ABSTRACT

Abundance variations of benthic foraminifera in upper Oligocene and Miocene strata of south Louisiana were studied in order to evaluate trends of species dominance, associations, and faunal diversity. Fifty-seven well cuttings samples were obtained from eleven wells in six parishes: Vermilion, St. Mary, St. Martin, Assumption, Terrebonne, and Lafourche. In addition, core samples were obtained from two wells in Terrebonne Parish.

The bulk of the stratigraphic section represents environments ranging from outer shelf to upper slope. These samples are dominated by species of Lenticulina and costate species of Uvigerina. A few samples represent the inner shelf. These are dominated by species of Hanzawaia and by Ammonia parkinsoniana. Ammonia parkinsoniana is extremely abundant in a few samples from marginal marine brackish environments. Samples from upper to middle slope environments contain abundant costate Uvigerina spp., Cyclammina cancellata, and Eggerella bradyi. Diversity of the fauna increases from the inner to outer shelf, and then apparently decreases seaward of the shelf edge.

Comparison of the core samples to cuttings from correlatable units reveals that faunal features are similar among samples in different wells within the same stratigraphic interval. Results of a multivariate cluster analysis also indicate that species dominances and associations are similar among samples in different wells within the same stratigraphic interval. This evidence indicates that the observed faunal trends are not obscured by contamination, and that well cuttings serve as suitable sample material for quantitative faunal studies.

Benthic foraminifera from both geopressured and non-geopressured shales indicate their deposition was in outer shelf to upper slope environments.