Includes west-east, stratigraphic section in southern Piceance Basin showing sandstones, coals, flooding surfaces, and progradational sequences on logs and measured section. Discusses stratigraphic setting of the Mesaverde Group, Piceance Basin. Covers regional genetic stratigraphy, coal occurrence, and cross section of the Upper Cretaceous Mesaverde Group, Williams Fork Formation. 42 pages. 1 color plate. OF-95-02D
Excerpted from the Abstract:
The principal Upper Cretaceous coal-bearing horizons and coalbed gas exploration and production targets in the Piceance Basin occur in the Williams Fork Formation, Mesaverde Group, and are associated with the progradational Rollins-Trout Creek shale and sandstone shoreline sequence. We have genetically defined the bottom of the Williams Fork Formation as the base of the Rollins-Trout Creek Shale (Mancos Tongue, maximum flooding surface), above which a series of at least seven seaward-stepping, progradational sequences extend the Rollins-Trout Creek shoreline sandstone from R97W to R89W. …
The Williams Fork Formation can be further subdivided into several genetic depositional sequences bounded by regionally extensive, low-resistivity shale markers that represent marine flooding surfaces. The first regionally correctable genetic depositional sequence, genetic unit 1, is a clastic wedge that extended coal-bearing coastal plain deposits beyond the present-day basin margin. Three depositional systems are recognized in genetic unit 1: (1) a linear shoreline (strandplain/delta plain) system, backed landward by (2) a coastal plain system, traversed by fluvial systems feeding the advancing shoreline, which in turn grade into (3) an alluvial plain system.