WESTERN INTERIOR SEAWAY PROJECT (WISP)
Image from top left and going clockwise: Blair Delta, Rusty Estuary, Trail Brackish Interval, Sandbody Correlation, Blair Beds, Possible Lowstand Model (from D. Mellere)
PIs: R.Steel, R. Martinsen, J. Crabaugh
Cretaceous sandstone tongues that penetrate far out into the Western Interior Basin have generated debate over the last 25 years because (1) they are anomalously coarse-grained potential reservoirs, (2) the mechanism of their dispersal, far out onto shelf sites, has been especially problematic and debated, and (3) they can appear to be unpredictable. Their basinal location, downdip of highstand shoreline tongues that have transited the shelf for 10s to 100s of km, dictate that they derive from falling-stage and lowstand shorezones. This is reinforced by their occurrence near the leading edge (progradational maximum) of larger-scale, 3rd-order clastic wedges, and their ocasional time-equivalence with fluvially-eroded valleys in updip areas. They are therefore ramp-basin analogs to deepwater slope and basin-floor sands in basins with a shelf-slope break, and probably have as much variability. This variability arises because such lowstand shorezones have been partly or entirely reworked by tidal and other currents in the narrow, lowstand seaways of the broken Campanian foreland (embryonic Laramide basins). This class of ‘shelf’ sands (falling stage, lowstand and transgressive) is likely to include (a) forced regressive/lowstand deltas with minimum transgressive reworking, (b) transgressive shelf-sand ridges, entirely reworked/detached from host lowstand tract, (c) transitional cases where lowstand deltas are overlain by transgressive sands, though skewed into the adjacent seaway. In this project we will focus our study on the variability of the basinal 'shelf' sandbodies, but will also tie them back northwards and norhwestwards to updip highstand equivalents.
WISP PROJECT PROPOSAL
INTRODUCTION & REGIONAL SETTING
It’s long been recognized that the western margin of the Cretaceous Interior Seaway was strongly wave-dominated (Ericksen and Slingerland, 1990). Shoreline deposits consist of sandstones that are hummocky and swaley cross-stratified to bioturbate in their lower parts and trough cross-stratified to sub-horizontally laminated in their upper parts and are typically overlain by coastal plain deposits. Deltas were so strongly wave-dominated that distinguishing deltaic from inter-deltaic successions commonly hinges on whether or not they are associated with a distributary channel (Hendricks, 1983; Rice and Gautier, 1983).
In recent years however, an increasing number of current-dominated shoreline deposits have been identified (Martinsen, 2001; Figure 1). These deposits are much more heterolithic than wave-dominated shoreline deposits. Sandstones are rippled to trough cross-stratified, and often contain mudstone interbeds and clasts. Currents dominantly flowed south. Evidence of tidal influence includes: organic rich mud couplets, reverse flow ripples, and sigmoidal to tidally bundled cross-beds. Although these deposits show evidence of waves, hummocky cross-stratification is usually noticeably absent. Furthermore, they are not often closely associated with coastal plain deposits but encased in marine shale. Their anomalous characteristics compared to wave-dominated successions lead to previous interpretations as shelf sandstone (e.g. Shannon Sandstone) and submarine fan (e.g. Lower Sandy Member, Blair Formation) deposits.
It is proposed that these current-dominated deposits represent the distal lowstand shorelines of low-order depositional sequences. The change from wave to current-dominated shorelines is attributed to a change in basin physiography. Evidence indicates the Western Interior Cretaceous Basin was tectonically partitioned into uplifts and basins (broken foreland of Jordan, 1995) long before the major episode of Laramide deformation (Weimer, 1978, 1983; Schwartz & DeCelles, 1988; Ambadar and Vondra, 1989; Dolson et al., 1991; Steidtmann, 1993; Jordan, 1995; Martinsen, 2003). It is likely that throughout the Cretaceous variations in tectonic subsidence produced lateral variations in topography that during (at least some) lowstands created embayments. Tidal processes were amplified within these embayments and storm wave processes dampened. Furthermore, fluvial drainages and shoreline progradations changed from overall west-to-east to south-southeast (Posamentier and Morris, 2000; Martinsen, 2003, 2004; Fitzsimmons, 1995; Van Wagoner et al., 1990).
Few studies exist however, that document the precise stratigraphic relationships between lowstand and highstand deposits. Where such studies have been undertaken there is often a lack of consensus among workers regarding stratigraphic correlations. Unfortunately, many of the cross-sections constructed to document depositional system linkages are oriented east-to-west. If these lowstand systems more often prograded south, then linkages to more proximal highstand and falling stage deposits should be studied along north-south transects.
PROPOSED RESEARCH
The project will study linkages between highstand and lowstand shoreline systems along several north-south transects. Each transect will focus on Lower to lower Upper Campanian age (Mesaverde Group) depositional systems.
Transect one.
The first transect will be constructed mostly from subsurface data and will run from the Powder River Basin of Wyoming north into Montana. Formations of interest are the problematic Shannon and Sussex Sandstone Members of the Mesaverde Group and outcrop and core data will be integrated along portions of the transect where possible.
Transect two.
The second transect will consist of a well log cross section that runs from the southern Washakie Basin of Wyoming north through the Great Divide Basin. Units of interest are within the Haystack Mountains Formation and include the Tapers Ranch (~ equivalent to the Shannon Sandstone), O’Brien Springs and Hatfield sandstones. At attempt will be made to continue transect two north into the Wind River and Bighorn basins. However, it is uncertain that this task can be accomplished because of the lack of data on the intermountain uplifts between the Rawlins Uplift, Wind River and Bighorn basins.
Transect three.
The third transect will be based mostly on outcrop correlations and will run from northwestern Colorado north along the Rawlins Uplift, approximately parallel to the southern portion of transect two. Interval of interest is the upper Mancos (northwest Colorado)/Steele (Wyoming) shale and the overlying Mesaverde Group (Iles, Haystack Mountains Formation).