Abstract: Determination of Salinity Based on the Morphology of Ammonia parkinsoniana from Laguna Madre and Baffin Bay, Texas

COLBURN, DENISE F. and JON A. BASKIN
Department of Geosciences, Texas A&M University-Kingsville, Kingsville, TX 78363

Ammonia parkinsoniana is one of the most widely distributed and abundant foraminiferans living in nearshore environments of the north-west Gulf of Mexico. It is also a most commonly occurring fossil in Neogene Gulf Coast sediments and sedimentary rocks. Three samples of A . parkinsoniana from Baffin Bay and Laguna Madre, Texas, were investigated to assess the effects of temperature and salinity on morphology, in order to determine if morphology of this species could be used to interpret paleoenvironmental setting. Salinity appears to be the most important factor affecting morphology. Specimens collected at a time of higher salinity (50 ppt) differ significantly (p < .01) from those collected at lower salinity (14 ppt) in having a larger proloculus and fewer chambers in whorls 1 and 2. They differed marginally significantly (p < 0.1) in having fewer total number of chambers and somewhat smaller umbo diameter. Multivariate statistical analyses also reveal a statistically significant relationship between environment and test morphology.

This South Texas sample supports the recognition of A parkinsoniana as a distinct species and not as an ecophenotypic variant of A. beccarii, because it is the dominant foraminiferan taxon regardless of temperature or salinity. Very few (usually <5%) of the Ammonia specimens from Baffin Bay were A. tepida over a range of salinities from 19 to 54 ppt. This is contrary to what would be expected if these two species were ecophenotypes. All specimens of A. beccarii appear to be megalospheric.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas