CHARACTERIZATION OF FLUVIAL SAND BODIES IN THE LOWER WILLIAMS FORK FORMATION (CAMPANIAN), COAL CANYON AREA, COLORADO

The lower Williams Fork Formation in the southwestern Piceance Basin is a classic low net-to-gross sequence deposited in a lower coastal-plain setting adjacent to the Western Interior seaway. Fluvial systems were sinuous to anastomosing, flood plains poorly drained, and peat-forming mires, marshes, swamps, and lakes common. Detailed sedimentological examination and mapping of fluvial sand bodies in the lower Williams Fork (500 to 700 ft thick) was conducted in Coal Canyon, near Palisade, Colorado. Within the study area (~800 acres), 136 fluvial sand bodies were mapped using GPS receivers. Stratigraphic control for the sand bodies was defined by 15 measured sections (total = 4,441 ft). The field data indicate a range in average thickness from 0.5 to 29.0 feet (mean average = 9.3 feet), and a range in apparent sand-body width from 40.1 to 2,791.1 feet (average = 528.4 feet). Paleocurrent data (N = 1,646) collected from 99 of the 136 sand bodies indicates a unimodal distribution with a vector mean of 75º. Sand bodies in the study area can be grouped into five genetic types. Type A sand bodies were deposited by anastomosed channels. Type B sand bodies were deposited by sinuous channel systems that did not develop significant meander belts. Type C sand bodies, on the other hand, were deposited by more robust sinuous channel systems that did produce significant meander belts. Type D and E sand bodies are interpreted as crevasse-channel and crevasse-splay, facies, respectively.