Luncheon – February 2, 2001

Title: Interplay of Paleostructure, Sedimentation and Preservation of Middle Jurassic Rocks, Bighorn Basin, Wyoming

Speaker: by David E. Schmude, (AEC Oil & Gas USA, Inc.)

Date: Friday, February 2, 2001

Publication: The Outcrop, February 2001, p. 4

The Middle Jurassic rocks of the Bighorn Basin are an ideal sequence in which 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. An understanding of the interaction of paleostructure, sedimentation, and preservation, provides insight into the complex geologic history of this region.

Rock units of interest to this study are the Gypsum Spring, Piper and lower Sundance formations. Three significant uplift episodes are also evident in this sequence, represented by the J1, J2 and J4 unconformities. The position and timing of these tectonic events resulted in changes in the preservation potential of the underlying sedimentary package.

The J1 unconformity represents uplift and erosion prior to Gypsum Spring deposition. The J2 unconformity reflects uplift and erosion of the Gypsum Spring Formation, which set up a complex bathymetry for the advancing Sundance Sea. A large north- to northwesterly-oriented paleohigh (Black Mountain High), developed in the southeastern portion of the Bighorn Basin, influencing sedimentation during lower Sundance deposition. The final uplift and erosion event is expressed by the J4 unconformity. This unconformity caps the Middle Jurassic sequence. Data suggest that the Sheridan Arch was uplifted during this time, not post Gypsum Spring time as previously suggested.