Sedimentology and Stratigraphy of the Upper ...

Sedimentology and Stratigraphy of the Upper Cretaceous (Cenomanian) Frontier Formation, Northeast Bighorn Basin, Wyoming, U.S.A.1 ANDREW J. HUTSKY2 CHRISTOPHER R. FIELDING2 TREVOR J. HURD2 C. KITTINGER CLARK2

1. Manuscript received November 30, 2011; Accepted March 24, 2012 2. University of Nebraska-Lincoln, Department of Earth & Atmospheric Sciences, 214 Bessey Hall, Lincoln, NE 68588, [email protected], [email protected], [email protected], and [email protected]

ABSTRACT A detailed facies analysis of the Upper Cretaceous (Cenomanian) Frontier Formation was conducted over a ~ 35 kilometer outcrop belt in the northeast Bighorn Basin, Wyoming, U.S.A. Nine recurring lithofacies are identified: 1. dark-gray, laminated mudstone with bentonite intervals (offshore marine), 2. dark-gray, laminated mudstone with thin sandstone intervals (prodelta), 3. thinly interbedded siltstone and fine-grained sandstone (distal delta front), 4. thickly bedded, sharp-based sandstones with thin siltstone partings (middle delta front), 5. amalgamated, sharp-based sandstones (proximal delta front/river mouth), 6. non-bioturbated, fine-grained sandstone lacking siltstone partings (upper shoreface), 7. brown, fissile, laminated siltstone with abundant plant debris (coastal/alluvial floodplain), 8. trough cross-bedded sandstone (coastal fluvial channel), and 9. erosional based, low-relief, laterally extensive pebble/cobble accumulations (transgressive lag). Correlation of key stratigraphic surfaces (pebble lags, bentonites) and individual sandstone beds, and analysis of vertical facies stacking patterns indicate the preservation of multiple coarsening-upward cycles. Such cycles consist of basal, offshore marine/prodelta facies, overlain by progressively more proximal deposits containing sedimentary features consistent with the deposition and progradation of wave- (hummocky cross-stratification, symmetrical ripples) and tide(bimodal cross-bedding) influenced, fluvially dominated (syneresis cracks, current ripples and cross-bedding, impoverished trace fossil suites) deltas. Southward-directed cross-bedding (Peay and Torchlight members) and gently dipping clinoforms (Facies 4, 5; Peay Sandstone Member) indicate the broadly southward progradation of digitate delta fronts into shallow-marine settings under accommodation-limited conditions. Cycles lacking significant sandstone accumulations (Facies 4, 5) likely represent distal expressions of individual progradational events. Occurrences of cycle-capping pebble lags (Facies 9) suggest that transgressions led to the significant winnowing and top-truncation of accommodation-limited accumulations, generating isolated, mudstone-encased sandstone bodies observed throughout the Frontier Formation.

Hutsky, Andrew J., Christopher R. Fielding, Trevor J. Hurd, and C. Kittinger Clark, 2012, Sedimentology and stratigraphy of the Upper Cretaceous (Cenomanian) Frontier Formation, northeast Bighorn Basin, Wyoming, U.S.A. The Mountain Geologist, v. 49, no. 3, p. 77-98. www.rmag.org

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Andrew J. Hutsky, Christopher R. Fielding, Trevor J. Hurd, and C. Kittinger Clark

Facies 6—Upper Shoreface . . . . . . . . . . . . . . . . . . 89 Facies 7—Coastal/Alluvial Flood Plain . . . . . . . . . 90 Facies 8—Coastal Fluvial Channel . . . . . . . . . . . . 90 Facies 9—Transgressive Lag . . . . . . . . . . . . . . . . . . 90 DEPOSITIONAL ENVIRONMENT . . . . . . . . . . . . . . 90 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . 95 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . 78 METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 STRATIGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 FACIES ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Facies 1—Offshore Marine . . . . . . . . . . . . . . . . . . 81 Facies 2—Prodelta . . . . . . . . . . . . . . . . . . . . . . . . 81 Facies 3—Distal Delta Front . . . . . . . . . . . . . . . . . 85 Facies 4—Middle-Proximal Delta Front . . . . . . . . . 85 Facies 5—Proximal Delta Front/River Mouth . . . . . 89

INTRODUCTION

METHODS

The Upper Cretaceous Frontier Formation in the Western Cordilleran foreland basin of North America is economically important, hosting exploitable resources of coal (Kirschbaum et al., 2000), and oil and gas (Bighorn Basin Assessment Group, 2010). The unit is widespread across northern Utah, Wyoming, and much of Colorado, progressively becoming thinner and more distal in lithology eastward. The Frontier Formation has previously been interpreted in terms of coastal plain, deltaic, and shallowmarine environments of deposition (Barlow & Haun, 1966; Merewether et al., 1979; Tillman & Almon, 1979; Winn, 1991; Merewether et al., 1998; Bhattacharya & Willis, 2001). A significant body of literature places the Frontier Formation in its regional stratigraphic context (Cobban & Reeside, 1952; Kirschbaum et al., 2009) (Fig. 1). The internal character of the Frontier Formation and its constituent members is well-documented in some areas, such as in the Powder River Basin of eastern Wyoming (Willis et al., 1999; Bhattacharya & Willis, 2001; Gani et al., 2007; Vakarelov & Bhattacharya, 2009), but significantly less so in others. One such area from which little published information on the Frontier Formation is available is in the Bighorn Basin of northern Wyoming (Fig. 2). Despite being a productive reservoir for oil and gas in the basin, the Frontier Formation has received little research attention in this area, one recent exception being the stratigraphic analysis of Kirschbaum et al. (2009). In this paper, we provide a full review of the stratigraphy of the Frontier Formation in the northern Bighorn Basin based on continuous, high-quality surface exposure northwest of Greybull, Wyoming (Fig. 2), supplemented by data from subsurface drilling. We document its depositional environments via a facies analysis and summarize sediment dispersal direction from paleocurrent data. Such information will add to the understanding of Cenomanian history of the Western Cordilleran foreland basin and may assist in future efforts to characterize depositional systems in this formation.

A detailed sedimentologic and ichnologic analysis was performed along a ~35 kilometer (km), northwest-southeast-trending outcrop belt of the Frontier Formation within the northeast Bighorn Basin. Twenty-six outcrop sections were measured from Goose Egg Anticline in the northwest to the Greybull River Cliffs and Potato Ridge in the southeast (Fig. 2). Sections were spaced approximately 1 km apart so as to facilitate a high-resolution analysis of the outcrop belt. Sedimentologic and ichnologic data, including fossils (body and trace), sedimentary structures, vertical and lateral bedding trends, grain-size trends, lithological contacts, body thicknesses and paleocurrent data were collected, forming the basis for facies and depositional environmental analyses. Vertical stacking patterns and lateral variations formed the foundation for the construction of two cross-sections (depositional dip and strike), generating a regional three-dimensional reconstruction of the Peay and Torchlight sandstone body geometries. Age control from laterally extensive synchronous horizons (Clay Spur and ‘X’ bentonites), as well as the verification of vertical stacking trends and lateral variations of facies and body geometries between adjacent outcrop sections, aided in the correlation of key geologic surfaces and sediment bodies.

The Rocky Mountain Association of Geologists

STRATIGRAPHY In the northeast Bighorn Basin (Fig. 2), the Frontier Formation is of mid-late Cenomanian age (97.17 +/- 0.69 million years ago (Ma) – ~ 94.71 Ma; Obradovich, 1993), conformably overlies the Mowry Shale and is conformably overlain by the Cody Shale (Cobban & Reeside, 1952) (Fig. 1). Within the Frontier Formation of the northern Bighorn Basin, only two specific, sandstone-dominated intervals (the lower Peay and upper Torchlight members) have been given Member status by Hintze (1915) prior to the current work (Clark, 2010; Hutsky, 2011). Accordingly, it is difficult to make unambiguous reference to other parts of the formation. In this work, we provide informal naming details

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SEDIMENTOLOGY AND STRATIGRAPHY OF THE UPPER CRETACEOUS FRONTIER FORMATION, NORTHEAST BIGHORN BASIN, WYOMING, U.S.A.

Figure 1. Biostratigraphic, lithostratigraphic, and chronostratigraphic correlation of the Frontier Formation along a west-east transect through Wyoming (see Fig. 2B for locations). Correlation of Frontier Formation intervals between basins (Green River, Bighorn, Powder River) is constrained by radiogenic isotope ages on bentonites, as well as the first occurrences of specific inoceramids and ammonites.

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Figure 2. Paleogeographic and regional location maps of the Frontier Formation within the Bighorn Basin, Wyoming. A. Paleogeographic reconstruction of the western interior of North America during the Late Cretaceous (Cenomanian-Coniacian). As shown, several deltas were sourced from the Sevier Orogenic Belt and deposited into the Cretaceous Western Interior Seaway (KWIS). These include the Frontier (FD), Vernal (VD), Last Chance (LCD), and Notom/The Post (ND/TPD) delta complexes. B. Present-day Wyoming showing Laramide-age basins and ranges. The area of interest for this study (inset box) is within the northeastern Bighorn Basin in north-central Wyoming. C. Map of study area with measured outcrop sections along a ~ 35 km, northwest-southeast trending outcrop belt, northwest of Greybull, Wyoming.


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SEDIMENTOLOGY AND STRATIGRAPHY OF THE UPPER CRETACEOUS FRONTIER FORMATION, NORTHEAST BIGHORN BASIN, WYOMING, U.S.A. FACIES ANALYSIS

for those portions of the Frontier Formation, using the coding scheme of Kirschbaum et al. (2009) with some additional horizons, so as to facilitate description and interpretation of the entire unit (Figs. 3, 4). Historically, the Mowry-Frontier contact has been placed at a prominent, altered volcanic ash (bentonite) bed, named the Clay Spur Bentonite (Hintze, 1915; Kirschbaum et al., 2009). However, the Clay Spur Bentonite is not ubiquitous across the study area, and other bentonitic beds are present in the upper part of the underlying Mowry Shale. A more reliable, and mappable, criterion for the Mowry/Frontier contact is given by a distinct, laterally continuous change from light-gray, porcellaneous siltstones and sandstones (Mowry Shale) to dark-gray/black mudstones (Frontier Formation) observed everywhere in outcrop (F600, Fig. 3). Following the nomenclature methodology of Kirshbaum et al. (2009), the Frontier Formation is herein divided into seven intervals, most of which are shallow-marine sandstones abruptly overlain by marine mudstones. Stratigraphically, from oldest to youngest, these units are named the F550, Peay Sandstone (F500), F400, F350/F300, Torchlight Sandstone (F200), and F175 intervals. The basal Mowry/Frontier contact (F600) is laterally extensive throughout the study area. A basal, laterally continuous unit (30–40 meters (m) thick) consisting of darkgray shales that coarsen up into interbedded siltstone and sandstone is herein named the F550 interval (Fig. 3). Originally defined by Hintze (1915), the Peay Sandstone Member (F500 interval) is a sandstone unit that forms ridges and cliffs, overlying the basal shales and coarseningupward siltstone-sandstone interbeds of the F550 interval (Fig. 3). A laterally extensive pebble horizon above the sandstone defines the top boundary (F500) of the Peay Sandstone Member. A regionally extensive, coarseningupward succession (1–5 m thick) of interbedded siltstones and sandstones between the top of the Peay Sandstone Member and the base of the ‘X’ Bentonite (defined by local bentonite mining operations) corresponds to F400 interval in Kirschbaum et al. (2009) (Fig. 3). Two prominent coarsening-upward cycles (F350, 300) occupy the stratigraphic interval between the base of the ‘X’ Bentonite and the base of the cliff-forming sandstone of the Torchlight Sandstone Member (Fig. 3). The Torchlight Sandstone Member (F200; Hintze, 1915) is a prominently outcropping sandstone that gradationally overlies the F300/350 interval and is capped by a laterally persistent conglomerate horizon (F200) (Fig. 3). Finally, it is proposed that a set of typically two coarsening-upward intervals (each 10–15 m thick) above the Torchlight Sandstone Member (F200) are considered as an uppermost interval (F175) of the Frontier Formation, previously undefined by Kirschbaum et al. (2009). This interval lies below the Cenomanian-Turonian F150 unconformity of Kirschbaum et al. (2009) and is determined to be of late Cenomanian age (Fig. 3).

The typical facies assemblage observed throughout the Frontier Formation consists of a coarsening-upward sequence from basal dark-gray shales, interbedded siltstones and sandstones (sandstone ratio increases up-section) into thickly bedded, sharply based amalgamated sandstones, capped by a laterally extensive, pebble lag (Fig. 5). Partial to complete expressions of this assemblage occur repetitively throughout the Frontier Formation. Evidence for tidal, wave, and fluvial influences, and unidirectional paleocurrent trends point to a broadly deltaic environment of deposition for the various intervals. A detailed facies analysis is presented below and in Table 1. Facies 1 —Offsh or e Mar in e Descr iption Facies 1 consists of finely laminated, fissile, organic-rich claystone and siltstone, with interbedded pale-yellow (fresh) to light-gray (weathered) bentonitic intervals (Table 1). Deeply weathered bentonite exhibits a ‘popcorn’ texture, while physically unaltered bentonite is blocky in character. Facies 1 is repetitive throughout the Frontier Formation, occurring as the basal facies in several coarseningupward cycles. Intervals are approximately 5 - 10 m thick, showing a decrease in the clay-to-silt ratio up-section and lateral continuity in successive outcrop sections. Individual bentonitic intervals range in thickness from 20–80 centimeters (cm), with thicker units being regionally correlateable. A sharp, relatively planar lower contact at the base of Facies 1 is observed, while the occurrence of thin, finegrained, laterally discontinuous sandstone stringers marks the gradational upper contact with Facies 2. In ter pr etation Facies 1 is interpreted as offshore-marine deposits with the dominance of claystone and siltstone and a paucity of coarser-grained sediment. Clay- and silt-sized particles were likely entrained in buoyant hypopycnal plumes and transported 10’s of km from a terrestrial source and slowly deposited from suspension. Sulfides, a lack of bioturbation, and high organic carbon values suggest anoxic and possible euxinic conditions during deposition. Bentonite intervals represent the accumulation of ash derived from volcanic activity. Facies 2—Pr odelta Descr iption Facies 2 comprises dark-gray to black, fissile, organicrich siltstone and irregularly distributed, very fine- to fine-

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Figure 3. Composite log of the Frontier Formation based on 26 measured outcrop sections throughout the northeast Bighorn Basin. Thicknesses of each member are generalized. Facies, sedimentary structures, bioturbation intensity (BI), and trace and body fossils are shown. Red arrows denote stratigraphically important bentonite beds. Surface data were also used to calibrate regional subsurface correlations (via geophysical logs from well API# 4900320303, Alkali Anticline Field–N 44.71225˚, W 108.32171˚). The Rocky Mountain Association of Geologists

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Figure 3. Legend.

Figure 4. Panoramic view (looking south) of a reference section (top 110 m of the Frontier Formation) on the west flank of Goose Egg Anticline (CKC 1–Fig. 2C), extending from the top of the Peay Member (PM) to the top of the F175 interval (F175). Structural dip is to the west (right) at 35˚. The top of the Frontier Formation is delineated by a sharp lithology and color contrast between the F175 (brown-orange sandstone and septarian nodules) and the Upper Cretaceous Cody Shale (dark-gray mudstone). The apparent angular unconformity between the Torchlight Sandstone Member and the F175 interval is an artifact. F400: F400 Interval, X: ‘X’ Bentonite, F300/350: F300/350 interval, TM: Torchlight Member. Photo by Tracy Frank.

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Figure 5. Cliff exposure (~ 500 m long) of the Peay Member along the Bighorn River, Greybull, WY, displaying a coarsening-upward cycle consisting of basal prodelta (Facies 2, black), distal delta front (Facies 3, dark gray), and middle/proximal delta front (Facies 4, light gray) facies. Facies 5 is exposed above the cliff line, while Facies 1 occurs below river level. Gently dipping clinoforms (< 5°, arrows) occur in Facies 4, suggesting the progradation of a gently sloping delta front.



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SEDIMENTOLOGY AND STRATIGRAPHY OF THE UPPER CRETACEOUS FRONTIER FORMATION, NORTHEAST BIGHORN BASIN, WYOMING, U.S.A. In ter pr etation Facies 3 is interpreted as distal delta-front deposits, formed up-dip of the prodeltaic deposits of Facies 2. Preserved sedimentary structures indicate that fluvial outflow currents and combined wave-current flow substantially influenced deposition (Dumas et al., 2005). Siltstone intervals containing phytodetrital deposits likely developed from high fluvial runoff events (freshets) that transported terrestrially derived organic matter seaward, while also introducing zones of fresh water near the sediment-water interface (MacEachern et al., 2005; MacEachern & Bann, 2008). Such events generate brackish conditions and a stressful environment for marine organisms. Horizontal traces along bedding planes likely developed between turbid river outflow events. An increase of storm-generated structures (HCS, swaley cross-stratification (SCS), wave/interference rippled bed tops) and a decrease in the siltstone-to-sandstone ratio up-section represents greater influence of storm wave action (Southard et al., 1990). This facies is comparable to distal-to-medial deltafront deposits within the broadly time-equivalent Ferron Sandstone Member of the Mancos Shale of south-central Utah (Fielding, 2010).

grained sandstone (Table 1). Thin sandstone beds (~ 1–5 cm thick) are laterally discontinuous and have sharp upper and lower contacts. Bodies are internally dominated by flat to low-angle and combined-flow ripple cross-lamination, while unidirectional current structures and syneresis cracks are preserved on bedding plane tops and bases, respectively. Bioturbation is absent within the siltstones, while sandstones exhibit low to modest faunal activity. Overall, Facies 2 is a laterally persistent sediment package (~ 3–15 m thick) within the F550, F400, F300/350, and F175 intervals, often enclosed by Facies 1 below and Facies 3 above. In ter pr etation Facies 2 is interpreted as prodeltaic sediments deposited below storm wave base in an environment slightly more proximal to a terrestrial source than Facies 1. Large stormflow deposits were responsible for the accumulation of thin sandstones with combined-flow and unidirectional current structures (Aigner & Reineck, 1982; Bhattacharya & MacEachern, 2009). A lack of bioturbation, and the presence of dispersed sulfides and preserved organics suggest anoxic conditions during the suspension settling of finegrained muds. Planolites, Diplocraterion, Thalassinoides, and sediment swimming traces within the sandstone beds represent trophic generalists that exploited brief periods of bottom-water oxygenation following tempestite deposition on a regularly thixotropic (fluid mud) substrate (Bhattacharya & MacEachern, 2009). A gradual transition to an increasingly proximal depositional environment is inferred as sandstone beds increase in abundance, thickness, and lateral continuity up-section.

Facies 4—Middle-Pr ox imal Delta Fr on t Descr iption Facies 4 comprises intervals of massive, fine-grained sandstone punctuated by thin (~ 0.3–0.5 m), interbedded siltstone/sandstone intervals. Interbedded intervals consist of finely laminated siltstone and thin, sharp-bounded sandstone internally dominated by planar and combined-flow ripple cross-lamination (Table 1). Syneresis cracks, waveripple cross-lamination, and interference ripples are pervasive along bedding plane surfaces. Massive cliff forming intervals are characterized by storm wave-generated structures internally and along bedding plane surfaces. Syneresis cracks and sole marks also occur on basal bedding planes. An impoverished expression of the Cruziana/Skolithos Ichnofacies is preserved along distinctive horizons within massive sandstone bodies. Overall, the unit is 10–15 m thick and is present only in the Peay Sandstone Member (F500). Beds thicken up-section, and are laterally extensive throughout the study area, forming gently dipping clinoform sets extending over large distances (~ 1 km) before tangentially downlapping onto Facies 3 below. Paleocurrent data from small-scale cross-bedding and clinoform dip surfaces indicate southward sediment dispersal (Fig. 6).

Facies 3—Distal Delta Fr on t Descr iption Facies 3 consists of lenticular to wavy-bedded intervals of siltstone and thin, very fine- to fine-grained sandstone (Table 1). Internally, sandstone beds are dominated by flat to low-angle cross-lamination, ripple-scale hummocky cross-stratification (HCS), and combined-flow ripple crosslamination. Symmetrical and interference ripples and syneresis cracks are common along bedding-plane surfaces. Bioturbation is focused on specific horizons and markedly declines up-section. Locally, ~ 1–2 m thick sandstone beds develop in the upper portions of Facies 3, coinciding with an upward trend of increasing sandstone body thickness and lateral continuity. Such bodies contain similar sedimentary structures as their thinner counterparts. Facies 3 ranges from 1–10 m in thickness and is best developed in the F550 interval where it is laterally extensive and gradationally passes into Facies 4 above and Facies 2 below.

In ter pr etation Facies 4 records middle-proximal delta-front environments up-dip from Facies 3, between storm and fair-weather

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Lithology

Fissile black to dark-gray claystones, siltstones, and interbedded bentonitic intervals (20–80 cm), claystone vs. siltstone ratio decreases up-section, plant debris in mudstones.

Lenticular bedding in a unit consisting of dark gray laminated, fissile siltstones (80–90 %) interbedded with irregularly distributed (0.02–0.06 m spacing), thin (~ 0.01–0.05 m thick), sharp-based, very fine-grained, lithic sandstone intervals (10–20 %), plant debris in siltstones; sandstones increase in abundance and thickness up-section.

Lenticular to wavy bedded siltstone and sandstone intervals (~ 1.0–2.0 m thick), siltstones are dark gray–black, fissile, finely laminated, organic (woody/coaly debris) rich, and occur as distinct partings (~0.05–0.2 m), sandstones (≤ 0.30 m) are fine grained, composed of quartz, lithic grains, and black chert pebbles, local isolated sandstone beds (1.0–2.0 m thick) develop in upper portions of unit.

Facies

1

2


3

86 Sandstones are internally dominated by flat to low-angle cross-lamination (~ 1 cm), small-scale (micro) hummocky cross-stratification, combined-flow ripple cross-lamination (locally strongly aggradational), and small-scale cross-bedding (local), bedding plane surfaces commonly exhibit symmetrical and interference ripples, syneresis cracks, load casts, and sole structures, siltstones are finely laminated with locally occurring sulfur patches.

Sandstones are internally dominated by flat to low-angle cross-lamination, combined-flow ripple crosslamination (with occasional organic-rich mud drapes), and linsen (pinstripe) lamination, asymmetrical current ripples on bed tops, syneresis cracks on basal surfaces (local), siltstones exhibit sulfur staining.

Patches of sulfur staining within siltstones.

Physical Structures

Lower sandstone bodies exhibit moderate bioturbation (BI = 2–3) and trace diversity, mainly pronounced along bedding plane surfaces (principally Planolites and Thalassinoides, with minor Gyrochorte and Rhizocorallium), sandstones internally contain low to moderate bioturbation (BI = 0–3); principally small and large Diplocraterion, with minor occurrences of Asterosoma, Lockeia, Schaubcylindrichnus, Teichichnus, Phycosiphon, Cylindrichnus, and fugichnia, trace abundance and diversity declines up-section (BI = 0 -1; Planolites along bedding planes), siltstones lack bioturbation.

Sandstone bedding plane tops are sparsely bioturbated (BI = 0–2, 2–4 locally; principally Planolites and Thalassinoides), sandstones internally bioturbated locally (BI = 0–2; Diplocraterion and navichnia), no observed bioturbation in siltstones (BI = 0).

BI = 0

Biogenic Structures

Distal Delta Front

Prodelta

Offshore Marine

Interpretation

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Unit is laterally continuous (7–10 m thick), individual sandstone bodies pinch out over short distances (lensoid geometries). Sandstone beds thicken up-section, coinciding with a decrease in the presence of siltstone partings.

Unit displays lateral continuity (3–15 m thick), thin sandstone intervals display variability in lateral extent. Sandstones increase in abundance, thickness, and lateral continuity up-section.

Entire unit is laterally continuous (5-10 m thick), thicker bentonite beds more laterally continuous.

Lateral & Vertical Variations

The facies assemblage of the Frontier Formation within the northeast Bighorn Basin is derived from the careful observation of lithology, physical sedimentary structures, ichnology, and spatial variations throughout the study area.

Table 1


Lithology

Fine-grained, sharp-based, massive, cliff-forming sandstones (~ 2–5 m) with thin (~0.3–0.5 m) sandstone/siltstone recessive intervals (50:50 sandstone vs. siltstone, +/- ironstone concretions), siltstones contain abundant plant debris, sandstones (both massive and thin interbeds) are lithic-dominated with wellrounded black chert pebbles dispersed throughout.

Amalgamated, sharp-based, fineto medium-grained lithic sandstone with well-rounded chert pebbles, boulder-sized iron concretions throughout.

Fine- to medium-grained lithic sandstone, dispersed, black chert pebbles, local accumulations of ironstone.

Thinly laminated, fissile gray to brown siltstone unit (~ 0.4 m thick), abundant plant debris throughout.

Facies

4

5

6

87

7

Sandstones internally dominated by flat to low-angle cross-bedding, with minor unidirectional cross-bedding, hummocky cross-stratification, and mud-draped ripples.

Sandstone beds internally dominated by flat to low-angle crosslamination, hummocky and swaley cross-stratification, and small trough cross-bedding sets (local; ~ 0.2–0.4 m), syneresis cracks, wave, and interference ripples occur on bedding plane surfaces.

Sandstone/siltstone intervals: siltstones are finely laminated, sandstones are internally dominated by flat and undulatory lamination, and combined-flow ripple cross-lamination; bed planes contain syneresis cracks, symmetrical, and interference ripples Massive sandstones: internally dominated by large-scale flat to low-angle cross-lamination and hummocky cross-stratification (local); basal surfaces contain syneresis cracks, sole marks, and load casts; bed tops contain wave and interference ripples.

Physical Structures

BI = 0

Sparse bioturbation (BI = 0 -1; rare fugichnia).

Low bioturbation (BI: 0–2) along bed planes (Planolites, Thalassinoides) and within sandstone bodies (Diplocraterion, Ophiomorpha, Conichnus, and Lockeia), bioturbation decreases up-section.

Sandstone/siltstone intervals: minimal bioturbation (BI = 1–2; Planolites and Lockeia) Massive sandstones: Bed tops are irregularly moderately bioturbated by low diversity, horizontal trace genera (BI = 3–4; principally Planolites, Thalassinoides, Gyrochorte, and un-named arthropod claw marks); beds are internally low to moderately bioturbated (BI = 1–3; Diplocraterion, Schaubcylindrichnus, Cylindrichnus, Ophiomorpha, and Taenidium); bioturbation decreases up-section (BI = 1–2; principally vertical burrows).

Biogenic Structures

Table 1 (Cont’d)

Laterally and vertically discontinuous (lensoid geometry) near Section 1.

Upper contact is erosive and unit may have been erosionally incised by overlying facies, succession is regionally discontinuous (2–6 m thick).

Unit is laterally continuous, with thickness variations along depositional dip and strike (10–35 m), regionally form gently dipping (≤ 5˚) clinoform sets.

Succession is laterally continuous (10–15 m thick) along depositional strike with thickness variations up and down depositional dip, individual sandstone beds regionally form gently dipping clinoforms (≤ 5°), up-section, siltstone partings disappear and sandstone intervals thicken.


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Coastal/Alluvial Flood Plain

Upper Shoreface

Proximal Delta Front/River Mouth

Middle/Proximal Delta Front

Interpretation



Lithology

Medium- to coarse-grained, lithic, poorly cemented sandstone bodies (~ 3–7 m thick) with sharp, erosionally based upper and lower contacts, wellrounded black chert pebbles dispersed along bedding planes and scour surfaces.

Two variants recognized: 1. Well-rounded pebbles and cobbles, clast-supported: clasts include chert, quartzite, andesitic porphyry, +/- granodiorite, +/- diorite 2. Coarse- to very coarse-grained lithic sandstone or matrix- to clast-supported, well-rounded black chert gravel.

Facies

8

9

The Rocky Mountain Association of Geologists Gravel-filled symmetrical ripples with interference patterns.

Sandstones contain bimodal trough cross-bedding (~ 0.2–0.7 m thick; smaller cross-beds prevalent lower in the succession and locally contain plant debris mud drapes), minor flat to low-angle cross-lamination, wave, and interference ripples.

Physical Structures

BI = 0; dispersed fish vertebrae, crocodile teeth, petrified wood, bone fragments.

BI = 0; bivalve shell molds ~ 1 m from top of unit (local), petrified fossil tree trunks (local).

Biogenic Structures

Table 1 (Cont’d)

Laterally continuous throughout study area, forms regionally traceable horizons.

Unit is laterally persistent along depositional dip and strike (3–7 m thick), individual sandstone bodies coalesce to form multilateral, multistory, tabular bodies.


Transgressive Lag

Coastal Fluvial Channel

Interpretation


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Figure 6. Paleocurrent measurements from trough cross-bedding (Peay, Torchlight sandstones) and clinoform dip directions (Peay Sandstone) indicate south-southeastward sediment dispersal for these major sandstone intervals within the Frontier Formation.

wave base. Bed thicknesses (2–5 m) and gently dipping, elongate clinoform sets suggest deposition and progradation on a gentle depositional gradient. Fluvial outflow currents likely interacted with storm processes, as reflected in the suite of preserved sedimentary structures (Dumas et al., 2005). Trace fossil distribution implies opportunistic biogenic activity during fair weather and between periods of high sediment output from a nearby river mouth (Bann & Fielding, 2004; MacEachern et al., 2005; MacEachern & Bann, 2008). Facies 4 closely resembles proximal delta-front deposits in the Ferron Sandstone Member of the Mancos Shale of south-central Utah interpreted by Fielding (2010).

ded, sharply based, tabular, amalgamated sandstone bodies with macroform inclined bedding represent progradation of bedforms down a gentle depositional slope. Storm wave-generated structures, as well as a lack of interbedded mudstones and siltstones, indicate significant sediment reworking in shallow-water, high-energy environments. Syneresis cracks along sandstone-bed bases suggest a similar fluvial influence as interpreted for Facies 3 and 4. The trace-fossil suite represents faunal activity by trophic generalists during brief periods of fair weather and between high sediment delivery events from the river mouth, similar to Facies 4.

Facies 5—Pr ox imal Delta Fr on t/River Mouth

Facies 6—Upper Sh or eface

Descr iption Facies 5 comprises 10-35 m thick, amalgamated intervals of sharp-based, fine- to medium-grained sandstone beds (Table 1). In outcrop, the unit exhibits a flaggy (thinly bedded) appearance due to weathering. Large, brown siderite nodules, formed from secondary processes, are also distinctive. Storm wave-generated features dominate the suite of preserved sedimentary structures. Bioturbation is exceptionally rare and sporadic. Facies 5 is exclusive to the Peay Sandstone Member (F500), exhibiting a gradational basal contact with the underlying Facies 4 and is sharply overlain by a pebble conglomerate horizon (see Facies 9). Regionally, the unit forms gently dipping clinoform sets with Facies 4. Paleocurrent data from cross-bedding and clinoform dip surfaces indicate sediment dispersal to the south-southeast with a minor northeast mode, similar to Facies 4.

Descr iption Facies 6 is a coarsening-upward unit of sparsely bioturbated, fine- to medium-grained sandstone internally dominated by flat to low-angle bedding (Table 1). Exclusive to the Torchlight Sandstone Member (F200), the interval ranges between 2 - 6 m thick. The lower contact with Facies 3 is not readily observable, while the upper contact with Facies 7 or 8 is sharp, erosional, and correlateable regionally. In ter pr etation Facies 6 is interpreted as strongly storm-influenced, upper-shoreface deposits. Flat to low-angle cross-bedded sandstone lacking any observable bioturbation implies a near-constant reworking of sediments following deposition. The regionally persistent sharp upper contact is indicative of significant widespread erosion before the accumulation of overlying facies (Facies 7 or Facies 8). While the lower contact is not readily observable, Facies 6 is interpreted to be genetically related to underlying facies (Facies 3). Together, Facies 3 and Facies 6 represent a coarsening-upward cycle. A lack of bioturbation, plant

In ter pr etation Facies 5 is interpreted as a proximal delta front/deltaic mouth bar depositional environment up-dip from Facies 4, and above fair-weather wave base. Medium- to thick-bed-

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debris, trough cross-bedding, and siltstone partings suggests the accumulation of Facies 6 in an environment different from that of a delta front (Facies 4, 5).

influence. Thicknesses of constituent sandstone bodies (3–7 m) suggest deposition in relatively shallow water. The regional erosional surface does not exhibit significant relief and therefore does not indicate incised valley formation. A lack of bioturbation can be attributed to both high energy and brackish-to-fresh water conditions within the fluvial environment. Bimodal trough cross-bedding, preservation of plant debris mud drapes in the lower portions of Facies 8, and the occurrence of bivalve molds in the upper portions of Facies 8 may suggest a coastal/tidal influence upon the deposition of Facies 8.

Facies 7—Coastal/Alluvial Flood Plain Descr iption Facies 7 comprises a discrete accumulation of fissile, plant debris-rich siltstone (Table 1). A localized occurrence within the Torchlight Sandstone Member (F200; Section 1), Facies 7 is ~ 40 cm thick and extends laterally (~ 10–20 m) before being erosionally truncated. The lower contact with Facies 6 is sharp while the upper contact with Facies 8 is undulating and erosional. Overall, Facies 7 exhibits a discontinuous lensoid geometry in the north-south trending outcrop.

Facies 9—Tr an sgr essive Lag Descr iption Facies 9 is a suite of pebble-to-cobble conglomerate beds preserved at several stratigraphic levels throughout the Frontier Formation (Table 1). The thickest accumulation occurs as a clast-supported horizon at the top of the Torchlight Sandstone Member (F200), although very coarse-grained sandstone/pebble to cobble matrix- to clastsupported conglomerate also occurs within the Peay Sandstone (F500) and the F300/350 interval. In situ exposure of Facies 9 is limited. Weak cementation is observed in a scattered pebble/cobble horizon adjacent to the Torchlight Member. Where in situ, the conglomerate lacks physical sedimentary structure and displays a fining-upward trend from basal cobbles to pebbles and granules. Facies 9 above the Peay Sandstone is dominated by large gravelfilled symmetric ripples with interference patterns (e.g. Forbes & Boyd, 1987). Regionally, the multiple occurrences of Facies 9 (Peay and Torchlight sandstone members) are laterally extensive and display sheet-like geometries with planar, erosional basal contacts.

In ter pr etation Facies 7 is interpreted as alluvial or coastal flood-plain deposits. These non-bioturbated, plant debris-dominated siltstones likely represent one or several periods of floodplain inundation from substantial fluvial outflow events. The juxtaposition of Facies 7 above shallow-marine accumulations (Facies 6) suggests significant erosional discontinuity. Furthermore, Facies 7 is absent over much of the outcrop belt, where Facies 6 is erosionally overlain by fluvial channel deposits (Facies 8—see below), suggesting that regional fluvial downcutting likely removed significant thicknesses of shallow marine sediments. Facies 8—Coastal Fluvial Ch an n el Descr iption Facies 8 consists of medium- to coarse-grained, buff-colored sandstone (Table 1). Internally, Facies 8 is dominated by trough cross-bedding with bimodal paleocurrent distributions. Individual cross-sets coalesce to form multilateral and multistory, sheet-like, sharp-based sandstone bodies that lack bioturbation. This facies forms the uppermost part of and is exclusive to the Torchlight Sandstone Member (F200). Overall, Facies 8 has erosional upper and lower contacts with enclosing facies. The lower contact with Facies 6 or 7 (where present) is a regionally traceable erosional surface. The upper contact with Facies 9 is a lowrelief erosional surface. Paleocurrent data suggest south-southeastward sediment dispersal with a minor north-northeast component.

In ter pr etation Facies 9 is interpreted as regional transgressive lag deposits. Low-relief and consistently widespread deposits represent depositional hiatal events that regionally truncated underlying strata in a planar-to-sub-horizontal fashion (Martinsen, 2003). Lags with wave and interference ripples suggest that the associated wave action scoured the underlying sediments, winnowing away fine-grained sediments (Weimer, 1988; Hwang & Heller, 2002). Localities displaying reverse-graded lag deposits overlain by offshore mudstones (Facies 1) record a period of high-energy wave ravinement that subsequently waned through time.

