Abstract: Valley-Fill and Shelf-Ridge Sandstones of Cenomanian Age, Frontier Formation, Central Wyoming

Roderick W. Tillman

Numerous 400 foot long outcrop measured sections of Cenomanian age Belle Fourche deposits in the Tisdale Mountain area of central Wyoming include laterally correlatable up to 100 foot thick valley-fill sandstone deposits and up to 170 foot thick shelf-ridge sandstone deposits. The interval in which these deposits occur is bracketed by the Clay Spur Bentonite (base of Frontier Formation) and the oldest readily identifiable suite of Cenomanian macrofauna; the Thatcher fauna.

The valley-fill interval is included in the 1020 foot long USGS; Bailey Flats No.2 core located near Kaycee, Wyoming. In the core the previously unrecognized valley fill mudstones occur in association with a previously unrecognized Sequence Boundary and are nearly 100 feet thick.

In outcrop, brackish water estuarine deposits form the valley-fill, and include tidally deposited channel-fill sandstones and mudstones. As is typical of many coarse-grained estuarine tidal deposits marine indicators are rare. A flooding surface at the top of the valley-fill is marked by a thin pebbly marine sandstone and marine shales.

The shelf-ridge sandstone is correlatable for 17 miles from southwest to northeast. A marine silty-shale deposit is correlatable with facies of the shelf-ridge. The shelf-ridge is dominated by wave deposited sandstone, however, tidally deposited and storm-current deposited sandstones are also observed. Recognition of this here-to-for unrecognized Sequence Boundary overlain by thick estuarine tidally dominated valley-fill sandstones adds new dimensions to interpretation of the Belle Fourche Member of the Frontier Formation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994