shelf-edge architecture and bypass of sand to ... - GeoScienceWorld

Journal of Sedimentary Research, 2009, v. 79, 652–672 Perspectives DOI: 10.2110/jsr.2009.074

SHELF-EDGE ARCHITECTURE AND BYPASS OF SAND TO DEEP WATER: INFLUENCE OF SHELF-EDGE PROCESSES, SEA LEVEL, AND SEDIMENT SUPPLY CRISTIAN CARVAJAL* AND RON STEEL Jackson School of Geosciences, University of Texas, Austin, Texas 78713, U.S.A. e-mail: [email protected]

ABSTRACT: Analysis of two Maastrichtian depositional cycles from the Lewis–Fox Hills shelf margin (S. Wyoming, U.S.A.) demonstrates that rivers played the driving role in the bypass of sand to deep water. Waves and tides were of less importance, but they had a significant influence on the relative storage and architecture of sand along the shelf edge, which in turn appear to have been driven by relative sea level and sediment supply. Clinothem 9 shows main sandstone accumulations in an extensive deltaic sandstone belt along the shelf edge and two deepwater fans. Along the deltaic sandstone belt, river channels fed sand down to deep-water areas, whereas shelf-edge segments with storm-wave domination and tidal influence did not produce channels, but passed basinwards to muddy, turbidite-starved slopes. The wave and tidal regimes, however, likely produced an eastward along-shore drift that fed sediment along the shelf edge and possibly occasionally to deep water via canyon heads. Aggradation (50 m) of the storm-wave deltas indicates a rising relative sea level, which likely favored strong wave influence by allowing open-ocean swells to shape the aggrading, shelf-edge sediment masses. Aggradation also suggests a high sediment supply for deltas to have reached the shelf edge, to have aggraded there, and to have bypassed large volumes of sand to deep water. Clinothem 10 shows main sandstone accumulations to occur in an incised shelf-edge area with a linked, large deep-water fan. The former exhibits characteristic flat-laminated sandstones (hyperpycnal flows?) and tidally influenced facies in the delta front that are cut by a prominent fluvial incision. Fluvial and overlying tidally influenced estuarine deposits infill the incision, indicating that it is a valley, probably cut during relative-sea-level fall. Outside the valley the shelf edge lacks a continuous and thick sandstone belt, suggesting that wave influence along this shelf edge was relatively weak. A falling relative sea level and wide river incision would have interrupted the otherwise common littoral drift and would have diminished wave influence. At the same time the incised segments of the shelf edge would have provided confined areas for tidal-current amplification and for focused, efficient downslope delivery of sand.

INTRODUCTION

The presence of deltas at the shelf edge is one of the most favorable scenarios to allow bypass of significant volumes of sand from the shelf to the deep-water slope and basin floor (Edwards 1981; Suter et al. 1987; Pore˛bski and Steel 2003). In this scenario, fluvial processes in the deltas have been emphasized because most modern and ancient deep-water fans are clearly linked to rivers (Kolla and Perlmutter 1993; Flood and Piper 1997; Posamentier and Allen 1999; Plink-Bjo¨rklund and Steel 2004; Johannessen and Steel 2005; Petter and Steel 2006). However, waves and tides also influence shelf-edge deltas (Cummings et al. 2006; Uroza and Steel 2008), although we know much less about how they interact with river processes, or how they may buffer or enhance the delivery of sand to deep-water areas by the shelf-edge river. Understanding this process interaction is important, because it contributes to better characterize, and thus predict, the overall architecture of the linked shelf-edge to deepwater system and the timing and location of sand bypass to the slope and basin floor. Furthermore a proper characterization provides the * Present address: Energy Technology Company, Chevron Corporation, 1500 Louisiana, Houston, Texas, 77002, U.S.A.

Copyright E 2009, SEPM (Society for Sedimentary Geology)

foundation to evaluate the relative role of allogenic variables driving processes and shelf-margin growth such as sediment supply and relative sea level. The former is often under-evaluated or ignored. Nonetheless, as intimately linked to river discharge, sediment supply contributes to enhance fluvial processes and is often critical to the formation of highstand shelf-edge deltas (Uroza and Steel 2008), to induce vigorous shelf-margin growth, and to bypass large volumes of sand to deep water (see Wetzel 1993; Carvajal and Steel 2006). The sea-level ‘‘drive’’ producing sand bypass in conventional sequence stratigraphy (Posamentier et al. 1988), is much less reliant on high sediment supply, but also influences processes during the shelf transits of deltas and during their time at the shelf edge (Pore˛bski and Steel 2003; Yoshida et al. 2007; Ainsworth et al. 2008; Ponteń and Plink-Bjo¨rklund 2009). In this paper we aim at documenting wave-, tide-, and river-influenced facies in shelf-edge deltas and estuaries, and we explore their influence on shaping the architecture of the shelf edge and on sand bypass to deep-water areas. In addition, we evaluate the influence of relative sea level and of sediment supply on such processes, architecture, and bypass. We address the problem through the Maastrichtian Lewis–Fox Hills shelf margin in the Washakie and Great Divide basins of southern Wyoming.

1527-1404/09/079-652/$03.00

JSR

INFLUENCE OF SUPPLY, SEA LEVEL, AND SHELF-EDGE PROCESSES ON DEEP-WATER BYPASS GEOLOGIC SETTING

A well developed shelf, shelf break, and fronting deep-water system existed in the Washakie and Great Divide basins of southern Wyoming during the Maastrichtian (Asquith 1970; Winn et al. 1987; McMillen and Winn 1991; Pyles 2000; Pyles and Slatt 2000; Carvajal and Steel 2006; Carvajal 2007; Pyles and Slatt 2007) (Fig. 1). Although at present these basins are separated by the Wamsutter Arch and each of them is an individual structural trough, during the Maastrichtian they constituted a large, single Laramide depocenter that reached deep-water conditions (Fig. 1). The presence of bathyal (up to 500 m undecompacted) water depths in this basin at this time is in sharp contrast to the earlier very shallow-water Late Cretaceous foreland basin (DeCelles 2004). During the Maastrichtian, however, the Wind River Range, Granite Mountains, and Rawlins Uplift area underwent significant thick-skinned uplift, causing rapid tectonic subsidence of the adjacent basin (Reynolds 1976; MacLeod 1981; Blackstone 1991; Steidtmann and Middleton 1991; Connor 1992; Pyles and Slatt 2002; Carvajal 2007; Pyles and Slatt 2007). Localized rapid uplift and subsidence was the hallmark of the early stages of the Laramide Orogeny, an episode of mountain building and basin development that affected most of the Rocky Mountain Region, breaking up the former wide foreland basin into smaller depocenters (Dickinson et al. 1988; Steidtmann and Middleton 1991; Steidtmann 1993). Maastrichtian infilling of the study basin was initially outpaced by subsidence, resulting in development of deep water and an accreting E– W-oriented shelf margin with a clear topset, slope, and basin-floor morphology (Fig. 1). With inclinations , 1–2 degrees, the slope height reached , 430 m (undecompacted), providing a minimum estimate for basinal water depth from shelf edge to basin floor. Water depth was not the same across the deep-water areas; subsidence was greater toward the eastern margins of the basin, where uplift and crustal loading was likely more active, and so clinoform height (and water depth) increases from west to east. The integrated fluvial-to-shelf-to-deep-marine depositional system on the margin is grouped in the Lance Formation, the Fox Hills Sandstone, and the Lewis Shale (Love and Christiansen 1985; Winn et al. 1987; Perman 1990; McMillen and Winn 1991; Carvajal and Steel 2006) (Fig. 1). It spans a 1.8 My interval in the early Maastrichtian and includes the Baculites eliasi, B. baculus, B. grandis, and B. clinolobatus ammonite zones (Winn et al. 1987; Kauffman et al. 1993; Carvajal 2007). The Lewis Shale contains deep-water mudstones and abundant deep-water sandstone (informally referred as the Dad sandstone) in successions up to 762 meters thick (Winn et al. 1987). The Fox Hills Formation represents the sand-prone shoreline-to-shelf succession and is up to 214 meters thick in southern Wyoming (although estimates of maximum thickness may vary significantly according to different authors; see Gill et al. 1970; Steidtmann 1993), whereas the Lance Formation, also exceeding 200 meters in the Rock Spring Uplift, is a coal-bearing paralic to alluvial-plain succession. The vertical succession of these three lithostratigraphic units mirrors a partial lateral time equivalence. The upstream Lance fluvial system fed sediment out into the Fox Hills deltas and shorefaces, and the latter, in turn, fed sediment down into the deeperwater Lewis system (Weimer 1961; Land 1972; Winn et al. 1987). Sourced mainly from the North, clinoforms prograded southward (Winn et al. 1987; Perman 1990; McMillen and Winn 1991; Pyles and Slatt 2007) at an unusually high rate (. 48 km/My) due to high sediment supply (Carvajal and Steel 2006; Carvajal et al. in press). METHODOLGY

The study integrates outcrop facies analysis with subsurface well-log correlation. We describe two consecutive cycles of basin infill: clinothems 9 (older) and 10. Both cycles are within the B. clinolobatus (lower Maastrichtian) ammonite zone (Weimer 1961; Gill and Cobban 1973;

653

Kauffman et al. 1993; Carvajal 2007). In the subsurface, we mapped the clinothems in 3-D by correlating , 500 wells which provide a basin-scale coverage of the Washakie and Great Divide basins (Fig. 1). Most wells have gamma-ray (or spontaneous-potential) and conductivity logs. We used genetic sequences (Galloway 1989) or clinothems (Rich 1951), which emphasizes transgressive shales, to subdivide the stratigraphy (Fig. 1). We chose this approach not because we advocate or prefer a particular stratigraphic methodology (for a review see Catuneanu et al. 2009a,b; Helland-Hansen 2009), but because it works well in our data set. Strata just below Clinothem 9 correlate with B. clinolobatus along both east and west margins of the basin, confirming the basin-wide correlation (Weimer 1961; Gill et al. 1970; Carvajal 2007). In addition, closely spaced wells and multiple correlation loops add certainty to the correlation, though uncertainty increases in the coastal plain and areas of sparse well control. We normalized gamma-ray logs (or if not present, spontaneous-potential logs) and used them to build isopach maps for sandstone, which are extremely useful to delineate sand depositional environments. Segments of the shelf edge of clinothems 9 and 10 are cropping out along the western and eastern margins of the basin respectively. The outcrops show an instructive contrast in processes and architecture already noted in 2002 by an AAPG field trip led by the authors, Roger Slatt, David Pyles, and others (see Steel et al. 2002). We use standard sedimentological and stratigraphic methods to characterize and correlate facies associations, and document processes and stratigraphic architecture. Most of the exposures are good, but excavated trenches were dug in the upper segments of the Clinothem 10 outcrop. Our sections are closely spaced, and we can walk many beds and surfaces along exposures, facilitating correlation. In addition, to improve outcrop correlation and to calibrate well-log trends to possible processes, we used a hand-held scintillometer to collect gamma-ray logs for each stratigraphic section at a 0.25–1 m resolution (average of three readings per point). These logs are also very useful for correlating the outcrops with subsurface well-log cross sections, which extend to near the exposures and therefore improve confidence in the subsurface-outcrop tie. CLINOTHEM 9

Regional Paleogeography Figure 2 presents uninterpreted and interpreted sandstone maps of Clinothem 9 through much of the Washakie and Great Divide basins. The map clearly shows that sandstone is preferentially stored in two main depocenters: one curved, extended in a NE to E direction along the shelf edge and the other to the south, down on the basin floor in two submarine fans. Sandstone along the outer-shelf and shelf-edge depocenter is 24– 32 m thick (and thicker in the west). The sandy fans are , 60 m thick, and Carvajal (2007) calculated a sandstone volume of about 48 km3 for the two fans combined. The lobate geometry of these fans with their apex pointing N to NW clearly indicates feeding from this direction, and sandstone thickness northwards of the fans delineates the position of the slope feeder channels (Fig. 2). On the topset landward of the shelf edge, well-log character and sandstone thickness suggest that in the center of the basin there are two main fluvial channel belts that meander through the coastal plain but have an overall N–S orientation. Towards the shelf edge, these belts occupy the area just updip from the slope channels. The sandstone distribution therefore strongly suggests sand feeding downslope from river mouths. A segment of the shelf edge of Clinothem 9 crops out near the western edge of the basin (Figs. 2, 3). Land (1972) initially studied this outcrop extensively, though he did not deal with the linkage to the coeval shelfedge and deep-water deposits. Figure 3A shows a clinoformal cross section from this outcrop down through the slope and to the basin floor. The cross section demonstrates clearly that the slope is shale-prone and contains only very minor amounts of sandstone. The outcrop analysis

654

C. CARVAJAL AND R. STEEL

JSR

JSR

INFLUENCE OF SUPPLY, SEA LEVEL, AND SHELF-EDGE PROCESSES ON DEEP-WATER BYPASS

indicates that some of these are thin hummocky cross-stratified sandstones. The basin floor immediately down slope is also shale-prone. The muddiness of the slope with its conspicuous absence of channels is also proved in the outcrop (Fig. 3B). The deposits themselves at the shelf-edge outcrop are composed of deltaic, estuarine, and coaly swamp strata (Figs. 3B, 4). In the lower two thirds of this succession, prominent progradational to aggradational packages of deltaic sandstones contain clear evidence for strong stormwave influence (Fig. 4). These are the dominant sandstone facies through the outcrop (see also Land 1972) and therefore testify to dominant stormwave processes during much of Clinothem 9 times. The wave-dominated deltaic deposits are capped by tidally influenced estuarine sandstones with landward and along-shore oriented, stacked cross strata, a change interpreted in terms of the onset of transgression across the topset (Fig. 4). Continued aggradation of Clinothem 9 topsets during transgression produced swamps with peat. Thin mudstones overlying lag concentrations of marine shells mark the climax of transgression across this clinothem, after which there was renewed shoreline progradation. Fluvial feeder channels are scarce along the study outcrop, consistent with the regionally interpreted position of the main fluvial channel systems focused farther east towards the center of the basin (Figs. 2, 3). Storm-Wave-Dominated Shelf-Edge Delta Facies Association Upper and Lower Delta-Front Deposits.—These deposits consist of very fine- to fine-grained sandstones and shales organized into bedsets or parasequences exhibiting upward-coarsening and -thickening motifs (, 20 m thick) (Figs. 3, 4). Bedsets consist of a lower heterolithic interval composed of sandstones (beds , 30 cm) and shales (, 30 cm) with a combined thickness of , 10 m. These sandstones are relatively continuous, with local and minor variations in thickness. The lower interval grades upwards to amalgamated sandstones (, 13 m), forming most of the prominent outcrop sandstone belt. In a given bedset, sandstones commonly exhibit symmetrical ripples, low-angle laminations, hummocky cross-stratification, and occasional swales or troughs towards the top (Figs. 3 4). Sandstone basal contacts with shales are commonly sharp and somewhat flat (i.e., without any appreciable relief), are occasionally gradational, and in one instance exhibit a hummock-filled gutter cast (Fig. 3C). There are rare convoluted beds and pockets (, 15 cm wide) of sandstone pebbles (, 3 cm). Bioturbation is generally low in the sandstones, although some of the uppermost sandstones have abundant Ophiomorpha traces exhibiting branching and long vertical burrows, leaving only remnants of low-angle laminations. Shales are light gray, generally flaggy to fissile, calcareous, and with some organic material (, 3%). Interpretation.—The dominance of hummocky, swaly and low-angle laminations suggest marine shoreline deposition from strong oscillationdominant combined flows during storms (Harms et al. 1982; Dumas and Arnott 2006). The presence of the fluvial channels in the center of the basin (Fig. 2) suggests that these wave shorelines were segments of a larger storm-wave dominated delta. Upward coarsening and thickening of beds indicates upward shallowing during delta progradation. Amalgamated sandstones in the upper levels of bedsets represent an upper delta-front setting where persistent high energy prevented mud preservation. The lower, more heterolithic levels represent lower delta

655

front below fair-weather wave base, as indicated by preservation of intervening mudstone beds, but above storm wave base, as suggested by preservation of the hummocks. The somewhat flat and relatively sharp basal sandstone contacts with shales and gutter casts (albeit scarce) indicate abrupt storm deposition of sand in the lower delta front, probably accompanied by some scouring and erosion of the sea floor (Einsele and Seilacher 1991; Seilacher and Aigner 1991), but minor, to judge by the lack of any major relief or channels at the contact. The total lack of channels in the association and the relatively good lateral continuity of the beds suggest that offshore-directed storm flows were unchannelized or relatively unfocused and expanded. Low bioturbation in the sandstones resulted from the rapid sedimentation, erosion, and episodic fresh-water inflow that characterized the open-ocean segments of the storm wave-dominated delta (MacEachern et al. 2005). Along more sheltered segments of the deltaic coast however, more tranquil conditions brought more abundant sand bioturbation. Prodelta to Shelf to Upper-Slope Deposits.—These deposits consist of mudstones with very thin sandstones, and they conformably underlie and interfinger with the sand-prone, lower delta-front strata (Figs. 3, 4). They form thick (tens of meters) outcrop successions below the sandstones and thinner (, 10 m) interfingering mudstone tongues. Mudstones are carbonaceous, dark gray, and calcareous, commonly with some few very thin sets of very fine-grained and hummocky cross-stratified sandstone beds that are relatively continuous laterally with gradual pinchouts. Sandstones exhibit burrows of Arenicolites and Rhizocorallium (Land 1972) and organism trails on bedding planes. Interpretation.—The conformable contact and interfingering with deltaic facies, the mudstone-rich composition and the increasing basinward abundance indicate that this lithofacies association represents prodelta to shelf deposits, grading out onto an upper-slope environment where the muddy units become much thicker. The area was below fairweather wave base, episodically accumulating hummocky sand during storms (Frey and Pemberton 1984; MacEachern et al. 2005). The good lateral continuity of the sandstones and the lack of channels indicate relatively unconfined offshore-directed flows. Estuary and Swamp Facies Association Sandy Estuary Deposits.—These deposits are composed mainly of finegrained sandstones in stacked (up to 12 m thick) sets of planar and trough-cross strata (Figs. 3, 4). They contrast greatly with, and lie immediately above, the wave-dominated shelf-edge delta deposits, on an irregular, low-relief contact across which grain size increases slightly (Land 1972). Cross-strata sets thin upwards from , 70 cm at the base to , 10–15 cm toward the top. Thick sets tend to taper laterally, forming planar-wedge sets, and have ripple cross lamination in their bottom sets. Thick sets show prominent organic drapes on their foresets and bottomsets, at times draping bundled foresets, and draping ripples and erosional surfaces. Herringbone cross strata and reactivation surfaces are present at some locations (Fig. 4). Cross-stratal foresets mainly dip towards the E–NE with subordinate SE and NW components. Abundant small wood fragments (, 10 cm) and rare bivalve casts occur on some bedding planes. Bioturbation is relatively low; we observed Planolites, and Teredolites occurs in some fossil wood. The upper contact of this

r FIG. 1.—Geologic map of southern Wyoming (compiled from Love and Christiansen 1985 and Blackstone 1991) showing the outcrop of the Lewis Shale, the Fox Hills Sandstone, and the Lance Formation in the Great Divide and Washakie basins; the location of the main mountain ranges; and the well-log data base correlated and integrated with the outcrop for the basin-scale study. The cross section (red line on the map) shows the well-developed topset, slope, and deep-water basin floor through the basin infill. The study clinothems are 9 and 10. Notice the abundant turbidites on the basin floor and the relatively flat shelf-edge trajectory of Clinothem 10.

656


JSR

JSR


facies association with the overlying coaly succession is sharp and flat, commonly showing branching root traces that penetrate , 5 cm downwards into the sandstone. Interpretation.—Wood fragments, root traces, lack of a marine fauna, low bioturbation, position immediately above deltas, close proximity to the shelf edge, and presence of Teredolites indicate a brackish, marginal marine depositional environment. We suggest an estuary because of the prominent presence of high-energy 2-D and 3-D dunes, and subsurface indications of linked fluvial channels to the east. Organic drapes, sometimes in bundles, and herringbone cross strata strongly suggest tidal currents (see also Nio and Yang 1991). The basal irregularity and relief suggests tidal scour of the estuary bottom to produce a tidal ravinement surface (TRS). Given the overall E–NE orientation of the shelf-edge shoreline in this area (Fig. 2), E–NE-dipping foresets suggest shorelineparallel currents possibly from an eastward shoreline drift, whereas the NW and SE dips may represent flood and ebb tidal currents. The upwarddecreasing thickness of cross-sets, top-contact roots, and overlying coaly succession indicate upward-shallowing during estuary filling, and transition to salt marshes and supratidal environments (Dalrymple and Choi 2007), characteristics typical of wide, laterally accreting estuarine channels. Swamp and Salt-Marsh Deposits.—These deposits (, 15 m thick) consist of coal beds and organic-rich shales that lie on the estuarine strata with a sharp and root-marked contact (Figs. 3, 4). Usually less than 50 cm thick, the coal horizons are tabular and occur in uneven numbers in different sections, suggesting that they have only local extent, but that there are several in the area. Shales tend to be dark, organic rich, some of them with abundant plant fragments. Sandstone beds exist but are sporadic. At some localities, the upper contact of this facies is marked by an oyster conglomerate, occasionally quite thick (, 70 cm). Interpretation.—The presence of coal and plant-rich shales in this facies, and the close proximity to the ocean evidently demonstrate a terrestrial but subaqueous highly vegetated marginal marine environment, possibly a swamp. The organic-rich composition and the scattered presence of sandstone indicate a minimal supply of coarse clastics and probably an ephemeral surface connection to river outflows or to the ocean. The aggradational stacking of this facies association (12 m thick) indicates a rising water table and therefore a rising relative sea level, inasmuch as in marginal marine environments sea level largely controls water-table position (Bohacs and Suter 1997). Wave-Dominated Bay-Head Delta Deposits.—Overlying the swamp strata (Figs. 3, 4), these deposits (, 8 m thick) are composed of a basal shell conglomerate, followed upwards by mudstones, and these in turn by an upper heterolithic (sandstone–shale) upward-coarsening and -thickening bed set. The shell-supported conglomerate is , 70 cm thick, and sandstones in the overlying upper heterolithic strata are cross-stratified and with little bioturbation. Towards the succession top there is a 2-mthick ripple cross-laminated sandstone. Interpretation.—We interpret the basal shell conglomerate as a transgressive lag on a wave ravinement surface (WRS) produced by waves as they transgress over the swamps. The upward thickening and coarsening trend indicates upward-shallowing and outbuilding of a

657

marginal marine bar, further supporting the landward migration of more marine settings. Deposition apparently occurred under a fair-whether wave regime and/or in a protected shoreline segment, to judge from lack of hummocks or swales but presence of wave ripples and crossstratification. The limited thickness of the overall succession (, 8 m) and its position immediately above swamp strata indicate that the upward-shallowing succession was built within a flooded swamp rather than in open ocean waters, suggesting that it represents a prograding bayhead delta. Fluvial Channel Deposits.—Present toward the outcrop top and fully contained within the swamp strata, these deposits crop out only at one location (Fig. 3). They consist of a laterally discontinuous or lenticular, upward-fining, relatively coarse-grained succession (, 7 m thick) with a basal, matrix-supported conglomerate (, 2 m thick). The conglomerate is composed of subangular mudstone clasts (, 5–10 cm) within a mediumgrained sand matrix and grades upwards to trough cross-stratified, medium-grained sandstones with conglomerate lenses towards the base. Sandstone foresets dip eastward (basinward). Interpretation.—The coarse-grained character of this facies association along with its lenticular outcrop architecture and upward fining of grain size clearly indicates a river channel with preserved basal lag deposits. The scarcity of this facies association indicates that eastward-flowing rivers did exist in the area, but they were small and probably few in number. CLINOTHEM 10

Regional Paleogeography Clinothem 10 contains a large accumulation of sandstone on its basinfloor reaches (Fig. 5). Carvajal (2007) estimated a 53 km3 volume of sandstone in this fan (however, this value probably underestimates total volume, because some of the fan toward the east is missing by erosion). The fan correlates to fluvial channels and deltas at the shelf edge (Figs. 5– 9). This correlation and the presence of fluvial channels at the outcrop was initially documented by Pyles (2000) and Pyles and Slatt (2002, 2007), who used one regional N–S cross section integrated with the outcrop. The present study builds on this work and contributes: (1) a three-dimensional perspective by placing the outcrop and its linkage to deep water within a basinal paleogeography and stratigraphy derived from multiple cross sections and regional isopach maps (Fig. 5); and (2) new elements to the outcrop description and interpretation (incised valley, estuary, and mixed fluvial–tidal character of the delta front) (Figs. 5–9). In contrast to Clinothem 9, the shelf-edge area of Clinothem 10 is not prominently marked by a strike-extensive sandstone belt and the outcrop shows no evidence of a strong wave regime. Sandstone is 15–25 m thick (and even , 15 m in places) through large areas of the shelf edge (in Clinothem 9, sandstone is . 24–32 m and thicker in the west; Fig. 2). The N–NE oriented apex of the sand lobe on the toe of slope and basin floor suggests a sand-feeding fairway from this direction along the path of slope channels and fluvial channels. The Clinothem 10 outcrop exhibits well-developed tidal and fluvial current indicators (Figs. 5–9) and a prominent incision marked by a surface that sharply truncates underlying delta-front foreset strata (Figs. 6, 7, 8, 10). Subsurface correlation indicates that this major surface extends westwards for 30 km, showing significant relief (Fig. 10B) and delineating a large-scale, eroded

r FIG. 2.—A) Uninterpreted sandstone isopach map of Clinothem 9. The map delineates the main depositional environments of the sand: two basin-floor lobes and a sand belt along the shelf edge (see Fig.1 for outcrop legend). Notice that the sandstone is . 24 m thick over most of the outer-shelf to shelf-edge area (compare with sandstone storage in C10, Fig. 5). B) Cross section (red line in Part A) through the basin floor, showing stacked fans. C) Interpreted isopach map.

658


JSR

JSR


container. This incision contains the fluvial channel deposits and much of the overlying unit of tidal bars and shale (Fig. 6), the latter interpreted as an estuary. Therefore at the Clinothem 10 outcrop the combined thickness of delta-front and overlying strata within the container is less than about 25 m (contrast with the 50 m of aggrading storm-wave deposits in the Clinothem 9 outcrop; Figs. 3, 4). The scale, extent, and significant relief of this container and its dual fluvial and estuarine infill strongly suggest that it is not a regular river channel, but an incised valley formed during relative-sea-level fall. A fall of sea level is consistent with the flat shelf-edge trajectory that Clinothem 10 shows in a basinal cross section just west of the outcrop (Fig. 1), which itself suggests stable to falling relative sea level (Carvajal and Steel 2006). Fluvial–Tidal Shelf-Edge-Delta Facies Association River-Dominated Delta-Front Deposits (Clinoform Set 1).—Along the northern edge of the outcrop, river-dominated delta-front strata extend basinward for 300–400 m, exhibiting well-developed clinoforms that are , 10 m high and dip basinwards at steep angles (up to 10–15u) (Figs. 6, 7, 8). A river-channel erosion surface (valley base) sharply truncates the clinoform top. The clinothem strata consist mainly of fine-grained sandstone beds (typically , 70 cm thick). They are continuous and relatively tabular, with some tapering toward the clinothem toe (Figs. 7, 8), and exhibit both thickening and thinning upwards. Sandstones typically show a sharp, shallow-scoured base followed upwards by flat lamination and in some cases toward the bed top current-ripple crosslamination (Fig. 8). Normal grading occurs in some layers. A thin bed or lamina of mudstone drapes each sandstone. Bioturbation in the sandstone is rare, but Palaeophycus traces and subvertical to slightly inclined burrows, possibly Skolithos, are present. Interpretation.—The clinoform architecture of the strata, their truncation by a fluvial channel, and their outer-shelf location (as seen when tied to the subsurface data) suggest that these strata are deltaic foresets at the shelf-edge area (Fig. 6). The relatively steep foreset inclination is probably due to: (1) deepening of water toward the shelf edge (Pirmez et al. 1998; Plink-Bjo¨rklund and Steel 2005), (2) small mud percentage in the river effluent (Pirmez et al. 1998), and (3) possible delta progradation into and healing of a slope canyon or collapse scar on the delta front (e.g., see Cummings et al. 2006; Deptuck et al. 2008). Upward thickening and thinning within bed sets probably represent autogenic shifting of foreset sheets or systematic changes in the volume of the river discharge. The shallow scours at bed bases followed upwards by flat lamination in the bed suggest only minor erosion during flow emplacement, and transition to upper-flow-regime deposition, sustained for some time, to judge from a few thick beds (several tens of centimeters thick). Currentripple lamination towards some bed tops, thin muddy caps, and normal grading indicates evolution to lower flow regime and waning flow. The tabular bedding and absence of significant erosional structures suggest that (at the outcrop scale) the flows were unconfined sheets. They likely represent fluvial discharge that, upon debouching at the river mouth, expanded subaqueously and radially on the delta front as sheets, maintained by turbulence within a jet plume such as has been described from Wax Lake delta-mouth bars (Wellner et al. 2005). These beds are

659

likely to be the main signal of river domination on the delta front, possibly from hyperpycnal flows (see also Soyinka and Slatt 2008) further ignited by mouth-bar collapse, high foreset gradients, and rapid fluvial sedimentation and dewatering. Tidally Influenced Delta-Front Deposits.—The river-dominated deltafront strata pass southwards to heterolithic strata. The transition area (Fig. 7) exhibits a marked but gradual appearance of thicker mud layers and dunes that dip landwards against and up the clinoform slope. Further southwards (section 2, Figs. 6, 9), where the overlying fluvial channel is thickest, fine- to medium-grained cross-stratified sandstone sets (, 60 cm) are prominent. In this area, clinoform slope has become gentler or dips into the outcrop. Sandstone sets are lenticular and contain 2D dunes that dip mostly E to slightly SE (Fig. 9D) and have tangential and ripple cross-laminated bottomsets. Mudstone occurs as beds (typically , 10– 20 cm) separating sandstones; as drapes (, 3 cm) over dune foresets, bottomsets, and ripples, occasionally forming wavy, flaser, and lenticular bedding (Fig. 9); and in many cases it appears remarkably unlaminated and nonbioturbated. Shallow scours and small (, 20 cm) load structures are locally present. Bioturbation abundance is low, and there are traces of Planolites, Arenicolites, Chondrites, Conichnus, and Thalassinoides (at times in Glossifungites ichnofacies). Towards the south end of the outcrop (section 3, Figs. 6, 9), sandstones become thin-bedded and very-fine grained, sometimes rhythmically alternating with muddy and organic-rich thin layers and exhibiting upward-thickening and -coarsening trends, with ripple cross lamination in places grading upwards to dunes with bidirectional foresets. Interpretation.—The vertical juxtaposition of these strata below the erosional fluvial channel and their gradual transition to the riverdominated delta-front facies indicate that these deposits represent a deltafront environment that interfingered with the river-dominated delta. The heterolithic bedding, mudstone drapes, and flaser, lenticular, and wavy bedding suggest that this delta front was strongly influenced by tidal currents. Dominance of eastward-dipping dune foresets suggests that tidal currents flowed mostly eastwards along the shore. The unlaminated and unbioturbated thicker mud layers strongly suggest fluid muds generated from the mixing of fresh and marine waters (see Steel et al. 2008). The grain-size fining of this facies in the southern outcrop reaches would represent a more distal or marginal delta area, but still influenced by tides, to judge from the bidirectional dune foresets. Examples of shelfedge, tidally influenced deltas are rare (but see Cummings et al. 2006). We speculate that thickening of the distributary-channel deposits above and the steep foresets in the river-mouth-bar deposits may indicate an incised area favoring the amplification of tidal currents. Incised-Valley Facies Association Fluvial Channel Deposits.—These deposits consist of fine- to mediumgrained sandstone (6–12 m thick) traceable all along the outcrop length (, 1.7 km; Fig. 6). The base of the unit is a surface that sharply truncates underlying strata (Figs. 7, 8, 9) and across which sand grain size typically increases from lower fine to upper fine or medium. The surface can be traced westward for several kilometers (Fig. 10). Overlying the surface, in

r FIG. 3.— Transects through Clinothem 9 (see red lines in inset maps for location): A) cross section from the shelf edge to the slope and basin floor, showing a well developed sandy shoreline at the shelf edge and a muddy slope and basin floor with only thin sands (thick sands below Clinothem 9 are the lateral margin of an older fan fed from the center of the basin); and B) cross section through the shelf-edge outcrop, showing a stacking of wave-dominated deltaic parasequences followed upwards by estuarine and swamp deposits. Both cross sections show a lack of any shelf-edge and slope incisions linked with the wave and tidal deposits of the shelf. Instead, there are thin hummocky beds and scarce turbidites.

660


JSR

JSR


some outcrop locations there are thin (up to 20 cm) and discontinuous, clast-supported mudstone–pebble conglomerates, isolated tree-trunk casts (, 1 m), mudstone cobbles (, 20 cm), fossil wood pervasively burrowed by Teredolites, and rarely thin sets of ripple cross-laminated sandstones, draped by mudstone caps. Upward from the basal surface, in the body of the unit, the sandstones are dominated by stacked sets of trough cross strata (3D dunes, up to 40 cm high) that exhibit mostly S– SW-dipping foresets with a subordinate NE component (Fig. 8F). Spectacular convolute bedding is present in the southward reaches and the lower few meters of the outcrop (section 3, Figs. 6, 9), where deformation of the trough cross strata progresses from small patchy areas to larger (few-meter scale) deformed intervals. Interpretation.—Basal truncation and mainly southward dune foresets strongly indicate significant erosion and sediment bypass prior to the onset of main deposition in river channels. Basal conglomerates, isolated mud cobbles, and tree trunks are interpreted as lags at the channel bottom. Mostly S–SW foresets suggest that rivers fed sediment in this direction toward the slope and basin floor. Subordinate mud drapes and eastward-dipping foresets point to likely influence by tidal currents and a brackish setting close to the ocean, also supported by Teredolites tree borings. Convolute bedding most likely resulted from increasing gradients inducing deformation toward the shelf edge, and enhanced by normal fast deposition and dewatering in channels. Tidal-Sand-Bar and Shale Deposits.—These deposits lie above the distributary-channel strata on a relatively flat contact surface and are composed of mostly fine (lower)-grained sandstones (Figs. 6, 8) and some shale, which according to subsurface correlation to the west are at least partially contained within the incised valley (Fig. 10B). Sandstones contain sets of cross strata (with abundant 2D dunes), prominent between sections 2 and 3 (Fig. 6). Sets are , 30–40 cm thick and at times exhibit sigmoidal geometry with ripple cross lamination in tangential herringbone. Most foresets dip basinwards, but we also observed occasional herringbone cross stratification. Thick ripple-cross-laminated intervals exist locally as well as irregular surfaces toward the top. Shales are gray and typically covered. Bioturbation intensity is low. Interpretation.—The lack of organization into upward-thickening and -coarsening trends, lack of clinoform surfaces, and lack of an erosional base to these deposits precludes a delta-front, shoreface, or fluvialchannel interpretation. Instead, the close proximity to the shelf edge, their position above the fluvial deposits, and the fact that these strata are at least partially contained within a major incision suggest an estuary setting. We propose that tidal currents, common in estuaries (Dalrymple and Choi 2007), formed the abundant 2D dunes, and tides are further supported by the presence of occasional herringbone cross strata. Estuary-Mouth Deposits.—Estuary-mouth deposits are organized into: (1) basal shell (with oysters) conglomerates traceable through the outcrop with variable thickness (, 20 cm) and discontinuous with an irregular (possibly erosional) base; (2) a middle, partially covered sandstone (, 3 m), fine to very fine grained, with organic material and toward the base with wave-ripple laminae, irregular surfaces (every 20 cm or so), and small-scale cross-strata (including 3D dunes); and (3) an upper bed set (,

661

5 m) of very fine- to fine-grained sandstones (, 40–80 cm) that show upward coarsening and thickening, and low-angle, basinward-dipping clinoforms (Clinoform set 2, Figs. 6, 7) traceable through the outcrop (, 1.7 km). In Clinoform set 2, sandstones sporadically exhibit planeparallel up to ripple lamination and mudstone caps and others show normal or reverse grading. The uppermost sandstone (80 cm) on the clinoforms is completely bioturbated with numerous traces of Ophiomorpha and has a conformable top (Figs. 6, 7, 8). It is difficult to evaluate how much of these strata are contained within the valley, and they may have accumulated mainly after the valley was filled. Interpretation.—We interpret the basal conglomerate surface as a waveravinement surface (WRS) on which shell lags were accumulated during transgression. In the overlying middle sandstone, wave ripples and trough cross-strata are consistent with wave influence. Toward the top, the upward thickening and coarsening, and basinward-prograding clinoformed sandstones (Clinoform set 2), indicate a regressive event punctuating the transgression. The lower clinoform angle compared to Clinoform Set 1 reflects progradation in shallower water, a few kilometers landwards from the contemporaneous shelf edge (Figs. 5, 6). In the clinoformed sandstones, upward transition from flat to ripple cross lamination and to muddy tops, albeit sporadic, suggest upper-flowregime traction followed by deceleration and waning currents, possibly related to sediment gravity flows linked to rivers; but the lack of a topping fluvial channel in this area makes it hard to prove river domination. The abundant Ophiomorpha and bioturbation in the top sandstone is likely due to more marine conditions during rising relative sea level related to overlying shales. Therefore, the complete succession from the basal shell conglomerates through Clinoform Set 2 indicates more marine conditions and thus the landward transit of estuary-mouth areas, suggesting overall transgression. Delta Facies Association Prodelta to Delta-Front Deposits.—Lying on the estuary-mouth strata, these deposits (described mostly from hand-dug trenches) are composed of a basal shale (, 6 m, with organics) that correlates across outcrop sections (Fig. 6) and pass upwards to heterolithic strata (, 18 m). The transition shows upward grain-size coarsening, bed thickening, and increasing sandstone content. Sandstones in the heterolithic strata are very fine to medium grained and , 1 m thick; in the lower meters they show gradations from flat lamination to ripple lamination and to mudstone caps; in the remaining strata they show ripple cross lamination, lenticular and flaser bedding, dune foresets (likely 2D) with few mudstone drapes, and hints of upward thickening and coarsening. Shales (with organics , 3 mm) are , 20 cm thick. Bioturbation is low, with few examples of small sandy burrows of Glossifungites-like ichnofacies, and possible Conichnus and Rosselia traces. Elsewhere, hummocky-bedded sandstones occur capping the outcrop. Interpretation.—The gradual transition from the basal shale to the heterolithic strata suggests progradation and evolution from prodelta to delta-front environments. We favor a delta because subsurface and outcrop data indicate in the area there are fluvial channels which are likely to feed a delta. Lenticular and flaser bedding and foreset mud

r FIG. 4.—Facies and depositional environments in Clinothem 9 outcrop (see Fig. 3 for legend): A) swaley cross-stratification, B) Ophiomorpha trace, C) gutter cast, D) hummocky cross-stratification, E) bivalve imprints, F) cross-strata with organic-rich (dark) laminations, G) rose diagram with paleocurrents in estuary facies (n 5 21, corrected for structural attitude), H) herringbone cross-strata, I) roots at the top of the estuary, J) transition from swamp to wave ravinement and to marine tongue, K) wood imprints in organic-rich shales, and L) shells. Notice the aggradational (50 m) and unincised stacking of the wave-dominated shorelines, a sign of high and rising relative sea level (see text).

662


JSR

FIG. 5.—A) Interpreted sandstone isopach map of Clinothem 10, showing that the main accumulations of sand are on the basin floor, in the deltas, and in incisions feeding the fan; along most of the shelf edge, sandstone is , 24 m thick, and even , 15 m thick over large areas (contrast with Fig. 2). B) Clinothem 10 outcrop to deepwater cross section showing fluvial deposits at the shelf edge (see also Figs. 6, 7), numerous slope channels, and abundant sand in deep-water fans (contrast with Figs. 2 and 3).


FIG. 6.—Cross section through the shelf-edge outcrop of Clinothem 10 near the eastern margin of the basin, showing the high degree of incision (see also Figs. 7, 8), and fluvial domination that coexists with tidal influence at the shelf edge (wave influence is minor or absent). The combined thickness of the delta front, the overlying fluvial channel, and tidal sand bars and shale unit is , 25 m, a clear sign of lower accommodation than in Clinothem 9, whose wave-dominated deltas reach 50 m at the outcrop (see Fig. 3).

JSR 663


FIG. 7.—A) Panoramic mosaic of Clinothem 10 in proximal outcrop areas; B) enlarged clinoform sets 1 and 2, also showing the prominent erosional surface truncating set 1; C) close-up of clinoform sets 1 and 2, showing similar features, and the area (left of the panel) where clinoform set 1 facies start exhibiting tidal indicators.

664

JSR

JSR


drapes suggest tidal influence, although flat lamination to ripple lamination to mud cap transitions may indicate some river influence and mixed tide-river processes. The hummocky beds document stormwave influence toward the end of deposition. Because of the poor exposures, our interpretations are tentative. DISCUSSION: RIVERS, WAVES, AND TIDES AT THE SHELF EDGE: THEIR INFLUENCE ON ACUMULATION OR BYPASS OF SAND AT THE SHELF EDGE IN DIFFERENT SEA-LEVEL REGIMES

Role of Rivers The location of the fans directly basinwards and downslope from shelfedge rivers in both clinothems (Figs. 2, 5) demonstrates that river processes played the driving role in sand bypass (for instance, compare with Hueneme Fan in the California Borderland (Normark et al. 1998) and the Rhone Neofan in France (Torres et al. 1997)). The pivotal role of rivers comes from their potential to (1) supply sediment, (2) channel the shelf edge, and (3) ignite turbidity currents. On the latter aspect, riverignited turbidity currents may result from hyperpycnal discharge (Mulder et al. 2003; Plink-Bjo¨rklund and Steel 2004; Petter and Steel 2006) and from rapid mouth-bar loading and subsequent collapse. Shelf-edge river channels can continue through subaqueous slope channels to form efficient, well fed conduits that reach the basin floor. In this way, semipermanent conduits for sediment bypass can be maintained in front of the river mouth, provided that the river or delta remains at the shelf edge. In addition, the river drainage area influences the volume of sediment ultimately bypassed to slope and basin floor (Wetzel 1993). Role of Waves Inefficient Deep-Water Delivery.—The study case suggests strongly that waves by themselves are likely to have limited potential to create their own conduits to the basin floor. The thick, shale-rich slope succession below the wave-dominated deltas of the western outcrop of Clinothem 9 contains only occasional thin sheet sandstones, and subsurface cross sections show a highly mud-prone slope (Figs. 3, 4). Despite abundance of sand at the shelf edge (50 m thick), there are few indications that wave-triggered offshore flows (rip currents, storm surges, etc.) bypassed significant volumes of sand directly to the deep-water slope. This is predictable because although waves can easily create offshore flows toward the slope during storms (Dumas and Arnott 2006), these flows seem to have had an insignificant potential to channel the slope, so the downslope sand delivery was as unconfined flows tending to lose their momentum and deposit their load in thin sand sheets, as documented here in the outcrop. Deposition in deep water below storm wave base would prevent further wave reworking of these sediments and they would remain on the slope to become buried by mud. The present study case therefore indicates that waves alone are inefficient at triggering bypass to the slope and basin floor. Importance of Wave Drift Plus Canyon Intersection.—On the other hand, the study case does indicate that waves are very efficient at storing sand in elongate sand belts along and parallel to the shelf edge, when the coastline is in that position (Fig. 2). It is very common for belts like this to be associated with significant transport and reworking of sediments from alongshore drift currents. If these drifts intersected river mouths or slope canyon heads, they would have supplied sand for deep-water bypass. These means of sand transport are well known from the present highstand world (Weber et al. 1997; Michels et al. 2003; Covault et al. 2007; Boyd et al. 2008). For instance, on the Bangladesh shelf, the ‘‘Swatch of No Ground’’ is a canyon deeply incised into the Bengal shelf, directly connecting inner-shelf areas with deep-water settings. During fair weather the canyon head receives mud from tidal currents, and during storms it receives mud and sand (Michels et al. 2003). Weber et al. (1997) have

665

documented that turbidity currents through the canyon have resulted in some growth of the Bengal Fan during transgression and highstand of sea level. Coastal drift as a supplier to deep water via slope canyons on narrow shelves has been well described in the California borderland (Piper and Normark 2001). Here, for example, the small Dume Fan is thought to receive sand solely by alongshore drift. Covault et al. (2007) have documented that the La Jolla fan system has received much sand through slope canyons fed by littoral drifts since 13 ka to the present. These drifts have captured distant river discharges apparently without storing much of it in coastal systems, in contrast to the study case. Instead much of the drift sediment has been efficiently supplied to the La Jolla canyon head which, through the present highstand, has not been connected to a river mouth. The La Jolla fan contains an estimated highstand bulk sediment volume of 38 km3, slightly greater than the combined lowstand volumes of the Oceanside (25 km3) and Carlsbad (12 km3) fans, underlining the efficiency of the highstand drift system and the fact that (at sea-level lowstand) multiple incisions through the shelf edge broke drift cells and partitioned river-supplied sediment volumes into more numerous deep-water depocenters. Boyd et al. (2008) have also documented that sand drifts and tidal currents transport sand to deep water along the southeastern margin of Australia. Here a vigorous sand drift carries about 500,000 m3/y (or 0.5 km3/ky) of sand each year, and part of this sand is at present bypassing to the slope and basin floor, although over much of the past 750 ky the sand has been stored in shelf sand islands along the coast. Asymmetry around River Mouths.—Another important effect of alongshore drift of sand is that it can produce an asymmetric distribution of sand on either side of deflecting river mouths, as Bhattacharya and Giosan (2003) have documented in modern wave-influenced deltas such as the Danube (Black Sea), the Brazos (Gulf of Mexico), the Guadiana (Spain and Portugal), the Nile (Mediterranean Sea), and other deltas. Such delta asymmetry results from the interplay between wave-driven alongshore currents and strength of direct fluvial discharge. Despite wave-induced drift, the strong river discharge acts as a ‘‘fence,’’ tending to retain and accumulate the largest sand volume on its updrift side. During times of decreased discharge or weak fluvial outflows, the river-mouth bar may even become deflected in a downdrift direction, with an extreme deflection when river discharge is very weak relative to the strength of alongshore currents. In the study case, the Clinothem 9 isopach map (Fig. 2) indicates that the distribution of sandstone with respect to the main river mouths is asymmetric with the most widespread sandstones just westwards of the main river-mouth sites (Fig. 2). Such a distribution suggests an eastward drift (Bhattacharya and Giosan 2003), which we cannot confirm with paleocurrents from the storm-wave dominated deltas because their hummocks and swales prevent paleocurrent analysis. It is interesting to note, however, that the estuary sandstones overlying the deltas show an E paleocurrent trend (Fig. 4G), which in preceding sections we attributed to a possible E drift because it is relatively parallel to the shelf edge in the area. If an eastward drift existed during the stormwave deltas, then sands delivered from small rivers in the western reaches of the basin would have had a decreased potential to pass eastwards of the main river mouths (despite eastwards drifting) and would have been captured and accumulated along the western shelf edge or delivered directly to canyon heads in the basin center. In addition, such a drift would have caused N–S fluvial channels to swing into NW–SE orientations, and these in turn would perhaps have driven the observed NW–SE orientation of slope channels and deep-water fans (Fig. 2). Role of Tides Almost no descriptions exist of tide-influenced deltas at the shelf edge (but see Cummings et al. 2006) and therefore the role that tides may play

666


JSR

JSR


in the delivery of sand to deep-water areas has been less explored. In some ancient cases and at present, there is convincing evidence pointing to tidal currents in canyons (see Shepard 1976; Shepard et al. 1979; Shanmugam 2003). Bottom currents in many modern canyons show up-canyon and down-canyon flow directions (e.g., Congo, Santa Monica, Monterrey, Rio Balsas, and Wilmington canyons) with the periodicity of daily tidal cycles (e.g., Hueneme Canyon) and with velocities commonly in the range of 25–50 cm/s, but reaching even 70–75 cm/s, i.e., velocities able to transport coarse sand. Inasmuch as tides are largely confined to the canyon, they may only rework sediment already provided by rivers or drifts and not add to the budget available for bypass. In our study case, it is quite possible that Clinothem 9 tides in the estuarine facies association provided sand eastwards via littoral drifts, as previously discussed. In Clinothem 10, tides in the delta front are dominated by E–SE paleocurrents (Fig. 9), parallel to the shoreline, which could also have been part of an eastward drift, although the absence of a main source of sand west and outside of the shelf-edge incision (Figs. 5, 10; contrast with Fig. 2) suggests only a minor sand supply to canyon heads by alongshore drifts. In addition, the incised character of the shelf-edge coast in Clinothem 10 would have tended to segment littoral drifts, diminishing their capacity to transport sediment along the coast (Covault et al. 2007). However, other areas of the same delta in Clinothem 10, or areas of the overlying estuary may well have been dominated by S or SW currents, which would have contributed sediment to downslope transport. Tides may become more important in cases where they are able to transport sand from distant places (e.g., Clinothem 9), thereby increasing the river budget of sand for bypass. Influence of Sea Level and Sediment Supply on Shelf-Edge Architecture and Processes Clinothems 9 and 10 present an interesting contrast as regards the distribution, thickness, and volume of sandstone along their shelf edges. As described, shelf-edge sandstone in Clinothem 9 is thicker and more continuous than in Clinothem 10, in which it is more restricted to the area of incision (Figs. 2, 5, 10). Through most of the shelf-edge region, sandstone is 24–32 m thick (and thicker in the west) in Clinothem 9 (Fig. 2) and 15–25 m thick in Clinothem 10 (even , 15 m). At the outcrop, Clinothem 9 shows 50 m of aggradation from storm-wavedominated deltas, whereas in Clinothem 10 the combined thickness of delta front and overlying strata within the valley is less than about 25 m (Figs. 3, 6, 10). As we have suggested, such differences probably point to a strong wave regime and probably significant alongshore drift during development of Clinothem 9 and a weaker wave regime and alongshore drift during Clinothem 10 growth. This is consistent with the evidence from the outcrops, inasmuch as we do observe strong wave regime indicators in Clinothem 9 and few signs of strong wave activity in Clinothem 10. The question then arises as to what drives this marked difference. Strong wave influence along a coast results from climate, coastal morphology, and/or sea level (Pore˛bski and Steel 2006; Yoshida et al. 2007). A climatic change through time, by its potential to influence coastal storminess, could explain the decreased wave regime from Clinothem 9 to 10, but we do not have any independent evidence to suggest that such a change took place. Based on the architecture of both clinothems, we suggest that relative sea level may be driving the observed differences:

