Sequence Stratigraphy of Coal-Bearing Strata, Upper Cretaceous, Washakie Basin, SW Wyoming

R. T. Beaubouef, P. P. McLaughlin, K. M. Bohacs, W. J. Devlin, J. R. Suter

An integrated outcrop - well-log - seismic study of the sequence stratigraphy of the Blair and Rock Springs formations of the Washakie basin provides criteria for recognition of sequences and systems tracts in marine to non-marine transition zones and enhances our understanding of the distribution of paralic coals. Shoreline trends are generally SW-NE, with predominantly marine shales and sandstones in the southeastern part of the study area that change to non-marine coaly coastal plain deposits and fluvial sandstones to the northwest.

Seismic character has been mapped and calibrated to lithofacies in outcrop and well-log data.

From this, integrated recognition criteria have been established for sequence boundaries, transgressive surfaces, and maximum-flooding surfaces in shallow-marine, mixed marine/paralic, and paralic/non-marine depositional environments. Five major sequence boundaries, numerous transgressive surfaces, and two major maximum flooding surfaces can be correlated based on seismic data across the marine to non-marine transition. Most of the transgressive surfaces are marked by abrupt vertical facies changes whereas most of the sequence boundaries are show less abrupt changes. As a result, the transgressive surfaces produce the strongest seismic response and are most easily carried regionally. These facies stacking patterns probably reflect enhanced relative sea level rises and diminished relat ve sea level falls resulting from the interplay of eustasy and high rates of tectonic subsidence.

Seismic character of the paralic and non-marine paleoenvironments is strongly influenced by the thickness and lateral continuity of coaly intervals. In the northwestern part of the study area, vertical facies stacking patterns in non-marine coal-bearing strata appear to be controlled by both long- and short-term accommodation changes. Coals are thickest, most abundant, and most laterally extensive in intervals that can be correlated seismically to transgressive surfaces in the marine paleoenvironments of the southeastern part of the study area. Thus, coal is best developed in the upper part of the lowstand systems tract through the lower part of the trangressive systems tract. Other regionally extensive coaly intervals are recognized within the lower to middle part of some highstand s stems tracts. Both maxima of coal occurrence are associated with predominantly aggradational shoreline stacking patterns.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995