HOGE, BRADLEY EARLE, Rice University, Houston, TX

ABSTRACT: Marsh Thanatocoenoses: Using Microfossils to Define Wetland Paleoenvironments

There is growing sedimentologic evidence that shows sea level has transgressed at both 4 Ka and 2.5 Ka in the Gulf of Mexico. One way todetermine sea-level history is to analyze the microfossil assemblages from coastal and bay-fringing wetlands. By taking both surface samples and coresfrom each marsh type in the Anahuac National Wildlife Refuge, and cores from the Holocene record of the incised valley fill of the Trinity River, it is possible to determine the wetland sequences of the Trinity delta and Galveston Bay.

Foraminiferal assemblages can delineate salt to brackish marsh environments, but are absent in fresh marsh sediments. Ostracoda assemblages are consistent in fresh to brackish marshes, but unpredictable in the salt watermarsh. Diatom biocoenoses are good indicators of each marsh type, but the thanatocoenoses are often too similar to the biocoenosis for taphonomic studies. Taken separately none of these assemblages can provide an effective method of determining paleosalinity of marsh sediments.

By analyzing the three assemblages together, however, it is possible to determine how combined thanatocoenoses indicate the spatial paleodistribution of marsh facies. This is because taphonomic effects on the three groups overlap. Calcium carbonate and silicate dissolve according to pH, while the chitinous cement of some agglutinated foraminifera with stands dissolution and is removed only through oxidation-reduction reactions. A combined thanatocoenosis composed of species with similar taphonomic signatures but different ecological requirements can, therefore, provide an effective method of identifying paleosalinity and taphonomic history.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.