TECTONIC CONTROL ON THE SEQUENCE STRATIGRAPHY...

Journal of Sedimentary Research, 2007, v. 77, 239–255

Research Article

DOI: 10.2110/jsr.2007.021

TECTONIC CONTROL ON THE SEQUENCE STRATIGRAPHY OF NONMARINE RETROARC FORELANDBASIN FILLS: INSIGHTS FROM THE UPPER JURASSIC OF CENTRAL UTAH, U.S.A.

XAVIER ROCA* AND G.C. NADONDepartment of Geological Sciences, Ohio University, Athens, Ohio 45701, U.S.A.

ABSTRACT: Continental successions of the North American Western Interior retroarc foreland basin provide an excellentopportunity to evaluate the tectonic controls on nonmarine sequence stratigraphy. The transition between the Upper JurassicBrushy Basin Member anastomosed fluvial system of the Morrison Formation and the gravelly braided-river deposits of theBuckhorn Conglomerate has been studied to assess the dispersal of coarse clastics and the development of associated basin-wideunconformities in a sequence stratigraphic framework. The sharp contact between the two members is interpreted to beconformable based on stratigraphic, sedimentologic, and petrologic data collected at and near Cedar Mountain in central Utah,while a regional, mature paleosol at the top of the Buckhorn Conglomerate indicates the presence of a major sequenceboundary. These interpretations are combined with paleoflow data and fluvial architectural analysis to reconstruct the evolutionof the alluvial equilibrium profiles that controlled deposition of the succession.

The Buckhorn Conglomerate is interpreted as an example of a post-orogenic deposit on the basis of (1) its tabular geometry,(2) distance from the contemporaneous thrust belt, (3) shift to transverse paleoflow direction from the preceding Brushy Basinaxial drainage, (4) conformable lower contact, and (5) presence of an overlying sequence boundary with a westward-expandinghiatus eroding the coeval foredeep. The onset of a protracted period of low rate of orthogonal convergence between the Farallonand North American Plates resulted in tectonic quiescence and consequent isostatic rebound of the Middle to Late Jurassicthrust belt and the contiguous contemporaneous foredeep, as well as dynamic uplift. The ensuing basin-wide decrease insubsidence rate induced the progradation of the Buckhorn alluvial plain into the Morrison back-bulge depozone, and thesubsequent generation of the overlying pedogenic unconformity, which separates the deposits of the Jurassic flexural event fromthe overlying Sevier tectonostratigraphic unit.

INTRODUCTION

The development of a process model for the formation of forelandbasins (Jordan 1981; Beaumont 1981) provided a framework withinwhich the spread of coarse clastics can be explained as the result of eitherthe initiation or the cessation of thrusting in the adjacent orogen. Ensuingconceptual and quantitative models recognized that flexural deflection ofthe foreland area, resulting from the structural evolution of the thrustbelt, controlled the stratigraphic configuration of the basin fill (Heller etal. 1988; Flemings and Jordan 1989; Jordan and Flemings 1991). Inretroarc settings, the influence of static loading and unloading as well asof dynamic subsidence and topography on the evolution of fluvial profilesand the resulting changes in accommodation rate within the basin must betaken into account (Mitrovica et al. 1989; Gurnis 1992; Catuneanu et al.1997). In this tectonic framework, the geometry, paleoflow direction, andextent to which coarse sediments prograde into the basin, combined withthe location and extent of associated unconformities, can be used to infera syn-tectonic or post-tectonic origin for the coarse clastics (Heller et al.1988; Flemings and Jordan 1989). A syn-tectonic origin requires anunconformity below the coarse sediments, whereas a post-tectonic origininvolves an unconformity above.

Denudation of the topographic load during tectonic quiescence leads tothe isostatic rebound of the orogenic lithosphere and the subsequentuplift and erosion of the adjacent wedge-top and foredeep depozones. Theproximal basin settings become part of the source area of post-tectonicdistal coarse sediments, which consequently correlate with a foredeepunconformity. As isostatic adjustment continues and residual accommo-dation diminishes, the proximal unconformity advances progressivelybasinward, eventually developing above the post-tectonic coarse clastics.In retroarc foreland basins, cessation of thrusting may be associated withcessation of dynamic subsidence leading to steeper fluvial gradients andfacilitating the transport of gravel into distal parts of the basin, enhancingthe development of the ensuing regional unconformity.

In fully nonmarine successions where sea level is not an effectivecontrol on alluvial style, the development of a sequence stratigraphicframework relies on the recognition of tectonically driven unconformitiesto identify sequence boundaries. In foreland basins, a sequence boundarymay be formed by isostatic uplift at the end of a tectonic cycle. In theabsence of key stratigraphic surfaces, the interpretation of alluvialsystems tracts can be based on changes in the accommodation ratedepicted by channel geometry and connectedness, coarse clastics tomudstone ratios, and paleosol maturity of the fluvial succession (Shanleyand McCabe 1993; Wright and Marriott 1993).

The Middle Jurassic to Early Eocene Western Interior Basin of NorthAmerica is one of the best-preserved and most intensely studied examples

* Present address: Department of Earth Sciences, University of Western

Ontario, London, Ontario, N6A 5B7, Canada

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-239/$03.00

of a retroarc foreland. The Buckhorn Conglomerate is one of a number ofcoarse clastic fluvial units that overlie the Upper Jurassic alluvial strata ofthe Morrison and coeval formations throughout the basin (Fig. 1A). Inthe northwestern part of the Colorado Plateau, the sharp contact betweenthe Buckhorn Conglomerate and the underlying Brushy Basin Member ofthe Morrison Formation has traditionally been considered to be a basin-wide, low-order unconformity and a major nonmarine sequenceboundary (Heller and Paola 1989; Currie 1997, 1998a) (Fig. 2). However,soft-sediment deformation at the base of the Buckhorn Conglomerate(Kirkwood 1976; Yingling 1987) and the interfingering between theBuckhorn and the Morrison Formation (Aubrey 1998) suggest that theircontact is conformable.

Several tectonic mechanisms have been presented to explain thedeposition of the Buckhorn Conglomerate. Some workers have viewedthe conglomerate as a Late Jurassic, pre-Sevier tectonic event (e.g., Hellerand Paola 1989; Yingling and Heller 1992; Aubrey 1998; Heller et al.2003), whereas others have interpreted it as an Early Cretaceoussynorogenic deposit of the Sevier foreland basin (e.g., Armstrong andOriel 1965; Wiltschko and Dorr 1983; Currie 1997, 1998a). Currie (1997)presented the only sequence stratigraphic framework to date, in which theBuckhorn was interpreted as the fill of a paleovalley incised duringforebulge uplift generated by thrusting in the Sevier belt.

Despite their importance to both tectonic and sequence stratigraphicmodels of the early evolution of the Western Interior Basin, the age andtime span of the Buckhorn Conglomerate are poorly constrained becauseof a lack of fossils and interbedded bentonites. Accordingly, in order todetermine the tectonic process controlling Buckhorn sedimentation, itsrelationship to the Sevier orogeny, and the location of the associatedsequence boundary, it is necessary to locate unconformities withina largely unfossiliferous alluvial succession. To address all thesequestions, stratigraphic, sedimentologic, and petrologic data from theBrushy Basin Member and the Buckhorn Conglomerate were collected inand around the Buckhorn type section at Cedar Mountain in centralUtah.

REGIONAL SETTING

Biostratigraphic and geochronologic data from the terrestrial sedi-ments that straddle the Jurassic and Cretaceous Periods throughout theNorth American Western Interior retroarc foreland basin place theboundary between the two periods at a basin-wide, low-order un-conformity, known as K-1, across which as much as 20 My of section areabsent (McGookey et al. 1972; Pipiringos and O’Sullivan 1978; Imlay1980). Structural and stratigraphic data also show that the K-1

FIG. 1.— General location map of the studyarea showing major geologic features. A) Ap-proximate geographic extent and paleoflowdirections (arrows) of the Lower CretaceousConglomerates of the North American WesternInterior retroarc foreland basin (paleocurrentdata from Heller and Paola 1989 and Aubrey1992). B) Main structural elements of theColorado Plateau. Uplifts are indicated bydouble-headed arrows. Note the location of theMiddle to Late Jurassic and Early to LateCretaceous thrust fronts. SJB, San Juan Basin;SRS, San Rafael Swell; UMU, Uinta MountainUplift. C) Morrison and Cedar Mountainformations outcrops in central Utah. Note thelocation of Cedar Mountain on the northwesternflank of the San Rafael Swell, and the location ofthe exposed Canyon and Pavant Range thrustsof the Sevier thrust-and-fold belt (modifiedfrom Yingling and Heller 1992).

240 X. ROCA AND G.C. NADON J S R

unconformity separates two tectonostratigraphic units that originatedfrom different flexural events: the Middle to Late Jurassic Idaho–UtahTrough and its Canadian counterpart foreland basin (Brenner 1983;Saleeby and Busby-Spera 1992; Bjerrum and Dorsey 1995; Leckie andSmith 1992; Stott 1993), and the latest Jurassic(?)–Early Cretaceous toearly Cenozoic Sevier foreland basin (Armstrong 1968; Cowan and Bruhn1992; Leckie and Smith 1992; Pang and Nummedal 1995). In addition toflexural subsidence, the broad geographic extent of the Jurassic andCretaceous sedimentary wedges requires the contribution of dynamicsubsidence (Lawton 1994; Currie 1998b). The lack of overwhelmingevidence for a Middle to Late Jurassic orogenic belt has led previousworkers to attribute the generation of accommodation in the basin duringthis period to dynamic subsidence alone (Lawton 1994).

Deposition of the Morrison Formation is considered to have takenplace within the back-bulge depozone of a retroarc foreland basin systemon the basis of long-term sediment accumulation and the westward onlaponto Middle Jurassic strata in central Utah, interpreted as the coevalforebulge (DeCelles and Currie 1996; Currie 1998a; DeCelles 2004). Thecontribution of dynamic subsidence to the generation of accommodationis suggested by the exceedingly large geographic extent and tabulargeometry of the Morrison Formation (Lawton 1994; DeCelles 2004). Theabrupt increase in thickness of the overlying Lower Cretaceous rockstowards the thrust belt records renewed flexural subsidence at the onset ofthe Sevier orogeny and the eastward advance of the foredeep into centralUtah (Yingling 1987; Yingling and Heller 1992; Currie 1998a, 2002).Consequent eastward migration of the flexural forebulge has beensuggested as the cause for the generation of the K-1 unconformity in theColorado Plateau area (DeCelles and Currie 1996; Currie 1998a). Thegeographical extent of the stratigraphic lacuna exceeds that predicted byflexural models, and consequently dynamic uplift has also been suggestedto contribute to its basin-wide development (Currie 1998b).

Associated with the K-1 unconformity are a series of discontinuous,coarse-grained fluvial deposits, which extend from southern Utah towestern Alberta, referred to as the Lower Cretaceous conglomerates byHeller and Paola (1989) (Fig. 1A). Although biostratigraphic andgeochronologic control is limited, the similar stratigraphic position,depositional environment, Late Paleozoic western provenance, andgeneral eastward paleoflow suggest that the various conglomeratic unitsare roughly contemporaneous (Stokes 1944; Armstrong 1968; Heller andPaola 1989; Heller et al. 2003). Traditionally, the K-1 unconformity isplaced at the base of these conglomerates because its erosional nature isinterpreted to indicate an unconformable contact (Stokes 1944, 1952;Young 1960; Mirsky 1962; Heller and Paola 1989; Meyers et al. 1992;Way et al. 1994; May et al. 1995; Currie 1997, 1998a, 1998b), but otherstudies have not found evidence to corroborate this interpretation(Rapson 1965; Furer 1970; Gibson 1985; Winslow and Heller 1987;DeCelles and Burden 1992).

In the northwestern part of the Colorado Plateau, the BuckhornConglomerate is underlain by the Brushy Basin Member of the MorrisonFormation, and is overlain by the Upper Shale Member, which togetherconstitute the Cedar Mountain Formation. The Late Jurassic age of thefluvio-lacustrine deposits of the Morrison Formation has been wellestablished by geochronologic ages from bentonites (Kowallis et al. 1991;Kowallis et al. 1998), charophytes (Schudack et al. 1998), palynomorphs(Litwin et al. 1998), and magnetostratigraphy (Steiner et al. 1994; Steiner1998) (Fig. 2). Deposition of the terrigenous strata of the Upper ShaleMember took place from the Late Hauterivian (?) or earliest Barremian tonear the boundary between the Albian and the Cenomanian, as indicatedby charophytes (Mitchell in Aubrey 1998), palynomorphs (Tschudy et al.1984), dinosaur fauna (Kirkland 1992), and bentonite dates (Cifelli et al.1997) (Fig. 2). Consequently, a major hiatus is inferred to exist betweenthe Brushy Basin and Upper Shale members. Traditionally, the K-1unconformity has been placed at the sharp base of the BuckhornConglomerate west of the Colorado River (Fig. 3). To the east, theunconformity was considered to lie at the base of the Burro CanyonFormation, a fluvial unit interpreted to be entirely equivalent to theUpper Shale Member of the Cedar Mountain Formation (Stokes andPhoenix 1948; Craig 1981; Currie 1998a).

Aubrey (1998) challenged this long-held interpretation on the basis ofthe interfingering of the Morrison Formation with overlying deposits(Craig et al. 1961; Ekren and Houser 1965; Aubrey 1992). Aubrey (1998)noted that only the upper part of the Burro Canyon Formation containsabundant carbonate nodules and associated hardpan calcrete horizonscharacteristic of the Upper Shale Member of the Cedar MountainFormation, and correlated the lower part of the Burro CanyonFormation with the upper part of the Morrison Formation to the west.

FIG. 2.—Synthetic stratigraphic column of the Morrison and Cedar Mountainformations along the northwestern part of the Colorado Plateau. Note the agedifference between the Brushy Basin and the Upper Shale members and theunknown age of the Buckhorn Conglomerate. The exact location of the K-1unconformity is controversial. Different workers have placed the hiatus at thesharp base of the Buckhorn Member or at the top of the regional calcrete thatoverlies the Buckhorn Conglomerate. Age data are from Cifelli et al. (1997),Kirkland (1992), Kowallis et al. (1991), Kowallis et al. (1998), Litwin et al. (1998),Mitchell in Aubrey (1998), Schudack et al. (1998), Steiner et al. (1994), Steiner(1998), Tschudy et al. (1984).

SEQUENCE STRATIGRAPHY OF NONMARINE FORELAND BASIN FILLS 241J S R

Aubrey (1998) placed the K-1 unconformity at the oldest calcrete bed,which locally exceeds 10 m in thickness, or at the base of a channel fillcontaining the first calcrete intraclasts where the calcrete bed is absent.Because the lowermost calcrete horizons overlie the Buckhorn Conglom-erate in the San Rafael Swell (Conley 1986; Currie 1997, 1998a), Aubrey(1998) argued that deposition of the Buckhorn Conglomerate precededboth the generation of the K-1 unconformity and the onset of the Sevierorogeny, and considered the conglomeratic member to be part of theMorrison Formation (Fig. 3).

STUDY AREA AND METHODS

Cedar Mountain is located approximately 100 km east of the exposedfront of the Sevier fold-and-thrust belt, on the northern part of thewestern flank of the San Rafael Swell, a Laramide anticlinal uplift in east-central Utah (Fig. 1B, C). The lack of vegetation in the area offersexcellent exposures of the Buckhorn Conglomerate, including its typesection, and the underlying Morrison Formation (Fig. 4). The gentlynorthwest-dipping, very resistant Buckhorn Conglomerate forms a south-east-trending topographic feature, the outline of which defines theoutcrop belt. A total of 30 stratigraphic sections of the upper BrushyBasin Member, the Buckhorn Conglomerate, and, where present, thelower Upper Shale Member of the Cedar Mountain Formation weremeasured between Cleveland Lloyd dinosaur quarry and the southern

flank of Little Cedar Mountain (Fig. 5). Stratigraphic data aresummarized in a cross section transverse to the orientation of CedarMountain along structural strike (Fig. 6).

Paleoflow indicators, thin sections, and clast lithologies were used todetermine provenance of the two units. A total of 1,112 paleoflowmeasurements from cross-bedded sandstones of the Brushy Basin andBuckhorn members were taken. A minimum of 400 grains was countedfrom thin sections of three medium-grained sandstones in each member.Pebble counts were performed by sampling chips from a minimum of363 clasts/m2 from eight sites within the Buckhorn Member and bycollecting large samples from the same number of granular conglomeratesof the Brushy Basin Member and examining them under a binocularmicroscope. Lithoclast types of the two members were compared in orderto assess any difference in source area between the members.

FACIES AND FACIES ASSOCIATIONS

The measured sections within the study area contain a total of 13 faciesgrouped into six facies associations: three in the Brushy Basin Memberand three in the Buckhorn Conglomerate (Fig. 7). All the facies representdeposition in terrestrial environments. The facies associations in eachmember are presented separately below, followed by a discussion of thefluvial architecture.

FIG. 3.— Variations in the placement of theK-1 unconformity in the San Rafael Swell. Notedifferent assessments of the age of the BuckhornConglomerate over the years. The duration ofthe hiatus between the Morrison Formation andUpper Shale Member of the Cedar MountainFormation is tentative in all the suggestedchronostratigraphies. Time scale is fromGradstein et al. (2004).


Brushy Basin Member

The Brushy Basin Member has a tabular geometry with a meanthickness of 75 m. Variegated smectitic claystone is the diagnostic featureof this member, and makes up approximately 77% of the succession.Sandstone (15.3%), conglomerate (2.3%), thin siltstone beds (4.5%), andfreshwater limestone beds (0.7%) are present, commonly toward the baseof the unit (Fig. 8A). Where thick Buckhorn sediments overlie the BrushyBasin Member, the latter comprises a pale greenish to gray mudstone withrare coarser clastics. Where the Buckhorn conglomerate is not wellcemented, discontinuous, or absent, the upper mudstones are commonlyintense red to dark purple, and sandstones and conglomeratic strata aremore common, showing a characteristic white bleaching and poorcementation at or near the top of the Morrison Formation.

Facies Association BB1: Mudstones and Carbonates.—Facies Associa-tion BB1 is composed of five facies. Claystone to silty claystone and raresiltstones are the predominant lithologies of the Brushy Basin Member.The claystones and silty claystones (Facies Fb) contain rootlets, havea blocky texture, and weather to a characteristic popcorn texture. Nocarbonate nodules are present in the mudstones. Meter- to decimeter-

scale color banding that crosscuts bedding, and millimeter-scale mottlingin the finer grained sediments, are ubiquitous. A series of laterallyinterfingering horizons of purple and greenish hues are common directlybelow the contact with the Buckhorn Conglomerate. Near the top of theBrushy Basin Member is a bentonite bed. Centimeter- to decimeter-thicklayers or bodies of brown to purple platy mudstones (Facies Fp), whichshow vertical branching structures up to 25 cm long at Cleveland LloydDinosaur quarry, are present in some outcrops directly above the contactwith the Buckhorn Conglomerate or near the top of the Brushy BasinMember where the Buckhorn is thin or absent.

The contact between the Brushy Basin and Buckhorn members showstwo types of postdepositional deformation. Flame structures are locallypresent at several locations (Fig. 9A). At one site there are roundeddepressions in the uppermost Brushy Basin Member mudstones that arefilled with Buckhorn Conglomerate (Facies BC3). These depressionssuperficially resemble load casts (Fig. 9B). The lamination of themudstone is clearly deformed under the conglomerate, but the layeringin the fill is horizontal.

Gray lime mudstone beds (Facies Fsc) are common in the lower andmiddle part of the Brushy Basin Member. Although these beds reach upto 64 cm in thickness, they are rarely thicker than 20 cm. They commonly

FIG. 5.— Study-area map showing the loca-tion of measured sections and paleocurrent datacollected from the Brushy Basin and BuckhornConglomerate. Well-cemented Buckhorn faciescontrol the location of Cedar Mountain andLittle Cedar Mountain, whereas poorly cemen-ted Buckhorn deposits enable the erosionalretreat of the succession cropping out as a narrowbelt along structural strike north of CedarMountain. Note the parallelism between thesoutheasterly trend of Cedar Mountain and theBuckhorn Conglomerate paleoflow.

FIG. 4.—The Morrison Formation overlying the red beds of the Summerville Formation and overlain by the Buckhorn Conglomerate at Little Cedar Mountain, 3 kmsouth of the Buckhorn type section.


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extend for a few tens of meters before pinching out into Brushy BasinMember mudstones and weather as thin resistant ledges. Mollusks andcharophytes are the most common fauna. Sporadic massive, thin(, 0.3 m thick) laterally continuous siltstone beds constitute the coarsestfacies (Facies Fsi). These beds extend for a few tens of meters and havesharp bases and tops.

Interpretation: Facies Association BB1 is composed of variousfloodplain facies. The presence of roots and a uniform blocky textureindicates that Facies Fb represents floodplain mudstones that werealtered to paleosols. Facies Fb is interpreted as a series of stackedProtosols (Mack et al. 1993) equivalent to modern Inceptisols (SoilSurvey Staff 1975) on the basis of their poor horizonation. The popcornweathering pattern is typical of mudstones that have a devitrified volcanicash component (Owen et al. 1989). Branching and horizontal structures inthe uppermost blocky purple Brushy Basin Member mudstones may bepedogenic features. These mudstone structures are consistent withprevious descriptions of kaolinite recrystallization in the same intervalinterpreted as a lateritic soil (Bilbey 1992). The purple and green bandingin the mudstones directly below the Buckhorn Conglomerate suggestsa diagenetic rather than a depositional origin, possibly due to variationsin the paleo-groundwater level.

Beds of Facies Fsi are interpreted to represent the distal portions ofcrevasse-splay deposits, on the basis of the coarser grain size relative toother facies of the association. The thin beds of facies Fcs representbiochemical sedimentation in small, short-lived lakes or ponds on thefloodplain. All the components of facies Association BB1 are consistentwith deposition on a floodplain under a relatively high accommodationrate (Wright and Marriott 1993).

The flame structures, which are the result of the injection of theuppermost Brushy Basin Member mudstones into the overlying Buck-horn Conglomerate, indicate that the Brushy Basin sediments wereunconsolidated at the onset of Buckhorn Conglomerate deposition. Theconglomerate-filled ‘‘load casts’’ are interpreted to be dinosaur tracks onthe basis of the presence of horizontal bedding at the top of the structure,indicating that the Buckhorn Conglomerate passively infilled a preexistingdepression on the floodplain (Nadon 1993, 2001). The presence of thesetracks means that at this location there was essentially no erosion of themudstones by the conglomerate. The combination of the flame structuresand the track impressions means that deposition of the conglomeratetook place shortly after that of the underlying mudstones.

FIG. 7.— The facies and facies associations of the upper Brushy Basin andBuckhorn Conglomerate members.

FIG. 8.—General photos of the Brushy Basin and Buckhorn members. A) Brushy Basin Member near Cleveland Lloyd Dinosaur Quarry. B) Well-cemented BuckhornConglomerate at Cedar Mountain. C) Poorly cemented, muddy-matrix-rich Buckhorn Conglomerate north of Cedar Mountain.


Facies Association BB2: Sandstone Sheets.—Sandstone sheets tend tobe light gray to tan but locally have the same color as the surroundingmudstones. Sandstones close to the top of the member have a character-istic white color and are extensively weathered. The sandstones rarelyexceed 0.5 m in thickness, are sharp-based, and are typically less than25 m, but locally up to 200 m, in lateral extent. These sandstones arecommonly very fine-grained and tend to fine upward slightly. The mostcommon sedimentary structures are horizontal laminae (Facies Shl)capped by ripple cross-lamination (Facies Sr). The thicker sandstones areplanar cross-bedded (Facies Sp).

Interpretation: The thin, tabular sandstone bodies are interpreted ascrevasse-splay deposits. The beds with Facies Sr overlying Shl indicatea decrease in flow regime with time (Collinson and Thompson 1982) andare interpreted to represent deposition under waning flow. The thickersandstones (Facies Sp) represent sediments deposited in standing waternear the mouth of the crevasse channel. Crevasse splay deposits tend to bepresent near facies association BB3, suggesting a genetic link. The rarityof sandstone sheets in the Brushy Basin Member suggests that the fluvialsystem transported little bedload.

Facies Association BB3: Ribbon Sandstones and Conglomerates.—Lenticular sandstones and small pebble conglomerates that vary in

thickness from 2 to 5 m and 0.1 to 3.2 m, respectively, rarely exceed 30 min width and yield W/T ratios that range from 15 to 2. These lenticularunits are scattered throughout the Brushy Basin Member. The ribbonstend to fine upward from an erosional base whereas the top is eitherabrupt or gradational into overlying mudstones. Trough cross-bedding isthe most common sedimentary structure, but planar cross-bedding wasalso observed. Plane-laminated sandstones are present near the top ofsome ribbon bodies. Intraclasts of cobble- to pebble-size mudstone andfine-grained sandstone, which also include scattered dinosaur bone andwood fragments, are common at the bases of the ribbons. Measurementsfrom the cross-bedded sandstones show a northeastward paleoflowdirection with low dispersion (Fig. 5).

Trough cross-bedded conglomerate and sandstone ribbons arecommon near the top of the member in the northern localities wherethe Buckhorn is very thin or absent. These coarse-grained ribbonsdiffer from those lower in the section in having a strong whitecoloration and being weakly cemented. In most of these localities,conglomeratic ribbons near the top of the formation contain bright-colored granules and small pebbles, are similar to ribbons observed inthe lower parts of the Brushy Basin Member in the study area, and areconsidered to be characteristic of the regional Morrison Formation(Peterson 1988).

FIG. 9.— Sedimentary structures at the contact between the Brushy Basin Member and the Buckhorn Conglomerate. A) Flame structures formed by injection ofMorrison mud into the overlying Buckhorn Conglomerate. B) Depressions in the top of the Brushy Basin Member filled by the overlying Buckhorn conglomerate. Thedeformation of underlying strata and the horizontal infill indicates that the depressions are dinosaur tracks. C) Subparallel furrows at base of the BuckhornConglomerate, interpreted to be rill marks.


Interpretation: The sandstone and conglomeratic ribbons representisolated channel-fill deposits on a mud-dominated, low-gradient flood-plain. In spite of forming a low percentage of the total volume, andhaving a rather fine grain size, their presence indicates that thepredominantly fine-grained Brushy Basin fluvial paleosystem alsotransported coarse clastics prior to the deposition of the BuckhornConglomerate. The northeastward paleoflow direction is consistent withmeasurements previously reported form the Brushy Basin Member (Craiget al. 1955; Cadigan 1967; Peterson 1984; Yingling and Heller 1992;Currie 1998a).

The sandy and conglomeratic ribbons near or at the top of the membernorth of Cedar Mountain are considered, on the basis of theirstratigraphic position and occurrence where the Buckhorn Conglomerateis abnormally thin or absent, to be chronostratigraphically coeval withBuckhorn deposits defined at the type section farther south (Stokes 1944).However, since these Brushy Basin facies are clearly different from theBuckhorn Conglomerate but are similar to underlying coarse-grainedribbons in the Brushy Basin Member, we do not consider it appropriateto include them as part of the lithostratigraphic Buckhorn Conglomerate.Deep and protracted weathering is inferred to be the cause of the whitecolor and poor cementation. The presence of horizontal and branchingkaolinite-rich structures (see above) in the mudstones overlying bleachedsandstones suggests that sandstone discoloration was a product oflaterization (Bilbey 1992).

Buckhorn Conglomerate

The Buckhorn Conglomerate consists of clast-supported pebble andcobble conglomerate (72.1%) with interbedded sandstones and pebblysandstones (27.9%), and very rare blocky mudstone lenses (, 0.1%). Theunit is a sheet that ranges in thickness from 0 to . 16 m with a W/T ratioof approximately 3,000. Although the thickness of the Buckhorn variesthroughout the study area, the maximum thickness tends to be along thesoutheast–northwest axis of Cedar Mountain (Figs. 5, 6). The Buckhorngradually thins and fines to a pinch-out south of Little Cedar Mountain.A northern depositional pinch-out occurs southwest of the ClevelandLloyd dinosaur quarry road (Figs. 5, 6). Approximately 500 m northwestof this locality, the pebbly sandstones of the Buckhorn Conglomerateinterfinger locally with mudstones of the Brushy Basin Member (betweensections ZB and PB in Fig. 6).

The top of the Buckhorn Conglomerate is placed directly below thefirst appearance of a calcrete horizon. The basal calcrete bed consists ofa massive, indurated carbonate sheet that varies in thickness from a fewdecimeters to 2 m, and can be classified as a hardpan calcrete (Wright andTucker 1991) (Fig. 10). Chert grains ranging in size from sand to smallpebbles float in the carbonate matrix. Multiple generations of brecciatedcarbonate intraclasts are present, suggesting a very advanced degree ofpedogenesis. White and red chert veins are also common. Where thecalcrete is absent due to erosion, the top of the Buckhorn is placed at thebase of the first occurrence of pebbly sandstones and sandy conglomer-ates containing calcrete intraclasts or, if these are not present, at the baseof the carbonate-nodule-bearing light purple mudstones of the UpperShale Member.

