Subsurface Stratigraphy of the Upper Cretaceous Lower Williams Fork Formation, Piceance Basin, Northwestern Colorado

The lower Williams Fork is divided into five informal intervals which consist largely of interbedded sandstones, siltstones, shales, and coal deposited in shoreface, river channel, overbank, crevasse-splay, and swamp settings. The lower Williams Fork was deposited within the highstand-systems tract of a third-order sequence. This HST is bounded by a maximum flooding surface at the top of the underlying Cozzette (75.1 Ma) and a sequence boundary at the base of the overlying middle Williams Fork (73.5 Ma). The width of the coal zone in the lower coastal-plain varied through time from 30-40 km for intervals 1 and 3, to about 15-20 km for intervals 2 and 4. The differences in widths are likely associated with variation in accommodation.