Stratigraphic Variability of Sandstone-Body Dimensions in the Williams Fork Formation: Outcrop Data from the Southwest Piceance Basin, Colorado

Rex D. Cole¹ and Matthew J. Pranter^2
1Physical and Environmental Sciences, Mesa State College, Grand Junction, CO
²Geological Sciences, University of Colorado, Boulder, CO

This paper summarizes data on the size and architecture for 912 fluvial sandstone bodies or channel-form elements in the Upper Cretaceous Williams Fork Formation in Coal, Main, and Plateau Creek Canyons, near Palisade, Colorado. Methods used included high-resolution aerial LiDAR, digital orthophotos, outcrop photomosaics, and field mapping. In the study areas, the Williams Fork is approximately 1,800 ft thick and consists of two informal units. The lower one-third has a low net-to-gross ratio (40 to 70% mudrock) and was deposited in a coastal-plain setting. The remainder of the Williams Fork has a moderate to high net-to-gross ratio (25 to 45% mudrock) and was deposited in an alluvial-plain setting.

Individual sandstone bodies in the low net-to-gross interval were deposited primarily by meandering fluvial systems, and occur as three types: single-story channel fills (SSCF), multi-story channel fills (MSCF), and crevasse-splay deposits (CSD). In a 900-foot thick, 5.7-mile long transect of Coal Canyon, 158 SSCF, 321 MSCF, and 320 CS sandstone bodies were documented. The SSCF sandstone bodies range in thickness form 2.6 to 21.1 ft and in apparent width from 44 to 1,700 ft. The MSCF types are 4.5 to 32.5 ft thick and 38 to 2,791 ft wide, whereas the CS sandstone bodies are 0.5 to 9.1 ft thick and 40 to 1,661 ft wide. The moderate to high net-to-gross interval, which crops out in Main and Plateau Creek Canyons, is characterized by sheet-like sandstone bodies deposited by low-sinuosity to braided fluvial systems. These sheets, which consist of numerous amalgamated channel-form (ACF) components, can be traced up to five miles in an east-west direction and may be up to 200 ft thick. Measurement of 113 ACF components in a single sheet shows thickness to range from 8.7 to 54.3 ft and apparent width from 204 to 2,566 ft.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado