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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1997)109<1206:SSONJC>2.3.CO;2 1997;109;1206-1222Geological Society of America Bulletin

Brian S. Currie foreland-basin system

Cretaceous rocks, central Cordilleran−Sequence stratigraphy of nonmarine Jurassic

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ABSTRACT

An analysis of the Upper Jurassic–Lower Cretaceous Morrison and CedarMountain Formations of Utah and Coloradohas resulted in a general sequence-strati-graphic model for nonmarine rocks. In thismodel, nonmarine deposition is governed bychanges in basin accommodation develop-ment and corresponding shifts in depositionalbase level. These fluctuations result in deposi-tion of systematically varying facies and ar-chitectural elements that allow nonmarinedepositional sequences to be recognized. In-ternally, nonmarine depositional sequencescomprise three systems tracts—degradational,transitional, and aggradational—which areanalogous to the lowstand, transgressive, andhighstand systems tracts of marine deposi-tional sequences. Degradational systemstracts overlie sequence-bounding unconformi-ties and consist of relatively coarse-grained,low-sinuosity fluvial deposits that are eithercontained within incised valleys or depositedas widespread, thin sheets above shallow ero-sion surfaces. Transitional systems tracts rep-resent an increase in basin accommodationdevelopment following degradational systems-tract deposition. They are marked by the tran-sition from laterally continuous, low-sinuosityfluvial channel sandstones and conglomeratesto lenticular and ribbon-like, meandering andanastomosing channel sandstones. Aggrada-tional systems tracts are characterized by me-andering-anastomosing channel sandstonesand abundant fine-grained overbank and la-custrine deposits.

The Upper Jurassic–Lower Cretaceousnonmarine rocks of the study area containthree depositional sequences. The first ofthese, the UJ-1 sequence, consists primarilyof an aggradational systems tract overlain bya sequence-bounding unconformity. How-ever, the lower parts of this sequence are

transitional with older marine rocks and canbe considered the late stages of a marinehighstand systems tract. The upper Morri-son sequence (UJ-2) consists of degrada-tional, transitional, and aggradational sys-tems tracts. Above the UJ-2 sequence are asequence-bounding unconformity and degra-dational and transitional systems tracts ofthe LK-1 sequence represented by the Buck-horn Conglomerate. The Buckhorn is over-lain by a sequence-bounding unconformityand transitional-aggradational systemstracts of the LK-2 sequence that is composedof the upper part of the Cedar MountainFormation. The Upper Jurassic–Lower Cre-taceous sequences in Utah and Colorado canbe traced regionally and correlated with non-marine depositional sequences in centralUtah and sequences that contain nonmarine,transitional, and marine rocks in centralWyoming. These sequences were deposited inthe back-bulge, forebulge, and distal fore-deep depozones of the Late Jurassic–EarlyCretaceous foreland-basin system.

INTRODUCTION

Since its inception, sequence-stratigraphicanalysis has become a valuable tool for interpret-ing extrinsic controls on, and correlating marinestrata in sedimentary basins (e.g., Vail et al.,1977; Van Wagoner et al., 1988; Baum and Vail,1988). On the basis of studies of marine rocks,sequence-stratigraphic concepts recently havebeen proposed for, and applied to, fluvial-domi-nated, nonmarine basins (e.g., Posamentier andAllen, 1993, Wright and Mariott, 1993; Shanleyand McCabe, 1991, 1994; Olsen et al., 1995). Al-though these models have established a frame-work for expected stratigraphic relationships inalluvial depositional sequences, field documen-tation of nonmarine depositional sequences, theirbounding surfaces, internal architecture, and re-lated systems tracts, remains sparse.

One of the basic tenets of sequence stratigra-phy is that depositional sequences are depositedin response to changes in the magnitude and rate

of accommodation development (Posamentier etal., 1988). In fluvial-dominated nonmarine sys-tems, the development of accommodation (orspace for potential sediment accumulation) isgoverned by the equilibrium stream profile andbase level, to which the profile ultimately isgraded (e.g., Mackin, 1948; Shanley and Mc-Cabe, 1991). Fluctuations in base level, such asa rise or lowering of relative sea level, can causereadjustment of fluvial equilibrium profiles andan increase or decrease in nonmarine accommo-dation (Fig. 1; Posamentier and Vail, 1988;Miall, 1991; Shanley and McCabe, 1994). How-ever, variations in sediment supply, stream dis-charge, or intrabasin tectonism may also resultin the readjustment of fluvial equilibrium pro-files and the creation or destruction of accom-modation (e.g., Lane, 1955; Bull, 1991; Blum,1992; Posamentier and James, 1993).

Variations in the amount and rate of accom-modation development may result in depositionof systematically varying facies assemblagesthat have distinctive architectural relationships(Shanley and McCabe, 1991, 1994; Wright andMarriott, 1993; Olsen et al., 1995). Identifica-tion of these assemblages, their related architec-tural elements, and bounding unconformities al-lows nonmarine depositional sequences andtheir internal systems tracts to be recognized.Such recognition permits prediction and re-gional correlation of genetically related pack-ages of rocks based on sequence position andorganization, even if there are limited chrono-stratigraphic data. This, in turn, may allow re-construction of basin deposystems throughtime, and assist in identifying factors that influ-enced basin development.

Throughout the western interior UnitedStates, Upper Jurassic–Lower Cretaceous strataare characterized by alternation of coarse-grained, low-sinuosity fluvial deposits and fine-grained, high-sinuosity–anastomosing fluvialand lacustrine deposits. This stratigraphic rela-tionship and the widespread distribution of theserocks make them prime candidates for se-quence-stratigraphic interpretation. Using se-quence-stratigraphic concepts, four depositional

Sequence stratigraphy of nonmarine Jurassic–Cretaceous rocks,central Cordilleran foreland-basin system

Brian S. Currie* Department of Geosciences, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721

GSA Bulletin; September 1997; v. 109; no. 9; p. 1206–1222; 13 figures; 2 tables.

*E-mail: [email protected]

currie.qxd 10/31/97 3:06 PM Page 1206



sequences have been identified in the Juras-sic–Cretaceous nonmarine rocks of the centralCordilleran foreland-basin system in Utah,Wyoming, and Colorado. These sequences arerecognized on the basis of changes in deposi-tional architecture and the presence of regionalunconformities. Deposition of these sequencescan be directly tied to the evolution and migra-tion of individual foreland-basin system depo-zones through time (DeCelles and Currie, 1996;Currie, 1997).

The purpose of this paper is to present a gen-eral sequence-stratigraphic paradigm for nonma-rine rocks that is based on fluctuations in basinaccommodation. After describing the model andits application in nonmarine foreland-basin sys-tems,the Upper Jurassic–Lower Cretaceous non-marine rocks of northeastern Utah and north-western Colorado are analyzed, and changes indepositional architecture are placed in a se-quence-stratigraphic framework. A regional cor-relation based on the sequence-stratigraphicmodel is presented for the Upper Jurassic–LowerCretaceous nonmarine rocks of the Western Inte-rior, and the relationship between accommoda-tion development and evolution of Late Jurassicand Early Cretaceous Cordilleran foreland-basinsystem depozones is discussed.

SEQUENCE-STRATIGRAPHIC MODELFOR ALLUVIAL STRA TA

The sequence-stratigraphic model utilized inthis study is based on changes in the magnitudeand rate of basin accommodation developmentin fluvial-dominated nonmarine basins. Becausenumerous factors can influence the creation ordestruction of accommodation within a basin(i.e., eustasy, tectonics,sediment supply, and cli -mate), the particular controls on the develop-ment of space for potential sediment accumula-tion are not specified. In general, however, adrop in relative sea level, an increase in fluvialdischarge, tectonic uplift within the basin,or adecrease in sediment supply will cause a down-ward shift of graded fluvial profiles, and anoverall decrease in basin accommodation (Posa-mentier and Vail, 1988; Bull,1991; Blum,1992;Shanley and McCabe, 1994). As fluvial systemsattempt to reestablish their equilibrium profiles,sediment bypass and stream incision may occur(Mackin, 1948; Leopold and Bull,1979).

An upward shift in equilibrium fluvial pro-files,due to a rise in relative sea level, decreas-ing discharge, basin subsidence, or increasedsediment supply, will generate space for poten-tial sediment accumulation (Posamentier andVail, 1988; Bull,1991; Posamentier and Allen,1993; Shanley and McCabe, 1994). This will bemanifested by an increase in channel and flood-

plain aggradation as fluvial systems deposit sed-iment to maintain or reestablish a graded profile(Mackin, 1948; Leopold and Bull,1979).

During periods of reduced accommodation,sediment bypassing and valley incision mayerode the preexisting flood plain and drasticallyreduce sediment accumulation in the basin(Posamentier and Vail, 1988). An increase instream power (due to an increase in slope or dis-charge) and the ability of f luvial systems totransport coarse-grained sediment may be asso-ciated with accommodation reductions (Bag-nold, 1977; Leopold and Bull,1979). This maybe accompanied by a transition to low-sinuos-ity–braided channel morphologies as fluvialsystems adjust to increased slope, proportion ofbedload transport, or discharge variations(Schumm,1981; Ferguson,1987; Orton andReading, 1993). Low aggradation rates mayalso promote extensive reworking of fine-grained overbank material due to lateral chan-nel migration or avulsion (Posamentier andVail, 1988; Holbrook,1996). As a result,fluvialsediments deposited during low rates of accom-modation development may be dominated byrelatively coarse-grained channel bodies thatare thin but laterally widespread (Wright andMarriott, 1993). If valley incision occurs duringthese periods,associated fluvial deposits shouldbe restricted to paleovalleys (Wright and Mar-riott, 1993; Shanley and McCabe, 1994).

The factors producing an increase in accom-modation may result in a reduction in streampower and proportion of transported bedload andfacilitate a change from low-sinuosity–braided to

high-sinuosity–anastomosing channel morpholo-gies (Schumm,1981; Ferguson,1987; Orton andReading, 1993). This may result in stabilizationof the flood plain,decreased rates of lateral chan-nel migration, and preservation of increasingamounts of fine-grained material (Posamentierand Vail, 1988; Shanley and McCabe, 1991).High rates of aggradation will produce channelsandstones that are isolated both vertically andlaterally by fine-grained overbank material (e.g.,Allen, 1978; Bridge and Leeder, 1979). Duringthe latest stages of an overall period of accom-modation increase, decreasing fluvial aggrada-tion will produce increasingly amalgamated flu-vial channels and a transition from isolated tomore laterally continuous channel forms (Wrightand Marriott, 1993; Shanley and McCabe, 1994).

These facies assemblages can be described insequence-stratigraphic terms as degradational,transitional,and aggradational systems tracts,each respectively related to increasing rates ofaccommodation development. The new termsare contrasted with the terms lowstand, trans-gressive, and highstand used in marine se-quence stratigraphy, because architectural anddeposystem characteristics in nonmarine basinsare not necessarily related to fluctuations in eu-static or relative sea level, only to increasing ordecreasing amounts and rates of accommoda-tion development within a basin. As stated ear-lier, overall, variations in nonmarine accommo-dation may be influenced by tectonics,climate,and sediment supply in addition to eustasy. Asnoted by Shanley and McCabe (1994),it maybe impossible to correlate corresponding sys-

SEQUENCE STRATIGRAPHY OF NONMARINE JURASSIC–CRETACEOUS ROCKS

Geological Society of America Bulletin,September 1997 1207

Figure 1. The effects on nonmarine accommodation due to changes in base level. (a) Loweringof base level from 1 to 2 results in reduction in accommodation and incision of the preexistingfluvial equilibr ium profile. (b) Increased fluvial accommodation due to a rise in base level. Notethat variations in sediment supply, stream discharge, or intr abasin tectonism may also result inthe readjustment of fluvial equilibr ium profiles and may increase or reduce nonmarine accom-modation. Modif ied from Posamentier and Vail (1988) and Shanley and McCabe (1994).




tems tracts in coeval nonmarine and marine set-tings in areas where the nonmarine-marine tran-sition is not preserved or exposed. This does notaffect the utility of classifying architecturalchanges in alluvial stratigraphic sequences interms of systems tracts. As shown in the follow-ing, this scheme allows regional stratigraphiccorrelations and architectural predictions.

The architectural elements associated withdegradational, transitional,and aggradationalsystems tracts are shown in Figure 2. The degra-dational systems tract is shown at time 1. Dur-

ing the highest rates of accommodation de-crease, a drop in the graded stream profile mayresult in sediment bypass,widespread flood-plain erosion,and stream incision (Posamentierand Vail, 1988). Following the initial phases oferosion,fluvial systems deposit coarse-grainedsediment in low-sinuosity braided fluvial chan-nels within the valley (Wright and Marriott,1993; Shanley and McCabe, 1994; Olsen et al.,1995). The unconformity associated with thedevelopment of a degradational systems tractmay permit well-developed paleosol zonation

(Wright and Marriott, 1993) and diagenetic al-teration of preexisting alluvial sediments in in-terfluve areas.

At time 2,an increase in basin accommoda-tion development marks the beginning of transi-tional systems-tract deposition. As rates of ac-commodation increase, fluvial aggradation fillsthe valley. Decreasing stream power accompa-nying this filling may result in deposition offiner grained channel sandstones and increasingamounts of overbank material (Wright and Mar-riott, 1993; Shanley and McCabe, 1994). Oncethe valley is filled, deposition occurs above theunconformity surface on the preexisting inter-fluve flood plain (Posamentier and Vail, 1988).In addition, flood-plain stabilization that accom-panies this change may result in preservation ofthin paleosols (Wright and Marriott, 1993).

The aggradational systems tract is shown attime 3. During the highest rates of accommoda-tion development within the basin,increasedvertical aggradation results in deposition of lat-erally discontinuous (meandering-anastomos-ing) channel sandstones and abundant fine-grained overbank and lacustrine sediments(Shanley and McCabe, 1991,1994; Wright andMarriott, 1993; Olsen et al.,1995).

At time 4,as rates of accommodation devel-opment decrease, aggradation slows and chan-nel bodies become increasingly amalgamated(Wright and Marriott, 1993; Olsen et al.,1995).Preserved overbank sediments may display in-creasingly mature paleosols as flood-plainaggradation decreases. A summary of the non-marine systems tracts including accommoda-tion trends,depositional responses,and internalarchitectural elements is listed in Table 1.

Overall, increasing rates of basin accommo-dation development are represented by a higherproportion of preserved overbank material anddecreasing amounts of fluvial channel sand-stone interconnectedness. This trend is sup-ported by modeling results that predict increas-ing amounts of fine-grained overbankpreservation with increased rates of fluvialaggradation (Leeder, 1978; Allen, 1978; Bridgeand Leeder, 1979). The assumption behindthese models is that channel avulsion frequencyis random and independent of sedimentationrate. However, increased sedimentation rates as-sociated with an accommodation increase maypromote more rapid rates of channel avulsionand a higher proportion of channel intercon-nectedness (Heller and Paola,1996). Analysisof associated architectural elements (such as thematurity of flood-plain paleosols,changes influvial channel morphology, and the presence orabsence of unconformities) may allow the ac-tual ties between aggradation rates and fluvialarchitecture to be determined.

B. S. CURRIE

1208 Geological Society of America Bulletin,September 1997

Figure 2. The fluvial architectural response to changes in basin accommodation and corre-sponding systems tracts (see text for explanation). Time 0—initial f luvial architecture; coarse-grained, laterally discontinuous channels. Time 1—degradational systems tract; coarse-grained fluvial sediment is deposited within incised valley margins, while preexistingflood-plain sediments are subjected to paleosol formation or early diagenetic alteration. Time2—transitional systems tract; increased accommodation results in decreased fluvial gradientsand aggradation of the incised valley. Time 3—aggradational systems tract; continued ac-commodation development results in deposition of laterally discontinuous channel sandstonesand abundant fine-grained overbank sediments. Time 4—late aggradational systems tractshowing increase in channel density and amalgamation. Modif ied from Shanley and McCabe(1994) and Wr ight and Marr iott (1993).




Sequence-Bounding Unconformities

One of the keys to interpreting the architec-tural assemblages related to alluvial depositionalsequences is recognizing sequence-boundingunconformities. In general, there are three typesof unconformities:type 1 develops during majorreductions in basin accommodation; type 2 is as-sociated with a minor decrease in accommoda-tion within the basin; and type 3 forms becauseof localized or subregional uplift within thebasin (Posamentier and Vail, 1988) (Fig. 3).

A type 1 sequence-bounding unconformitywill develop if the amount of accommodation re-duction is relatively large (Fig. 3a). Erosion dur-ing the development of a type 1 unconformity ischaracterized by valley incision and concentra-tion of through-going fluvial systems within val-ley margins. The preexisting alluvial surface isabandoned, producing widespread, well-devel-oped paleosols outside valley margins.

A type 2 unconformity can be thought of as acontinuation of the architectural trends thatoccur during the late stages of aggradational sys-tems-tract deposition. During the late-stage dep-osition of an aggradational systems tract, re-duced accommodation development producesan upsection increase in fluvial channel bodyamalgamation (Fig. 3b, time 0). In terms of flu-vial facies architecture, channel bodies are morelaterally continuous and are separated verticallyby lesser amounts of fine-grained overbank sed-iment (Wright and Marriott, 1993; Olsen et al.,1995). A minor decrease in accommodation fol-lowing deposition of the late-stage aggradationalsystems tract may result in widespread, shallowerosion of the preexisting depositional profile.However the decrease may not be great enoughto produce valley incision (Fig. 3b, time 1). Thedegradational systems tract following this typeof unconformity would consist of a widespread,thin accumulation of coarse-grained sedimentabove late-stage aggradational facies.

A type 3 unconformity develops in responseto abandonment of the preexisting alluvial sur-face due to localized or subregional uplift of thebasin floor. Because of the sensitivity of fluvialsystems to minor adjustments in basin relief, theinitiation of intrabasinal uplift may cause preex-isting depositional sequences to be rapidly aban-doned. In addition, erosion of the uplifted areasmay cause truncation of underlying depositionalsequences. These factors may cause aggrada-tional systems tracts underlying a type 3 uncon-formity to be incomplete or missing entirely.During development of type 3 unconformities,uplifted areas of the basin may also undergo ex-tensive pedogenesis. If the uplifted area of thebasin is not beveled to horizontal by erosion,al-luvial deposits of the next depositional sequence

may onlap the unconformity surface (Fig. 3c).In general, both type 1 and type 2 unconfor-

mities are identified by an overall increase inthe caliber of sediment deposited directly abovethe unconformity surface. However, the initialsediment deposited above the unconformitymay be dominantly fine grained, if coarse-grained detritus is absent in the basin. Similarly,a type 3 unconformity that is progressively on-lapped by fine-grained alluvial deposits maylack coarse-grained deposits directly above theunconformity surface. In the case where overly-ing coarse-grained degradational and transi-tional systems tracts are absent,an unconfor-mity may be recognized by the erosionaltruncation of stratigraphic units,widespread,well-developed pedogenesis and early diage-netic alteration not associated with valley inci-sion,or large gaps in the chronostratigraphic orbiostratigraphic record.

FORELAND-BASIN SYSTEM SEQUENCE STRATIGRAPHY

Although the above sequence-stratigraphicmodel can be applied to nonmarine basins re-gardless of tectonic setting, expected changes inaccommodation associated with developmentof foreland-basin systems through time requirespecial consideration. A foreland-basin systemis a zone of potential sediment accommodationthat develops in the foreland of a contractionalorogenic belt (e.g., Price, 1973; Dickinson,1974; Jordan,1981; Beaumont,1981). Fore-land-basin systems contain four depozones thatform due to the flexural and structural responseto thrust loading in the orogenic belt (DeCellesand Giles,1996). These depozones are referredto as wedge-top,foredeep, forebulge, and backbulge, and they occupy the area between the

thrust belt and the undeformed craton (Fig. 4)(DeCelles and Giles,1996).

Sediment accommodation in foreland-basinsystems is produced primarily by flexural sub-sidence due to thrust loading in the orogenicwedge. However, in retro-arc foreland-basinsettings,far-field, subduction-related dynamicsubsidence may contribute significantly to ac-commodation development (e.g., Mitrovica etal., 1989; Gurnis, 1992). In addition to thesetectonic controls,eustasy, climate, and sedimentsupply can also influence accommodation inforeland-basin system depozones (Flemingsand Jordan,1989; DeCelles and Giles,1996).

In terms of the nonmarine sequence-strati-graphic model presented above, degradational,transitional,and aggradational systems tractscan exist in all foreland-basin system depo-zones,although system-tract thickness and dis-tribution may vary. In the wedge-top depozone,sediment accumulates on top of the frontal partof the thrust belt. Accommodation in this depo-zone results from the competing influences ofthrust-load–driven flexural subsidence andstructural uplift of the orogenic wedge (De-Celles and Giles,1996). The wedge-top is char-acterized by an abundance of coarse sediment,and regional thinning of wedge-top strata to-ward the hinterland (DeCelles and Giles,1996).Because sediment accumulation in the wedge-top depozone occurs in an area of active foldingand thrust fault-related uplift,it is commonly anarea of sediment bypass and unconformity de-velopment (types 1,2, and 3) (Vergés andMuñoz,1990; DeCelles,1994). For this reason,nonmarine wedge-top sequences may be re-stricted to degradational and transitional sys-tems tracts; aggradational systems tracts areeroded soon after deposition,or they are neverdeposited (Fig. 5). However, aggradational sys-



TABLE 1. GENERALIZED NONMARINE SYSTEMS TRACT CHARACTERISTICS

Systems tract Grain size Architectural elements Facies

Degradational Coarse grained Incised valleys Braided channel sandstones and conglomerates

Laterally continuous Thick paleosol zonationchannel forms adjacent to valley

marginsTransitional Coarse to fine grained Transition from laterally Transition from braided to

continuous to isolated meandering channelchannel forms sandstones

Increasing preservation ofoverbank sediment andthin paleosols

Aggradational Dominantly fine grained Lenticular, isolated Abundant fine-grainedwith coarse-grained channel forms overbank and lacustrinefluvial channels facies

Late stage increase in Anastomosing channel channel frequency and sandstones and amalgamation conglomerates

Late stage transition to braided channel sandstones and conglomerates




tems tracts may be preserved in local,struc-turally ponded wedge-top basins (e.g., Lawtonet al.,1993). Stratal geometries in wedge-topdepositional sequences commonly onlap or of-flap positive structural features associated withactive thrust deformation. This may producenumerous local unconformities or condensedintervals that grade into conformities transverseto structural strike.

The foredeep depozone is a region of thickaccumulation between the wedge-top and theproximal side of the forebulge (DeCelles andGiles,1996). Accommodation in the foredeep isgenerated primarily by flexural subsidence dueto crustal loading in the orogenic wedge (Price,1973; Dickinson,1974; Beaumont,1981; Jor-dan,1981). Potential accommodation in theforedeep decreases rapidly toward the fore-bulge, producing stratal geometries that thinaway from the thrust belt (Flemings and Jordan,1989; Sinclair et al.,1991). Because of the rapidflexural subsidence generated in the foredeepdepozone, nonmarine depositional sequencesmay be dominated by aggradational systemstracts (Jordan and Flemings,1991). However,basin-scale reductions in nonmarine accommo-dation may produce type 1 or type 2 unconfor-mities,and result in deposition of associateddegradational and transitional systems tractsacross the basin (Shanley and McCabe, 1991).Unconformity development and degradational-transitional systems-tract deposition will bebest developed on the margins of the foredeepdepozone, where generation of flexural accom-modation is reduced (Fig. 5). These unconfor-mities may pass laterally into conformities inthe rapidly subsiding axial portions of the fore-deep (Posamentier and Allen, 1993).

The forebulge depozone is a region of poten-tial sediment accumulation located on the flex-ural forebulge. Deposition in the forebulge de-pozone depends largely on the interplaybetween regional accommodation generation(primarily by far-field, subduction-related sub-sidence and eustasy) and accommodation re-duction produced by flexural forebulge uplift(DeCelles and Giles,1996). If the rate of fore-bulge uplift is greater than the rate of accom-modation generation, a type 3 unconformitywill f orm (Fig. 6a). This unconformity willmost likely grade laterally into conformities inthe foredeep and back-bulge depozones. How-ever, as the forebulge migrates,contemporane-ous back-bulge deposits may be uplifted anderoded. In this case, passage of the forebulgethrough the area will be documented only byforedeep transitional and aggradational systemstracts that onlap the unconformity surface. If theoverall rate of accommodation development inthe forebulge depocenter changes over time,

B. S. CURRIE


Figure 3. Sequence-bounding unconformities. (a) Type 1. Severe reduction in accommoda-tion causes valley incision and pedogenic alteration of preexisting flood plain. (b) Type 2. Mi-nor accommodation decrease produces widespread coarse-grained deposits,limited valley in-cision,and poorly developed soil and alteration horizons. (c) Type 3. Uplift of the basin at ratesgreater than rates of fluvial incision causes abandonment of the preexisting flood plain. If ero-sion of uplifted areas is minor, soil formation and alteration can occur. When accommodationdevelopment in the basin resumes,overlying sediments may onlap the topographic high pro-duced by the uplift. Litholo gic patterns are the same as in Figure 2.

Figure 4. Diagram showing a hypothetical foreland-basin system. The main depocenter inthe system is the flexurally subsiding foredeep located between the thrust belt and the flexu-rally produced forebulge. A secondary, “back-bulge” basin is located between the flexuralbulge and the craton. Sediment accommodation can also be generated in the wedge-top andforebulge depozones. With pr opagation of the thrust belt toward the craton, areas once occu-pied by the forebulge or back-bulge depozone can become incorporated into the foredeep andthe thrust belt. Modified from DeCelles and Giles (1996).




sediment delivered from the foredeep and/orback bulge may progressively onlap the fore-bulge unconformity produced during times ofreduced accommodation (Fig. 6,b–d).

If the rate of accommodation generation ex-ceeds the rate of forebulge uplift, depositionmay be continuous in the forebulge depozonealthough the depositional sequence may be con-densed. If rates of accommodation developmentare high enough,the forebulge may becomecompletely buried and morphologically sup-pressed (e.g., Flemings and Jordan,1989) (Fig.6d). If this occurs, depositional sequences de-posited in the forebulge depozone may show ac-commodation trends similar to distal foredeepand back-bulge depozone sequences.

The back-bulge depozone is a broad regionof potential accommodation between the fore-bulge and the undisturbed craton (Goebel,1991,Giles and Dickinson,1995). Accommo-dation in the back-bulge depozone is controlledby minor flexural subsidence and potentiallymore substantial dynamic subsidence (Gurnis,1992; DeCelles and Giles,1996). Nontectoniccontrols (climate, eustasy, sediment supply)also have a significant influence on accommo-dation in this zone. Stratal patterns in the back-bulge depozone commonly are subhorizontaland uniformly thick over long distances (De-Celles and Burden,1992). Due to relatively lowrates of accommodation development,the back-bulge depozone is characterized by abundanttype 1 and type 2 unconformities and numerousdegradational, transitional,and aggradationalsystems tracts. In the vicinity of the forebulge,back-bulge depositional sequences may be trun-cated by erosion caused by forebulge migration,onlap uplifted forebulge rocks,or be continuouswith strata deposited in the forebulge depozoneif the foreland-basin system filled beyond thecrest of the forebulge (Fig. 6).

Over time, as a thrust belt advances towardthe craton, the four depozones of the foreland-basin system may be stacked vertically, andback-bulge deposits are overlain by progres-

sively younger forebulge, foredeep,and wedge-top sediments (DeCelles and Giles,1996). Theexpected long-term sediment accumulationtrend should be roughly sigmoidal; a major un-



Figure 5. Schematic cross section of the wedge-top and foredeep depozones of a foreland-basin system showing distribution of nonmarinesystems tracts.

Figure 6. Schematic cross section of forebulge depozone. Ar ea above dashed line representsflexural forebulge. (a) Sediment is deposited on the forebulge when forebulge uplift (U) is lessthan accommodation (A) in the foreland-basin system. This relationship can change over time,causing the forebulge unconformity to be onlapped with sediment from the foredeep andback-bulge depozones (b). As the forebulge migrates (c),areas of the forebulge initiall y up-lifted are incorporated into the foredeep, while sediments deposited on the back-bulge side ofthe forebulge are uplifted and eroded. If accommodation development is great, the entire fore-bulge can be overtopped by sediment (d).




conformity may separate back-bulge from fore-deep depositional sequences (DeCelles andCurrie, 1996). However, fluctuations in the rateand magnitude of accommodation developmentthroughout the history of depozone migrationmay complicate this general pattern.

APPLICATION TO JURASSIC–CRETACEOUS ROCKS INNORTHEASTERN UTAH ANDNORTHWESTERN COLORADO

The relationships between accommodationdevelopment and nonmarine sequence deposi-tion can be observed in the Upper Jurassic–Lower Cretaceous rocks surrounding the UintaMountains in northeastern Utah and northwest-ern Colorado (Fig. 7). The Morrison (UpperJurassic–Lower Cretaceous?) and Cedar Moun-tain (Lower Cretaceous) Formations (Fig. 8)contain four depositional sequences defined bychanges in sedimentary architecture that re-sulted from variations in the rate of accommo-dation development.

In this region, the Morrison and CedarMountain Formations constitute approximately200–250 m of almost entirely nonmarine sand-stone, mudstone, and minor carbonate rocks.The Morrison overlies the Oxfordian RedwaterMember of the Stump Formation and containsfour formally recognized members: the WindyHill, Tidwell, Salt Wash,and Brushy BasinMembers (Turner, 1992). The Cedar MountainFormation overlies the Morrison and consists ofthe Buckhorn Conglomerate Member and anupper mudstone and sandstone member(Stokes,1952; Kirkwood, 1976).

Although these stratigraphic subdivisions arewidely recognized, internal similarities in faciesassemblages and depositional architecture allowrecognition of eight different systems tracts thatdefine four depositional sequences within theUpper Jurassic–Lower Cretaceous stratigraphicinterval (Fig. 9). For the purposes of this study,the depositional sequences in the Morrison For-mation are given the names UJ-1 and UJ-2,andthose sequences composing the Cedar MountainFormation are named LK-1 and LK-2.

UJ-1 Sequence

The lowermost depositional sequence in-cludes the Windy Hill, Tidwell, and Salt WashMembers of the Morrison Formation. Theserocks contain shallow-marine, nearshore, tidal,fluvial, and eolian facies that represent the tran-sition from marine to nonmarine depositionduring late Oxfordian and Kimmeridgian time(Currie, 1993). Although it has been suggestedthat an unconformity exists between these lower

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Figure 7. (A) Generalized map showing tectonic setting of the Cordiller an foreland basin,after Dickinson and Snyder (1978). Dashed barbed line shows approximate location of theJurassic–Cretaceous subduction zone; solid barbed line represents final location of the lead-ing edge of the Cordiller an fold-and-thrust belt. (B) Map of the south-central part of the fore-land basin showing location of the study area (inset C),nearby Laramide uplifts, individualsegments of the thrust belt, and locations mentioned in text. Abbreviations are as follows:WRR—Wind River Range; GFM—Granite and Freezeout Mountains; CW—location of cen-tr al Wyoming Jurassic–Cretaceous sections of DeCelles and Burden (1992); PR—ParkRange; UM—Uinta Mountains; SRS—San Rafael Swell; WP—Wasatch Plateau. (C) Detailedmap of the study area showing outcrops of Jurassic–Cretaceous nonmarine rocks (shaded ar-eas) and locations of measured sections (black dots) referred to in the text. Abbreviations formeasured sections are: IPR—Island Park Road; 1TD—Theropod Draw; TCW—Trail CreekWest; 1MHQ—Monument Headquarters; 1DP—Deerlodge Park. Section locations are listedin Table 2.

TABLE 2. UINTA MOUNTAIN STRATIGRAPHIC SECTION LOCATIONS

Section Location

IPR se Ú sec. 6 and sw Ú sec. 5, T. 4 S., R. 23 E.1TD se Ú sec. 26, T. 4 S., R. 23 E.TCW sw Ú sec. 3 and nw 1/4 sec. 10, T. 6 S., R. 25 E.MHQ ne Ú sec. 8, T. 3 N., R. 103 W.1DP nw Ú sw Ú and sw Ú nw Ú sec. 28, T. 6 N., R. 99 W.




Morrison members and the underlying marineRedwater Member of the Stump Formation(Pipringos and O’Sullivan,1978),facies rela-tionships in the study area indicate that the for-mations are conformable (Currie, 1993). Theassociation of marine and continental facies inthe UJ-1 sequence indicates progradation of ahighstand systems tract of marginal marine andnonmarine facies into a marine basin.

Fluvial facies in the UJ-1 sequence are pre-sent primaril y in the Salt Wash Member. Thisunit consists of as much as 35 m of sandstoneand chert-pebble conglomerate that were de-posited by a sandy and gravelly braided fluvialsystem (Currie, 1997). Within this unit,grainsize coarsens upsection,whereas fluvial channelbodies become more laterally continuous andvertically amalgamated. These trends suggestsan upsection reduction in the rate of basin ac-commodation development during deposition.In terms of the nonmarine systems tracts out-lined above, the Salt Wash Member representsthe late stages of an aggradational systems tract.

UJ-2 Sequence

An abrupt coarsening at the top of the SaltWash Member is marked in places by a lag ofpebbles,cobbles,and boulders (as much as 50cm in diameter) derived from underlying sand-stones and pedogenic carbonates (Fig. 9). Thisuppermost Salt Wash conglomerate is usuallyless than 3 m thick and is more laterally contin-uous than the underlying channel deposits.These characteristics indicate that the upper-most Salt Wash conglomerate was deposited asa degradational systems tract,and its base mayrepresent a type 2 unconformity. As such, theupper Salt Wash marks the boundary betweenthe UJ-1 and UJ-2 depositional sequences.

Above the Salt Wash Member are mudstoneand sandstone of the lower Brushy Basin Mem-ber. These lithologies show an upsection in-crease in mudstone and a transition frombraided to meandering fluvial channel mor-phologies (Currie, 1997). This apparent changein fluvial channel morphology and increasedpreservation of overbank sediments in the lowerBrushy Basin Member indicates an increase inbasin accommodation and represents a transi-tional systems tract following deposition of theUJ-2 degradational systems tract.

The upper Brushy Basin Member makes upthe next systems tract in the UJ-2 sequence.This interval is dominated by massive to thinlybedded, smectitic and siliceous mudstone, butalso contains laterally discontinuous lenses ofsandstone and conglomerate. Interbedded withthe mudstone are abundant bentonite and silici-fied volcanic ash beds that were preserved as

ash-fall tuffs or later reworked by fluvialprocesses. The upper Brushy Basin Member isinterpreted as deposits of an anastomosing flu-vial system with associated shallow lacustrineenvironments (Bell,1986; Currie, 1997). Thetransition between lower Brushy Basin braided-meandering facies and upper member anasto-mosing fluvial facies is interpreted as the bound-ary between transitional and aggradationalsystems tracts in the UJ-2 depositional sequence.

In the upper parts of the Brushy Basin Mem-ber, lenticular channel bodies become moreconcentrated both laterally and vertically and

are associated with stacked paleosols (Currie,1997; Demko et al.,1996). This suggests lateaggradational systems-tract deposition due toa decreasing rate of accommodation develop-ment (e.g., Wright and Marriott, 1993). Thissame interval shows evidence for early dia-genetic overprinting, such as leaching of fine-grained beds,and bleaching and dissolution ofclasts from coarse-grained units (Currie, 1997).This alteration may have resulted from intenseweathering during a long period of little or nodeposition during development of a type 1 un-conformity.



Figure 8. Generalized stratigraphic chart showing Upper Jurassic–Lower Cretaceousstratigraphy of the Uinta Mountain area and surrounding regions. Stage boundaries aretaken from the Jurassic–Cretaceous time scale of Gradstein et al. (1995). Modified from De-Celles and Burden (1992),Dolson and Muller (1994),and Dyman et al. (1994).




