Deltaic process and architectural evolution during cross-shelf transits, Maastrichtian Fox Hills Formation, Washakie Basin, Wyoming Mariana I. Olariu, Cristian R. Carvajal, Cornel Olariu, and Ronald J. Steel
ABSTRACT The topset compartments of two Maastrichtian basin-scale clinothems are characterized, with focus on the function they played in constructing the Lance–Fox Hills–Lewis shelf-margin sedimentary prism in the Laramide Washakie Basin, south Wyoming. Approximately 1000 well logs were used to map the delta lobes and complexes on the Fox Hills shelf and to detail their depositional character, dimensions, and orientation as they autogenically shifted during transit from an inner-shelf to shelf-edge position. The regressive transits of the deltas initiated up to 40 km (25 mi) landward from the preexisting shelfedge and preserved river and wave-dominated deltaic deposits that thicken and concentrate sand on the outer shelf. Tidally influenced deltas (now outcropping) also occur in localized areas along the paleoshelf edge, probably where wave influence was reduced along invaginated coastal segments. Net sandstone maps of the individual clinothem topsets show that (1) coeval delta lobes exist within each clinothem, suggesting multiple rivers; (2) delta lobes have a likely autogenic compensational stacking pattern; and (3) deltas thicken and storm-wave influence become dominant closer to the shelf edge. Our results support the ideas of (1) predictable increased wave influence and (2) change to strike-elongate architecture as deltas transit the shelf. In addition, along-strike changes in process dominance cause deltaic reservoirs to be highly variable in their orientation, external shape, and internal character.
Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved. Manuscript received August 9, 2011; provisional acceptance November 8, 2011; revised manuscript received February 29, 2012; final acceptance March 26, 2012. DOI:10.1306/03261211119
AAPG Bulletin, v. 96, no. 10 (October 2012), pp. 1931–1956
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AUTHORS Mariana I. Olariu Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas;
[email protected] Mariana I. Olariu is a Research Associate with the Jackson School of Geosciences at the University of Texas at Austin. She received her B.S. degree in 1995 from the University of Bucharest, Romania. She holds a Ph.D. in geosciences from the University of Texas at Dallas. Her research interests include clastic sedimentology and sequence stratigraphy, with a special interest in the architecture of shallowand deep-water sedimentary systems. Cristian R. Carvajal Energy Technology Company, Chevron Corporation, 1500 Louisiana, Houston, Texas;
[email protected] Cristian R. Carvajal is a geologist working with Chevron. He has B.S. (Universidad de Chile) and M.S. (Loma Linda University, California) degrees and a Ph.D. (University of Texas at Austin) in geology. Carvajal has worked on continental to deep-water strata from the Gulf of Mexico, offshore Angola, United States, and Peru. His main interest is on shelf-margin stratigraphy applied to reservoir prediction and characterization. Cornel Olariu Geo EcoMar, Bucharest, Romania; present address: Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas;
[email protected] Cornel Olariu is a research associate at the Jackson School of Geosciences, University of Texas at Austin. He earned a geology engineering degree in 1995 from the University of Bucharest, Romania. He received his M.S. degree in 2002 and his Ph.D. in 2005 both in geologic sciences from the University of Texas at Dallas. He is interested in clastic sedimentology, with focus on shallow-water depositional systems and more specifically on the mechanisms of sediment transfer from shallow to deep water. Ronald J. Steel Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas;
[email protected] Ronald J. Steel is professor and Davis Centennial Chair, as well as department chair,
at the Jackson School of Geosciences, University of Texas at Austin. He received his B.Sc. degree in 1967 and his Ph.D. in 1971 from the University of Glasgow, United Kingdom. His research is aimed primarily at using clastic sedimentology to address problems in basin analysis and, particularly, to decipher the signatures of tectonics, sea level change, climate, and sediment supply in stratigraphic successions.
Some process changes are interpreted to be autogenic responses during overall shoreline progradation. The study also provides new data on delta-lobe and delta-complex thicknesses as well as on deltaic coastline versus shelf-edge progradation rates.
ACKNOWLEDGEMENTS
Continental shelf successions are commonly built from the repeated cross-shelf transits of shorelines (mainly regressivetransgressive transits of deltas and estuaries) (Morton and Suter, 1996; Galloway, 2001; Steel and Olsen, 2002; Pratson et al., 2007; Steel et al., 2008), a process influenced mainly by relative sea level change, sediment flux, and autogenic responses of the delta complex to the forcing of these external drivers (Muto and Steel, 2002; Muto et al., 2007). The shoreline separates nonmarine delta plain and coastal plain from marine delta front or shoreface and shelf reaches of the system. Repeated transgressions and regressions of the shoreline, commonly on a time scale of less than 100 k.y. (Burgess and Hovius, 1998; Muto and Steel, 2002), produce basinward, lateral, and vertical migration of these facies belts, causing an overall aggrading and basinward-directed sedimentary succession, the shelf-margin prism. Basinal processes, such as tides and waves, additionally cause a significant influence on the river output at short time scales (Olariu and Steel, in press) and on the along-shelf sediment dispersal (Swift et al., 1991), including the development of muddy subaqueous clinoforms beyond the shoreline clinoform of many large deltas (Kuehl et al., 1986; Nittrouer et al., 1986). Many of these mud belts appear to have been built by oblique along-shore transport on the shelf (Driscoll and Karner, 1999; Slingerland et al., 2008). The shelf width and gradient, delta-front and coastalplain gradients, sea level behavior, and sediment flux will affect the way the shoreline reaches the shelf edge (Burgess and Hovius, 1998; Muto and Steel, 2002). Previous studies of longer term growth of shelves and shelf sedimentary prisms confirm that this happens primarily by delivery of sediment by currents, plumes, and underflows from deltas (Bates, 1953; Wright, 1977), although wave-, tide-, and wind-driven currents become increasingly important as this sediment arrives on the shelf. In the current study of Late Cretaceous deltaic coastlines and their long-term behavior for shelf-prism growth, several aspects of process change will be considered. Along-strike variability of process dominance is well known on modern deltas (Bhattacharya and Giosan, 2003), as are general
We thank RIOMAR sponsor companies (British Petroleum, Broken Hill Proprietry (BHP) Billiton, Shell, Statoil, Petróleos de Venezuela, S.A. (PDVSA), Conoco-Phillips, Devon, Woodside, Ente Nazionale Idrocarburi (Eni), Petrobras, British Gas (BG)Group) for their generous support of this research and company participants for their lively discussion and input. We also thank Janok Bhattacharya, Dale Leckie, and Randi Martinsen for their thoughtful suggestions, which helped improve the manuscript. We thank John Espy and his family and Gene Carrico for permission to access their property and do outcrop field work. The AAPG editor thanks the following reviewers for their work on this paper: Janok P. Bhattacharya, Dale A. Leckie, and Randi Martinsen.
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INTRODUCTION Shoreline and Shelf Growth
Deltaic Cross-Shelf Transits, Fox Hills Deltas, Washakie Basin
process changes on coastlines through time, with changing large-scale controls (Galloway, 2001). Change in the dominant process on deltas during cross-shelf transit is not well documented but has been proposed by Porebski and Steel (2006) and Yoshida et al. (2007)—an increase in storm-wave influence on deltas as they approach the shelf edge and encounter open ocean waves.
Objectives of Present Study The Maastrichtian Lance–Fox Hills–Lewis succession in southern Wyoming represents large-scale shoreline regression associated with the final exit of marine waters from the Western Interior of the United States (Winn et al., 1987; Pyles, 2000). A deep-water shelf margin was produced in the Washakie Basin because Laramide tectonics had created a basin that deepened more abruptly than it could be infilled, resulting in more than 500 m (>1640 ft) of water depth, as well as thick and extensive submarine fan deposits on the basin floor (Pyles, 2000; Carvajal and Steel, 2012). The presence of bathyal water depths at this time is in sharp contrast to the previous, very shallow water Late Cretaceous foreland basin (DeCelles, 2004). During the Maastrichtian, however, the Wind River Range, Granite Mountains, and Rawlins uplift underwent significant thick-skinned uplift, causing an abrupt tectonic subsidence of the adjacent basin (Reynolds 1976; Steidtmann and Middleton, 1991; Pyles and Slatt, 2002; Carvajal, 2007; Pyles and Slatt, 2007). Maastrichtian infilling of the Washakie Basin was initially outpaced by subsidence, resulting in the development of a deep-water and an accreting east-west–oriented shelf margin with a clear topset, slope, and basin-floor morphology (Figure 1). Carvajal (2007) documented a three-dimensional (3-D) tracking of 16 basinwide clinoforms throughout this linked fluvial to shelf to deep-marine depositional system. A mapped tie between ammonitebearing outcrop strata (Gill et al., 1970) and the subsurface clinothems provide an approximate 1.8-m.y. duration for the succession and an average clinothem duration of some 100 k.y. (Carvajal and Steel, 2009).
This study provides details of the Lewis–Fox Hills shelf and shelf-margin architecture within two of these clinothems (clinothems 9 and 10 of Carvajal et al., 2009) in terms of the dominant processes and sediment partitioning between depositional environments from shoreline to shelfslope break. Emphasis is placed on the delta complexes of the Fox Hills Formation through an analysis of the facies, dimensions, and orientation of delta lobes as they autogenically shifted during cross-shelf transit. Documented rates of movement of delta lobes and changes in their architecture as they reached the shelf edge as well as identification of exit points at the shelf edge through which sediment was delivered aid an understanding of how sediment was supplied to the slope and basin-floor reaches of clinothems.
GEOLOGIC SETTING During the early Maastrichtian, the Washakie Basin of southern Wyoming (later subdivided into Washakie and Great Divide basins by the Wamsutter arch; Figure 1) formed a sediment sink in response to Laramide orogenic movements along the Wind River Range, Granite Mountains, and Rawlins uplift (Dickinson et al., 1988; Steidtmann and Middleton, 1991). The Washakie Basin filled under greenhouse climate conditions (Miller et al., 2004), high subsidence rates (Hagen et al., 1985; Flemings et al., 1986; Shuster and Steidtmann, 1988), and high sediment supply conditions (Carvajal and Steel, 2006, 2012; Carvajal et al., 2009) during an approximate 1.8-m.y. interval. The Maastrichtian Basin was filled by a fluvial (Lance Formation) to shelf (Fox Hills Formation) to deep-marine (Lewis Shale) depositional system (Love and Christiansen, 1985; Winn et al., 1987; Perman, 1990; McMillen and Winn, 1991). The Lance Formation, more than 200 m (>656 ft) thick in the Rock Springs uplift, is a coal-bearing, paralic to alluvial plain succession (Steidtmann, 1993). The Fox Hills Formation represents a sand-prone shoreline-to-shelf succession and is up to 214 m (702 ft) thick (Gill et al., 1970; Steidtmann, 1993). The Olariu et al.
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Figure 1. Geologic map of the Washakie and Great Divide basins in southern Wyoming (modified from Carvajal and Steel, 2009). (A) Geologic setting with outcrop exposures, well locations (∼1000), and cross sections. (B) A two-dimensional tracking of individual fourth-order cycles through the linked fluvial to shelf to deep-marine depositional system of the Lance Formation (green), Fox Hills Formation (yellow), and Lewis Shale (mainly white, but with yellow submarine fans) is displayed on the NS cross section. MFS = maximum flooding surface; GR = gamma ray; SP = spontaneous potential; N = North; S = South; ID = Idaho; MT = Montana; WY = Wyoming; SD = South Dakota; NE = Nebraska; CO = Colorado; UT = Utah. 1934
Deltaic Cross-Shelf Transits, Fox Hills Deltas, Washakie Basin
Lewis Shale is a slope to basin-floor succession (Pyles, 2000; Pyles and Slatt, 2000, 2007) and contains deep-water mudstones and turbidite sandstones (referred as the Dad Sandstone Member) in successions up to 762 m (2500 ft) thick (Winn et al., 1987). This lithostratigraphic succession is crossed by a series of 16 chronostratigraphic units or clinothems (Figure 1), each with a topset, slope, and basinfloor component (Carvajal and Steel, 2006), such as would be recorded by a series of seismic reflectors cutting obliquely through the stratigraphy. Sourced mainly from the north, the shelf-margin clinoforms prograded southward at a high average rate (>48 km/ m.y. [>30 mi/m.y.]) and filled the basin with an accumulation rate of 267 m/m.y. (876 ft/m.y.) (Carvajal et al., 2009). The clinoforms have undecompacted amplitudes of approximately 430 m (∼1411 ft). With inclinations of less than 1 to 2°, the slope height (undecompacted) provides a minimum estimate for basinal water depth from shelf edge to basin floor. Sediment volumes and clinothem architecture suggest a two-stage tectonostratigraphic model for basin development and infill (Carvajal and Steel, 2012) (Figure 1). During the first stage of 1 m.y., clinothems C1 to C9 display an overall aggradational architecture indicated by thick clinothem topsets and increasing clinothem amplitude. This architecture is interpreted to result from high and increasing subsidence rates and high rates of relative sea level rise producing progressively greater accommodation and, combined with high supply, generated greater sediment volumes through time and, consequently, greater total clinothem volumes (Carvajal and Steel, 2012). Lower and decreasing rates of thrusting and reduced loading would have produced decreased subsidence rates and a diminishing rate of rise of sea level, resulting in smaller total clinothem volumes during the second stage (a span of ∼0.55 m.y.) (Carvajal, 2007). Clinothems C10 to C15 are markedly progradational and thinner, with more extensive terrestrial topsets, and display fairly constant to slightly increasing clinothem amplitudes (Figure 1). At the shelf margin, the locus of the shelf edge exhibits an alternation of flat and more steeply rising trajectories formed by repeated increments of margin accretion on an approximately 100-k.y. scale (Figure 1).
Before the subsurface mapping and interpretation of clinoforms 9 and 10 are made, outcrops of clinothems 9 and 10 along the western and eastern edges of the present basin will be briefly described.
Depositional Environments: Clinothem 9 Outcrops Large outcrop exposures of clinothem 9 along the western reaches of the basin margin display delta, estuary, and coal-swamp deposits (Land, 1972; Carvajal and Steel, 2009). Calibration of outcropping clinothem 9 (west side of basin in Figure 1) with subsurface mapped clinoform 9 was made by Carvajal (2007) on the basis of subsurface to surface correlations. This same calibration allowed the subsurface clinoforms to be positioned within the Maastrichtian ammonite zonation strata just below clinothem 9, which correlates in both basin margins with Baculites clinolobatus (Carvajal, 2007; see his figure 2.4). The lower half of the clinothem 9 succession exposes prominent progradational to aggradational parasequences of deltaic sandstones (Figure 2A) that contain clear evidence (symmetrical ripples, hummocky cross-stratification, swaly and low-angle laminations) of strong storm-wave influence (Land, 1972; Carvajal and Steel, 2009). The highly aggradational stacking pattern of the 60-m (197-ft)–thick western deltas suggests a significant relative sea level rise during accumulation. In the upper half of clinoform 9, the wave-dominated deltaic deposits are overlain by tidally influenced sandy estuarine deposits and some coaly mudstones (Carvajal and Steel, 2009), a change suggesting the onset of transgression across clinothem 9. Thin marine mudstones mark the climax of transgression across this clinothem so that the entire clinothem succession records an evolution from aggradational regression to transgression. A tie between these western outcrops of clinoform 9 and the nearby downdip wells (Carvajal and Steel, 2009, their figure 3) shows that the outcrops are some 10 km (6 mi) from the shelf-edge area. Along the eastern side of the basin (Figure 1), outcrop exposures of approximately 2 km (∼1 mi) Olariu et al.
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1936 Deltaic Cross-Shelf Transits, Fox Hills Deltas, Washakie Basin Figure 2. Facies and depositional environments in clinothem 9 outcrop. (A) Large outcrop exposures along the western reaches of the basin (Figure 1) display aggradational units of deltaic sandstone, with evidence of dominating storm-wave processes. (B) Outcrop exposure along the eastern side of basin (Figure 1) displays two upward-coarsening and thickening successions, indicating an upward shallowing during two stages of delta-lobe progradation. Vf = very fine; Fl = fine lower; Fu = fine upper; M = medium grain size.
display two upward-coarsening and thickening parasequences indicating upward shallowing during two stages of delta-lobe progradation (Figure 2B). The delta deposits show signs of both fluvial and stormwave influence. Storm-wave influence on the deltaic succession is recognized by the abundant waveripple lamination, occurrence of prominent hummocky and swaly cross-stratification on the delta front, and the clean character of the sediment. Fluvial influence is suggested by low marine trace-fossil abundance and diversity (Arenicolites, Skolithos, and Ophiomorpha) and the presence of sharp-based turbidite beds formed during the river floods. The presence of small-scale soft-sediment deformation structures (convolute lamination and slumped intervals) indicates abrupt deposition of sandstone and/or oversteepening of a fluvial and wave-dominated delta front. The sediment seems to be deformed in place without a long sliding distance (transport). Depositional Environment: Clinothem 10 Outcrop Large continuous outcrops of clinothem 10 on the eastern side of the basin exhibit significant changes in the dominant depositional processes over surprisingly short distances (3 km [2 mi]) as the deltas autogenically shifted (Olariu and Steel, in press) from fluvially to tidally dominated to tidally influenced shelf-edge deltas with significant soft-sediment deformation (Carvajal and Steel, 2009). The deposits are thought to be autogenic in nature (Muto et al., 2007) because they developed within the same parasequence under likely steady allogenic forcing (sea level, tectonics) during its progradation (Olariu and Steel, in press). The proximal reaches of the outcrop display fluvially dominated delta-front deposits with foresets that dip basinward at steep angles (up to 10–15°) (Figure 3A). The steepness is greater than some typical fluvially dominated deltas (Plink-Björklund, 2008; Enge and Howell, 2010; Olariu et al., 2010) and may be caused by a greater water depth in this near–shelfedge locality. The fluvially dominated delta-front deposits represented by planar-laminated sandstone beds pass southward (basinward) into coarser grained, but muddier strata. Heterolithic bedding, fluid-mud
drapes, and bidirectional dune foresets (Figure 3B) suggest a tidally influenced delta-front environment (Carvajal and Steel, 2009). Along the length of the outcrop, a thick (up to 12 m [39 ft]), trough crossstratified sandstone unit erosionally cuts into underlying delta foreset strata. The trough cross strata are mostly oriented basinward (south-southwest), with subordinate foresets dipping landward. Locally, organic matter drapes the foresets. At some outcrop locations, logs of fossil wood pervasively burrowed by Teredolites exist. The vertical juxtaposition of the tidally influenced deposits below the erosional fluvial channel and their gradual transition to the riverdominated delta-front facies indicates that these deposits represent a delta-front environment that interfingered with the river-dominated delta topset (delta plain). The southern reaches of the outcrop display spectacular, several-meters–thick folded and slumped intervals of trough cross strata. The erosively based cross-stratified sandstone that overlies the delta-front deposits is interpreted as a fluvial channel. The erosive features are formed by fluvial channel incision during normal regression such as the features observed in the modern Mississippi delta (Nittrouer et al., 2011). The large-scale deformation possibly indicates outershelf slope instability and nearness to the shelf-edge rollover or the entry of the river channel to a canyonhead reentrant (Carvajal and Steel, 2009). Paleocurrent directions measured on foresets indicate a progradation of the channel system toward the southsouthwest. Subordinate mud drapes and landwarddipping foresets (north-northeast) point to likely influence by tidal currents and a brackish setting close to the ocean, also supported by the Teredolites tree borings. The southernmost reaches of the outcrop, close to the shelf edge, exhibit two upward-coarsening and thickening parasequences (Figure 3C). The lower part of each parasequence displays a heterolithic interval with thin-bedded, very fine sandstones (∼10 cm [∼4 in.] thick), with wave ripples and parallel lamination interbedded with mudstones. The sandstones are continuous with local and minor variations in thickness. Locally deformed strata are also present. The upper part of each parasequence is dominated by plane-parallel-laminated Olariu et al.
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1938 Deltaic Cross-Shelf Transits, Fox Hills Deltas, Washakie Basin
Figure 3. Facies and depositional environments of clinothem 10 outcrop on the eastern side of the basin. (A) Proximal reaches of the outcrop display fluvially dominated delta-front deposits, with foresets that dip basinward at steep angles. (B) The fluvially dominated delta-front deposits pass basinward into coarser grained but muddier strata. Heterolithic bedding, mud drapes, and bidirectional dune foresets suggest a tidally influenced delta-front environment. (C) Distal reaches of the outcrop (close to the shelf edge) preserve two upward-coarsening and thickening successions with thin-bedded wavy sandstones and hummocky cross-stratified amalgamated sandstones, suggesting a wave-dominated delta-front environment. See Figure 2 for key to lithologic symbols. Vf = very-fine; FL = fine lower; FU = fine upper; M = medium grain size.
and hummocky cross-stratified, fine-grained thicker amalgamated sandstones. Low-angle lamination occurs in places; some sandstone beds are massive. Bioturbation is generally low with a low-trace fossil diversity (Skolithos, Cylindrichnus, Rosselia, and Ophiomorpha). The heterolithic interval is interpreted as lower delta front below fair-weather base as indicated by the preservation of mud, but above storm-wave base as suggested by the presence of hummocks. Soft-sediment deformation suggests sedimentary loading caused by deposition (high sedimentation rates) along a steep wave-dominated delta front. The amalgamated sandstones in the upper interval represent an upper delta-front environment where sustained wave erosion prevented mud preservation. Upward-coarsening and thickening successions record the last pulses of delta progradation before approaching the shelf-edge area, some 10 to 15 km (6–9 mi) away (Carvajal and Steel, 2009, their figure 5).
METHODOLOGY The topset compartments of clinothems C9 and C10 are described using subsurface (well-log) data. Each clinoform, in the sense described by Carvajal (2007), is referred to, for convenience, as a fourthorder unit because of the approximately 100-k.y. scale for the formation of a transgressive-regressive cycle as discussed above. Fourth-order regressive deposits occur from the maximum flooding interval in the basal clinothem shale upward to the first transgressive surface, marking the interval of basinward migration of the shoreline, whereas fourthorder transgressive deposits occur between the first transgressive surface and the thin shale (again, maximum flooding interval) bounding the top of the clinothem. The regressive part consists mainly of alluvial plain, coastal plain, and deltaic and shelf deposits, whereas the transgressive part is made up of estuarine, lagoonal, and barrier bar and shelf deposits. The regressive intervals of fourth-order cycles are further subdivided into fifth-order units that are bounded by marine flooding surfaces and are simply referred to as parasequences. Each parasequence might represent several laterally offset,
coeval, spatially distinct delta lobes. The coeval delta lobes might be formed by the same river or distinct rivers. Approximately 1000 wells, mostly with gammaray logs, and also spontaneous potential and conductivity logs, have been used for basinwide subsurface correlations. The coastal plain reaches are characterized by serrated, blocky, and bell-shaped (fining-upward) log motifs at small (meter) scale, which is typical for heterolithic and coaly floodplain deposits dissected by sandy fluvial channels. The prograding delta-front deposits are characterized by funnel shapes on gamma-ray logs (upward-coarsening and thickening units), sometimes with more blocky or bell-shaped log motifs toward the top when they are truncated by their own fluvial distributaries. Deltaic parasequences are separated by high-frequency (fifthorder) flooding surfaces marked by muddy sediments (high gamma-ray counts) of local to subregional extent (Figure 4). Mapping of the fifth-order cycles is accomplished through the identification of individual parasequences within each well log. Each coarseningupward unit is topped by mud (fifth-order flooding surface) unless the units are amalgamated (Figure 5). These flooding surfaces can be traced laterally to the surrounding wells and their correlation is done in 3-D. Correlation of closely spaced (as close as 500 m [1640 ft]) well logs allowed identification of the regressive transits and the shifts of individual delta lobes (parasequences). This migration and shifting of parasequences describes the behavior of the shoreline trajectory over short time scales (Helland-Hansen and Hampson, 2009) and, on longer time scales, eventually the aggradation and progadation pattern of individual deltaic complexes (the fourth-order clinothems). On a yet longer time scale, the pathway of the point of maximum regression for a series of fourth-order clinoforms describes the trajectory of the shelf edge during the infilling of the deepwater basin. The condensed section (Asquith shale) near the base of the Lewis Shale is used as a datum (prominent volcanic-ash marker on logs—high gamma-ray values). This shale, a regionally recognizable maximum flooding surface within the lower Lewis, can be correlated throughout the Great Divide and Olariu et al.
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Figure 4. Description and interpretation of log facies. Gamma-ray (GR) logs have been normalized, and a cutoff value of 10 to 90 was chosen to define sandstone lithology. Fourth-order maximum flooding surfaces (MFS 9 and 10) are displayed in black. Deltaic parasequences are separated by high-frequency (fifth-order) flooding surfaces (dashed lines) marked by muddy sediments (high GR counts). Gamma-ray profiles display an upward-coarsening pattern (funnel shape) characteristic of deltaic parasequences. The bell shape or blocky pattern (fining-upward) represents heterolithic and coaly floodplain deposits dissected by sandy fluvial channels. An undifferentiated log pattern with high GR counts locally interrupted by bell-shaped (fining-upward) log motifs is characteristic of a muddy slope cut by channels.
Washakie basins and represents the maximum extent of the Lewis Sea (McMillen and Winn, 1991; Pyles and Slatt, 2000; Carvajal and Steel, 2006). Above the Asquith marker, the overlying sediments of the Lewis, Fox Hills, and Lance formations accomplished the final infilling of this last phase of the Cretaceous intracratonic seaway. Estimating lithologies (sandstone, shale, and coal) in each clinothem compartment requires log normalization and cutoff log values for each lithol1940
ogy. Gamma-ray logs have been normalized according to a type of gamma-ray curve (with 90 API for sandstone-shale cutoff and 10 API for sandstone-coal cutoff) and inspected to check that cutoff values adequately separate sandstone, shale, and coal through the log. Coals have been indicated by very low gamma-ray values (
