Major oil reserves of South Louisiana are found in Oligocene, Miocene, and Pliocene sandstone reservoir rocks. Increasing rate of sediment concentration in depocenters explains the hydrocarbon richness of the upper Miocene and Pliocene strata. This same factor led to increasing provincialism of faunas in post-Oligocene time. The paleontologists' problems in younger Tertiary strata are compounded by an apparent slowing of foraminiferal evolution. Whereas there are 20 regional Oligocene marker zones that divide about 10,000 ft of section, there are no more than five or six Pliocene zones to divide twice as thick a section. Recognition of temporal clines is vital to the stratigrapher who would subdivide an interval so nearly devoid of index species. Stratigraphic subdivision is not an end in itself, but is an indispensable step in the delineation of depocenters and depositional trends.

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