Modern depositional environments of the Texas coast

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Morton, Robert A.

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University of Texas at Austin. Bureau of Economic Geology


The Texas Coastal Plain is ideal for studying physical processes and the late Quaternary sedimentological record. Together, the diversity of depositional environments, the moderate climate, and the accessibility to most areas provide unique opportunities for (1) conducting geological investigations of modern sediments and the hydrodynamics responsible for their formation and (2) developing models suitable for interpreting ancient sediments. Within a span of about 350 mi (564 km), a broad spectrum of depositional systems is found. These systems include coarse-grained and fine-grained fluvial channels, bayhead and oceanic deltas, coastal lagoons, transgressive and regressive barriers, and a host of other nearshore deposits that are commonly preserved in ancient sedimentary basins and recognized in outcrop or by applications of subsurface methods. Morphology and facies distribution within the Coastal Zone are responses to climatic gradients, low wave energy, and low tidal range that characterize the northwest Gulf of Mexico. Within this microtidal, storm-dominated region, sediment dispersal is controlled largely by wind forcing and river flooding. Wind forces are responsible either entirely or partly for aeolian activities, wind tides, bay circulation, wave generation, shelf circulation, and longshore currents. River discharge provides the primary mechanism for sediment transport into the Coastal Zone. The Coastal Zone is a dynamic area, as evidenced by monitoring of physical and biological parameters during historical time. Proper analysis of extant conditions requires an appreciation for the temporal and spatial variations in processes that are attributed to both natural changes and human modifications. Human alterations are clearly responsible for some significant coastal changes. For example, progradation of the Brazos and Colorado deltas, closing of Packery Channel, and lateral infilling of Pass Cavallo attendant with spit accretion are all related to major engineering projects. Documentation of specific historical conditions, such as overbank flooding and storm washover, is invaluable for understanding sediment transport and deposition during these low-frequency, high-energy events. When placed in context, such historical documentation permits hydrodynamic reconstructions for the modern sediments, which in turn can be transferred to the ancient rock record. Vertical successions of stratification types, textural variations, crosscutting relationships, and sand-body geometry, as well as three-dimensional facies distribution, are diagnostic of particular depositional systems. This is demonstrated by discussions covering the broad spectrum of Texas coastal environments from the fluvial systems to the inner shelf.


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