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STONE, DONALD S.
6178 So Lakeview St., Littleton, CO 80120

Abstract: Geology of the Cloud Creek Impact Structure on the Casper Arch, Central Wyoming

Located near the center of the Casper Arch in central Wyoming and buried below the Late Triassic-Middle Jurassic (T-J) unconformity at a depth of ~1130 m, the 7-km-diameter Cloud Creek structure displays the morphological characteristics of a complex terrestrial impact crater. Geologic interpretation of the Cloud Creek structure is based on reflection seismic data, supplemented by gravity and magnetics mapping, and by data from ten exploratory wells that penetrated some part of the circular Cloud Creek structure. There is a 1.4-km-diameter central peak enclosed by a ring fault zone, and a 1.6 km-wide encircling skirt sloping into an annular trough that is overlain by a fault-bounded, partial rim anticline. Within the central peak, strongly deformed Paleozoic strata are centripitally uplifted ~520 m (Precambrian uplifted ~200 m) above extracrater levels and truncated by members of the Middle Jurassic Sundance Formation. Crater morphology has been modified by Early Jurassic erosion and an important Laramide overprint.

The Cloud Creek impact event occurred near the lower end of the 40-m.y. time gap spanned by the T-J unconformity, i.e., near the close of Triassic (~190 +/- 30 Ma). Cloud Creek may be part of a crater chain including the Saint Martin (Manitoba) and Red Wing Creek (North Dakota) impact structures.

Optical petrographic examination of poor-quality drill cuttings from five boreholes has been disappointing. There is evidence of concussion fracturing in tectosilicate quartz fragments, but evidence of higher-pressure, shock metamorphism (e.g., PDFs) has not been identified. Nevertheless, in the absence of evidence of endogenic activity (e.g., volcanism, salt diapirism) anywhere on the Casper Arch, meteorite impact is clearly the preferred interpretation of the extensive geophysical and borehole database.

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