A High Frequency Sequence Stratigraphic Framework for Shallow through Deep-Water Deposits of the Lewis Shale and Fox Hills Sandstone, Great Divide and Washakie Basins, Wyoming

Pyles, D.R. and R.M. Slatt, Dept Of Geology And Geological Engineering, Colorado School of Mines, Golden, Colorado

Abstract

A detailed study of outcrops, cores, and well-logs in the Upper Cretaceous Lewis Shale and Fox Hills Sandstone of south-central Wyoming indicate that the formations were deposited in a transitional shelf-slope-basin setting during a time of rapidly fluctuating relative sea level. The fluctuations in relative sea level effected the location and style of sand and mud deposition.

Sequence analysis at outcrops, cores, and in well log correlations have been used to interpret a high-frequency sequence stratigraphic framework along a regional (50 miles long) depositional dip-oriented cross section.

High frequency transgressive, highstand, and lowstand systems tracts are interpreted in this system. Transgressive systems tracts comprise highly continuous, organic-rich shales (below storm wave base) and continuous interbedded sand stones and mudstones (above storm wave base) that onlap the coastline and downlap the basin floor.

Highstand systems tracts comprise continuous, thick-bedded sandstones and thin-bedded mudstones (above storm wave base) and continuous thin-bedded sandstones and mudstones (below storm wave base) that onlap above the coastline and downlap on the shelf and upper slope.

Lowstand systems tracts can be divided into the basin floor fan, slope fan, and prograding complex. The basin floor fans comprise continuous thick-bedded sandstones and thin-bedded mudstones that bi-directionally downlap the basin floor. Slope fans comprise channelized, discontinuous sandstones, thin-bedded continuous sandstones, and continuous mudstones that downlap the basin floor fan and onlap the slope. Prograding complexes comprise thin-bedded, continuous sandstones and mudstones that downlap the slope fan and basin floor fan and onlap the upper slope and shelf.

The high-frequency sequence stratigraphic framework presented in this study may provide a bridge between subsurface data and rock attributes for formations where outcrop and core data are not available.