91Lucas, S.G. and Spielmann, J.A., eds., 2007, Triassic of the American West. New Mexico Museum of Natural History and Science Bulletin 40.

STRATIGRAPHY, SEDIMENTOLOGY, AND SEQUENCE STRATIGRAPHY OF THELOWER TRIASSIC SINBAD FORMATION, SAN RAFAEL SWELL, UTAH

THOMAS H. GOODSPEED AND SPENCER G. LUCAS1Kohala Mountain Road, Kamuela, Hawaii 96743;

2New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico

Abstract—The Lower Triassic Sinbad Formation (Thaynes Group) in the San Rafael Swell of east-central Utah(11-18 m thick) represents the maximum flooding zone and at least part of the highstand systems tract of a 185+m thick depositional sequence. The Sinbad Formation conformably overlies siliciclastic tidal flat, fluvial channeland carbonate peritidal deposits of the Black Dragon Formation, a transgressive systems tract, and is overlain byprodelta, delta front and fluvial deposits of the Torrey Formation. Vertical facies relationships in the SinbadFormation in the San Rafael Swell and Capitol Reef indicate initial deepening followed by shallowing. Thecarbonate peritidal deposits of the upper part of the Black Dragon Formation represent the regional onset ofcarbonate sedimentation during the Early Triassic. These intertidal deposits are overlain by an offshore facies atthe base of the Sinbad Formation that represents maximum flooding. The offshore facies thickens to the north(oceanward) and contains an open-marine fauna dominated by ammonoids, bivalves, gastropods and scaphopods.The overlying deposits are restricted foreshoal/shoal, peloidal-oolitic grainstone, backshoal/lagoonal skeletal-intraclastic-oolitic grainstone and mudstone (with a depauperate fauna composed of gastropods, bivalves andostracods). These deposits also contain tidal channel skeletal-intraclastic-oolitic grainstone. A shoreface/shoal,skeletal-peloidal-oolitic grainstone caps the Sinbad Formation regionally.

INTRODUCTION

In the San Rafael Swell, east-central Utah, the Lower TriassicSinbad Formation (Thaynes Group) is a conspicuous marine carbonateunit within a dominantly siliciclastic, transgressive/regressive sedimen-tary sequence (Figs. 1-4; Blakey, 1974). The Sinbad Formation repre-sents the most extensive marine transgression in the western USA duringthe Early Triassic and plays a key role in understanding the relationshipbetween the Early Triassic, dominantly siliciclastic deposits of theMoenkopi Group and the time-equivalent marine deposits of the ThaynesGroup to the northwest and west.

This study investigates how vertical changes in sediments andfossils in 18 measured stratigraphic sections of the Sinbad Formation(Fig. 5) indicate temporal changes in depositional environments. Withsuch excellent two-dimensional coverage, much can be inferred about thethree-dimensional depositional environment from the vertical stacking offacies. Stratigraphic, sedimentologic, and paleontologic evidence fromthe Sinbad Formation of southeastern Utah better constrains and docu-ments its age and relationship to the first and most extensive sea-levelrise and fall during the Early Triassic in the western USA. Additionaldetailed data drawn on here (e.g., thin section petrography, descriptionsof measured stratigraphic sections) can be found in Goodspeed (1996).

REGIONAL SETTING AND STRATIGRAPHIC FRAMEWORK

In the western USA, the dominantly siliciclastic Moenkopi Group(Moenkopi Formation of most authors) has a maximum thickness ofmore than 600 m and is exposed in portions of Wyoming, Idaho, Utah,Colorado, New Mexico and Arizona (e.g., McKee, 1954; Stewart et al.,1972). The State Bridge Formation in Colorado and the Chugwater, RedPeak, Alcova and Crow Mountain formations in Wyoming are parts ofthe same lithosome. These dominantly terrestrial units interfinger withthe marine carbonate rocks of the Thaynes Group (Thaynes Formationof most authors) exposed to the west and northwest (Fig. 2).

Lower Triassic marine deposits of the Moenkopi and ThaynesGroup were deposited on a pericratonic ramp that deepened to thenorthwest (Figs. 1-2). This pericratonic ramp was between 500 and 160km wide (Blakey, 1974), and was part of an inland sea that extended, atmaximum transgression, from southeastern Utah into British Columbia

(Fig. 1). The ramp was bordered on the east by the Uncompahgre high-land and the Defiance uplift, to the west by a topographic barrier inNevada, and to the south by the Mogollon highland of Arizona (Paull andPaull, 1994).

FIGURE 1. Early Triassic paleogeographic features showing the location ofthe San Rafael Swell in southeastern Utah and the extent of the shelf tobasinal carbonate marine depositional facies of the Thaynes Group.

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The most significant tectonic features influencing Early Triassiccarbonate sedimentation on the shallow part of the pericratonic ramp insoutheastern Utah were the Emery uplift and the Kaibab uplift (Blakey,1974). The Emery uplift was a positive submarine topographic featureduring the Early Triassic, extending from the Orange Cliffs through theSan Rafael Swell and into central Utah (Fig. 1; Blakey, 1974). The Kaibabuplift, to the southwest of the San Rafael Swell, was either uplifted byfaulting (Marzolf, 1993) or by gypsum doming (Nielson, 1993).

The Sinbad Formation is a tongue of the lower Thaynes Groupintercalated with the lower Moenkopi Group in east-central and south-eastern Utah (Fig. 2). It represents the most extensive regional floodingof the Early Triassic sea, with a shoreline located in southeastern andeast-central Utah (Fig. 1; Blakey, 1974; Blakey et al., 1993). In the SanRafael Swell, the Sinbad Formation thickens from 10 to 16 m to the northand northwest. Eighty km to the south and southwest, it thickens slightlyin Capitol Reef National Park. To the southeast the unit thins to 1 m andpinches out in Canyonlands National Park (Lucas, 1995; Lucas et al.,1997).

Age-diagnostic fossils (especially ammonoids) provide biostrati-graphic control and indicate that the Sinbad Formation is correlative withthe lower Thaynes Group of Wyoming, Idaho, Nevada, and Utah andwith the Sinbad Formation of southwestern Utah (Stewart et al., 1972;Blakey, 1974; Dean, 1981; Lucas et al., 2004, this volume). Based onregional ammonoid and conodont biostratigraphy, the Sinbad Formationis of late Smithian age (zone of Anasibirites kingianus: Lucas et al., thisvolume).

SINBAD FORMATION

Previous Work

Gilluly and Reeside (1928) named the Sinbad Limestone for expo-sures in the Sinbad “Country” of the San Rafael Swell, Emery County,Utah (Fig. 3). The San Rafael Swell is an antiformal dome that has beenbreached by erosion, exposing Permian to Cretaceous strata. Althoughthe Sinbad Limestone has long been accepted as a formal lithostratigraphicunit, a type section was not established. A lectostratotype section is

described here (Fig. 5, section 10 at Rods Valley).The Sinbad Formation has been examined as part of regional stud-

ies of the Moenkopi Formation (McKee, 1954; Stewart et al., 1972;Blakey, 1974), with Blakey (1974) providing the most comprehensivediscussion of the unit. The Sinbad Formation is also recognized in thesubsurface between the San Rafael Swell and Capitol Reef and inCanyonlands National Park, and core samples of the unit have beenanalyzed to the north of the San Rafael Swell in Carbon County, Utah.Strata formerly termed Timpoweap Member of Moenkopi Formation insouthwestern Utah are considered by us to belong to the Sinbad Forma-tion (Lucas et al., 2004, this volume).

In addition to the Sinbad Member of the Moenkopi Formation,Blakey (1974) recognized a subjacent Black Dragon Member and thesuperjacent Torrey and Moody Canyon members (Figs. 2, 4). Based onthe carbonate petrology and depositional environments of the SinbadMember from the San Rafael Swell, he distinguished three facies: (1) abasal skeletal calcarenite; (2) a middle silty, peloidal calcilutite; and (3) anupper, dolomitized calcarenite (Blakey, 1974).

The most detailed study of the Sinbad Formation to date ad-dressed exposures in Capitol Reef National Park (Dean, 1981). Deanrecognized six lithofacies: (A) stromatolitic boundstone, oolite-peloidpackstone, dolomicrite, channel conglomerate, and evaporite; (B) skel-etal packstone and pelletal wackestone; (C) dolomicrite; (D) oolite-mol-lusk packstone and peloidal mudstone-wackestone; (E) dolomitizedgrainstone; and (F) dolomitic claystone. Dean (1981) envisioned these aspart of three depositional phases (an initial transgressive tidal flat-sabkhaphase, a subtidal, tidal-channel and shoal-protected lagoonal phase, anda final regressive, moderate-energy shoal phase) of the Sinbad Forma-tion.

The base of the Sinbad Formation has been defined differently inthe San Rafael Swell and in Capitol Reef National Park. Blakey (1974)and Ochs (1988) placed the base of the formation at a gray skeletalpackstone that forms the ledges and cliffs around the San Rafael Swell.Dean (1981) defined the base of the Sinbad Formation in Capitol ReefNational Park as the “lithologic break from pale reddish-brown siltyshale of the underlying Black Dragon Member to pale yellow-gray

FIGURE 2. Time-rock north-northwest to south-southeast cross section of the Lower Triassic of Idaho, Wyoming and Utah.

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dolomicrite of the basal Sinbad Member” and suggested that this contactis largely gradational.

However, the contact between the Sinbad Formation and thesuperjacent Torrey Formation has been defined similarly in both areas.Blakey (1974) described the top of the Sinbad Formation in the SanRafael Swell as being capped by a distinct, dolomitized calcarenite inter-preted as a carbonate shoreline prograding seaward over offshore depos-its. Blakey’s study never clearly defined whether the contact betweenthe Sinbad and Torrey formations is gradational or abrupt; however, itdid interpret the Torrey Formation as a typical prograding deltaic epi-sode with the deposits representing an overall deep to shallow watertransition. So, the contact must represent a rather sudden deepening ofthe sea from the tidal flat-sabkha environment to a relatively deeperwater prodelta environment.

Dean (1981) described the boundary between the Sinbad and theTorrey formations in Capitol Reef National Park as the upper boundary

of his dolomitic claystone facies. The contact was marked at the “sharp,conspicuous color change between the gray green claystones ... and thepale reddish brown silty shale of the lower Torrey [Formation.]” Hisstudy suggests that, although the contact is gradational in most areas, itcould be determined by an increase in ferric iron coloration, an increase inclastic content and coarseness, and a decrease in dolomitic cement. Dean(1981) interpreted the dolomitic claystones to represent fine-grainedprodelta sediments indicating progradation of the overlying Torrey delta.

Type Section and Associated Facies

A type section (lectostratotype) of the Sinbad Formation is hereestablished in the San Rafael Swell in Utah at the mouth of Rod’s Valleyin the SW 1/4 NW 1/4 SE 1/4 sec.33, T23S, R10E, Emery County, Utah(Figs. 4-5, section 10). At this site, the Sinbad Formation is 10.5 metersthick. It conformably overlies the Black Dragon Formation and is con-formably overlain by the Torrey Formation (Blakey, 1974; Ochs, 1988;Ochs and Chan, 1989).

The contact between the Black Dragon and the Sinbad formationsat the type section is characterized by a change in outcrop pattern, colorand lithology between the peritidal facies at the top of the Black DragonFormation and the offshore facies at the base of the Sinbad Formation.Here, siliciclastic, ripple-laminated siltstone, bioturbated carbonatewackestone, laterally-linked hemispheroidal pelletal grainstone (stroma-tolites?) and skeletal-intraclastic oolitic grainstone of the “transitionalfacies” of Blakey (1974) and Ochs (1988; Ochs and Chan, 1989) of theBlack Dragon Formation are overlain by ledge-forming skeletal, intraclasticpackstone and grainstone of the Sinbad Formation. The color changesfrom yellowish gray to dominantly light gray to pale yellowish brown.The marked change in lithology from the slope-forming upper BlackDragon to the predominantly ledge-forming Sinbad Formation definesthe formational contact. The Sinbad Formation typically forms a promi-nent ledge and holds up many of the mesas throughout the central SanRafael Swell.

The facies at the proposed type section begin with 2.5 m ofoolitic-algal laminated facies at the top of the Black Dragon Formation.Meter-scale packages of siltstone with gypsum and mud intraclasts gradevertically into bioturbated layers of skeletal wackestone to packstonewith gypsum replacement nodules (1.5 m). These beds laterally gradeinto massively bedded intraclastic oolitic grainstone. The skeletalwackestone to packstone contains an assemblage of bivalves, gastropodsand scaphopods. Above this and capping the peritidal facies lies a 1-m-thick layer with peloidal grainstone composed of laterally linkedhemispheroid algal laminae (stromatolies) modified slightly by verticaland horizontal bioturbation. This facies is interpreted to be peritidal, as

FIGURE 3. General map of the San Rafael Swell showing the location ofmeasured sections (Fig. 5) in this study.

FIGURE 4. Selected outcrops in the San Rafael Swell. A, Overview of section near I-70 roadcut. B, Type section of Sinbad Formation.

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it contains meter-thick packages of subtidal, lagoonal, tidal channel rocks,and intertidal to supratidal rocks (see Table 1).

Cyclic, graded skeletal grainstone to wackestone with bioturbatedtops compose the lowest 2.0 m of the Sinbad Formation above theperitidal facies (Fig. 4; Table 1). This unit is graded beds with skeletal-intraclastic packstone to grainstone grading into skeletal wackestone topackstone. These packages compose the prominent ledge former in thetype section and across the study area. The following three lithologiescharacterize this unit: (1) thin- to medium-bedded, trough crossbeddedskeletal-intraclastic grainstone, (2) bioturbated skeletal-intraclasticpackstone to grainstone, and (3) skeletal wackestone to packstone con-taining vertical burrows about 2 cm long. The sedimentary structurespreserved in this facies are graded beds 12-16 cm thick with troughcrossbeds at the base of the cycles; vertical bioturbation and disruptedlaminae found near the cycle tops; and abraded and aligned shell hash aswell as whole pristine fossils. Fossils recognized within the lowest unitof the type section are bivalves, gastropods, scaphopods and ammonoids.Codiacean algae preserved in dolomite rhombohedrals, and echinodermspines and plates have been found in other correlative facies within thefield area. The color of this facies ranges from light gray (N7) to paleyellowish brown (10YR6/2). This facies varies in thickness across thestudy area from 1.5 m in the Keesle Country (Fig. 5, section 19) andLone Man Draw (Fig. 5, section 17) sections and thickens to the north to5 m in the Cottonwood Draw B (Fig. 5, section 1) and White HorseCanyon (Fig. 5, section 5) sections.

The next 4 m of the type section consist of two coarsening-upward packages of pelletal/peloidal grainstone to oolitic grainstone (Fig.5). This unit occurs in all sections across the field area and interfingerslaterally with the subjacent facies. These units contain two lithologies: athinly laminated (3-6 mm), fine- to thick-bedded pelletal/peloidalgrainstone with low-angle cross laminae and trough crossbeds (3-10 cmsets), and a thinly laminated and trough crossbedded, fine skeletal, pelletal/peloidal/oolitic grainstone. Sedimentary structures associated with thisunit are low-angle and trough crossbeds (3-10 cm sets), thin (3-6 mm)mechanical laminae, soft sediment deformation, hints of graded beds,low-angle cross laminae, 2 cm sets of bi-directional ripple or herringboneripple laminae, and occasional larger-scale trough crossbeds. Lenses ofthis facies contain bivalves. The color ranges from light brown (5YR6/4)to pale yellowish brown (10YR6/2) and, in general, is lighter than theunderlying facies. It is 4 m thick in the type section and thickens to 6 mto the north in the White Horse Canyon section and thins to 3 m in theTemple Mountain section (Fig. 5, section 4).

The next 1 m of the type section and in correlative units containsmm-thick laminated, dolomitic mud to siltstone that grades into 0.5-m-thick pelletal/peloidal grainstone with mud intraclasts and ripple lamina-tions. Two meters of a coarsening-upward package of pelletal to peloidalgrainstone with intraparticle porosity near the top of the peloidalgrainstone lie above the intraclastic grainstone. The uppermost 2 m ofthe section is composed of a similar, coarsening-upward interval of dolo-mitic mud-siltstone capped with oolitic grainstone. The dolomitic mud-

FIGURE 5. Stratigraphic correlation of 18 measured sections of the Sinbad Formation from north-northeast to south-southwest within the San RafaelSwell, Utah (Fig. 3), showing lithologies and correlation of facies. For detailed descriptions and map coordinates of measured sections see Goodspeed(1996).

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siltstone contains mm-thick suspension laminae and some ripple laminaenear the boundary of the coarser, cross-laminated grainstone. The colorof the dolomitic mud-siltstone ranges from pale olive (10Y6/2) to yel-lowish gray (5Y7/2), and the peloidal to oolitic grainstone ranges frommoderate yellowish brown (10YR5/4) to pale yellowish orange (10YR8/6). The oolitic grainstone contains trough-crossbedded ooids. The oolitic-skeletal grainstone dominates the northern sections, and the deposits ofmud and bioturbated wackestone dominate in the eastern and southernsections. It varies in thickness from 2 to 5 m across the study area (Fig.5).

Across the San Rafael Swell, a dolomitized oolitic-skeletal-intraclastic grainstone facies caps most of the Sinbad sections (Fig. 5).This oolitic-skeletal-intraclastic grainstone contains herringbone crossstratification, ripple laminae and graded beds with oolitic-skeletalgrainstone that grade up into pelletal-skeletal wackestone drapes. Thoughthis facies does not occur in the type section, it is found in the easternand northern sections and is most predominant in the Temple Mountainand Cottonwood Draw sections (Fig. 5).