667

Clinothem 9.—The 50 m of storm-wave aggradation exhibited by Clinothem 9 at the outcrop clearly indicates sustained relative-sea-level rise to provide space for the aggrading deltas. In addition, as tracked into the subsurface these deltas downlap onto a maximum flooding surface (Fig. 9) and therefore are likely to represent ‘‘highstand’’ sea-level conditions. The rising sea level thus favored the development of aggrading and voluminous masses of sediment at the shelf edge prone to wave domination, because open-ocean swells that propagated in deep water seaward from the shelf edge (deep-water waves) would have been minimally attenuated by sea-floor interaction and therefore upon breaking along the shallower water shelf edge would have had a pronounced impact on the coast. They would have imposed a strong wave influence and alongshore drift of sand, accentuated during storms, leading to the spread of much sediment along the coast and the creation of a widespread sand belt along the shelf edge. This has also been suggested for Paleogene shelf edges of the Gulf of Mexico (Galloway 2001) and has been suggested conceptually (Pore˛bski and Steel 2006). Therefore it seems likely that a rising relative sea level, with its tendency to deepening of water and straightening of coastlines, favors wave influence on the coastline, especially when at a shelf-edge position. In the above scenario sediment supply becomes quite critical, because a rising sea level necessarily causes transgression unless sediment supply is high enough to drive deltas to the shelf edge and maintain them there. In Clinothem 9, the fact that the coastal plain aggraded, that a significant sandstone belt is present at the shelf edge, and that a relatively large, deep-water fan formed strongly indicate a high sediment supply. Carvajal and Steel (2006) have also suggested that the high rates of progradation (. 48 km/My) of the Lewis–Fox Hills shelf margin themselves indicate a high sediment supply, which is also consistent with the relatively large size of the deep-water fans as compared with fans in other shelf margins of similar dimensions (Carvajal et al. in press). In analogous literature examples of highstand shelf-edge deltas where sediment supply is relatively low across shelf margins (e.g., Uroza and Steel 2008) there is the tendency that no basin-floor fan is formed and that the shelf edge accretes only at slow rates. In the Lewis–Fox Hills margin the large supply of sediment is thought to have resulted from the high relief generated by high uplift rates during thick-skinned deformation and mountain building in the Wind River Range, Granite Mountains, and Rawlins Uplift area (Reynolds 1976; Carvajal 2007; Pyles and Slatt 2007; Fig. 1). Studies in modern systems demonstrate that high relief, as a proxy for high uplift rate, leads to higher sediment supply (Syvitski et al. 2003; Syvitski and Milliman 2007). Clinothem 10.—In contrast, it is likely that there was relative-sea-level fall during the development of Clinothem 10, as inferred from the presence of an incised valley, from the thin and deeply incised deltas of Clinothem 10 (, 10 m) (Figs. 6, 10), and from the flat shelf-edge trajectories of Clinothem 10 elsewhere in the basin (Fig. 1). A falling relative sea level would have induced a particular shoreline character; the delta at the shelf edge would have been much less developed, because falling sea level would have prevented delta aggradation, impeding the accumulation of thick deltaic wedges. On the contrary, it is most likely that deltas were continuously degraded, as falling sea level led to continued delta destruction by its own fluvial feeders, which incised deeper as sea level continued falling, resulting eventually in incised valleys. Consequently, as the delta reached the shelf edge it would have survived only as much thinner and restricted wedges of sediment perched

r FIG. 8.—Facies and depositional environments in Clinothem 10 outcrop: A) bedding in proximal segments of clinoform set 1, B) flat lamination in set 1, C) currentripple cross-lamination in set 1, D) erosional surface truncating set 1 (valley base), E) trough cross bedding in fluvial channel, F) paleocurrent measures in fluvial channel (n 5 25, corrected for structural attitude), G) tidal bars in estuary, and H) shell lags near the base of clinoform set 2.

668


JSR

FIG. 9.—Facies in Clinothem 10 (see Fig. 8 for legend) outcrop: A) bedding in tidal facies, B) thin mud drapes on foresets, C) mud drapes on ripples, D) paleocurrents in tidal facies showing strong eastward trend parallel to the shoreline (n 5 32, corrected for structural attitude), E) distal tidal facies, F) mud clast above erosional surface, G) convolute bedding in fluvial channel, a sign of proximity to the shelf edge, and H) valley base in distal outcrop area.


FIG. 10.—Cross sections through the outer-shelf to shelf-edge area: A) Clinothem 9 showing a highly continuous sandstone belt composed of relatively smooth, gradually decreasing gamma-ray motifs (i.e., funnellike), interpreted to represent a storm-wave-dominated shoreline; B) Clinothem 10, showing thinner and much less developed funnels that pass upwards, often sharply, to more blocky patterns (logs 4–13); the outcrop indicates that the transition occurs at a surface of fluvial erosion, interpreted to be the base of an incised valley.

JSR 669

670

on the outermost shelf or uppermost slope and under continued erosion by river channels as the study delta front in Clinothem 10. Other examples of such deltas have been described by Petter and Steel (2006), and by Ponteń and Plink-Bjo¨rklund (2009) from shelf-edge exposures in Spitsbergen. Thinner and restricted deltas under continued erosion as well as the development of local embayments due to incision, bird-foot delta geometries, and shelf-edge scars would have tended to inhibit and limit wave influence due to the generation of protected shoreline segments. Also, coastal segmentation would tend to limit the development of extensive alongshore drifts, largely diminishing the potential of these drifts to supply sand to canyon heads. In the California borderland, Covault et al. (2007) inferred that the present strong and continuous littoral drift along the coast was much less important during the last lowstand of relative sea level because at that time it was segmented into numerous smaller cells due to the canyons and river effluents at the shelfedge coast. On the other hand, such coastal morphology would have induced tidal amplification in confined settings such as shelf-edge channels and upper-slope canyons. It may be due to this coastal morphology, induced by relative-sea-level fall and fluvial erosion, that Clinothem 10 shows a decreased wave influence and increased tidal regime in the delta front and that only minor volumes of sediment were spread along strike on the shelf edge. We are cautious to suggest that tidal influence is restricted or most accentuated at lowstand; it is well known that significant tidal influence accompanies transgression of the shelf (Dalrymple et al. 1992; Yoshida et al. 2007) and in general tidal influence can increase or decrease throughout a relative-sea-level cycle provided that coastal morphology and tidal regime are adequate. For instance, confined areas in Clinothem 9, as those within distributary channels, may also show increased tidal influence (e.g., see Ponteń and Plink-Bjo¨rklund 2009). We suggest rather that a fall of relative sea level likely enhanced tidal influence because it contributed to generate confined shelf-edge coastal morphologies that favored tidal amplification. CONCLUSIONS

Sedimentary and stratigraphic analysis in two shelf to basin-floor clinothems of basin infill in the Washakie and Great Divide basins of southern Wyoming indicates that: 1.

2.

3.

JSR


Rivers played the driving role in the bypass of large volumes of sand to deep-water areas. River processes are most effective because the natural fluvial scour of the river creates channels at the shelf edge and, because either through hyperpycnal flow or mouth-bar collapse, rivers can ignite turbidity currents which can further channel the slope to reach the basin and form sandy fans. In this way long-lived conduits can be established to transfer large volumes of sediment to deep-water areas. Waves and tides by themselves appear to have a smaller potential to bypass significant volumes of sand to the basin floor, because they have limited potential to channel the shelf edge and create conduits for sediment bypass to deep water. However, waves and tides can contribute sand to canyon heads via longshore drift, although the same drifts can also cause trapping and retention of very large volumes of sediment along the coast itself. Drifts can also trigger deflection of river mouths at the shelf edge and so influence the direction of downslope river feeding. Development of a strongly wave-generated architecture in the shelfedge delta is favored by a rising relative sea level. The latter encourages development of thick and aggrading packages of sediment that are shaped by the strong wave regime induced by deep-water waves. Alongshore drift can extend for long distances on these open, linear coasts. In contrast, a falling relative sea level tends to produce thin and poorly developed deltas and a more incised shelf-edge coast. Wave influence is damped along protected

4.

areas of such coasts and drifts become segmented, significantly decreasing the alongshore spread of sand. Confinement in coastal indentations may tend to amplify tides, which can partly overwhelm river process signals. During rising relative sea level, the success of a mixed wave–riverinteraction delta to generate widespread shelf-edge sand belts and to bypass large volumes of sand to the basin floor is largely dependent on sediment supply. A small supply system is likely to allow only shelfmargin aggradation and progradation without significant bypass. ACKNOWLEDGMENTS

We appreciate the funding and data provided for this research by Devon Energy Corporation (Dale Reitz), A2D Technologies (Bill Ross), companies of the Wolf Consortium Industry Associates, the Geology Foundation at the Jackson School of Geosciences (University of Texas at Austin), the Geological Society of America, and the Wyoming Geological Association. We are grateful to the Espy family for allowing us to do field work on their property. CC thanks Rick Peters for field assistance and participants of the Dynamic Stratigraphy Group at the Jackson School of Geosciences for lively discussions regarding delta and deep-water processes. David Mohrig, Lesli Wood, and William Fisher are thanked for critical reviews of early versions of this article. Suggestions from Bill Galloway and Craig Fulthorpe have greatly improved the manuscript, as well as the editorial review by JSR corresponding editor John B. Southward. REFERENCES

AINSWORTH, R.B., FLINT, S.S., AND HOWELL, J.A., 2008, Predicting coastal depositional style: influence of basin morphology and accommodation to sediment supply ratio within a sequence stratigraphic framework, in Hampson, G.J., Steel, R.J., Burgess, P.M., and Dalrymple, R.W., eds., Recent Advances in Models of Siliciclastic ShallowMarine Stratigraphy: SEPM, Special Publication 90, p. 237–263. ASQUITH, D.O., 1970, Depositional topography and major marine environments, Late Cretaceous, Wyoming: American Association of Petroleum Geologists, Bulletin, v. 54, p. 1184–1224. BHATTACHARYA, J.P., AND GIOSAN, L., 2003, Wave-influenced deltas: geomorphological implications for facies reconstruction: Sedimentology, v. 50, p. 187–210. BLACKSTONE, D.L., JR., 1991, Tectonic relationships of the southeastern Wind River Range, southwestern Sweetwater Uplift, and Rawlins Uplift, Wyoming: U.S. Geological Survey of Wyoming (Laramie), Report of Investigations 47, 24 p. BOHACS, K., AND SUTER, J., 1997, Sequence stratigraphic distribution of coaly rocks: Fundamental controls and paralic examples: American Association of Petroleum Geologists, Bulletin, v. 81, p. 1612–1639. BOYD, R., RUMING, K., GOODWING, I., SANDSTROM, M., AND SCHRO¨DER-ADAMS, C., 2008, Highstand transport of coastal sand to the deep ocean: A case study from Fraser Island, southeast Australia: Geology, v. 36, p. 15–18. CARVAJAL, C., 2007, Sediment volume partitioning, topset processes and clinoform architecture—understanding the role of sediment supply, sea level and delta types in shelf margin building and deep-water sand bypass: The Lance–Fox Hills–Lewis system in S. Wyoming [unpublished Ph.D. thesis]: The University of Texas, Austin, 171 p. CARVAJAL, C.R., AND STEEL, R.J., 2006, Thick turbidite successions from supplydominated shelves during sea-level highstand: Geology, v. 34, p. 665–668, doi 10.1130/ G22505.1. CARVAJAL, C., STEEL, R.J., AND PETTER, A., in press, Sediment supply: the main driver of shelf-margin growth: Earth-Science Reviews. CATUNEANU, O., ABREU, V., BHATTACHARYA, J.P., BLUM, M.D., DALRYMPLE, R.W., ERIKSSON, P.G., FIELDING, C.R., FISHER, W.L., GALLOWAY, W.E., GIBLING, M.R., GILES, K.A., HOLBROOK, J.M., JORDAN, R., KENDALL, C.G.St.C., MACuRDA, B., MARTINSEN, O.J., MIALL, A.D., NEAL, J.E., NUMMEDAL, D., POMAR, L., POSAMENTIER, H.W., PRATT, B.R., SARG, J.F., SHANLEY, K.W., STEEL, R.J., STRASSER, A., TUCKER, M.E., AND WINKER, C., 2009a, Towards the standardization of sequence stratigraphy: Earth-Science Reviews, v. 92, p. 1–33. CATUNEANU, O., ABREU, V., BHATTACHARYA, J.P., BLUM, M.D., DALRYMPLE, R.W., ERIKSSON, P.G., FIELDING, C.R., FISHER, W.L., GALLOWAY, W.E., GIBLING, M.R., GILES, K.A., HOLBROOK, J.M., JORDAN, R., KENDALL, C.G.St.C., MACuRDA, B., MARTINSEN, O.J., MIALL, A.D., NEAL, J.E., NUMMEDAL, D., POMAR, L., POSAMENTIER, H.W., PRATT, B.R., SARG, J.F., SHANLEY, K.W., STEEL, R.J., STRASSER, A., TUCKER, M.E., AND WINKER, C., 2009b, Reply to the comments of W. Helland-Hansen on ‘‘Towards the standardization of sequence stratigraphy’’: Earth-Science Reviews, v. 94, p. 98–100, doi 10.1016/j.earscirev.2009.02.004. CONNOR, C.W., 1992, The Lance Formation; petrography and stratigraphy, Powder River basin and nearby basins, Wyoming and Montana: U.S. Geological Survey, Bulletin, B 1917-I, 17 p.

JSR


COVAULT, J.A., NORMARK, W.R., ROMANS, B.W., AND GRAHAM, S.A., 2007, Highstand fans in the California borderland: The overlooked deep-water depositional system: Geology, v. 35, p. 783–786. CUMMINGS, D.I., ARNOTT, R.W.C., AND HART, B.S., 2006, Tidal signatures in a shelfmargin delta: Geology, v. 34, p. 249–252. DALRYMPLE, R.W., AND CHOI, K., 2007, Morphologic and facies trends through the fluvial–marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation: EarthScience Reviews, v. 81, p. 135–174. DECELLES, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, Western U.S.A.: American Journal of Science, v. 304, p. 105–168. DEPTUCK, M.E., PIPER, D.J.W., SAVOYE, B., AND GERVAIS, A., 2008, Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica: Sedimentology, v. 55, p. 869–898, doi 10.1111/j.1365-3091.2007.00926.x. DICKINSON, W.R., KLUTE, M.A., HAYES, M.J., JANECKE, S.U., LUNDIN, E.R., MCKITTRICK, M.A., AND OLIVARES, M.D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: Geological Society of America, Bulletin, p. 1023–1039. DUMAS, S., AND ARNOTT, R.W.C., 2006, Origin of hummocky and swaley crossstratification—the controlling influence of unidirectional current strength and aggradation rate: Geology, v. 34, p. 1073–1076. EDWARDS, M.B., 1981, Upper Wilcox Roseta Delta System of S. Texas: growth-faulted shelf-edge deltas: American Association of Petroleum Geologists, Bulletin, v. 65, p. 54–73. EINSELE, G., AND SEILACHER, A., 1991, Distinction of tempestites and turbidites, in Einsele, G., Ricken, W., and Seilacher, A., eds., Cycles and Events in Stratigraphy: New York, Springer-Verlag, p. 377–381. FLOOD, R.D., AND PIPER, D.J.W., 1997, Amazon Fan sedimentation; the relationship to equatorial climate change, continental denudation, and sea-level fluctuations: Proceedings of the Ocean Drilling Program, Scientific Results, v. 155, p. 653–675. FREY, R.W., AND PEMBERTON, G.S., 1984, Trace fossil facies models, in Walker, R.G., ed., Facies Models: Geological Association of Canada, Reprint Series 1, p. 189–207. GALLOWAY, W.E., 1989, Genetic stratigraphic sequences in basin analysis: I, Architecture and genesis of flooding-surface bounded depositional units: American Association of Petroleum Geologists, Bulletin, v. 73, p. 125–142. GALLOWAY, W.E., 2001, Cenozoic evolution of sediment accumulation in deltaic and shore-zone depositional systems, northern Gulf of Mexico Basin: Marine and Petroleum Geology, v. 18, p. 1031–1040. GILL, J.R., AND COBBAN, W.A., 1973, Stratigraphy and geologic history of the Montana Group and equivalent rocks, Montana, Wyoming, and North and South Dakota: U.S. Geological Survey, Professional Paper P 0776, 37 p. GILL, J.R., MEREWETHER, E.A., AND COBBAN, W.A., 1970, Stratigraphy and nomenclature of some Upper Cretaceous and lower Tertiary rocks in south-central Wyoming: U.S. Geological Survey, Professional Paper 667, 53 p. HARMS, J.C., SOUTHARD, J.B., AND WALKER, R.G., 1982, Structures and Sequences in Clastic Rocks: SEPM, Short Course 9, 249 p. HELLAND-HANSEN, W., 2009, Towards the standardization of sequence stratigraphy: Earth-Science Reviews, v. 94, p. 95–97, doi 10.1016/j.earscirev.2008.12.003. JOHANNESSEN, E.P., AND STEEL, R.J., 2005, Clinoforms and their exploration significance for deepwater sands: Basin Research, v. 17, p. 521–550. KAUFFMAN, E.G., SAGEMAN, B.B., KIRKLAND, J.I., ELDER, W.P., HARRIES, P.J., AND VILLAMIL, T., 1993, Molluscan biostratigraphy of the Cretaceous Western Interior Basin, North America, in Caldwell, W.G.E., and Kauffman, E.G., eds., Evolution of the Western Interior Basin: Geological Association of Canada, Special Paper 39, p. 397–424. KOLLA, V., AND PERLMUTTER, M.A., 1993, Timing of turbidite sedimentation on the Mississippi Fan: American Association of Petroleum Geologists, Bulletin, v. 77, p. 1129–1141. LAND, C.B., JR., 1972, Stratigraphy of Fox Hills Sandstone and associated formations, Rock Springs Uplift and Wamsutter Arch Area, Sweetwater County, Wyoming: a shoreline–estuary sandstone model for the Late Cretaceous: Colorado School of Mines, Quarterly, v. 67, p. 1–69. LOVE, J.D., AND CHRISTIANSEN, A.C., 1985, Geologic Map of Wyoming: U.S. Geological Survey, 1: 500,000. MACEACHERN, J.A., BANN, K.L., BHATTACHARYA, J.P., AND HOWELL, C.D.J., 2005, Ichnology of deltas: Organism responses to the dynamic interplay of rivers, waves, storms, and tides, in Bhattacharya, J.P., and Giosan, L., eds., River Deltas— Concepts, Models, and Examples: SEPM, Special Publication 83, p. 49–85. MACLEOD, M.K., 1981, The Pacific Creek Anticline: buckling above a basement thrust fault: University of Wyoming, Contributions to Geology, v. 19, p. 143–160. MCMILLEN, K.M., AND WINN, R.D., JR, 1991, Seismic facies of shelf, slope, and submarine fan environments of the Lewis Shale, Upper Cretaceous, Wyoming, in Weimer, P., and Link, M.H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 273–287. MICHELS, K.H., SUCKOW, A., BREITZKE, M., KUDRASS, H.R., AND KOTTKE, B., 2003, Sediment transport in the shelf canyon ‘‘Swatch of No Ground’’ (Bay of Bengal): Deep-Sea Research. Part II: Topical Studies in Oceanography, v. 50, p. 1003–1022. MULDER, T., SYVITSKI, J.P.M., MIGEON, S., FAUGERES, J.-C., AND SAVOYE, B., 2003, Marine hyperpycnal flows: initiation, behavior and related deposits: a review: Marine and Petroleum Geology, v. 20, p. 861–882.

