in high numbers from and are found throughout the Chuar Group in shale and mudstone. upper Awatubi and lower Walcott mudstones that are otherwise Although it is likely that these ornamented forms are eukaryotic devoid of diverse acritarch assemblages (Table 1; Nagy et al., (Javaux et al., 2003), paleontologists aren’t exactly sure what kinds 2009). Two taxonomic interpretations have been suggested for of eukaryotes they represent. It has long been assumed that they S. variabilis. The fi rst view is that it was a cyanobacterium that are the remains of phytoplankton—tiny algae that fl oat in the water bloomed under high-nutrient (eutrophic) conditions, living in a column—but recent discoveries suggest that at least some species thin layer of oxygenated surface waters above an anoxic water may have been fungi (Butterfi eld, 2005), and some may represent column (e.g., Moorman, 1974; Cloud et al., 1975; Knoll et al., animal eggs and embryos (Xiao and Knoll, 2000; Yin et al., 2004). 1981; Mansuy and Vidal, 1983). The second view is that it was It is also clear that not all fl oated in the water column; some fossils an anoxygenic (non-oxygen producing) photosynthetic bacte- are preserved attached to sand grains, suggesting that they lived rium that thrived in anoxic waters much like sulfur bacteria in on the seafl oor (e.g., Butterfi eld, 1997). Many acritarchs appear to stratifi ed lakes today (Repeta et al., 1989; Vidal and Nystuen, have been cysts, dormant structures formed when a cell is exposed 1990). In either case, its presence in the upper Chuar Group to stressful conditions, such as nutrient depletion or changes in suggests a transition to eutrophic and anoxic conditions dur- water salinity or temperature. Evidence that some acritarchs are ing late Chuar time, an inference corroborated by the presence cysts comes from excystment structures, circular “escape hatches” of gammacerane, a biomarker indicative of anoxic waters, in (Fig. 13P) or medial splits (Fig. 13Q) through which the cell exited upper Chuar rocks, and by recent iron speciation and sulfur iso- the cyst once its dormant stage was over. The diversity of orna- tope analyses (see above and Table 1; Summons et al., 1988; mented acritarchs—including species covered with tiny cone-like Sinninghe Damsté et al., 1995; Vogel et al., 2005; Johnston et projections, concentric circular ridges, and intricate wrinkles—and al., 2010). Downloaded from specialpapers.gsapubs.org on January 24, 2013

66 Dehler et al.

SHAKING SANDS AND MOVING CONTINENTS the Chuar syncline and Butte fault are in Nankoweap, Kwagunt, Carbon, and Lava Chuar drainages. The Carbon–Lava Chuar In Chapter 2 (Timmons et al., this volume) the history of loop hike offers a fabulous view, but the ultimate view may be Unkar Group sedimentation and deformation was described, pro- from the North Rim (e.g., Point Imperial), where views into Nan- viding a framework for understanding the relative importance koweap Canyon display the syncline in stunning detail. Desert and timing of the subsequent Chuar-age basin formation. We now View on the South Rim also presents an impressive view; from know that late Unkar Group deformation predated Nankoweap there the Chuar syncline can be seen within the Carbon and Lava and Chuar Group deposition and was different in deformational Chuar Canyons. The hinge line of the fold is doubly plunging, style from Chuar-related faulting and syncline development. meaning that within the axis of the syncline, beds in some areas Unkar Group rocks were faulted and tilted along NW-trending (Nankoweap Canyon) dip toward the south, and in other areas fault systems like the Palisades fault (Fig. 2). In contrast, Chuar (Lava Chuar Canyon) beds dip toward the north (Fig. 2). The rocks were folded by the Chuar syncline, paralleling the north- Chuar syncline parallels the trace of the Butte fault, suggesting trending Butte fault. The different styles of deformation and fi eld a genetic relationship between the syncline and the fault. The observations suggest that the Unkar and Chuar Groups record relative timing of syncline development can be determined in the two separate deformational events in the middle to late Protero- fi eld by examining the contact of the Chuar Group with the over- zoic. The remainder of this section will further describe the style lying Tapeats Sandstone where the Tapeats truncates the Chuar and relative timing of Chuar Group deformation and its impor- syncline, indicating that the Chuar syncline is Proterozoic in age tance for understanding the tectonic history of the western United (Fig. 14). Another key characteristic of the Chuar syncline is that States in late Proterozoic time. lower formations of the Chuar Group are more tightly folded The relative importance of Chuar Group deformation, than upper beds (Fig. 15). This is an unusual relationship in most including movement of the Butte fault, development of the Chuar folded terranes and suggests that the syncline was forming dur- syncline, and formation of intraformational faults was docu- ing deposition of the Chuar Group. To test the hypothesis that mented by Timmons et al. (2001). In the absence of absolute age the Chuar syncline, Butte fault, and Chuar Group deposition are determinations from the Chuar Group, early workers lumped the linked, we must examine the preserved rock record. faulting and tilting of the Unkar Group (described in Chapter 2 Field observations collectively suggest that the Chuar Group [Timmons et al., this volume]) and faulting and folding in the was deposited in a shallow-marine extensional basin related to Chuar Group into a single tectonic episode that affected much the earliest development of the Cordilleran margin, which was of western North America (Noble, 1914). This tectonic episode then the western edge of Laurentia (Sears 1990; Dehler et al., propagated through the geologic literature as the Grand Canyon 2001; Timmons et al., 2001). Chuar Group sediments were “Revolution” (Maxson, 1961), “Disturbance” (Wilson, 1962; deposited synchronously with movement on a series of N-S– Elston and McKee, 1982), and “orogeny” (Elston, 1979). The striking normal faults. The principal fault exposed is the Butte common model envisioned for this tectonic event was broadly fault; however, owing to the dominantly fi ne-grained nature of analogous with Basin and Range deformation seen in the mod- the Chuar Group, it is reasonable to conclude that Chuar depos- ern landscape in the western United States. All these early work- its originally overlapped this fault zone (no fault scarp is indi- ers placed the timing of this deformational event in latest Chuar cated by Chuar Group deposits, e.g., thicker and coarser grained Group and Sixtymile Formation time. More recently, workers deposits). Faults, though, work as networks, and so what we learn have come to realize through a combination of better geochronol- about the Butte fault and subordinate faults tell us much about ogy, more detailed geologic mapping, and sedimentary-tectonic regional stresses and what was happening within the greater interpretation that the Unkar and Chuar Groups record distinct Chuar basin. Some of these subordinate faults are intraforma- tectonic and sedimentary events separated by more than 200 m.y. tional, meaning that the faults die out up section and lower beds of time (Timmons et al., 2001, 2005). have more displacement than upper beds, suggesting that exten- The Butte fault is a major north-trending normal fault that sional faulting was concurrent with deposition. The amount of records west-side-down Neoproterozoic displacement (see Chapter displacement across these minor faults is typically very small, 2 [Timmons et al., this volume] for description of common fault usually <2 m of displacement, and likely did not form scarps at types). The amount of displacement across the Butte fault is large the surface. Rather, they were likely buried faults during deposi- by Colorado Plateau standards and varies along the trace of the tion (i.e., they did not rupture the surface), much like the larger fault. The maximum amount of Proterozoic displacement is esti- Butte fault. The subordinate faults do, however, provide clues as mated at 1800 m; however, the actual amount of displacement may to when faulting occurred in the basin. Intraformational faults have been greater. Erosion of the late Proterozoic landscape prior to are found throughout the Chuar Group strata, including the basal Cambrian (Tapeats) time precludes an accurate estimation of fault Tanner Member and middle Carbon Canyon and Carbon Butte displacement. Subordinate faults within the Chuar basin parallel the Members, suggesting that faulting was occurring throughout trace of the Butte fault and also record normal-sense movement. Chuar Group deposition. The Chuar syncline is a broad asymmetric fold compris- One of the most striking observations is that Chuar Group ing Chuar Group strata (Fig. 14). Some of the best places to see deposits are not known to be preserved east of the Butte fault, Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 67

5 4 3 12 Lava Chuar Canyon 0

10 Basal stromatolite marker (Boxonia) of the Awatubi Member 20

Shale and mudstone of the Carbon

Meters Butte Member 30

Fine- to medium-grained sandstone 40 of the Carbon Butte Member W 50 E 2000

Walcott Member Tonto Group Carbon Butte Member Awatubi Member 1 5 2 4 3 Duppa Member Tonto * * Group 1000 Jupiter Member Carbon Canyon Member

Elevation (m) Dox A = partial measured A ? section in Carbon Can- yon Member * = measured sections in member not presented A A′ Figure 14. East-west geologic cross section and measured sections of the Carbon Butte Member in Lava Chuar Canyon. The locations of the measured sections are numbered on the cross section. The overall form of the Chuar syncline is asymmetric, with a steeper limb on the east and a shallower limb on the west. The syncline is shown tightening with depth, as suggested by ﬁ eld observations. The Cambrian Tonto Group is not folded by the Chuar syncline, indicating that the syncline is Proterozoic in age.

Sixtymile Figure 15. Field photo of the Chuar syncline, looking toward the north along its axis. The Carbon Butte Sand- Walcott stone (at right) is steeply dipping toward the west, and, at the skyline, the Awatubi 70 Sixtymile Formation caps Nankoweap Butte. Note that the syncline axial plane dips to the east and that upper beds are folded less tightly. This is a hallmark of growth synclines. Downloaded from specialpapers.gsapubs.org on January 24, 2013

68 Dehler et al. indicating that movement of the Butte fault preserved deposits bon Canyon Member was measured on the east limb and within to the west and exposed the deposits to the east to erosion, all the axis of the syncline (Fig. 16). The fi rst observation is that before the Tapeats Sandstone was deposited. When this rela- within the axis of the syncline the marker bed interval is thicker tionship fully developed is a mystery, but one possibility is that than the same interval on the east limb; however, the thickness syncline development, fault movement, and deposition were con- changes are largely confi ned to mudstones. Carbonate beds and current. To address this possibility, numerous measured sections sandstones do not express this thickness change and likely refl ect were completed at multiple locations with particular interest in either short depositional episodes and/or periods of relative fault understanding how deposits vary in thickness and type across inactivity. Sections measured above in the Carbon Butte Mem- the syncline. An interval bound by marker beds within the Car- ber also refl ect a similar relationship, with the thinnest intervals

Axis of Chuar syncline East limb of Chuar A syncline B Baicalia

40 ? ? ? 30

? ?

Figure 16. Measured partial sections of the Carbon Canyon Member in Carbon 20 Depth (m) Canyon (A) and Kwagunt Canyon (B). Note lateral continuity of thicker carbonate marker beds and discontinuity of other rock types. Within the synclinal axis (section A) the mudstone beds are thicker, suggesting that the syncline axis was 10 creating more accommodation for deposition of mud. Does this reﬂ ect episodic development of the Chuar syncline and Butte fault, or do mudstone facies record more geologic time and thus more incre- "Polygonal" marker bed mental synclinal development?

0 Partial section of <0.5 km as projected into cross sections Partial section of Carbon Canyon Member Carbon Canyon in Carbon Canyon Member in Kwagunt Canyon Duppa KEY Member variegated mudrocks Marker bed black mudrocks interval Carbon sandstone Canyon dolomite Member stromatolites

Jupiter crinkly laminations large mudcrack Member planar laminations trough cross-beds GALEROS FORMATION symmetric ripple calcareous mudcrack intraclasts Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 69

occurring on the eastern limb of the Chuar syncline (Fig. 14). the end of Chuar time, incremental fault movement forms the Here again, the mudstone units refl ect the syncline form, with Chuar syncline while the fault itself remains buried. During this thicker accumulations within the axis; sandstone beds are rela- interval, it is possible that some of the fi ne-grained sediments tively uniformly thick across the syncline. Finally, within the deposited east of the fault would have been eroded and recycled upper units of the Chuar Group the syncline form becomes even into the deposits on the west side of the fault. Sometime after more pronounced, and it is manifested in different units, includ- Chuar Group time, the fault must have penetrated to the surface ing carbonate beds (Fig. 17). At this interval, carbonate beds and created a scarp that would encourage the complete stripping pinch out to zero thickness toward the Butte fault. If the deposits of the Chuar Group from the east side of the fault (Fig. 18C). were accumulating in an incrementally developing syncline, this The record of the emergent Butte fault may be recorded is the relationship one would predict. in the deposits of the Sixtymile Formation. This formation is The above observations collectively point to the conclusion truly a unique section in the Grand Canyon Supergroup. Work- that the Chuar syncline was developing during deposition and ers have long recognized that the Sixtymile Formation marks faulting of the Chuar Group. The Butte fault and subordinate dramatic faulting along the Butte fault system and represents structures were instrumental to the development of the Chuar the principal extensional event in the Neoproterozoic (Elston, syncline, but remained buried during deposition of the Chuar 1979). Although the Sixtymile Formation does refl ect a dra- Group (Fig. 18A). Early in Chuar time, intraformational faults matic change in depositional style, we prefer the interpretation offset Tanner Member deposits and the synclinal form began to that the Sixtymile Formation records a continuation of Butte take shape with incremental faulting (Figs. 18A and 18B). By fault movement.

Nankoweap Butte Sixtymile Canyon (X and Y) (Z) sandstone with conglomerate interbeds; breccia at base 60 (W) (E) Cambrian Tapeats red to white siltstone with silicified

upper member beds, breccias and convolute beds (channel fill)

40 dolomite

shale middle (siltstone) member 20 (locally brecciated and slumped)

Sixtymile brecciated siltstone Walcott soft-sediment siliceous layers deformation slumped horizon 0 red

Depth (m) 742 Ma ash black Walcott Walcott Butte fault scarp concretion in black shale with ~700 m evidence for rotation and minor translation toward the to the east 30m axis omitted 30m omitted dolomite doublet pinches out toward Butte fault in Sixtymile Canyon

syncline axis

Figure 17. Measured sections and schematic diagram of deformational features in the upper Chuar Group and Sixtymile Formation. The view is toward the north, parallel to the trace of the Butte fault and Chuar syncline. Carbonate beds of the upper Walcott Member pinch out toward the Butte fault, suggesting that syncline development continued through Walcott time. The end of Walcott Member deposition is marked by very large, disarticulated blocks of carbonate on a mudstone slip horizon. Sixtymile time is marked by deposition of clastic red beds and the development of incised channels within the formation. Downloaded from specialpapers.gsapubs.org on January 24, 2013

70 Dehler et al.

The Sixtymile Formation is present in only four places in the Chuar Valley, with outcrops atop Nankoweap Butte, and within Awatubi and Sixtymile Canyons (Fig. 2). The thickness of this Jupiter Member formation was measured in multiple localities and is as thick as Tanner dolomite Tanner Member 60 m, though the actual depositional thickness remains unknown owing to erosion prior to deposition of the Tapeats Sandstone. Within this section a number of fi eld observations indicate that the Unkar Group formation represents a fundamental change in depositional envi- A ronments and tectonic activity (Fig. 17). Near the base of the section are blocks of carbonate that are surrounded by fi ner grained siltstone and mudstone. The blocks lie within a narrow interval, Carbon Butte Member and individual blocks appear to be rotated, suggesting that they Duppa Member were transported on bedding-parallel slip planes or slumps. In Carbon Canyon Member fact, early workers postulated that these blocks were derived from Chuar deposits east of the Butte fault, specifi cally the “doublet Jupiter Member Tanner Member beds” of the Walcott Member (Elston, 1979). Our more reserved Tanner Member interpretation is that the carbonate blocks do not appear to be B Unkar Group equivalent to the doublet, and the doublet beds pinch out west of the Butte fault (Fig. 17). More likely, the carbonate blocks were derived from another part of the Chuar section, but we have not Sixtymile Formation been able to correlate these blocks directly to lower beds in the Jupiter Duppa Carbon Walcott Chuar Group. Soft sediment deformation of fi ner grained beds Butte Tanner within the same interval suggests that the blocks and deformation Member Awatubi Carbon Canyon Member occurred while the deposits were still soft and wet. Unkar Group Farther up section the Sixtymile Formation is dominated by Jupiter Member thinly bedded siltstone. The siltstone beds are incised and infi lled by locally derived silicifi ed siltstone clasts. The incised channels Tanner Member are steep sided and locally as deep as 15 m (Fig. 17). The channel Unkar fi ll is weakly stratifi ed with interbedded sandstone and locally Group C derived breccia. The incision into siltstone of the Sixtymile For- mation is interesting in that it suggests that there was some sort of change in relative base level, meaning either the land surface Paleozoic rocks Walcott was uplifted relative to sea level, or sea level dropped. In either Carbon Butte Awatubi case the change in depositional environments had a profound Duppa effect on the development of the Butte fault and Chuar syncline. Paleozoic rocks Jupiter Carbon Without fi ne-grained deposits continually burying the Butte fault Canyon zone, continued incremental movement along the Butte fault Tanner Unkar likely reached the surface and created a fault scarp, as suggested Group D Unkar by lower beds in the Sixtymile Formation. The Butte fault likely continued to move after Sixtymile time, resulting in the com- Figure 18. Schematic time slices illustrating the growth history of the plete removal of Chuar deposits from the east side of the fault Chuar syncline at key intervals. (A) Time slice shows the truncation of the Tanner dolomite against a subordinate normal fault, with fault (Fig. 18D). The fi nal extensional movement of the Butte fault overlapped by Tanner Member shale, indicating that extensional fault- system ended late in Neoproterozoic time, prior to Tapeats Sand- ing was synchronous with Tanner Member deposition. (B) Time slice stone deposition, but much of that record was lost to erosion. illustrates the growth nature of the Chuar syncline during deposition of the Carbon Butte Member; note the postulated depocenter in the syn- THE NEOPROTEROZOIC EARTH SYSTEM AS cline and thinning of units over the footwall of a “blind” Butte fault. (C) Time slice displays our interpretation of syncline/fault relation- “SEEN” THROUGH THE CHUAR GROUP ships during Sixtymile Formation time. Note the pinch-out of dolomite units in the Walcott Member, the apparent truncation of the Chuar The Chuar Group captures a critical “snapshot” in Earth syncline by the Sixtymile Formation, and the tightening of the syncline systems history. The combined data sets suggest that the Chuar at depth. (D) Time slice shows the present fault-syncline geometry in Group was deposited during, or just before, the onset of low- Lava Chuar Canyon (includes Laramide movement on the Butte fault). Unshaded layers or portions of layers indicate those rocks that have latitude glaciations, and during the early rifting of the supercon- been eroded. tinent Rodinia. Researchers are currently pursuing the Chuar Group and correlative strata to fi nd clues as to why the ensuing Downloaded from specialpapers.gsapubs.org on January 24, 2013

The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon 71 large-scale (possibly snowball-Earth-style) glaciations occurred The Butte fault and Chuar syncline provide a unique win- and how these changes affected biotic evolution. Understanding dow into the regional tectonics of the western United States dur- the timing and tectonic style of the Chuar basin provides geolo- ing Neoproterozoic time. Along much of the western margin of gists with more documentation on the complex disassembly North America, remnants of late Proterozoic deposits are pre- of Rodinia, a problematic question owing to the long duration served in isolated outcrops (Fig. 19). The combined record of (200 m.y.) between initial rifting and the development of the Cor- these deposits marks the incipient phases of a continental rift dilleran miogeocline (e.g., Bond and Komintz, 1984; Timmons et margin. Starting ~800 m.y. ago, the western margin of the North al., 2001; Colpron et al., 2002). American crust began to thin and stretch as North America began

Baltica Windermere .75 North Group w America 0.58 Australia 0.78 50° N

0.55

0.82 East Antarctica 800-700 Ma incipient rifting of Rodinia 45° N

a Figure 19. Index map of Neoprotero- c

i zoic sedimentary rocks (shown in black) r and inferred Neoproterozoic structures. e The Neoproterozoic N-S tectonic grain m h A is inferred from the extent of Laramide r t (reactivated) structures and other N-S o features. Inset shows a proposed Neo- 40° N N Uinta Mountain f proterozoic plate reconstruction (after o Group Brookﬁ eld, 1993; Karlstrom et al., 1999;

e Burrett and Berry, 2000). Neoproterozo-

g ic sedimentary basins (900–600 Ma) are

d E shaded, and ages of maﬁ c dikes are in billions of years.

35° N Pahrump Group Chuar Group

30° N

Caborca GroupGroupGroupGroup 500 km

120° W 115° W 110° W 105° W Downloaded from specialpapers.gsapubs.org on January 24, 2013

72 Dehler et al.

to rift away from other continental blocks. Within the Winder- nity of ornamented acritarchs (Nagy et al., 2009), consistent mere Group of British Columbia, the Pahrump Group of Death with other evidence that suggests that eukaryotes were diver- Valley, and the Uinta Mountain Group of Utah, workers have sifying during this time (Porter, 2004). In the upper Awatubi identifi ed syn-extensional deposits, much like the Chuar Group Member, however, ornamented acritarchs disappear, replaced (Link et al., 1993; Prave, 1999; Rybczynski, 2009; Dehler et al., in both shallow- and deeper water environments by blooms of 2010). The combined studies point to a major continental-scale the bacterium Sphaerocongregus variabilis. This fossil is typi- rifting event that affected much of western North America, and is cally associated worldwide with “snowball Earth” glaciations; a proxy for what was going on worldwide at ca. 750 Ma. indeed, blooms of S. variabilis have been recovered from syn- The carbon-isotope excursions in the Chuar Group are of glacial deposits (sediments deposited during glaciations) (Knoll similar magnitude and age to strata in other places in the world, et al., 1981). The appearance of bacterial blooms in the Chuar and indicate that the Chuar curve may represent a global-scale Group coincides with evidence for increased ice volume on the carbon cycle signature (Dehler et al., 2005). The cyclicity in the globe (inferred from stratigraphic cycles; see above), as well as Chuar Group strata also refl ects a global phenomenon: fl uctua- biomarker evidence that the Chuar sea may have become eux- tions in global sea level and ice volume throughout the history inic (anoxic and sulfi dic) at this time. Interestingly, this shift of the basins, with increasing ice volume by 750 Ma (Dehler et also coincides with the fi rst appearance of VSMs—fossils of al., 2001). On a more local scale, changes in the shale geochem- heterotrophic protists that may have proliferated because of the istry suggest changing rainfall patterns throughout Chuar time. organic-rich environment. These data sets all point to a climate-regulated carbon cycle. Research continues on the Chuar Group, and perhaps in the One possible scenario is that many Chuar-type intracratonic rift future there will be more concrete answers as to what caused the basins formed in low-mid latitudes as the supercontinent was unusual changes recorded in these strata. If you are lucky enough breaking up. The basins acted as sediment traps and, in concert to see the Chuar Group from one of the canyon rims, from the air, with changing sea level and local rainfall patterns, buried enough or on a hike, think of these strata not only as the gorgeous painted carbon to cause the radical shifts in the carbon curve as seen, rocks of the Grand Canyon desert but as a “snapshot” in time, for example, in the upper Chuar Group. When enough carbon ~750 m.y. ago. Along this ancient, warm, quiet shoreline, shallow was removed from the atmosphere and buried in sediments, this seas came in and out as glaciers waxed and waned in other nearby

could have caused a global CO2 drawdown from the atmosphere, latitudes, diverse protists and bacteria ﬂ oated in the water or grew potentially signiﬁ cant enough to bring glaciers to lower latitudes on the surface of carbonate sands, and the ground occasionally and elevations. shook from movement on the nearby Butte fault. The paleontological record of the Chuar Group provides indirect evidence for the onset of low-latitude glaciation as well. The lower Chuar Group documents a diverse commu- MANUSCRIPT ACCEPTED BY THE SOCIETY 6 JANUARY 2012

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