Abstract: Sequence Stratigraphy and Sedimentology of Cedar Mountain-Dakota Interval, Western and Southern Piceance Creek Basin, Colorado

Rex D. Cole, George E. Moore

This study (1992) is based on sedimentologic and stratigraphic examination of outcrops (N = 40), subsurface cores (N = 9), and recent-vintage well logs (N = 80) of the Cedar Mountain-Dakota interval (Neocomian-Cenomanian) in and around the Piceance Creek basin and adjacent Douglas Creek arch.

In the study area, the Cedar Mountain (Burros Canyon) Formation ranges in thickness from zero to 125 feet and is composed of sandstone, conglomeratic sandstone, and mudrock deposited in a system of incised valleys. Thicker parts of the Cedar Mountain have as many as five valley-fill sequences, although only three sequences appear to correlate regionally. Major incised valleys may have been as wide as 15 miles and had complex longitudinal orientations. Fluvial systems within the valleys had low sinuosities in the western and southern parts of the study area but more moderate sinuosities to the east. The lower sequence boundary of the Cedar Mountain is represented by a major unconformity, or possibly a series of stacked unconformities. Isolated packages of granule-pebble conglomerate (" uckhorn") occasionally occur at the basal sequence boundary.

The Dakota Formation rests unconformably on the Cedar Mountain, ranges in thickness from 65 to 110 feet, and is composed of a complex system of fluvial-deltaic, coastal plain, estuarine, shoreface, and inner-shelf strata. Mapping of facies tracts suggests a northwest-southeast paleo-shoreline trend. In the southwestern part of the study area, the Dakota consists of four major sequences, which grade into the Mowry Shale to the north and northeast.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994