FLUCTUATING SALINITY GRADIENTS IN GALVESTON BAY AND HOW TO CONFUSE TRANSGRESSIVE BARRIER ISLAND DEPOSITS WITH REGRESSIVE DELTAS (Invited Presentation)

Salinity in Galveston Bay on the Texas gulf coast, is greatest during the summer months, when runoff from the Trinity and San Jacinto rivers is at a minimum, and the inner bay reaches 15–20 ppt, while the outer bay, including the back-barrier landward of the barrier island, reaches 20–25 ppt. Freshwater influx during flooding of the Trinity River overwhelms the bay, creating a plume that surges oceanward from the tidal inlet and carried by longshore currents along the seaward face of Galveston and Follets Island. Salinity was 25 ppt in the surf ~64 km (40 miles) southwest of the tidal inlet in March, 2016, following major flooding of the Trinity River. Additionally, groundwater salinity in the beach showed a strong gradient of 25–0 ppt over a 50-m transect from the low-tide line to the eolian dune field, demonstrating the importance of salinity as well as wave energy and grain size when examining ancient shallow marine, intertidal, and supratidal deposits. Burrowing organisms and their associated incipient ichnofossils within the bay reflect this dynamic physicochemical environment with polychaetes, crabs, bivalves, and shrimp constructing small Psilonichnus, Thalassinoides, Ophiomorpha, Siphonichnus, Skolithos, and Polykladichnus in the inner bay, proximal to the bayhead delta when salinity levels are high. Larger burrows are constructed by animals living in the higher salinity portion of the bay, especially mantis shrimp (Squilla sp.) that construct incipient, large Psilonichnus, Pholeus, and Teichichnus. More diverse polychaetes faunas construct a variety of Cylindrichnus, Skolithos, and Paleophycus. When salinities are low and/or reach freshwater levels, marine organisms are no longer active, and their dead bodies typically found in burrows. The result of this salinity-dependent partitioning is discrete distribution of ichnotaxa and maximum burrow diameter that can be recognized in the ancient record, especially in transgressive deposits. Of particular significance in the modern example is the presence of longshore-driven freshwater plumes that emanate not from a delta, but from an estuary. The resulting signature on the burrowing organisms of the downdrift barrier island shoreface would be easy to mistake for a regressive, wave-dominated deltaic deposit instead of a transgressive, barrier island.