ENVIRONMENTAL INTERPRETATION OF CARBONATE BEDS IN THE BAHIA GRANDE WETLANDS, SOUTH TEXAS

The Bahia Grande wetlands are a unit of Laguna Atascosa National Wildlife Refuge, located northeast of the Port of Brownsville in South Texas. This area is a major coastal wetland restoration project adjacent to Lower Laguna Madre and part of the Rio Grande Delta system. During the geological mapping of the region, several unusual carbonate beds were located. The carbonate beds occur in higher elevation lomas (hills) within the wetland complex. These lomas appear at about 7.5 to 10 masl. They also form peninsulas indicating they are more resistant to weathering than the underlying and overlying mudstones. The carbonates are low-density micrities with vesicles towards the upper surfaces. They contain rounded frosted quartz grains. The surrounding mudstones are discolored to a pinkish color. The upper surfaces of the carbonates are irregular.

We used stable isotopic data (δ¹³C and δ¹⁸O) and ground penetrating radar to help determine the depositional environment of these lenticular beds. δ¹³C values ranged from -14.03 to -25.41 indicating the origin of the material was likely was formed by algal growth or terrestrial plant material. These data are consistent with deltaic deposition. δ¹⁸O values ranged from -2.02 to -18.87 indicating the depositional environment was likely brackish water with approximate salinity levels from 4 ppt to 27 ppt. These data also support a deltaic depositional environment.

Using ground-penetrating radar we collected vertical images in parallel transects. Using spatial analysis and GIS, which we converted to two-dimensional images into three-dimensional models. We used these models to help us determine the shape, size and extent of these beds. The beds are irregular in shape and taper towards the edges, indicating they formed in isolated pond within the delta.

Today these carbonate beds created topographic highs that are resistant to weathering and erosion. The ponds were filled in with the material including plant matter, later lithafied over time, forming carbonates. The biological decay and diagenisis created vesicular structures in these carbonates. These weather and erosion resistant carbonate beds contribute to the progradation of the Rio Grande Delta into the Gulf of Mexico.