Abstract: Deposition of the Upper Cretaceous Lance Formation in Central Wyoming

JOHNSON, RONALD C., WILLIAM R. KEEFER, C. WILLIAM KEIGHIN, and THOMAS M. FINN

The Lance Formation in central Wyoming consists of largely continental rocks deposited during Late Cretaceous Maastrichtian time following the eastward retreat of Lewis seaway. In the Powder River basin and the Greater Green River Basin the Lance directly overlies the marine Lewis Shale, but in the Wind River and Bighorn basins, the Lance overlies the coaly coastal plain deposits of the Meeteetse Formation that accumulated along the west margin of the seaway. The Lance appears conformable with the overlying Paleocene Fort Union Formation in the deeper parts of all the Laramide basins in central Wyoming. Uplift during Paleocene time, however, resulted in the truncation of all or part of the Lance along the margins of these basins.

Deposition of the Lance coincided with the initial stages of the Laramide orogeny (Late Cretaceous through Eocene). Only the Wind River Range and possibly the Granite Mountains, however, were actively rising during Lance deposition. Stream flow patterns were altered near these uplifts, but elsewhere in central Wyoming, stream flow patterns continued a general eastward flow typical of pre- Laramide trends. Regional subsidence trends were also disrupted near these rising uplifts. A rapidly subsiding east-west trending basin began to form along the structural axis of the Wind River Basin as it existed during the early stages of the Laramide. As much as 6,000 ft of Lance sediments were deposited in the basin, including sandstones that now produce gas in several fields in the region. The north limb of the basin was overridden by the rising Owl Creek Mountains to the north and its eastern extension was cut off by the Casper Arch to the east during the later stages of the orogeny.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado