PALEO-EROSIONAL FEATURES OF THE TRINITY RIVER FLOODPLAIN IN DALLAS, TEXAS

Two- and three-dimensional terrain maps of a section of the Trinity River in the Dallas metropolitan area were created from discrete geologic boring data and light detection and ranging (LIDAR) imagery. The terrain maps were then used to identify and analyze the relationships between paleo-erosional features at depth and in adjacent river terraces. Subsurface terrain maps show large paleo-channels deeply incised in late-cretaceous-age bedrock overlain by more recent fluvial sediments. Channel dimensions suggest that the Trinity River once had a significantly higher discharge rate than at present. The observed erosional unconformity is thus attributed to a high-energy, erosion-dominated depositional environment that occurred as a result of a drop in base level. This conclusion confirms the existence of an erosional unconformity between the Eagle Ford Shale and Austin Chalk units, and overlying fluvial sediments. Surface terrain maps show numerous cut-bank features in the terraces adjacent to the floodplain. These features are attributed to paleo-channel migration in a higher-energy meandering river system. Differences in elevation and morphology between features indicate that they are of different ages. Surface and subsurface morphology indicate that there is a relationship between the incised paleo-channels and terrace cut-banks. This relationship may be used to accurately determine the age of the various meander belts within the Trinity River Floodplain. Accurate determination of the age, morphology, and spatial relationships of observed paleo-erosional features is necessary to constrain the geologic history of the Trinity River Floodplain. The methods used in this study may provide a framework for similar investigations in metropolitan areas where surface expressions of morphologic features are obscured by anthropogenic activities.