LK-1 and LK-2 Sequences

The LK-1 and LK-2 depositional sequencesare represented by the Buckhorn ConglomerateMember and the upper Cedar Mountain Forma-tion. The Buckhorn Conglomerate Member iscomposed of as much as 30 m of clast-supportedpebble-cobble conglomerate, medium- to coarse-grained sandstone, and minor mudstone (Currie,1997). Conglomerate and sandstone grain sizedecrease upsection,whereas the thicknesses ofassociated mudstone beds increase. Thesefacies indicate deposition in a gravelly-sandy,braided fluvial system that shows a general de-crease in stream power upsection (Kirkwood,1976; Currie, 1997).

The Buckhorn Conglomerate unconformablyoverlies the Brushy Basin Member and is pre-sent in a 25-km-wide west-east–oriented out-crop belt across the southern and eastern parts ofthe study area (Fig. 10) (Currie, 1997). The unitprogressively decreases in thickness from a

maximum of ~30 m near Dinosaur, Colorado,tozero edges that are symmetrically disposedalong a southwest-northeast–oriented axis.Where the Buckhorn Conglomerate is present,the altered upper 20 m of the Brushy BasinMember are absent. Paleocurrent data indicatethat the Buckhorn fluvial system flowed north-eastward (Fig. 10).

Laterally confined conglomerate distribution,symmetrical thinning patterns,and apparent ero-sional truncation of the underlying upper BrushyBasin Member indicate that the Buckhorn Con-glomerate represents the fill of a northeastward-sloping valley. Decreasing upsection grain sizeand increasing mudstone content within the con-glomerate permit interpretation of the valley fillas degradational and transitional systems tracts.The valley was erosionally cut into the underly-ing Morrison Formation during development ofa type 1 unconformity (Currie, 1997).

Above the Buckhorn is the upper member ofthe Cedar Mountain Formation (Kirkwood,

1976). At the base of the upper Cedar Mountain,above both the Buckhorn Conglomerate andupper Brushy Basin Member where the Buck-horn is absent,is a regionally extensive, massiveto nodular calcrete zone. This zone is locallymore than 10 m thick and completely replaces ordisplaces the host sediment. This interval is in-terpreted as a thick pedogenic or ground-water–related calcrete that formed across the re-gion after the Buckhorn paleovalley was filled(Currie, 1997). The great thickness of this zoneindicates long-term stability of the landscapeand low rates of deposition during the time ofcalcrete formation. The calcrete may thereforerepresent an unconformity that developed withinthe basin following deposition of the BuckhornConglomerate (Kirkwood, 1976; Currie, 1995).

Above the calcrete zone, the upper CedarMountain consists of 15–40 m of laterally dis-continuous channel sandstone, lacustrine lime-stone, and overbank mudstone. Well-developedcalcic paleosols are also present in the lower

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Figure 9. Generalized stratigraphic sections and sequence diagram for the Upper Jurassic–Lower Cretaceous rocks of the study area. Sec-tion locations are shown in Figure 4. Sections are hung on the calcrete zone at the base of the Cedar Mountain Formation (for abbreviations,see Fig. 7). Sections are capped by a type 1 unconformity and the Dakota Formation. The Buckhorn Conglomerate is not present at the 1DP,1TD, and IPR sections,although alteration of the upper 20 m of the Upper Morr ison sequence occurred during Buckhorn valley incision andfilling . Formation Symbols:Jsr—Stump Formation, Redwater Member. Morr ison Formation: Jmw—Windy Hill Member; Jmt—T idwellMember; Jmsl—lower Salt Wash Member; Jmsu—upper Salt Wash Member; Jmbl—lower Brushy Basin Member; Jmbu—upper BrushyBasin Member. Kb—Buckhorn Conglomerate. Kcm—Cedar Mountain Formation. Kd—Dakota Formation.




15–20 m of the unit,although thickness of indi-vidual paleosols and overall paleosol abundancedecrease upsection. These facies represent tran-sitional and aggradational systems-tract,anasto-mosed, fluvial channel sandstones and associ-ated overbank and lacustrine mudstones (Fig. 9;Currie, 1997). The stacked paleosols in thelower half of the interval developed duringdeposition of the transitional systems tract.Fluvial channels in the upper part of the CedarMountain aggradational systems tract are morenumerous and laterally continuous than thosenear the base. This relationship is similar to thatobserved in late aggradational systems-tractdeposits of the upper Brushy Basin Member.

The calcrete zone at the base of the CedarMountain Formation presents a problem in thesequence-stratigraphic model outlined above.The great thickness and well-developed natureof the zone suggests a period of little or no dep-osition across the region, although transitionaland aggradational systems-tract deposition ofthe upper Cedar Mountain Formation indicatesa continuation of the accommodation trends ini-tiated during deposition of the Buckhorn Con-glomerate transitional systems tract. One possi-ble explanation is that the calcrete formedduring initiation of fluvial deposition in areasoutside the Buckhorn paleovalley. Once the val-ley was filled, fluvial systems would havespread laterally above the preexisting unconfor-mity surface. With sediment being distributedacross a broader region, rates of sediment accu-mulation may have become relatively low, al-lowing formation of a thick pedogenic orground-water calcrete. In addition, with fillingof the Buckhorn paleovalley, regional water ta-bles may have been elevated, contributing tocalcrete development. As basin accommodationincreased, aggradational systems-tract deposi-tion of the upper Cedar Mountain Formationbuilt up the alluvial surface, outpacing the rateof calcrete formation. This would indicate thatthe Buckhorn Conglomerate and upper CedarMountain Formation are parts of the same dep-ositional sequence.

Alternatively, the calcrete may be related to aperiod of reduced accommodation due to slightuplift or tilting of the basin. Across the studyarea,the thickness of the Cedar Mountain se-quence ranges from ~40 m in the west to ~15 min the east (Fig. 9). Along this same trend, thecalcrete zone at the base of the Cedar Mountainthickens from about 1 m to 10 m. This relation-ship may have formed due to upwarping or tilt-ing of the basin following Buckhorn Conglom-erate deposition. This intrabasinal uplift mayhave been superimposed on the overall basinaccommodation increase that was initiated dur-ing transitional systems tract filling of the

Buckhorn paleovalley. Ultimately, the rate ofuplift outpaced the rate of accommodation de-velopment and produced the unconformity atthe top of the Buckhorn. Through time, theslightly uplifted area may have been progres-sively onlapped and eventually overtopped byrelatively fine-grained Cedar Mountain alluvialdeposits. Because areas to the east were ex-posed for longer periods of time, the calcretezone in northwestern Colorado became thickerand better developed. As such, the basal calcretezone may represent a type 3 unconformity thatdeveloped across the study area followingdegradational and transitional systems-tractdeposition of the LK-1 sequence represented bythe Buckhorn Conglomerate. The upper CedarMountain Formation therefore represents tran-sitional and aggradational systems tracts of theLK-2 depositional sequence following a periodof uplift-induced unconformity development.On the basis of regional chronostratigraphiccorrelations (discussed in the following sec-tion), this second alternative is preferred.

Above the LK-2 sequence is a type 1 se-quence-bounding unconformity and nonmarine,transitional and marine facies of the Lower Cre-taceous Dakota Formation. The basal Dakota isdominated by braided fluvial facies that fill ero-sional valleys that are incised as much as 20 minto the underlying Cedar Mountain (Vaughnand Picard, 1976). The Dakota contains at least

two depositional sequences that were depositedduring fluctuations in relative sea level alongthe margin of the Western Interior seaway dur-ing late Albian time (Ryer et al.,1987; Currie etal.,1993; Dolson and Muller, 1994).

REGIONAL CORRELA TIONS

A test of the sequence-stratigraphic frame-work outlined above is the regional continuityof the proposed depositional sequences. Figure11 shows a generalized sequence-stratigraphiccorrelation between the San Rafael Swell ineast-central Utah,the study area in northeasternUtah and northwestern Colorado,and centralWyoming. For the Wyoming correlations,litho-stratigraphic and chronostratigraphic data fromthe Morrison and Cloverly Formations pre-sented by DeCelles and Burden (1992) and Dol-son and Muller (1994) were utilized.

Central Utah

Facies assemblages and depositional archi-tecture in the Morrison,Cedar Mountain,andDakota Formations of northeastern Utah andnorthwestern Colorado are similar to those inthe same units in central Utah. In the MorrisonFormation of the San Rafael Swell region, flu-vial and lacustrine facies of the Tidwell Mem-ber are overlain by upward-coarsening, increas-



Figure 10. Paleocurrent and isopach map of the Buckhorn Conglomerate in the study areashowing northeast trending distribution of the unit. Each arrow represents the average troughaxis determined from 10 or more measurements of trough limbs per station according tomethod I of DeCelles et al. (1983). Open circles represent well locations used in subsurfacethickness calculations. Isopach contours are in meters.




ingly amalgamated, braided fluvial sandstonesof the Salt Wash Member (Fig. 11). Channelbodies at the top of the Salt Wash are abruptlycoarser and more laterally continuous thanthose in the lower Salt Wash. This pattern issimilar to the architectural characteristics in theUpper Salt Wash Member in northeastern Utahand northwestern Colorado. These channel de-posits are overlain by anastomosing fluvial fa-cies of the Brushy Basin Member (Currie,1997). Like the Brushy Basin in northeasternUtah,the lenticular channel bodies in this inter-val become more numerous and vertically con-densed upsection.

The architectural elements in the Morrison

Formation of central Utah indicate aggrada-tional systems-tract deposition of the Tidwelland lower Salt Wash Members. The aggrada-tional systems tract was followed by degrada-tional systems-tract deposition of the upper SaltWash Member, and transitional-aggradationalsystems-tract deposition of the Brushy BasinMember. These units represent the UJ-1 depo-sitional sequence. In central Utah,the Tidwelland lower Salt Wash Members represent the UJ-1 depositional sequence, and the Upper SaltWash and Brushy Basin Members represent theUJ-2 sequence.

Unconformably overlying the Brushy BasinMember are gravelly-sandy braided fluvial fa-

cies of the Buckhorn Conglomerate. The Buck-horn in central Utah has clast compositions,pa-leocurrent orientations,and an upward-fininggrain-size trend that are similar to its counterpartto the northeast (Currie, 1997). The northeast-trending, symmetrically thinning distribution ofconglomerate and the well-developed valleymargin paleosol horizons in northeastern Utahand northwestern Colorado are also present inthe San Rafael Swell region. This suggests thatthe Buckhorn paleovalley, incised during devel-opment of type 1 unconformity within the basin,extended from the eastern Uinta Mountains re-gion into central Utah (Yingling and Heller, 1992;Currie, 1997). This period of valley incision was

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Figure 11. Regional correlation and sequence diagram of Upper Jurassic–Lower Cretaceous rocks between central Wyoming and the SanRafael Swell in central Utah. Stratigraphic sections are generalized compilations from each region (cf. DeCelles and Burden,1992; Yingling,1987). Correlation of members and formations incorporates sequence-stratigraphic and geochronologic data referred to in the text. Symbolsused on the diagram include: a—stratigraphic levels of bentonites dated by 40Ar/ 39Ar techniques; p—stratigraphic levels of age-diagnosticpalynomorph-bearing mudstones; f—stratigraphic level of fission-track dated ash horizons; k—stratigraphic levels of K/Ar dated bentonite;u—stratigraphic levels of U/Pb dated bentonites. Ages of formations and members are shown in Figure 8.

currie.qxd 10/31/97 3:06 PM 16



then followed by degradational-transitional sys-tems tract filling of the northeast-trending Buck-horn paleovalley during deposition of the LK-1sequence.

The thick calcrete zone that lies above theBuckhorn in northeastern Utah and northwesternColorado is also present in central Utah. This in-dicates that the type 3 unconformity that devel-oped following deposition of Buckhorn to thenortheast also developed in the San Rafael region.

Above the regionally extensive calcrete aremeandering-anastomosing fluvial and lacus-trine facies of the Cedar Mountain Formation.These strata are similar to those in northeasternUtah and northwestern Colorado and representtransitional and aggradational systems-tractdeposition of the LK-2 sequence during a bas-inwide increase in accommodation.

In central Utah,the Cedar Mountain Forma-tion is unconformably overlain by braided flu-vial facies of the Dakota Sandstone (Yingling,1987; Currie, 1997). The boundary is character-ized by incised valleys that have been cut asmuch as 10 m into the underlying Cedar Moun-tain (Molenaar and Cobban,1991). This repre-sents development of a type 1 unconformity fol-lowing aggradational systems-tract depositionof the upper Cedar Mountain. Dakota fluvialfacies contained within these valleys displayupward-fining, valley-fill characteristics (Ying-ling, 1987) similar to the lower Dakota Forma-tion sequence in northeastern Utah. However,regional stratigraphic correlations indicate thatthe Dakota in central Utah is Cenomanian inage and thus younger than the late AlbianDakota Formation found to the northeast(Molenaar and Cobban,1991). In addition, theunconformity separating the Dakota from theCedar Mountain Formation in northeasternUtah may pass into a conformity to the south-west. This is supported by the fact that theupper 6–20 m of the Cedar Mountain Formationin the San Rafael Swell contains carbonaceouschannel sandstones and overbank mudstone thatare similar to the lithologies of the Dakota Forma-tion to the northeast.

Central Wyoming

Depositional sequences similar to those in thestudy area are also present in the Upper Jurassicand Lower Cretaceous rocks of centralWyoming. In the area surrounding the Granite-Freezeout and Laramie Mountains,the MorrisonFormation and lower Cloverly mudstone containlithologic and architectural characteristics simi-lar to those in the UJ-1 and UJ-2 sequences inthe Uinta Mountains region. The UJ-1 sequenceis represented by transitional marine and fluvial-lacustrine facies of the lower sandstone and mid-

dle mudstone units of the Morrison Formation.The UJ-2 sequence is represented by braidedfluvial and alluvial-lacustrine facies of the upperMorrison sandstone and lower Cloverly mud-stone. The lower Cloverly mudstone in centralWyoming has in the past been considered EarlyCretaceous in age (DeCelles and Burden,1992).However, lithostratigraphic and chronostrati-graphic correlations indicate that the lowerCloverly may be related to the Brushy BasinMember of the Morrison Formation in Utah andColorado (see following discussion).

The Buckhorn Conglomerate in the study areais similar to the upper Cloverly conglomerate inWyoming in terms of lithology and distribution.Both the Buckhorn and Cloverly conglomerateswere deposited in ~30-km-wide, northeast-trend-ing belts and contain similar north-northeast–di-rected paleocurrent indicators (DeCelles andBurden,1992; Currie, 1997). Compositionally,the Buckhorn and Cloverly conglomerates aredominated by gray and black chert clasts and con-tain minor amounts of white chert and quartzite.These compositional and distributional similari-ties suggest that the Buckhorn and Cloverly con-glomerates may be genetically related units thatwere deposited in a northeastward-flowing fluvialsystem (DeCelles and Burden,1992). The uncon-formity at the base of the Buckhorn Conglomer-ate in Utah and northwestern Colorado is at astratigraphic position similar to that between theCloverly conglomerate and lower mudstone incentral Wyoming. In both cases,erosional inci-sion and initial deposition of both the Buckhornand Cloverly conglomerates represent the transi-tion from aggradational to degradational-transi-tional systems-tract deposition within the basinduring Early Cretaceous time.

Lithologic and stratigraphic similarities be-tween the study area and central Wyoming endat the top of the Buckhorn and Cloverly con-glomerates. In the study area the Buckhorn isoverlain by calcrete and fluvial-lacustrine faciesof the Cedar Mountain Formation. In Wyoming,the Cloverly conglomerate is overlain by ma-rine sandstone and black shale. The transitionbetween the Cloverly and the overlying marinerocks in Wyoming is a continuous succession offluvial conglomerate and sandstone overlain byestuarine and shallow marine sandstone andshale (DeCelles and Burden,1992). Althoughthe marine rocks overlying the Cloverly wereassigned to the Fall River Sandstone and Ther-mopolis Shale by DeCelles and Burden (1992),regional correlations by Dolson and Muller(1994) indicate that these marine lithologiesmay be related to the unnamed shale member ofthe Skull Creek Shale. In eastern and centralWyoming the unnamed shale is separated fromthe Cloverly and Lakota Formations by a trans-

gressive surface and is unconformably overlainby the Fall River Sandstone (Dolson andMuller, 1994). Similarly, a transgressive marineunit overlies the Cedar Mountain Formation inthe subsurface along the Moxa arch in south-western Wyoming (Ryer et al.,1987; D. A.Pivnik, 1995,personal commun.).

The marine sequence-stratigraphic relation-ship between the Cloverly conglomerate andthe overlying marine rocks in central Wyomingappears to be lowstand, transgressive, and high-stand systems tracts,and incised valley-fill andestuarine deposits of the Cloverly are transi-tional with the shallow-marine unnamed shale.Deposition of this marine interval may have oc-curred during the initial flooding of the WesternInterior seaway during early Albian time (Dol-son and Muller, 1994).

The early Albian age of the marine rocks over-lying the Cloverly in Wyoming creates a problemin correlating the Buckhorn and Cloverly con-glomerates. On the basis of the late Neoco-mian–Aptian age of the Cedar Mountain Forma-tion in the Wasatch Plateau region of Utah,theBuckhorn in central Utah is most likely mid-Neocomian in age (Weiss and Roche, 1988;Sprinkel et al.,1992; Currie, 1997). However, ifthe nonmarine-marine transition between theCloverly and the unnamed shale occurred duringthe Albian,a 10–15 m.y. age difference may existbetween the Buckhorn Conglomerate in Utahand the Cloverly conglomerate in Wyoming.

One explanation for this apparent age dis-crepancy is that the Buckhorn and Cloverlyconglomerates may contain multiple intrafor-mational unconformities and may representsporadic, long-term degradational systems-tractdeposition throughout Neocomian–Aptian time.During this period of intermittent deposition,coarse-grained sediment initially deposited bythe Neocomian Buckhorn fluvial system (LK-1depositional sequence) may have been recycledand incorporated into younger Cloverly fluvialsystems (LK-2 depositional sequence) withinsimilar northeast-trending paleovalleys (e.g.,Dolson and Muller, 1994). Erosion and recy-cling of Buckhorn clasts into Cloverly streamsin Wyoming may have occurred during forma-tion of the calcrete zone at the top of the Buck-horn Conglomerate in Utah.

