TOOLS FOR EXAMINING THE ORIGIN OF THE ASYMMETRIC STRATIGRAPHIC ARCHITECTURE OF THE EARLY FILL OF THE WESTERN INTERIOR BASIN

Examination of Jurassic strata of the Sundance Formation in the Black Hills region of South Dakota and Wyoming provides insight into controls of the evolution of stratigraphic architecture of basin fill of an evolving foreland basin (the Sundance Sea in this interval) during an period that was dominated by Green House climatic conditions. The study area is located on what was the tectonically quiet, passive margin of the “craton-ward” side of the basin, so eustatic and climatic change were the primary drivers of stratal arrangement. The stratigraphy displays a long-term trend of trangression followed by regression that is punctuated with multiple parasequences. During this Green House interval, ice sheets were absent, so the rates of eustatic change were relatively slow and the amplitudes of sea level fluctuation were small. Consequently, “high frequency” variation in stratal architecture observed in outcrop and well log cross-sections was shaped by cyclic variation in sediment supply that was governed by climate change. At the time of submission of this abstract we hypothesize that the long-term change in shoreline position is associated with relatively small and slow sea level change associated with temperature change while the parasequences are associated with climatically driven, cyclic pulses of sedimentation. Results of this investigation can serve as a control that can be compared with studies of age equivalent strata from the western flank of the basin to assess the significance of tectonic activity in shaping the stratigraphic architecture of terresterial and marginal marine successions in the portion of the basin, that due to its close proximity to the area of intense crustal deformation, was subjected to a reversed trend in the relationship between the position of terrestrial through marine environments and the distribution of areas of highest subsidence, along with rates of subsidence that were much higher. Combined these yield insight into the evolution of the asymmetric fill of the older portion of the fill of the Western Interior Basin.