STEEL, R., J. CRABAUGH, B. McLAURIN, and J. SCOTT
Department of Geology and Geophysics, University of Wyoming, Laramie, WY

Abstract: Channels, channel-belts and sequences in the Campanian Castlegate and Ericson Formations

Thalweg-fill, supraplatform bar, progradational crevasse splay, abandoned channel fill and overbank facies associations are the building blocks of Campanian alluvium within the Castlegate and Ericson Formations of SW Wyoming and NE Utah.

Combinations of these components form simple channel belts (channel bar- to-avulsion units), probably best developed in high-avulsion rate settings. Complex channel belts have multiple thalweg-fill tiers and develop from repeated reoccupation of older channels (within a single belt), and thus symptomatic of low avulsion rates. Complex belts are also capped by an avulsion suite of muddy abandoned channels and crevasse complexes.

Alluvial sequences typically show a time trend from complex up to simple channel belts, though there can be some reversal of this trend towards the sequence top. Characteristic features of sequences are an upward decrease in erosion-surface density, in channel-belt interconnectedness and in overall sand-shale ratio, as well as an upward increase in muddy crevasse units and in brackish/tidal indicators. Both third- and fourth-order sequences with these characteristics can be recognised in the Castlegate and Ericson successions.

Both channel belts and sequences are progressively more difficult to recognise as the alluvial succession amalgamates, for example across slightly active, Laramide structural highs. Conversely, the hierarchy of simple belts, complex belts, and fourth-order sequences are easier to document in basinal areas where accommodation to sediment supply ratio is higher.

It is likely that the erosion-surface clusters (unconformities) in the lower parts of fourth-order alluvial sequences correlate with episodes of shoreface and lowstand delta progradation in the time-equivalent, marine Iles Formation in northern Colorado.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas