Sequence Stratigraphy Condensed Low Accommodation Succession

Sequence stratigraphy of acondensed low-accommodationsuccession: Lower Upper

divided into two condensed top-truncated stratigraphic se-quences, the upper of which contains two parasequences. The

AUTHORS

Jonathan Antia Core Laboratories, 6316Windfern Road, Houston, Texas 77040;[email protected]

Jonathan Antia received his Ph.D. in geologyfrom the University of Nebraska-Lincoln in 2009.He currently works as a staff geologist at CoreLaboratories in Houston, Texas. His academicresearch focused on coastal to shallow marinesiliciclastic depositional systems.

Christopher R. Fielding GeociencesDepartment, 214 Bessey Hall, University ofNebraska, Lincoln, Nebraska 68588-0340;[email protected]

Chris Fielding holds the Mr. & Mrs. J.B. CoffmanChair in sedimentary geology at the Universityof Nebraska-Lincoln. He received his Ph.D. fromthe University of Durham (United Kingdom)in 1982 and previously worked for BP Explora-tion and the University of Queensland in Bris-bane, Australia. His research interests lie in thestratigraphy of continental, coastal, and shallowmarine successions.

ACKNOWLEDGEMENTS

The work presented in this article forms part ofa Ph.D. dissertation at the University of Nebraska-Lincoln, supported financially by the Mr. &Mrs. J.B. Coffman Endowment in SedimentaryGeology, and the AAPG Grants-in-Aid program.We thank Mark Kirschbaum (U.S. GeologicalSurvey, Denver) for his guidance in the initialstages of fieldwork, Lauren Birgenheier fordrafting Figure 1, and the editor (Gretchen Gillis)and referees (Soledad Garcia Gil, Tom Ryer,Janok Bhattacharya) for their reviews of thesubmitted manuscript.The AAPG Editor thanks the following reviewers fortheir work on this paper: Janok P. Bhattacharya,Soledad Garcia Gil, and Thomas A. Ryer.basal parts of both sequences are composed of braided fluvialconglomerates and sandstone overlain by tidally influencedfluvial sandstone, inclined heterolithically stratified estuarinemudstone, carbonaceous shale, and coal. The overlying para-sequences consist of coarsening-upward lower to upper shore-face mudstone, sandstone, tidal inlet deposits, and oyster shellconcentrations. These facies define tripartite subdivisions ofdepositional environments typical of wave-dominated estuaries.

Copyright 2011. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received November 8, 2009; provisional acceptance January 12, 2010; revised manuscriptreceived April 3, 2010; final acceptance June 30, 2010.DOI:10.1306/06301009182Cretaceous DakotaSandstone, Henry Mountains,southeastern UtahJonathan Antia and Christopher R. Fielding

ABSTRACT

Cretaceous low-accommodation deposits have been exten-sively studied in the subsurface of the Western Interior ofNorth America because of their prolific hydrocarbon pro-duction and remaining potential. Understanding the strati-graphic complexities of these deposits in the subsurface reliesstrongly on detailed outcrop analogs. In this study, the DakotaSandstone was examined along 100 km (62 mi) of semicon-tinuous outcrop between the towns of Hanksville and Ticabooin the Henry Mountains of southeastern Utah. This regionrepresented a low-accommodation setting located over theforebulge of the Cretaceous Western Interior Basin during ac-cumulation of the unit. The Dakota Sandstone is 0 to 38 m(125 ft) thick, of Cenomanian age, and records multiple cy-cles of sediment accumulation. The Dakota Sandstone is sub-AAPG Bulletin, v. 95, no. 3 (March 2011), pp. 413447 413

tions. Many studies have characterized these units

in the subsurface of the region (PattisonandWalker,1994; Peper, 1994; Walker and Wiseman, 1995;MacEachern et al., 1998; Snedden and Bergman,1999; Bhattacharya and Willis, 2001; Zaitlin et al.,2002;Leckie et al., 2004;Ratcliffe et al., 2004;Crerarand Arnott, 2007; Currie et al., 2008; Feldman et al.,2008) and fewer others in outcrop (Willis, 1997;am Ende, 1991; Leckie and Singh, 1991; Ulin,1999;Holbrook, 2001;Holbrook et al., 2006; Laurinand Sageman, 2007). However, the complexityof these units is commonly difficult to fully assessfrom subsurface data because of the laterally lim-The fluvial deposits may represent lowstand de-posits, but overall sediments accumulated duringtransgressive systems tracts (TST). The parase-quences recorded in the Henry Mountains are sim-ilar to the Dakota Sandstone of northwestern NewMexico and to high-frequency sequences identi-fied in the Kaiparowits Plateau, approximately80 km (~50 mi) to the southwest, which suggestseustatic driving mechanisms.

The best potential for hydrocarbon reservoirsoccurs in fluvial sandstones and conglomerates.

INTRODUCTION

Objectives

The Cretaceous Western Interior seaway (KWIS)of North America represents one of the best stud-ied sedimentary basins in the world, partly becauseit contains multiple prolific hydrocarbon-producingstratigraphic units throughout its geographic ex-tent (e.g., the D and J sandstones and the Fall RiverFormation in the Denver-Julesberg Basin of Col-orado and Nebraska, the Muddy Sandstone andparts of the Frontier Formation in basins through-outWyoming, the Dakota Sandstone in the UintaBasin of Utah and the San Juan Basin of New Mex-ico, the (EllerslieMember (also known as the BasalQuartz Member) of the Manville Group and theViking Formation in Alberta, Canada). Many ofthese units show laterally and vertically complexstratigraphic stacking patterns because of their ac-cumulation under accommodation-limited condi-414 Dakota Sandstone Sequence Stratigraphy, Henry Mountainited nature of core and well-log information. There-fore, well-exposed outcrop equivalents of theseunits are crucial to the understanding, exploration,and development of these units in the subsurface.

The Dakota Sandstone represents up to 38 m(125 ft; average, 10 m [33 ft]) of the lower UpperCretaceous section in exposures around the HenryMountains syncline of southeastern Utah (Figure 1).The unit is predominantly sandstone, lies withina mudrock-dominated succession, and is hydrocar-bon prospective in the nearby San Juan and Uintabasins (Figure 2). The Dakota Sandstone is superblyexposed in the study area along both limbs of theHenry Mountains syncline in an outcrop belt thatextends for approximately 100 km (62mi) from thetown of Hanksville in the north to Ticaboo in thesouth (Figure 1). This area is surrounded by well-studied areas such as the Kaiparowits Plateau ap-proximately 80 km (50 mi) to the west-southwest(am Ende, 1991; Ulin, 1999; Laurin and Sageman,2007), the Uinta Basin nearly 80 km (50 mi) tothe north (Figure 2) (Currie et al., 2008), and theSan Juan Basin 100 to 200 km (62124 mi) tothe southeast (Owen, 1969; Aubrey, 1989; Lucaset al., 1998). In the Kaiparowits Plateau, multiplehigh-frequency cycles have been reported fromthe Dakota Sandstone (Ulin, 1999; Laurin andSageman, 2007) and interpreted to be approxi-mately 100 k.y. in duration and involving relativesea level fluctuations of 10 to 20 m (3366 ft). Inthe San Juan Basin, the unit also contains multiplecycles of sediment accumulation. The stratigraphiccomplexity observed in the surrounding areas sug-gests that the stratigraphy of the Dakota Sandstonein the Henry Mountains may be similarly complex.Therefore, this unit represents an excellent outcropanalog for similar units in the subsurface throughoutthe Cretaceous Western Interior of North America.

The Dakota Sandstone successions in the Kai-parowits Plateau and in the San Juan Basin are sig-nificantly thicker than in the Henry Mountains andrepresent higher accommodation settings associ-ated with different tectonic realms. Thus, compar-ison of the Dakota Sandstone from the KaiparowitsPlateau, through the Henry Mountains, and intothe San Juan Basin provides a cross-orogen transectof the KWIS during the Cenomanian.s, Southeastern Utah

Figure 1. Location maps showing (A) the Henry Mountains within the state of Utah (left), (B) a close-up of the Henry Mountains including the location of all measured sections (blackdots), and lines of cross sections AA and BB, and (C) a close-up of the Blue Valley area depicting lines of cross section CC and DD. Abbreviations refer to names given to measuredsections and include BC = Bitter Creek; BT12 = Blind Trail 12; BHG12 = Bloody Hands Gap 12; BV1 = Blue Valley 1; CQ 13 = Caineville quadrangle 13; CCB = Cedar CreekBenches; CC = Clay Canyon; CW = Collie Wash; CB 12 = Copper Creek Benches 12; DC = Dugout Creek; Eg = Eggnog; HC2 = Halls Creek 2; HO = Halls Creek Overlook; HCE or W =Hansen Creek East or West; JB = Jet Basin; NCR = North Caineville Reef; SP = Saleratus Point; SW = Saleratus Wash; TP12 = The Post 12.

Antiaand

Fielding415

Figure 2. Index map ofthe Western Interior ofthe United States of Amer-ica with reference sites(gray dots) used for theregional stratigraphicframework (Figure 4) ofthe Dakota Sandstone aswell as relevant geologicfeatures. Abbreviations in-clude B = Bighorn Moun-tains; BB = Bighorn Basin;BH = Black Hills Uplift;DB = Denver Basin; FR =Front Ranges; GRB = GreenRiver Basin; HM = HenryMountains; KP = Kaiparo-wits Plateau; L = LaramieRange; MR = MadisonRange; PRB = Powder RiverBasin; S = Sierra MadreUplift; SJB = San Juan Ba-

sin; TCA = Transcontinental

416 Dakota Sandstone Sequence Stratigraphy, Henry MountainThe purpose of this research was to develop asequence-stratigraphic framework for the DakotaSandstone in the Henry Mountains of southeasternUtah and compare it with adjacent areas to betterunderstand the patterns of deposition in this unit,the tectonic configuration of the KWIS, and thepossible factors controlling cycles of sediment ac-cumulation associated with this unit. Such a strat-igraphic framework will be based on physical cor-relation of lithofacies and key surfaces throughoutthe study area, with emphasis given to the ichnol-ogy of each facies to strengthen the sedimentologicinterpretations.

The main hypotheses to be tested are that theDakota Sandstone in the study area (1) representslow-accommodation depositional settings, (2) hasa condensed and complex record ofmultiple cyclesof sediment accumulation similar to adjacent re-gions, (3) preserves both lowstand systems tract

(LST) and TST deposits, and (4) contains evidenceof the dominance of a eustatic mechanism drivingcycles of sediment accumulation in the KWIS dur-ing the Cenomanian.

GEOLOGIC SETTING

The Dakota Sandstone in the Henry Mountains re-gion preserves a record of middle to late Ceno-manian continental and shallow marine sedimentaccumulation (Hunt et al., 1953; Lawyer, 1972;Peterson and Ryder, 1975;Merewether and Cobban,1986; Cobban et al., 2000). During this time, theHenry Mountains area was located near the west-ern margin of the North American KWIS, approx-imately 200 km (124 mi) east of the orogenic frontof the Sevier fold and thrust belt (Figure 3) (Huntet al., 1953; Kauffman, 1977; Peterson and Smith,

Arch; UB = Uinta Basin;UM = Uinta Mountains;WB = Williston Basin;WRB = Wind River Basin;WRM = Wind RiverMountains.s, Southeastern Utah

Figure 3. Paleogeographic reconstruc-tion of the Western Interior of the UnitedStates during the Late Cenomanian trans-gression. References used to compile thisfigure include Scott (1970), Vuke (1981),and Laurin and Sageman (2007).1986; Currie, 2002;White et al., 2002). The KWISformed as a retroarc foreland basin by flexural sub-sidence of the crust caused by thrust loading andcrustal compression associated with the Sevierorogeny (Armstrong, 1968; Cross, 1986; Angevineand Heller, 1987). Deformation along preexistingtectonic trends also occurred and may have influ-enced sediment dispersal in some areas (Picha,1986; Currie, 2002). In general, the basin is divis-ible into four longitudinal realms: the foredeep inArizona, Utah, and Wyoming; the forebulge nearthe Utah and Colorado border; an axial basin thatat its maximum coverage extended from Texasthrough Colorado, Alberta, and the Northern Ter-ritories of Canada; and a hinge zone to the NorthAmerican Cratonproduced by differential sub-sidence away from the orogenic front (Kauffmanand Caldwell, 1993; White et al., 2002). Duringaccumulation of the Dakota Sandstone, the studyarea lay within the forebulge realm, where sedimentaccommodation was significantly lower in compar-ison with adjacent areas such as the KaiparowitsPlateau to the west-southwest, the Uinta Basin to

the north, and the San Juan Basin to the southeast(Currie, 2002).

Paleogeographic reconstruction of middle to lateCenomanian places most of the midwestern UnitedStates within 30 to 45N of latitude (Figure 3)(Laurin and Sageman, 2007). At this time, theboreal and southern seas of the KWIS were fullyconnected and extended between western Iowaand central Utah (Kauffman and Caldwell, 1993).

The name Dakota Sandstone, in the ColoradoPlateau, refers to a succession of Cenomanian-agerocks rich in carbonaceous mudstone overlying theCedar Mountain Formation on the eastern parts ofthe plateau and progressively older units towardthe west, and overlain by the lower Mancos Shale(Figure 4) (Young, 1960, 1965). In the early stagesof development of the stratigraphic nomenclaturefor Colorado, the name Naturita Formation wasproposed by Young (1960) for exposures of theCenomanian-age rocks near the town of Naturitain southwestern Colorado. However, the name wasnever adopted widely in the literature. Further-more, the unit has not been assigned a type sectionAntia and Fielding 417

Figure 4. Regional stratigraphic framework for the Dakota Sandstone and equivalent units throughout the Western Interior of North America. The location of reference sections isshown in Figure 2. SB = Sequence Boundary. References used to compile this figure include Ellis (1963), Haun (1963), Harms (1966), Merewether and Cobban (1986), Dyman et al.(1994), Hamilton (1994), Scott et al. (1998), Brenner et al. (2000), Cobban et al. (2000), Scott et al. (2001), Kirschbaum and Roberts (2005), Currie et al. (2008). The time scale used isthat of Gradstein et al. (2004).

418Dakota

SandstoneSequence

Stratigraphy,HenryMountains,Southeastern

Utah

Figure 5. Stratigraphiccolumn for the AlbianTuronian (Cretaceous)section in southeasternUtah, sea level curvesfor the North AmericanWestern Interior seaway(Weimer et al., 1988;Kauffman and Caldwell,1993) and eustatic curves(Haq et al., 1987; usedwith permission from Sci-ence). In the sea levelcurves, deepening is to theleft and shallowing is tothe right. The stratigraphiccolumn was compiledwith reference to am Ende(1991), Cobban et al.(2000), and Currie et al.(2008). The time scaleused is that of Gradsteinet al. (2004). SB = se-quence boundary.because of marked variations in its stratigraphiccharacter throughout the Colorado Plateau (Young,1960). Several authors broadly subdivide the Da-kota into three members: a basal chert pebbleconglomerate of fluvial origin; a middle memberconsisting of interbedded sandstone, carbonaceousmudstone, and coal interpreted as deltaic to coastalplain and flood-plain deposits; and an upper mem-ber consisting of sandstone and mudstone that inter-tongue with overlying marine shale (Hunt et al.,1953; Young, 1960, 1965; Peterson and Ryder,1975; Ryer, 1984). Such descriptions are valid froma lithostratigraphic perspective but do not providea sequence-stratigraphic context for the unit andhave been shown to be simplistic in most areas,including the area of this study (am Ende, 1991;Ulin, 1999; Laurin and Sageman, 2007; Currieet al., 2008).

Regionally, the Dakota Sandstone has beenshown to become progressively younger from theeast toward west and northeast toward southwest

across the Colorado Plateau based on relative dat-ing of invertebrate fossil content, including paly-nomorphs, ammonites, and other molluskan faunawithin it or in adjacent units, and by absolute agecalculations from bentonite layers in overlying units(Merewether and Cobban, 1986; Cobban et al.,2000; Currie, 2002; Currie et al., 2008). From eastto west, between the towns of Delta, Colorado,and Castle Dale, Utah (Figure 2), the Dakota Sand-stone has been shown to span the entire Ceno-manian stage (Merewether and Cobban, 1986).Correlation to the east suggests that the DakotaSandstone in the Colorado Plateau (Figure 4) isstratigraphically equivalent to units between theHorsetooth Member of the Muddy Sandstone atits earliest and the Thatcher Limestone Memberof the Graneros Shale at its latest, in the DenverBasin of eastern Colorado (Mellere, 1994; Currie,2002). Deposition of the lower parts of theDakotaSandstone, however, has been interpreted as dia-chronous and progressively younger from the eastAntia and Fielding 419

Table 1. Description of Facies Identified in the Dakota Sandstone in the Henry Mountains

Facies Lithology Sedimentary Structures Ichnology Facies Associations Depositional Environment

F1 Poorly sorted and well-roundedconglomerate with clasts 0.512 cm in diameter, 80% chert,and minor quartzite, milkyquartz, limestone, sandstone,and mudstone clasts. Angularpetrified log clasts occur rarely.

Massive or planar and troughcross-bedded with some clastimbrication. Contain lenses ofplanar and trough cross-beddedcoarse- to fine-grained sandstonewith common graded bedding.

No trace fossils Erosional lower contact with theCedar Mountain Formation.Variably sharp or gradationalupper contact with F2 andtypically sharp or irregularwith F3. In some sections,lenses of conglomerate occurwithin F2.

Braided fluvial

F2 Medium- to coarse-grained sandstonewith scattered chert pebbles andgranule to pebble conglomeratelenses. Typically brownish red toorange on weathering surfaces andlight gray, white, or beige on freshsurfaces. Mudstone rip-up clastsoccur sparsely at the base.

Massive, planar, and trough cross-stratification. Cross-bed setsrange in thickness between0.1 and 1.5 m and commonlycontain graded bedding. Softsediment deformation structuresare common. Nested channelforms exist in some areas.

No trace fossils Erosional lower contact withthe Cedar MountainFormation or variably sharpor gradational with F1. Theupper contact may be sharpor gradational with F3.

Braided fluvial

F3 Light gray and yellow, very fine tomedium-grained sandstone andinclined heterolithically stratified(IHS) or horizontal pinstripe-laminated very fine to fine-grainedsandstone and carbonaceousmudstone.

Trough and planar cross-bedded,parallel horizontal, current, andclimbing ripple-laminated tabularto lenticular layers. Herringbonecross-stratification, sigmoidalbedding, paired mud drapes,and lateral accretion surfacesexist in some areas. Syneresiscracks, root traces, coaly plantdebris, and leaf imprints arecommon. Pyrite occurs sparsely.

Bioturbation is low to moderate(BI 24), increases toward thetop of the unit, and is typicallyhigher in mud layers and atthe top of cross-bed sets.Common trace fossils includediminutive Teichichnus,Diplocraterion habichi,Skolithos, Planolites, andChondrites. Stressed variantsof the Skolithos and Cruzianaichnofacies assemblages.

Sharp basal contact with F1,F2, or the Cedar MountainFormation. Gradational lowercontact with F2 also exists.Grades upward into or isoverprinted by F4 or issharply overlain by F8.

Tidally influenced estuarinechannel and basin

420Dakota

SandstoneSequence


Utah

F4 Dull coal, carbonaceous shale, andmottled light gray to purplishsiltstone with sparse maceratedplant debris. Cleat fractures arewell developed in the coal. Sulfurstaining occurs commonly.

Leaf imprints and coalified plantdebris are abundant in thecarbonaceous shale. Coaly,iron-stained root traces andspheroidal weathering arecommon. Root traces up to50 cm in length and 1 cmthick exist in some sections.

Low bioturbation in thecarbonaceous shaledominated by Teichichnusand Planolites. The top ofthe coal contains palimpsestThalassinoides, Diplocraterion,and Rhyzocorallium tracesindicative of a Teredolitesichnofacies assemblage.

Gradational lower contact withF3 or moderate to intenseoverprinting of F3 and F5.The upper contact is sharpand erosional with F8.

Coastal mire and paleosols

F5 Horizontal heterolithically stratifiedsiltstone and very fine grainedsandstone.

Most commonly low-angleparallel laminated with sparsecurrent ripple lamination.Short carbonaceous root traces(5 cm) and syneresis cracksoccur sparsely. Minutecarbonaceous debris areabundant.

Trace fossils are rare in thisfacies with diminutivePlanolites and Skolithosoccurring in some areas.

Abrupt lower contact with theCedar Mountain Formationand F2. Interfingers with F2in some areas. Irregular anderosional upper contact withF2 and F7.

Flood plain

F6 Fine-grained sandstone alternatingwith siltstone lenses or drapesin coarsening-upward intervals.

Flaser to wavy bedding withcurrent and combined flowripple-lamination and parallelhorizontal bedding.

Moderate to high bioturbationconcentrated on siltstonelayers and the top of beddingplanes. Common tracesinclude diminutive Planolites,Thalassinoides, Ophiomorpha,Diplocraterion, Skolithos,Teichichnus, Gyrochorte, andother unidentified locomotiontraces similar to Cruziana orProtovirularia.

Gradational lower contact withF3 in a coarsening-upwardtrend or sharp with F2 or F3.Sharp upper contact with F3or F8.

Tidal flat

F7 Medium- to coarse- grainedsandstone with scattered chertpebbles.

Planar and trough cross-beddingoften associated with lateralaccretion surfaces and channelbodies. Parallel horizontal andcurrent ripple-laminated layersas much as 0.6 m thick existsin the upper parts. Sparse pairedmud drapes.

Low bioturbation typicallyconcentrated on beddingplanes. Identified tracesinclude Thalassinoides,Diplocraterion, Gyrochorte,Palaeophycus, and rareRhyzocorallium, Asterosoma,and Roselia.

Sharp lower contact withF2, F3, F9, or the CedarMountain Formation.Sharply overlain by F8.

Tidal inletAntiaand

Fielding421

F8 Tan, yellow, and brownish green,fine- to medium-grained sandstonewith scattered oyster shells andchert granules. Chert granule topebble conglomeratic sandstoneis common at the base of the unit.

Massive, planar and hummockycross-stratification within shingledor horizontal bedding. Oystershells become increasinglycommon toward the top. Brown,laterally extensive concretionsare common in the upper parts.

Moderate to high bioturbation(BI 46) dominated by robustThalassinoides, Skolithos,Ophiomorpha, Diplocraterion,Conichnus, Planolites, andfugichnia and less commonRosselia, Cylindrichnus,Teichichnus, and Asterosoma.Skolithos ichnofacies assemblage.Glossifungites ichnofaciesassemblages are common atthe base and top of the unit.

Sharp lower contact withF1-4, F6-7, the CedarMountain, or MorrisonFormations. Gradationallower contact with F9.The upper contact is oftenmarked by condenseddeposits of F11 gradingupward into the TununkShale.

Upper shoreface

F9 Thickly bedded heterolithic brownishgreen very fine to fine-grainedsandstone and gray mudstone.

Massive, parallel horizontal andcurrent ripple-laminations. Oystershells occur sparsely toward thetop of the unit.

Biotubation is high overall(BI = 46) and concentratedon the upper parts ofsandstone layers and withinmudstone layers. Identifiabletraces include robustThalassinoides, Teichichnus,Diplocraterion, and Planolites.Cruziana ichnofacies assemblage.

The unit coarsens upwardinto F8 and sharply overliesF2, F4, F8, or F11.

Lower shoreface

F10 White (fresh) or orange to reddishbrown (weathering) bentoniticmudstone.

Parallel horizontal laminations No bioturbation within butpalimpsest traces ofThalassinoides, Planolites,and Diplocraterion identifiedas a Glossifungites ichnofaciesassemblage occur at the top.

Occurs within F8 or F11. Volcanic ash

F11 Well-cemented to uncementedand framework-supported oystershell concentrations with a fine-to coarse-grained sandstonematrix grading upward into theTununk Member.

Unstratified matrix and randomlyoriented shells.

Moderate bioturbation withobvious Ophiomorpha tracesin some places. The base ofthis unit is often marked bya Glossifungites ichnofaciesassemblage. Borings exist insome shells.

Occurs at the top and withinF8 and grades upward intothe Tununk Member at thetop of the Dakota Sandstone.

Oyster shell concentrationsin the shoreface

422Dakota

SandstoneSequence


Utah

east (Figure 7). Erosional relief of this facies intothe underlying Cedar Mountain Formation locallytoward the west, from the Denver Basin towardthe Colorado Plateau (Merewether and Cobban,1986; Mellere, 1994).

In the Henry Mountains, the Dakota Sand-stone was mapped and studied by Hunt et al.(1953) and later considered by Davidson (1967),Lawyer (1972), Peterson and Ryder (1975), andRyer (1984). These authors show that in the studyarea, the Dakota Sandstone ranges in thicknessbetween 0 and 38 m (125 ft), weathers to form athin series of ledges at the base of broad slopes ofthe overlying Tununk Member of the MancosShale, and was deposited across the beveled CedarMountain Formation or Brushy Basin Member ofthe Morrison Formation (Upper Jurassic). For thepurpose of discussion, these authors subdivide theunit into a lower or nonmarine member and anupper or marine member but indicate that no onesection of this unit can be considered as typical.Overall, these authors interpreted the unit to rep-resent the transgressive littoral deposits that wereformedwhen the Upper Cretaceous sea first spreadwestward across this region (Hunt et al., 1953).

Index fossils indicate that at least the upperpart of the Dakota Sandstone in this area is late Ce-nomanian in age and spans the ammonite zones be-tweenConlinoceras tarrantense (ca. 95.73 0.61Ma)and Vascoceras diartianum (ca. 93.99 0.72 Ma)(Peterson and Ryder, 1975; Ulin, 1999; Cobbanet al., 2000; Cobban et al., 2006; Laurin andSageman, 2007). Corbula, Inoceramus, and the oys-ter shells Flemingostrea prudentia (White), Pycno-donte newberryi (Stanton), and Exogyra (Costagyra)olisisponensis (Sharpe) in the upper Dakota Sand-stone are characteristic of normal marine inner-shelfenvironments (Peterson and Ryder, 1975; Frsichand Kirkland, 1986; Cobban et al., 2000). Lawyer(1972) determined that water depths associatedwith deposition of the upper Dakota Sandstonewere generally less than approximately 5 m (16 ft)in the northeastern part of the region (Lawyer,1972). The lowermost units of the Dakota Sand-stone have not been dated in this area, but the oc-currence of late Albian palynomorphs in the upperparts of the Cedar Mountain Formation in thesouthern margin of the Uinta Basin suggests thatthe Dakota Sandstone is likely contained entirelyreaches up to 6 m (20 ft). However, these con-glomerates form laterally extensive sheetlike de-posits that appear to be contained within broadMultiple cross sections were made from north tosouth and east to west to constrain the lithostrati-graphic and sequence-stratigraphic variations in theunit along directions approximately parallel to de-positional strike and dip.

Eleven environmentally significant lithofacieswere identified throughout the study area (Table 1).These are interpreted to represent environmentsranging from braided fluvial at the base of the unit,through estuarine channel and basin, tidal flat, tidalinlet, and coastal swamp in the central part of theunit, into upper and lower shoreface sandstoneswith condensed oyster shell deposits and sparsebentonite layers in the upper part.

Facies 1: Fluvial Conglomerate

Conglomerates (Table 1, Figure 6A) occur through-out most of the study area at the base of the DakotaSandstone and become finer grained from westto east. Units are commonly 5 m (16 ft) or less inthickness, but bodies up to 16 m (52 ft) thick occur(e.g., southeast of Blue Valley, section 17). Paleo-current directions measured within this facies showrelatively low divergence about mean directionsand commonly trend toward the east and north-within the Cenomanian Stage (Figure 5) (Currieet al., 2008).

FACIES ANALYSIS

This study of the Dakota Sandstone involved mea-suring, describing, and physically correlating strat-igraphic sections around the Henry Mountainssyncline. Sections were measured at a variety of lo-cations covering the entire region (Figure 1). Care-ful consideration was given to the sedimentology,ichnology, paleocurrent directions, and internal ar-chitecture in each section as well as to key surfacesthat could be traced laterally into adjacent sections.Antia and Fielding 423

Figure 6. Photographs of fa-cies identified in the DakotaSandstone: (A) conglomerate(facies 1); (B) trough cross-bedding in fluvial sandstone(facies 2); (C) sigmoidal cross-bedding in tidally influencedsandstone (facies 3); (D) mod-

erate bioturbation dominated by

424 Dakota Sandstone Sequence Stratigraphy, Henry Mountainpaleovalleys or channel belts, typically more than1 km (0.6 mi) in width and showing up to 20 m(66 ft) of erosional relief.

Rare lenses of coaly and carbonaceous mud-stone exist within conglomerates in some sections(e.g., Blue Valley sections 2 and 15) and are docu-

mented along with facies 4. In general, the dom-inance of coarse clastic sediments, rarity of finesheetlike geometry, and relatively low paleocurrentvariability suggest that facies 1 represents depositsof braided streams during the initial stages of de-position of the Dakota Sandstone.

Skolithos (s) trace fossils in facies 3.(E) Moderate to intense biotur-bation in inclined heterolithicmudstone (facies 3) dominatedby traces of Teichichnus (T),Planolites (P), and sparse syner-esis cracks (Sy). (F) Pinstripe-laminated siltstone of facies 3sharply overlain by conglom-eratic shoreface sandstone.(G) Coal seam containing pa-limpsest Thalassinoides (Th) bur-rows representing a Teredolitesichnofacies assemblage and atightly folded sandstone-filledfracture (IF). (H) Root traces (rt)in pinstripe-laminated siltstone(facies 3).s, Southeastern Utah

Facies 2: Fluvial Sandstone

This facies (Table 1, Figures 6B, 8A) occursthroughout most of the study area, interfinger-ing with or gradationally overlying conglomeratesof facies 1. It exists at two levels within the basalparts of the Dakota Sandstone and is composedof nonbioturbated sandstone in both sheet andmultistory multilateral amalgamated channel forms(Figure 8A). In some sections, these sandstones

form composite channel bodies less than 20 m(

Figure 8. (A) Photomosaic of section 6 in the Blue Valley area showing a composite fluvial channel body. (B) Photomosaic of section 3 in the Blue Valley area showing a cross section ofan estuarine channel dominated by inclined heterolithic stratification sharply overlying conglomerates (facies 1) and abruptly overlain by shoreface sandstones (facies 8).

426Dakota

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Utah

andst theparaic offSandstone and Carbonaceous Shale

Cross-sets in this facies (Table 1, Figures 6CF, 8B)are up to 30 cm (12 in.) thick in the lower parts ofthe unit (Figure 6CD), becoming thinner upwardwith massive ripple or planar laminations towardthe top. Paleocurrent indicators in this facies typi-interpreted as braided fluvial deposits because ofits sheetlike and multistorey multilateral amalgam-ated channel character and close association withconglomeratescharacteristics that are compara-ble to those of Mialls (1992) models of alluvialdeposits.

Facies 3: Tidally Influenced Estuarine

Figure 9. Photographs of facies identified within the Dakota Ssequence 1 showing transition from tidally influenced sandstones agrading upward into shoreface sandstone. (B) Coarsening-upwardmeters south of The Post 1 section with thickly bedded heterolithshoreface sandstone with chert pebbles and sparse oyster shells.cally have a multimodal to broadly bimodal (north-west to southeast) distribution (Figure 7). Channelequal in thickness to mudstone layers. Lateral ac-cretion surfaces are evident in the Blue Valley areawith internal cross-bedding (Figure 7) dipping athighly oblique angles to the accretionary surface.

Lateral accretion surfaces in combination withIHS occur continuously over several kilometers,which suggest that this facies represents intertidalpoint-bar deposits in belts of sinuous estuarinechannels (Dalrymple et al., 1992; Boyd et al.,2006). Sharply based lenses of pinstripe-laminatedsandstone and carbonaceous mudstone are inter-preted to represent abandoned estuarine channels.Bann et al. (2004) have described analogous faciesin the Pebbley Beach Formation in Sydney Basin,Australia.tone. (A) Contact zone between the base of cycle 2 and para-base, paleosols, and coal, abruptly overlain by offshore mudstonesequence at the top of the Dakota Sandstone near a few hundredshore mudstone and sandstone at the base grading upward intoforms up to 250 m (820 ft) in width and 5 m (16 ft)in thickness, filled with inclined heterolithicallystratified (IHS) carbonaceous mudstone and veryfine to fine-grained sandstone, are associated withthis facies in the northern part of the study area(Figure 8B) (e.g., Blue Valley sections 3 and 11). Insome sections, the upper part of the channel isfilled by sharply based lenses, about 50 m (164 ft)in width and as much as 1.5 m (5 ft) in thick-ness, consisting of horizontally to slightly inclinedpinstripe-laminated carbonaceous mudstone andvery fine to fine-grained sandstone (Figure 6F).Sandstone layers in these lenses are typically lessthan or equal to 0.6 cm (0.2 in.) thick and sub-

Facies 4: Paleosols, Coal, andCarbonaceous Shale

Paleosols (Table 1, Figure 6GH) typically under-lying coal seams or carbonaceous shale (O horizon)and consisting of moderately to poorly developedC horizons (parent rock material showing variabledegrees of enrichment of clay material, iron, alu-minum, or organic compounds) are preserved inthe northern and central parts of the study area inthe middle of the Dakota Sandstone (Figures 6H,9A). Coal and carbonaceous shale are common inoutcrops north of the Freemont River in the BlueValley area, in North Caineville Reef, and betweenBlind Trail and The Post. Typically, only one coalAntia and Fielding 427

Figure 10. Photographs of fa-cies identified within the DakotaSandstone. (A) Flood-plain mud-stone (facies 5). (B) Wavy toflaser-bedded sandstone inter-preted as tidal flat deposits(facies 6). (C) Part of a channelbody within the tidal inlet facies

showing lateral accretion sur-

428 Dakota Sandstone Sequence Stratigraphy, Henry Mountainseam can be identified and traced laterally for hun-dreds of meters to several kilometers. The thick-ness of the coal seam ranges between 0 and 65 cm(26 in.) with an average thickness of approximately30 cm (12 in.). Two coal seams, 30 and 15 cm (12and 6 in.) thick, occur in Jet Basin.

