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Abstract: Marine-Shelf Model for Upper Cretaceous Hygiene Member of Pierre Shale, West Denver Basin, Colorado

Karen W. Porter

Outcrop and subsurface studies of the Hygiene Member suggest that deposition of this thick sequence was in an offshore marine-shelf environment. Three facies can be identified in a stratigraphic model showing an overall coarsening-upward sequence of (a) 600 to 700 ft (180 to 210 m) of bioturbated, muddy sandstones and sandy mudstones (shelf or interbar facies); (b) 2 to 20 ft (0.6 to 6 m) of indistinctly cross-stratified, well-burrowed sandstone (bar-margin facies); and (c) 50 to 70 ft (15 to 21 m) of moderately sorted, trough cross-stratified sandstone with mudstone interbeds (central-bar facies). The sandstones are glauconitic litharenites bound by a matrix of compacted argillaceous lithic grains and authigenic clay.

Late in early Campanian time an arcuate segment of the western shoreline of the interior seaway extended into south-central Wyoming as the Parkman delta. Storm-intensifed and storm-generated shelf currents swept predominantly fine-grained material from this delta southward across the existing shaly lower Pierre shelf. Sand was transported by particularly strong storm systems. In nonstorm intervals thorough mixing of sediments by burrowing organisms occurred. As sediments built to water depths regularly within reach of storm waves and currents, predominantly sand was transported and deposited. Periodic storms winnowed and redeposited the sand, forming local bars across broad sandy areas of the shelf. Finer materials were scoured from adjacent shelf areas and, as the storm passed, settl d as a thin mantle and thus formed interbeds between successive storm cosets.

Development of stacked sandstone zones in the upper part of the Hygiene Member may reflect local tectonic influence on Hygiene shelf sedimentation. The sharp contact of upper Hygiene cross-stratified sandstones against mudstones of the overlying Terry Member, however, records a widespread deepening of water over the northern Colorado shelf, perhaps reflecting a eustatic change in sea level.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado