Janok P. Bhattacharya¹, Brian J. Willis², Charles D. Howell¹

(1) University of Texas at Dallas, Richardson, TX
(2) State University of New York at Oswego, Oswego, NY

ABSTRACT: Top-Truncated, Lowstand Deltas in the Cretaceous Interior Seaway of North America

The Cretaceous Interior Seaway has been at the center of debates about the origin of elongate "shelf" sandstones encased in marine shales. Although many basin-distal sandstones have been interpreted to be deposits of offshore bars or shelf isolated valley fills, examples from the Cretaceous Interior Seaway of North America suggest that many of these deposits may be top-eroded deltas formed where rivers delivered sediment to lowstand coastlines. These sandstones have a lobate to elongate geometry, upward coarsening facies successions, basinward dipping internal clinoform bedding, and radiating paleocurrents. Low to moderate degree of shallow marine burrowing, a high proportion of non-marine spores and pollen, and lack of foraminifera, within the mudstones, directly indicate river-influenced deposition.

Delta plain "topset" facies were eroded during transgression, placing marine mudstone on top of delta front sandstones. The capping erosion surfaces are the only stratal discontinuities that can be mapped regionally, compared to lowstand surfaces of erosion recording the bypass of sediments basinward. A low accommodation setting left little room for sandstones to stack vertically, and successive episodes of delta progradation were offset along strike. More tide- and river-influenced delta deposits formed within shoreline embayments defined by the topography of older wave-influenced delta lobes and subtle syndepositional deformation of the basin floor.

Sequence stratigraphic terminology is difficult to use because sandstones do not show simple stacking patterns, major discontinuities form by processes other than fluvial erosion, and minor syndepositional deformation of the basin floor exerts a first-order influence on sediment deposition and preservation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado