Genetic Sequences and Unconformities in Shallow Marine to Fluvial Depositional Systems, Mesaverde Group, North-Central Wyoming

KLUG, BERND, Universitat Bonn, Bonn, F.R.G., CARL F. VONDRA,* Iowa State University, Ames, IA, and PAUL WURSTER, Universitat Bonn, Bonn, F.R.G.

Continuous exposures of the Mesaverde Group (Campanian) in the Bighorn basin area, Wyoming, were utilized to establish regional facies architecture and to test sequence stratigraphic concepts along and perpendicular to the general trend of the shoreline of the Western Interior Cretaceous Seaway.

Sections along the west flank of the basin begin with stacked seaward stepping, wave-dominated beach sandstones (Eagle equivalent) that are fed by widely spaced river systems. These sandstones grade eastward into storm influenced intercalated shale/sandstone beds of the lower shoreface-shelf transitional zone. The uppermost beach sequence is frequently incomplete. Bioturbated lower and upper shoreface deposits are often truncated by a laterally continuous erosion surface and overlain by coastal swamp and channel deposits, suggesting a regional regressive unconformity. The overlying fluvial units (Parkman and Judith River equivalents) exhibit a distinct transition in architecture from single and multistoried, lens-shaped, avulsion-controlled, low sinuosity channel bodies to single-stor ed sheets of high sinuosity channels that consist exclusively of gently dipping, heterolithic lateral accretion units. This fluvial interval in the west correlates with a series of complexly superimposed beach and shelf deposits in the eastern part of the basin, corroborating base-level control on the observed changes in fluvial style.

The uppermost depositional sequence of the Mesaverde is the Teapot Sandstone, a conspicuous multistoried sheet sandstone that consists of laterally amalgamated, vertically stacked low to high sinuosity channels. Floodplain sediments are only represented by shale rip-up clasts in channel lags. Laterally persistent ferricrete horizons, containing plant impressions, are time significant surfaces within the Teapot and indicate a rhythmic pattern of sedimentation, nondeposition, and pedogenesis. The base of the Teapot unconformably overlies weathered lower shoreface sandstone along the east flank of the Bighorn basin and thus represents a regional sequence boundary.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)