The fundamental stratigraphic unit of Gulf Coast subsurface geology is the biostratigraphic zone. These zones are named after benthonic Foraminiferida and are used widely in a time-rock sense by most workers in the region. Zonal nomenclature has developed informally within competitive oil companies. The middle Miocene Cibicides carstensi zone has been generally recognized for at least 15 years, but use of the name has been inconsistent, because no type section had been designated. The C. carstensi zone of south-central Louisiana, at its type locality, consists of four subzones. Boundaries of the zone and its components are defined on the basis of thin Foraminiferida-rich beds and distinctive electric-log markers.

Relative abundance of foram tests in sediments provides an index to rate of clastic deposition. Lentils containing abundant Foraminiferida in a predominantly clastic sequence record episodes of relatively slow deposition. Abundance and composition of faunas in benthonic communities are controlled, in large part, by depositional rates and nature of substrate. Both these factors are dependent on tectonic history in the Gulf Coast. In that province stratigraphic thickness and sequence are closely related to faulting. Episodes of fault movement can be dated accurately by their influence on depositional thickness and lithologies in correlative strata on opposite sides of a fault. Stratigraphy, paleontology, and structural history consequently are inseparable. All three must be considered s multaneously in any geologic interpretation.

Hydrocarbons of the C. carstensi zone accumulated in blanket sand bodies deposited under conditions of tectonic stability. Sands which have a sharp basal contact tend to be erratic and limited in areal distribution. These sands are thought to have been deposited on a steeply inclined or irregular submarine topography, during or immediately following spasms of tectonic activity. Such sands rarely produce hydrocarbons.

Nodosaria angularis, new species is restricted to down-dip parts of the C. carstensi and underlying Textularia stapperi zones.