Subsurface Geometry of Currently and Recently Active Faults, Arcola Area, Brazoria County, Texas

Justine A. Boccanera, David H. Melnyk

A study of currently and recently active faults in northwest Brazoria County, Texas, used aerial photography, well logs, and seismic data to prove the long-established Arcola and Iowa Colony faults to be, respectively, a major down-to-the-coast growth fault and a relatively young antithetic structure. Several previously unknown faults were identified as part of this system, including relatively minor synthetic and antithetic features, and an earlier generation antithetic to the Arcola--the Stevens Road fault.

Most faults within the Arcola-Iowa Colony system exhibited growth character. Expansion indices indicate three major events for this system: (1) a decrease in movement rate on the Arcola since the Oligocene, corresponding to a concomitant decrease in sedimentation rate, (2) initiation of the Stevens Road fault as the major antithetic fault to the Arcola during upper Vicksburg deposition, and (3) activation of the Iowa Colony fault as the primary antithetic fault to the Arcola during earliest Miocene. Structure contour maps show that successful wells in the Arcola oil field, bounded by this fault system, are located on the closure developed by rollover into the Arcola and Stevens Road faults. Numerous dry holes along the Arcola-Iowa Colony rollover anticline suggest that some early expl rationists were misled by this younger structural feature.

All faults in this system are listric in shape with initial dips of 70°, decaying to approximately 54°-59° at 8,000-ft depths. Application of the Gibbs model to describe the listric shape of these faults did not account for the observed geometries. Predicted fault dips and depth to detachment were much too shallow. However, a model invoking sediment compaction rate as the principle factor determining growth fault geometry was found to be successful.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.