Top-Truncated, Structurally-Confined Lowstand Deltas

By

Janok P. Bhattacharya¹, Brian J. Willis², Charles D. Howell¹, and Cornel Olariu¹

¹Geosciences, University of Texas at Dallas, P.O. 830688, Richardson, TX, 75083

²Department of Geology and Geophysics, 3115 Texas A&M University, College Station, TX 77843-3115

The Cretaceous Interior Seaway has been at the center of debates about the origin of elongate "shelf” sandstones encased in marine shales. Some of these basin-distal sandstones have been interpreted to be tidally-influenced incised-valley fill deposits. We suggest that many of these deposits are top-eroded lowstand deltas, as indicated by lobate to elongate geometries, upward coarsening facies successions, basinward dipping internal clinoform bedding, and radiating paleocurrents. Low abundance and diversity of ichnofacies and preponderance of non-marine derived microfossils indicate river-influence. Tidal sedimentary structures indicate tidal modulation during progradation.

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, versus an underlying erosional surface. A low accommodation setting left little room for sandstones to stack vertically, and successive episodes of delta progradation were offset along strike reflecting autocyclic controls. 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 related to foreland tectonics.

AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003