ABSTRACT

Regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply-buried linear shale masses characterized by low bulk density and high fluid pressure. From seismic data these masses, commonly tens of miles in length, have been observed to range in size up to 25 miles in width and 10,000 ft. vertically. These features, aligned subparallel to the coast are en echelon or branching in pattern, and represent residual masses of undercompacted sediment between sand-shale depoaxes in which greater compaction has occured. Most regional contemporaneous fault systems in the Texas coastal area were formed during times of shoreline regression when periods of fault development were relatively short, and where comparatively simple down-to-the-basin fault patterns were formed. In cross sectional view, faults in these systems flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses. Data indicate that faults formed during time of shoreline regression were developed primarily through differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sand-shale section.

Where subsidence exceeded the rate of deposition, gravitational faults developed where basinward sea floor inclination was established in the immediate area of deposition. Some of these faults became bedding plane type when the inclination of basinward-dipping beds equaled the critical slope angle for gravitational slide. Fault patterns developed in this manner are comparatively complex and consist of numerous antithetic faults and related rotational blocks.

Nondepositional (structural) faults are common on the landward flanks of deeply buried shale masses. Many of these faults dip seaward and intersect the underlying low density shale at relatively steep angles.

Conclusions derived from these observations support the concept of regional contemporaneous fault development through sedimentary processes where thick masses of shale are present and where deep seated tectonic effects are minimal.