Top-Truncated, Structurally-Confined Lowstand Deltas
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.Major discontinuities form by processes other than fluvial erosion and minor syndepositional deformation of the basin floor controls sediment deposition and preservation. Tidal facies may be found in lowstand deltas built onto undulating seascapes associated with tectonically produced embayments. Tidal facies are not limited to transgressive systems tracts and should not automatically be interpreted as being deposited within fluvially-incised valleys.
AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003