SEDIMENTARY ENVIRONMENTS IN A SHALLOW, GULF COAST ESTUARY-LAVACA BAY, TEXAS

Lavaca Bay is a broad, flat, and shallow (< 2 m) microtidal estuary within the upper Matagorda Bay system of the central Texas coast. The distributions of sedimentary facies were determined by mapping the entire bay at 250 m spacing with sidescan sonar and chirp seismic profiler. The mapping effort identified four dominant facies in Lavaca Bay, which are: 1) muddy estuarine bottom; 2) shelly sandy shoal; 3) emergent live oyster reef; 4) clayey sandy shoal. Of these four facies the estuary consists of mainly estuarine mud with emergent oyster reefs conjugating on bathymetric highs. Extending from the river mouth to the bay mouth, sediment textures within the Lavaca Bay system are composed of clayey silts, clays and sands. Chirp data shows that within the upper seabed, sediment thickens towards the center of the bay from both river and marine control sources. South Lavaca Bay consists of shell-fragmented sand that correlates to an ancient flood tidal delta that has been re-exposed.

As revealed from a series of 15 cores, x-radiographs, and radioisotope and grainsize profiles showed frequent hurricanes and tropical storms induce deep mixing and significant sediment transport. Between these storm processes, bioturbation, tidal cycles and passage of fronts frequently drive shallow seabed mixing. Pre and post Hurricane Claudette cores showed an maximum ephemeral erosion rate of 17 cm, and an ephemeral deposition rate of 7 cm, but long term accretion were ~ 0.2 cm/yr.