In ter pr etation Facies 8 is interpreted to represent the accretion, accumulation, and south-southeastward progradation of subaqueous barforms and bedforms in rivers with tidal The Rocky Mountain Association of Geologists

DEPOSITIONAL ENVIRONMENT Sandstone body geometries, facies stacking patterns, and paleocurrent data observed throughout the Frontier 90

Figure 7. The correlation of key stratigraphic horizons and sandstone intervals observed in outcrop is illustrated by depositional dip and strike cross-sections oriented in both NW-SE (depositional dip) and W-E (depositional strike) directions. A. NW-SE oriented cross-section through the Frontier Formation along the interpreted depositional dip direction (Fig. 2). Major sandstone intervals (Peay & Torchlight members) display dip-elongate geometries.


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Figure 7B. A W-E oriented cross-section along the interpreted depositional strike (Fig. 2). The Peay Sandstone thins dramatically west-east over 10 km, from ~ 50 m thick (BRS) to ~ 15 m thick (PR). Both cross-sections are drawn using the top Torchlight Member as the datum.

study area for regional incision; however, progradation into distal basin locations suggests deposition during sealevel lowstand. Phytodetrital deposits, current-derived sedimentary structures, syneresis cracks, and trace fossil suites indicating physico-chemical stress (Gingras et al., 2011) within Facies 3 - 4 suggests outflow-dominated deposition on a delta front. Up-section (Facies 4 - 5), storm wave-modified structures (HCS, SCS, wave and interference ripples, Skolithos Ichnofacies) outnumber fluvially derived features. This likely represents significant post-depositional reworking of sediments derived from fluvial outflow events as the depositional profile shallows into the storm and fair weather wave base window, and not an overall fundamental change to a storm wave-dominated system. The planform geometry (Fig. 8) (north-south elongate, laterally restricted, digitate) further supports a fluvially dominated delta depositional system with a storm wave overprint (Fig. 9). The Torchlight Sandstone Member (F200) differs sedimentologically and geometrically from the Peay Sandstone Member (F500), suggesting a different formative environment. The Torchlight sandstone consists of nearshore marine (Facies 6), coastal flood plain (Facies 7), and coastal fluvial channel (Facies 8) sediments. South-southeastwardoriented trough cross-bedding with a modest north-northeast bimodal overprint (Fig. 6; Facies 8) suggests the progradation of channel mouth bars with a slight tidal influ-

Formation are consistent with tide- and wave-influenced, fluvial-dominated delta-front progradation into a shallowmarine, low sea-floor gradient and low accommodation setting. Several coarsening-upward cycles, which consistently show an upward transition from offshore to shallow-marine sediments, are interpreted as a record of multiple episodes of delta progradation. The F550 and Peay Sandstone Member (F500) intervals display the most complete deltaic vertical facies succession within the Frontier Formation (Fig. 3), while high-frequency, small coarsening-upward cycles observed in outcrop within the F400, F300/350, and F175 intervals (Fig. 3) represent distal or off-flank equivalents of distinctive progradational events. Presented below is a depositional environmental analysis of two significant sandstone-prone intervals within the Frontier Formation in the study area, the Peay (F500) and Torchlight (F200) members. Paleocurrent data from gently-dipping clinoform sets (< 5˚) and small-scale trough cross-bedding in middle-proximal delta front and river mouth bar facies (Facies 4 - 5) indicate south-southeastward progradation of the Peay Sandstone Member (F500) delta front. The gentle dips and thickness of individual clinoforms (≤ 5 m) (Fig. 5), as well as an overall north-south elongate, east-west laterally restricted geometry (Figs. 7A, 7B, 8), suggest that the Peay delta prograded into low accommodation, low sea-floor gradient settings. No persuasive evidence exists within the


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Figure 8. Net sand isolith maps of Frontier Formation gross sandstone body thicknesses within Big Horn and Washakie counties, northeast Bighorn Basin. Subsurface sandstone body thicknesses were determined using a half-way cutoff on gamma ray logs (Fig. 3). A. Peay Sandstone (F500 of Kirschbaum et al., 2009). Accumulations display a north-south elongate, east-west thinning trend, characteristic of a southwarddirected digitate system. B. F400 Interval. An east-west elongate sandstone body geometry is observed from subsurface correlations. C. F300/350 Interval. A northwest-southeast elongate, linear body is observed regionally. D. Torchlight Sandstone (F200 of Kirschbaum et al., 2009). Widespread coastal marine and fluvial deposition observed within the Torchlight Sandstone is consistent with regional sheet-like geometry with a linear trend.

ence. Development of a regional, chert pebble-filled scour surface that separates Facies 8 (above) and Facies 6–7 (below) likely indicates unrepresented geologic time with the removal of sediments by high-energy fluvial activity. As such, deposition of the Torchlight Sandstone Member (F200) likely coincided with a tidally influenced, coastal plain fluvial environment and not fluvially dominated delta front progradation (Peay delta). A possible mechanism for the southward deflection of prograding Frontier Formation deltaic sediments is the

interaction between river outflow events and geostrophic current deflection (Slingerland et al., 1996). Shore-parallel geostrophic currents were generated through the interaction between the Coriolis Effect, surface wind stress, and a seaward dipping slope, and developed from the introduction of fresh outflow water and saline sea water (Slingerland et al., 1996). This relationship established a counterclockwise gyre that deflected outflow plumes southward on the western margin of the Cretaceous Western Interior Seaway (Slingerland et al., 1996). Seasonal counterclockwise

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Figure 9. Interpreted planform geometry of Frontier Formation deltaic environments, based on the Peay Member in the study area. Sediments prograded into distal portions of the Cretaceous Western Interior Seaway during sea-level lowstand, resulting in an elongate, digitate, laterally restricted geometry consistent with fluvially dominated deltas. Deltaic sediments of the study area are shown as detached from their contemporaneous shoreline, which cannot be located at this time. The Rocky Mountain Association of Geologists

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cyclonic events also possibly contributed to the southward deflection of the delta front (Slingerland & Keen, 1999). Another example of south-deflected, shore-parallel shallow marine delta sediments within the Upper Cretaceous Western Interior Seaway stratigraphic record is given by the Ferron Sandstone Member of the Mancos Shale in southcentral Utah (Fielding, 2010). Deposition of the Po River Delta, Italy is invoked as a modern day analog (Cattaneo et al., 2003). Low-relief, sheet-like pebble lags (Facies 8) that toptruncate coarsening-upward cycles record significant reworking and winnowing of nearshore and coastal marine sediments during relative sea-level transgression (Weimer, 1988; Hwang & Heller, 2002). Such low-relief and consistently widespread deposits represent regional events that effectively truncated underlying strata in a planar to subhorizontal fashion (Martinsen, 2003). Planar erosion of deltaic sediments should result in a sandstone body that thins down depositional dip while thickening up-dip. The Peay Sandstone Member (F500), however, displays a thickness variability along transect A-A’, with an arched area of thinning in the central portions (Lovell Draw, Fig. 7A, 7B) and subsequent thickening (Greybull, Fig. 7A, 7B) down depositional dip. This behavior possibly represents differential erosion resulting from localized tectonic uplift. Sealevel transgressions removed significantly larger amounts of sediment from uplifted locations, while thicker accumulations were preserved in off-flank (up and down depositional dip) areas.

deltas. Coastal alluvial floodplain and channel deposits, as well as a regional sheet-like geometry, suggest deposition in a tidally influenced coastal environment for the Torchlight Sandstone Member (F200) and not delta-front deposition as observed within the Peay Sandstone Member (F500). Sediment dispersal patterns within all members indicate a south-southeastward direction, recording the basin margin-parallel deflection of eastward-dispersed sediments, resulting from a Late Cretaceous counterclockwise gyre within the Western Interior Seaway. ACKNOWLEDGMENTS The authors would like to thank the Mr. & Mrs. J.B. Coffman Endowment in Sedimentary Geology (University of Nebraska-Lincoln) for contribution to field costs; AAPG Grants-in-Aid program for the 2010 Rodney A. Bernasek Memorial Grant; Executive Editor Joyce Trygstad Nelson, Assistant Editor Melissa (Mel) Klinger, Mark A. Kirschbaum, and Lee T. Shannon for their constructive reviews of the submitted manuscript; and Mr. Mark Mathison and the Iowa State University Field Camp, Shell, WY for field work accommodations. REFERENCES Aigner, T., and H.E. Reineck, 1982, Proximality trends in modern storm sands from the Helgoland Bight (North Sea) and their implications for basin analysis: Senckenbergiana Maritima, v. 14, p. 183–215. Bann, K.L., and C.R. Fielding, 2004, An integrated ichnological and sedimentological comparison of non-deltaic shoreface and subaqueous delta deposits in Permian reservoir units of Australia, in D. McIlroy, ed., The application of ichnology to paleoenvironmental and stratigraphic analysis: Geological Society of London Special Publication, v. 228, p. 273–310. Barlow, J.A., and J.D. Haun, 1966, Regional stratigraphy of Frontier Formation and relation to Salt Creek Field, Wyoming: AAPG Bulletin, v. 50, p. 2185–2196. Bhattacharya, J.P., and B.J. Willis, 2001, Lowstand deltas in the Frontier Formation, Powder River Basin, Wyoming: Implications for sequence stratigraphic models: AAPG Bulletin, v. 85, p. 261–294. Bhattacharya, J.P., and J.A. MacEachern, 2009, Hyperpycnal rivers and prodeltaic shelves in the Cretaceous seaway of North America: Journal of Sedimentary Research, v. 79, p. 184–209. Bighorn Basin Assessment Group, 2010, Petroleum systems and the geologic assessment of oil and gas in the Bighorn Basin province, Wyoming and Montana: USGS Digital Data Series, 69-v. Cattaneo, A., A. Correggiari, L. Langone, and F. Trincardi, 2003, The late-Holocene Gargano subaqueous delta, Adriatic Shelf: Sediment pathways and supply fluctuations: Marine Geology, v. 193, p. 61–91.

CONCLUSIONS High-quality, continuous exposure of the Frontier Formation in the northern Bighorn Basin, northwest of Greybull, WY facilitates a full review of Frontier Formation stratigraphy and provides new insight into the depositional history of regionally important hydrocarbon reservoirs. Nine facies are identified (offshore marine to coastal alluvial channel sediments) that, in part, compose several incomplete coarsening-upward cycles present in the vertical succession, recording multiple cycles of delta-front deposition. Facies stacking patterns, sandstone body geometries, and paleocurrent data within the Peay Sandstone Member (F500) reveal the progradation of a gentlysloping, digitate, tide- and wave-influenced, fluvially dominated delta front under conditions of low accommodation. The progradation of elongate bodies into distal portions of the basin suggests deposition during sea-level lowstand; however, persuasive evidence is lacking in the absence of any indication of large-scale, regional incision. Intervals lacking significant accumulations of proximal delta-front facies within the F400, F300/350, and F175 intervals likely represent distal equivalents of prograding 95



Kirschbaum, M.A., and L.N.R. Roberts, 2005, Stratigraphic framework of the Cretaceous Mowry Shale, Frontier Formation and adjacent units, southwestern Wyoming Province, Wyoming, Colorado, and Utah, in USGS Southwestern Wyoming Province Assessment Team, ed., Petroleum systems and geologic assessment of oil and gas in the southwestern Wyoming Province, Wyoming, Colorado, and Utah: USGS, v. 1, p. 1–31. Kirschbaum, M.A., E.A. Merewether, and S.M. Condon, 2009, Stratigraphy and age of the Frontier Formation and associated rocks, central and southern Bighorn Basin, Wyoming–Surface to subsurface correlation: The Mountain Geologist, v. 46, p. 125–147. Kirschbaum, M.A., L.N.R. Roberts, and L.R.H. Biewick, 2000, Geologic assessment of coal in the Colorado Plateau; Arizona, Colorado, New Mexico, and Utah: USGS Professional Paper, p. 1625-B. Martinsen, R.S., 2003, Depositional remnants part 1: Common components of the stratigraphic record with important implications for hydrocarbon exploration and production: AAPG Bulletin, v. 87, p. 1869–1882. Merewether, E.A., and W.A. Cobban, 1986, Biostratigraphic units and tectonism in the mid-Cretaceous foreland of Wyoming, Colorado and adjoining areas, in J.A. Peterson, ed., Paleotectonics and sedimentation in the Rocky Mountain region, United States: AAPG Memoir, v. 41, p. 443–468. Merewether, E.A., M.C. Huff, W.A. Cobban, and G.L. Skipp, 1998, Stratigraphy of marine sandstone in the Upper Cretaceous Frontier Formation, Johnson and Natrona Counties, Wyoming in Cretaceous and Lower Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming Geological Association, 49th Annual Field Conference Guidebook, p. 43–58. Merewether, E.A., P.D. Blackmon, and J.C. Webb, 1984, The midCretaceous Frontier Formation near the Moxa Arch, southwestern Wyoming: USGS Professional Paper, P 1290, 29 p. Merewether, E.A., W.A. Cobban, and E.T. Cavanaugh, 1979, Frontier Formation and equivalent rocks in eastern Wyoming: The Mountain Geologist, v. 16, p. 67–102. Merewether, E.A., W.A. Cobban, and R.W. Tillman, 2010, Outcrops, fossils, geophysical logs, and tectonic interpretations of the Upper Cretaceous Frontier Formation and contiguous strata in the Bighorn Basin, Wyoming and Montana: USGS Scientific Investigations Report, 2009-5256, 49 p. MacEachern, J.A., and K.L. Bann, 2008, The role of ichnology in refining shallow marine facies models, in Hampson, G.J., R.J. Steel, P.M. Burgess, and R.W. Dalrymple, eds., Recent advances in models of siliciclastic shallow-marine stratigraphy: SEPM Special Publication, v. 90, p. 73 –116. MacEachern, J.A., K.L. Bann, J.P. Bhattacharya, and C.D. Howell Jr., 2005, Ichnology of deltas: Organism responses to the dynamic interplay of rivers, waves, storms, and tides, in Giosan, L., and J.P. Bhattacharya, eds., River Deltas–Concepts, Models, and Examples: SEPM Special Publication, v. 83, p. 49–86. Obradovich, J.D., 1993, A Cretaceous time scale, in Caldwell, W.G.E., and E.G. Kaufmann, eds., Evolution of the Western Interior Basin: Geological Association of Canada Special Paper, v. 39, p. 379–396. Slingerland, R., and T.R. Keen, 1999, Sediment transport in the Western Interior Seaway of North America; predictions from a climate-ocean-sediment model, in Bergman, K.M., and Sned-

Clark, C.K., 2010, Stratigraphy, sedimentology, and ichonology of the Upper Cretaceous Frontier Formation in the Alkali Anticline region, Bighorn County, Wyoming: Master’s Thesis, University of Nebraska-Lincoln, Lincoln, Nebraska, 57 p. Cobban, W.A., 1990, Ammonites and some characteristic bivalves from the Upper Cretaceous Frontier Formation, Natrona County, Wyoming: USGS Bulletin, 1917-B, p. 1–15. Cobban, W.A., and J.B. Reeside, Jr., 1952, Frontier Formation, Wyoming and adjacent areas: AAPG Bulletin, v. 36, p. 1913–1961. Cobban, W.A., E.A. Merewether, T.D. Fouch, and J.D. Obradovich, 1994, Some Cretaceous shorelines in the Western Interior of the United States, in M.V. Caputo, J.A. Peterson, and K.J. Franczyk, eds., Mesozoic systems of the Rocky Mountain region, USA: SEPM Rocky Mountain Section, p. 393–414. Cobban, W.A., I. Walaszczyk, J.D. Obradovich, and K.C. McKinney, 2006, A USGS zonal table for the Upper Cretaceous Middle Cenomanian-Maastrichtian of the Western Interior of the Uinted States based on ammonites, inoceramids, and radiometric ages: United States Geological Survey Open-File Report 2006-1250, p. 1–45. Dumas, S., R.W.C. Arnott, and J.B. Southard, 2005, Experiments on oscillatory-flow and combined-flow bed forms: Implications for interpreting parts of the shallow-marine sedimentary record: Journal of Sedimentary Research, v. 75, p. 501–513. Fielding, C.R., 2010, Planform and facies variability in asymmetric deltas: Facies analysis and depositional architecture of the Turonian Ferron Sandstone in the western Henry Mountains, south-central Utah, USA: Journal of Sedimentary Research, v. 80, p. 455–479. Forbes, D.L., and R. Boyd, 1987, Gravel ripples on the Scotian Shelf: Journal of Sedimentary Research, v. 57, p. 46–54. Gani, M.R., and J.P. Bhattacharya, 2007, Basic building blocks and process variability of a Cretaceous delta: Internal facies architecture reveals a more dynamic interaction of river, wave, and tidal processes than is indicated by external shape: Journal of Sedimentary Research, v. 77, p. 284–302. Gani, M.R., J.P. Bhattacharya, and J.A. MacEachern, 2007, Using ichnology to determine relative influence of waves, storms, tides, and rivers in deltaic deposits: Examples from Cretaceous Western Interior Seaway, USA, in MacEachern, J.A., K.L. Bann, M.K. Gingras, and G. Pemberton, eds., Applied Ichnology: Society of Economic Petrologists and Mineralogists, Short Course Notes, v. 52, p. 209–227. Gingras, M.K., J.A. MacEachern, and S.E. Dastgard, 2011, Process ichnology and the elucidation of physico-chemical stress: Sedimentary Geology, v. 237, p. 115–134. Haas, O., 1949, Acanthoceratic Ammonoidea from near Greybull, Wyoming: American Museum of Natural History, v. 9, p. 7–38. Hintze, F. F., 1915, The Basin and Greybull oil and gas fields: Wyoming Geological Survey Bulletin, v. 10, 62 p. Hutsky, A.J., 2011, Stratigraphic analysis and regional correlation of isolated, top-truncated shallow marine sandstone bodies within the Upper Cretaceous Frontier Formation, Bighorn and Washakie counties, Wyoming: Master’s Thesis, University of Nebraska-Lincoln, Lincoln, Nebraska, 99 p. Hwang, I.G., and P.L. Heller, 2002, Anatomy of a transgressive lag: Panther Tongue Sandstone, Star Point Formation, central Utah: Sedimentology, v. 49, p. 977–999.


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Vakarelov, B.K., J.P. Bhattacharya, and D.D. Nebrigic, 2006, Importance of high-frequency tectonic sequences during greenhouse times of Earth history: Geology, v. 34, p. 797–800. Wegemann, C.H., 1912, The Powder River oil field, Wyoming: USGS Bulletin, B 0471-A, p. 56–75. Weimer, R.J., 1988, Record of relative sea-level changes, Cretaceous of the Western Interior, USA, in Wilgus, C.K., ed., Sealevel changes, an integrated approach: Society of Sedimentary Geology Special Publication, v. 42, p. 285–288. Willis, B.J., J.P. Bhattacharya, S.L. Gabel., and C.D. White, 1999, Architecture of a tide-influenced river delta in the Frontier Formation of central Wyoming, USA: Sedimentology, v. 46, p. 667–688. Winn, R.D., Jr., 1991, Storm deposition in marine sand sheets: Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming: Journal of Sedimentary Petrology, v. 64, p. 86–101.

den, J.W., eds., Isolated shallow marine sand bodies; sequence stratigraphic analysis and sedimentologic interpretation: SEPM Special Publication, v. 64, p. 179–190. Slingerland, R., L.R. Kump, M.A. Arthur, P.J. Fawcett, B.B. Sageman, and E.J. Barron, 1996, Estuarine circulation in the Turonian Western Interior Seaway of North America: GSA Bulletin, v.108, p. 941–952. Southard, J.B., J.M. Lambie, D.C. Federico, H.T. Pile, and C.R. Weidman, 1990, Experiments on bed configurations in fine sands under bidirectional purely oscillatory flow, and the origin of hummocky cross-stratification: Journal of Sedimentary Research, v. 60, p. 1–17. Tillman, R.W., and W.R. Almon, 1979, Diagenesis of Frontier Formation offshore bar sandstones, Spearhead Ranch Field, Wyoming, in Tillman, R.W., and C.T. Siemers, eds., Siliciclastic shelf sediments: Tulsa Society of Economic Paleontologists and Mineralogists Special Publication, v. 34, p. 85–142. Vakarelov, B.K., and J.P. Bhattacharya, 2009, Local tectonic control on parasequence architecture: Second Frontier sandstone, Powder River Basin, Wyoming: AAPG Bulletin, v. 93, p. 295–327.

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THE AUTHORS

ANDREW HUTSKY ANDREW HUTSKY is a doctoral student in Geology at the University of Nebraska-Lincoln. He holds an M.S. degree in Geology from UNL and a B.S. degree in geology from the University of Pittsburgh-Johnstown, PA. His current research interests involve the sedimentology, stratigraphy, and ichnology of coastal and shallow-marine successions of the Cretaceous Western Interior Seaway.

CHRIS FIELDING CHRIS FIELDING holds the Mr. & Mrs. J.B. Coffman Chair in Sedimentary Geology at the University of Nebraska-Lincoln. He received his Ph.D. from the University of Durham (United Kingdom) in 1982 and previously worked for BP Exploration and the University of Queensland in Brisbane, Australia. He has served as President of SEPM (2011–2012). His research interests lie in the stratigraphy of continental, coastal and shallow-marine successions.

TREVOR HURD TREVOR HURD holds an M.S. degree in Geology from the University of Nebraska-Lincoln and a B.S. degree in Geology from Florida State University. His research focused on the sedimentology and stratigraphy of shallow marine deltaic depositional systems of the Cretaceous Western Interior Seaway and their application to resource exploration. He is currently employed by Chesapeake Energy Corporation in Oklahoma City, OK.

CHARLES KITTINGER CLARK CHARLES “KIT” CLARK holds an M.S. degree in Geology from the University of Nebraska-Lincoln and a B.S. degree in Geology from Colby College, ME. He currently works for Anadarko Petroleum Corporation in Houston, TX.


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