ABSTRACT

The hydrocarbon endowment of the Frio and Anahuac Formations of Louisiana and Texas has made them the most thoroughly studied geologic strata in the Gulf Coast Basin. A review of previous studies of the stratigraphic frame-work of the Frio and Anahuac Formations in the Louisiana and Texas Gulf Coast Basin reveals that researchers have placed the boundary between the Frio and Anahuac in this basin at different biostratigraphic and lithostratigraphic levels. This review also reveals that in some previous studies the subdivision of the Frio in Texas into lower, middle, and upper zones does not conform to the traditional biostratigraphic zonation scheme.

The index fossils used to mark the boundary between the Frio and Anahuac differ in the Texas Gulf Coast from those employed in the Louisiana Gulf Coast. In the sub-surface Frio of Louisiana, the paleontologic top of the Frio is historically placed at the highest occurrence of Camerina sp. A. In the subsurface Frio of Texas, the paleontologic top of the Frio is historically marked by the highest occurrence of Cibicides hazzardi. Although microfossils characteristic of the Camerina A zone are reported from wells penetrating the subsurface Anahuac and Frio, their occurrence has proven too erratic to be employed in a regionally correlatable biostratigraphic framework. This review also shows that some workers place the lithostratigraphic top of the Frio in the Texas Gulf Coast Basin just above the first sandstones lying below the Anahuac shale wedge. These sandstones are actually lower Anahuac Marginulina sandstones in the lower and middle Texas Gulf Coast and upper Frio Cibicides hazzardi sandstones in the upper Texas Gulf Coast, depending on the dip position of the particular well.

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