Regardless of the precise relationship be-tween the Buckhorn and Cloverly conglomer-ates,deposition of these coarse-grained litholo-gies represents a regional Early Cretaceousdecrease in accommodation and widespreaddeposition of one or more degradational sys-tems tracts. Similarly, the transgressive-high-stand systems tracts of the upper Cloverly con-glomerate and unnamed shale in centralWyoming correspond to an overall increase in






basin accommodation during the Albian (e.g.,Vuke, 1983; Dolson and Muller, 1994). Theearly Albian increase in accommodation associ-ated with initial marine flooding may have re-sulted in coeval deposition of nonmarine LK-2sequence transitional and aggradational systemstracts in Utah and northwestern Colorado.

The unnamed shale in Wyoming is uncon-formably overlain by a type 1 unconformity andincised valley fill sandstones of the Fall RiverSandstone (Dolson and Muller, 1994). This un-conformity may correspond to the type 1 un-conformity above the LK-2 sequence at thebase of the Dakota Formation in northeast Utahand northwest Colorado.

SUPPORTING CHRONOSTRATIGRAPHIC CONTR OL

Although age control for the Upper Juras-sic–Lower Cretaceous nonmarine rocks of thecentral Cordilleran foreland basin is limited, re-ported geochronologic ages support the se-quence-stratigraphic correlations discussedherein. The age of the UJ-1 sequence betweenthe study area, central Utah, and centralWyoming is documented by palynological andisotopic dates. In central Wyoming, DeCelles

and Burden (1992) recovered a palynomorphassemblage from the lower Morrison indicatingan Oxfordian age. This interpretation is sup-ported by 40Ar/39Ar sanidine ages from a ben-tonite in the Tidwell Member of northeasternUtah that yielded an Oxfordian age of 154.9 ±1.5 Ma,using the time scale of Gradstein et al.(1995) (Peterson,1992). 40Ar/39Ar dating of anash bed in the Tidwell of central Utah yieldedan almost identical age of 154.8 ± 0.6 Ma(Kowallis et al.,1993).

Chronostratigraphic data from the UJ-2 se-quence of the Uinta Mountain region and centralUtah also indicate similar ages of deposition.The Salt Wash Member in the study area hasyielded Tithonian palynomorphs from carbona-ceous mudstone above the uppermost degrada-tional systems-tract conglomerates (Currie,1997). This age is supported by U/Pb dating ofzircons from bentonites in the middle BrushyBasin Member from the study area that yielded aTithonian age of 147.2 ± 2.5 Ma (G. Gehrels,1994,personal commun.). In addition,40Ar/39Ardates from bentonites in the lower and middleBrushy Basin Member in southeastern Utahhave yielded Tithonian ages ranging from 149 ±0.4Ma to 145.2 ± 1.2 Ma (Kowallis et al.,1991).A U/Pb zircon date from a bentonite at the base

of the lower Brushy Basin Member in the SanRafael Swell of central Utah also yielded aTithonian age of 149.7 ± 0.4 Ma (G. Gehrels,1994,personal commun.). A bentonite bed in theupper part of the Brushy Basin Member in thestudy area yielded a biotite K/Ar date of 135 ±5 Ma, suggesting that the top of the formationmay be Early Cretaceous in age (S. A. Bilbey,1993,personal commun.). However, an 40Ar/39Ardate from plagioclase crystals from the samehorizon yielded a Jurassic age of 152.9 ± 1.2 Ma(Kowallis et al.,1991),leaving the actual age ofthe upper Morrison in question. In addition, itshould be noted that the uppermost 20–30 m ofthe formation in Utah has not been dated.

Age correlation of the UJ-2 depositional se-quence between Utah and central Wyoming isnot as well constrained. Fission-track dating ofzircons from the lower Cloverly mudstone incentral Wyoming indicates an age of 129 Ma,although the large error associated with thisdate (±27 m.y.) easily overlaps the Tithonian40Ar/39Ar and U/Pb ages of the middle andlower parts of the Brushy Basin Member inUtah. Palynomorphs from the lower Cloverlymudstone just below the Cloverly conglomeratein central Wyoming indicate a Berriasian age(DeCelles and Burden,1992). This age is simi-

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Figure 12. Isopach maps of the (A) Morr ison Formation and Buckhorn Conglomerate and (B) Cedar Mountain Formation in Utah andwestern Colorado. Contours are in meters. Note the change in contour interval for the Cedar Mountain Formation.




lar to the Early Cretaceous K/Ar date from theBrushy Basin Member in the study area.

The only age-diagnostic fossils from the LK-2sequence in northeastern Utah were reported byPeck and Craig (1962),who noted Early Creta-ceous charophytes and ostracodes from theCedar Mountain Formation along the north flankof the Uinta Mountains. Nonmarine mollusks(Katich, 1951; Stokes,1952) and plant macro-fossils (Simmons,1957; Thayn, 1973) from theCedar Mountain Formation of central Utah indi-cate an Aptian–Albian age of deposition. In theBurro Canyon Formation, which is a westernColorado correlative of the Cedar Mountain(Stokes, 1952; Craig, 1981), Tschudy et al.(1984) collected an Aptian–early Albian paly-nomorph suite, although the authors suggestedthat the assemblage may be as old as Barremian.The same study documented a late Albian paly-nomorph suite from the upper Cedar MountainFormation in east-central Utah. A late Albian agefor the upper part of the Cedar Mountain is cor-roborated by a 100 Ma zircon U/Pb date from abentonite 10 m below the top of the formation onthe southwest flank of the San Rafael Swell (G.Gehrels,1996,personal commun.).

The overall Aptian–Albian age of the LK-2sequence in Utah corresponds with the limitedchronostratigraphic evidence from marine unitsof Wyoming. An 40Ar/39Ar date from a ben-tonite in the Skull Creek Shale, a correlative tothe Thermopolis Shale in the northern BlackHills, indicates an Albian age of deposition(104.4 Ma) (Dyman et al.,1994; Gardner et al,1994). Similarly, an 40Ar/39Ar date from theMuddy Sandstone in the same region hasyielded a late Albian age (98.9 Ma) (Dyman etal., 1994). Regional correlations of these lateAlbian marine units in Wyoming suggest thatthey correspond to the Dakota Formation innorthwestern Utah and northwestern Colorado(Ryer et al.,1987). At least two unconformitiesare present in the Dakota Formation in this re-gion (Currie et al.,1993). The 100 Ma age ofthe upper Cedar Mountain Formation in centralUtah suggests that these Dakota unconformitiesmay grade into conformities to the southwest.Thus the upper Cedar Mountain Formation inthe San Rafael Swell region may be correlativeto the Dakota Formation in the study area(Molenaar and Cobban,1991)

To summarize the chronostratigraphic data inrelation to the sequence-stratigraphic correla-tions, the UJ-1 depositional sequence in Utahand Wyoming was deposited during Oxfordianand Kimmeridgian time. This was followed bydevelopment of a type 2 unconformity and bydeposition of the UJ-2 sequence degradational,transitional,and aggradational systems tracts.The majority of the UJ-2 sequence was de-

posited during Tithonian time, but depositionmay have extended into early Neocomian time.UJ-2 sequence aggradational systems-tract dep-osition was followed by development of the re-gional type 1 unconformity. This was followedby deposition of degradational and transitionalsystems tracts of the LK-1 sequence repre-sented by the Buckhorn Conglomerate in Utah.Although there is no independent age determi-nation for the Buckhorn, the ages of the se-quences above and below the conglomeratesuggest a mid-Neocomian age of deposition.Deposition of the Buckhorn conglomerate mayhave been associated with a prolonged Neoco-mian–Aptian degradational systems tract con-taining multiple unconformities. During thistime the widespread calcrete above the Buck-horn and Morrison Formations developed ineastern Utah,and the Cloverly conglomeratewas deposited in central Wyoming. This wasfollowed by Aptian–Albian transitional andaggradational systems-tract deposition of theLK-2 sequence in eastern Utah,and Albian ma-rine transgressive and highstand systems-tractdeposition of the upper part of the Cloverly con-glomerate and unnamed shale in Wyoming.

The initial Albian marine incursion into theWestern Interior was followed by a drop in rel-ative sea level and the development of a type 1unconformity across the region. A subsequentincrease in accommodation resulted in deposi-tion of the Fall River Sandstone and Thermopo-lis Shale in Wyoming, and the lower DakotaFormation in northeastern Utah and northwest-ern Colorado.

DISCUSSION:THE LA TEJURASSIC–EARLY CRETACEOUSFORELAND-BASIN SYSTEM

Deposition of the Upper Jurassic–Lower Cre-taceous sequences in Utah,Colorado, andWyoming can be related to changes in accom-modation produced by the cratonward migra-tion of foreland-basin system depozones and re-gional eustatic influences. Regional isopachcontours of the UJ-1,UJ-2,and LK-1 deposi-tional sequences (Morrison Formation andBuckhorn Conglomerate) show that these rockswere deposited in a broad, relatively shallowbasin extending from central Utah eastward intoColorado (Fig. 12A). The westward pinchout ofthese sequences is both erosional and deposi-tional; in northern Utah the upper members ofthe Morrison Formation are erosionally trun-cated, whereas in central Utah the upper mem-bers of the Morrison onlap Middle Jurassicrocks (Peterson,1988; Currie, 1997). Paleocurrentorientations from fluvial channel sandstoneswithin these sequences indicate sediment transport

from the west and southwest (Craig et al.,1955;Peterson,1988; Yingling, 1987; Currie, 1997).

The UJ-1,UJ-2,and LK-1 depositional se-quences are interpreted as the deposits of theforebulge and back-bulge depozones of theCordilleran foreland-basin system (Fig. 12A)(Currie, 1994; DeCelles and Currie, 1996). Thedepositional onlap observed in the MorrisonFormation in central Utah represents the loca-tion of the eastern margin of the Late Jurassicforebulge depozone. Sediment was transportedinto the back-bulge region by fluvial systems en-tering the basin from the southwest and overtop-ping the forebulge from the west (Currie, 1994).UJ-1 and UJ-2 sequence deposition eventuallyfilled accommodation in the back-bulge depo-zone and onlapped the forebulge located inwest-central Utah (Fig. 13A) (Currie, 1994).

Early Cretaceous migration of the forebulgecaused erosion of the UJ-1 and UJ-2 sequencesin northern Utah and areas west of the SanRafael Swell in central Utah. This initial migra-tion also coincided with a basinwide accommo-dation decrease in the back-bulge depozone.This resulted in generation of a type 1 uncon-formity and degradational systems-tract deposi-tion of LK-1 sequence fluvial systems acrossUtah,Colorado,and Wyoming (Fig. 13B) (De-Celles and Currie, 1996; Currie, 1997).

Following deposition of the LK-1 sequence,continued migration of the forebulge upliftedareas in eastern Utah and northwestern Col-orado,while west-central and northern Utah un-derwent flexural subsidence in the encroachingforedeep depozone (Currie, 1997). At the sametime, degradational systems-tract depositioncontinued in the back-bulge region in Wyomingand Colorado (Currie, 1995). This produced atype 3 unconformity extending from the studyarea into south-central Utah,allowing the thickcalcrete to form above the Buckhorn Conglom-erate and upper parts of the Morrison Formation(Currie, 1997).

Regional isopach contours of the LK-2 depo-sitional sequence (Cedar Mountain and correla-tive formations) in northern Utah and westernColorado show that these rocks thicken signifi-cantly toward the west,exhibit a zone of re-gional thinning extending from south-centralUtah into northwestern Colorado,and thickenslightly into western Colorado (Fig. 12B).These patterns have been interpreted to indicatethe presence of a flexurally subsiding foredeepin west-central Utah (Schwans,1988; Yinglingand Heller, 1992),a northeast-trending fore-bulge stretching from the southeastern SanRafael Swell region into northwestern Col-orado,and a back-bulge depozone extendingeastward into Colorado (Fig. 12B) (Currie,1995). LK-2 sequence transitional and aggrada-






tional systems tracts in northeast Utah andnorthwest Colorado were deposited as the Apt-ian–Albian foredeep filled and accommodationwas generated in the forebulge depozone (Fig.13C). The overall accommodation increase that

resulted in deposition above the Early Creta-ceous forebulge may have been enhanced by theinitial f looding of the Western Interior seawayduring early Albian time (Kirkwood, 1976; Cur-rie, 1997).

Whereas the foredeep, forebulge, and back-bulge depozones of the Early Cretaceous foreland-basin system can be observed in the present-daySevier foreland and thrust belt,only the back-bulge depozone of the Late Jurassic foreland-basin system is preserved today. This is due to theuplift and erosion of Late Jurassic foredeep sedi-ments during Early Cretaceous through Paleogeneeastward propagation of the Sevier thrust belt(Royse, 1993; DeCelles and Currie, 1996; Cur-rie, 1997).

CONCLUSIONS

The sequence-stratigraphic model discussedabove is based primarily on interpreted changesin basin accommodation development duringdeposition of Upper Jurassic–Lower Cretaceousrocks in northeastern Utah and northwesternColorado. These sequences can be subdividedinto regionally traceable systems tracts on thebasis of changes in fluvial architecture and theposition of sequence-bounding unconformities.

Sequence bounding unconformities definedin the model can be of three types. Type 1 un-conformities form during major reductions inbasin accommodation and are characterized byvalley incision. Type 2 unconformities formduring minor reductions in nonmarine accom-modation and result in widespread shallow ero-sion of the preexisting alluvial plain. Type 3 un-conformities form during localized tosubregional uplift within the basin.

The depositional sequences bounded bythese unconformities can be divided into threesystems tracts:degradational, transitional,andaggradational. Degradational systems tractsoverlie regional unconformities and are charac-terized by relatively coarse grained fluvial stratadeposited within incised valleys or as thinsheets across type 2 unconformity surfaces.Transitional systems tracts are characterized bya change in fluvial architecture, from laterallycontinuous braided channel sandstones to finergrained, laterally discontinuous,meanderingand anastomosing channel bodies. Aggrada-tional systems-tract deposits are characterizedby isolated sandstones deposited by meander-ing-anastomosing fluvial channels and abun-dant fine-grained alluvial and lacustrine sedi-ments. Fluvial channel bodies deposited duringthe late stages of aggradational systems tractsmay be increasingly amalgamated due to de-creasing rates of accommodation development.

The Upper Jurassic–Lower Cretaceous non-marine rocks of northeastern Utah and north-western Colorado can be separated into threedepositional sequences. The Morrison Forma-tion contains two sequences. The lower Morri-son sequence (UJ-1) consists primarily of a ma-

B. S. CURRIE


Figure 13. Paleogeographic maps of east-central Utah and western Colorado during depo-sition of the Morr ison,Buckhorn, and Cedar Mountain Formations. Interpreted sedimentdispersal pathways taken from Craig et al. (1955),Craig (1981),Yingling (1987),and Peter-son (1988). (A) During initial LK-2 sequence deposition, sediment was delivered to the back-bulge region by Upper Salt Wash Member braided fluvial systems that breached the forebulgeto the west. (B) Prior to initial LK-1 sequence deposition,basin drainage systems were recon-figured into one northeast-flowing tr unk system during development of a type 1 unconfor-mity. Sediment was transported into the basin by the Buckhorn trunk stream from the south-west and by tr ibutar ies that breached the forebulge. (C) Eastward migration of the forebulgein the Early Cretaceous resulted in foredeep development in the preexisting back-bulge re-gion. LK-2 sequence deposition occurred as foredeep fluvial systems onlapped the upliftedforebulge (arrows). During this time, back-bulge deposition continued in western Colorado,back-bulge fluvial systems possibly onlapping the forebulge from the east. Location of pre-sent-day San Rafael Swell (SRS) is shown as a reference point.




rine highstand-nonmarine aggradational sys-tems tract comprising the Windy Hill, Tidwell,and lower Salt Wash Members. The UJ-1 se-quence is separated from the upper Morrisonsequence (UJ-2) by a type 2 unconformity anddegradational systems-tract deposits of theupper Salt Wash Member. Transitional andaggradational systems-tract deposits of theBrushy Basin Member make up the upper partsof the UJ-2 sequence. The UJ-2 sequence isoverlain by a type 1 unconformity and theBuckhorn Conglomerate.

The Buckhorn Conglomerate represents thethird nonmarine depositional sequence in thestudy area (LK-1). It was deposited during fillingof the valley incised during development of thetype 1 unconformity found at the base of the unit.

Separating the Buckhorn from the overlyingCedar Mountain Formation is an unconformityrepresented by the calcrete zone found through-out Utah and northwestern Colorado. This un-conformity may have been produced as the areawas uplifted by migration of the Early Creta-ceous flexural forebulge. Transitional andaggradational systems-tract deposits of theCedar Mountain Formation (LK-2 sequence)were deposited as sediment onlapped upliftedareas of the basin. Accommodation develop-ment in the forebulge depozone may have beenenhanced by the initial Early Cretaceous trans-gression of the Western Interior seaway duringearly Albian time.

ACKNOWLEDGMENTS

This study was partly supported by NationalScience Foundation grants EAR-9316462 andEAR-9526991 to P. G. DeCelles,and summerresearch grants from the Colorado Scientific So-ciety, Geological Society of America,Chevron,U.S.A., and Sigma Xi. I thank C. E. Turner andF. Peterson for sharing their vast knowledge ofthe Morrison Formation, and field assistants K. C. Pankow, B. D. Ritts,J. M. Trop,and R. R.Rasmussen for their help in gathering data. Ialso thank D. Chure and the staff at DinosaurNational Monument for their hospitality, and T. M. Demko for his informative insight on theMorrison Formation. I am grateful to P. G. DeCelles for his guidance and informal reviewsof the manuscript. Reviews by T. F. Lawton,G.Smith, and F. Peterson greatly improved thequality of this paper. Special thanks goes to R. K. Schwartz, whose encouragement and as-sistance helped make this investigation possible.

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