Steel, Ron¹, Jeff Crabaugh², Bob Dalrymple³, Randi Martinsen², Szczepan Porebski⁴, Shuji Yoshida¹ 
(1) The University of Texas at Austin, Austin, TX 
(2) University of Wyoming, Laramie, WY
(3) Queen's University, Kingston, ON 
(4) Polish Academy of Sciences, Krakow, Poland

ABSTRACT: Isolated Basinal Sandbodies of the Western Interior Basin Revisited

Cretaceous sandstone tongues that penetrate unusually 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 preferential occurrence near the leading edge (progradational maximum) of larger-scale, 3rd-order clastic wedges, and their frequent 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. Examples in this spectrum are given from S.Wyoming and N.Colorado. The analysis is hampered by lowstand and transgressive ‘products‘ being preferentially imprinted by tidal currents, partly a function of coarse grain sizes made available from falling-stage by-pass.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.