ABSTRACT: Sequence Stratigraphic Controls on Coal Distribution, Campanian, Mesaverde Group, Sand Wash Basin, Colorado

Larry B. Kellison

The Iles and Williams Fork formations of the Mesaverde Group in the Sand Wash basin provide an excellent opportunity to assess tectonic, eustatic, and paleoclimatic controls on coal deposition within a framework of intertonguing marine and nonmarine strata. Estimated coal reserves exceed 1 billion tons of surface minable coal and approximately 12 tcf of coalbed methane.

The Mesaverde Group thickens from 800 m near Steamboat Springs to over 1000 m west of Craig. Typical shoreline parasequences are 12 to 60 m thick comprising bioturbated mudstone with thickening upward hummocky cross-stratified sandstone overlain by massive, swaley to bidirectionally trough cross-bedded sandstone capped by parallel to low-angle cross-beds. Thick coastal plain deposits overlying these parasequences are comprised of coal and carbonaceous shale, crevasse splay and meandering stream deposits, tidally influenced, lowsinuosity fluvial channels, and lagoonal shale and sandstone. Low ash, low sulfur coal beds up to 15 m thick are split by crevasse splay or meandering stream deposits suggesting deposition in low-lying mires.

High-resolution facies analysis and subsurface and paleontologic control suggest local subsidence rates exceeded rates of third-order eustatic change resulting in the deposition of high-stand, shelf margin, and transgressive system tracts. Coals in the seaward-stepping portion are thin and discontinuous. Coastal plain deposits of the regressive maxima contain the thickest and most laterally consistent coal beds. In the landward-stepping portion of the Mesaverde Group coals are numerous, but laterally discontinuous.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990