The boundary between the Sinbad and Torrey formations is char-acterized by a lithologic change from the dolomitized grainstone andmud-siltstone of the upper Sinbad Formation to the slope-forming dolo-mitic and siliciclastic siltstone and claystone of the lower Torrey Forma-tion (Figs. 4-5). The dolomitized grainstone of the upper Sinbad Forma-tion has a distinct orange to pale yellowish-brown color and caps all ofthe sections in the San Rafael Swell in places where the Torrey Forma-tion and the upper mud-siltstone of the Sinbad Formation have beeneroded away. The dolomitized skeletal-peloidal-oolitic grainstone often

contains beds with mud intraclasts (up to 10 cm) with minor quartz andmuscovite and limonite nodules (Table 1). In the northern sections,intraparticle porosity is present as well.

REGIONAL FACIES AND DEPOSITIONAL ENVIRONMENTS

We recognize five regional facies indicative of separate but relateddepositional environments in the Sinbad Formation of the San RafaelSwell (Table 1). They indicate a progression from subtidal, shallow wa-ter to supratidal conditions during Sinbad deposition (Fig. 5). The fivefacies we recognize are peritidal, offshore, foreshore/shoal, restrictedlagoonal/backshoal and tidal-channel.

Peritidal Facies

The two peritidal subfacies recognized by Ochs (1988) and thisstudy are oolitic and algal limestone and bioturbated pelletal/peloidalwackestone. The following description of the peritidal facies is fromOchs (1988 p. 49-53):

The oolitic and algal limestone (OAL) facies ...occursthroughout the San Rafael Swell and consists of 4-5 m ofalternating beds of oolitic, algal and fenestral limestone, andcalcareous siltstone, shale, and limy mud clasts. Ooliticpackstone and grainstone are the predominant lithologiesof the OAL facies. Ooids occur in either massive or troughcross-stratified beds and may be admixed with molluscanshell hash. In the Cottonwood Draw area, oolite grainstonelayers overlie a 0.5-0.8 m thick bed of algal laminated lime-

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stone. Fenestrae form planar to bubble-shaped disconnectedcalcite-filled voids which occur in and adjacent to frag-ments of algal lamination (3 mm to 4 cm thick). Intraclastsinterbedded with carbonate mudstone and laminated algaealso occur between and above oolitic and fenestral lime-stone layers and mark the contact with the Sinbad Lime-stone.

The peritidal facies was originally described as part of the BlackDragon Formation (Blakey, 1974). This facies represents the first domi-nantly carbonate, subtidal deposits within the depositional sequence.Dean (1981) included it in the Sinbad Formation and made its base theformational contact in Capitol Reef National Park. In the San RafaelSwell, the peritidal facies is also present in the more eastern sectionstowards the top of the formation and is interbedded with tidal channel,and lagoonal deposits.

This facies is correlative with Dean’s (1981) lithofacies A of Capi-tol Reef National Park, which contains stromatolite boundstone, oolite-peloid packstone, dolomicrite, channel conglomerate and evaporitesubfacies. The stromatolitic boundstone records an environment similarto that proposed for many ancient and modern algal mats. This facieswas probably confined to the intertidal and supratidal zone, which al-lowed algal mats to be periodically submerged but reduced the popula-tion of algal-grazers such as cerithiid gastropods (Lucia, 1972; Dean,1981).

Dean (1981) interpreted this facies as high intertidal to supratidaldeposits with tidal channels meandering through the tidal flats. Ochs(1988) suggested that the algal mats and fenestral limestone formed inrestricted supratidal environments on the landward side of shallow, subtidaldune complexes. The algal-laminated and lime mud intraclasts were rede-posited during storm events. Dean (1981) pointed out that on such abroad shelf, storm events would be damped by the time they reached thesupratidal region, and that the intraclasts are a result of desiccation andshrinking of algal mats. Ochs (1988) suggested that this facies “reflectscessation of clastic deposition which allowed carbonate accumulationduring the final phase of Black Dragon deposition.”

The displacive oval gypsum nodules (mostly replaced by calcite)and discrete beds of selenite within the wackestone suggest an arid cli-mate. Dean (1981) described an evaporite subfacies within his lithofaciesA of Capitol Reef that is more abundant in his northern sections andsimilar to modern deposits described from the Persian Gulf (Schneider,1975). The displacive growth of the nodules suggests that the gypsumprecipitated in situ from saturated pore waters. The bedded gypsumprobably resulted from remobilization of the gypsum during diagenesisso that the original gypsum within nodules was replaced by calcite andreprecipitated into thin beds of gypsum; alternatively, these beds mayrepresent coalescence of individual gypsum nodules (Kinsman, 1969).

TABLE 1. Sinbad Formation facies.

97Further study of this facies is required to determine whether the beddedgypsum represents early diagenetic processes or whether it is a modernfeature.

Based on the displacive evaporite nodules and bedded gypsum,Dean (1981) interpreted this facies to have been deposited within upperintertidal to lower supratidal environments. The highly bioturbated na-ture of the wackestones suggest subtidal deposition within shallowsubtidal waters. The presence of some bivalves and gastropods, ripplelaminae to bioturbated packstone to grainstone, suggest part of thisfacies was deposited in subtidal environments.

Offshore Facies

The offshore facies (Table 1) occurs across the entire study area atthe base of the Sinbad Formation. It thickens towards the north andnorthwest (Fig. 5) and is composed of three subfacies: (1) troughcrossbedded, skeletal-intraclastic grainstone; (2) bioturbated skeletal-intraclastic packstone to grainstone; and (3) vertically bioturbated skel-etal packstone.

Three lines of sedimentologic evidence suggest that this facies wasdeposited in an offshore environment near fairweather wave base. First,a diverse fauna including ammonoids indicates open-marine depositionin normal salinity waters. This facies is the most fossiliferous unit in thestudy area. Fossil preservation varies from abraded and aligned fossils ofbivalves to layers of whole, unabraded fossils with higher concentrationsof ammonoids. Second, the variation in grain sizes indicates a consider-able range of depositional energies. Graded beds, crossbedded, and scoureddeposits dominate this facies and probably indicate waning energy re-lated to storm processes. Skolithos-type vertical burrows within thegrainstone of these deposits also suggest moderate-to-high energy condi-tions. Rock types vary from bioclastic grainstone to micrite. The pres-ence of micrite suggests periods of low energy deposition. The micriteintraclasts, vertical burrows, and larger scale cross beds indicate a moder-ate to high energy environment. Finally, stratigraphic relationships sug-gest a shallowing-upward sequence, and the overlying facies clearly rep-resents more nearshore marine deposits, whereas the underlying stratashows an overall deepening trend (Blakey, 1974; Ochs, 1988).

The offshore facies described here is the same as Blakey’s (1974)calcarenite facies of the Sinbad Formation in the San Rafael Swell, but ourinterpretation of depositional settings diverges somewhat. Blakey pro-posed that the mud and whole fossils were deposited on the leeside ofcalcarenite (grainstone) bars, and that these bars acted as a subtidal bar-rier to the lagoonal deposits and carbonate shoreline landward (Blakey,1974). However, we found no evidence of subaerial exposure within thisfacies, and the moderate- to high-energy deposits overlying the offshorefacies (see below) suggest that the offshore facies did not effectivelyprotect the shoreward facies from wave energy. Rather, the offshorefacies is similar to the paleoenvironment proposed by Dean (1981) forhis lithofacies D in Capitol Reef National Park (Dean 1981)—normal,open-marine setting in water depths greater than fairweather wave base.Similar modern and ancient deposits containing graded beds with wholeand abraded shell hash, and vertical bioturbation, have been interpretedas storm-dominated, offshore deposits by Aigner (1982, 1984, 1985) inthe Triassic upper Muschelkalk of Germany, and by Calvet and Tucker(1988) in the middle and upper Muschelkalk of Spain.

Foreshoal/Shoal Facies

The foreshoal/shoal facies is composed of three subfacies: (1)trough-crossbedded, laminated peloidal to oolitic grainstone, (2) trough-crossbedded, bivalve oolitic grainstone, and (3) dolomitized, skeletal-oolitic grainstone. This facies is typically found both superjacent to theoffshore facies and at the top of the formation. It consistently formsslopes where it lies on top of the more resistant offshore facies, butforms benches where it caps the formation.

The trough crossbedded to laminated peloidal-to-oolitic grainstone

subfacies was deposited in subtidal environments above fairweather wavebase. Coarsening-upward trends indicate an increase in energy or a low-ering of relative sea-level in this subfacies that could be due to eithersediment aggradation and/or local progradation of shoals. Soft sedimentdeformation observed at the base of the subfacies at some sections sug-gests a high sedimentation rate but does not rule out shoal migration.

The bivalve oolitic grainstone subfacies (Table 1) may either rep-resent a transition from normal marine conditions to more restrictedconditions of the high-energy oolitic shoals or it may represent localfluctuations in the degree of restriction in energy and/or sedimentationrates. It usually occurs where the foreshoal/shoal facies interfinger withthe underlying offshore facies. This high-energy environment coupledwith high sedimentation rates probably restricted the fluctuating bottomsediments from most benthic organisms. With slightly lower sedimenta-tion rates, the bottom may have become tolerable, allowing some bivalvesto move into the environment. Dean (1981) concluded that “the upwarddecrease in bioclasts throughout [this facies] is more than a result ofselective sorting and suggests original scarcity of organisms.”

The dolomitized skeletal-oolitic grainstone occurs above andinterfingered with the lagoonal and tidal-channel deposits. It forms benchesacross the study area, caps most sections across the San Rafael Swellwhere the Torrey Formation has been eroded, and interfingers with themud-siltstone and oolitic grainstone of the tidal channels of the lagoonal/backshoal facies. It is massive- to medium-bedded and contains ooids,peloids and beds with mud intraclasts and minor quartz and muscovitegrains, and limonite nodules. It is trough crossbedded with some planarlaminae, and the micritic intraclasts fill the troughs. This subfacies eitherrepresents a deepening and return to subtidal, more open-marine condi-tions and/or simply local migration of oolitic shoals in relation to thebackshoal and lagoonal deposits. The presence of lagoonal/backshoalmud-siltstone and oolitic tidal-channel deposits in some sections sup-port the latter interpretation. Large limonite nodules occur near the topof this subfacies, and greater porosity is also observed, which may sug-gest leaching related to exposure or low sedimentation rates from rapidsea level rise. This facies was probably deposited in a similar environ-ment as the peloidal-oolitic subfacies.

The three subfacies that compose the foreshoal/shoal are similarto modern carbonate environments in the Persian Gulf where tidal chan-nels cut across the barrier shoals and coated grains develop (Loreau andPurser, 1973). The ooids and amalgamated pellets are dispersed from thetidal channels and reworked by moderate- to high-energy currents orwaves along the barrier shoals. In the Sinbad Formation, the more mas-sive peloidal to oolitic grainstone units with local micritic intraclastsprobably represent moderate- to high energy subtidal barrier shoals. Thecross-bedded oolitic grainstone represents the tidal channels that cutacross the shoals and provided a source for the peloids and ooids. Thepelletal to peloidal grainstone of the foreshoal/shoal facies probablyrepresent foreshoal or backshoal deposits surrounding the barrier shoals.The limited fossils within these shoal deposits may be a result of restric-tion due to high salinity and constantly shifting substrates and highsedimentation rates (Sellwood, 1978), or simply indicate the depositswere originally scarce in fauna (Dean, 1981).

The “foreshoal/shoal facies” corresponds with Blakey’s (1974)silty, peloidal calcilutite facies which he interpreted as representing re-stricted lagoonal deposits behind the skeletal calcarenite (grainstone)bars. The presence of high-energy ooid and peloid grainstone depositsare more consistent with a foreshoal/shoal than a restricted lagoon inter-pretation.

