SEDIMENT CHARACTERIZATION IN LAVACA BAY, TEXAS, USING CHIRP SONAR PROFILING

Lavaca Bay is a shallow, nearly flat-bottomed estuary typical of insular bays on the middle Texas coast. As are other Texas estuaries, it was filled by coarse and fine-grained riverine sediments deposited since the stabilization of sea level at the end of the last deglaciation. Also similar to many other Texas estuaries, it contains extensive oyster beds and has been modified by channel dredging. The locations and extents of oyster beds as well as the distribution of sediment types is poorly known in Lavaca Bay because it has not previously been surveyed with modern techniques. With the goal of improving knowledge of oyster reef and sediment type distribution, we surveyed Lavaca Bay using side-scan sonar and chirp sonar subbottom profiler, positioned with high-accuracy GPS. A total of 1745 line-km of geophysical data were collected, mostly on survey lines spaced 250 m apart. In general, chirp sonar acoustic profiles show a distinctive stratigraphic layering, except where disturbed by acoustic anomalies likely caused by organic materials in the sediments. The unperturbed layering appears as a mostly acoustically transparent layer atop a section with moderate to strong layering. We interpret these two zones to represent coarse, basal sediments deposited during the early filling of the bay surmounted by fine-grained bay-fill muds. Perturbations to the normal stratigraphy appears in two general forms: (1) hazy zones or “wipeout” zones where subbottom layers are weak or absent and (2) surface and subbottom strong reflectors that often mask deeper layers or cause acoustic multiples because of strong reflection coefficients. The first type is common around the periphery of the bay and may be a result of gas in the sediments (which masks subbottom layers) or the absence of internal layering in bay-edge sediments. The latter could occur, for example, if bay-edge sediments are reworked more than those nearer the center of the bay. The second type is strongly correlated with oyster beds and mounds. These strong reflectors are probably hard layers caused by the accumulation of oyster shells. Interestingly, buried strong reflectors are more extensive than present-day oyster mounds, suggesting that oyster beds may have been more widespread in the past.