Glacioeustatic Influence on Lower Miocene Coastal Sand Distribution, South-Central Texas: Calibration and Testing of a Calculated Neogene Glacioeustatic Sea Level Curve

William E. Galloway, Quicheng Ye, R. K. Matthews, Cliff Frohlich

The Lower Miocene succession in south-central Texas was deposited within a streamplain/shore-zone/shelf/slope depositional system tract. Principal supply of sand to the barrier islands and strandplains that form the sandy core of the shore zone system was along shore from adjacent delta systems in the Rio Grande and Houston embayments. Such a strike-fed shoreline provides a particularly sensitive barometer of relative sea level fluctuation. Sediment supply variability was suppressed by the long distance of strike transport, a broad shelf fronted the shoreface, and the coastal plain was low-relief and easily flooded.

Initial spectral analysis showed that 95, 54, 41, and 30 KY frequencies are prominent in cyclic distal shoreface to shelf successions. A shoreline trajectory plot was derived from interpretation of a regionalized stratigraphic dip cross section. Comparison of shoreline trajectory, deep-ocean oxygen isotope data, and calculated glacioeustatic curve reveals good correlation between predicted and observed fall and low-stand intervals that are spaced at 2 million year intervals. Each of these long-term falls and low stands is characterized by prominent episodes of shore-zone progradation. The initial low stands, which were contemporaneous with the time of highest sediment supply, were also times of shelf-to-slope sand bypassing. In the context of long-term sediment supply fluctuation, the calculated glacioeustatic curve clearly predicts specific times of most probable coastal offlap and shelf sand bypassing.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995