Geomorphology of the Trinity River floodplain in Dallas County, Texas
Abstract
Data from more than 1,800 geologic borings and over 500 cone penetrometer tests (CPTs) were used to characterize the geomorphology of the Trinity River floodplain in the Dallas Metropolitan Area. Historical maps, aerial photographs and other published information were used to prepare a preliminary geomorphic map. Boring logs and CPT data were then used to refine the preliminary map, produce a series of two-dimensional (2D) and three-dimensional (3D) cross sections, and interpret the recent geologic history of the area. Geomorphologic interpretations - most importantly the locations of paleo-channel deposits of sands and gravels - were used to identify reaches of the levees managed by the United States Army Corps of Engineers (USACE) and the City of Dallas that may be at significant risk for under-seepage. Boring logs and CPT data collected atop the levees were used to assess through-seepage risks. Local bedrock is comprised of cretaceous-age Eagle Ford Shale and Austin Chalk. Depth to bedrock in the study area averaged 14.6 m (47.8 ft). The uppermost surface of bedrock has been deeply incised by a meandering river. Vertical relief between the shallowest bedrock sections and deepest portion of the incised paleo-channel is more than 15 m (50 ft). In places the incised paleo-channel is more than 0.8 km (0.5 mi) wide. These data confirm the presence of an erosional unconformity between local bedrock and overlying quaternary floodplain deposits. The observed erosional unconformity is attributed to a higher-energy fluvial environment that occurred as a result of a drop in base level. Recent floodplain deposits consist of interlobate point bar, channel and overbank sediments that are generally distributed in a fining-upward sequence. Buried channel dimensions vary widely, but are more than 250 m (820 ft) in some areas - much larger than the current channel. A semi-continuous basal layer of quaternary sands and gravels approximately 2 to 5 m (7 to 16 ft) thick exists in much of the study area. These data indicate that the discharge rate of the Trinity River today is at its lowest since quaternary deposition began. Large terraces of varying geometry and elevation observed at the edge of the floodplain show that the Trinity River has been geographically constrained to its current floodplain since the late Cretaceous, and perhaps even earlier.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H53B1022H
- Keywords:
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- 1625 GLOBAL CHANGE / Geomorphology and weathering;
- 1819 HYDROLOGY / Geographic Information Systems;
- 1825 HYDROLOGY / Geomorphology: fluvial;
- 9800 GENERAL OR MISCELLANEOUS