XUE, LIANG, and WILLIAM E. GALLOWAY, Department of Geological Science, Univ. of Texas at Austin, Austin, TX

ABSTRACT: High-Frequency Genetic Stratigraphic Sequences of the Middle Wilcox Subgroup in the Northwest Gulf of Mexico Basin

The Middle Wilcox Subgroup in the Northwest Gulf of Mexico Basin is consistently defined by maximum flooding surfaces associated with the Big Shale at the bottom and the Yoakum Shale at the top and dated at 56.5 Ma and 55.0 Ma respectively. The structure maps at the base and top of the Middle Wilcox Subgroup illustrate the gentle southeastward-dipping arcuate monocline, reflecting the stable tectonic setting. Based on the detailed correlations of up to 700 well logs, two high-frequency Middle Wilcox genetic stratigraphic sequences were delineated. They encompass a 1.5 Ma time span and are equivalent to the upper half part of TA2.1 3rd-order cycle of Haq and others (1988). Each Middle Wilcox sequence had a maximum duration of less than a 1 million years and provided high-resolution str tigraphic framework for lithofacies mapping and paleogeographic reconstruction. The isopach maps of both Middle Wilcox sequences show the discontinuous, thick strike-oriented sediment accumulation along the ancient shelf edge, especially concentrated along the growth-faulted continental margin. The net-sandstone maps of both Middle Wilcox sequences depict the strong dip-dominated trends in the Houston and Rio Grande Embayments and strike-oriented patterns in the San Marcos Arch. There is no major reorganization of sediment supply axes along strike during Middle Wilcox deposition; whereas downward shifts of deposition beyond the highstand shelf edge occurred during lowstand time. The regional depositional systems for each Middle Wilcox sequence are outlined on the basis of a comprehensive interpretation of the well-log data.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.