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Geological Society of America Special Papers Downloaded from specialpapers.gsapubs.org on January 24, 2013 Geological Society of America Special Papers 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) Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Special Papers Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes © 2012 Geological Society of America Downloaded from specialpapers.gsapubs.org on January 24, 2013 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 50 112° 35'W 111° 47'W Downloaded from 36° 25'N 36° 25'N E a N s t K a i b a specialpapers.gsapubs.org b m o no cli ne Bass Canyon Butte fault onJanuary24,2013 l Dehler etal. Bright AngelAnge K7-115-3 Pa lis ad Chuar Group e s Cardenas Basalt and Diabase Unkar Group Granite Gorge Metamorphic Suite Proterozoic normal fault, ball on the downthrown side 10 km n o y Proterozoic monocline Buried Proterozoic fault n Vishnu Canyon a C d Laramide monocline Approximate trace of Laramide monocline Re 35° 58'N 35° 58'N 112° 35'W 111° 47'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). 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 tluaf ettuB tluaf mi. 61 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.
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