Surface faults in the gulf coastal plain between Victoria and Beaumont, Texas

Abstract

Displacement of the land surface by faulting is widespread in the Houston-Galveston region, an area which has undergone moderate to severe land subsidence associated with fluid withdrawal (principally water, and to a lesser extent, oil and gas). A causative link between subsidence and fluid extraction has been convincingly reported in the published literature. However, the degree to which fluid withdrawal affects fault movement in the Texas Gulf Coast, and the mechanism(s) by which this occurs are as yet unclear. Faults that offset the ground surface are not confined to the large (>6000-km 2 ) subsidence “bowl” centered on Houston, but rather are common and characteristic features of Gulf Coast geology. Current observations and conclusions concerning surface faults mapped in a 35,000-km 2 area between Victoria and Beaumont, Texas (which area includes the Houston subsidence bowl) may be summarized as follows: (1) Hundreds of faults cutting the Pleistocene and Holocene sediments exposed in the coastal plain have been mapped. Many faults lie well outside the Houston-Galveston region; of these, more than 10% are active, as shown by such features as displaced, fractured, and patched road surfaces, structural failure of buildings astride faults, and deformed railroad tracks. (2) Complex patterns of surface faults are common above salt domes. Both radial patterns (for example, in High Island, Blue Ridge, Clam Lake, and Clinton domes) and crestal grabens (for example, in the South Houston and Friendswood-Webster domes) have been recognized. Elongate grabens connecting several known and suspected salt domes, such as the fault zone connecting Mykawa, Friendswood-Webster, and Clear Lake domes, suggest fault development above rising salt ridges. (3) Surface faults associated with salt domes tend to be short (<5 km in length), numerous, curved in map view, and of diverse trend. Intersecting faults are common. In contrast, surface faults in areas unaffected by salt diapirism are frequently mappable for appreciable distances (>10 km), occur singly or in simple grabens, have gently sinuous traces, and tend to lie roughly parallel to the ENE-NE “coastwise” trend common to regional growth faults identified in subsurface Tertiary sediments. (4) Evidence to support the thesis that surface scarps are the shallow expression of faults extending downward into the Tertiary section is mostly indirect, but nonetheless reasonably convincing. Certainly the patterns of crestal grabens and radiating faults mapped on the surface above salt domes are more than happenstance; analogous fault patterns have been documented around these structures at depth. Similarly, some of the long surface faults not associated with salt domes seem to have subsurface counterparts among known regional growth faults documented through well logs and seismic data. Correlations between surface scarps and faults offsetting subsurface data are not conclusive because of the large vertical distances (1900- 3800 m) involved in making the most of the inferred connections. Nevertheless, the large number of successful correlations - in trend, movement sense, and position - suggests that many surface scarps represent merely the most recent displacements on faults formed during the Tertiary. (5) Upstream-facing fault scarps in this region of low relief can be significant impediments to streams. Locally, both abandoned, mud-filled Pleistocene distributary channels and, more commonly, Holocene drainage lines still occupied by perennial streams reflect the influence of faulting on their development. Some bend sharply near faults and have tended to flow along or pond against the base of scarps; others meander within topographically expressed grabens. Such evidence for Quaternary displacement of the ground surface is widespread in the Texas Gulf coast. In the general, however, streams in areas now offset by faulting show no disruption of their courses where they cross fault scarps. Such scarps are probably very young, and where they can be demonstrated to partly or wholly predate fluid withdrawal, very recent natural fault activity is indicated. (6) Early aerial photographs (1930) of the entire region and topographic maps (1915-16 surveys) of Harris County (Houston and vicinity) show that many faults had already displaced the land surface at a time when appreciable pressure declines in subjacent strata were localized to relatively few areas of large-scale pumping. Prehistoric faulting of the land surface, as noted above, appears to have affected much of the Texas Gulf Coast. (7) A relation between groundwater extraction and current motion on active faults is suspected because of the increased incidence of ground failure in the Houston-Galveston subsidence bowl. This argument is weakened somewhat by recognition of numerous surface faults, some of them active today, far beyond the periphery of the strongly subsiding area. Moreover, tilt beam records from two monitored faults in northwest Houston and accounts of fault damage from local residents demonstrate a complex, episodic nature of fault creep which can only partially be correlated with groundwater production. Nevertheless, although specific mechanisms are in doubt, the extraction of groundwater from shallow (<800-m) sands is probably a major factor in contributing to current displacement of the ground surface in the Houston-Galveston region. Within this large area, the number of faults recognizable from aerial photographs has increased at least tenfold between 1930 and 1970. Elsewhere in the Texas Gulf Coast only a moderate increase has been noted, some of which is possibly attributable to oil and gas production. Surface fault density in the Houston-Galveston region is far greater than in any other area of the Texas Gulf Coast investigated to date. A plausible explanation for these differences is that large overdrafts of groundwater over an extended period of time in the Houston-Galveston region have stimulated fault activity there. Throughout the Texas Gulf Coast, however, a natural contribution to fault motion remains a distinct possibility.