Subsidence of the Texas coast: inferences from historical and late Pleistocene sea levels

Abstract

Changes in sea level observed at tide gauges are caused by actual changes in water level and by changes in elevation at the observing station. Recent research has focused on the relationship between climatic change and sea level, but vertical land movement can be just as important, particularly in subsiding sedimentary basins. The purpose of this study is to compare long-term rates of subsidence estimated from upper Pleistocene strata along the central Texas coast with historical subsidence rates from the same area obtained from geodetic surveys and tide gauge data. This comparison shows that historical subsidence rates are much greater than long-term averages and are equal or greater than actual sea-level change along the Texas coast south of Galveston Bay. Long-term (~ 10 5 yr) subsidence rates were estimated by establishing the extent of marine, marine-influenced, and nonmarine strata within the upper Pleistocene Beaumont Formation in the Copano Bay area of the central Texas coast, and comparing the maximum elevation of in-place, marine-influenced deposits with published maximum sea level estimates of 5–8 m above mean sea level (MSL) from correlative, well-dated coral terraces from stable and uplifted areas. In-place, shell-bearing horizons deposited at or below sea level occur no higher than 2 m MSL in the Copano Bay area, suggesting that there has been no more than 6 m of subsidence since the probable time of deposition during the Sangamon interglacial at ~ 120 ka. The long-term, average subsidence rate for this part of the Texas coast is thus 0.05 mm/yr or less. Historical subsidence rates were obtained by: 1. (1) calculating relative elevation changes between National Geodetic Survey first-order leveling surveys conducted in the early 1950s with those conducted in the late 1970s to early 1980s; 2. (2) normalizing the relative elevation differences between surveys to annual rates of change relative to an arbitrarily chosen benchmark; 3. (3) referencing these lines to sea level at three tide gauges; and 4. (4) comparing calculated rates of relative sea-level (RSL) rise along the lines with estimates of eustatic sea-level (ESL) rise. Rates of RSL rise for the Texas coast south of Galveston Bay were generally 4–8 mm/yr; locally, rates were as high as 23 mm/yr. These rates are significantly higher than global averages of ~ 1 mm/yr. Much of the difference is probably caused by subsidence of the Texas coastal zone at rates of 1–22 mm/yr, or 20–440 times the long-term average of 0.05 mm/yr. The highest subsidence rates were found locally where there has been historical water-level decline in shallow aquifers. Lower subsidence rates of 3–7 mm/yr occur regionally where groundwater decline is minimal or nonexistent. Increased subsidence over the long-term average in these areas may be caused by pressure decline in underlying oil and gas reseivoirs.