BRIGHAM YOUNG UNIVERSITY
GEOLOGICAL SOCIETY OF AMERICA
FIELD TRIP GUIDE BOOK
1997 ANNUAL MEETING SALT LAKE CITY, UTAH
PAR'
EDITED BY PAUL KARL LINK AND BART J. KOWALLIS
VOIUME 42 I997 PROTEROZOIC TO RECENT STRATIGRAPHY, TECTONICS, AND VOLCANOLOGY, UTAH, NEVADA, SOUTHERN IDAHO AND CENTRAL MEXICO Edited by Paul Karl Link and Bart J. Kowallis
BRIGHAM YOUNG UNIVERSITY GEOLOGY STUDIES Volume 42, Part I, 1997
CONTENTS
Neoproterozoic Sedimentation and Tectonics in West-Central Utah ...... Nicholas Christie-Blick 1
Proterozoic Tidal, Glacial, and Fluvial Sedimentation in Big Cottonwood Canyon, Utah ...... Todd A. Ehlers, Marjorie A. Chan, and Paul Karl Link 31
Sequence Stratigraphy and Paleoecology of the Middle Cambrian Spence Shale in Northern Utah and Southern Idaho ...... W. David Liddell, Scott H. Wright, and Carlton E. Brett 59
Late Ordovician Mass Extinction: Sedimentologic, Cyclostratigraphic, and Biostratigraphic Records from Platform and Basin Successions, Central Nevada ...... Stan C. Finney, John D. Cooper, and William B. N. Beny 79
Carbonate Sequences and Fossil Communities from the Upper Ordovician-Lower Silurian of the Eastern Great Basin ...... Mark T. Harris and Peter M. Sheehan 105
Late Devonian Alamo Impact Event, Global Kellwasser Events, and Major Eustatic Events, Eastern Great Basin, Nevada and Utah ...... Charles A. Sandberg, Jared R. Morrow and John E. Warme 129
Overview of Mississippian Depositional and Paleotectonic History of the Antler Foreland, Eastern Nevada and Western Utah ...... N. J. Silberling, K. M. Nichols, J. H. Trexler, Jr., E W. Jewel1 and R. A. Crosbie 161
Triassic-Jurassic Tectonism and Magmatism in the Mesozoic Continental Arc of Nevada: Classic Relations and New Developments ...... S. J. Wyld, and J. E. Wright 197
Grand Tour of the Ruby-East Humboldt Metamorphic Core Complex, Northeastern Nevada: Part 1- Introduction & Road Log ...... Arthur W. Snoke, Keith A. Howard, Allen J. McGrew, Bradford R. Burton, Calvin G. Barnes, Mark T. Peters, and James E. Wright 225
Part 2: Petrogenesis and thermal evolution of deep continental crust: the record from the East Humboldt Range, Nevada ...... Allen J. McGrew and Mark T. Peters 270 Part 3: Geology and petrology of Cretaceous and Tertiary granitic rocks, Lamoille Canyon, Ruby Mountains, Nevada ...... Sang-yun Lee and Calvin G. Barnes
Part 4: Geology and geochemistry of the Hanison Pass pluton, central Ruby Mountains, Nevada ...... Bradford R. Burton, Calvin G. Barnes, Trina Burling and James E. Wright
Hinterland to Foreland Transect through the Sevier Orogen, Northeast Nevada to North Central Utah: Structural Style, Metamorphism, and Kinematic History of a Large Contractional Orogenic Wedge ...... Phyllis Camilleri, W. Adolph Yonkee, Jim Coogan, Peter DeCelles, Allen McGrew, Michael Wells
Part 2: The Architecture of the Sevier Hinterland: A Crustal Transect through the Pequop Mountains, Wood Hills, and East Humboldt Range, Nevada ...... Phyllis Camilleri and Allen McGrew
Part 3: Large-Magnitude Crustal Thickening and Repeated Extensional Exhumation in the Raft River, Grouse Creek and Albion Mountains ...... Michael L. Wells, Thomas D. Hoisch, Lori M. Hanson, Evan D. Wolff, and James R. Struthers
Part 4: Kinematics and Mechanics of the Willard Thrust Sheet, Central Part of the Sevier Orogenic Wedge, North-central Utah ...... W. A. Yonkee
Part 5: Kinematics and Synorogenic Sedimentation of the Eastern Frontal Part of the Sevier Orogenic Wedge, Northern Utah ...... W. A. Yonkee, F! G. DeCelles and J. Coogan
Bimodal Basalt-Rhyolite Magmatism in the Central and Western Snake River Plain, Idaho and Oregon ...... Mike McCurry, Bill Bonnichsen, Craig White, Martha M. Godchaux, and Scott S. Hughes
Bimodal, Magmatism, Basaltic Volcanic Styles, Tectonics, and Geomorphic Processes of the Eastern Snake River Plain, Idaho ...... Scott S. Hughes, Richard l? Smith, William R. Hackett, Michael McCuny, Steve R. Anderson, and Gregory C. Ferdock
High, Old, Pluvial Lakes of Western Nevada ...... Marith Reheis, and Roger Momson
Late Pleistocene-Holocene Cataclysmic Eruptions at Nevado de Toluca and Jocotitlan Volcanoes, Central Mexico ...... J. L. Macias, F! A. Garcia, J. L. Arce, C. Siebe, J. M. Espindola, J. C. Komorowski, and K. Scott A Publication of the Department of Geology Brigham Young University Provo, Utah 84602
Editor
Bart J. Kowallis
Brigham Young University Geology Studies is published by the Department of Geology. This publication consists of graduate student and faculty research within the department as well as papers submitted by outside contributors. Each article submitted is externally reviewed by at least two qualified persons.
Cover photos taken by Paul Karl Link. Top: Upheaval Dome, southeastern Utah. Middle: Luke Bonneville shorelines west of Brigham City, Utah. Bottom: Bryce Canyon National Park, Utah.
ISSN 0068-1016 9-97 700 23348124218 Preface
Guidebooks have been part of the exploration of the American West since Oregon Trail days. Geologic guidebooks with maps and photographs are an especially graphic tool for school teachers, University classes, and visiting geologists to become familiar with the temtory, the geologic issues and the available references. It was in this spirit that we set out to compile this two-volume set of field trip descriptions for the Annual Meeting of the Geological Society of America in Salt Lake City in October 1997. We were seeking to produce a quality product, with fully peer-reviewed papers, and user-fnendly field trip logs. We found we were buck- ing a tide in our profession which de-emphasizes guidebooks and paper products. If this tide continues we wish to be on record as producing "The Last Best Geologic Guidebook." We thank all the authors who met our strict deadlines and contributed this outstanding set of papers. We hope this work will stand for years to come as a lasting introduction to the complex geology of the Colorado Plateau, Basin and Range, Wasatch Front, and Snake River Plain in the vicinity of Salt Lake City. Index maps to the field trips contained in each volume are on the back covers. Part 1 "Proterozoic to Recent Stratigraphy, Tectonics and Volcanology: Utah, Nevada, Southern Idaho and Central Mexico" contains a number of papers of exceptional interest for their geologic synthesis. Part 2 "Mesozoic to Recent Geology of Utah" concentrates on the Colorado Plateau and the Wasatch Front. Paul Link read all the papers and coordinated the review process. Bart Kowallis copy edited the manu- scripts and coordinated the publication via Brigham Young University Geology Studies. We would like to thank all the reviewers, who were generally prompt and helpful in meeting our tight schedule. These included: Lee Allison, Genevieve Atwood, Gary Axen, Jim Beget, Myron Best, David Bice, Phyllis Camillen, Marjorie Chan, Nick Christie-Blick, Gary Christenson, Dan Chure, Mary Droser, Ernie Duebendorfer, Tony Ekdale, Todd Ehlers, Ben Everitt, Geoff Freethey, Hugh Hurlow, Jim Garrison, Denny Geist, Jeff Geslin, Ron Greeley, Gus Gustason, Bill Hackett, Kimm Harty, Grant Heiken, Lehi Hintze, Peter Huntoon, Peter Isaacson, Jeff Keaton, Keith Ketner, Guy King, Me1 Kuntz, Tim Lawton, Spencer Lucas, Lon McCarley, Meghan Miller, Gautarn Mitra, Kathy Nichols, Robert Q. Oaks, Susan Olig, Jack Oviatt, Bill Peny, Andy Pulham, Dick Robison, Rube Ross, Rich Schweickert, Peter Sheehan, Norm Silberling, Dick Smith, Bany Solomon, K.O. Stanley, Kevin Stewart, Wanda Taylor, Glenn Thackray and Adolph Yonkee. In addition, we wish to thank all the dedi- cated workers at Brigham Young University Print Services and in the Department of Geology who contributed many long hours of work to these volumes.
Paul Karl Link and Bart J. Kowallis, Editors Proterozoic Tidal, Glacial, and Fluvial Sedimentation in Big Cottonwood Canyon, Utah
TODD A. EHLERS MARJORIE A. CHAN Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112
PAUL KARL LINK Department of Geology, Idaho State University, Pocatello, Idaho 83209
ABSTRACT Spectacular outcrops of Mesoproterozoic to Cambrian deposits illustrating a variety of depositional envi- ronments are exposed along the steep walls of Big Cottonwood Canyon of the central Wasatch Range. This field trip is designed to examine the unique aspects of tidal and estuarine deposition in the 4.8 km thick -900 Ma Big Cottonwood Formation, which contains the oldest known examples of tidal rhythmites. The trip also includes stops illustrating Neoproterozoic glaciation in the Mineral Fork Formation, braided fluvial deposition in the Mutual Formation, and shallow marine deposition in the Tintic Quartzite.
INTRODUCTION known examples of tidal rhythmites and related tidal structures (~hanet al., 1994;-Archer, 1997; Kvale et al., Big Cottonwood Canyon, just east of Salt Lake City, 1997). These structures provide a new perspective on the contains easily accessible outcrops of the Big Cottonwood tectonic setting and paleogeography of the unit (Ehlers and Formation, Mineral Fork Formation, Mutual Formation, Chan, 1996a, 1996b), as well as insight into Proterozoic and Tintic Quartzite, which (with intervening unconfor- Earth-moon orbital mechanics (Sonett et al., 1996; Kvale mities) collectively span as much as 470 my of earth history et al., 1997). from -1000 to 530 Ma (Crittenden et al., 1952; Ojakangas The trip route traverses eastward and up section (Figs. and Matsch, 1980; Christie-Blick, 1983; Christie-Blick and 4 and 5) on Highway 190 (old Highway 152), from the Big Link, 1988) (Fig. 1 and 2). These Proterozoic formations Cottonwood Formation at the mouth of the canyon (stops fall within Precambrian successions B and C of western 1-5; Fig. 2), to the Mineral Fork Formation at Mineral North America, as defined by Link et al. (1993) (Fig. 3). Fork (stop 6), the Mutual Formation (stop 7), and lastly the The outcrops in Big Cottonwood Canyon lie structurally Tintic Quartzite (stop 8). The first portion of this trip will below the far-traveled thrust sheets of the Sevier orogenic be run in conjunction with the companion three day belt. Though they do lie above thrust faults with small (few Neoproterozoic trip led by Christie-Blick (1997). km) displacement, they are basically in place with respect to their underlying cratonic basement and their depositional HISTORICAL PERSPECTIVE site. They are thus parautochthonous, and were deposited tens of km to the east of the bulk of the Meso- and Neo- Big and Little Cottonwood Canyons are two prominent proterozoic strata exposed along the western margin of drainages that dissect the Wasatch Front (Fig. 1) and are North America, which is largely allochthonous (Young, 1979; well known for their scenery and recreational attractions. Crittenden et al., 1983; Christie-Blick, 1983; Levy and These two canyons also have a colorful mining history. Christie-Blick, 1989; 1991; Winston, 1991; Skipp and Link, Oligocene intrusive activity of the Little Cottonwood, Alta, 1992; Link et al., 1993; Rainbird et al., 1996). and related stocks affected strata that had been folded and The Big Cottonwood Formation has received little atten- thrust faulted during Cretaceous and Paleocene time. The tion since the 1970s. However, despite mild metamorphic stocks generated hydrothermal systems, which produced overprinting (greenschist facies), sedimentary features are skarns, veins and replacement deposits in deformed Paleo- well preserved. Renewed interest in the Big Cottonwood zoic limestones stratigraphically above the Proterozoic rocks Formation has been sparked by the discovery of the oldest examined in this field trip. The Big and Little Cottonwood 32 BYU CEO1,OGY STlTDIES 1997, VOL. 42, PAKT I
GEOLOGIC SETTING The Big Cottonwood Canyon area lies astride the
Quaternary Deposits Cordilleran hinge line (CVasatch Line) (Crittenden, 1976, p. 363), across which Neoproterozoic and Paleozoic pas- sive-margin strata thicken drastically wes~ard.An approx- imately 6 krn tl~ickcomposite strr-ltigr~phicsection of Meso- proterozoic through Jurassic sedimentary rocks is exposed. The Mesoproterozoic througlr Cambrian rocks which will be exanlined on this field trip are exposed in the lower, western part of the canyon. The upper lialf of the canyon exposes late Paleozoic liniestones (Fig. 5) with kf.fcsozoic Figure 1. Generalized locutiorz inc~pof Big Cottonwoorl Cunyon, continental to shallow nrarine deposits comprising the sootlx~~mtof Salt Lake City. higl-rest canyon ridges (Crittenden, 1976; Stokes, 1986; ETintze, 1988). In the Wasatch Range, Proterozoic through hfesozoic mining districts produced over $34,000,000 from 1867 to strata below the Absaroka, Willard and Charlestoii thrust 1940, from silver, lead and copper ores (Calkins and Butler, faults of the Sevier orogenic belt are parautochthonoris, 1942; Crittenderl et al., 1952; James, 1979). By the 1940s, but strata now located al~ovethese thrust fi~~rltshave been mining activity had subsided, with the prinrary use of transported tens of kilometers to tl~eeast with respect to the canyon turning towards skiing and other recreational the underlying craton (Crittenden, 1976; Brulm et al., 1986; outlets. Levy and Christie-Blick, 1989; Cowan and Bruhn, 1992;
Undivided Neoproterozoic - Mississippian Mineral Fork Formation 0 1 mile - Cambrian Meso to Neoproterozoic 0 1 kilometers I Maxfield Limestone Big Cottonwood Formation Ophir Formation
Figure 2. Locution map of field trip stops and lithologies in the lowcr hulfof Big Cottonwoorl Canyon. i\focliJieclfrom Jnrn~s(1979). EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 33 Smith et al., 1989) and exhumed 11 krn of rock near Little and Big Cottonwood Canyon to expose the rocks to their present position (Parry and Bruhn, 1987; Pany et al., 1988; Kowallis et al., 1990).
STRATIGRAPHY Big Cottonwood Formation Our understanding of the geologic relations in Big Cottonwood Canyon is based largely on the work of Max D. Crittenden, Jr., who worked in the northern Wasatch Range for the U.S. Geological Survey over a period of 30 years (Crittenden et al., 1952; Crittenden and Wallace, 1973; Crittenden, 1976). He defined mappable formations in the canyon and distinguished the prominent quartzite and shale lithologies within the Big Cottonwood Forma- tion. James (1979) later summarized the geology and history of the Big Cottonwood mining district, with emphasis on the mineral deposits. The geology of the Big Cottonwood area is shown on the Mount Aire, Dromedary Peak, Sugar House, and Draper 7 112 minute geologic maps (Crittenden 1965a; 1965b, 1965c, 1965d). The most recent geologic compilation of the area is in Bryant (1990). The thickest, and best preserved sequence of the Big Cottonwood Formation is exposed in Big Cottonwood Canyon (type area). Smaller sections (800 m or less) that have been mapped as Big Cottonwood Formation are ex- posed in Slate Canyon, East Tintic Mountains, Stansbury Island, and Carrington Island. These western sections show some lithologic similarities to a few of the facies in Big Cottonwood Canyon, but have not been studied re- cently. The present discussion focuses only on the thick and relatively continuous exposures present in Big Cottonwood Canyon. The base of the Big Cottonwood Formation, where accessible, contains a thin conglomerate with clasts derived from the unconformably underlying Paleoproterozoic Little Figure 3. Stratigraphic comelation chart of Proterozoic succes- Willow Formation (Crittenden, 1976). The Big Cottonwood sion B and C of western North America. White spaces represent missing record, or igneous rocks not shown on diagram. Modiied Formation is unconformably overlain by the Neoproterozoic from Link et al., 1993 (DNAG Precambrian volume). Mineral Fork Formation where present, and the Mutual Formation elsewhere. Regionally, the Big Cottonwood Formation is correlat- Christie-Blick, 1997). Starting with the Late Cretaceous ed to the Uinta Mountain Group of northeast Utah, and (Maastrichtian), the Wasatch Range experienced east-west the Chuar Group of northern Arizona (Fig. 3) on the basis compression (Sevier orogen~)followed by late Paleocene of paleomagnetic data (Elston, 1989; Link et al., 1993) and north-south compression (Laramide orogeny) which re- the commonalty of microfossils, including acritarchs, and sulted in the formation of the east-trending Uinta Arch Melanocyrillium (Hofmann, 1977; Knoll et al., 1981; Vidal that extends westward through Big Cottonwood Canyon and Ford, 1985). Crittenden and Wallace (1973, p. 117) (Crittenden, 1976; Bruhn et al., 1983; 1986). made an initial Mesoproterozoic age assignment for the Following Oligocene plutonism (including emplace- Big Cottonwood Formation and correlated it to the Uinta ment of the Little Cottonwood and Alta Stocks), Basin and Mountain Group. Further, they made what is now thought Range extension initiated along the Wasatch fault approxi- to be an incorrect correlation of these strata with the Belt mately 12-15 Ma (Zoback, 1983; Smith and Bruhn, 1984; Supergroup, which fostered decades of misconception 34 BYU GEOLOGY STUDIES 1997, \'(>I, 42, I'd4KI‘ I
Stop 6,7,8 Channeled Quartzite 13ral Fork Foiation (-0.7 -0.8 Ga) 4-1 1 Transitional Argillite Mud-cracked Argillite Channeled Quartzite (Moss Ledge) 1-1 Sheet Quartzite I
Mud-cracked Argillite
Sheet Quartzite (?) Laminated Argillite Mud-cracked Argillite Channeled Quartzite
Sheet Quartzite
Sheet Quartzite (?)
Laminated Argillite
Mud-cracked Argillite (?) 4-+ stop 3 1 Sheet Quartzite (Storm Mt.) Little Willow Fm. (-1.6+ Ga) J Mudcracked Argillite
E'igurc, 4. C;i~nc~rciEizcdrne~asur~tl sfrcrtigruplzic section of fhr' Big C'ottonzcootl firincifion. Location,\ ctf' rtopc O)o,rcs) czrltl litfrc$~cirr Irzliels ~rcshowrt to the I-ig11tof ecrch colzcittrr. Erorioncd nnconfonnitics arc? ~hotctl,roitll thick 1ir1c.s tl~rorigftocitthe, ,scctioi~
al~outcorrelation of Mcsoproterozoic strata of the western represents the global Stitrtian glaciation at i11)out 725 Ma U.S. The Belt Supergroup is now thought to be older (C~ittendenct al., 1983, Link et al., 1994). The Miner'il Fork (1430-1200 Ma) than tile Uinta Xlountain Group (-1000 Formation 01 crlics thc Big Cottonwoocl I%rmation in the to -800 Ma) (see discussion it1 Link et a]., 1993). The Big fielcl trip area, btit to the north the Big Cottorrwood Fornla- Cottonwood Eiormation, Uinta hlountain Group and Chuar tion has been eroded arlcl the klineral Fork overlies tile Group are collectively placed irk the late Meso to early kchean and Paleoprotero~oicFaimington Canyon Cornples, Neoprotero~oic,although no reliable radiometric dates as it does to the northwest at Antelope Island (C11risti~- are availalde, except for the top of the Chuar Croup (Sixty Blick, 1983). Milt Fortilatiorl) where a K-Ar age between 855-790 llineral Fork strata now occupy west-tr~nciing,stcep- (~nean823) 52a has been acquired (Elston ancl Mcliee, walled (to 40 degrees) glaciall> scourecl valle) s up to 900 1982). n1 decp. Their origin likely involved n~~tltiplecpi5odes of incision, augnrented 11) Ncoprotero~oicglacial deepening Mineral Fork Formation (Christie-Blick, 1983; Christie-Blick et al., 1989). Clasts in In Big Cottonwood Canyon, the Big Cottonwood Fornia- tlic Mineral liork cliarnictites were likelj glacially trans- tion is overlain 11y up to 800 1n of glacioge~iicdiamictite, ported hrri eroded outcrop5 of the miderly~ngBig Cotton- siltstone, sandstorie and conglomerate ofthe Mineral Fork wood Foruiation drld other sotirces to tlie east. The Forn1:ttion (Ojakangas arrd Matscli, 1980; Christie-Blick, Xfineral I'ork krniation contains carl~oriateclasts fro111 ,ln 1983; Christie-Blick and Link, 1988). The 11anie Mineral inferred post Big Cotton\vt>odFonn'ltion car1)orlate deposit Rtrk firmation is used in parautochthonous settiilgs ill of intermediate tllickness (Christie-Blick, 1997). The Llin- the IVasatch Range and in the Charleston thri~stsheet to era1 Fork \va\ deposited in contact with glacial ice, in sub- tli~soutll. Thc hlineral Fork correlates wit11 much thicker aqueou5 environments. Christie-Rlick (1997) ft~rtherde- glaciogenic deposit5 of the forrrtation of IJerly Canyon anel scril~esthe details of' sequence bourtclariies, and tc,ctono- Pocatello Forniation in the M7illardthrust shect, and likely stratigriiphic relations of tlic Virleral Fork Fornlation. El11,ERS. <:Ii.%N, LINK. PROTEROZOIC TIDAL, (;l,ACIAl,. AN11 F1,UVIAL SEI1IMENTATI0S 35 I / siltstone; lithology is quite consisterlt across tlre large area of exposure. Generally this formation is interpreted to Park City Fm represent a large braicl-plain, locally occupying incised Weber Quartzite valleys, and representing a time of increaseel coarse clastic Round Valley Ls Dou hnut Fm supply and relatively low sea level (Crittendetl ct a]., tium%u Fm ~eserep~s 1971; Link et al., 1993). Discontintlous dtstone beds and Gardison Ls pocls suggest local flood plain or lake deposits. Maxfield Ls Ophir Fm. 'The hfutual Rormation contains two stratigraphic se- Tintic Quartzite qltences, ant1 sequence boundaries, in Big Cottonwood Canyon (Christie-Blick and Levy, 1989; Levy, 1991; Le\y Mutual Fm. and Christie-Blick, 1991, p. 379). The lower part (30 m ex- Mineral Fork posed l~ehveenerosional contacts) correlates with the tipper Fm. Caddy Canyou axid Illkom Forinations of sol~thcastIctalio and \~est-centr~~lUtah. 'The upper portion is part of a rela- tively unifi)rrrm lrlanket of braidecl flrtvial blutual t.'o~~liation. This may represent highstanel atid tran\grcs5ivc 5y5terns Big Cottonwood Fm. tracts deposited cturing a period of lowered sea level dur- ing the late stages of the 610580 hla Varanger glaciatio~~ (Christie-Blick, 1907). The blanket geometry of the Sltttud Formation suggests that accommodation space was con- meters trolled by late ?'aranger glacio-eustatic rise of sea level and not by differential thermal subsidence.
ITigzrr-e 5 Get~er-(llizedstratigrnpllic section of rocks expo~erl Tintic Quartzite alorlg Hightccly 190 it1 Big Cottorzwood Carz!joa. ~l.lodtfic.tlfrom The Lower atid Middle Camhriarr Tintic Quartzite is Ilinfzr ('1988). ,;L,out 240 m thick in Big Cottonwood Canyon, ancl con- sists of white to pink, textnrally mature, cross-bedcied coarse- to medium-grained quartz areliite with a local Mutual Forn~a t' lon basal conglomerate. The formation passes upward to the 'The Neoproterozoic Mutrtal Forrnation is up to 370 m firs,iliferous hfidcfle C;imbrian Ophir Shale (Bryant, 1990). thick in Big Cottonwood Canyotl and unconformahly oLJer- The Tintic Quartzite it1 Big Cottonwood Callyo11 repre- lies the Mirieral Fork Forntatiou, or where tlle Mineral sents the \tratigraphically youngest, eastern feather edge Fork Forlnatioxl is missing, the Big Cottot~woodForma- of a great latest Neoprotero~oicto Middle Cambrian, west- tion. The Mutual Formation is overlain to the north (near ward-thickening (to over 2,000 m), 5hallow marine quartz IIuntsville, Utah) 1ry tlre Browns I-lole For~tlatiolk,which \and wedge co~lsistirlgt~f the upper Brigham Group (Chncl- contains volcaviiclastics and felsic volcanic rocks ectinlated back Mountain and (kertset-r Cal-rjoti Quartzites) in Idaho to he 580 %fa 1)y Ar-Ar dating (Crittenden et a]., 1971; and the northern ?%sate11 Range, and the Prospect hfoun- Christie-Blick et al., 1989; Link et a]., 1993, p. 542). tail1 and Tintic Quartzite5 in central and western Utah The hlutual Formation is part of the Neoproterozoic (Calkins and Butlt>l; 1932; Stewart, 1972; IIintze, 1988; and Camlrriar~ Brigham Group, which contains thick 1,evy and Christie-Blick, 1991: Link et al., 1993). Tile base quartzitic sandstones that lie above the Sturtian glacial <:eertsetl Canyon-Camel1)ack Mountain QuartLite sequence deposits (Crittenden ct al., 1971; Link et al., 1987). Levy bounclary of the miogeocli~leis represented 11y the uncon- and Christie-Blick (1991) surnmari~ea sequetlcc strati- formable base of the Tir~ticQuart~ite in Big Cottonwood graphic framewtxk for the Brigham Croup ancl co~l-elative Canyon, ancl the sequence houlidary spat~sconsiderably rocks, with ti)tir regionall! recognizable Neoprotero~oic Inore titne in Big Cottonmrood Canyon than in allochtho- sequence boundaries from southeasterr] Iclaho to eastern nous locations, u~hichwere west of the Corclilleran hinge- Nevada. line (Levy and Christie-Blick, 1991). l'lle Mutual Forll~atiorris present it1 several thr~tstsheets Because the Tiritic Quartzite is generally poorly fossil- in a large area of soutl~c~asteniIdaho and northern Utah, iferour, it has receivec$ much less atterltio~lthru~i the over- but its thick~lessis generally less than 850 m. The Mutual lying Cambria~iOp11ir Shale ancl 14:ufieltl Li~nestone,antf Forniation contains grayish red to reel-purple, pcblrly has not i)een described inuch since the work of Critteu- arkosic sandstone, wit11 lenses of variegated red atlcl green clerl ct al. (1952, 1971) and Crittenden (1976). 36 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
TIDAL CYCLES AND LUNAR PERIODICITIES - A M Oceanic tides produce deposits that are both signifi- cant and widespread in the geologic record. Our under- 0 standing of ancient clastic tidal deposits in the last decade Daily N has increased through important comparative studies on Subordinate + Dominant + modern tidal processes and preserved structures and T sequences. The recognition of diagnostic tidal structures and processes have useful application to interpretation of H both outcrop and subsurface deposits as well as to explo- A ration, basin analysis, and sequence stratigraphy. This sec- Neap '112 tion highlights portions of previous work to provide a I - - background understanding of diagnostic structures such A M 3 6 as tidal rhythmites (Fig. 6), and their lunar relationships 0 "'0 N (Fig. 7). For summaries of tidal depositional systems the (d l Spring T .E E reader is referred to Nio and Yang (1991), and Dalrymple H s (1992). $0 There are a variety of criteria that have traditionally 4 been used to interpret tidal environments, such as the * 7 presence of sigmoidal bundles, tidal rhythmites, mud cou- plets or mud drapes, current ripples with crests rounded Figure 6. Idealized diagram of a heterolithic tidal rhythmite from by back flow, reactivation surfaces, herringbone cross bed- a diurnal tidal regime (with no diurnal inequality). Two neap- ding, flaser bedding, mud cracks, and more. Two of these spring cycles are shown in this figure where each cycle represents structures, sigmoidal bundles and tidal rhythmites (also half a synodic month (modern length of 14.77 days). Relatively referred to as cyclic rhythmites or small-scale tidal bun- coarse grained layers are deposited during the dominant tide, dles), have cyclic variations in laminae thickness. Sigmoidal and mud drapes from the subordinate tide. Neap-spring cycles in bundles occur as a series of laterally accreted, S-shaped, the Big Cottonwood Formation range between 0.3 to 20 cm. alternating sand and mud layers. They commonly occur in Similar examples of heterolithic rhythmites are at stop 3 of the tidal channels. Tidal rhythmites commonly contain stacked trip. sets of mud-draped ripples, or stacked sets of alternating flat laminae of mud and silt to sand. These stacked sets and laminations are organized into vertically accreted rhythmite deposition in a diurnal tidal environment is thickening and thinning tidal bundles/packages which con- given in Figure 6. These periodicities have been described tain periodic variations in thickness as a result of changing from a number of both modem settings (e.g., Dalrymple et current velocities invoked by lunar cycles. The cyclicity in al., 1991; Tessier, 1993) and ancient settings (e.g., Sonett laminae thicknesses in sigmoidal bundles and tidal rhyth- et al., 1988; Kvale et al., 1989, 1997; Tessier and Gigot, mites is primarily a result of lunar orbital variations and 1989; Williams, 1989; Archer et al., 1991; Archer and Feld- the alignment of the Earth and Moon with respect to the man, 1994). Identification of tidal periodicities in a se- Sun (Archer et al., 1991). Varying configurations in the quence of cyclic tidal deposits is accomplished with time position of the moon results in changes in the gravitational series analysis using such methods as the Fourier trans- pull on the Earth's oceans, causing periodic oscillations in form to match cycles to known, diagnostic tidal lunar cycles tidal amplitudes on short (less than a year) and long (1 to (Schureman, 1958; Home and Baliunas, 1986; Archer et al., 1991; Martino and Sanderson, 1993; Archer, 1994, 1996; 104 years) time scales. Although longer lunar periods are Kvale et al., 1997). not discussed here, summaries are presented in MacMillan (1966) and Nio and Yang (1989). Big Cottonwood Rhythmites The shorter term lunar cycles of interest to this study are: 1) semidaily to daily tides, 2) semimonthly and The Big Cottonwood Formation contains the oldest monthly phase changes (neap-spring) of the moon; and known (-900 Ma) example of tidal rhythmites (Chan et 3) semiyearly periods from alignment of the sun and the al., 1994). The tidal rhythmites in the Big Cottonwood major axis of the moon's orbit. The relationships of the Formation (e.g. see Fig. 8) are typically vertically stacked main lunar periods affecting tidal amplitude are shown in rhythmic alternations of light (siltlsand-rich) and dark Figure 7, and the affect of the lunar synodic month on (mud-drape) laminations. The smallest individual laminae EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 37
determine the tidal periodicities, and various smoothing and interpolation programs are used where some of the laminae may be truncated or difficult to distinguish. Lamina and bundle counts from cores drilled through the Big Cottonwood tidal rhythmites have been used examine Proterozoic orbital parameters. The analysis indi- cates Proterozoic tides acted in a similar fashion to mod- em tides (Chan et al., 1994; Sonett et al., 1996). However, the Proterozoic Earth was spinning faster than it does now; the angular velocity has slowed over time. Thus, Pro- terozoic days were shorter (at - 18 hourslday vs. present 24 hourslday), and lunar months were also shorter (- 25 days/month vs. present 29 dayslmonth) (Sonett et al., 1996). Additionally, modern values of the lunar retreat rate away from the earth (based on NASA Apollo space research) are in the range of 3.8 cmlyear, in contrast to Proterozoic lunar retreat rates of about 2 cmlyear (Sonett et al., 1996).
Figure 7. Primary lunar periodicities affecting tidal amplitudes. BIG COTTONWOOD LITHOFACIES The synodic (a) half-month reflects lunar phases and has a mod- ern half-period (time between new and full moon) of 14.77 days. Several major lithofacies are distinguished in the Big The synodic half-month is visible in tidal rhythmites in the form Cottonwood Formation. Thicknesses shown in the com- of neap-spring cycles. The tropical half-month (b) reflects varia- posite stratigraphic section (Fig. 4) are based on detailed tions in lunar declination. The modern tropical half-month is measurements by Ehlers of the upper 213 of the section 13.66 days, the time required to move from a northern to south- with a Jacobs st&. Lithofacies within the less accessible ern declination. The anomalistic month (c) represents variations lower third of the formation were assigned thicknesses in lunar distance from the Earth. The modern anomalistic month based on geologic map outcrop width (James, 1979) sup- has a period from perigee to perigee) of 27.55 days. Mod$ed plemented by observations and measurements collected from Archer et al. (1991). at spotty outcrops. To augment this "one-dimensional" per- spective of the Big Cottonwood Canyon exposures, lateral relationships, geometries, and variability of lithofacies were generally represent- semidaily/daily tides with the siltlsand checked and verified by inspection of aerial photographs deposited by the dominant tide, and mud deposited by and reconnaissance studies along accessible trails to the the subordinate tide (Fig. 6). The individual laminae are high country (including Mount Olympus, Twin Peaks, vertically organized into packages of thick and thin lami- Lake Blanche, and Mill B North Fork). nae. These smallest laminae can be counted and mea- sured (much like counts of tree rings) with respect to lam- Description inae thickness. The packages of thick lamina (dominated by thicker siltlsand lamina) were created by stronger spring The Big Cottonwood Formation is composed of alter- tides when the gravitational pull is greatest due to the nating sequences of argillite and quartzite facies (Fig. 4) straight alignment of the moon, Earth, and sun (Fig. 7). with relative abundance of about 50% each (Crittenden Spring tides occur during new and full moon cycles. The and Wallace, 1973). Argillites are more common in the packages of thin lamina (dominated by thin mud lamina) lower half of the section. Five distinct lithofacies are fur- were created by weaker neap tides when gravitational ther broken out of the basic quartzite and argillite litholo- pull is slightly canceled out by the moon being out of gies: channeled quartzite, sheet quartzite, mud-cracked phase with the Earth and sun. Neap tides occur during argillite, laminated argillite, and transitional argillite (Ehlers 1st and 3rd quarters. Thus the pair of a thin mud lamina and Chan, 1996a, 1996b).The two quartzite facies (slightly package and thick sand lamina package comprises a neap- metamorphosed quartz arenites) have lithologic and some spring cycle, or a half month. Four packages of laminae (2 internal structure similarities, but exhibit different lateral mud lamina packages and 2 sand lamina packages) com- geometries. The three argillite facies are slightly meta- prise a full lunar month (synodic month). The counts and morphosed mudstones to siltstones characterized by dis- thickness spacing of the neap-spring bands literally allows tinctly different internal structures. These three argillite the determination of the number of days in a month, how types are typically notable in outcrop due to their differ- many months in a year, etc. Harmonic analyses are used to ence in color. The characteristic stratigraphic position, 38 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I geometries, lithologies, and sedimentary structures for each facies are summarized in Table 1. The channeled quartzite facies is largely distinguished by individual channels that are laterally extensive on the order of hundreds of meters, with a thickness on the order of tens of meters. The facies commonly exhibits a sharp scoured base that is lined with a coarse lag of quartz pebbles and/or argillite rip-up clasts. There is a general upward fining within this facies and a corresponding upward thin- ning of bed size. Internally, the channeled quartzite typi- cally contains tabular and trough crossbedding, some soft sediment deformation, and current ripples. The sheet quartzite facies exhibits a more tabular geom- etry that is laterally extensive on the order of hundreds of meters to several km,and hundreds of meters thick. Where exposed, the facies tends to be laterally traceable over several kilometers although it is difficult to trace individu- ally stacked beds or sets any distance due to the terrain. The internal structure appears to be stacked cross bed- ding although outcrop weathering makes it difficult to dis- tinguish. Wave ripples are present along some bedding plane outcrops. The mud-cracked argillite is typically a weakly bedded, reddish purple mudstone to siltstone, with common mud crack structures. Where coarser (silt to sand size) fractions are present, associated features include mud-cracks, rain drop impressions, adhesion structures and wave ripples along bedding planes. The laminated argillite is a black shale to siltstone with some thin sandstone beds. This argillite facies typically contains finely alternating mud- and sand-rich, mm-lami- nae herein termed rhythmites (e.g. Fig. 8, dark colored, mm scale rhythmites). The rhythmites range from being heterolithic, meaning a mix of grain sizes, or "unlike" lithologies (typically sand and mud) to those that have very little sand and therefore become more difficult to distinguish. Where the vertically organized heterolithic rhythmites occur, daily, semimonthly, and monthly cycles can be observed in both outcrop and in hand specimens. Syneresis cracks (possibly subaqueous shrinkage cracks?) are common along exposed bedding planes in the lower portions of the facies. Thin sandstone beds typically show small load casts and physical deformational features. Diagenetic pyrite is also common within the black lami- nated argillite. Some large and deformed blocks of the rhythmites occur in association with channeled quartzite. The transitional argillite is green-gray in color and typ- ically fines upwards into the reddish purple mud-cracked argillite. It is well bedded and also contains some rhyth- Figure 8. Ext~inpleso$/)lnnc~r /nttlinc~tetl r/zrlthinites loctited in the mites although not as finely developed as those that occur Big Cottontoootl Formation. R/zytlzinites like these are typically within the black laminated argillite. These rhythmites forined in zones of turl7idity intlxirnuin in macro-tidal estuaries. appear to represent annual cycles whose thickness can be Neap-spring cycle thicknesses average arounc-l 3-4 cm (a), and range from as much as 15 cin (I]), to as little as seoeral rn7n (c). verified with harmonic analyses and where daily and EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 39
Table 1. Summay of lithofacies observed in the Big Cottonwood Formation
Facies Name Sedimentary Structures Facies Association Interpretation - - - -- Channelized Channeled, upward fining Grades into all the Tidal fluvial Quartzite successions 10s m thick argillite facies channel Cross bed sets 0.3-1.0 m thick Sigmoidal bundles Scoured bases, lags Current ripples
Sheet Thick tabular bodies: Overlies and Sand sheet Quartzite 14m thick underlies or tidal by 10-200 m long all facies sand bar Internal, upward fining Wave ripples
Mud-cracked Mud cracks, rain Overlies channeled Inter- to possibly Argillite drop impressions quartzie or supratidal Adhesion structures green-gray Weakly bedded agrillite Wave ripples
Laminated Thinly laminated at base Generally overlies Sub- to Argillite with heterolithic rhythmites channeled intertidal Diagenetic pyrite quartzite Syneresis cracks
Transitional Heterolithic rhythmites at Overlies channeled Intertidal at base Argillite base (with annual cycles quartizite to supratidal Thin (<30 cm) discontinuous at top quartzite and argillite beds Current ripples with crests rounded by backflow Clay-draped reactivation surfaces 10-20 m thick
monthly signals are not well preserved. This transitional upper 213 of the formation were evaluated. These mea- facies contains small scour and truncations surfaces, small surements were corrected for two tectonic events to arrive load structures, and climbing ripples. at a dominant westward paleoflow direction. Within the composite section through Big Cottonwood Two important vertical relationships of facies and tran- Canyon (Fig. 4), the mud-cracked argillite constitutes 40% sitions are recognized: 1) The sand sheet quartzite gener- of the section and is found mainly in the lower half of the ally has relatively straight, sharp contacts with adjacent section, whereas the laminated argillite constitutes 12% of argillite facies (e.g., stop 3). Although the contacts are sharp, the section and is found only in the upper half of the section. there is little evidence of scouring (lags, rip-up clasts) at The transitional argillite is located in the upper 300 m of the base of the this facies. 2) The channeled quartzite the section, and constitutes only 4% of the section. The characteristically has a scoured base with pebble size rip- channeled and sheet quartzite are interspersed throughout up clasts and lags. The top of the channeled quartzite fines the section, and constitute 30% and 14% of the section, upward into overlying transitional argillite. The thinning respectively. One hundred and fourteen paleocurrent mea- and fining nature as well as some preservation of rhyth- surements from crossbedding and current ripples in the mites between the underlying quartzite and overlying 40 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I argillite suggests the gradual transition from a higher to rel- interpret the sedimentary structures and lithofacies sum- atively lower energy environment (e.g., stops 3,4, and 5 of marized in Table 1. this trip). The sheet quartzite lithofacies is located in the lower two thirds of the Big Cottonwood Formation section and ranges Interpretation in thickness from 100400 m. This facies shows little evi- dence of fluvial influence, and the presence of wave ripples Diagnostic structures of heterolithic rhythmites with is suggestive of marine processes. This facies is interpreted daily, semi-monthly and annual lunar periodicities, as well to represent subtidal sand waves and/or tidal sand bars as sigmoidal bundles, current ripples with crests rounded (Fig. 9) which were deposited in the marine-dominated by back flow, and clay draped reactivation surfaces clearly mouth of the estuary. Within the section, this facies is gen- indicate the strong tidal influence on the depositional set- ting. The association of quartzite and argillite lithofacies erally vertically adjacent with the mud-cracked argillite and interpreted to have formed laterally adjacent to the suggest a depositional system that incorporates aspects of a tidal signature along with river-like channels and some thick successions of mud-cracked argillite. shallow marine influence. We argue that the Big Cotton- The channeled quartzite lithofacies with cross bedding wood facies of this area illustrate deposition in a tide- and current ripples is most common in the upper half of dominated estuary as discussed below. the section and is vertically associated with the laminated The precise interpretation of tidal environments is some- and transitional argillite. The channeled quartzite may be what problematic because tidal processes can overlap into similar to two types of tidal fluvial channels found in the a number of clastic shoreline settings. There is also confu- upper reaches of modern estuaries. First, when associated sion in nomenclature used to classify tidal deposits partly with the laminated argillite, this facies may represent the because of differences in scale and gradations between "main" tidal fluvial channel (Fig. 9). Second, when associ- coastal systems (e.g., see discussion in Reading and Collin- ated with the transitional argillite, this facies may represent son, 1996, p. 182). The combination of both tidal and fluvial smaller tributary tidal channels which were oblique to influence suggests a transitional environment in which sub-parallel to the main channel. A basis for a distinction both processes can operate. Tidal deltas are recognized as between channels lies in the associated argillite facies. areas where constricted exchange through a barrier (e.g., The laminated argillite lithofacies with syneresis cracks, barrier island) disperses sediment load on either side of heterolithic rhythmites, and diagenetic pyrite likely formed the barrier as flood-tidal and ebb-tidal deltas. Nothing in the sub- to intertidal zone. The dark color of the shale within the Big Cottonwood Formation indicates this kind with diagenetic pyrite suggests reducing conditions and of tidal delta setting. the presence of organic carbon that might have been best Another transitional interpretation to be considered is preserved at subtidal depths. Heterolithic rhythmites with a tide-dominated river delta. Shoreline deltas typically daily lamina and neap-spring cycles suggests that deposi- exhibit delta plain and delta front facies with well-devel- tional rates were relatively high and at a place where tidal oped, upward-coarsening sequences formed from delta currents were strong enough to deposit sand during the lobe progradation and/or sea level change. However, no dominant tide. This facies occurs in both thick successions well-developed upward-coarsening successions were rec- and thin units associated with channeled quartzite in the ognized within the Big Cottonwood Formation in Big upper half of the section (Fig. 4). Where thin units (1-10 Cottonwood Canyon. m scale) of laminated argillite flanked the main tidal chan- An alternative interpretation we believe to be more nel in the subtidal zone, they may have capped a chan- appropriate to the Big Cottonwood facies is deposition in neled sequence, or were flanking heterolithic rhythmites an estuary. An estuary spans a broader range of geomor- that were occasionally cut and collapsed along channel phic and environmental features than most of the delta bank margins with shifting of the tidal channel (Figs. 17 terminology, conveys a coastal embayment or geomorphic and 18, and stops 4 and 5).Thick successions (100-300 m) reentrant with a drowned river valley. This setting allows of the laminated argillite are present in six locations large- for prominent tide-dominated structures (e.g., tidal chan- ly in the lower half of the section (Fig. 4). At the base of nels, tidal sand bars, and tidal flats) and a paleogeographic the thick successions of laminated argillite, heterolithic tie to a major river system. Previous studies of modern rhythmites are also well preserved suggesting subtidal estuarine environments (e.g., Dalrymple et d., 1991, 1992, deposition adjacent to a tidal channel. However, the upper and Tessier, 1993) provide a model (Fig. 9) that illustrates portion of the thick successions lack rhythmites and synere- the facies of a tide-dominated estuary. The tidal limit typi- sis cracks and may therefore represent more intertidal mud- cally extends up into the mouth of the alluvial valley. flat deposition as the succession progressively shallows. Modem analogs provide the conceptual basis for how we Unfortunately, the upper portions of these thick succes- EHLEKS, CHAN, LINK: PROTEROZOIC T113AL, CI,ACIAI,, AND FLUVIAL SEDIbfENTKrION 41
Estuary arriiual cycles might be preserved. Modeni analogs intlicate that hydrologic conditions required for episodic deposi- I Mixed- 'River- -' /-Ft%:ated Energy l~ominated tion of this hcies are present in the upper reaches of the estuary, near the ticlal limit, and removecl from sedirnenta- tion associated with the inain tidal channel. The mud-cracked argillite lithofacies is most abundant in thick successions (100-500 m) in the lower half of the section, although thin layers (2-4 m) are found in the u 1 Tidal Fiuvial upperniost part of the section where it caps the transition- f Channel I al argillite (Figs. 3 and 19). We interpret the thick succes- sions of this facies to represent intertidal mud-flats close to the rnouth of the estuary (Fig. 9). In modern analogs, tidal mud-flats are laterally extensive and co~itinuousover several kilometers (Dalrymple, 1992). Similarly, in the Big Cottonwood Formatioti, this facies is laterally continuous on the same scale. The thick successions of the mud- cracked argillite in tlie Rig Cottonwood For~rlationare (Sheet Quartzite) I I vertically associated with the sheet quartzite hcies (Fig. I I Alluvial I 4). The thinner units of mud-cracked argillite differ from I Sediments the thick units in that they are part of a fluvial upward fin- ing and thinning sequence (stop 5; Fig. 19). These thinner mud-cracked argillites were likely deposited towards the landward end of the estuary adjacent to the tidal-fluvial channels as a hybrid tidal flat to flood plain deposit. In
Tidal Sand Uppet elow Regime modern settings, this area in the landward end of tlie estu- Bars Sand Flat ary is recogiiized as a salt nrarsh fkcies. I-Iowever, in the Big Cottonwood setting with no plants, such a facies is not Figure 9. Estucirini. depositiotzcil modr~lfor the Big Cuttonzcood distinguishable. Forrrrcilion. Channeled r~uortzitefiicies, aizcl argillites with het- Vertical fzicies associations in the Big Cottonwootf For- erolithic rhyt1zrnitt.s were most Eik~lyrlcpositf~tl in the rrzixed ener- mation, and the locatiorl behveen f~iciesin which rhyth- gy erzziironrn~rtt where both jlurial aizrl tidal sedimentation mites are fouticl are also analogous to xiioderii estuarine occurretl. Saratl sheet facies (with wal;e ripples) werP rrtost likely environments. Tessier (1993) notes hvo important condi- clt7posited near the tnoutlt i$thr estuary where marine processrs tions fbr the preservation of modern rhythmites: a protect- clorninatecl. hloclern ~iepositioncilenoirontrzents are laheled in the i~tiddleportion of the figure, and Big Cottorlwoorl Forrncltion ed environnient is needed to limit erosion from wave facies erluiralet~tsare itz italics and pat-entlzcsis Mod$ied -from action or highly energetic tidal currents; and suspended Dalryrr~pleet ul., 11992). sediment concentrations must be high so that thick sand- mud couples are deposited. Both these conditions are found at the upper end of estuaries close to (within 5-15 sion5 are general11 fissile and sedimentary structures are km of) the tidal limit. In the Bay of Frtndy and the Bay of not well preserved. Mont-Saint-Mielid rhythniites typically form adjacent to The transitional argillite litliofacies with annual cycle tidal-fluvial channels in sub- to intertidal parts of tidal flats, rhythrnites, current ripples with crests round by l>ackflow, or in a1)andoned tidal channels. In the forrner c:tse, rhyth- and clay draped reactivatioii surfaces is located in the mites form adjacent to tidal channels on the mud flat as a uppermost part of the section (Stop 5; Figs. 4 and 19). The type of "overbank" deposit in a tidal flat/flootlplain area presence of annual cycles in heterolithic rhythmites sug- where deposition occurs with every tide. The best preser- ge$ts that this ficies was deposited farther allray from tlie vation of the rhythmites is in the inner portion of the estu- main tidal channel, perhaps in the uppermost intertidal, ary where wave action is riiinimal ancl sedimentatiori rates or supratidal zone arid where only extreme hydrologic con- are high. This location of rhythniite deposition accounts for ditions (high reasonal runoff, storm events) cartsed redi- the slump blocks (bank collapse structures) of rhythrnites mcntation. Thi5 facie5 is characteristically associated with preserved in the channeled quartzite of thc Big Cotton- the channeled quartzite as part of an upward fining and wood Formation. In the latter case where rhythmites form thinning sequence, and suggestive of deposition along in abandoned tidal channels, modern analogs recorct a tributary tidal channels where thinner (more condensed) upward-fining succession with sands frorn the tidal channel at the base and the11 gradatioir into rhytlirrtites and then silts. Sonre diflkrences cxist between the lithof~tciesof the Big Cottollwood Formation 'tntl those found in the mod- ern analogs. The most notable differelices observed III the Big Cottonwootl Ei,ntr,itio~r include a broader aid rilore sheet-like deposition of fkies, particularly in tllc. qlrart71te fitcirs, md tltc lack of a salt m,ri-sh f'icies per stL.\VCC hc- lieve these ditre~cncesare a result of tlie lack of heget '1 t'1011 1x1 the Proterozoic. 111 modern esttlaric5, vegetation in salt marshes Itolcts up steep (1-3nt high) hanks which latcually constndit tidal clirrents to intertidal murlflats. The lack of Ftgnre 10 I'cl/r,ogrogrtrl~lzy rind fcc torlrt rc ttzrre of fllc BIL' vegetation in the Big Cottonwood Formiition posiildy (,'otto~luoorJ Fontzntzori trw? Crztltit LJI~)~l~af~~~~(:1011/1 lJoitittoti of resulted in Inore subclued topographq and .l broadcl; more the Bzg Criftont~oo~lfi~i,t.mrrtzon trlltl t r,ltu AJfr,rtr~ltitiiLr orildiitrr c laterally extensive distribution of fxies. The lithofacici of hr'ol c,orrci tcd fiir rr 13 and -10 rlc,ctr,i, iittiitrot,, rc'ii)tv hi (It1 tlre Big Cottorrcvood firn~~ition'ire geuerall) laterally exten- hnrerl ott ti'tc ~~~l~ortz(tgrz~~tr(.dilt(1 ~f f,l\ii)n cBt (il IE~ILuiA i t (I/ , sive fi-om 8-11 km, ancl are prcse~~to\er a greater l'tteral 1993) Thtr roric,ctiori I c?cfoicJ\ fl~r*rc~irr~rti to tirezi or rr,itattot, (it extent than ii~nildrfacics of rnodern :inalogs. Because of the fhc tzmi? of ~frl>osrtrorr 7%~c,~.tott of tftc~bii~~tr f/iat ot)i)~nssi'~i lack of egetation in tlie Big Cottonwoocl Fortr~ation,\ve tizesr tlepoitfi rr ~rttkiiozcti, ~tdtiic orrfr I cll\ti ilrctlzoi~rrr ~lrts fig ti^-^ t~ 111~i(11ri~ (1s tltt~rt/)rf'terrt cJnrl otitr tops believe the Proterozoic etllliviilent of a s,ilt rr~arslrwould 1)e a tributai-y tidd cltanirel floocj pl'iin. This 'incieilt eclui\ - alerit of ,r. salt marsh woultl for111in the, uppernrost inter- fine-gr'iinetl n,ttiirc nol~ldnl'lkc tiit~rr:rtiotr, cIrftrr.tr!t to tidal sone. away from the main tidal fluvial chani~cl.'itid recogrllzt, would be l~l~~~rketedh) fine grained srtlirnent during tlaily 1nte.rprctatri)n of ~n
NEOPROTEROZOIC TECTONIC SYNTHESIS mechanisms. The connection to this ocean body could be AND WORKING HYPOTHESES by means of a narrow seaway that focused tidal energy into the craton, somewhat analogous to the Bay of Fundy. A reasonable Neoproterozoic tectonic synthesis is as Connection to an ocean by a narrow seaway would leave follows (Levy and Christie-Blick, 1991; Link et al., 1993; little evidence of its presence, and therefore making it dif- Christie-Blick, 1997). The Mineral Fork Formation and ficult to find in the geologic record. A second mechanism Pocatello Formation represent the Sturtian glaciation could be that western Laurentia was relatively low, and (750-700 Ma), with synchronous continental rifting and sea level high; meaning that a tide-dominated shelf envi- bimodal volcanic activity. Above the last glacial lowstand, ronment over southwestern Laurentia transmitted tidal strata deposited during the post-glacial transgression and energy to the Big Cottonwood Formation estuary. This subsequent thermal subsidence of the incompletely rifted second mechanism is similar to the modern tide-dominat- continental margin are represented by the upper Pocatello ed shelf environment of the North Sea which locally has and lower Brigham Group strata up to the lower part of the macrotidal amplitudes. A tide-dominated shelf environ- Caddy Canyon Quartzite. The Mutual Formation in Big ment would presumably cause some deposition across Cottonwood Canyon contains both the upper Caddy Can- western Laurentia, for which no evidence has yet been yon-Inkom sequence and the Mutual sequence, which may found. However, evidence of a post-Big Cottonwood Fonna- have been deposited during transgressive and highstand tion carbonate platform is preserved in clasts of the Neo- phases of glacio-eustatic cycles during the Varanger glacia- proterozoic Mineral Fork Formation. If a marine shelf en- tion (610-580 Ma). The Geertsen Canyon-Camelback vironment did provide macrotidal amplitudes for the Big Mountain-Tintic sequence represents thermal subsidence Cottonwood Formation, then the post-Big Cottonwood of the Cambrian Cordilleran passive margin. Formation carbonates preserved in the Mineral Fork The problem arises in terms of how to fit the early Neo- Formation might represent a subsequent period of marine proterozoic macrotidal Big Cottonwood Formation into sedimentation over southwestern Laurentia. the previous tectonic scenario, which suggests a tectoni- In the companion field guide on Neoproterozoic sedi- cally quiet intracratonic setting prior to rifting associated mentation and tectonics of west-central Utah, Christie- with the Mineral Fork Formation. We offer speculation as Blick (1997) suggests that sedimentation in the Big Cotton- to how the Big Cottonwood Formation and Uinta Mountain wood Formation is a result of Late Mesoproterozoic to Group might relate to regional paleotectonics and the Early Neoproterozoic (-1.1-0.8 Ga) crustal extension fol- Rodinia Supercontinent (Dalziel, 1991; Moores, 1991; lowed by thermally driven subsidence and deposition of Unrug, 1997). The proposed scenarios are based on: macro- the overlying carbonate unit, which was removed by gla- tidal deposition in the Big Cottonwood Formation; a nor- ciation (-750 Ma) subsequent to lithification. The main mal bounding fault synchronous with deposition in the distinction between our tectonic hypothesis and that of Uinta Mountain Group and possibly the Big Cottonwood Christie-Blick lies in the time of rifting and deposition. Formation; and westward paleocurrent flow in the Big Christie-Blick suggests deposition was unrelated to the Cottonwood Formation and a majority of the Uinta Moun- breakup of Rodinia (-750 Ma), and is the result of a sepa- tain Group. rate rifting event of moderate magnitude that occurred in One hypothesis is that the Big Cottonwood Formation southwestern Laurentia while western Rodinia was still and Uinta Mountain Group were deposited prior to the assembled. Both hypotheses are plausible. However, at -750 Ma breakup of western Rodinia between south- this time they require further development including an western Laurentia and East Antarctica along the SWEAT examination of Big Cottonwood Formation deposits west connection (Dalziel, 1991; Moores, 1991; Powell et al., of the Wasatch Range. A goal would be calculation of pre- 1993; Torsvik et al., 1996; Dalziel, 1997). If deposition dicted tectonic extension and thermal subsidence amounts occurred at this time then macrotidal amplitudes in the using McKenzie (1978) type stretching models, though Big Cottonwood Formation would have been generated lack of geochronometric data will probably preclude all through a -300400 km connection of this unit to a but the most general models. If predicted Big Cottonwood southern ocean body. Formation extension amounts are large, then perhaps A second hypothesis is that deposition is associated with deposition was associated with the breakup of Rodinia the breakup of Rodinia allowing for a shorter (-200 km) since no other evidence of large magnitude extension has western connection of the Big Cottonwood Formation to been documented in the Late Mesoproterozoic to Early ocean waters that inundated between western Laurentia Neoproterozoic of southwestern Laurentia. Future work and East Antarctica (Dalziel, 1997). in the Big Cottonwood Formation will focus on the sparse For both of these cases, macrotidal amplitudes could western outcrops located in Slate Canyon, the East Tintic have been transmitted into the craton by two different Mountains, Stansbury Island, and Carrington Island. 46 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
SUMMARY were exhumed approximately 11 km (Parry et al., 1988; Parry and ~ruhn,-1987) as result of and Range Several Proterozoic formations exposed in Big Cotton- a s as in extension. The seismically active Wasatch fault (a normal wood Canyon illustrate a range of clastic depositional sys- fault at the eastern boundary of the Basin and Range tems from contrasting paleogeographic and tectonic re- province) crosses the highway just east of the parking lot gimes. Within the thick, Big Cottonwood Formation there and is capable of generating M 7.0-7.5 earthquakes is remarkable preservation of sedimentary structures which (Arabasz et al., 1990). From this location, the road ascends suggest tidal deposition in an estuarine setting. Paleocur- rapidly into the rugged topography of Big Cottonwood rent measurements indicate a dominant westward direc- Canyon. tion of flow, fed by the easterly fluvial systems of the Uinta Mountain Group. Five distinct lithofacies in the Big Stop 2: Remnants of an ancient sea sign-Big Cotton- Cottonwood Formation include: fluvial-dominated chan- wood Formation (mile 2.3, 3.8 km) neled quartzite (tidal channel); subtidal sheet quartzite (sand sheets at mouth of estuary); inter- to supratidal mud- Location. Pull out is located long the south side of cracked argillite (probably gradational to a tidal channel Highway 190, at the National Forest Sign reading "Rem- flood plain); sub- to intertidal laminated argillite (in the nants of an Ancient Sea." inner portion of the estuary where wave action is mini- Description: This stop is located about 113 of the way mal); and a inter- to supratidal transitional argillite (toward up the stratigraphic section of the Big Cottonwood Forma- the upper portion of the estuary). These facies suggest tion (Figs. 2 and 4) at an outcrop of the purple-colored, deposition in an estuarine setting with river dominance in mud-cracked argillite facies. This facies typically overlies the upper reaches (tidal limit), to wave influence in tidal the channeled quartzite or transitional argillite facies, and sand bars in the lower reaches (subtidal). Comparisons is hundreds to several meters thick. The lateral extent of the with modern estuarine environments are used to infer mud-cracked argillite is generally 1 km or more. Several that the Big Cottonwood experienced macro-tidal ampli- distinct structures are preserved in this weakly bedded tudes in a diurnal tidal regime. Harmonic analysis of tidal facies (particularly along the bedding plane exposures) rhythmites indicate that the Proterozoic tides acted in a including mud cracks, rain drop impressions, adhesion similar fashion to modern tides and record daily, semi- structures, and wave ripples. monthly, monthly, and annual cycles. Green-colored beds indicate more permeable fine- The relatively younger Neoproterozoic Mineral Fork grained sandstone and siltstone. Fluids resulting from em- Formation and Mutual Formation, and the Cambrian Tintic placement of the Little Cottonwood Stock (-30 Ma) moved Quartzite indicate a shift in depositional and tectonic along the zone of the Wasatch Fault to produce hydrother- styles from that of the Big Cottonwood Formation. The mal alteration (including white vein quartz w~thhematite Mineral Fork Formation suggests deposition in temperate and pyrite) in the fine-grained beds. glacial-marine glacially sculpted fjord. The Mutual Forma- Interpretation: The mud-cracked argillite is interpret- tion records deposition in a braided fluvial environment ed to have formed in intertidal to possibly supratidal flat and contains a regional stratigraphic sequence boundary. area within the tidal limit of the estuarine system. This Deposition in the Cambrian Tintic Quartzite suggests facies is vertically associated with the sheet quartzites, marine passive-margin sedimentation. interpreted to be subtidal sand bars, suggesting that the mud-cracked argillite facies was a mud flat located towards DESCRIPTION OF STOPS the mouth of the estuary. This facies could also have been transitional to tidal channel flood plain deposits. The pres- The mileage of all the stops is measured from the mouth ence of mud cracks and rain drop impressions indicates of Big Cottonwood Canyon at the parking lot. The green subaerial exposure. The small wave ripples superimposed mile markers along Highway 190 do not represent the on the mud-cracked bedding planes imply shallow water mileage from the mouth of the canyon. and oscillatory flow. It is difficult to determine whether other parts of the facies might be supratidal due to lack of Stop 1: Overview of the Wasatch Range (mile 0) any paleosols (correlated with a lack of vegetation in the Location: Parking lot at the mouth of Big Cottonwood Precambrian). Canyon. Description: This stop provides an overview of the Stop 3: Storm Mountain quartzite sign-Big Cotton- Wasatch Front. Oligocene magmatism resulted in the em- wood Formation (mile 2.9, 4.8 km) placement of plutons such as the quartz monzonite visible high on the south side of the canyon across from the park- Location: Located just past the Storm Mountain picnic ing lot. The rocks in the central Wasatch range (at this stop) area and amphitheater. Pull off along the south side of the EHLERS, CHAN, LINK: PROTEROZOIC TIDA L, GLACIAL, AND FLUVIAL SEDIMENTATION 47
Figure 11. Photograph of mud cracked argillite (stop 2). Figure 12. Sand sheet quartzites located behind Storm Mountain amphitheater (looking northwest from stop 3). Approxiinate dis- tance across the l?ottom of thefigure is 300 In. road at the "Storm Mountain Quartzite" sign. The second part of this stop will involve walking to an outcrop of well developed rhythmites on the north side of the Storm Moun- talus slope shows an example of soft sediment deforma- tain Reservoir (on the north side of Highway 190). tion and truncation within the laminated argillite facies. Description: Laminated argillite and channeled quart- Interpretation: The preservation of the rhythmites (Fig. zite facies are exposed just north of the reservoir where 8 and 15), the finely laminated structures, and diagenetic lenticular geometries are visible. Numerous, stacked sheet pyrite within the black argillite suggest a reducing sub- to quartzite bodies form Storm Mountain to the southwest, intertidal environment with the presence of organic carbon. and the steep quartzite outcrops by the amphitheater (Fig. Although littlelno carbon is currently left in this black 12). The black, laminated argillite crops out immediately argillite, correlative units such as the Chuar Group of to the northeast of the pullout. Syneresis (subaqueous Arizona are currently being explored as Precambrian shrinkage?) cracks (Fig. 13) are common along bedding source rocks (e.g., Summons et al., 1988; Elston, 1989; plane exposures, and diagenetic pyrite is present within Chidsey et al., 1990; Uphoff, 1997). some of these laminated argillites. Immediately to the The laminated argillite facies at this stop is traceal~leto east, over the knoll, an example of heterolithic rhythmites the south up Broads Fork, all the way to the top of Twin (with daily and semi-monthly cycles) deposited over about Peaks (elevation: 11,328 ft.). Along Broads Fork, the lami- 8 months can be seen in the gradational contact between nated argillite contains -similar sedimentary structures the underlying channeled quartzite and black argillite. (rhythrnites, syneresis cracks and diagenetic pyrite) to At the top of the knoll, there are blocks of channeled those seen at this stop. Although tidal rhythmites are visi- quartzite facies which contain coarse-grained lags and ble in this facies near the top of Twin Peaks, the neap- cross bedding (Fig. 14) . One area at the top of the slippery spring cycles are generally not as well developed. 48 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
Figure 13. Syneresis cracks of tlze laminated argillite (stop 3).
Figlire 14. Cross beds ctf the clzannelad quartzite fiicies (stop 3). Wllite itlarks on sccrle are at 10 c~ninteroals. EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 49
Figure 16. Tidal rlzythmites by the Storm Mountain reseruoir. These rlzytlzmites are located in a transition zone between the channeled quartzite (left) ancl laminated argillite (right, 3 m off Figure 15. Tidal rhythmites with climl7ing ripples collected from edge of photograph) litlzofacies (stop 3). core. Dark arrows represent the location of neap cycles. Tlzis core sample was collected near stop 3. at the edge of the outcrop and follow it across the flat, grassy meadow to the outcrop where channeled quartzite The channeled quartzite with lags is interpreted to grades into laminated argillite. represent tidal channel deposits. The one deformed block The outcrop provides a view of over 2 m of heterolithic (with soft sediment deformation) is interpreted to be a rhythmites (Fig. 16). This outcrop also demonstrates the tidal fluvial channel bank collapse structure. The chan- typical vertical facies arrangement in which rhythmites neled quartzite is also visited in more detail at the next form. The left (west) side of the outcrop is composed of two stops. Figure 8 shows examples of tidal rhythmites channeled quartzite facies. This facies grades into well- collected from this area. developed planar laminated tidal rhythmites which con- tain visible daily, semi-monthly, and monthly cycles. To North Side of Storm Mountain Reservoir the right (east) of the rhythmites, the section progressively For the second half of this stop, walk down the talus fines upward into -300 m of laminated argillite. A similar slope to the road on the east side of the knoll. Continue rhythmite outcrop to this one is also located at mile 3.1 walking up the road (-30 m) across the bridge over Big along the north side of the road (50 m up the steep hill Cottonwood Creek. After the bridge, turn left (west) by slope just south of the large boulder next to the road). the chain link fence and walk along the creek past the out- This locality at mile 3.3 also contains well developed crop of laminated argillite on your right. Turn right (north) neap-spring cycles in the laminated argillite. A 1.5 m core 50 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
Figure 17. Slump block with heterolithic rhythmites presemed in n tidnlfluainl channel nt the S-crrrces (stop 4).
of these neap-spring cycles has been collected, and con- well developed in these heterolithic rhythmites (distinct tains approximately 10 years of continuous deposition. white sand laminae alternating with dark, thin clay drape laminae). Some of the daily rhythms are also preserved in Stop 4: S-curve channeled quartzite-Big Cottonwood climbing ripples. Formation (mile 4.7, 7.8 km) Internretation: The sedimentary structures within the channeled quartzite suggest a current-dominated tidal flu- Location: Outcrop located about 0.1 mi. past the sec- vial channel. The block of rhythmites (originally deposited ond hair pin "S-curve." Use the pullout about 0.2 mi. from adjacent to or near channel margins) (Fig. 17) is interyret- the second curve and walk west (back down the canyon) ed as a channel bank collapse structure adjacent to the to the outcrop along the north side of the road across from fluvial-tidal channel. Figure 18 is a photomosaic of the the concrete rail, east of Hidden Falls. Please watch care- channeled quartzite geometries. The white lines in this fully for cars. figure outline the channel geometries and the crassbed Description: This stop includes a good lateral and ver- sets which internally thin upwards. Also note the scoured tical view of the channeled quartzite facies including a bases with lags. large rotated block, containing about 18 months of tidal rhythmites. The geometries can be viewed from the south Stop 5: Moss Ledge picnic area-Big Cottonwood For- side of the road. This is one of the localities of drilled core mation (mile 5.1, 8.5 km) used in analysis of tidal rhythmites (Chan et a]., 1994; Sonett et a]., 1996). Quartzite structures observed here include Location: Park along the north side of the road at the west-dipping crossbed sets, scoured basal contacts, and Moss Ledge picnic area parking lot. current ripples. Daily rhythms and neap-spring cycles are Description: This stop highlights upward thinning and EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 51
Figure 18. Plaotomosaic of the S-curve channel geometries. White lines (dashed where inferred) indicate the bounrlnry between cross- bed sets, which fine and thin upwards. Other noteworthy sedimentar!~features are Zal7eled (stop 4). fining successions of the channeled quartzite, and the black laminated argillite. A thin (2-4 m) layer of purple green-gray transitional and purple mud-cracked argillite mud-cracked argillite (similar to the argillite seen at stop facies. There are four stacked facies successions (chan- 2) overlies the transitional argillite and caps the upward- neled quartzite to transitional argillite) similar to the gen- fining sequence. The mud-cracked argillite is in turn over- eralized succession shown in Figure 19. The top of the lain by the channeled quartzite followed by more stacked Big Cottonwood Formation is truncated at the top of the fining-upward successions (Fig. 19). ridge by the Mineral Fork Formation. Sigmoidal bundles Interpretation: This facies is interpreted to represent a are present in the quartzite from along the road (just west fluvial channel with intermittent tidal influence, probably of the parking area). In the channeled quartzite above the in the upper reaches of the estuary close to the tidal limit. picnic area, there is a good exposure of the scoured basal The scoured base, and internal upward thinning and fin- contact with coarse grained rip-up clasts, overlain by dis- ing of bed size and grain size respectively suggest channel tinct crossbed sets that thin upwards. The green-gray infilling and shallowing (Fig. 19). The annual cycles in the transitional argillite facies (exposed to the west of the pic- sand-mud couplets suggest tidal deposition in the upper nic table) contains sand-mud couplets with yearly period- end of the estuary during episodes of annually high sedi- icities (Fig. 20, determined from spectral analysis; Erik mentation (high seasonal water levels, storm events). Only Kvale, pers. commun., 1996), current ripples with crests one of the channeled quartzites at this Moss Ledge locali- rounded by back flow, some load and soft-sediment defor- ty is aerially extensive over the entire outcrop area north mational structures, and clay-draped reactivation surfaces. and south of Big Cottonwood Canyon, and appears to be Inferred tidal rhythms as well as sedimentary structures correlative to the quartzite near the top of Mount Olympus in this facies are significantly different from those in the (9,026 ft) about 9 km to the northwest. 52 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
CHANNELED QUARTZITE MUD-CRACKED ARGlLLlTE - mudcracks
t mm scale laminations TRANSITIONAL ARGlLLlTE
t cm scale laminations
x-bed thickness - 2-10 cm
CHANNELED QUARTZITE
x-bed thickness - 30+ cm
rip up clasts - scoured base MUD-CRACKED ARGlLLlTE
Figure 19. q/pical stratigraphic succession of the Moss Ledge Picnic area (stop 5). About 4 of these channeled quartzite and transitional argillite successions are stacked close to the top of the Big Cottonwood Formation. Successions are t!/pically 60-90 m in thickness.
Figure 20. Heto-olitl~icrlt!ltltinites c!f'i/tefrcrnsitiottcrl clrgi/lifc ~ritlt Stop 6: Mineral Fork Formation (mile 5.9, 9.9 km) unnual /rrnrrr pel-iorlicitics (stop 5). Location: Continue east, up the canyon from the Moss Ledge picnic area approximately 0.8 miles. A pullout for 1994). Note the variety of clasts in the diamictite (includ- Mineral Fork is located along the south side of the road ing orthoquartzite, carbonate, fine-grained sedimentary immediately after the brown sign that reads "No Dogs rocks, and sparse granite and gneiss), which suggests that Allowed." A narrow dirt road continues south past the the source terrane which supplied coarse sediment to the iron gate. Mineral Fork lobe of Neoproterozoic Sturtian (750-700 Descri~tion:We will stop at the mouth of the Mineral Ma) glaciers contained quartzite and carl~onaterocks with Fork canyon and walk up the trail for about twenty min- little basement contribution (C~ittendenet al., 1952; Varney, utes just past the fourth switchback to the first outcrop of 1976; Ojakangas and Matsch, 1980; Christie-Blick, 1983). poorly sorted, black pebble diamictite (Fig. 21). Boulders Deposition was in a temperate glacia I -marine environ- of the Mineral Fork Formation with mm-cm scale angular ment, relatively close to the grounding line of a partially clasts are visible along the hike up the trail. The diamictite buoyant ice sheet (Christie-Blick, 1983; 1997; Christie- is in distinct contrast to the other siliciclastic rocks pres- Blick and Link; 1988; Christie-Blick et al., 1989; Link et al., ent in Big Cottonwood Canyon since it is poorly sorted, 1994). The Mineral Fork Fornlation is dominated by proxi- high in clay matrix, and was deposited and lithified under mal facies, but lacks major internal erosional surfaces. reducing conditions which preserved its black color. Other This suggests that it represents an overall retreat of the distinctive diamictite and laminated sandstone and silt- ice sheet, with minor oscillations in the ice margin . stone lithologies are evident in the talus and alluvium. Interpretation: The Mineral Fork Formation represents Stop 7: Mutual Formation across from Mineral Fork (mile the complex deposits of a sub-glacial fjord carved into the 5.9+, 9.9+ km) flat-lying subjacent Big Cottonwood Formation (Christie- Blick, 1983; Christie-Blick and Link, 1988; Link et al., Location: The outcrop of the Muhral Fo~mationis located EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 53
Interpretation: The depositional environment of this outcrop was likely a high-energy stream system, with local erosive channels and areas of lower regime flow over a fine- grained substrate. Regionally the Mutual Formation con- tains less than 850 m of trough cross-bedded pebbly sand- stone with local and variable pods of siltstone (Christie- Blick, 1982). It was deposited as a stratigraphic blanket, in a braided stream system with local lakes and flood basins (Link et al., 1987; Levy and Christie-Blick, 1991). Accommo- dation space was apparently controlled by late Varanger post-glacial sea level rise rather than by differential ther- mal subsidence (Christie-Blick, 1997).
Stop 8: Cambrian Tintic Quartzite (mile 6.7, 11.0 km) Location: Continue up the canyon approximately 0.5 miles from Mineral Fork. Field trip participants can easily walk up the canyon. There is a parking area on the south side of the road where the shoulder widens. The best out- crops of tan quartzite are on the north side of the road, adjacent to a thicket and across about 10 m of field grass. Immediately down the canyon is a more continuous, stra- tigraphically lower, roadside exposure (Fig. 24). Description: The Tintic Quartzite in the thicket (Fig. 25) is coarse- to very coarse-grained, planar cross-bedded sandstone. It is texturally more mature than the Mutual For- mation, suggesting its interpretation as a shallow marine sandstone. Interpretation: The Middle Cambrian Tintic Quartzite represents the immediate post-rift shallow-marine phase Figure 21. Large boulder (1.5 m diameter) of Mineral Fork Foma- of development of the Cordilleran passive continental tion located along the trail to the outcrop (stop 6). This boulder margin, which had entered the carbonate platform phase contains rnm-cm scale angular clasts within a black, silt!]mc~trix. by Early Cambrian time. The Tintic here represents the parautochthonous, uppermost eastern feather edge of the regional, west-thickening wedge of shallow marine sand- about 0.1 mi up canyon and across the road from the pull- stone represented by the Geertsen Canyon, Camelback out for Mineral Fork. Walk up the canyon to the first out- Mountain and Prospect Mountain quartzites. This wedge crop of purple quartzite, adjacent to the green sign that is overlain by fossiliferous Lower and Middle Cambrian reads "mile 8." mudrock, and was deposited in accommodation space cre- Description: The Mutual Formation here contains pur- ated by thermal subsidence of the Neoproterozoic and ple to pinkish gray, medium- to coarse-grained sedimentary Paleozoic Cordilleran miogeocline (Christie-Blick, 1982; litharenite (quartzite), and interbedded argillite (Fig. 22 and Christie-Blick et al., 1989; Christie-Blick and Levy, 1991; 23). The sandstone is medium-bedded, with a general up- Link et al., 1993). ward-fining succession from left to right. Some of the fine- sand beds contain current ripples on bedding planes. One OTHER OPTIONAL STOPS internal contact is channelized into the underlying dark- colored argillite. The basal bed of the channel fill contains The eastward continuation of Highway 190 up Big pebble conglomerate and mudstone rip-up clasts. Cottonwood Canyon traverses through Late Paleozoic The type section of the Mutual Formation is located at limestones (Mile 7.0 at "Mississippian Marble" sign) and the old Mutual Mine here in Big Cottonwood Canyon. passes the lowest point that Pleistocene glaciers extended Although not shown on geologic maps of this area, the Alta down the canyon (Mile 8.6 at "Meeting of the Glaciers" thrust repeats the Mutual Formation type section on both sign). The Highway ends in a loop at the Brighton Ski sides of Big Cottonwood Canyon. Resort. 54 BYU GEOLOGY STUDIES 1997, VOL. 42, PART I
Figure 22. Outcrop of Mutual Formation located about 0.1 mi, up the roacl from Mineral Fork. Alternating lalyers of qzcarkite and argillite laaoe normal grading from left to right. The quartzite layer on the right laancl side of tlze figure has a scoured hase that cuts rloton into the unrlerl!/ingargillite. Also oisilde on the lefi are current ripples along the be(lrliizg plone oftlie qitartzitc (stop 6).
ACKNOWLEDGMENTS Archer, A,\\!, 1996, Relia1,ility of lunar orl>ital periods extractetl from ancient cyclic tidal rhythmites: Earth and Planetan Science Letters, The research presented here is a result of funding by v. 141, p. 1-10. NSF grant EAR 9315937 (to Chan), and GSA research Archel; A., 1997 In Press. Modeling of cyclical rhythmites (Carl>oniferor~s grant 5813-96 (to Ehlers for Uinta Mountain Group work). of Indiana and Kansas, Precambrian of Utah, U.S.A.) 21s a hisfor Thoughtful discussions and rhythmite core interpretation reconstruction of intertidal positioning and paleotidal regimes: Setli- mentolog): by A1 Archer and Erik Kvale contributed greatly to this Archer, A.W, and Feldmnn, H.R.. 1994, Tidal influence in Carhoniferocts work. We acknowledge a most helpful review of the man- fine-grained limestones [Rhythmites tidales dans les calcaires caho- uscript by Nicholas Christie-Blick. niferes aux Etats-Unis]: Geol~ios,v. 16, p. 283-291. Archer, A.\lr., Kvale, E.P, and Johnson, H.R., 1991, Analysis of niodem REFERENCES CITED equatorial tidal periodicities as a test of information encotletl in ancient tidal rl~~thniites,in Smith, D.G., Reinson, G.E., Zaitlin. R.A.. Arabasz, W.J., Pechniann, J.C., aiid Brown, E.D., 1990, Observational Rahmanin, R.A., etl., Clastic tidal sedimentology: Canadian Society seismology and the evaluation of earthquake hazards and risk in the of Peh-oleum Geologists Memoir 16, p. 189-198. Wasatch Front area, Utah, Assessment of regional earthquake hazards Bruhn, R.L., Picard, M.D., and Beck, S.L., 1983, Mcsozoic and early and risk along the Wasatch front, Utah, US. Geological survey profes- Tertiary stiucture and seditncntology of the central Wasatch Moun- sional paper 1500-D, p. Dl-D36. tains, Uinta Mountains ant1 Uintn basin: Utah Geological ant1 hlin- Archer, A.W., 1994, Extraction of sedimentologic information via com- eralogical SunfeyBulletin 59, p. 63-105. puter-lmsed i.nage analyses of gray shales in Carboniferous coal- Bnthn, R.L., Picard, M.D., Isb): J.S., 1986, Tectonics ant1 sedimentology bearing sections of Indiana ant1 Kansas, USA: Mathematical Geology, of Uinta Arch, westem Uinta ilfountains, and Uinhc Basin, ill Peterson. v. 26, p. 47-65. J.A., etl., Paleotectonics ant1 Sedimentation in the Rocky Mountain EHLERS, CHAN, LINK: PROTEROZOIC TIDAL, GLACIAL, AND FLUVIAL SEDIMENTATION 55
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5.1 '-. * '.. , . f'" ;;;,.% .; .; Iri...q..v3
Figure 24. Outcrop of Tintic Quartzite -0.5 mi froin Mineral Fork. The contcict hettoeen the Mtittccrl Fonnation ant1 Tintic Qzmrtzite i.s uisible on the ridge in tlze background (0.1 mi prior to stop 8).
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