671

NIO, S.-D., AND YANG, C.-S., 1991, Diagnostic attributes of clastic tidal deposits: a review, in Smith, D.G., Reinson, G.E., Zaitlin, B.A., and Rahmani, R.A., eds., Clastic Tidal Sedimentology: Canadian Society of Petroleum Geologists, Memoir 16, p. 3–28. NORMARK, W.R., PIPER, D.J.W., AND HISCOTT, R.N., 1998, Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California: Sedimentology, v. 45, p. 53–70. PERMAN, R.C., 1990, Depositional history of the Maastrichtian Lewis Shale in southcentral Wyoming: deltaic and interdeltaic, marginal marine through deep-water marine, environments: American Association of Petroleum Geologists, Bulletin, v. 74, p. 1695–1717. PETTER, A., AND STEEL, R.J., 2006, Hyperpycnal flow variability and slope organization on an Eocene shelf margin, Central Basin, Spitsbergen: American Association of Petroleum Geologists, Bulletin, p. 1451–1472. PIPER, D.J.W., AND NORMARK, W.R., 2001, Sandy fans: from Amazon to Hueneme and beyond: American Association of Petroleum Geologists, Bulletin, v. 85, p. 1407–1438. PIRMEZ, C., PRATSEN, L.F., AND STECKLER, M.S., 1998, Clinoform development by advective diffusion of suspended sediment: modelling and comparison to natural systems: Journal of Geophysical Research, v. 103, p. 24,141–24,157. PLINK-BJO¨RKLUND, P., AND STEEL, R.J., 2004, Initiation of turbidity currents: outcrop evidence for Eocene hyperpycnal flow turbidites: Sedimentary Geology, v. 165, p. 29–52. PLINK-BJO¨RKLUND, P., AND STEEL, R.J., 2005, Deltas on falling-stage and lowstand shelf margins, the Eocene Central Basin of Spitsbergen: importance of sediment supply, in Giosan, L., and Bhattacharya, J.P., eds., River Deltas—Concepts, Models, and Examples: SEPM, Special Publication 83, p. 179–206. PONTEŃ, A., AND PLINK-BJO¨RKLUND, P., 2009, Process regime changes across a regressive to transgressive turnaround in a shelf–slope basin, Eocene Central Basin of Spitsbergen: Journal of Sedimentary Research, v. 79, p. 2–23. PORE˛BSKI, S.J., AND STEEL, R.J., 2003, Shelf-margin deltas: their stratigraphic significance and relation to deepwater sands: Earth-Science Reviews, v. 62, p. 283–326. PORE˛BSKI, S.J., AND STEEL, R.J., 2006, Deltas and sea-level change: Journal of Sedimentary Research, v. 76, p. 390–403. POSAMENTIER, H.W., AND ALLEN, G.P., 1999, Siliciclastic Sequence Stratigraphy— Concepts and Applications: SEPM, Concepts in Sedimentology and Paleontology 7, 204 p. POSAMENTIER, H.W., JERVEY, M.T., AND VAIL, P.R., 1988, Eustatic controls on clastic deposition: I, conceptual framework, in Wilgus, C.K., Hastings, B.S., Ross, C.A., Posamentier, H.W., Van Wagoner, J., and Kendall, C.G.St.C., eds., Sea-Level Changes: An Integrated Approach: SEPM, Special Publication 42, p. 109–124. PYLES, D., 2000, A high-frequency sequence stratigraphic framework for the Lewis Shale and Fox Hills Sandstone, Great Divide and Washakie basins, Wyoming [unpublished M.Sc. thesis]: Colorado School of Mines, Golden, 261 p. PYLES, D., AND SLATT, R., 2000, A high-frequency sequence stratigraphic framework for shallow through deep-water deposits of the Lewis Shale and Fox Hills Sandstone, Great Divide and Washakie basins, Wyoming, in Weimer, P., Slatt, R.M., Coleman, J., Rosen, N.C., Bouma, H.C., Styzen, M.J., and Lawrence, D.T., eds., Deep-Water Reservoirs of the World: SEPM, Gulf Coast Section, 20th Annual Research Conference, Houston, p. 836–861. PYLES, D., AND SLATT, R., 2002, Almond, Lewis, Fox Hills and Lance systems in the Greater Green River Basin Wyoming, in Steel, R.J., Crabaugh, J., Carvajal, C., Slatt, R.M., Pyles, D., and Olson, M., eds., Introduction to the Lance, Fox Hills, and Lewis Formations on the Eastern Margin of the Great Divide and Washakie Basins, American Association of Petroleum Geologists, Rocky Mountain Section, Annual Meeting (Laramie), Fieldtrip #1 Guidebook, p. 1–46. PYLES, D., AND SLATT, R., 2007, Stratigraphy of the Lewis Shale, Wyoming, USA, applications to understanding shelf edge to base-of-slope changes in stratigraphic architecture of prograding basin margins, in Nilsen, T.H., Shew, R.D., Steffens, G.S., and Studlick, J.R.J., eds., Atlas of Deep-Water Outcrops: American Association of Petroleum Geologists 56, CD-ROM. REYNOLDS, M.W., 1976, Influence of recurrent Laramide structural growth on sedimentation and petroleum accumulation, Lost Soldier area, Wyoming: American Association of Petroleum Geologists, Bulletin, v. 60, p. 12–33. RICH, J.L., 1951, Three critical environments of deposition and criteria for recognition of rocks deposited in each of them: Geological Society of America, Bulletin, v. 62, p. 1–20. SEILACHER, A., AND AIGNER, T., 1991, Storm deposition at the bed, facies, and basin scale: the geologic perspective, in Einsele, G., Ricken, W., and Seilacher, A., eds., Cycles and Events in Stratigraphy: New York, Springer-Verlag, p. 249–267. SHANMUGAM, G., 2003, Deep-marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons: Marine and Petroleum Geology, v. 20, p. 471–491. SHEPARD, F.P., 1976, Tidal components of currents in submarine canyons: Journal of Geology, v. 84, p. 343–350. SHEPARD, F.P., MARSHALL, N.F., MCLOUGHLIN, P.A., AND SULLIVAN, G.G., 1979, Currents in submarine canyons and other sea valleys: American Association of Petroleum Geologists, Studies in Geology 8, 173 p. SOYINKA, O.S., AND SLATT, R., 2008, Identification and micro-stratigraphy of hyperpycnites and turbidites in Cretaceous Lewis Shale: Sedimentology, v. 55, p. 1117–1133.

672


STEEL, R.J., CRABAUGH, J., CARVAJAL, C., SLATT, R.M., PYLES, D., AND OLSON, M., 2002, Almond, Lewis, Fox Hills, and Lance Systems in the Greater Green River Basin, Wyoming: American Association of Petroleum Geologists, Rocky Mountain Section, Annual Meeting (Laramie), Fieldtrip #1 Guidebook. STEIDTMANN, J.R., 1993, The Cretaceous foreland basin and its sedimentary record, in Snoke, A.W., Steidtmann, J.R., and Roberts, S.M., eds., Geology of Wyoming: Geological Survey of Wyoming, Memoir 5, p. 250–271. STEIDTMANN, J.R., AND MIDDLETON, L.T., 1991, Fault chronology and uplift history of the southern Wind River Range, Wyoming: Implications for Laramide and postLaramide deformation in the Rocky Mountain foreland: Geological Society of America, Bulletin, v. 103, p. 472–485. SUTER, J.R., BERRYHILL, H.L., AND PENLAND, S., 1987, Late Quaternary sea-level fluctuations and depositional sequences, Southwest Louisiana continental shelf, in Nummedal, D., Pilkey, O.H., and Howard, J.D., eds., Sea-Level Fluctuation and Coastal Evolution: SEPM, Special Publication 41, p. 199–219. SYVITSKI, J.P.M., AND MILLIMAN, J.D., 2007, Geology, geography, and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean: Journal of Geology, v. 115, p. 1–19. SYVITSKI, J.P.M., PECKHAM, S.D., HILBERMAN, R., AND MULDER, T., 2003, Predicting the terrestrial flux of sediment to the global ocean: a planetary perspective: Sedimentary Geology, v. 162, p. 5–24. TORRES, J., DROZ, L., SAVOYE, B., TERENTIEVA, E., COCHONAT, P., KENYON, N.H., AND CANALS, M., 1997, Deep-sea avulsion and morphosedimentary evolution of the Rhone fan valley and neofan during the late Quaternary (north-western Mediterranean Sea): Sedimentology, v. 44, p. 457–477.

JSR

UROZA, C.A., AND STEEL, R.J., 2008, A highstand shelf-margin delta system from the Eocene of West Spitsbergen, Norway: Sedimentary Geology, v. 203, p. 229–245. WEBER, M.E., WIEDICKE, M.H., KUDRASS, H.R., HUEBSCHER, C., AND ERLENKEUSER, H., 1997, Active growth of the Bengal Fan during sea-level rise and highstand: Geology, v. 25, p. 315–318. WEIMER, R.J., 1961, Uppermost Cretaceous rocks in central and southern Wyoming, and northwest Colorado, in Wiloth, G.J., ed., Symposium on Late Cretaceous Rocks: Wyoming Geological Association, Annual Field Conference Guidebook 16, p. 17–28. WELLNER, R., BEAUBOUEF, R., VAN WAGONER, J., ROBERTS, H.H., AND SUN, T., 2005, Jetplume depositional bodies: the primary building blocks of Wax Lake Delta: Gulf Coast Association of Geological Societies, Transactions, v. 55, p. 867–909. WETZEL, A., 1993, The transfer of river load to deep-sea fans: A quantitative approach: American Association of Petroleum Geologists, Bulletin, v. 77, p. 1679–1692. WINN, R.D., JR., BISHOP, M.G., AND GARDNER, P.S., 1987, Shallow-water and substorm-base deposition of Lewis Shale in Cretaceous Western Interior seaway, southcentral Wyoming: American Association of Petroleum Geologists, Bulletin, v. 71, p. 859–881. YOSHIDA, S., STEEL, R.J., AND DALRYMPLE, R.W., 2007, Changes in depositional processes: an ingredient in a new generation of sequence stratigraphic models: Journal of Sedimentary Research, v. 77, p. 447–460.

Received 28 March 2008; accepted 5 March 2009.

shelf-edge architecture and bypass of sand to ... - GeoScienceWorld

shelf-edge architecture and bypass of sand to ... - GeoScienceWorld

Suggest Documents

Stratigraphic architecture, magnetostratigraphy ... - GeoScienceWorld

VisÃ©an tectonostratigraphy and basin architecture ... - GeoScienceWorld

sand bank and dune facies architecture raukelv

Sensitivity of fluid flow to fault core architecture ... - GeoScienceWorld

Gravel and Sand Flotation: A Sediment Dispersal ... - GeoScienceWorld

RESEARCH Architecture of the hidden Penokean ... - GeoScienceWorld

Facies architecture of Miocene subaqueous ... - GeoScienceWorld

Mapping the internal structure of sand dunes with ... - GeoScienceWorld

Multiscale Gilbert-type delta lobe architecture and ... - GeoScienceWorld

Turning Stone to Sand

Of Sea and Sand

Tectonic and glacial contributions to focused ... - GeoScienceWorld

Upper- and middle-crustal response to ... - GeoScienceWorld

Introduction to Weathering and Slope Movement ... - GeoScienceWorld

letter to the editor - GeoScienceWorld

Foxground and Berry bypass

to deep-crustal levels - GeoScienceWorld

Riverdominated, shelfedge deltas: delivery of ... - Wiley Online Library

bypass. hypothermic and normothermic

PURIFICATION AND PROPERTIES OF ... - GeoScienceWorld

Application of Generalized Pareto Distribution to ... - GeoScienceWorld

reinterpretation of climactichnites logan 1860 to ... - GeoScienceWorld

micromechanics of sand grain failure and sand compaction

Experimental measurement of wind-sand flux and sand transport for ...