Abstract: Paleoenvironments of Frontier Formation (Upper Cretaceous), Northwestern Big Horn Basin, Wyoming

Charles T. Siemers

The Frontier Formation, approximately 166 m (545 ft) thick, in the vicinity of Cody, Wyoming, overlies the marine Mowry Shale, and displays a single "regressive-transgressive" prograding delta-plain cycle of sedimentation. It is overlain by the marine Cody Shale.

Five major paleoenvironmental facies, including 12 well-defined subfacies, are delineated. The lower one-half to two-thirds of the Frontier consists of alternating marine sand-bars and interbar-shale units which grade upward through a prodelta facies into an eastward-prograding delta-plain and delta-margin facies. The deltaic units are overlain sharply by a thin transgressive sandstone and variable nearshore marine sediments which extend upward to the marine Cody Shale.

Paleoenvironmental facies can be differentiated easily on the basis of stratigraphic and lithologic characteristics. Sandstones were categorized as "chert litharenite and feldspathic litharenite;" however, the deltaic sandstones contained only about half as much quartz, twice as much feldspar and more than one and one-half as much rock-fragment material as the marine sandstones. Glauconite was present in most marine and marginal-marine sandstones but never in the deltaic deposits. Clay minerals in the Frontier shales and mudstones are dominated by well crystallized montmorillonite, but illite is common in marginal-marine shales and kaolinite is common in the nonmarine claystones of the delta-plain interval.

Varied trace fossils are present throughout the marine and marginal-marine sediments of the Frontier; however, none were observed in the nonmarine delta-plain sediments. Specific trace-fossil genera are indicative of specific paleoenvironments.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA