Tectogenic Sedimentary Fill and Subsidence History of Hanna Basin, Southeastern Wyoming: ABSTRACT

Abstract

The Hanna basin in southeastern Wyoming if unique because of its small size, and great depth, and extremely thick upper Cretaceous through Tertiary tectogenic sedimentary fill. The basin filled from the north, where proximal conglomeratic sediments were shed by the rising Sweetwater arch, and from the southwest, where more distal sandy sediments prograded into the basin. At the same time, vast coal deposits were accumulating in the center of the basin. Subsidence analysis, together with detailed stratigraphic and structural studies along the northern basin margin, show that the Hanna basin did not form as a flexural response to tectonic loading by the Shirley thrust. Constraints on potential mechanisms for Hanna basin evolution include (1) approximately 43,000 ft of basement offset adjacent to the Shirley thrust, (2) nearly 25,000 ft of basement relief adjacent to Simpson Ridge, (3) deposition of about 30,000 ft of Late Cretaceous through Tertiary tectogenic sedimentary fill, and (4) a pre-Shirley fault source for feldspathic sediments from the north. Their current modeling suggests that the present basin configuration is the result of at least three evolutionary phases: (1) initial uplift of the Sweetwater arch and associated downwarping of the adjacent syncline to the south, (2) breakup andmore » deepening of the synclinal depression, possibly by basement-block rotation and associated extension, and (3) post-early Eocene compression that activated the Shirley thrust and molded the present structural configuration of the Hanna basin.« less