The foreshoal/shoal facies from the San Rafael Swell correlateswith Dean’s (1981) lithofacies E in Capitol Reef National Park. Heinterpreted this lithofacies as oolitic shoals and barriers based on thegrainstone texture and typical high-energy sedimentary structures. Thisstudy corroborates Dean’s interpretation. The foreshoal/shoal facies rep-resents an overall shallowing- and coarsening-upward succession depos-ited in a shallow subtidal environment. The presence of cross-laminae

98and grainstone textures suggest deposition above fairweather wave basein high-energy subtidal, oolitic shoals.

Lagoonal/Backshoal and Tidal Channel Facies

Modern backshoal environments are defined as the region land-ward of subtidal barriers containing an open-marine to restricted faunaand subject to episodic high energy storms and tides. Strata from thisdepositional environment interfinger with restricted low-energy lagoonsand high-energy tidal channel deposits that cut across the barriers. In theSinbad Formation, the lagoonal/backshoal/tidal channel facies contains avariety of subfacies, including: (1) graded, skeletal-oolitic grainstone towackestone, (2) bioturbated, skeletal wackestone to packstone with thinlaminite interbeds, (3) thinly laminated lime mudstone and siltstone, and(4) herringbone cross-bedded skeletal-oolitic-intraclastic grainstone. Theskeletal, oolitic grainstone subfacies is thickest to the northwest andgrades landward to the east and south into the skeletal-oolitic and theskeletal packstone/wackestone and lime mudstone/siltstone. The her-ringbone cross-bedded, skeletal-oolitic-intraclastic grainstone interfingerswith all the subfacies.

The graded beds of the skeletal-oolitic grainstone to wackestonewere probably deposited in a restricted subtidal, backshoal environmentassociated with or close to the tidal channels, lagoon, and the leeside ofthe oolitic shoals. The ooids and intraclasts within this facies suggestthat storms and/or tides transported material landward. The skeletalwackestone containing a restricted fauna of bivalves and ostracods indi-cates periods of quiet-water deposition and suspension settling.

The thinly-laminated lime mudstone to siltstone subfacies is in-terpreted to have been deposited in a low-energy, shallow subtidal la-goon. Units composed of this subfacies are thin (0.25 m) and generallyform slopes or recesses between oolitic shoal deposits, algal laminite-capped skeletal packstone, or foreshoal/shoal deposits. Although poorlyexposed throughout the field area, no evidence of subaerial exposure wasobserved. The only sedimentary structures observed are thin, suspen-sion laminae in the mudstone, and ripple cross laminae within the silt-stone layers. Sparse ostracods may suggest somewhat restricted condi-tions.

Skeletal wackestone to packstone capped by algal laminae suggesta relatively landward position within a lagoonal and intertidal environ-ment (Table 1). Within the study area, this subfacies is only found in theeasternmost section (I-70 and I-70 View Area; Fig. 5, sections 8, 12)where it is associated with the lime mudstone of the lagoonal facies.However, a similar, possibly correlative, thin skeletal-oolitic-pelletaland algal- laminated subfacies is present in Canyonlands National Parksoutheast of the San Rafael Swell (Blakey, 1974). With regard to deposi-tional setting, this subfacies was deposited at the edge of a lagoon andinterfingers with the peritidal facies at the top of the Black DragonFormation (Table 1; Ochs, 1988).

The herringbone cross-bedded, skeletal-oolitic-intraclasticgrainstone was most likely deposited in tidal channels associated withlagoonal/backshoal facies and the upper part of foreshoal/shoal facies, asindicated by the mixture of ooids, molluscs and intraclasts and the chan-nel morphology. The tidal channels cut across the subtidal barriers of theforeshoal/shoal facies as indicated by the herringbone cross-stratifieddeposits and channels. This facies is the same as the tidal-channel subfaciesof Dean’s (1981) lithofacies E in Capitol Reef National Park.

Several lines of evidence suggest that these subfacies were depos-ited in close proximity within a backshoal, lagoonal, and tidal channelenvironment. The presence of graded beds with skeletal wackestonedrapes indicate intermittent high-energy events. Given the consistentlymoderate- to high-energy nature of the subtidal barrier system, this fa-cies probably formed on the leeside of the barriers and was subjected toperiods of storms or tidal processes that breached the shoals or thelevees of the tidal channels behind the barriers. The association of thesedeposits with the lime mudstone, the subtidal packstone and algal

laminites, and the restricted fauna of bivalves, gastropods and ostracods,strongly suggest deposition of this facies in an environment where thebackshoal, restricted lagoon and tidal channels interfinger immediatelylandward of the subtidal shoals.

Loreau and Purser (1973) described mixed oolitic, pelletal, skel-etal sands from the Persian Gulf. These mixed sands form mainly withinand adjacent to the axis of the tidal channel that supplies sediment to theseaward tidal delta complex and barrier system. They suggest that theooids formed near the channel levees and are deposited in the channelwhere they are mixed with skeletal grains and lime mud of the tidalchannel. The ooids that are carried seaward through the channels onto thetidal delta are swept laterally and back to the barrier beaches or subtidalshoals where they are mixed with skeletal grains of the shoreface.

REGIONAL FACIES RELATIONSHIPS AND DISTRIBUTION

Four facies characterize the type section and most of the SinbadFormation in the San Rafael Swell (Table 1). They are (from base to top):offshore, foreshoal/shoal, lagoonal/backshoal and tidal channel facies (Fig.5). The peritidal facies of the Black Dragon Formation represents theregional start of carbonate sedimentation with more rapid transgressionof the seas cutting off the siliciclastic sediment supply to the ramp. Theoffshore facies consists of skeletal packstone to grainstone and is the(relatively) deepest water facies containing the most open-marine fauna.It thickens in the northern part of the San Rafael Swell (Fig. 5) and thinsto the south and east. The foreshoal/shoal facies is composed of peloidal-oolitic, trough-crossbedded grainstone and consistently is foundinterfingered with the subjacent offshore facies and interfingered later-ally with the lagoonal/backshoal facies. The lagoonal/backshoal facies isthe most variable facies. In modern carbonate tidal flat and subtidalenvironments numerous depositional environments may exist contem-poraneously in close proximity both normal and parallel to the shorelineso that vertical and lateral successions, such as are seen in the SinbadFormation, record rapid, local facies variations and may not reflect re-gional relative sea-level fluctuations (James, 1979).

The units observed in the San Rafael Swell have affinities to thosedescribed in Dean’s (1981) Sinbad Formation units of Capitol Reef Na-tional Park. A combination of Dean’s (1981) lithofacies A, B, and Cclearly correlate with the peritidal facies in the San Rafael Swell. Thispackage contains interbedded stromatolitic boundstone, oolite-peloidpackstone, dolomicrite, channel conglomerate, and evaporite facies thatare closely associated with a subtidal sketetal packstone and pelletalwackestone facies and a dolomicrite facies. Dean’s (1981) lithofacies D(oolite-mollusc packstone and peloidal mudstone-wackestone facies)correlates to Blakey’s calcarenite facies and this study’s offshore facies.It represents the deepest water facies and maximum flooding in theregion and contains the most abundant and diverse fossil assemblageranging from whole, pristine fossils (especially ammonoids) to lenses ofabraded and aligned fossils (storm deposits). The most notable and cor-relative unit of the Sinbad Formation from the San Rafael Swell to theCapitol Reef National Park is the well-sorted, dolomitized, cross-strati-fied grainstones of this study’s oolitic shoal/foreshoal facies and Dean’s(1981) dolomitized grainstone facies of his lithofacies E. In both regions,a dolomitized claystone facies marks the contact between the SinbadFormation and the overlying Torrey Formation.

DEPOSITIONAL MODEL

A model of deposition of the Sinbad Formation in the San RafaelSwell was developed from vertical and lateral facies relationships, sedi-mentary structures, skeletal and trace fossils, and grain size and textures(Fig. 6). The deepest water facies in the Sinbad Formation is representedby the offshore facies, which was deposited at or just below fairweatherwave base. Open-marine carbonate sedimentation with normal salinityand storm-wave agitation were the dominant processes affecting thesedeposits.

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Offshore facies grade landward into foreshoal/shoal facies thatwere deposited in high-energy environments above fairweather wavebase (Fig. 6). Fairweather waves, high sedimentation rates, and tideswere the dominant processes affecting this environment. Subtidal barri-ers within the foreshoal/shoal damped the energy of the waves, generat-ing quiet-water conditions landward of the shoals. Tidal channels cutacross the shoals and supplied the coated grains from the channel leveesand peloids to the subtidal barriers and tidal deltas. They also suppliedsediment landward to the backshoal and parts of the lagoonal environ-ments.

Leeward of the shoals were three closely associated depositionalenvironments—the backshoal, lagoonal, and tidal channels. Breaching ofthe shoals by storm waves, tidal-channel levee spill over during hightides, and quiet-water deposition characterized the backshoal environ-ments. The dominant processes occurring within the restricted lagoonwas deposition of lime mud, and reworking by burrowing organisms.The tidal channels cut across both of these facies and were dominated byebb and flood tide currents; they connected the more restricted lagoonalwaters with the open-marine waters seaward of the shoals. Lagoonalfacies are capped by intertidal algal laminites at the base of the formationand in the middle of the easternmost sections of the San Rafael Swell(Fig. 5).

The distribution of facies and thickness trends within the SinbadFormation suggest both regional and local paleogeographic controls. Inmodern coordinates, the paleoshoreline was to the southeast with theopen ocean to the north and northwest (Fig. 1). In the San Rafael Swell,the Sinbad Formation is thickest to the north at the Cottonwood Drawand the White Horse Canyon sections (Fig. 5). The open marine, off-shore facies is thinnest to the south at the Keesle Country section and tothe east in the I-70 view area section and tends to thicken to the north andwest part of the study area, suggesting more open marine deposition inthis region (Fig. 5). The peritidal facies at the top of the Black DragonFormation (Ochs, 1988; Ochs and Chan, 1989) also thins to the south-east. Restricted lagoonal, backshoal and associated tidal-channel depos-its are more prevalent to the east and south, suggesting that thepaleoshoreline was to the southeast (Blakey, 1974; Blakey et al., 1993).A thin limestone composed of intertidal algal laminites and oolitic-skel-etal grainstone (Blakey’s dolomitized calcarenite facies) in the Canyonlandsregion is correlative with the Sinbad Formation and pinches out to thesoutheast, confirming this interpretation (Blakey, 1974; Goodspeed etal., 1992; Blakey et al, 1993; Lucas, 1995; Lucas et al., 1997). This thinlimestone correlates with the Sinbad Formation of the San Rafael Swellto the northwest, and probably correlates with the offshore facies of thelower Sinbad Formation in the San Rafael Swell and represents the maxi-mum flooding zone.

During the Early Triassic, the Emery uplift was a positive topo-graphic feature that influenced Sinbad sedimentation patterns (Blakey,1974). Overall thinning over the middle of the study area and the preva-lence of shallow-water deposition in these sections suggests the pres-ence of a subtle paleogeographic high (Fig. 5). The fluvial, trough-crossbedded sandstone facies of the underlying Black Dragon Formation(Blakey, 1974; Ochs, 1988) pinches out near I-70, and thickens to thenorth, with paleocurrent trends indicating northward flow (Ochs, 1988).

This indicates that the Emery uplift also influenced Black Dragon depo-sitional patterns.

DEPOSITIONAL SEQUENCES ANDSEQUENCE STRATIGRAPHY

To better understand the Early Triassic relative sea-level historyof the region, the Sinbad Formation and adjacent formations were inter-preted in terms of sequence stratigraphy (Fig. 7). Depositional patternsof the Black Dragon Formation were largely controlled bypaleotopographic relief preserved on underlying Permian strata (Blakey,1974; Ochs, 1988). Fluvial to tidal deposits of the Black Dragon Forma-tion were deposited on an erosional surface cut into the Permian WhiteRim Sandstone and Kaibab Limestone. During initial rise in base level,chert-pebble conglomerate and trough cross-stratified sandstone facieswere deposited in incised valleys by braided streams eroding and drain-ing the Permian highlands to the south and southeast (Blakey, 1974;Ochs, 1988). The overlying deposits are composed predominantly ofinterbedded sandstone, siltstone, and shale facies, which are interpretedas intertidal to supratidal deposits (Ochs, 1988). These deposits areabruptly overlain by carbonate strata of the upper Black Dragon peritidalfacies. These carbonates mark the first evidence of subtidal deposition

FIGURE 6. A cartoon of the depositional profile or model of the SinbadFormation’s paleoenvironment based on vertical stacking patterns of thefacies.

FIGURE 7. Sequence stratigraphy of the Black Dragon Formation, theSinbad Formation and the Torrey Formation in the San Rafael Swell, Utah.TST=Transgressive Systems Tract; MFZ= Maximum Flooding Zone; HST=Highstand Systems Tract.

100within the study area. The peritidal facies grades vertically and horizon-tally into the offshore facies of the lower Sinbad Formation. This up-ward-deepening trend in the Black Dragon Formation and lower SinbadFormation is interpreted as a relative sea level-rise or the transgressivesystems tract (TST: Fig. 7).

The offshore facies of the lower Sinbad Formation is the deepestmarine facies preserved within the sequence and represents maximumflooding (MFZ) of the Early Triassic seas during the Smithian (Fig. 7).The offshore facies grades upsection to foreshoal/shoal facies and thenbackshoal, lagoonal and tidal-channel facies. Regionally, the backshoal,lagoonal, and tidal-channel facies are overlain by trough-crossbedded,skeletal, intraclastic grainstone (upper foreshoal/shoal facies) with a nor-mal-marine fauna of bivalves, gastropods and ammonoids (Table 1). Thesedeposits may represent a renewed transgression and relative sea-levelrise or possibly the migration of subtidal shoals over intershoal/backshoaldeposits. These facies relationships reflect maximum flooding followedby shallowing to backshoal, lagoonal and, shoreward, to intertidal waterdepths (highstand systems tract: HST).

The overlying lower Torrey Formation is composed of siliciclasticand dolomitic siltstone and mudstone and is interpreted as delta slope toprodelta deposits marking the return of siliciclastic deposition to theregion (Blakey, 1974). Episodic flooding of the delta is indicated by afew, thin marine carbonates in the basal portion of the siltstone andmudstone facies (Blakey, 1974). The Torrey Formation grades upsectionfrom siliciclastic and dolomitic siltstone and mudstone into thicker sand-stone as the delta prograded northwestward over the Sinbad Formation(Blakey, 1974).

Two interpretations have been suggested to explain the relation-ship of the overlying Torrey Formation with the Sinbad Formation. Oneinterpretation suggests that there is a sequence boundary at the top ofthe Sinbad Formation, with the lower Torrey Formation lyingunconformably over the Sinbad Formation. This implies that the TorreyFormation represents a relative deepening followed by shallowing as thedelta prograded northwestward over prodelta deposits; i.e., a TST that isequivalent to the Virgin Limestone of southwestern Utah and the Tirolitesbeds of the Thaynes Group to the northwest (Fig. 2). Alternatively, theTorrey Formation may simply represent renewed siliciclastic input andcessation of carbonate deposition as a delta complex prograded into thestudy area. This interpretation implies that the Torrey Formation repre-sents the HST with the sequence boundary lying in the Torrey Forma-

tion (Fig. 7). Further study is necessary to determine whether there isevidence of subaerial exposure at the top of the Sinbad Formation.

The results of this study suggest that the interbedded Moenkopiand Thaynes Groups (185 m thick) in the San Rafael Swell represent onethird-order depositional sequence formed by a relative sea-level rise andfall (Fig. 7). The Black Dragon Formation (~70 m thick) represents theTST, with the offshore facies of the lower Sinbad Formation (1.5-5 m)representing the MFZ. The middle and upper Sinbad Formation (4-16m) and lower Torrey Formation represents the HST. If there is a se-quence boundary at the top of the Sinbad Formation, then the lowerTorrey Formation represents the succeeding TST.

Supporting evidence for this Lower Triassic transgressive/regres-sive facies trend is also found in northwestern Utah to southeasternUtah, in Capitol Reef National Park and near St. George in southwesternUtah (Fig. 1). The TST shown in a regional cross-section from northwestto southwest Utah (Fig. 2) is represented by deepening and onlap of theWoodside Formation, the lower Thaynes Group and the Black DragonFormation. The MFZ occurs within the Meekoceras gracilitatis-bearingbeds of the Thaynes Group of Utah, Nevada, California, Idaho andWyoming, the lower Sinbad Formation in the San Rafael Swell and Capi-tol Reef, and the Sinbad (=Timpoweap) Formation of southwesternUtah. The HST is represented by shallowing and progradation of themiddle and upper Sinbad Formation, the Torrey Formation and the up-per Thaynes Group (Fig. 2). If the upper Sinbad and lower Torreyformations represent the succeeding TST, then this part of the sequenceprobably correlates with the Virgin Limestone of southwestern Utah andthe Tirolites/Columbites beds of the upper Thaynes Group in Utah,Nevada, Idaho, and Wyoming (Fig. 2). Biostratigraphy suggests that thedepositional sequence exposed within the San Rafael Swell region spannedat least part of the duration of the Smithian substage of the OlenekianStage of the Lower Triassic.

ACKNOWLEDGMENTS

Todd Lamaskin, John Rogers and Amy Thompson assisted in thefield. The Department of Earth and Planetary Sciences, University ofNew Mexico, and the New Mexico Museum of Natural History andScience provided financial support of this research. This study is basedon a masters thesis (Goodspeed, 1996), and the assistance and com-ments of Maya Elrick and Barry Kues are much appreciated. Karl Krainerreviewed the manuscript and his comments improved it.

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