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Abstract: Interplay of Uplift, Erosion, Sedimentation and Preservation of Middle Jurassic Rocks, Big Horn Basin, Wyoming

The Middle Jurassic stratigraphic sequence in the Big Horn Basin provides an ideal situation to study the interaction of uplift and erosion and their influences on marine deposition and preservation. The Basin contains thousands of well penetrations and excellent outcrops. Rock units of interest include the Gypsum Spring and Lower Sundance Formations.

Three significant uplift episodes are evident in this sequence and are represented by the J1, J-2 and J-4 unconformities. Uplift and erosion prior to Gypsum Spring deposition is represented by the J-1 uncofformity. The J-2 unconformity reflects uplift and erosion of the Gypsum Spring Formation, which set up a complex bathymetry for the advancing Sundance Sea. A large north-northwesterly oriented paleohigh (Black Mountain High), developed at this time in the southeastern portion of the Big Horn Basin. This feature appears to influence deposition during Lower Sundance time. The northeasterly oriented Sheridan Arch does not appear to be a significant positive element within the study area at this time. The final uplift and erosion event is expressed by the J-4 unconformity, which caps the Middle Jurassic sequence. Data suggest that ther Sheridan Arch was uplifted during J-4 time and that the obvious thinning observed on a regional isopach of the Lower Sundance is due to erosion during J-4 time.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana