Large-scale regressive-transgressive (R-T) packages of the Cretaceous Western Interior seaway are commonly attributed to eustatic and, less dominantly, tectonic forcing. It is now known that considerable along-strike variability exists between the timing and architecture of these R-T packages (e.g., Krystinik and DeJarnett, 1995) and that the timing of many packages differs from those predicted by the Haq et al. (1988) curve. These suppositions are possible because of tight ammonite biostratigraphic control. Genesis of such variability is commonly attributed to local tectonic activity.

We propose that large-scale regressive episodes and subsequent transgressions may be related to drainage reorganisation in the Sevier orogenic wedge associated with the morphometric growth of frontal thrust structures Modern drainage systems, like those active in the Sub-Himalaya, illustrate that river deviations are associated with the progressive lateral growth of anticlines above laterally propagating blind thrust tips. Drainage reorganisation by the growth of structurally controlled topography influences the location and number of river outlets at mountain fronts, and exerts considerable control on sediment dispersal into the Ganges foreland basin.

We suggest that the position, architecture, and timing of many R-T packages is primarily influenced by the interactions of drainage systems and frontal structures. This produces both drainage amalgamation at discrete entry points into the foreland basin and also storage of sediment within piggy-back basins (e.g., Tucker and Slingerland, 1996) for later delivery to the sedimentary basin.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas