Randi S. Martinsen1
(1) University of Wyoming, Laramie, WY
ABSTRACT: Wave Dominated versus Current Dominated Shorelines in the Cretaceous Western Interior Seaway
It's long been recognized that the western margin of the Cretaceous Interior Seaway was strongly wave dominated. 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 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.
In recent years however, an increasing number of current dominated shoreline deposits have been identified. 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 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 sandstones. It is believed that these current dominated deposits represent the distal lowstand shorelines of 3rd order depositional sequences. The change from wave to current dominated shorelines is attributed to a change in basin physiography. Throughout the upper Cretaceous variations in tectonic subsidence produced lateral variations in topography that during some lowstands created embayments. Tidal processes were amplified within these embayments and storm wave processes dampened.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado