New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 © 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. NEW INSIGHTS ON THE IMPACT OF TIDAL CURRENTS ON A LOW-GRADIENT, SEMI-ENCLOSED, EPICONTINENTAL BASIN—THE CURTIS FORMATION, EAST-CENTRAL UTAH, USA Valentin Zuchuat, Arve R.N. Sleveland, Douglas A. Sprinkel, Algirdas Rimkus, Alvar Braathen, and Ivar Midtkandal GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover View from the Lower South Desert Overlook, Capitol Reef National Park, displaying the earthy facies of the Callovian Entrada Sandstone (reddish-colored sand- stone), overlain by the Oxfordian, tidally influenced Cur- tis Formation (light-colored sandstone). The two units are separated by the J-3 unconformity, which coincides here with the Major Transgressive Surface (MTS) at the base of the informal middle Curtis. Note the evidences of tidal ravinement at the base of the middle Curtis. The middle Curtis consists mainly very fine to fine-grained sandstone, and corresponds to a sub- to intertidal chan- nel-dune-flat complex depositional setting. The middle Curtis gradually grades into the thinner-bedded, very fine-grained, sub- to intertidal heterolithic flat deposits of the upper Curtis, which is conformably overlain by the Summerville Formation. Note the geologist in the lower right quadrant of the photograph for scale. i 2018 President Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2018 President-Elect Peter Nielsen peternielsen@utah.gov 801.537.3359 2018 Program Chair Emily McDermott emcdermott@utah.gov 801.537.3389 2018 Treasurer Zach Anderson zanderson@utah.gov 801.538.4779 2018 Secretary Christopher Kravits ckravitsgeo@gmail.com 2018 Past President Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Board UGA Committees Education/Scholarship Loren Morton lmorton@utah.gov 801.536.4262 Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Terry Massoth twmassoth@hotmail.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Public Education Paul Jewell pwjewell@mines.utah.edu 801.581.6636 Matt Affolter gfl247@yahoo.com Publications Roger Bon rogerbon@xmission.com 801.942.0533 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017–2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committe UGA Representative Jason Blake blake-j@comcast.net 435.658.3423 UGA Newsletter Newsletter Editor Bill Lund uga.newsletter@gmail.com 435.590.1338 UGA Website www.utahgeology.org Webmasters Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Lance Weaver lanceweaver@utah.gov 801.403.1636 Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. If you would like to participate please contact the current president or committee member corresponding with the area in which you would like to volunteer. Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. Volume 5 2018 This is an open-access article in which the Utah Geological Association permits unrestricted use, distribution, and reproduction of text and figures that are not noted as copyrighted, provided the original author and source are credited. Earthquake Safety Committe Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Utah Geological Survey 801.391.1977 GIW@utahgeology.org Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 131 ABSTRACT Based on a methodic sedimentological analysis, the Late Jurassic (Oxfordian) Curtis Formation unravels the intricate facies variability which occurs in a tide-dominated, fluvially starved, low-gradient, semi-enclosed epicontinental basin. This unit crops out in east-central Utah, between the eolian deposits of the underlying Middle Jurassic (Callovian) Entrada Sandstone, from which it is separated by the J-3 unconformity, and the conformable overlying supratidal Summerville Formation of Oxfordian age. A high-resolution sedimentary analysis of the succession led to the recognition of eight facies associations (FA) with six sub-facies associa- tions. Based on the specific three-dimensional arrangement of these eight facies associations, it is proposed to separate the Curtis Formation into three sub-units: the lower, middle and upper Curtis. The J-3 unconformity defines the base of the lower Curtis, which consists of upper shoreface to beach deposits (FA 2), mud-domi- nated (FA 3a) and sand-dominated heterolithic subtidal flat (FA 3b), sand-rich sub- to supratidal flat (FA 4a) and correlative tidal channel infill (FA 4c). It is capped by the middle Curtis, which coincides with the sub- to intertidal channel-dune-flat complex of FA 5, and its lower boundary corresponds to a transgressive surface of regional extent, identified as the Major Transgressive Surface (MTS). This surface suggests a potential correla- tion between the middle and the upper Curtis and the neighboring Todilto Member of the Wanakah Forma- tion or Todilto Formation. The upper Curtis consists of the heterolithic upper sub- to intertidal flat (FA 6) and coastal dry eolian dunes belonging to the Moab Member of the Curtis Formation (FA 7), and it conformably overlies the middle Curtis. The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis For- mation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation. This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over- pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis. New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Valentin Zuchuat1, Arve R.N. Sleveland1, Douglas A. Sprinkel2, Algirdas Rimkus1, Alvar Braathen1, Ivar Midtkandal1 1 University of Oslo, Department of Geosciences, Sem Sælands vei 1, 0371, Oslo, Norway; valentin.zuchuat@geo.uio.no 2 Utah Geological Survey, PO Box 146100, Salt Lake City, Utah 84114; douglassprinkel@utah.gov Citation for this article. Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I., 2018, New insights on the impact of tidal currents on a low-gradient, semi-enclosed, epicontinental basin—the Curtis Formation, east-central Utah, USA: Geology of the Intermountain West, v. 5, p. 131–165. © 2018 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. 132 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 INTRODUCTION The complex aspect of tide-dominated environ- ments was reported as early as first century AD, when Pliny the Elder, in his Historia Naturalis, wondered whether such areas “invaded twice each day and night by the overflowing waves of the ocean…are to be looked upon as belonging to the land, or whether as forming portion of the sea?” (translation from Bostock and Riley, 1855). Since then, it has been shown that the essence of tidal deposits resides within their typical three-dimensional intricate assemblage of heterolithic facies, which distribution is dictated by a fine equilib- rium of sediment input, basinal hydrodynamic forces, as well as avulsing and migrating channels (Davis and Dalrymple, 2012; references therein). This complexity is further enhanced by temporal and cyclical variations of tidal currents in a basin, which unevenly impact the erosion-transport-deposition mechanisms of the different grain classes within the system (Kvale, 2012; Wang, 2012; references therein). Adding to the intrinsi- cally dynamic system, the effects of fluctuating rates in relative sea-level variation are amplified by a low-gra- dient shelf through shifting facies belts over great dis- tances (Midtkandal and Nystuen, 2009). Poor time constraints, limited but changing accommodation, and pre-existing basin floor relief further complicate the ac- curate interpretation and correlation of such deposits. Despite highly complex depositional scenarios, these conditions may produce substantial volumes of reser- voir-grade sandstone, and represent potentially viable aquifers, CO2-injection targets, or petroleum reservoirs (Martinius and others, 2005; Halland and others, 2014). Research focusing on siliciclastic tide-dominat- ed environments recognizes four main categories: (1) the upward fining, transgressive estuarine, (2) the (semi-)protected lagoonal systems, (3) the prograding tide-dominated deltas, as well as (4) the open-coast tidal flats (e.g., Boyd and others, 1992; Dalrymple and others, 1992; Fan, 2012). Whereas modern analogs can help scientists identifying such depositional systems in the rock record, Tape and others (2003) highlight- ed the fact that some tidally influenced sedimentary units do not belong to any of the above-mentioned classes and cannot be illustrated by modern equivalent environments either, as they were deposited on broad and shallow epicontinental shelves. The outstanding outcrop quality and the significant internal variability of the Middle Jurassic Entrada Sandstone and Late Ju- rassic Curtis-Summerville Formations of east-central Utah allow a detailed investigation of gradual, subtle, and intricate interactions within such a low-gradient, continental to subtidal epeiric system, characterized by a starvation of major fluvial input (Kreisa and Mo- iola, 1986; Caputo and Pryor, 1991; Wilcox and Currie, 2008). The main objective of this study is to develop a detailed data-driven classification of heterolithic facies and facies associations present on a tidally influenced siliciclastic-dominated shelf of regional extent, repre- sented by the Curtis Formation of Early Oxfordian age (about 161–159 Ma) (Kreisa and Moiola, 1986; Capu- to and Pryor, 1991; Wilcox and Currie, 2008; Ogg and others, 2016). This work establishes the sedimentary basis for deconstructing the growth and infill dynamic of such a tide-dominated basin, which will be analyzed in a separate study. The overarching goal of this venture is to generate a multi-disciplinary predictive protocol to assess the combined seal-reservoir properties of such heterolithic deposits, notably for carbon capture and storage purposes. GEOLOGICAL SETTING Basinal Setting Since the Mesozoic, Utah’s geological history has been profoundly influenced by several tectonic events, markedly by the development of the North American Cordillera and its cascade of orogenies; key orogenies, partly overlapping in time and space (Bump and Da- vis, 2003; Hintze and Kowallis, 2009; Thorman, 2011; Anderson, 2015; Yonkee and Weil, 2015; and references therein), are (1) the Middle Jurassic-Early Cretaceous Nevadan orogeny, whose remains notably consist of small granitic batholiths at today’s Utah-Nevada bor- der, (2) the Middle Jurassic Elko orogeny, which differs from the other orogenies by alternating extensional and contractional tectonic events, (3) the Early Cretaceous to Paleogene Sevier orogeny, with its resultant thin- skinned contractional structures associated with a fore- 133 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 land basin development, and (4) the Late Cretaceous to Paleogene Laramide orogeny seen as basement-root- ed monoclines, from which the San Rafael Swell and other large uplifts emerged. Additionally, rocks from east-central to southeastern Utah were also affected by movements of the Paradox Basin salt deposits (Trudg- ill, 2011), regional uplifts of the Colorado Plateau, and related extensional events (Levander and others, 2011; Murray and others, 2016). There were also intrusive and extrusive magmatic episodes during the Middle to Late Oligocene; the latter are expressed by the Henry, La Sal, and Abajo Mountains igneous complexes (Sullivan and others, 1991; Nelson, 1997). Burial history data com- piled by Nuccio and Condon (1996) and Petrie and oth- ers (2017) suggest that the Curtis Formation was buried to depths of 2.45 km and 2.86 km near the San Rafael Swell. Stratigraphy The westward-thickening sedimentary succession of the San Rafael Group was deposited in neighboring areas of the Utah-Idaho trough, a north-northeast to south-southwest-trending distal retroarc foreland ba- sin parallel to the Elko highlands (figure 1) (Anderson and Lucas, 1994; Brenner and Peterson, 1994; Peterson, 1994; Bjerrum and Dorsey, 1995; Thorman, 2011). This basin recorded multiple transgressive-regressive marine cycles (Anderson and Lucas, 1994). The marine and in- tertonguing eolian sediments of the coastal paleo-erg of the Middle Jurassic (Aalenian to Bajocian) Temple Cap Formation in southwestern and central Utah un- conformably overlies the eolian Navajo Sandstone of Early Jurassic age on the J-1 unconformity (Pipiringos and O’Sullivan, 1978; Peterson and Pipiringos, 1979; Sprinkel and others, 2011). Paleowind indicators for the Navajo Sandstone suggest a north-northwest to south-southeast wind direction, whereas paleowind in- dicators for the Temple Cap Formation suggest a north- east to southwest wind direction (Parrish and Peterson, 1988; Peterson, 1988; Hartwick, 2010). To the east in south-central Utah, the former basal Harris Wash Mem- ber of the Page Sandstone unconformably overlies the Navajo Sandstone, above the J-2 unconformity of Pipir- ingos and O’Sullivan (1978) and Peterson and Pipirin- gos (1979). Isotopic ages obtained from ash beds in the Harris Wash Member indicated they were time equiva- lent to the Temple Cap Formation and hence pre-dat- ed the basal Carmel Formation (Kowallis and others, 2001; Dickinson and others, 2010; Sprinkel and oth- ers, 2011). This stratigraphic relationship brought into question the validity of the J-2 unconformity and led to the recommendation to re-assign the beds represent- ing the Harris Wash Member of the Page Sandstone to the Temple Cap Formation (Sprinkel and others, 2011; Doelling and others, 2013). The shallow marine Middle Jurassic Carmel Formation (Gilluly and Reeside, 1928) conformably overlies the Temple Cap Formation, re- flecting marine incursions from the northern Sundance seaway from Bajocian to early Callovian time (Ander- son and Lucas, 1994; Brenner and Peterson, 1994; Pe- terson, 1994; Hintze and Kowallis, 2009; Sprinkel and others, 2011). However, the Carmel Formation does unconformably overlie the Navajo Sandstone in places where the Temple Cap Formation is missing because of irregular deposition of the Temple Cap on a pre-exist- ing paleotopography and its depositional pinch-out in eastern Utah (Sprinkel and others, 2011; Doelling and others, 2013). Continental conditions returned with the deposi- tion of the Middle Jurassic (Callovian) Entrada Sand- stone (Peterson, 1994: Hintze and Kowallis, 2009), formally defined by Gilluly and Reeside (1928). This sedimentary formation is divided into two units: (1) the basal Slick Rock Member, which consists of alternating eolian dune and interdune intervals, and (2) the over- lying informal, intermittently vegetated, earthy facies, which was deposited in a marginal marine setting (Wit- kind, 1988; Crabaugh and Kocurek, 1993; Carr-Cra- baugh and Kocurek, 1998; Mountney, 2012; Doelling and others, 2015). The Entrada Sandstone and correl- ative formations (Twist Gulch and Preuss Formations) thicken westward towards the Utah-Idaho trough and northwards towards the Sundance Seaway (Imlay, 1980; Kocurek and Dott, 1983). Dickinson and Gehrels (2009, 2010) showed that the siliciclastic grains of the Entra- da Sandstone were mostly recycled from river systems sourced from the Appalachian Mountains, on the east- ern side of the continent. The Entrada Sandstone recorded four erg con- 134 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 struction-destruction cycles (sensu Mountney, 2006), dictated by regional variations of the paleo-water ta- ble, themselves related to relative sea level fluctuations (Carr-Crabaugh and Kocurek, 1998; Mountney, 2012). The entire system is truncated along its top by the re- gional J-3 unconformity (Pipiringos and O’Sullivan, 1978; Hintze and Kowallis, 2009), with local relief of up to about 23 m. In areas of south-central Utah, the J-3 unconformity truncates subtle, large-amplitude folds developed in the Entrada Sandstone and underlying formations, exhibiting distinctive angular relationships along the unconformity (see figure 7 of Wheatley and Price Moab Hanksville Salt Lake City Uinta Mountains UTAH Green River San Rafael Swell 109°0'0"W111°0'0"W113°0'0"W 38 °0 '0" N 40 °0 '0" N 42 °0 '0" N 100 km B 1000 kmA N B 24 191 24 191 95 25km N Lacustrine In te rc on ne ct ed se co nd ar y re se rv oi rs Re gi on al re se rv oi r Re gi on al re se rv oi r se co nd ar y re se rv oi rs Lacustrine Mixed Tidal Mixed Aeolian Marginal Marine Aeolian Fluvial Re gi on al ca pr oc k AGE FORMATION LITHOLOGY CR ET AC EO U S JU RA SS IC M AN CO S SH AL E M O RR IS O N Summerville Curtis MT Carmel Page sst Navajo Sandstone Blue Gate Ferron Sst Tunuk Shale Dakota Shale Mbr BuckhornCe da r M t Brushy Basin Salt Wash Tidwell Central UtahC In te rc on ne ct ed Entrada Slick Rock Earthy Facies 10 11 1 2 3 4 5 6 7 8 9 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Grid Eas�ng Northing 1 Sulphur Canyon 27.06.2015 12 S 538598 4354691 2 Stove Gulch East 23.06.2015 12 S 541150 4354677 3 Humbug Flats East 30.06.2015 12 S 545496 4349054 4 Neversweat Wash 01.07.2015 12 S 546582 4345984 5 Middle Canyon 21.09.2015 12 S 545582 4340631 6 Dry Mesa 02.07.2015 12 S 546370 4336290 7 Cur�s Point 03.05.2016 12 S 549241 4328385 8 Wet Gulch 04.05.2016 12 S 550874 4325353 9 Sven's Gulch 22.09.2015 12 S 552531 4323706 10 Smith's Cabin 03.07.2015 12 S 553584 4319583 11 Rabbit Gulch 19.05.2016 12 S 553369 4314960 12 Interstate 70 06.05.2016 12 S 550457 4308362 13 Uneva Mine Canyon 12.05.2016 12 S 547594 4304403 14 Crystal Geyser 15.09.2015 12 S 575060 4310623 15 Lower San Rafael Rd 20.05.2016 12 S 570049 4295643 16 Ruby Ranch Meandre 04.06.2016 12 S 576420 4298161 17 Ruby Ranch Road 03.06.2016 12 S 583113 4296930 18 Duma Point 01.06.2016 12 S 588894 4293333 19 Horse Flies Gulch 31.05.2016 12 S 590606 4293184 20 Dune Mesa 30.05.2016 12 S 590620 4290953 UTM - coordinates Log N° Log name Date Grid Eas�ng Northing 21 Ten Mile Rd 28.05.2016 12 S 594368 4291828 22 Petrified Tree Gulch 29.05.2016 12 S 596564 4287509 23 Dubinky Well Rd 27.05.2016 12 S 595351 4284912 24 Safari Road 26.05.2016 12 S 601764 4282137 25 Bartle� Wash 16.06.2015 12 S 605257 4286436 26 Salt Valley 21.05.2016 12 S 608768 4302663 27 Lost Spring Canyon 23.05.2016 12 S 624118 4294979 28 Dewey Bridge 10.05.2016 12 S 647467 4298329 29 Big Pinto Mesa 22.05.2016 12 S 654182 4294917 30 Goblin Valley 29.04.2017 12 S 522572 4269001 31 Li�le Flat Top 08.05.2016 12 S 544488 4266008 32 Hanksville 05.05.2017 12 S 525317 4248736 33 Notom Ranch 09.05.2017 12 S 492291 4226559 34 Cainville Airstrip 11.05.2017 12 S 495813 4244029 35 L. South Desert Ov. 12.05.2017 12 S 481839 4250657 36 LCD the Two Towers 27.04.2017 12 S 477493 4275207 37 LCD Road Cut 25.04.2017 12 S 472428 4279105 38 Salt Wash View Area 16.05.2017 12 S 490555 4298727 39 Sid and Charley 15.05.2017 12 S 500015 4311719 40 Lower Cedar Mt Rd 14.05.2017 12 S 518557 4340815 Log name Date UTM - coordinates Log N° Figure 1. (A) Maps of the study area. (B) Green dots represent visited localities where the Curtis Formation crops out, where- as red dots (not numbered) illustrate its absence. Each dot number on the map refers to a specific locality in the attached table (geological units after Hintze, 1980; Witkind, 1988; Doelling, 2001; and Doelling and others, 2015). (C) Schematic stratigraphic column of the area (modified from Ogata and others, 2014). 135 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 others, 2016). Depending on location, this boundary can be characterized as a conformable contact, a para- conformity, a disconformity, or an angular unconfor- mity. It is important to note that the precise time en- capsulated in the composite J-3 unconformity remains unknown. Since the definition of an unconformity im- plies a “lacuna of substantial duration” (sensu Holbrook and Bhattacharya, 2012), the nature of the J-3 uncon- formity remains a point of discussion. Peterson (1994) argued that a regional tectonic uplift in the west played the major role in the development of this unconformi- ty, whereas Caputo and Pryor (1991) as well as Eschner and Kocurek (1988), respectively, advocated that (un- identified) marine or tidal currents during the earliest stage of the Curtis transgression reworked the substra- tum. The overlying tidally influenced Curtis Formation of Early Oxfordian age (Kreisa and Moiola, 1986; Ca- puto and Pryor, 1991; Wilcox and Currie, 2008; Ogg and others, 2016) was first formally defined by Gilluly and Reeside (1928), and its type section of Curtis Point is located about 5.3 km south of Dry Mesa, along the northeastern margin of the San Rafael Swell (UTM coordinates: 12S 547430/4331169). The formation is characterized by its greenish-whitish color due to the presence of glauconite or chlorite (Gilluly and Reeside, 1928; Caputo and Pryor, 1991; Peterson, 1994). Its strik- ing color contrast to the underlying earthy red Entra- da Sandstone is readily identifiable in outcrops. The typical thickness ranges between 30 and 80 m around the San Rafael Swell (Caputo and Pryor, 1991). Never- theless, as it was deposited in the foredeep basin of the Elko orogeny (Thorman, 2011; Anderson, 2015), the formation pinches out southwards towards Tergeson Flats, about 38 km southwest of Hanksville, as well as eastwards in the vicinity of Duma Point, about 28 km south-southeast of Green River (figure 1) (Gilluly and Reeside, 1928; Caputo and Pryor, 1991; Peterson, 1994). As a note, the coastal paleo-erg of the Moab Member of the Curtis Formation is the lateral equivalent to the marine beds of the Curtis towards the east of the study area (Wright and others, 1962; Caputo and Pryor, 1991; Peterson, 1994; Doelling, 2001). The Summerville Formation conformably over- lies the Curtis Formation in the San Rafael Swell and Henry Mountains basin and is characterized by dark-red and chocolate-brown hypersaline sab- kha deposits, including evaporative ponds, which resulted in precipitation of gypsum and anhydrite (Gilluly and Reeside, 1928; Caputo and Pryor, 1991; Peterson, 1994; Lucas, 2014). Peterson (1994) de- scribed the marine Curtis Formation and the con- formably overlying supra-tidal and sabkha deposits of the Summerville Formation as representing the fifth transgressive-regressive cycle within the Juras- sic System of the Western Interior basin. This cycle potentially corresponds to Haq and others (1987) LZA-2.3 third-order transgressive-regressive inter- val, after calibrating their curve onto Wilcox and Currie (2008) age and Ogg and others (2016) time scale. The Curtis-Summerville interval correlates to the Redwater Shale Member of the Sundance Forma- tion (Imlay 1947, 1980) in Wyoming, the Stump For- mation around the Wyoming-Idaho border (Mans- field and Roundy, 1916; Pipiringos and Imlay, 1979; Imlay, 1980), and the Stump Formation in the Uin- ta Mountains of northeastern Utah (Pipiringos and Imlay, 1979; Imlay, 1980; Wilcox and Currie, 2008), reflecting the same transgressive-regressive period of the Sundance Sea (Pipiringos and O’Sullivan, 1978; McMullen and others, 2014). In the Four Corners area, the Curtis Formation has been correlated to the Todilto Member of the Wanakah Formation, where- as the Summerville Formation is replaced by the Be- clabito Member of the Wanakah Formation (Condon and Huffman, 1988). Note that Anderson and Lucas (1994) used a different nomenclature for the same in- terval; they regarded the Todilto as a formation rather than a member, whereas the Summerville Formation extends into the Four Corners area. The Summerville Formation is capped by the J-5 unconformity (Pip- iringos and O’Sullivan, 1978), which resulted from the fall of the regional base level (Caputo and Pryor, 1991; Peterson, 1994), generating a relief of at least 20 m, before being overlain by the fluvial sediments of the Tidwell Member, the lowermost unit of the continental Morrison Formation south of the Uinta Mountains (Waldschmidt and LeRoy, 1944; Peterson, 1988; Turner and Peterson, 1999). Figure 1C displays a summary stratigraphic column of the study area. 136 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 METHODS AND DATA In order to understand the genesis of the Curtis Formation in the study area, fieldwork campaigns were conducted in 2015, 2016, and 2017. The data includ- ed in this paper comprise (1) 40 detailed sedimenta- ry logs (figure 1), which locations were chosen based on outcrop exposure quality, accessibility, and regular distance between each measured section; (2) pictures taken at and between log-sites by all members of the research group involved in the project, as well as Un- manned Aerial Vehicles (UAVs), aerial photographs, satellite images, and open-source imagery available from Google and Microsoft Bing databases; and (3) re- cordings of paleocurrent directions and other structur- al data. Three-dimensional (3D) virtual outcrops were generated for selected localities following Westoby and others (2012) structure-from-motion photogrammetry principles, in order to assess the architecture of the sed- imentary succession. UAV data were processed using PhotoScan Pro by AgiSoft (Agisoft LLC, St. Petersburg, Russia), whereas the 3D-generated models were subse- quently analyzed and interpreted in Lime (developed by the Virtual Outcrop Geology (VOG) Group from the Universities of Bergen and Aberdeen). Traditional sed- imentologic methods were applied, such as identifica- tion of depositional sub-environments and correlation across short and long distances in order to reconstruct the spatial and temporal distribution of sub-units of the target strata. The resultant assimilation of data allows the construction of an improved depositional model for the Curtis Formation, and sheds light on how sediments are dispersed across a shallow shelf in general. RESULTS Twenty-four sedimentary facies were recognized and summarized in table 1. Rock fabric, composition, and structure(s) are the key to interpreting the process- es and conditions under which these sediments were deposited. As a result, these facies have been organized in eight main facies associations (FA 1 to FA 8) with six sub-facies associations (FA 1a, FA 1b, FA 3a, FA 3b, FA 4a, and FA 4b), which are summarized in table 2 and carefully described below. These facies associations are not homogeneously distributed across the study area (figures 2 and 3). It is important to mention that the datum on which the measured sections are aligned on figure 3 corresponds to the J-3 unconformity. To increase the visibility of the correlation, 19 out of 41 visited localities were select- ed based on spatial distribution and completeness of the sedimentary succession. Pie charts reflect the ratio between the different facies associations present in the Curtis Formation, whereas the Entrada Sandstone and the Summerville Formation are neglected. Based on the spatial distribution of these various facies associations, it is possible to divide the study area into sectors 1, 2 and 3 (figure 3), which are discussed further below. FA 1 – Coastal Wet Eolian Deposits Description This unit corresponds to the cross-stratified eolian dunes and sandy interdunes (Facies A in table 1) of the Slick Rock Member (Entrada Sandstone, FA 1a), which crops out in the eastern and southeastern part of the study area (figure 1). Towards the west, FA 1 coincides with the parallel-laminated to mottled deposits (Facies B) dominating the earthy facies (FA 1b) of the Entrada Sandstone, with interfingering trough cross-stratified sandstone (Facies C), rippled cross-stratified sandstone (Facies U), and isolated coastal eolian dunes (Facies A). The hoodoos of Goblin Valley State Park mainly consist of structureless sandstone (Facies D). This sedimentary package thickens westward. Only at Safari Road (fig- ure 1), FA 1 is capped by a rusty-red, calcite-cemented, thoroughly bioturbated, fine-grained sandstone (Facies E). Facies C (trough cross-stratified sandstone) occurs sporadically within FA 1, reaching a maximum thick- ness of about 1 m and is located 13 m below the base of the Curtis Formation at the Crystal Geyser section (fig- ure 1). Here, the well-sorted foresets alternate between coarse and fine-grained sand with potential double mud drapes. Facies A, Facies B, Facies C, and Facies T are characterized by a sharp contact at their base, which can also be erosive, especially for Facies A, C, and T. Facies T usually appears both at the base and at the top of eolian dune packages (Facies A) but can also crop out 137 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Fa cie s De sc rip tio n St ru ct ur es Gr ai n  Si ze * In te rp re ta tio n Fo rm at io n A Cr os s‐s tra tif ie d  sa nd st on e Un id ire ct io na l ta ng en tia l cr os s‐b ed de d ve ry fin e to fin e‐ gr ai ne d sa nd st on e, al te rn at in g gr ai n flo w an d gr ai n fa ll de po sit s, sh ar p ba se ,r us ty re d or w hi te ,l oc al ly bl ea ch ed , lo ca l oc cu rre nc e of rh izo lit hs , va ry in g be df or m /b ed fo rm se ts siz e, m ax im um in di vid ua ld un e th ick ne ss 15 m .P ot en tia lo cc ur re nc e of co un te r‐r ip pl es at  th e  to e  of  th e  fo re se ts VF  ‐  F Eo lia n  du ne  d ep os its , l oc al ly  in flu en ce d  by  a   dy na m ic  an d  m ig ra tin g  w at er  ta bl e/ sa tu ra te d  le ve l En tra da  Ss . M oa b  M br . B Pl an e  pa ra lle l‐l am in at ed  to  m ot tle d  m ud st on e  w ith  lo ca liz ed  e va po rit es Da rk re d sil ty m ud st on e w ith pa le ye llo w to w hi te ve ry fin e to fin e‐ gr ai ne d sa nd le ns es , pl an e pa ra lle l‐l am in at ed to ‐st ra tif ie d or m ot tle d, po te nt ia l bl ea ch ed pa tc he s ar ou nd rh izo lit hs ,l oc al ize d ev ap or ite ‐ri ch ho riz on s, m ax im um in di vid ua l ho riz on  1  cm Si  ‐ C l Eo lia n  in te rd un e  de po sit s s ho w in g  oc ca sio na l  flo od in g  w ith  d ev el op m en t o f s ab kh a‐ ty pe  d ep os its   an d/ or   s up er fic ia l v eg et at io n En tra da  Ss . Su m m er vil le  Fm . C Tr ou gh  cr os s‐s tra tif ie d  sa nd st on e Tr ou gh cr os s‐s tra tif ie d ve ry fin e to m ed iu m ‐g ra in ed sa nd st on e, po te nt ia l m ud dr ap es an d rip ‐u p m ud cla st s, ev en tu al de sic ca tio n cr ac ks an d/ or ev ap or ite ‐ri ch ho riz on s.  Th ick ne ss  ra ng in g  be tw ee n  dm ‐ t o  m  va lu es VF  ‐  M Ti da lly  in flu en ce d  m ig ra tin g  3D ‐d un es En tra da  Fm . Cu rti s F m . Su m m er vil le  Fm . D St ru ct ur el es s f lu id ize d  sa nd st on e De fo rm ed to st ru ct ur el es sf lu id ize d of gr ee n to pi nk sil tt o fin e‐ gr ai ne d sa nd st on e, lo ca lf lu id ‐e sc ap e an d lo ad in g st ru ct ur es st ill vis ib le ,s om et im es vis ua lly ex pr es se d as w el lr ou nd ed sa nd st on e bo ul de rs w ith in je ct ed m ud st on e, m ax im um bo ul de r di am et er  (Ø ) 2 5  cm ,   m ax im um  b ed  th ick ne ss  2  m Si‐ F De st ru ct io n  of  o rig in al  se di m en ta ry  st ru ct ur es  d ue   to  fl ui ds  fl ow in g  th ro ug h  th e  sa nd st on e  be d  or   th ro ug h  liq ue fa ct io n  of  w at er ‐sa tu ra te d  ho riz on s En tra da  Ss . Cu rti s F m . E Th or ou gh ly  bi ot ur ba te d  co nd en se d  sa nd st on e Ru st y‐ re d co nd en se d, ce m en te d, fin e‐ gr ai ne d sa nd st on e, th or ou gh ly bi ot ur ba te d. , m ax im um  th ick ne ss  2 5  cm F Se di m en t s ta rv at io n  in  a  se m i‐a rid  co as ta l p la in   se tti ng En tra da  Ss . F M at rix ‐su pp or te d  ba sa l c on gl om er at e Ro un de d to w el l‐r ou nd ed ,m at rix ‐su pp or te d ba sa lc on gl om er at e, no pr ef er re d cla st or ie nt at io n bu t th ei r lo ng ax is te nd to be pa ra lle lt o th e be dd in g pl an e, m at rix co ns ist s of fin e‐ to m ed iu m ‐g ra in ed sa nd st on e, m ax im um cla st Ø 8 cm ,m ax im um be d  th ick ne ss  2 0  cm F‐ Pb Fla sh  fl oo d  de po sit s Cu rti s F m . ? G Pl an ar ‐ t o  lo w  a ng le  cr os s‐s tra tif ie d  sa nd st on e Pl an e‐ pa ra lle lt o lo w ‐a ng le cr os s‐s tra tif ie d, ve ry fin e to fin e‐ gr ai ne d, gr ay to gr ee n to w hi te sa nd st on e, po te nt ia lh er rin gb on e cr os s‐l am in at io n, cu rre nt an d os cil la tio n rip pl e‐ la m in at io n, as w el l as dm ‐sc al e so ft‐ se di m en t de fo rm at io n, m ax im um in di vid ua l b ed  th ick ne ss  6 0  cm VF  ‐  F Up pe r s ho re fa ce  to  b ea ch  d ep os its  w ith  ti da l  in flu en ce Cu rti s F m . H Ta ng en tia l c ro ss ‐st ra tif ie d  gr av el ly  sa nd st on e M at rix ‐su pp or te d co ng lo m er at ic du ne , hm ‐sc al e la te ra l ex te nt , su b‐ ho riz on ta l er os ive ba se ,r ip ‐u p m ud cla st s, ex tra ‐b as in al su b‐ to ro un de d cla st s, m ax im um cla st Ø 2. 5 cm , un id ire ct io na lc ur re nt tro ug h cr os s‐s tra tif ica tio n, m ax im um in di vid ua l du ne  th ick ne ss  2 .5  m M  ‐  Gr Hi gh  e ne rg y,  a sy m m et ric  ti da l f lo w  p at te rn  w ith in  a   la te ra lly  re st ric te d  em ba ym en t  Cu rti s F m . I Ti da lly  in flu en ce d  cr os s‐s tra tif ie d  co ng lo m er at ic  sa nd st on e M at rix ‐ to cla st ‐su pp or te d le ns e‐ sh ap ed in tra fo rm at io na l co ng lo m er at e of re st ric te d la te ra le xt en t, lo ca lly de ve lo pe d an d am al ga m at ed in tid al bu nd le s, rip ‐u p m ud cla st s, ex tra ‐b as in al su b‐ to ro un de d cla st s, m ax im um cla st Ø 2. 5 cm , bi di re ct io na lc ro ss ‐st ra tif ica tio n w ith su pe rim po se d cu rre nt rip pl es ,m ax im um be d th ick ne ss  6 0  cm F ‐  G r Hi gh ‐e ne rg y t id al  ch an ne ls– in le ts Cu rti s F m . J Pl an ar  cr os s‐s tra tif ie d  sa nd y  co ng lo m er at e Cl as t‐ to m at rix ‐su pp or te d co ng lo m er at e, hm ‐sc al e la te ra l ex te nt , co nv ex ‐d ow n er os ive ba se ,f la tt op ,e xt ra ‐b as in al su b‐ to ro un de d cla st s, m ax im um cla st Ø 2. 5 cm , pl an ar  cr os s‐s tra tif ica tio n,  m ax im um  in di vid ua l t hi ck ne ss  3 .0  m M  ‐  Gr Po in t b ar  la te ra l a cc re tio n  w ith in  a  m ig ra tin g  tid al   ch an ne l Cu rti s F m . K Pl an e  pa ra lle l‐l am in at ed  m ud ‐ t o  sil ts to ne Pl an e pa ra lle l‐l am in at ed m ud to sil ts to ne , sc at te re d bi di re ct io na l cu rr en t rip pl e cr os s‐s tra tif ica tio ns , gr ay to gr ee n, oc ca sio na l de sic ca tio n cr ac ks , sp or ad ic bi ot ur ba tio ns  b ot h  pa ra lle l a nd  n or m al  to  th e  be dd in g  pl an es Si  ‐ C l Ge nt le  fl ow  a ct ivi ty  w ith  ti da lly  re la te d  cu rre nt   re ve rs al s Cu rti s F m .              * Cl =c la y,  Si =s ilt , V F= ve ry  fi ne , F =f in e,  M =m ed iu m , G r= gr av el , P b= pe bb le ; G ra in  si ze  in  p ar en th es e  de no te s r ar el y   pr es en t a nd  b ra ck et ed  d en ot es  a t t he  b ou nd ar y b et w en  V F a nd  F T ab le 1 . F ac ie s d es cr ip tio n fo r t he E nt ra da S an ds to ne , C ur tis F or m at io n, a nd S um m er vi lle F or m at io n (c on tin ue d on fo llo w in g pa ge ). 138 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Fa cie s De sc rip tio n St ru ct ur es Gr ai n  Si ze * In te rp re ta tio n Fo rm at io n L He te ro lit hi c s ilt ‐ a nd  sa nd st on e  w ith   le nt icu la r b ed di ng Ri pp le d ve ry fin e to fin e‐ gr ai ne d sa nd st on e, gr ay ish le ns es co nt ai ni ng he rr in gb on e an d cu rr en t rip pl e cr os s‐ st ra tif ica tio ns w ith in a m at rix of la m in at ed gr ay to gr ee n m ud ‐ to sil ts to ne s, oc ca sio na l de sic ca tio n cr ac ks , sp or ad ic bi ot ur ba tio ns bo th pa ra lle l a nd  n or m al  to  th e  be dd in g  pl an es Si  ‐  F Cu rr en t r ev er sa ls  in  lo w er  su bt id al  zo ne Cu rti s F m . M He te ro lit hi c s ilt ‐ a nd  sa nd st on e  w ith   w av y  be dd in g Ri pp le cr os s‐ st ra tif ie d ve ry fin e to fin e‐ gr ai ne d gr ay ish sa nd la ye rs , w ith bi ‐ di re ct io na l cu rr en t in di ca to rs an d in te rb ed de d w ith la m in at ed gr ay to gr ee n sil ts to ne ,o cc as io na ld es icc at io n cr ac ks ,s po ra di c bi ot ur ba tio ns bo th pa ra lle la nd no rm al  to  th e  be dd in g  pl an es , v ar yi ng  a m ou nt  o f o rg an ic  m at te r Si  ‐  F Cu rr en t r ev er sa ls  in  su bt id al  zo ne  (s ha llo w er  th an   Fa cie s L ) Cu rti s F m . M oa b  M br . N He te ro lit hi c s an ds to ne  w ith  fl as er   be dd in g Ri pp le an d he rr in gb on e cr os s‐ st ra tif ie d ve ry fin e to fin e‐ gr ai ne d gr ay to gr ee n to w hi te sa nd st on e, sc at te re d m ud le ns es ,a s w el la s sin gl e an d do ub le m ud dr ap es , va ry in g  am ou nt  o f o rg an ic  m at te r VF  ‐  F Up pe r s ub ‐ t o  lo w er  in te rti da l s an dy  fl at Cu rti s F m . O Sa nd st on e  w ith  cl im bi ng  ri pp le s Cl im bi ng  ri pp le  cr os s‐ st ra tif ie d  ve ry  fi ne  to  fi ne ‐g ra in ed  sa nd st on e,  g ra y  to  g re en VF  ‐  F Ti da l c ha nn el  o ve rb an k  sp ill  o n  tid al  fl at , U pp er  su b‐   to  lo w er  in te rti da l s an dy  fl at Cu rti s F m . P Cr os s‐ st ra tif ie d  sa nd st on e  ar ra ng ed  in   w el l‐d ef in ed  rh yt hm ic  tid al  b un dl es Ve ry fin e to fin e‐ gr ai ne d (m ed iu m ‐g ra in ed ra re ly pr es en t) gr ay to gr ee n to w hi te sa nd st on e, ar ra ng ed in tid al bu nd le s, w ith oc ca sio na la nt i‐r ip pl es do cu m en te d fro m th ei rt oe se ts ,v ar yi ng am ou nt of or ga ni cm at te r VF  ‐  F  (‐M ) Ti da l i nl et s,  lo w er  e ne rg y  th an  F ac ie s I Cu rti s F m . Q St ru ct ur el es s s an ds to ne Ve ry fin e to fin e‐ gr ai ne d gr ai ne d gr ay to gr ee n to w hi te sa nd st on e, m as siv e, w ith po te nt ia l sc at te re d sin gl e an d‐ or do ub le m ud dr ap es . Us ua lly ro un de d an d sm oo th ly  w ea th er ed VF  ‐  F Th e  na tu re  o f t he  la ck  o f s tru ct ur e  m ig ht  o nl y  be   du e  to  in te ns iv e  su rfa ce  w ea th er in g.  P re se nc e  of   m ud  d ra pe s i nd ica te  su b‐  o r i nt er tid al  e nv iro nm en t Cu rti s F m . R Th or ou gh ly  b i‐d ire ct io na l r ip pl ed   cr os s‐ st ra tif ie d  sa nd st on e Th or ou gh ly rip pl ed sil tt o ve ry fin e gr ai ne d sa nd st on e, do m in at ed by he rr in gb on e cr os s‐ st ra tif ica tio ns S  ‐ V F De ep  su bt id al  e nv iro nm en t w ith  n ea r e qu al  fl oo d  an d  eb b  tid al  cu rr en t c on di tio ns . N ot e  th at  th e  w ea th er in g  ca n  in  so m e  ca se s e ra se  m os t o f t he   se di m en ta ry  st ru ct ur es Cu rti s F m . M oa b  M br . S Pl an e  pa ra lle l‐s tra tif ie d  sa nd st on e Pl an e pa ra lle l‐s tra tif ie d ve ry fin e to fin e‐ gr ai ne d sa nd st on e w ith sc at te re d cu rr en t rip pl e la m in at io n, w hi te ,p in k or gr ee n. No te th at th e w ea th er in g ex pr es sio n of th is fa cie sv ar ie sb et w ee n th e di ffe re nt un its of th e Cu rti sF m .P ot en tia lm ud cr ac ks an d so ft‐ se di m en t d ef or m at io n (S i‐)  V F  ‐ F Sa nd y  tid al  fl at , u pp er  fl ow  re gi m e  (to  lo w er   an tid un e‐ re gi m e? ).  Do cu m en te d  m ud  cr ac ks   in di ca te  sh or t‐l iv ed  su ba er ia l e xp os ur e Cu rti s F m . M oa b  M br . T Co nd en se d  sa nd st on e  Th in , ye llo w st ru ct ur el es s sa nd st on e, oc ca sio na lly di sp la yi ng lo w ‐a m pl itu de un du la tio ns , ex clu siv el y ob se rv ed ca pp in g th e M oa b M em be r of th e Cu rti s Fm . M ax im um  b ed  th ick ne ss  1 0  cm [V F‐ F]  F Co nd en se d  ho riz on M oa b  M br . U Ri pp le d  cr os s‐ st ra tif ie d  sa nd st on e Un du la te d to rip pl ed cr os s‐ st ra tif ie d, ve ry fin e to fin e‐ gr ai ne d, gr ay to br ow n sa nd st on e, w ith 3D cu rr en t rip pl es , po ss ib le in te rfe re nc e rip pl es , po te nt ia lm ud cr ac ks  a nd  so ft‐ se di m en t d ef or m at io n VF  ‐  F 3D  m ig ra tin g  rip pl es  u nd er  u ni di re ct io na l c ur re nt   co nd iti on s En tra da  S s. Cu rti s F m . M oa b  M br . Su m m er vi lle  F m . V Pl an e  pa ra lle l‐l am in at ed  si lts to ne Da rk re d so ft slo pe ‐fo rm in g sil ts to ne , m os t pr ob ab ly pl an e pa ra lle l‐l am in at ed , sc at te re d pa le w hi te bl ea ch ed le ns es an d ev ap or ite s Si Su pr at id al  p la in Su m m er vi lle  F m . W Iro n  ric h  rip pl e‐  a nd  p ar al le l‐ la m in at ed  sa nd st on e Da rk re d to br ow n ce m en te d ve ry fin e to fin e‐ gr ai ne d sa nd st on e, ge nt le rip pl e cr os s ‐ st ra tif ica tio n,  p ot en tia l d es icc at io n  cr ac ks VF  ‐  F Fl uv ia l o ve rb an k  flo od in g  de po sit s Su m m er vi lle  F m . X Pa le os ol Da rk  p ur pl e  m ud  o r s ilt Cl ‐S i Su b‐ ae ria lly  e xp os ed  su rfa ce  w ith  su pe rfi cia l s oi l  de ve lo pm en t En tra da  S s. Cu rti s F m . M oa b  M br . Su m m er vi lle  F m .              * Cl =c la y,  Si =s ilt , V F= ve ry  fi ne , F =f in e,  M =m ed iu m , G r= gr av el , P b= pe bb le ; G ra in  si ze  in  p ar en th es e  de no te s r ar el y   pr es en t a nd  b ra ck et ed  d en ot es  a t t he  b ou nd ar y b et w en  V F a nd  F T ab le 1 (c on tin ue d fr om p re vi ou s p ag e) . F ac ie s d es cr ip tio n fo r t he E nt ra da S an ds to ne , C ur tis F or m at io n, a nd S um m er vi lle F or m at io n. 139 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 as individual beds within a mottled interval (Facies B). Note that Facies B can locally grade up-section into pa- leosols (Facies X). Bleached patches of rock commonly underlie and overlie paleosols or are found in the direct vicinity of such horizons, contributing to the mottled expression (Blodgett, 1988). The top of FA 1 is capped by regional J-3 unconformity of Pipiringos and O’Sulli- van (1978). Facies  Association Depositional Environment Facies Included Formation FA 1a Coastal wet eolian dune system (Kocurek and Havholm, 1993;  Mountney, 2012), with episodic (marine) partial flooding of  interdunes deposits and superficial development of soil‐ and  vegetated horizons A, C, X Entrada Ss. Slick Rock Mbr. FA 1b Coastal wet eolian interdune system (Kocurek and Havholm,  1993; Mountney, 2012), with episodic (marine) partial  flooding of interdune deposits and superficial development of  soil and vegetated horizons B, C, D, X Entrada Ss. earthy facies FA 2 Beach deposits to upper shoreface deposits, with potential  associated tidal channel cut‐and‐fill C, G, S, U Curtis Fm. FA 3a Subtidal heterolithic mud, silt,  and very fine grained  sandstone, generally coarsening up from laminated  mudstone, wavy bedding, scarsely bioturbated H, I, J, K, L, M Curtis Fm. FA 3b Subtidal heterolithic vf‐ to f‐grained sandstone generally  coarsening up from wavy bedding to flaser bedding, scarsely  bioturbated H, I, M, N Curtis Fm. FA 4a Sandy tidal flat with correlative major tidal channels, having  potential subaerial exposures S, U, X Curtis Fm. FA 4b Tidal channel infills and splays, distal correlative of FA 4a in  the northern areas C, H, I, L, M, N, S,  U Curtis Fm. FA 5 High energy, sub‐ to intertidal sand ‐dominated  environments, encompassing tidal flats, tidal channels, and  beaches C, G, (K, L, M,) N,  O, P, Q, R, S(, X) Curtis Fm. FA 6 Upper intertidal heterolithic channels and flats complex,  fining up, with intermittent prolonged subaerial exposures  and rare bioturbation, indicator of a more stressed  environment than FA 3 K, L, M, N, Q, S, U,  X Curtis Fm. FA 7 Coastal dry eolian dune field (Mountney, 2012), arranged in  four to five sequences separated by supersurfaces, upon  which transgressive water‐carried sediments and/or paleosol  can be observed A, N, Q, S, T, U, X Curtis Fm. Moab Mbr. FA 8 Supratidal lower coastal plain, with episodic marine flooding U, V, W, X Summerville Fm. Table 2. Facies associations for the Entrada Sandstone, Curtis Formation, and Summerville Formation. 140 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 v v v v v v v v v v v v v v v v v v v 35 30 2545 40 20 15 10 5 C LA Y Si V F F M SA N D 35 30 25 20 15 10 5 C LA Y Si V F F M SA N D 45 40 35 30 25 20 15 10 5 C LA Y Si V F F M C V C G SA N D C LA Y Si V F F M C SA N D 80 75 70 6595 90 85 60 55 50 45 40 35 30 25 20 15 10 5 C LA Y Si V F F M C SA N D 80 75 70 6595 90 85 60 55 50 45 40 35 30 25 20 15 10 5 C LA Y Si V F F M 45 40 35 30 25 20 15 10 5 35 30 254550 40 20 15 10 5 C LA Y Si V F F M SA N D v v v v v v v v v v v v v v v v v v v 65 60 55 50 45 40 35 30 25 20 15 10 5 C LA Y Si V F F M SA N D 24 19 1 24 19 1 95 25 km N U TA H 10 0 km N CL AY Si VF F M C VC G PE B. Su pr at id al m ud � at Su pr at id al sa nd � at Sc he m at ic lo g Ar ch ite ct ur al e le m en ts Sa nd -ri ch su b- to su pr at id al � at a nd co rre la tiv e tid al ch an ne l i n� l Ti da l b un dl es Su bt id al sa nd -d om in at ed h et er ol ith ic � at Up pe r s ub - t o in te rt id al sa nd -d om in at ed he te ro lit hi c c ha nn el -� at co m pl ex Up pe r s ub - t o in te rt id al m ud -d om in at ed he te ro lit hi c � at Su bt id al co ng lo m er at ic d un es Su bt id al m ud -d om in at ed h et er ol ith ic � at Su b- to in te rt id al ch an ne l-d un e- �a t c om pl ex Co as ta l i nt er du ne s Co as ta l d un es Up pe r s ho re fa ce to b ea ch d ep os its Co as ta l d un es , b le ac he d FA 1 a FA 1 b FA 2 FA 3 a FA 3 b FA 4 b FA 4 a FA 5 FA 6 FA 8 Pr ox im al D is ta l FA 7 CL AY Si VF F upper C. lower C.middle C. J- 3 M TS Su lp hu r C an yo n m id dl e C ur tis lo w er C ur tis up pe r C ur tis lo w er C ur tis Su m m er vi lle Fo rm at io n En tra da Sa nd st on e ea rth y fa ci es En tra da Sa nd st on e Sl ic k R oc k M b. N ev er sw ea t W as h Sv en ’s G ul ch In te rs ta te 7 0 H an ks vi lle N ot om R an ch Bi g Pi nt o M es a G ob lin V al le y 11 .5 k m N or th So ut hw es t Ea st 24 k m 15 .5 k m 48 k m 20 .5 k m 40 k m 17 5 km J- 3 un co n. M TS Fa ci es A ss oc ia tio ns FA 8 S up ra tid al � at FA 7 C oa st al d ry e ol ia n du ne s i M oa b M em be r FA 4 S an d- ric h su b- to su pr at id al � at a nd co rre la tiv e tid al ch an ne l i n� ll FA 3 b Su bt id al sa nd -d om in at ed he te ro lit hi c � at FA 6 U pp er su b- to in te rt id al h et er ol ith ic � at FA 3 a Su bt id al m ud -d om in at ed he te ro lit hi c � at FA 5 S ub - t o in te rt id al ch an ne l-d un e- � at co m pl ex FA 1 b Ea rt hy fa ci es FA 1 a Co as ta l d un es a nd S lic k Ro ck M em be r FA 2 U pp er sh or ef ac e to b ea ch de po sit s �g ur e 4c , d Facies associations Height [m] Paleocurrent Fi gu re 2 . N or th -s ou th w es t- ea st c ro ss se ct io n ac ro ss th e st ud y ar ea sh ow in g th e sp at ia l d ist rib ut io n of th e m ai n fa ci es a ss oc ia tio ns id en tifi ed w ith in th e up pe rm os t s tr at a of th e En tr ad a Sa nd st on e, th e Cu rt is Fo rm at io n an d th e lo w er m os t s tr at a of th e Su m m er vi lle F or m at io n al on g th e w es te rn m ar gi n of th e Sa n Ra fa el S w el l a nd s ou th to th e H en ry M ou nt ai ns . Th e Bi g Pi nt o M es a m ea su re d se ct io n re pr es en ts th e ty pi ca l e xp re ss io n of th e M oa b M em be r o f t he C ur tis F or m at io n. D et ai le d se di m en ta ry in fo rm at io n on th e va rio us a rc hi te ct ur al e le m en ts p re se nt a t t he se le ct ed lo ca lit ie s i s di sp la ye d on th e m ea su re d se ct io ns , w he re as fa ci es as so ci at io ns ar e c ol or -c od ed in ea ch su m m ar y co lu m n ne xt to th ei r r es pe ct iv e m ea su re d se ct io n. 141 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 10 m 24 19 1 24 19 1 95 25 km N U TA H 10 0 km N 0. 1 % F A 6 SC N SW SG I7 0 G V H K N R LS D LC M R S& C SW VA LC D LF TLS RR D P TM R N o D at a N o D at a N o D at a N o D at a N o D at a SR BP M LS C SC N SW SG I7 0 G V H K N R LC D LC M R S& C SW VA LS D LF T LS RR D PTM R SR BP M LS C SCN SW SGI7 0 G V H K N R LS D LC M R S& C SW VA LC D LF T LS RR D P TM R SRBP M LS C Su lp hu r C an yo n N ev er sw ea t W as h Sv en ’s G ul ch In te rs ta te 7 0 G ob lin V al le y H an ks vi lle N ot om R an ch Lo w er S ou th D es er t O ve rlo ok Lo w er C ed ar M ou nt ai n Ro ad Si d & C ha rle y Sa lt W as h Vi ew A re a La st C ha nc e D es er t Li tt le F la t T op Lo w er S an R af ae l R oa d D um a Po in t Te n M ile R oa d Sa fa ri Ro ad Bi g Pi nt o M es a Lo st S pr in g Ca ny on 38 .2 3 m 62 .0 0 m 80 .5 0 m 38 .7 5 m 26 .0 0 m 41 .5 5 m 33 .6 5 m 29 .0 0 m 63 .2 5 m 34 .4 0 m 56 .4 0 m 67 .0 5 m 35 .8 0 m 13 .6 0 m 01 .3 0 m 14 .2 5 m 28 .6 0 m 65 .0 5 m Co de Se ct io n N am e Cu rt is F m . le ng th 40 .1 0 m FA 8 S up ra tid al fl at FA 7 C oa st al d ry e ol ia n du ne s i m M oa b M em be r Fa ci es A ss oc ia ti on s: FA 4 S an d- ric h su b- to s up ra tid al fl at a nd iiic or re la tiv e tid al c ha nn el in fill FA 3 b Su bt id al s an d- do m in at ed h et er ol ith ic fla t FA 6 U pp er s ub - t o in te rti da l h et er ol ith ic fla t FA 3 a Su bt id al m ud -d om in at ed h et er ol ith ic fla t FA 5 S ub - t o in te rti da l c ha nn el -d un e- fla t c om pl ex FA 2 U pp er s ho re fa ce to b ea ch d ep os its FA 1 b Ea rth y fa cie s FA 1 a Co as ta l d un es a nd S lic k Ro ck M em be r Cu rti s Fo rm at io n upper C. lower C.middle C. En tra da Sa nd st on e J- 3 un co nf or m ity Lo we r O xf or di an Upper Jurassic Middle Up pe r Ca llo via n A B Se ct or 1 - Lo w er , M id dl e an d U pp er C ur tis Se ct or 2 - M id dl e an d U pp er C ur tis Se ct or 3 - M oa b M em be r FA 2 -3 -4 D at a: 1 32 N FA 5 D at a: 1 69 N FA 6 -7 D at a: 2 2 N Fi gu re 3 . ( A ) 3 D c or re la tio n be tw ee n se le ct ed lo ca lit ie s. A s th e se di m en ta ry s ec tio ns a re a lig ne d on th e J- 3 un co nf or m ity (r ed li ne ), th e En tr ad a Sa nd st on e ap pe ar s b el ow th e m ap , w he re as th e Cu rt is Fo rm at io n re m ai ns a bo ve it . P al eo cu rr en t m ea su re m en ts a re a rr an ge d st ra tig ra ph ic al ly fr om bo tto m to to p. (B ) P ie ch ar ts re pr es en tin g th e r at io b et w ee n th e d iff er en t f ac ie s a ss oc ia tio ns b el on gi ng to th e C ur tis F or m at io n at ea ch lo ca lit ie s. Se e te xt fo r d isc us sio n of u nu su al p at te rn in L itt le F la t T op (L FT ). 142 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 On a structural geology note, numerous conjugat- ed extensional faults, associated fractures, and remobi- lized, injected and disintegrated sand (Facies D), as well as hydroplastic deformation occur in the upper Entrada Sandstone in the Humbug Flats area, at Smith’s Cabin in the north, farther south between Interstate 70 and Uneva Mine Canyon, and north of Hanksville Airport (figure 1). Fractures typically feature a bleached front in their direct vicinity (figure 4A). Fault planes are mostly planar; however, growth-fault geometry on south-fac- ing faults is found near Smith’s Cabin. Faults show me- ter-scale offset in the earthy facies of the Entrada Sand- stone, whereas most of them are concealed by the base of the Curtis Formation, which remains undisturbed. Faults strike east-west around the Humbug Flats-Smith’s Cabin area in the north; whereas south of Interstate 70 they strike north-south. Note that bleaching is common along fractures and faults within FA 1 (figure 4a). How- ever, 2.1 km north of Hanksville Airport, the earthy fa- cies (FA 1b), was impacted by syn- to post-depositional localized extensional and contractional faults, as well as synchronous erosion, generating a 2 to 3 m relief at the top of the Entrada Sandstone (figure 4F). These fault clusters display a semi-circular surface expression. The heterolithic deposits of lower Curtis (FA 3-4) passively filled the preexisting topography, before being rapidly overlain by the middle Curtis sediments (FA 5), which abruptly collapsed by faulting while or shortly after being deposited. It remains unclear how precisely and when these processes jolted the deposits of the middle Curtis. Interpretation Interbedding of eolian dune, interdune, sabkha, and tidal deposits are typical features for coastal wet eolian desert environments, as interpreted and described by several authors (Crabaugh and Kocurek, 1993; Kocurek and Havholm, 1993; Mountney, 2012). Occurrence of marginal marine sandstones, gypsum-rich beds, and pa- leosols horizons within an interval dominated by Facies B are related to partial marine flooding and/or relative water table rises within the sediments (Carr-Crabaugh and Kocurek, 1998; Mountney, 2006). On the contrary, intervals dominated by eolian dunes reflect short-lived relative base level falls (Mountney, 2006, 2012). Mottling is ascribed to forced disturbance from roots and has been enhanced by circulation of organic acids through the tight mudstones (Blodgett, 1988), most probably flowing along root burrows. Also, the development of the condensed and bioturbated sandstone bed (Facies E) at the top of FA 1 indicates an extended period of sediment starvation (Urash and Savrda, 2017) around the area known today as Safari Road (figure 1). Conse- quently, FA 1 is considered to represent a coastal eolian system, where marine processes and water table varia- tions jointly and increasingly influenced the sedimen- tary development of the Entrada Sandstone. Bleached fronts observed along fractures in the earthy facies oc- curred as a consequence of post-depositional reducing fluid circulation within a naturally CO2-charged system (Kampman and others, 2013; Ogata and others, 2014; Skurtveit and others, 2017; Sundal and others, 2017). As conjugated fault sets mostly offset the strata of the earthy facies, consequent extensional faulting appears to have occurred post-earthy facies deposition, but pre-Curtis sea transgression. The occurrence of disturbed earthy facies layers, as well as lower and middle Curtis stra- ta 2.1 km north of Hanksville Airport implies that the lowermost Late Jurassic strata responded to episodes of sand mobility which impacted the surface morphology. FA 2 – Beach to Upper Shoreface Deposits Description FA 2 is recorded at Curtis Point, Sven’s Gulch, In- terstate 70, and Uneva Mine Canyon measured sections (figure 1). It reaches a maximum thickness of about 1.70 m at Sven’s Gulch. FA 2 shows a clear upward-fin- ing trend, where the lowermost plane parallel-strat- ified sandstone of Facies S and planar to low-angle cross-stratified sandstone of Facies G are overlain by the trough cross-stratified sandstone of Facies C and/or rip- ple-laminated sandstone of Facies U. Mud drapes and rip-up clasts are also documented at Interstate 70 and Uneva Mine Canyon. FA 2 overlies FA 1, from which it is separated by the J-3 unconformity, locally displaying load structures (figure 5). The transition between FA 2 and the overlying FA 3 occurs either as a gradual and 143 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Figure 4. Caption on the following page. 144 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 fining-upward changeover or corresponds to a sharp and erosive contact (figures 4a and 5). Note that the lat- eral extent of FA 2 reaches 200 to 500 m, and at the mea- sured section Interstate 70, FA 2 is arranged in laterally accreting sandstone bodies, interbedded with FA 3 finer heterolithic material. Interpretation The high sand content of these rocks combined with the extremely low mud content indicate a high-energy marine environment. The deformation of the J-3 un- conformity and the underlying FA 1b by loading sug- gests a poorly lithified Entrada Sandstone at the time of deposition (Owen and others, 2011). The occurrence of oscillation ripple lamination, as well as planar to low-angle cross-stratified sedimentary structures are clear indicators of upper shoreface to beach deposits. The occurrence of rip-up clasts and mud drapes testi- fies of secondary tidal action over the system. The over- all fining-upward trend from FA 2 into FA 3 indicates a gradual transgression of the Curtis sea over the J-3 unconformity, whereas the restricted character of FA 2 suggests a direct influence of the pre-existing erosion- al relief over the distribution of these marginal marine deposits. They represent the onset of marine deposition into an erosional topography, and thus initially filled the available accommodation in the dips and furrows. FA 2 also displays the same deepening-upward development at Sven’s Gulch. However, its base is characterized by a short-lived shallowing-upward event, as recorded by the basal oscillation ripple-laminated sandstone of Fa- cies U (ripple-laminated sandstone). It is then overlain by the 3D migrating dunes of Facies C before grading into the high-energy deposits of Facies S (plane-par- allel stratified sandstone) and G (planar to low-angle cross-stratified sandstone). It subsequently follows the same deepening-upward pattern as mentioned above. The fact that these marginal marine-beach deposits are only documented from the measured sections south of Curtis Point (figure 1) suggests a flooding of the south- ern areas from the northeast, before expanding towards the northern higher grounds. FA 3 – Heterolithic Subtidal Flat and FA 4 – Sand-Rich Sub- to Supratidal Flat and Correlative Tidal Channel Infill Description FA 3 (figure 4) displays the lowermost sedimen- tary package, which represents the Gilluly and Ree- side (1928) type section of the Curtis Formation and crops out mostly in sector 1, as well as Little Flat Top and Hanksville (figure 3). It is separated into a lower mud-dominated interval (FA 3a), which grades into an Figure 4 (figure on the previous page). Overview of the lower Curtis in sector 1. See figure 3 for sectors, and figure 1 for pho- tograph locations. (A) The beach to upper shoreface deposits of FA 2 overlie the earthy facies of Entrada Sandstone (FA 1b) at Sven’s Gulch. FA 2 is truncated at its top by a Regressive Surface a Marine Erosion (RSME), corresponding to the base of FA 3b (Sand-Dominated Subtidal Heterolithic Flat). Note the plume geometry of the bleached zones below the J-3 unconfor- mity, suggesting a trapping of the reducing fluids below the sand of FA 2 as they circulate along fractures within the Entrada Sandstone (white arrows; Skurtveit and others, 2017, Sundal and others, 2017). (B) The three parasequences occurring in the lower Curtis, as observed at the Sid and Charley sections on the western margin of the San Rafael Swell. Note the occurrence of two small tidal channels (white surfaces) at the base of RSME 1 (FS: Flooding Surface). The lower Curtis is capped the a Major Transgressive Surface (MTS), which can be traced across the study area. (C and D). Major tidal incision observed at Sven’s Gulch, carved during a short-lived regressive phase within Parasequence 2. The red arrow points at a boulder of Entrada Sandstone within a matrix of Curtis Formation, indicating that the Entrada was poorly lithified when incised. The presence of this boulder, as well as FA 3 cannibalising its substratum, show that this depression was actually carved into the Entrada Sandstone by tidal currents, rather than being a pre-existing relief subsequently filled by the Curtis Formation. Note also the bi-truncation of Parasequence 2 during the early transgression of Parasequence 3, followed by the cannibalisation by FA 4b of its substratum during a short-lived regressive phase within Parasequence 3. (E) Display of two incision phases within the FA 4b deposits of Parasequence 3. (F) Collapse structure complex linked to sand mobility in the lower and middle Curtis cropping out 2.1 km north of Hanksville Airport. White lines indicate normal faults, whereas the black lines highlight contractional structures. 145 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 upper sand-dominated association (FA 3b) (table 2). FA 3a and FA 3b dark-green to gray color makes it readily identifiable in the field, where their combined thickness can vary significantly over a few hundred meters, rang- ing from less than a meter to about 30 m. FA 3 (dis)con- formably overlies or onlaps the J-3 unconformity which caps FA 1 notably in the northern part of the study area (figure 1). It can also overlie FA 2 from which it fines upward or erodes into (figures 4 and 5). FA 3 displays at least two major coarsening-upward parasequences (Parasequence 2 and Parasequence 3) (sensu Catunea- nu and others, 2009). A third parasequence (Parase- quence 1) has been observed at Sven’s Gulch and the Sid and Charley section (figure 1). Both Parasequence 2 and Parasequence 3 comprise a suite of sedimentary facies commonly associated with tidal deposits: FA 3a includes laminated mudstone (Facies K), lenticular (Fa- cies L), and wavy bedding (Facies M), whereas FA 3b comprises wavy bedding (Facies M) and flaser bedding (Facies N), with the presence of straight, sinuous-crest- ed, and linguoid current ripples, as well as herringbone cross-lamination. Sub-vertical and sub-horizontal bio- turbations are recorded in FA 3a and FA 3b; the degree of disturbance varies, but remains limited, and corre- sponds to Droser and Bottjer’s (1986) ichnofabric in- dex n°3. Furthermore, their diversity is circumscribed to only a few types, such as Thalassinoides, Cruziana, or Gyrochorte comosa ichnofossils, similarly reported from the Carmel Formation (De Gibert and Ekdale, 1999). FA 3 increases in grain size towards the south, with a gen- erally higher sand-to-mud ratio on the western margin of the study area. Its upper boundary is truncated by FA 5, which can, however, be locally conformable. A significant tidal incision can be observed at Sven’s Gulch, carving about 15 m into its substratum (FA 1b and FA 2) and reaching a width of about 60 m (figures 4C and 4D). The infill of that incision shows an intricate architecture, which mostly consists of FA 3b material, as well as one boulder of earthy facies (FA 1b). Meter-scale, runnel-shaped gravelly bodies (Facies I) are also docu- mented at all locations north of Sven’s Gulch, displaying evidence of tidal reworking within a heterolithic envi- ronment. The area north of Middle Canyon (figure 1) is marked by the occurrence of laterally extensive grav- el-rich compound dunes, displaying basinward-dip- ping, meter-thick tangential cross-stratification, with a non-erosive bedding-parallel base and a concave-up top surface (Facies H) (figure 6), and are usually found in the lowermost meters of the Curtis Formation. Con- glomeratic dunes are not to be mistaken with the later- ally accreting conglomeratic tidal channels, which are characterized by planar to tangential cross-stratification and a concave-down erosive base (Facies I and J). Also, at the measured section just south of Interstate 70, FA 2 is overlain by a 1-m-thick greenish-colored sandstone, Figure 5. Transition from the Entrada Sandstone into the lower Curtis as observed at Sven’s Gulch. FA 2 (beach to upper shoreface deposits) overlies the earthy facies of the Entrada Sandstone (FA 1b), from which it is separated by the J-3 unconformity. Note the presence of load casts at the base of FA 2 at some localities. FA 2 rapidly grades into FA 3a (mud-dominated heterolithic subtidal flat), which can be cannibalised by the sand-dominated subtidal deposits of FA 3b (RSME: Regressive Surface of Marine Erosion). 146 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Figure 6. (A) Outline of a westward laterally migrating tidal channel observed in the cliffs of Cedar Mountain, filled by the conglomeratic sandstone of Facies I. The transition from a cross-stratified conglomerate into a more parallel-bedded conglomerate reflects a change in the channel orientation, as the cross-stratified deposits correspond to a transversal cross section of the channel, whereas the parallel-bedded part of the channel represents a longitudinal cross section through the same channel. (B) Transversal section across a north-westward migrating conglomeratic dune (Facies H, table 1) in the cliffs of Last Chance Desert. This dune was influenced by tidal processes, as illustrated by the bundle-like regular and rhythmic thickness variations observed between the foresets. The biggest discrepancy between the tidal channel and the tidal dune resides within their respective base. The tidal channel displays a concave-down erosive base and a flat upper surface, whereas the dune is characterised by a non-erosive and bedding-parallel base and a convex upper boundary. 147 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 completely disturbed by processes related to liquefac- tion and water escape (Facies D), which has not been observed anywhere else in the study area. The road cut section measured at Last Chance Desert (figure 1) also displays a unique feature which has not been docu- mented anywhere else: a 20-cm-thick, matrix-support- ed basal conglomerate, with randomly oriented, round- ed to well-rounded extra-basinal clasts that are about 8 cm in diameter (Facies F). The outcrop exposure of this conglomeratic bed limits the exact measurement of its lateral extent, but reaches a minimum of 70 m. FA 4a conformably lies within and must interfinger with FA 3a and FA 3b within Parasequence 2. It rep- resents 30% to 50% of the Curtis Formation in Hanks- ville and Little Flat Top, but it has not been observed at the locations north of Smith’s Cabin nor on the western margin of the San Rafael Swell (figures 1, 2, 3, and 7). It is characterized by its dominant light-pink, plane par- allel-stratified or structureless, very fine to fine-grained sandstone (Facies S) and subordinate unidirectional straight-crested 2D and 3D current ripple-dominated intervals (Facies U), as well as centimeter-thick marine mudstone. Episodes of subaerial exposure are record- ed as dark-purple paleosol horizons (Facies X, figure 7) or desiccation cracks. Each individual bed measures as much as 1.50 m thick and can be laterally traced over several kilometers. Although appearing isopachous at outcrop scale, its thickness ranges from about 15 m at Rabbit Gulch to 2 m at Uneva Mine Canyon (figure 1), and it reaches a maximum of about 20 m thick at Little Flat Top (figure 1). About 250 m south of the Interstate 70 measured section, FA 4a forms a convex-down, flat- topped feature with sand-dominated heterolithic beds that thicken towards the center. It measures 6.25 m thick and about 45 m wide. At first glance, the architec- ture and shape resemble a channel infill succession, but it lacks any erosional scour at its base or internally. It only thickens in its central part due to differential load- ing (figure 7D). FA 4b has exclusively been document- ed in the Parasequence 3 interval at Sven’s Gulch (fig- ure 1), where the measured section traces a 15 m thick succession between two tidal channels that incise their FA 3 substratum by as much as about 10 m (figures 4C, 4D, and 4E). Note that at least two episodes of incision might have occurred during the deposition of FA 4b, as visible in Sven’s Gulch. The first incision carves the deepest into the FA 3a-3b deposits, whereas the second incision is shallower. The channel infills are dominated by Facies H, I, N, C, S, and U, with a very high sand- to-mud ratio. No evidence of subaerial exposure is re- corded in FA 4b. The channel margins contain Facies H and Facies I, which interfinger with Facies E, F, M, P, and Q. Individual beds are >1 m thick. FA 2, FA 3, and FA 4 are dominated by a north to north-northeast paleocurrent direction, with some northeast flows and with a subordinate and opposing south-southwest com- ponent (figure 3). Note the strong underrepresentation of west-southwest to southwest flow indicators (figure 3). Interpretation The heterolithic nature of FA 3 indicates a fluctu- ating energy level within the system (Kvale, 2012). Straight-crested to linguoid ripple marks suggest dif- ferent durations of flow events, as equilibrium linguoid morphology requires more time to form (Baas, 1999). The presence of Parasequence 0 exclusively at Sven’s Gulch suggests that this area was the first part of the system to be flooded. The base of each parasequence is marked by a flooding surface and the development of FA 3a, followed by the coarsening-upward tidally influ- enced sediments of FA 3b. This is envisioned as a result of increased energy within the system due to shallower water and an increasingly proximal subtidal to tidal flat setting. The same increased energy trend is also indicat- ed by the coarsening southward of FA 3, which suggests shallower water depth towards the present-day location of Hanksville. Whereas several flooding surfaces may be identified in outcrops, their exact correlation be- tween the different measured sections is impossible due to the extreme dynamic nature of tidal environments, and because these flooding surfaces might stem from local variations in relative sea level such as avulsions, rather than regional signals. When traced laterally, the infill of the steep and deep incision observed at Sven’s Gulch (figures 4C and 4D) belongs to Parasequence 2. The occurrence of an Entrada Sandstone boulder within a matrix of Curtis Formation indicates that the Entrada Sandstone was 148 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 poorly lithified as the tidal currents funneled into it, highlighting the poly-nature of the J-3 unconformity. Both the presence of this boulder and a FA 3 cannibal- izing its substratum show that this depression was actu- ally carved into the Entrada Sandstone by tidal currents during a short-lived regressive phase within Parase- quence 2, rather being a pre-existing negative relief sub- sequently and passively filled by the Curtis Formation. The limited bioturbation degree and diversity in- dicate stressed environmental conditions within a re- stricted, tidally influenced marginal-marine tidal flat setting, potentially indicating hypoxic conditions as well as salinity values superior to normal marine standards (Middleton, 1991; Nio and Yang, 1991; De Gibert and Figure 7. Overview of FA 4a. (A) Picture of Little Flat Top, where the light pink, isopachous sand-rich sub- to supratidal flat deposits of FA 4a overlie the earthy facies of the Entrada Sandstone (FA 1b). The exact location of the J-3 unconformity at that location remains uncertain due to the lack of an erosive, or flooding surface between these two facies associations, uncertainty reinforced by an unusual gradual color change between FA 1b and FA 4a. The black arrows indicate paleosol horizons which could potentially represent the lithostratigraphical boundary between the two formations. FA 4a is capped by the Major Transgressive Surface (MTS), and subsequently overlain by the sub- to intertidal deposits of FA 6, whereas the FA 5 is absent. (B) Double mud drape indicated by the white arrows, suggesting occasional subtidal depositional conditions. (C) Scattered unidirectional current ripple within a fine-grained sandstone dominated by upper-flow regime plane paral- lel-stratifications, which, together with the near-lack of clay material, suggest higher energy conditions in FA 4a with respect to FA 3. (D) Mini sag basin formed by the collapse of FA 3b deposits while being filled by the sandstone of FA 4a, just south of Interstate 70, near where the highway cuts through the eastern flank of the San Rafael Swell. 149 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Ekdale, 1999; Fan, 2012; Hughes, 2012; Daidu, 2013). The laterally extensive compound conglomeratic dunes of Facies H in the northern area require high energy within the system at time of deposition. This succession shows strong similarities with the modern submarine dune field in laterally restricted San Francisco Bay, es- pecially regarding the protracted extent of these fea- tures and their unidirectional, basinward development, suggesting a dominant ebb tide and subordinate flood tide as advocated by Barnard and others (2006). These conglomeratic dunes coexist with laterally migrating tidal channels (Facies J). The occurrence of cross-strat- ified conglomeratic lenses with interfingered green- ish-colored silt and mud implies significant variation and asymmetry in current velocities, at least locally. The dominant current will mobilize gravel and coarse sand, forming lens-shaped dunes, locally amalgamated with- in a tidal channel, whereas the subordinate current will cut the bedform, generating reactivation surfaces, and potentially develop counter ripples and small dunes. The replacement of the greenish-colored tidally in- fluenced deposits of FA 3 by the pinkish-colored rela- tively well-sorted plane parallel-stratified sediments of FA 4a suggests (1) higher energy and more stable current conditions within the system compared to FA 3, (2) a more oxygenated water column, oxidizing the iron present in the sediments, (3) a potential change in sediment sourcing, now originating from more homo- geneous very fine to fine-grained sand-rich continental strata with a potential higher K-feldspar content, and (4) a short-lived relative sea-level drop with subaerial exposure. The channels of FA 4b are regarded as distal subtidal channels, due to the lack of intertidal indica- tors and the absence of any evidence of subaerial ex- posure, as well as the interfingering with the subtidal deposits of FA 3a and FA 3b. They are also regarded as the distal time correlative unit of the more proximal FA 4a. Also, the two incision events of FA 4b documented at Sven’s Gulch (figure 4) might relate to the short lived relative sea-level fall and subsequent subaerial exposure episodes of FA 4a. The fact that the first incision carves deeper into the deposits of FA 3 implies a more import- ant relative sea-level fall during the first episode of inci- sion than during the second incision. FA 4 architecture and paleocurrent directions suggest a dominant basin- ward ebb-flow direction and a subordinate flood-tide in the lower part of the Curtis Formation. Interestingly, the basal conglomerate documented at Last Chance Desert (figure 1, Facies F) is interpret- ed as flash-flood deposits resting directly on Pipiringos and O’Sullivan’s (1978) J-3 unconformity, which implies that these deposits are older than the Curtis Formation as defined by Gilluly and Reeside (1928), yet younger than the Entrada Sandstone. FA 5 – Sub- to Intertidal Channel-Dune-Flat Complex Description FA 5 is present in every locality in this study, with the notable exception of Little Flat Top (figures 1 and 3). In comparison with the underlying FA 3 and 4 deposits, the sand-dominated interval corresponding to FA 5 is relatively homogeneous (figure 8). FA 5 is easily identi- fiable in the field by its light green-white color, as well as its polished weathering appearance. It generally overlies FA 3 or FA 4, but between the measured sections Inter- state 70 and Uneva Mine Canyon (figure 1), FA 5 caps FA 2 and FA 1 at a noticeable angle (figures 8F and 8G). FA 5 rests directly on FA 1 in sectors 2 and 3, with the exception of the Hanksville section, and is absent from Little Flat Top (figures 1, 2, and 3). The thickness and, to a lesser extent, the stacking architecture of this sed- imentary unit strongly vary between studied sections, nevertheless displaying a southward-thinning trend, reaching more than 45 m thick at Stove Gulch, about 15 m thick at Interstate 70, and only 4.60 m thick at Hanksville (figures 1, 2, and 3). It also thins towards the east, measuring only about 1.30 m thick at Duma Point and about 0.8 cm thick at Crystal Geyser, but never ex- ceeding 3.80 m thick in sector 3 (figures 1 and 3). Its lower sharp contact is either conformable with the un- derlying strata, onlapping its substratum, or erosive in nature, and corresponds to the MTS. FA 5 is dominated by sand-rich facies, featuring sedimentary structures such as flaser bedding (Facies N), climbing ripples (Fa- cies O), or thoroughly rippled intervals, dominated by herringbone cross-stratification (Facies R). Some bed- ding surfaces display interference ripple marks that are arranged in a nearly orthogonal pattern. Fine-grained 150 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Figure 8. Caption on the following page. 151 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 material arranged in laminated mudstone to lenticular to wavy bedding is recorded at Smith’s Cabin (Facies K, L, M, figure 1). Other sedimentary structures and type of architecture can be observed, such as (1) locally amalgamated cm- to m-scale tidal bundles with varying amount of organic matter captured within their toesets (Facies P), (2) 3D dunes cut by several reactivation sur- faces on top of which counter ripples can sometimes be seen (Facies C), and (3) plane parallel-stratified inter- vals (Facies S). These facies interfinger with one anoth- er over about 5 to 40 m. Both single and double mud drapes as well as desiccation cracks occur within several horizons included in FA 5. The fact that three shallow core-drilling attempts through these sandstone beds failed at providing any usable cored plugs suggests the unit is poorly consol- idated, at least through its first 20 cm, but the degree of consolidation varies significantly over tens of me- ters. Weathering can obscure sedimentary structures, which results in a structureless appearance (Facies Q). The stacking pattern of these different facies is extreme- ly intricate, and each locality has subtly unique archi- tecture. Therefore, the internal complexities of FA 5, coupled with distance between each location, render the detailed correlation between the measured sections very challenging and incompletely constrained. Note that the thickness, size, and wavelength of tidal struc- tures diminish up-section, whereas the lower half of FA 5 displays a relatively constant scale of sedimentary architecture. Towards the east, in sector 3, FA 5 is dom- inated by rippled cross-laminated to undulated beds, with scattered structureless intervals. Current data are shifted in comparison with the underlying units, from a north-dominated to a more symmetrical northwest to southeast trend (figure 3). However, FA 5 shows sim- ilarities with the underlying facies associations, as re- cordings of a west-southwest to southwest flow direc- tion are rare. Interpretation The overall high sand-to-mud ratio within FA 5 in- dicates an elevated energy level within the depositional environment, in comparison with the underlying units. However, this energy level fluctuated through space and time, as testified by the nature and the varying scale of the documented sedimentary structures, as well as by the intricate lateral and vertical interfingering and stacking pattern of the different facies. All of these fea- tures display the effect of individual and multiple tide cycles over a tidal flat (Facies C, L, M, N, O, and R), as well as neap-spring tidal cyclicity within a tidal channel (Facies P) (Kreisa and Moiola, 1986; Middleton, 1991; Nio and Yang, 1991; Fan, 2012; Hughes, 2012; Daidu, 2013). Ephemeral episodes of subaerial exposure oc- curred, as evident by desiccation cracks and, to a lesser extent, single intertidal mud drapes, typical for inter- tidal zones. Tidal channels are present, but never quite reach the size of the tidal channels observed in the un- derlying FA 3 as their lateral extent remains on the order of the decameter (dkm) with well-developed laterally accreting architecture (Facies I, figure 8E). The occur- rence of other tidal channels is indicated by the pres- ence of herringbone cross-stratification and sigmoidal tidal bundles (Kreisa and Moiola, 1986; Hughes, 2012). Considering this coastal setting, a highly various and undulating coastline is suggested, which Caputo and Figure 8 (figure on the previous page). Overview of FA 5 (middle Curtis). (A) Bidirectional tidal inlets (red and blue con- tours), and a third south-westward laterally accreting tidal channel (green contour) within a sub- to intertidal flat surround- ing environment. The respective migration direction of these three bedforms is color-coded on the rose-diagram, whereas the black line on the diagram illustrates the outcrop orientation. (B) Heterolithic sandstone with an alternation of flaser bed- ding (FB, Facies N) and wavy bedding (WB, Facies M). Lenses cap: 67 mm. (C) Enrichment of organic matter (OM) in the toesets of certain rhythmic tidal bundles (Facies P). (D) Weathered surface of a structureless sandstone. (E) South-westward laterally accreting tidal channel, incising into subtidal sandstone with flaser bedding (FB). The channel was subsequently overlain by 3D tidally influenced dunes (TD) migrating towards the northeast. (F and G) The two pictures are respectively taken 1.3 and 2.2 km south of the Interstate 70 measured section. They show the erosive and angular relationship between FA 5 and its substratum, as the Curtis Sea was transgressing its poorly consolidated and uplifted substratum. The MTS at the base of FA 5 can be traced over the entire study area. 152 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Pryor (1991) and Wilcox and Currie (2008) bring forth and visualize by their paleoenvironment reconstruction models. As depositional energy conditions increase basinward, the clear change in grain size in comparison with the underlying finer grained deposits of FA 2, FA 3, and FA 4 and the extended erosive tidal ravinement surface at the base of FA 5 suggest an overall transgres- sion within a context of limited available accommoda- tion space, with the emplacement of high energy tidal channels and bars system, shielding the back barrier in- tertidal mix-flat in the southeast (FA 6) (see Dalrymple and others, 2012). The overall FA 5 interval corresponds to Flemming’s (2012) bare tidal flat depositional system. The diminishing thickness, size, and wavelength of tidal structures up-section, as well as towards the east and the south, is interpreted to represent a decrease of tidal amplitude and influence over the study area as a result of the early stage of a prograding coastline within an asymmetrical, eastward-pinching foreland basin, which followed a period of architectural aggradation. FA 6 – Upper Heterolithic Sub- to Intertidal Flat Description FA 6 (figure 9) represents the topmost overall fin- ning-upward heterolithic interval of the Curtis For- mation documented throughout the whole study area, but its occurrence is limited in sector 3 in comparison with the neighboring sectors 1 and 2. It conformably overlies the sand-dominated FA 5, whereas, at Little Flat Top, it directly overlies FA 3. Its thickness is fair- ly constant, gently varying between about 7 and 17 m. Internal structures include asymmetrical current- and wave-ripple lamination of Facies L, M, and N, interbed- ded with laminated mudstone, and structureless or pla- nar parallel-stratified sandstone (Facies C, K, S). FA 6 is also marked by an increase of unidirectional current ripple lamination (Facies U), whereas bidirectional her- ringbone cross-laminations become increasingly sparse. Individual beds are as much as 10 to 40 cm thick. Dark- red paleosol horizons and scattered desiccation cracks, as well as scattered evaporite-rich levels are recorded throughout the successions. Reddish to orange-colored chert nodules often arranged along well-defined hori- zons are also common within FA 6. Paleocurrent data must be regarded with caution due to the low number of measurements (figure 3). Further, FA 6 sees the re- turn of bioturbation, exclusively consisting of root bur- rows. FA 6 is characterized by an extreme scarcity, and a dramatically reduced bioturbation size, which would correspond to Droser and Bottjer (1986) ichnofabric index n°2. Note that once again the Crystal Geyser sec- tion remains an outcast, as the 2 to 5 m of interpreted Curtis Formation displays only Facies S, U, and X. Interpretation The alternation of heterolithic deposits such as in FA 6 is closely related to systematic and periodic energy level fluctuations, typical of a tide-influenced environ- ment (Kvale, 2012). In comparison with the underlying units, the amplitude of flow velocity change, as well as the available accommodation is reduced, as suggest- ed by the limited thickness of each bed and the size of the bedforms populating them. The disappearance of relatively deep tidal channel-related herringbone cross-lamination and its gradual replacement by an in- creased frequency of unidirectional landward-oriented washover deposits directly reflects a reduced rate of rel- ative sea level rise, which marks the onset of a normal regression (Highstand System Tract [HSST]). The de- velopment of FA 6 is accompanied by a weaker tidal and overall marine influence over the intertidal flat, which generates extremely stressed habitability conditions within the system (Jaglarz and Uchman, 2010) coupled with episodes of subaerial exposure, as supported by precipitation of gypsum, the desiccation cracks, and the development of superficially vegetated paleosol hori- zons. FA 6 represents a more restricted and more prox- imal depositional environment than its FA 5 basinward equivalent. FA 7 – Coastal Dry Eolian Dune Field Description FA 7 corresponds to the eolian deposits of the Moab Member of the Curtis Formation, which crop out in the vicinity of Moab and Arches National Park, Utah (fig- ure 1). The overall thickness of FA 7 is as much as 50 m 153 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 in the easternmost visited localities and pinches out a few 100 m east of Duma Point (figures 1 and 3). It rests directly on the Slick Rock Member of the Entrada Sand- stone in the east and on the earthy facies towards Duma Point (figure 1). It is characterized at its base by a sedi- mentary package that mainly consists of light-green to white, structureless to ripple to trough cross-stratified, very fine to fine-grained sandstone (Facies Q and U), with greenish-colored silty sandstone intervals (Facies L), and superficial paleosols (Facies X). Facies Q and U are over-represented with respect to Facies X in the west, whereas this ratio inverts itself towards the east. Note that this facies has not been observed at Dewey Bridge section (figure 10). Soft-sediment deformation is common. This basal unit is overlain by a 1- to 3-m-thick, planar parallel-stratified, white fine-grained sandstone (Facies S), with locally occurring 0.3- to 0.5-m-thick tangential cross-stratified sandstone (Facies A) sets. The rest of the succession consists of four to five packages of amalgamated, large-scale (>2 m) tangential cross-strat- ified, fine-grained white sandstone (Facies A). The max- imum height of individual foreset reaches about 15 m. Each of these sedimentary packages is truncated at their top by a supersurface, upon which paleosols (Facies X) and/or fine-grained, undulating to rippled cross-strati- fied white sandstone (Facies U) that is as much as 1 m thick can be observed. Note that rhizoliths can be visible up to 2 m below these supersurfaces (figures 10C and 10D). FA 7 can be capped by a thin 10- to 20-cm-thick, fine-grained, structureless yellow sandstone (Facies T). Interpretation The thick, tangential cross-stratified sandstone beds are interpreted as migrating compound eolian dunes (Facies A), with alternating grain-fall and grain- Figure 9. (A) Photo illustrating the conformable contact between the middle Curtis (FA 5), the upper Curtis (FA 6), and the Summerville Formation (FA 8), accompanied their upward-thinning and upward-finning of the beds. (B) Close up of the conformable contact between the upper Curtis and the Summerville Formation. The lithostratigraphic boundary lies at the top the hammer handle. 154 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 Figure 10. (A) Picture of the Moab Member (MTMb; FA 5 and FA 7) at Big Pinto Mesa, where a 50-m-thick section was measured through the eolian deposits of FA 7. It can be divided in five sequences (a-e). The MTMb crops out in sector 3 (see figure 3), and it overlies the earthy facies (FA 1b) in the western part of sector 3, whereas it rests directly on the Slick Rock Member (FA 1a) towards the east. (B, C, D, and E) Close-up photographs of the lowermost meters of FA 7, which conform- ably overlies water-carried sediments of FA 5. Each eolian sequence is capped by a supersurface (SS). Rhizoliths and their precipitation fronts (white arrows, and close-up D) can be present up to several decimeters below such a supersurface. They indicate that vegetation developed prior to the deposition of the following sequence. 155 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 flow deposits on the foresets of each dune. The west- ward-pinching geometry of FA 7 indicates that the main eolian depocenter was, at the time, located in the vicinity of Big Pinto Mesa (figure 1). The extent of the Moab Member (FA 7) suggests that the paleo-erg cov- ered an area of at least 1800 km2. However, this surface is certainly underestimated, due to the low preservation potential of eolian deposits (Rodríguez‐López and oth- ers, 2014), as well as the limited area where the Moab Member has not yet been eroded. The vertical stacking architecture of FA 7 makes it no stranger to the Kocurek (1988, 1999), Kocurek and Lancaster (1999), and Mountney (2006) cyclic deposi- tional pattern: (1) construction, (2) accumulation, and (3) preservation phases of eolian deposits. The occur- rence of the basal Facies Q, U, and L implies a marine incursion within a coastal plain domain, during a peri- od of optimal climatic conditions allowing the develop- ment of (superficial) paleosols. Note that, when traced basinward, this basal tidally influenced marginal ma- rine interval corresponds to the transgressive deposits of FA 5, which highlights the spatial extent of this major transgressive event, whereas the intervals characterized by Facies U represent short-lived marine transgression as FA 6 was being deposited farther to the west. The transition from a “wet” environment to a dry phase of eolian accumulation is accompanied by the deposition of sand sheets (Facies S) and small-scale eolian dunes (Facies A). The onset of this shift results from an in- creased amount of loose sediment available for wind transport, as a consequence of a lowering of the satu- rated level within the sedimentary column (Kocurek, 1988, 1999; Kocurek and Lancaster, 1999; Mountney, 2012). The replacement of small-scale bedforms by large-scale eolian dunes represents the major phase of accumulation, also indicative of peak aridity and high- est loose sediment budget. Each eolian dune package (Facies A) is capped by laterally extensive deflation surfaces, which are referred to as supersurfaces (Brook- field, 1977; Talbot, 1985; Kocurek, 1988; Havholm and Kocurek, 1994), resulting from the cannibalization of the system down to the water saturated level, as the sand supply became exhausted. Similar to Mountney’s (2006) models from the Permian Cedar Mesa Sandstone, the observed supersurfaces are also characterized, at least locally, by “abundant calcified rhizoliths and bioturba- tion and which represents the end product of a wide- spread deflation episode” (Mountney, 2006), and the potential development of superficial vegetation. The stacking of four to five of these eolian sequences sug- gests cyclic climatic alternations between humid and arid episodes (Mountney 2006, 2012). The near absence of interdune deposits, and the wedge-symmetry of the succession, assign FA 7 to a dry temporally and spatially dynamic eolian system (Kocurek and Havholm, 1993; Mountney, 2012). FA 8 – Supratidal Flat Description FA 8 marks the stratigraphic top of this study and corresponds to the lowermost strata of the Summerville Formation, which is observed at each measured section (figure 9). FA 8 is characterized by a gradual yet short- lived transition from mostly greenish-colored tide-in- fluenced strata (FA 6) into a succession dominated by dark-red laminated mudstone (Facies V), paleosols (Fa- cies X), and evaporite-rich horizons. In the eastern part of the study area, FA 8 overlies FA 7, and the transition between the two units is abrupt, with no evidence of ero- sion. Centimeter- to decimeter-scale (cm- to dm-scale), light- to dark-green-colored, ripple-laminated strata (Facies U) and multi-dm-thick, trough cross-stratified sandstone beds (Facies I) occur repeatedly in FA 8. Fa- cies U occurs ubiquitously within FA 8, whereas Facies I has been documented only in the eastern part of the study area. Both their number and thickness diminish up-section. Episodic but rare interbedded, fluvial-dom- inated strata (Facies W) occur within the succession. Note that both Facies U and Facies W locally display desiccation cracks. Interpretation Fine-dominated grain size, fairly well-sorted, small sedimentary structures, architecture, dark-red colors and evaporites suggest a quiet and arid coastal envi- ronment, with episodic seasonal fluvial floods as well as periodic short-lived marine incursions, marked by the thin greenish-colored ripple-laminated and thicker 156 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 trough cross-stratified beds. The fact that marine floods diminish up-section indicates a basinward-prograding coastline during a phase of normal regression (HSST). DISCUSSION Sector 1, Sector 2, and Sector 3 The study of these facies associations shows that the Curtis Formation, as defined by Gilluly and Ree- side (1928) at their type section at Curtis Point (figure 1), can be divided in three lithostratigraphic sub-units easily identifiable in the field. These units are herein re- ferred to as the lower, middle, and upper Curtis (infor- mal nomenclature). The lateral equivalent Moab Mem- ber was originally defined by Wright and others (1962) as a member or a tongue of the Entrada Sandstone at the time. Doelling (2001) officially reassigned these eolian deposits as a member of the Curtis Formation. The spa- tial distribution of these three sub-units shows that the Curtis Formation displays three different expressions of itself over the study area, which are herein delimitated as sectors 1, 2, and 3. The complete sub-unit trilogy of the Curtis Forma- tion are only found in sector 1 (figure 3), which extends north of Uneva Mine Canyon on the eastern limb of the San Rafael Swell monocline; however, the triptych char- acter of the formation disappears south of Last Chance Desert on the western margin of the San Rafael Swell (figures 1 and 3). Sector 2 (figure 3) is dominated by the middle and upper Curtis, with the notable following ex- ceptions: (1) Duma Point, where only the middle Curtis is exposed, (2) Little Flat Top, where the middle Curtis is absent, and (3) Hanksville, where the three sedimen- tary sub-units crops out again (figure 1). On figure 3B, Little Flat Top’s pie chart displays a particular pattern in the sense that it reflects the uncertain location of the formation boundary between the Entrada Sandstone and the FA 4a of the Curtis Formation. At that specific locality, three lithological candidate boundaries remain, hence influencing the resulting ratio between the differ- ent facies associations present in the Curtis Formation. Sector 3 delineates the extent of the Moab Member of the Curtis Formation (figure 3). Lower, Middle, and Upper Curtis The lower Curtis crops out in sector 1, as well as at Little Flat Top and at Hanksville in sector 2 (figures 1, 2, and 3). Its base corresponds to the J-3 unconformity as defined by Pipiringos and O’Sullivan (1978). As shown in figures 4C and 4D, figures 5 and 6, as well as figures 8F and 8G, the unconformity is characterized by vari- ous types of relief formed by different processes, such as eolian deflation, and fluvial or tidal currents, which impacted the unconformity at various times. However, defining the base of the lower Curtis by this polygenet- ic and composite surface notably implies that the basal flash-flood conglomerates at Last Chance Desert (figure 1, Facies F) also belong to the Curtis Formation, despite predating and being genetically unrelated to the forma- tion. Instead, if it is decided to define the base of the formation using the multi-faceted transgressive surface while regrouping genetically related shallow marine de- posits only, then this basal conglomerate at Last Chance Desert would become a-formational, as it would neither belong to the Entrada Sandstone nor the Curtis Forma- tion. For lithostratigraphic convenience, it is suggested to use the highly diachronous J-3 unconformity of Pip- iringos and O’Sullivan (1978) as the boundary for the lower Curtis, and thus keep these basal conglomerate within this sub-unit. The lower Curtis is dominated by FA 3a-b dark- green to gray subtidal heterolithic strata, whereas FA 2 and FA 4 are present locally in the eastern part of sec- tor 1, notably at Neversweat Wash, Sven’s Gulch, Rab- bit Gulch, and Interstate 70 (figures 1, 2, and 3). The ratio between the mud-dominated (FA 3a) and the sand-dominated heterolithic succession (FA 3b) varies spatially. The highest concentration of coarser-grained material, including conglomeratic beds, occurs in the western part of sector 1 at Lower Cedar Mountain Road and Last Chance Desert, whereas, on the eastern mar- gin of sector 1, Sven’s Gulch and Rabbit Gulch show similar enriched sand-to-mud ratios (figures 1, 2, and 3). The exact provenance of the different conglomeratic facies occurring in the Curtis Formation remains inde- terminate. It is certain, however, that these extra-basinal lithoclasts are sourced from terrane(s?) exposed beyond the extent of the underlying strata of the Entrada Sand- 157 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 stone, due to the presence of metamorphosed polycrys- talline quartz within these gravels and pebbles. Skeletal carbonate fragments of unidentified bryozoan also ex- clude the Entrada Sandstone as a potential source for these conglomeratic beds, as opposed to the bulk of the Curtis Formation, which has a modal composition sim- ilar to the underlying Entrada Sandstone (Dickinson and Gehrels, 2009, 2010). Despite the lack of provenance data, we suggest that these conglomerate beds represent reworked material, potentially from flash flood depos- its, sourced from the uplifted highlands west of the study area (Thorman, 2011; Anderson, 2015) or from the nearby Uncompahgre highlands to the east of Moab (Otto and Picard, 1976; Scott and others, 2001). The fact that most dune-forming and channel-filling conglomer- atic facies are in the lowermost meters of the lower Cur- tis suggests that, as the overall Curtis transgression pro- ceeded, their sediment supply was exhausted. It seems that another sediment entry point emerged in the south to southeast parts of the study area, draining a different basin, from which reworked material from the Entrada Sandstone could have been assimilated into the Curtis Formation. Indeed, towards the south to southeast, FA 4a consists of relatively similar texture with respect to the underlying earthy facies of the Entrada Sandstone; it becomes increasingly dominant within the lower Cur- tis, as the heterogeneity and complex architecture of FA 3 is replaced by the laterally extensive, light-pink beds of FA 4a. The fact that no bleached front along fractures has been observed in this unit suggests that, unlike bleached fractures and corridors in the Carmel Forma- tion and in the Entrada Sandstone (Ogata and others, 2014), no reducing fluids have circulated within these rocks (Skurtveit and others, 2017; Sundal and others, 2017; figures 4a and 7). The middle Curtis consists exclusively of FA 5 de- posits (figure 3), and its lower boundary corresponds to the MTS, which can sometimes display an erosional relief of about 50 to 90 cm and is hence locally identified as a tidal ravinement surface. In sector 1, the middle Curtis overlies the lower Curtis in a possibly conform- able to disconformable to angular way, whereas in sector 2 and 3, the MTS merges with and partially reworks the J-3 unconformity (figure 8G). The angular unconformi- ty between the lower and middle Curtis, visible south of Interstate 70 (figures 8F and 8G), suggests a sub-region- al pre-middle Curtis uplift in the area which may signal regional yet unidentified tectonic activity over the area during the Late Jurassic, prior to the deposition of the middle Curtis. Further, local to sub-regional dome fea- tures, ascribed to sandy substratum mobility related to fluid overpressure and seismic activity (Jolly and Loner- gan, 2002; Wheatley and others, 2016), are for instance visible 2.1 km north of Hanksville Airport, where the lower and middle Curtis display a fault system driven by an underlying, circular sand pillow (figure 4F). Just as for the J-3 unconformity, the terms unconformity and disconformity are to be taken with caution, as they imply, by definition, a significant time gap between the adjacent units (Van Wagoner and others, 1988; Hol- brook and Bhattacharya, 2012), a temporal dimension, which, in the case of the Curtis Formation, cannot be assessed with great precision. The middle Curtis pinch- es out southward in sector 2 whereas it thins towards the east in sector 3. Due to the extreme low gradient present over the study area during the Jurassic (and through the Cretaceous) times (Heller and others, 1986; Fillmore, 1991; Lockley, 1991; Jones and Blakey, 1993), it is sug- gested that the time encapsulated in this transgression was short. This major and rapid transgression potential- ly reached far beyond the study area. Indeed, it may be linked to the deposition of the Todilto Formation and its calcareous saline sediments in southwestern Colo- rado and northwestern New Mexico (Lucas and Ander- son, 1997), which were deposited after a blitz-flooding of the Entrada Sandstone by a marine incursion (Benan and others, 2000). It is important to note that other re- gional marine embayments co-existed with the Cur- tis sea in similar low-gradient conditions, such as the Ralston Creek Lobe of Anderson and Lucas (1994) in southeastern Colorado. Thus, the Todilto Formation is likely related to the MTS and to the base of the middle Curtis, which implies that the paleo-erg of the Entrada Sandstone existed in New Mexico until “instantaneous” transgression of Benan and others (2000), whereas it was cannibalized by the lower Curtis transgression in Utah. The upper Curtis mainly consists of FA 6 deposits, with mud- and sand-dominated channel and tital-flat complexes. It has been recorded at each visited locality 158 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 in sectors 1 and 2, with the notable exception of Duma Point (figure 3). Its lower boundary is gradational, as it conformably overlies the middle Curtis. The main dif- ferences between the middle and upper Curtis reside in the up-section’s increased heterogeneity, as well as a more pronounced green color, and the gradual replace- ment of the multi-m-thick bedforms by smaller scale sedimentary structures and thinner strata. It implies that the rate of accommodation decreased in the basin, whereas the sediment supply remained either constant, or potentially increased. The upper Curtis corresponds to the laterally equivalent Moab Member, which crops out in sector 3. The Moab Member is characterized by the dominance of the eolian dunes of FA 7 arranged in four to five distinct sequences, representing at least 78% of the Curtis Formation in the eastern part of the study area. The remaining percentages are recorded as a thin basal middle Curtis interval, whereas marine in- cursions (FA 6) or superficial paleosols and supratidal deposits (FA 7) separate the different eolian sequenc- es (figure 10). Note that the base of the Summerville Formation (FA 7), which stratigraphically overlies the Curtis Formation conformably in the study area de- spite their obvious lateral stratigraphic relationship on a more regional scale, suffers from the same gradual tran- sition, from which no evident lithological boundary rises. It is therefore proposed to define the base of the Summerville Formation as the line where, over a meter of succession, more than 50% of the deposits belong to the supratidal FA 8. SPATIAL DISTRIBUTION OF THE VARIOUS FACIES ASSOCIATIONS: ILLUSTRATION BY MODERN ANALOGS No modern analogs exist to fully illustrate the over- all Curtis Formation depositional evolution. Never- theless, as shown in figure 11, the inner Gulf of Cali- fornia and the Wadden Sea are suggested to represent similarities to the aforementioned Curtis Formation subdivisions. Such comparisons have limitations, and mainly regard spatial extent of modern environments in comparison with the size of their respective Curtis Formation counterparts, as well as basinal geometry approximations. The modern analogs are not meant to represent a similar tectonic setting to the Curtis Forma- tion foreland basin conditions. The lower Curtis is compared the fluvially starved, arid and unprotected tide-dominated bay of Las Li- sas, Mexico (figure 11) (UTM coordinates: 12R 221191/3504767), on the northeastern margin of the Gulf of California. The bay offers a window on a proba- ble convoluted facies belt arrangement which occurred during one of the lower Curtis short-lived regressive episodes. In the case of Las Lisas, such a regression is linked to rift flank uplift in the Neogene (Mark and others, 2014). As displayed on figure 11, the area close to the shoreline features sand-rich deposits similar to FA 4a interfingering with finer-grained, supratidal sed- iments of FA 8 and possibly conformably overlying pre-existing substratum (equivalent of FA 1a). Farther into the basin, the heterolithic subtidal flat deposits of FA 3 are interfingering with gravel-bearing tidal chan- nels (FA 4b) and migrating dunes (Facies H). The middle Curtis is paralleled with the Dutch part of the shielded and tide-dominated shallow Wadden Sea (figure 11; UTM coordinates: 31U 650032/5902807), where intricate and dynamic interplay of sub- to inter- tidal channels, tidal flats, and migrating tidal dunes and shoals (FA 5) has developed over an extensive and gen- tly sloping area, behind barrier islands, as a result of a major and quick transgression (linked to Holocene gla- cio-eustatic sea-level rise in the Wadden Sea (Oost and de Boer, 1994)). It is important to point out that similar well-defined barrier islands are not recognized in the middle Curtis deposits (FA 5), although evidences for subaerial exposure episodes have been documented. The selected paragon for the upper Curtis is the fluvially starved and arid, back-barrier bay of La Pin- ta, located 75 km to the southeast of Las Lisas (figure 11) (UTM coordinates: 12R 286227/3460602). Here, the sub- to intertidal sediments (FA 6) deposited in this protected environment are being progressively overlain by supratidal deposits (FA 8), as the underfilled dep- ocenter morphs into an overfilled basin. It is accompa- nied by a seaward migration of the coastline, leading to the potential development of an eolian dune system (FA 7) in the neighboring area as sediments become sub- sequently available for transport (Carr-Crabaugh and Kocurek, 1998; Mountney, 2012). 159 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 UTM coordinates: 12R 221191/3504767 UTM coordinates: 12R 286227/3460602 UTM coordinates: 31U 650032/5902807 25 km 25 km N N lower Curtis - TR cycles middle Curtis - major transgression upper Curtis - normal regression A. Modern analogs of the Curtis Formation Las Lisas, Mexico B. Modern analogs transposed to the Curtis Formation basinal setting Wadden Sea, the Netherlands La Pinta, Mexico N 3 km SI O S N 4 km SI O S N 1km SI O SSIOS Satellite Image Orientation and Scale N 25 km Moab Green River Hanksville 100 km ColoradoUtah Wyoming GR HK MB GJ SLC Elko Orogeny Uncompahgre Study area FA 8 Supratidal �at FA 7 Coastal dry eolian dunes, Moab Member FA 4a Sand-rich sub- to supratidal �at FA 3b Subtidal sand-dominated heterolithic �at FA 6 Upper sub- to intertidal heterolithic �at FA 3a Subtidal mud-dominated heterolithic �at FA 5 Sub- to intertidal channel-dune- �at complex FA 1b Earthy facies FA 1a Coastal dunes and Slick Rock Member FA 2 Upper shoreface to beach deposits FA 4b Correlative tidal channel in�ll Legend Facies Associations Tectonic setting Oxfordian SLC - Salt Lake City. GR - Green River. HK - Hanksville. MB - Moab. GJ - Grand Junction Moab Green River Hanksville Moab Green River Hanksville Figure 11. (A) Modern analogs of the lower, middle, and upper Curtis Formation, respectively. Note that these pictures are not aligned to the true north, but are rotated in a way that allows them to fit the orientation of the Curtis basin (satellite im- ages from Microsoft Bing Maps). (B) Modern analogs draped with the corresponding facies associations from the Curtis For- mation. The change of scale between A and B illustrates how much bigger the Curtis basin setting is with respect to the three modern analogs. TR cycles – transgressive-regressive cycles. Tectonic setting after Heyman (1983) and Thorman (2011). 160 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 CONCLUSIONS 1. Eight main facies associations (FA), including six sub-facies associations, were identified based on lithology, internal sedimentary structures, archi- tectural arrangements, and spatial relationships. These facies associations are regarded as diagnostic expressions resulting from tidal processes in a flu- vially starved, low-gradient, semi-enclosed epicon- tinental basin. 2. It is proposed to divide the Curtis Formation into three sub-units: the lower, middle, and upper Curtis. The specific spatial distribution of these sub-units allows the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. 3. The lower Curtis consists of laterally restricted up- per shoreface to beach deposits (FA 2) overlain by a subtidal mud-dominated heterolithic succession (FA 3a), which grades into a sand-dominated sub- tidal flat (FA 3b). FA 3a and FA 3b interfinger with the more proximal and shallower FA 4a sand-rich sub- to supratidal flat, as well as with its more distal FA 4b correlative tidal channel infill. 4. FA 5, which corresponds to the middle Curtis, is mainly composed of a characteristic light green to white, very fine to fine-grained sandstone, appear- ing as an intricate arrangement of tidal channels, dunes, and tidal flats. The base of the middle Cur- tis coincides with the MTS, which can be traced throughout the entire study area. 5. The upper Curtis conformably overlies the middle Curtis and is characterized by heterolithic subtidal to intertidal deposits of FA 6, as well as their later- al and contemporaneous continental eolian dunes of the Moab Member (FA 7). The transition from the upper Curtis into the Summerville Formation is also gradual, with a progressively increasing oc- currence of supra-tidal deposits (FA 8) within the succession. 6. The lower, middle, and upper Curtis occur in sector 1, whereas only the middle and the upper Curtis crop out in sector 2 (with local exceptions). Sector 3 corresponds to the area where the Moab Member of the Curtis Formation was deposited. 7. It is possible to compare the lower Curtis to the Bay of Las Lisas in the Gulf of California, the middle Curtis to the Wadden Sea in the Netherlands, and the upper Curtis to the bay of La Pinta in the Gulf of California. 8. The Todilto Formation or Todilto Member of the Wanakah Formation, which crops out in south- western Colorado and northwestern New Mexi- co, is likely related to the major transgression that defines the base of the middle Curtis and is con- sidered its lateral contemporaneous equivalent. Hence, the upper Wanakah Formation is regarded as the lateral equivalent of the Summerville Forma- tion. 9. The middle Curtis overlies its substratum with an angular relationship on a sub-regional scale, sug- gesting an underlying and more regional deforma- tional event during the Late Jurassic. Further, with the occurrence of local- to sub-regional collapse features within the lower and middle Curtis, the Late Jurassic must been impacted by episode(s) of sand mobility, which influenced the local surface morphology. 10. The J-3 unconformity exposed a composite nature, as it was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. ACKNOWLEDGMENTS The Norwegian Research Council is to be sincere- ly acknowledged, with their awarded grant COPASS 244049, which funded the first author and allowed the ad hoc conduct of the required field campaigns for this research. The authors would like to extend their grati- tude to Dr. Anja Sundal, Anna van Yperen, Nathan “the Legend” Cote, Dr. Miquel Poyatos-Moré, Dr. Mark Mulrooney, Ole Rabbel, and Kristine Halvorsen for their assistance and helpful comments which elevated the standard of this work. Acknowledgments are to be extended to Grant C. Willis of the Utah Geological Sur- vey (UGS) for his fruitful comments and review of an 161 New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA Zuchuat, V., Sleveland, A.R.N., Sprinkel, D.A., Rimkus, A., Braathen, A., and Midtkandal, I. Geology of the Intermountain West 2018 Volume 5 early version of this manuscript. The authors also thank Kimm Harty (UGS) for her careful review and editing. Finally, we thank UGS technical editor Stephanie Car- ney, UGS Deputy Director Michael Hylland, and UGS Director Rick Allis for their reviews and support. 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