The C-horizon part of the paleosols is recog-nized by root traces (Figure 6H), spheroidal weath-ering, or complete homogenization of underlyinglithologies (Figure 9A). Commonly, only minutecoaly root traces are present immediately below acoal seam in carbonaceous shale and heterolithic

faces. (D) Internal cross-beddingwithin accretionary surfaces inthe panel C. (E) Shorefacesandstone showing a variablebut high degree of bioturbationand multiple bedsets. Mosttrace fossils visible in this imageconsist of long Skolithos (S) andOphiomorpha (Op). (F) Con-ichnus fugichnia (Fm) acrossmultiple bedding planes inshoreface sandstone (facies 8).(G) Oyster shell concentration(facies 11) at the top of the BitterCreek section composed ofExogyra (Ex) and Pycnodonte(Pn) oyster shells. (H) Pycnodontenewberryi oyster shell bed (fa-cies 11) at the top of the DakotaSandstone in the North CainevilleReef section.s, Southeastern Utah

(Figure 6G). Severe compaction of traces suggeststhat they were made while the coal was uncom-pacted. For this reason, the palimpsest traces areinterpreted as a Teredolites ichnofacies assemblageinto a transgressed peat substrate.

Lack of further horizonation in these paleosolssuggests that either the soil did not have enoughtime to become fully developed or that they rep-resent poorly drained substrates. Because of theclose association of this facies with coal seams andcarbonaceous shale, this facies is interpreted to rep-resent coastal mires, fringing estuarine basins, andabandoned estuarine channels.

Other coaly and carbonaceous mudstone lensesand drapes occur within conglomerates and sand-stone of facies 1 and 2 (e.g., Blue Valley sections 2and15). These lenses are lightly bioturbated, containabundant charcoal, and alternate with medium- tocoarse-grained pebbly sandstone layers. The car-bonaceous layers in this case range in thicknessbetween 0 and 30 cm (12 in.) and extend laterallytypically less than 10 m (

nogfaciem (

stone with 50% or more oyster shells commonlyoccur immediately below this layer and at multi-ple intervals within sandstones of facies 8. Thewell-cemented layers range in thickness from 30to 100 cm (1239 in.). The oyster shells consistmostly of Pycnodonte newberryi (Stanton), an un-identified pectinoid in the Blue Valley area, pre-dominantly Pycnodonte newberryi in the south (e.g.,Eggnog to Copper Creek Benches), a mixture ofPycnodonte newberryi, Exogyra (Costagyra) olisis-ponensis (Sharpe), and lesser amounts of Flemingo-strea prudentia (White) in the central part of theone layer of bentonite occurs in the northern partof the Blue Valley area extending continuouslybetween sections 11 and 8 (~1.5 km [0.9 mi]).Two layers of bentonite were identified south ofthe Freemont River between Blue Valley sections12 and 14 and one at the top of the Jet Basinsection. These layers occur within sandstones offacies 8 in the Blue Valley area and within oystershell concentrations (facies 11) in the Jet Basinsection.

Facies 11: Oyster Shell Concentrations

A bed up to 1.6 m (5 ft) thick of clast-supportedoyster shell breccia (Table 1, Figure 10G, H) existsat the top of the Dakota Sandstone throughoutthe study area. Well-cemented layers of sand-into shoreface sandstone (facies 8). The layers ofsandstone in this facies range in thickness from 5 to40 cm (216 in.) from the base to the top of theunit (Figure 9B). The topmost layer of sandstonecontains abundant chert pebbles and oyster shells.Shells are composed mostly of Exogyra costagyra(?) and Pycnodonte newberryii. This unit is inter-preted to represent lower shoreface environmentsof deposition and is similar to examples describedby Bann et al. (2004) from the Pebbley Beach For-mation in Sydney Basin, Australia.

Facies 10: Bentonites

Multiple 10- to 20-cm(48 in.) thick layers ofwhite,silty, reworked volcanic ash fallout (bentonites) oc-cur throughout the Blue Valley area both northand south of the Freemont River (Table 1). Onlystudy area (e.g., Blind Trail, The Post, Cedar CreekBenches, and Dugout Creek). Just north of ThePost section, a few articulated shells are present atthe top of the oyster bed, but in the rest of the studyarea, very few articulated shells were observed. Inparts of the BlueValley area and in some sections inthe southern part of the study area, the Pycnodonteconcentrations show paucity of the small caplikevalve and contain abundant chert granules. Over-all, any fragmentation of the oyster shells appearsto be caused by compaction, not transport. Manyof the shells are bored.

The almost intact preservation of very delicatepectinoid shells within the well-cemented layersin the Blue Valley area and lack of a hydrodynamic(convex up) orientation of the shells suggest thatdeposition occurred in an environment with mod-erate to low wave energy and cementation likelyoccurred rapidly. The large concentration of shellsand abundance of borings suggest that sedimenta-tion rates at the time of deposition must have beenlow. However, the moderate degree of sorting,which resulted in removal of the more delicatevalves of the Pycnodonte shells, and the presenceof chert granules along with the oysters are evi-dence to more energetic conditions involved inthe formation of these oyster shell concentrations.This perhaps indicates that the oyster concentra-tions formed under variable environmental en-ergy regimes or that the environment of deposi-tion changed through time.

Note that Flemingostrea prudentia is typicallyassociated with lagoonal brackish environments,whereas Pycnodonte newberryi and Exogyra areassociated with open-marine settings (Frsich andKirkland, 1986). The dominance of Pycnodonte andExogyra over Flemingostrea suggests that the en-vironment of deposition for this facies was mostlikely open marine. Therefore, these deposits areinterpreted as oyster accumulations on the shore-face. However, it is also plausible that fluvial out-put onto the shoreface promoted brackish waterconditions that allowed Flemisgostrea to temporar-ily colonize oyster shell lags on the sea floor, assupported by the presence of oyster shells in growthposition near the top of the section north of ThePost.Antia and Fielding 431

Figure 12. Cross sections and stratigraphic interpretations of the Dakota Sandstone in the Henry Mountains, southeastern Utah, including (A) cross section AA and (B) cross section BB.

432Dakota

SandstoneSequence


Utah

Figure 13. Cross-sections and stratigraphic interpretations of the Dakota Sandstone in the Blue Valley area in the Henry Mountains, southeastern Utah, including (A) cross-section CCand (B) cross section DD. Key to symbols in Figure 12.

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Overall, the low percentage of articulated shells,random orientation, and preferential preservationof themore robust shells, despite the low degree offragmentation, suggest physical reworking of theshells in situ. As suggested by Frsich and Kirkland(1986), in the short-term, condensed depositionlike this case is most easily accomplished by stormsand may range from winnowing of finer sedimentsbetween shells, leading to concentration of shellmaterial to short-term suspension of shells duringstorm surges followed by rapid redeposition. In thelonger term, however, this unit is interpreted torepresent condensed deposition caused by con-tinued transgression of the sea and starvation of thedeeper parts of the basin (Kidwell, 1989, 1991;Frsich and Pandey, 1999).

DISCUSSION

Sequence-Stratigraphic Framework

Several key stratigraphic surfaces were recognizedthroughout the study area using the facies schemepreviously developed (Figures 12, 13). Superposi-tion of the Dakota Sandstone on the scoured sur-face of the CedarMountain orMorrison formationsrepresents a major unconformity and a sequenceboundary. The hiatus associated with this boundaryhas been corroborated by other authors using paly-nomorph biostratigraphy. It has been determined tospan approximately the Late Albian and Early Ce-nomanian, where the upper Cedar Mountain For-mation is present and overall missing time increasestoward the west (Merewether and Cobban, 1986;Cobban et al., 2000; Currie et al., 2008).

