Stratigraphic, Geochemical, and Paleontologic Evidence for a Brackish, Restricted Seaway in Early Cenomanian (Mid-Cretaceous) Time, Eastern Wyoming

MEREWETHER, E. A., and D. L. GAUTIER, U.S. Geological Survey, Denver, CO

Unusual, concretion-bearing mudrocks of early Cenomanian age, which probably indicate a restricted embayment of the Cretaceous epeiric sea, have been recognized at outcrops in eastern Wyoming and in adjoining areas of Montana, South Dakota, Nebraska, and Colorado. In south-central Johnson County, Wyoming, these rocks are in the lower part of the Frontier Formation and are about 34 m (110 ft) thick; they consist of noncalcareous silty shale and clayey or sandy siltstone and bentonite. The shale and siltstone are either bioturbated or interlaminated. Burrows are common but represent only a few taxa. The rocks contain sparse arenaceous foraminifers, but no macrofossils.

In a drill-core from Johnson County, the lower part of the Frontier was divided into the following units, in ascending order: unit 3, mainly shale; unit 2, mostly concretion-bearing mudrocks; and unit 1, mainly clastic rocks. The composition of unit 2 contrasts with that of the underlying and overlying units. Unit 2 contains no pyrite or dolomite and much less sulfur than units 1 and 3. Carbon-sulfur-iron chemistry for unit 2 suggests that low sulfate availability limited sulfide formation. The proportions of sulfur, organic carbon, and iron in unit 2 and the major-element, isotopic, and paleontologic data suggest that the waters of the southern part of the late early Cenomanian epeiric sea were brackish to fresh, and oxic. The low salinity of this part of the sea might have been caus d by high meteoric runoff from adjoining lowlands and by an abnormal constriction in the seaway north of Montana.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)