SUBSURFACE GEOLOGY OF THE DALLAS FLOODWAY, DALLAS, TX

A subsurface investigation was performed as part of a larger geomorphic study of the Dallas Floodway. Local stratigraphy was characterized using historical soil borings from the 1930s to present together with Cone Penetrometer Test (CPT) data. The CPT is a geotechnical engineering tool for characterizing soil at ½ inch depth intervals. Using CPT data it is possible to generate depth plots and highly detailed cross sections of predicted soil types, soil strength, permeability, etc. Bedrock within the area is Cretaceous in age, and is comprised of the Eagle Ford shale and the Austin Chalk Formation. The Austin Chalk is present only in the easternmost part of the Floodway. Soil borings and CPT data show a fining upward sequence, with an apparent semi-continuous basal gravel and sand layer uncomformably overlying bedrock. Finer sediments overlie the basal sand and gravel, and in some areas the shallow subsurface has been modified and re-worked for flood control. Changes in environments of deposition (e.g., backswamp, point-bar) within the floodway have been identified. The stratigraphic sequence shows the system’s response to climatic variations over the last several thousand years, as well as more recent anthropogenic influences.