Another sequence boundary is identified in themiddle part of the Dakota Sandstone by the super-position of fluvial conglomerate and sandstone(facies 1 and 2) over estuarine carbonaceous shale(facies 3) (Figures 12, 13). Such disjoint super-position of facies is readily observed in the cen-tral and southern parts of the study area but ismore cryptic toward the north, where the fluvialdeposits amalgamate, pinch out, or completelyerode underlying deposits at the base of the Da-kota Sandstone.434 Dakota Sandstone Sequence Stratigraphy, Henry MountainThree regionally extensive flooding surfaces arereadily recognized in the upper parts of the DakotaSandstone where shoreface sandstone, mudstone,or tidal inlet facies overlie fluvial, estuarine, orcoastal mire deposits, or as coarsening-upward suc-cessions passing upward from lower shorefacemudstone to shoreface sandstone and oyster shellconcentrations (Figures 1214).

The identified key surfaces permit subdivisionof the Dakota Sandstone in the Henry Mountainsarea into two sequences and three parasequences(Figures 1214). The lowest sequence (cycle 1)consists of laterally extensive braided fluvial con-glomerates (facies 1), which in parts, grade upwardinto fluvial sandstone (facies 2) and fluvioestua-rine sandstone and mudstone (facies 3) or areabruptly overlain by fluvioestuarine facies. Suchchange in depositional regime from braided flu-vial into fluvioestuarine suggests a changing rela-tive base level, from a stillstand or lowering baselevel to a transgressive trend (Figure 14). Therefore,the conglomerates at the base of this sequence havebeen interpreted as LST.

In some areas, erosional relief of as much as20 m (66 ft) within less than or equal to 10 km(6 mi) along strike is infilled by fluvial and es-tuarine facies of the Dakota Sandstone, which sug-gests that at least parts of the unit represent incisedvalley fills (Figure 12B). In other parts, the fluvialand estuarine deposits of the Dakota Sandstonehave more of a sheetlike cross-sectional geometry,and incised valleys, if present, are more difficult todelineate.

Superposition of continental (facies 1 and 2)over coastal or paralic (facies 3) facies across theupper sequence boundary indicates a transitionfrom a transgressive depositional regime to a re-gressive one (Figure 14). The fluvial sandstonesoverlying this sequence boundary are conglomer-atic at the base in some areas and transition upwardinto tidally influenced sandstones and estuarinemudstone (facies 3). Similarly to the underlyingsequence, the conglomeratic part of the sand-stone in this case may represent LST or early TSTdeposits.

Physical correlation of facies from the centralpart of the study area toward the north shows thats, Southeastern Utah

Figure 14. Paleobathymetric curve forthe Dakota Sandstone in the Henry Moun-tains showing the position of key strati-graphic surfaces (SB = sequence bound-aries; FS = flooding surfaces) comparedwith the Bitter Creek and The Post 1 sec-tions. Key to symbols in Figure 12.the TST part of cycle 1 pinches out between thesections at Blind Trail 1 and Bitter Creek, and onlysparse remnants are preserved in the northern partof the study area between Blue Valley sections 4through 7 (Figure 13A). Such absence of the basalsequence in the northern part of the study areamay be caused by erosion of the cycle 1 duringdeposition of cycle 2 or lack of deposition of cycle1 in this area.

The TST part of cycle 1 is covered by collu-vium or does not crop out south of the The Posttoward Halls Creek Overlook, where only con-glomerates are present. Cycle 1 crops out again atEggnog as fluvioestuarine deposits at the base ofthe Dakota Sandstone (Figure 12A). Eastward,from Eggnog to Hansen Creek, west cycle 1 be-comes progressively thinner, and at Copper CreekBenches, it is not present.Antia and Fielding 435

sandstone in the northern parts of the study areaCycle 2 is composed of a basal succession offluvial sandstone (facies 2) fining upward into tid-ally influenced sandstones (facies 3), paleosols,carbonaceous shale, and coal (facies 4) (LST toTST), and two overlying parasequences that com-prise the highstand systems tract. These subdivi-sions are only readily recognizable in the centralpart of the study area, from where they are corre-lated both to the north and south. BetweenEggnogand Saleratus Point, the basal part of cycle 2 ischaracterized by flood-plain mudstone (facies 5)at the base overlain by braided fluvial sandstone(facies 2) and estuarine sandstone (facies 3) to-ward the top. This cycle is abruptly overlain byparasequences 1 and 2, which are each composedof lower shoreface mudstone or muddy sand-stone (facies 9) coarsening upward into shorefacesandstone (facies 8) and oyster shell concentra-tions (facies 11) at the very top. The bases ofparasequences 1 and 2 are typically marked byGlossifungites or Teredolites (in coal) ichnofaciesassemblages.

The top of the Dakota Formation representsthe topmost flooding surface associated with theparasequences of cycle 2, and it is marked byoyster shell concentrations and a regional changeinto the Tununk Member of the Mancos Shale(Figures 1214). Only the uppermost two flood-ing surfaces, which enclose parasequence 2, canbe traced throughout the entire study area. Theother (lowermost) flooding surface, at the base ofparasequence 1, is not distinguishable in the north-ern outcrops. The lack of such flooding surface inthis area could be attributed to local amalgamationof parasequences 1 and 2, erosion of parasequence 1before deposition of parasequence 2, or possiblyparasequence 1 being a lateral equivalent of thebasal part of cycle 2 and never having been de-posited in the northern parts of the study area.

Root traces at the base of parasequence 1 inThe Post section could be evidence that instead ofa flooding surface, this contact is actually a se-quence boundary, yet such evidence is not presentin any of the other measured sections. The pres-ence of such root traces at the base of parase-quence 1 at The Post may also be caused by lo-calized erosion of the substrate during marine436 Dakota Sandstone Sequence Stratigraphy, Henry Mountainindicates that there was erosion of underlying li-thologies before deposition of the overlying units.Therefore, the base of the shoreface sandstone(facies 8) in this area is interpreted as a wave ra-vinement surface and transgressive surface of ero-sion. Similarly, in the southern parts of the studyarea, the base of tidal inlet deposits (facies 7) isinterpreted as a tidal ravinement surface.

In the Blue Valley area, the sharp superposi-tion of shoreface sandstone over bentonite layers(facies 10) with firm ground ichnofacies assem-blages at the top represents locally preserved waveravinement and hiatus surfaces within the shore-face succession. Such surfaces suggest that multi-ple cycles of shoreface deposition may have oc-curred during deposition of the upper parts of theDakota Sandstone, but they have been obscuredby amalgamation of shoreface sandstone in otherparts of the study area.

The correlation of the identified sequences andparasequences to the east along cross sections DD(Figure 12C) is speculative because the units do notcrop out continuously from one section to the next.Therefore, the correlations shown in Figure 12Care meant to show the similarities in facies acrossthe direction of depositional dip. However, com-parison with examples from the literature (amEnde, 1991; Leckie and Singh, 1991; Willis, 1997;Ulin, 1999; Holbrook, 2001; Holbrook et al.,2006; Laurin and Sageman, 2007) shows thatsuch a correlation is likely too simplistic given thecomplexity observed within the unit in other partsof the study area. The facies scheme and sequence-stratigraphic considerations above were used to re-construct the paleogeography of the area for eachdepositional cycle as shown in Figure 15.

Nature of the Depositional System

On a broad scale, the parasequences within cycle 2define a tripartite zonation from predominantlytransgression and not necessarily recording relativesea level lowering throughout the region.

The presence of coal rip-up clasts within theconglomeratic interval at the base of the shorefaces, Southeastern Utah

Figure 15. Paleogeographic interpretations of cycles 1 and 2 associated with deposition of the Dakota Sandstone in the Henry Mountains region. Channel belts depicted in these mapswere interpolated among measured sections given general paleocurrent trends, similarities in lithofacies, and geographic position, but no subsurface data were used to corroborate theinterpretations.

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fluvial deposits at the base, grading upward intofluvioestuarine sandstone and estuarine mudstone,which are abruptly overlain by shoreface sand-stone and mudstone. Such a tripartite zonation hasbeen identified as typical of wave-dominated es-tuaries (Dalrymple et al., 1992; Boyd et al., 2006).On a smaller scale, the fluvioestuarine sandstonesand estuarine mudstones at the base of cycle 2show evidence of tidal modulation during sedi-ment accumulation. This indicates that inter-nally, the estuary associated with deposition ofthe Dakota Sandstone in the Henry Mountainsregion was influenced by tidal activity, but overall,the morphology of the system was dominated bywave activity.

Paleocurrent directions in the shoreface fa-cies atop the Dakota Sandstone indicate south-to-southwestward sediment transport (Figure 7).Beds have characteristics that suggest depositionduring storm events (e.g., long fugichnia traces andincreased bioturbation at the top of cross-bed sets;Figure 10E, F). Shingling of the cross-bed sets inthe shoreface sandstone toward the east and north-east in combination with fluvial paleoflow broadlyoriented toward the northeast suggests that theoverall direction of progradation for the DakotaSandstone in the study area was toward the eastand northeast (Figure 15). Such a direction of pro-gradation implies that the paleoshoreline asso-ciated with the Dakota Sandstone in this areawould have been oriented broadly northwest-southeast or north-south during cycle 2 (Figure 15).Therefore, the paleocurrent mode for shorefacesandstones may represent onshore-directed trans-port, longshore paleocurrents, or a combination ofboth. Analogous modern and ancient exampleswere described by Howard and Reineck (1981)and Clifton (2006).

Climate and oceanic circulationmodels suggestthat a counterclockwise water circulation gyre wasactive within the KWIS during the CenomanianTuronian (Slingerland et al., 1996). It has also beensuggested that mean annual wind fields over theKWIS consisted of easterly winds in the north andsouthwesterly winds in the south, with winter stormstracking far south over the seaway (Slingerlandet al., 1996). Such climate models emphasize the438 Dakota Sandstone Sequence Stratigraphy, Henry MountainUlin (1999) divided the Dakota Sandstone intosix units on the basis of recognition of major re-gionally traceable bounding surfaces (flooding sur-faces, sequence boundaries), comprising depositsof the following depositional environments frombase to top of the unit: braided fluvial (sequence 1),anastomosed fluvial (S2), tidally influenced fluvial(S3a), tide-dominated estuarine (S3b), offshoreto shoreface (S4 and 5), estuarine and offshore toshoreface (S6a), and estuarine and upper shore-face (S6b). Similar depositional environments arealso recognized in the Henry Mountains region.

The framework developedbyUlin (1999)waslater extended into the Paunsaugunt andMarkaguntTuronian period and not the Cenomanian, butgiven the paleocurrents recorded in the shorefacefacies of the Dakota Sandstone in the study area,these climatemodels can be considered applicableto the Dakota Sandstone in the Henry Mountainsregion.

The abundance of lower shoreface mudstonesbetween Blind Trail and The Post suggests thatthis area may have been at a relatively more distallocation compared with areas to the north andsouth (Figure 12B). Therefore, the central part ofthe study area may have been part of an embay-ment in the shoreline.

Comparison with the Kaiparowits Plateau,Uinta Basin, and Northwestern New Mexico

Recent studies have shown that multiple cyclesof deposition are preserved within the DakotaFormation in the Kaiparowits Plateau and UintaBasin (am Ende, 1991; Ulin, 1999; Laurin andSageman, 2007; Currie et al., 2008). In exposuresaround the southern margin of the Uinta Basin,Currie et al. (2008) subdivided the Albian to Ce-nomanian Dakota Sandstone into two storeys offluvial channel sandstone and conglomerate over-lain by gray smectitic and carbonaceous over-bank deposits, separated by a sequence-boundingunconformity.

In the Kaiparowits Plateau, Gustason (1989)first developed a detailed stratigraphic frame-work for the Dakota Sandstone, which was laterrefined by am Ende (1991) and Ulin (1999).s, Southeastern Utah

Figure 16. Comparison of facies and sequence-stratigraphic interpretations for the Dakota Sandstone in the Kaiparowits Plateau (Ulin, 1999), the Henry Mountains (this study), andthe southern Uinta Basin (Currie et al., 2008), used with permission from the Utah Biological Survey. Note that the side-by-side comparison of the three sequence stratigraphic models isnot meant to imply that the sequence-stratigraphic surfaces are the same age in all three locations. Instead, this figure aims to show the striking similarities among the three sections,both from a lithostratigraphic and sequence-stratigraphic perspective. FS = flooding surface; SB = sequence boundary; TS = transgressive surface of erosion.

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440 Dakota Sandstone Sequence Stratigraphy, Henry Mountainplateaus to the west of the Kaiparowits Plateauby Laurin and Sageman (2007), building uponthe previous work of Elder et al. (1994). Theseauthors developed a high-resolution time scale forthe unit based on fossil evidence and recognizedMilankovitch band cyclicity of ca. 65 to 160 kaand ca. 20 to 40 ka periodicities within the DakotaFormation. Such cyclicity is comparable to cyclesobserved in the equivalent Bridge Creek Lime-stone of the Greenhorn Formation in the Denver

Basin far to the east and is interpreted to indicateorbital forcing of insolation and climate during themiddle Cretaceous (Laurin and Sageman, 2007).

The sequence-stratigraphic facies subdivisionsof the Dakota Sandstone in the Henry Mountainsregion share significant similarities to the interpre-tations for the Kaiparowits Plateau about 80 km(50 mi) to the west-southwest (Gustason, 1989;am Ende, 1991; Ulin, 1999) and to the San Juanand Acoma basins 100 to 200 km (62124 mi) to

Figure 17. Comparison of the stratigraphy in the Kaiparowits Plateau (KP), the Henry Mountains (HM), and northwestern New Mexico(SJ). (A) Isopach map of Lower Cretaceous rocks in Utah and western Colorado and interpreted during the Early Cretaceous foreland-basin system depozone (source: Currie, 2002). (B) Comparison of the stratigraphic characteristics and thickness between the KP (takenfrom Ulin, 1999), HM, and San Juan Basin (based on discussions by Owen, 1969; Aubrey, 1989; Lucas et al., 1998). Note that the linesdrawn among the stratigraphic sections are not meant to imply that the sequence-stratigraphic surfaces are the same age in all threelocations. These lines are just meant to show the similarities among the three sections at similar stratigraphic levels. (C) Paleocurrents ofthe Dakota Formation in the Colorado Plateau as reported by Mellere (1994), Lucas et al. (1998), Ulin (1999), Currie et al. (2008), andthis study. (D) Time-stratigraphic diagram of the Dakota Sandstone among the KP, HM, and San Juan Basin. (E) Legend of symbols fordiagrams A to D.s, Southeastern Utah

the southeast (Figures 16, 17) (Owen, 1969;Aubrey, 1989; Mellere, 1994; Lucas et al., 1998).Overall, the identified facies are the same, exceptthat anastomosed fluvial deposits were not ob-served in this study and are not clearly delineatedin northwestern New Mexico, and a second storyof braided fluvial deposits, which do not occur inthe Kaiparowits Plateau or in northwestern NewMexico, is present in the Henry Mountains region(Figure 16). In this regard, the basal part of theDakota Sandstone in the Henry Mountains regionappears to be more similar to exposures in thesouthern margin of the Uinta Basin, where twostories of fluvial deposits overlain by estuarineand coastal heterolithic carbonaceous mudstoneshave been identified (Figure 16) (Currie et al.,2008).

Ulin (1999) showed that in the KaiparowitsPlateau, the Dakota Sandstone can be subdividedinto six units, and pointed out that evidence ofsubaerial exposure below the surfaces boundingsuch units indicates that these are stratigraphicsequences, not parasequences (Figure 16). Suchevidence of subaerial exposure at the boundariesof each unit was not detected in the Henry Moun-tains region. Thus, the units are interpreted asparasequences, not sequences. Any evidence ofsubaerial exposure between these parasequencesmay have been erased by erosion during depo-sition of the overlying parasequence. If this isthe case, then perhaps Ulins (1999) sequence-stratigraphic framework can be extended into theHenry Mountains region.

In this regard, note that the stratigraphic sec-tion studied by Ulin (1999) in the KaiparowitsPlateaumeasures close to 70m (230 ft) in the westand thins down to just more than 20 m (66 ft) inthe east over a distance of about 70 km (43 mi).The section in the Henry Mountains region, how-ever, measures at its thickest 38 m (125 ft) andaverages about 10 m (33 ft) overall. This showsthat theHenryMountains region represents amuchlower accommodation setting than the KaiparowitsPlateau and other areas to the east and southeastin western Colorado and northwestern New Mex-ico (Currie, 2002). It also implies that fluctuatingbase level would have had a stronger impact onthe stratigraphy of this area, with less preserva-tion potential of the geologic record. Hence, itseems likely that evidence of subaerial exposurebetween the identified parasequences may havebeen eroded away during deposition of the over-lying unit.

In northwestern New Mexico, the DakotaSandstone is also thicker (~80m [262 ft]) than inthe Henry Mountains and contains similar depositsto those in the Kaiparowits Plateau. The sequence-stratigraphic framework for the unit is not welldeveloped in this area. However, the sedimento-logic similarities are striking. In the San Juan andAcoma basins of northwestern New Mexico, theDakota Sandstone is subdivided into the braidedfluvial Encinal Canyon Member, overlain by carbo-naceous coastal plain deposits of the Oak CanyonMember, and multiple marginal marine sandstonebodies, namely Cubero, Paguate, and Twowellstongues, which interfinger with the Mancos Shale(Figure 17) (Owen, 1969; Aubrey, 1989; Mellere,1994; Lucas et al., 1998). It is unclear whether theOak Canyon Member contains deposits associatedwith anastomosing fluvial systems, nevertheless,it is obvious that the Kaiparowits Plateau sectionand the Dakota Sandstone in northwestern NewMexico are significantly similar. Furthermore,Mellere (1994), in a reassessment of the TwowellsTongue in the Acoma Basin, identified an uncon-formity at the base of the unit overlain by estuarinevalley-fill deposits that grade upward intomarginalmarine sandstone and the Mancos Shale, which isanalogous to the sequences in the upper parts ofthe Dakota Sandstone in the Kaiparowits Plateaudescribed by Gustason (1989), am Ende (1991),and Ulin (1999).

Two important differences between these areasare (1) the greater presence of marine shale inter-tonguing with the Dakota Sandstone in north-western New Mexico compared with the Kaiparo-wits Plateau and the Henry Mountains and (2)diverging paleocurrent directions in the fluvialfacies between the HenryMountains and adjacentareas to the east and west (Figure 17). The greaterpresence of marine shale intertonguing with theDakota Sandstone in northwestern New Mexicois to be expected because this section would haveAntia and Fielding 441

The observed variations in thickness of the unit

also indicate that the forebulge remained as a rela-tively elevated and nonsubsiding region comparedwith the foredeep and backbulge basins, which re-sulted in differential accommodation for accumu-lation of the Dakota Sandstone. The presence ofbraided fluvial deposits of similar thickness at thebase of the Dakota Sandstone across all three ofthese tectonic realms, however, suggests that dif-ferential subsidence of the foredeep and backbulgein this period did not become significant until afterdeposition of these fluvial lithofacies. Differentialsubsidence after deposition of the braided fluvialdeposits may explain the presence of anastomosingbeen in a more distal setting than the Kaiparowitsduring deposition of the unit. The fluvial paleo-currents, however, suggest that there might havebeen some structural control to the distributionof fluvial drainage in the Henry Mountains regionduring the time of deposition of the Dakota Sand-stone. Fluvial paleocurrents in the KaiparowitsPlateau and in northwestern New Mexico havesimilar northwest-to-southeast trends, whereas inthe Henry Mountains, the trend is predominantlytoward the northeast, parallel to the interpretedorientation of the forebulge (Figure 17).

Tectonic Configuration of the KWIS around theStudy Area

The variation in thickness of the Dakota Sandstone,approximately 70 m (~230 ft) in the western partsof the Kaiparowits Plateau, less than 38 m (

Characteristics of Potential Reservoirs

No hydrocarbon potential has been reported forthe Dakota Sandstone in the Henry Mountainssyncline. However, the well-exposed record of thisunit in the region can be used to guide explorationin analogous low-accommodation depositional set-tings throughout the KWIS and around the world.In this regard, the Dakota Sandstone in the HenryMountains has many elements that under the ap-propriate geologic conditions could result in a suc-cessful hydrocarbon play. The carbonaceous mud-rocks may be potential source rocks and seals alongwith the encasingmudrock-dominated units,MancosShale above and Cedar Mountain Formation below.

Potential reservoirs occur in fluvial conglom-erate and sandstone, tidal inlet facies, and shore-face sandstone. The more laterally continuous po-tential reservoir facies are shoreface sandstones.These sandstones represent multiple condensedparasequences and thus are likely to be verticallycompartmentalized. Also, of all the potential res-ervoirs, shoreface sandstones appear to contain sig-nificant amounts of mud in many areas, whichwould reduce the potential for desirable perme-ability in the unit. Tidal inlet facies are less muddythan shoreface sandstone, but they are typicallyenclosed by shoreface sandstone units. Thus, thetrap potential for this facies would likely be low.Fluvial conglomerates and sandstone, on the con-trary, are commonly encased within mudstone fa-cies (e.g., estuarine, coastal flood plain, and theCedar Mountain Formation), appear to containonly small percentages of fines (silt and clay), andcementation does not appear to be pervasive. Thisfacies appears to be contained within incised val-ley or channel belts and thus occurs predominantlywithin linear trends oriented broadly southwest tonortheast.

CONCLUSIONS

The Dakota Sandstone in the Henry Mountainsrepresents a low-accommodation depositional set-ting on the western margin of the CretaceousWestern Interior Basin. The unit preserves a recordof two sequences, the upper one of which containstwo parasequences. These sequences and parase-quences are dominated by continental to shallow-marine deposits. The basal conglomerates in thesesequences represent LST deposits, but overall, thesection is dominated by TST deposits.

The units recorded in the Henry Mountainsregion are similar to those reported from theKaiparowits Plateau by Ulin (1999) and fromnorthwestern NewMexico (Owen, 1969; Aubrey,1989; Mellere, 1994; Lucas et al., 1998); roottraces are associated with the flooding surfacesat The Post, but such evidence of subaerial expo-sure at the bounding surfaces was not detected inany other section. Therefore, the two upper unitsare classified herein as parasequences and not se-quences. However, top truncation by wave andtide ravinement caused by low accommodation inthe areamay have eroded any evidence of subaerialexposure between the units. Thus, these units pos-sibly are actually sequences as in the KaiparowitsPlateau.

The Dakota Sandstone in the Henry Moun-tains preserves a record of multiple cycles of sed-iment accumulation driven by fluctuations in rel-ative sea level. The presence of similar cycles inKaiparowits Plateau and in northwestern NewMexico, which represent the more rapidly sub-siding foredeep and backbulge tectonic realms,respectively, suggests that the dominant agentdriving such cycles must have been eustatic fluc-tuations. Such interpretation is reinforced by thehigh frequency of these cycles calculated by Ulin(1999) and Laurin and Sageman (2007), whichcorrelates to global sea level (Gale et al., 2008).

Cycle 2 in the middle to upper parts of theDakota Sandstone can be subdivided into threeparts: fluvial facies at the base, estuarine channelsand basin facies in the middle, and shoreface faciesat the top, which is analogous to facies models ofwave-dominated estuaries. Internally, however, theunit also shows evidence of tidal modulation ofsediment deposition.

Potential hydrocarbon reservoirs in settingssimilar to the Dakota Sandstone may be presentin shoreface sandstones, tidal inlet facies, and flu-vial sandstones and conglomerates. However, lowAntia and Fielding 443

Bhattacharya, J. P., and B. J. Willis, 2001, Lowstand deltas inthe Frontier Formation, Powder River Basin, Wyoming:Implications for sequence-stratigraphic models: AAPG

Bulletin, v. 85, no. 2, p. 261294

Boyd, R., R.W. Dalrymple, and B. A. Zaitlin, 2006, Estuarineand incised valley facies models, in H. W. Posamentierand R. G. Walker, eds., Facies models revisited: SEPMSpecial Publication 84, p. 171236.

Brenner, R. L., G. A. Ludvigson, B. J.Witzke, A. N. Zawistoski,E. P. Kvale, R. L. Ravn, and R. M. Joeckel, 2000, LateAlbian Kiowa-Skull Creek marine transgression, LowerDakota Formation, eastern margin of Western Interiorseaway, U.S.A.: Journal of Sedimentary Research, v. 70,accommodation in this setting resulted in a high de-gree of compartmentalization and abundant bafflezones in these potential reservoirs. The best reser-voirs would most likely occur in fluvial sandstonesbecause they are better sorted, are free of detritalmud, and are encased inmudstones that may act assealing lithologies in stratigraphic traps.

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