Recent foraminifera of Trinity Bay, Texas

[摘要] Trinity 3ay is a shallow, estuarine environment characterized by variability of hydrography and substrate. This environmental variation is reflected in the taxonomic composition, morphologic variation, and distribution of the Foraminifera. Two foraminiferal assemblages are recognized: 1) a bay assemblage characterized by low taxonomic diversity and the dominance of Ammotium and Ammonia; and 2) a deltaic marsh assemblage characterized by wholly agglutinated species. An ecotonal community characterized by the presence of Miliammina fusca and the abundance of Elphidium delicatulum inhabits portions of the deltaic margin. The bay assemblage is a mixture of species either endemic to the brackish-water environment or occurring in both brackish-water and marine habitats. Tests of the cosmopolitan species are calcareous whereas tests of all endemic species except one are agglutinated. Calcareous Foraminifera dominate the populations of the lower portion of Trinity Bay, but agglutinated species predominate in the upper bay areas. This variation in dominant faunal elements coincides with the salinity gradient within the bay. The ratio of agglutinated to calcareous tests in the samples is a useful index to the relative influence of marine and fluvial waters throughout the bay area. Intraspecific variation of test morphology is evident within several of the fonnliniferal species. Different phenotypic expressions of Ammonia beccarii are recognized in different areas of the bay, whereas in Ammotium salsum the morphologic variation displays no consistent distri-butional pattern. The density of living foraminiferal populations is correlated with the character of the substrate. The smallest populations are found in the coarsest and best sorted sands of the bay and delta margins and in areas of slow deposition in the lower bay. The largest populations occur in areas of relatively rapid accumulation of fine-grained sediments. This density distribution may coincide with the distribution of available organic nutrients. Comparison of living and total populations indicates that the total populationp have greater faunal diversity than the living populations. This probably reflects seasonal variability of the environment. It is not thought that living/total ratios are useful indices to relative depositional rates in this area. The foraminiferal faunas should be a major key to the variability of the hydrographic environment in studies of ancient bays.

[发布机构] Rice University