The Role of Crevasse Bodies in the Evolution of Fluvial Architecture, Iles Formation north of Rangely, Colorado

Anderson, Donna S., Sarah Coker
Colorado School of Mines, Golden, CO

Models of Rocky Mountain Cretaceous fluvial systems emphasize the importance of channelbelts and associated bodies such as point bars. Crevasse channels and splays, however, are less emphasized, possibly because such bodies are less documented from outcrop. A 3-dimensional 80-acre outcrop in the nonmarine portion of the Iles Formation north of Rangely, Colorado shows an upward evolution from crevasse splay and channel bodies to those of multi-lateral point bars of a fluvial meanderbelt. Bodies of all types are isolated, yet organized into discrete layers that are separated by laterally continuous deposits of coal, carbonaceous shale, and mudstone. The base of the 170 ft-thick succession consists of isolated crevasse splays erosively overlain by distal crevasse channel bodies. Higher in the section, isolated proximal crevasse channels form thick (up to 25 ft) multistory bodies dominated by trough cross bedding and down-stream accreting barforms. The constructional margins of the crevasse channels consist of current-rippled sandstone beds that change facies into laterally coeval mudstone. Crevasse channels have relatively non-erosive flat bases and concave-down tops. A series of amalgamated chute-modified point bars overlies the entire succession.

The crevasse-channel bodies at the Rangely outcrop are internally more homogeneous than the amalgamated point bars. Crevasse channel sandstones are better sorted than those of point-bars, yet they both have similar average grain-size. These characteristics suggest better potential reservoir quality than the point bars. A blocky outcrop gamma-ray pattern suggests that crevasse channel bodies could be mistaken for “braided river” deposits, if taken out of context from well-log gamma ray patterns, alone.