Clast size in the Buckhorn lacks spatial trends, but cementationpresents significant geographic variations. At the type section, theBuckhorn Conglomerate is well cemented, and this results in the positiverelief of both Cedar Mountain and Little Cedar Mountain (Figs. 5, 8B).North of Cedar Mountain, the Buckhorn is much less indurated, locallycontains a fine-grained matrix, and tends to be easily eroded, forminga poorly exposed unit of loose pebbles and disaggregated sandstonestained by hematite (Fig. 8C). The lateral boundary between the twostyles of cementation is apparently abrupt, but in detail, poorly cementedand well cemented units replace each other over decameter-scale

distances. The top of the Buckhorn Conglomerate has both pervasivesilica cementation and local vertical and horizontal silica veins at severalsites.

Facies Association BC1: Conglomerate.—Well-rounded, clast-sup-ported cobble, pebble, and granular conglomerate is characteristic ofthe Buckhorn Member. These lithologies normally occur in laterallydiscontinuous lenses ranging in thickness from 0.35 m to 4 m. Althoughthe conglomerate normally has a medium- to fine-grained sandstonematrix, some centimeter-scale conglomeratic lenses close to the base havevery abundant clay matrix and are locally mud-supported. North ofCedar Mountain, where the Buckhorn is weakly consolidated, a variegat-ed mudstone and sandstone matrix is locally present throughout the unit.

Three conglomerate facies are present in the Buckhorn. Lenses oftrough cross-bedded conglomerate (Gt) are very common, especiallytoward the top of the unit. Two types of Gt deposit can be distinguished.Narrow lenses with erosional bases are between 3 and 15 m wide andbetween 0.4 and 2.5 m thick. Wide lenses with relatively uniformthickness range from 0.5 to 2 m and extend laterally up to 150 m.Abrupt transitions from sandy conglomerate to pebbly sandstone areespecially common in the wide lenses. Planar cross-bedded conglomerate(Gp) tends to be thicker and more laterally continuous, sometimesreaching 25 m in width and between 0.5 m and 2 m in thickness.Although vertical gradation into other facies is present, the Gp beds tendto be bounded by erosion surfaces. Massive to crudely bedded layers(facies Gm) occur in lenses less than 10 m wide and between 0.35 m and2.5 m thick. These lenses have scoured lower contacts and may grade intoGt or Gp facies before ending in an erosional upper contact.

The bottom contact of the Buckhorn Conglomerate commonly exhibitsclosely spaced, somewhat V-shaped furrows that are centimeters deep,decimeters wide at the top, and up to several meters long (Fig. 9C). Thesestructures are commonly oriented east–west and may be parallel ordisplay a radial pattern. The sides of the deeper furrows are contorted.

Interpretation: The conglomerate facies were deposited in varioussubenvironments of a braided fluvial system. The laterally constrained Gtlenses with erosional bases represent local confluence scours in the deeperportions of the system (Miall 1996). The wider Gt deposits with a rapidtransition into pebbly sandstone are a product of dune migration inchannels (e.g., Miall 1996). The Gp lenses are inferred to be the productof bars growing from older channel bar remnants (Miall 1996). The Gm

FIG. 10.—Photo of the hardpan calcrete at the base of the Upper Shale Memberof the Cedar Mountain Formation. Our work suggests that this mature paleosolrepresents the sequence boundary between the Middle–Upper Jurassic and LowerCretaceous tectonostratigraphic units of the Western Interior Basin in theColorado Plateau. Stick (1.5 m long) for scale.


beds are interpreted as channel-lag deposits and longitudinal channel bars(Miall 1996). Different degrees of framework cementation, probablycontrolled by the amount of smectitic clay in the matrix, may explain thetransition between well cemented and poorly cemented conglomerate.

The V-shaped furrows at the base of the Buckhorn are interpreted asrill marks. Their presence indicates scour at the base of the Buckhorn butnot necessarily significant erosion. The contorted sides of the rills are theresult of differential compaction between the gravel filling the furrowsand the surrounding mud, following deposition of the BuckhornConglomerate. Such compaction would not be expected if the forma-tional contact represented a significant unconformity. If the contact hadbeen a surface of subaerial bypass, protracted exposure of the uppermostMorrison mudstones would have led to their early cementation. On theother hand, if the base of the Buckhorn were an erosional unconformity,previous burial of the Brushy Basin Member would have compacted thosemudstones before erosion of the rill marks. In both cases, contortion ofthe mudstone surrounding the conglomerate-filled rills would have beeninhibited. Accordingly, this feature is interpreted to suggest a ratherconformable contact.

Facies Association BC2: Sandstone.—Pebbly to clean, light brown totan, fine- to medium-grained sandstones are common in the BuckhornConglomerate. These sandstones tend to be more abundant, thicker, andmore laterally extensive in the upper part of the unit and toward itsmargins. Sandstones interbedded with conglomeratic facies near the baseof the Buckhorn are normally less than 0.4 m thick, rarely exceed 10 m inwidth, and show both erosional and gradational contacts.

Based on internal structures, four different facies are distinguished inthe sandstones. In order of decreasing abundance they are trough cross-bedded (St), planar cross-bedded (Sp), massive (Sm), and plane laminatedwith parting lineation (Shl) sandstones. Facies St is rarely thicker than2 m, whereas the less abundant Sp facies reaches 3 m in thickness. Thebeds of facies Sp and Sm are always much thinner, and never exceed a fewdecimeters in thickness. Paleoflow measurements from St sandstones weretaken mostly from the top of the unit in the southeastern part of the studyarea, because the weak cementation of the Buckhorn farther northprecluded measurements at several sites. The data show a strongsoutheastward paleoflow direction that coincides with the thickness axisof the Buckhorn Conglomerate (Fig. 5).

Interpretation: The sandstone facies are interpreted to representvarious channel-fill elements in a braided fluvial system. Facies St is theproduct of dune migration within channels. The thin Sp sandstones areconsidered to record 2-D dunes deposited during low flow regime withinchannels, whereas the thicker and more laterally extensive beds mayrepresent linguoid channel bars deposited under slightly deeper condi-tions. The Shl sandstones are considered to be channel-fill faciesdeposited during higher flow regimes. The correspondence between thepaleoflows measured in the Buckhorn sandstones and the topographictrend of Cedar Mountain strongly suggests that the latter preserves themain drainage axis of the Buckhorn alluvial plain.

Facies Association BC3: Mudstone Lenses.—Facies Fb, consisting ofmassive to blocky mudstone, is the least abundant facies in the BuckhornConglomerate Member. This facies is present as lenses bounded byerosional contacts at various stratigraphic levels in the member.Mudstone lenses close to the Buckhorn base tend to be thinner than20 cm. The largest mudstone lens occurs near the top of the Buckhornand has a maximum thickness of 1.2 m and a width of less than 5 m. Thelens is composed of smectitic blocky claystone with intense dark purpleand light green coloration. Several mudstone lenses were sampled forpalynology, but no pollen was recovered (R. Litwin, personal commu-nication 2003).

Interpretation: Buckhorn mudstone lenses are interpreted to representthe plugs of abandoned channels or fine-grained overbank deposits. Thesmall proportion of this facies suggests that either the Buckhorn fluvialsystem was strongly bedload dominated, or that channel migration wasfrequent, reworking most of the mud plugs. The absence of mudstoneintraclasts in the coarser Buckhorn deposits favors the first interpretation.

PETROGRAPHY AND PETROLOGY

Microcrystalline and chalcedonic chert, as well as undulatory and non-undulatory monocrystalline quartz, are the most common sand-sizelithologies in the Brushy Basin and Buckhorn members. Minor amountsof K-feldspar and plagioclase are also present. Modal percentages(Fig. 11A) indicate that the sandstones are litharenites (Pettijohn et al.1987) that lie in the ‘‘recycled orogen’’ field of Dickinson (1985). Thesmall variation in QFLt ratios between the two units is due to a slightincrease in chert, polycrystalline quartz, quartzite, and tuff fragments anda decrease in quartz and feldspar in the Buckhorn Conglomerate. Whenplotted with data from adjacent units (Crooks 1986; Currie 1998a),a pronounced peak in lithic grains associated with the Brushy Basin andBuckhorn members is observed (Fig. 11B), suggesting a common sourcearea. Cement phases of the sandstones of both members include quartzovergrowths, chert, calcite, and clays. Of special interest is a pervasivesilica cement in the uppermost Buckhorn sandstones observed in severallocalities, as well as poikilotopic calcite cementation of the upperBuckhorn sandstones underlying the basal Upper Shale Member calcrete.

Comparison of clast lithologies from the two members was difficult dueto the lack of conglomerates of similar clast sizes in the study area.Nevertheless, granule and small-pebble conglomerates from the BrushyBasin Member contained many of the same types of chert as were foundin the overlying coarser Buckhorn conglomerates. Chert-bearing con-glomeratic channel facies have also been widely recognized in the upperMorrison Formation of the San Rafael Swell (Crooks 1986; Conley 1986;Yingling 1987; Bilbey 1992; Currie 1998a, 1998b). Of special significanceis the observation that conglomerates of the two members east of Ferron(Fig. 1C) can be distinguished by their geometry but not by theircomposition (Conley 1986). Therefore, fluvial style, and not petrography,appears to be the main difference between the two members.

White, brown, and black are the most abundant chert colors, but pink,red, orange, blue, and green are also present. Pebble counts from theBuckhorn Conglomerate yield a mean clast composition of 70.25% chert,23.5% quartzite, 0.75% sandstone, and 5.5% other, with no significantgeographic trends. Buckhorn chert clasts contain silicified crinoid stemfragments, colonial corals, bryozoans, and fusulinids. Silicified fossilfragments were not found in the Brushy Basin Member granuleconglomerates, probably due to the smaller clast size, but quartziteclasts, although rare, are also present. The fossil content in the chert clastsis indicative of silicified upper Paleozoic rocks (Stokes 1944; Currie1998a), which could have been supplied from formations now exposed inthe Sevier thrust belt, but mainly from its hinterland (Yingling 1987),whereas quartzite clasts were derived from Devonian to Proterozoic(?)rocks present in northern and central Utah (Hintze 1988). The presence ofthese clast lithologies indicates that the source area had undergoneenough denudation to erode the supracrustal Mesozoic rocks prior to thedeposition of the Brushy Basin and Buckhorn members. Accessory lithicclasts found in the conglomeratic beds of both units comprise sandstoneclasts, as well as petrified wood and dinosaur bone fragments.

Provenance data are interpreted to indicate a western thrust belt tohave been the source area of the Buckhorn Conglomerate. The occurrenceof chert and quartzite granules and pebbles similar to those of theBuckhorn in the Brushy Basin Member, but absent in the underlyingmembers, indicates a growing contribution of the same source area duringdeposition of the upper Morrison. Accordingly, a gravelly ‘‘Buckhorn’’


facies is inferred to have accumulated updip from the more distal BrushyBasin fluvial system.

FLUVIAL ARCHITECTURE AND PALEODRAINAGE DIRECTION

The data collected in this study generally support the interpretations ofdepositional environments proposed by previous workers. The presenceof isolated channels with a low W/T ratio, the predominance of fine-

grained floodplain deposits with weakly developed paleosols, and thesubordinate ephemeral lacustrine environments in the Brushy BasinMember are consistent with deposition in a rapidly aggrading anasto-mosed fluvial system (Nadon 1994). Previous work on the depositionalenvironment of the upper Brushy Basin Member at the Cleveland Lloyddinosaur quarry reached the same conclusions (Kantor 1995; Richmondand Morris 1996). The coarse grain size, high W/T ratio, rapid faciestransitions, abundant cut-and-fill structures, irregular bedding contacts,and preserved longitudinal and linguoid bars support the braided-fluvialorigin for the Buckhorn Conglomerate, as suggested by numerous otherstudies (Young 1960; Crooks 1986; Conley 1986; Yingling 1987; Bilbey1992; Currie 1997, 1998a, 1998b).

Differentiating between local scour and regional erosion associatedwith a base-level fall is necessary for the interpretation of the controls onfluvial architecture (Salter 1993; Leeder and Stewart 1996). The abrupttransition between the Brushy Basin and Buckhorn members does notnecessarily imply an unconformable contact. The base of most riverchannels is sharp and highly irregular due to scour largely controlled bydischarge and competence (e.g., Leopold et al. 1964, their figs. 7–16). Weinterpret the rill marks reported here as evidence of local scour, and notevidence of significant downcutting and valley formation as previouslyinferred for the base of the Buckhorn Conglomerate (Currie 1997).Similar rill marks are reported and are interpreted to be the result of localscour by Gibling and Rust (1984) and Plint (1986). In addition, thecombination of flame structures, rill marks, and conglomerate-filleddinosaur tracks shows that the uppermost Brushy Basin Membermudstones underwent little or no compaction prior to the onset ofBuckhorn deposition.

The measured northeastward paleocurrent direction of the BrushyBasin Member fluvial system is in agreement with previous regionalpaleoflow studies of this unit (e.g., Yingling and Heller 1992; Currie1998a) (Fig. 12A). However, our work differs from those studies in thatwe interpret the paleocurrent structures and thickness trends to indicatethat the Buckhorn Conglomerate fluvial system flowed to the southeast(Fig. 12B). Other Buckhorn paleoflow directions reported from the samearea (Conley 1986) and the eastern flank of the San Rafael Swell(Osterwald et al. 1981) support our interpretation. An importantcomponent of this study is the recognition of the correspondence betweenthe mean paleocurrent direction of the Buckhorn Conglomerate and thetopographic trend of Cedar Mountain (Fig. 5). We interpret thiscoincidence to indicate that the geomorphology of Cedar Mountainrecords the axis of the southeast-directed Buckhorn braided alluvialsystem.

The southeastward drainage direction of the Buckhorn braid plain atCedar Mountain is consistent with the location of the Lake T’oo’dichi’,which was a regional topographic low, recorded in the uppermost BrushyBasin Member of the four corners area (Turner and Fisherman 1991)(Fig. 12A). Nevertheless, the basin-wide decrease in accommodation rateinferred to have resulted in southeastward progradation of Buckhornsediments predicts sediment bypass and transport of coarse clasticsfarther into the southeastern parts of the Colorado Plateau. According totheir conformable contact with the underlying Brushy Basin Member andtheir southeastward paleoflow direction, the Karla Kay Conglomerateand Jackpile Sandstone near and within the San Juan Basin (Aubrey1992) may represent distal deposits of the Buckhorn Conglomerate(Fig. 12B). Similar conglomeratic deposits in the Uinta Mountainsidentified as Buckhorn (e.g., Currie 1997, 1998a, 1998b) may behomotaxial with the type section, but there is no evidence to demonstratethat the fluvial paleosystems were physically connected. The change fromaxial to transverse drainage associated with the reduction in accommo-dation rate between the two members may indicate the transition from anunderfilled to overfilled foreland basin (Jordan 1995). Lawton et al.

FIG. 11.— QFLt diagrams from the Brushy Basin and Buckhorn members. A)Modal percentages of grain compositions measured from thin sections in theBrushy Basin and Buckhorn members. The similarity in composition suggestsa common provenance of both units. B) Comparison of petrographic data fromthe interval of interest to the adjacent strata in the study area. Data from Crooks1986, Currie 1998a, and this study.


(2003) have reported similar abrupt paleodrainage reorganization in thesame basin during the Late Cretaceous.

AGE AND LITHOSTRATIGRAPHIC NOMENCLATURE OF THE

BUCKHORN CONGLOMERATE

The stratigraphic, sedimentologic, and petrologic data from the BrushyBasin and Buckhorn members presented here suggest that their contact isconformable, and therefore the K-1 unconformity is not located betweenthese lithostratigraphic units. Instead, the mature calcrete and thecorrelative erosional surface at the base of the overlying Upper ShaleMember of the Cedar Mountain Formation is a better candidate for thediscontinuity (Roca 2003; Ayers and Nadon 2003; Ayers 2004).

The unfossiliferous nature of the Buckhorn Conglomerate and the lackof interbedded bentonites continue to hinder its direct dating, but theinterpreted genetic relationship with the underlying Brushy BasinMember can be used to infer the onset of Buckhorn deposition. On thebasis of the 148.1 6 0.1 Ma age yielded by a bentonite 1.5 meters belowthe contact between the two members at Cedar Mountain (Kowallis et al.1998), and using the Gradstein et al. (2004) chronostratigraphic chart, weconclude that deposition of the Buckhorn Conglomerate started in theTithonian. Considering that deposition of the Cedar Mountain mud-stones above the hardpan calcrete took place near the boundary betweenthe Hauterivian and the Barremian (Mitchell, in Aubrey 1998; Kirkland1992), it is inferred that approximately 12 My elapsed between thedeposition of the uppermost Brushy Basin Member and the Upper ShaleMember. We suggest that most of this span of geologic time is representedby the hardpan calcrete, but we have no means to quantify the timerepresented by the stratigraphic lacuna in order to establish the durationof the Buckhorn Conglomerate.

The genetic and age relationship between the Brushy Basin andBuckhorn members, and the evidence for the unconformable nature ofthe basal Upper Shale Member, make it more appropriate to consider theBuckhorn Conglomerate as the uppermost member of the MorrisonFormation. The lithologic criteria that support the revision of the formallithostratigraphy of the succession are the occurrence of clasts lithologiesin the underlying Brushy Basin Member similar to those of the Buckhorn

Member, and the lack of calcrete intraclasts in the Buckhornconglomerate, but which are present in the overlying coarse clastics ofthe Upper Shale Member. The revised lithostratigraphic nomenclaturetakes into account the presence of a hiatus at the contact between BrushyBasin and the Upper Shale Member conglomerates, and therefore it ismore suitable for studies of the depositional history of the succession. Theexclusion of the Buckhorn Conglomerate from the Cedar MountainFormation renders the name Upper Shale Member unnecessary. Themuddy strata above the Buckhorn Conglomerate could be termed simplyCedar Mountain Formation.

Our interpretation is consistent with a number of studies in Wyoming,Montana, and Alberta that have shown that there is no compellingevidence that the basal unit of the Lower Cretaceous conglomerate isseparated from the underlying Morrison Formation or correlative unitsby a low-order unconformity (Darton 1904; Moberly 1960; Rapson 1965;Suttner 1969; Furer 1970; Gibson 1985; Winslow and Heller 1987;DeCelles and Burden 1992). As in the Colorado Plateau, the basalconglomeratic unit in Wyoming and Montana differs from youngercoarse-grained intervals by the absence of carbonate intraclasts (Meyerset al. 1992; May et al. 1995). It is possible, therefore, that the BuckhornConglomerate has coeval deposits farther north, and that the K-1unconformity may not systematically lie at the base of the first prominentchert conglomerates throughout the Western Interior Basin.

DISCUSSION

The genetic relationship between the Brushy Basin and Buckhornmembers implies that their deposition can be explained in terms ofa common evolving base level. The decrease in overall thickness and fine-grained floodplain deposits, together with the increase in channelconnectedness and the reorganization of fluvial paleoflows from axialto transverse, indicate that the Buckhorn Conglomerate was deposited inan overfilled basin. Consequently, the change in fluvial style between theBrushy Basin and Buckhorn members is interpreted to be the product ofrapid progradation of the proximal braided alluvial plain into a moredistal setting prior to the development of the sequence boundaryrepresented by the regional calcrete above Buckhorn strata. Like

FIG. 12.— Paleogeographic maps of the studied interval in the Colorado Plateau showing the reorganization of the drainage direction. A) Brushy Basin Memberpaleogeography. Note the location of a Middle to Late Jurassic orogen in northeastern Nevada and northwestern Utah, the contiguous Utah–Idaho Trough, and the lakeT’oo’dichi’ in the Four Corners area. B) Buckhorn Conglomerate paleogeography. SJB, San Juan Basin (modified from Peterson 1994).


shoreface sediments deposited during a forced regression, Buckhornclastics were emplaced during the loss of accommodation coeval with thedevelopment of a sequence boundary. This interpretation differs from theExxon model (Posamentier et al 1988; Van Wagoner et al. 1990) in thatthe basinward shift in facies underlies the sequence boundary. By analogywith depositional sequences driven by relative-sea-level cycles, theBuckhorn Conglomerate represents an alluvial example of the falling-stage systems tract (Plint and Nummedal 2000).

The northward retreat of the Late Jurassic shoreline to a distantlocation near the U.S.–Canada border (e.g., Peterson 1972; Brenner 1983)eliminates sea level as a control on the evolution of the fluvial equilibriumprofile in this part of the basin. Instead, the Late Jurassic tectonic platereorganization that affected the western North American subductioncomplex (Ingersoll and Schweichkert 1986; May and Butler 1986; May etal. 1989) suggests that the resulting tectonic events in the orogen playeda major role in contemporaneous sedimentation in the basin. Conse-quently, we attempted to explain the deposition of the succession asa result of primarily tectonic controls. However, we recognized thattectonics might not be the only extrinsic control on the emplacement ofthe Buckhorn Conglomerate. Basin climate change, potentially resultingfrom the denudation of the orogenic belt (see below), and consequentreduction of a rain shadow effect over the retroarc basin or change in thearea of catchment basins, could have modified sediment supply and riverdischarge and consequently influenced alluvial base level.

Two tectonic models explain the deposition of coarse clastics witha stratigraphic signature similar to the Buckhorn Conglomerate. In onemodel the progradation of coarse sediments takes place during in-phaseadvance and uplift of the frontal part of the thrust belt, resulting inproduction of more sediment than the wedge-top depozone canaccommodate (DeCelles and Giles 1996). A second model explains thedispersal of coarse clastics as the result of isostatic adjustment of theorogen during tectonic quiescence (Heller et al. 1988; Jordan andFlemings 1991). Denudation of the topographic load leads to the flexuralrebound of the thrust belt, and the gradual basinward isostatic uplift ofthe wedge-top and foredeep depozones, as well as the subsidence of theforebulge. Both factors contribute to the transport of coarse sedimentsinto the back-bulge depozone. Unlike the syntectonic deposits, post-tectonic clastics correlate with a contemporaneous foredeep unconformi-ty. If the foredeep fill is not preserved, as in the case of the Buckhorn, it isnot possible to determine whether thrusting was contemporaneous withthe emplacement of the coarse clastics. In this case, the presence ofa proximal hiatus expanding toward the orogen may be the only means todistinguish between a syn-tectonic and a post-tectonic mechanism.

The Buckhorn Member is interpreted as a post-tectonic deposit on thebasis of the progressive westward expansion of the hiatus represented by

the K-1 unconformity as it cuts downsection into Middle Jurassic strataof the Idaho–Utah Trough (Pipiringos and O’Sullivan 1978; Hintze 1988;Peterson 1988) (Fig. 13). This depocenter has been interpreted torepresent the remains of a Middle–Late Jurassic foredeep depozone(Saleeby and Busby-Spera 1992; Bjerrum and Dorsey 1995) theuppermost portion of which, now eroded, accommodated the Morrisonforedeep. The recognition of the contemporaneous thrust belt (Burchfielet al. 1992; Allmendinger 1992) has been hindered by complex post-Jurassic tectonic overprinting. However, evidence of its existence has beendocumented by scattered Late Jurassic thrusting, plutonism, and regionalmetamorphism in eastern Nevada and western Utah (Allmendinger andJordan 1981; Miller and Gans 1989; Thorman et al. 1990, 1992; Hudec1992). Since this orogenic belt lies west of the Sevier belt, the post-tectonicinterpretation implies that the emplacement of the Buckhorn Conglom-erate preceded the onset of the Sevier orogeny.

The application of the subsidence-driven, two-phase model to explainthe stratigraphic record of an orogeny may be hindered by the absence ofepisodic thrusting and therefore tectonic cycles during its development.Indeed, kinematic studies of thrust belts suggest continuous crustalshortening (Jordan et al. 1993; DeCelles 1994) in agreement with criticaltaper theory (Davies et al. 1983; Stockmal 1983) and steady plate-convergence velocities. However, isostatic rebound of the tectonic wedgeoccurs at the end of the orogeny, which is precisely the tectonic situationinferred here to have led to the emplacement of the BuckhornConglomerate. For the same geodynamic reason, we consider theprotracted K-1 unconformity overlying the Buckhorn as evidence thatLate Jurassic thrusting was not part of the Sevier Orogeny.

In this post-tectonic model, the Brushy Basin Member recordssyntectonic fluvial deposition in an under-filled, back-bulge depozone(Fig. 14A). The end of the Middle–Late Jurassic orogeny led to isostaticuplift of the previous wedge-top and foredeep depozones, causing theprogradation of the Buckhorn Conglomerate into the back-bulgedepozone (Fig. 14B). Post-tectonic subsidence of the Morrison forebulgeas the result of the denudation of the Middle–Late Jurassic thrust beltenabled transport of coarse clastics into distal parts of the basin. Thecoeval progressive base-level fall led to the widespread development of theK-1 unconformity recorded by the regional calcrete that overlies theBuckhorn Member. Therefore, the Buckhorn Conglomerate is interpretedto represent the final component of the Morrison fluvial system, whichprecedes the generation of a basin-wide unconformity. Major calcretedevelopment records the hiatus between the two tectonostratigraphicunits.

Paleohydraulic studies show that dynamic tilting was required toproduce the stream gradients needed to transport the clast sizes present inthe Buckhorn (Heller et al. 2003). Therefore it is suggested that dynamic

FIG. 13.—Chronostratigraphic diagram of the Middle and Upper Jurassic of the Colorado Plateau. Note the temporal expansion of the K-1 hiatus to the west. SeeFigure 1B for location of cross section (modified from Bjerrum and Dorsey 1995).


uplift interacted with the isostatic rebound of the Late Jurassic thrust beltand contiguous foredeep to supply and transport the Buckhorn sedimentsinto the back-bulge depozone of the Western Interior retroarc forelandbasin. The required cessation of dynamic subsidence is inferred from theradical decrease in the rate of orthogonal convergence between the NorthAmerican and Farallon plates from the latest Jurassic to the middle EarlyCretaceous (Page and Engebretson 1984). This interpretation is re-inforced by the magmatic lull of similar age recorded in the Sierra Nevada(Armstrong and Ward 1993) and the absence of coeval volcanic ashdeposits in the contiguous basin (Christiansen et al. 1994). The occurrenceof the basinwide K-1 stratigraphic lacuna roughly coinciding with thisgeodynamic event suggests a causal relationship. Renewed plateconvergence in the late Early Cretaceous seems the most plausibleexplanation for the onset of Sevier deformation.

CONCLUSIONS

(1) The transition between the Brushy Basin and Buckhorn membersis sharp but conformable and records a drainage reorganizationindicating a change from an underfilled to an overfilled forelandbasin in the latest Jurassic.

(2) Deposition of the braided-river strata of the Buckhorn Conglom-erate took place as a result of a base-level fall in the Brushy BasinMember back-bulge anastomosed alluvial plain, preceding theinception of a basin-wide sequence boundary. Consequently, theBuckhorn Conglomerate represents a fully nonmarine example ofthe falling-stage systems tract.

(3) The regional calcrete overlying the Buckhorn Conglomeraterepresents the protracted hiatus separating the fine-grained alluvialdeposits of Morrison and Cedar Mountain Formations. TheBuckhorn Conglomerate should be regarded as the uppermost

member of the Morrison Formation and be distinguished from theoverlying calcrete intraclast-bearing conglomerates of the CedarMountain Formation.

(4) Deposition of the Buckhorn Member preceded the onset of theSevier orogeny.

(5) The Buckhorn Conglomerate is interpreted as an example of post-tectonic coarse-grained deposits of the subsidence-driven ‘‘twophase’’ stratigraphic-fill model of foreland basins. Post-tectonicisostatic rebound of the Middle–Late Jurassic thrust belt andcontiguous foredeep deposits of the Idaho–Utah Trough com-bined with dynamic uplift to supply the coarse materials and lowerequilibrium profiles to deposit the Buckhorn Conglomerate.

ACKNOWLEDGMENTS

We would like to thank James Ayers and John Bird for their assistance inthe field, and Mike Leschin for granting us access to the Cleveland LloydDinosaur Quarry. We are grateful to the Ohio University Department ofGeological Sciences and the Baker Fund for providing the funds that madethis project possible. We are also thankful to Guy Plint for reviewing severalversions of this paper. Comments from Paul McCarthy, Tim Lawton, andMichael Rygel greatly improved the content and clarity of the paper. Allerrors and omissions remain the responsibility of the authors.

REFERENCES

ALLMENDINGER, R.W., 1992, Fold and thrust tectonics of western United Statesexclusive of the accreted terranes, in Burchfiel, B.C., Lipman, P.W., and Zoback,M.L., eds., The Cordilleran Orogen: Conterminous U.S.: Geological Society ofAmerica, The Geology of North America, v. G-3, p. 583–607.

ALLMENDINGER, R.W., AND JORDAN, T.E., 1981, Mesozoic evolution, hinterland of theSevier orogenic belt: Geology, v. 9, p. 308–313.

ARMSTRONG, R.L., 1968, Sevier Orogenic Belt in Nevada and Utah: Geological Societyof America, Bulletin, v. 79, p. 429–458.

FIG. 14.—Tectonic model for the depositionof the studied succession. A) Deposition of theanastomosed Brushy Basin Member duringtectonic loading in the thrust belt and dynamicsubsidence inducing an underfilled basin withaxial drainage. The study area is located in theback-bulge depozone. B) Deposition of theBuckhorn Conglomerate as a result of isostaticand dynamic uplift of the orogen and contiguousforedeep. Note the subsidence of the fore-bulgeand the reorganization into a transverse basindrainage as the result of the uplift-inducedoverfilling of the basin.


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Received 20 February 2006; accepted 30 September 2006.


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