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A Numerical Index for Biostratigraphic Zonation in the Mid-Tertiary of the Eastern Gulf
Alan H. Cheetham, Phili B. Deboo
Continuous sections of abundantly fossiliferous rocks, such as occur in the mid-Tertiary of the eastern Gulf Coast, ought to provide an objective, natural, unequivocal basis for zonation. Recent statements of biostratigraphic principles, including the reports of the American Commission on Stratigraphic Nomenclature, recognize the practicability and desirability of basing zonal units on assemblages of fossils. However, there is no agreement among practitioners on objective criteria for establishing such units.
In almost any stratigraphic section, ecological variations of local nature obscure vertical differences of regional significance. Thus, a meaningful, objective measure of taxonomic resemblance must minimize differences in abundance and diversity between vertically successive faunules in order to emphasize regional changes in the fauna. For a slightly different purpose, Simpson introduced a numerical index, having just this property, 100 C/N1, in which C is the number of species common to two faunules and N1 is the number of species in the less diversified faunule.
To test the biostratigraphic usefulness of Simpson's index, 13 samples were collected from the lower 45 feet of section exposed in the north pit of St. Stephens quarry, Washington County, Alabama. The lithologic units sampled range from Pachuta Marl (Jacksonian) to Marianna Limestone (Vicksburgian) in a single, continuous exposure. Three major elements of the fauna were analyzed: cheilostome bryozoans (69 species), foraminifers (111 species), and ostracods (40 species). Two alternative methods, one using occurrences and the other using ranges, yielded distinctly low indices indicating a major faunal discontinuity between two of the samples for all groups, separately and combined. Two minor discontinuities are indicated for two other sample pairs by relatively low indices for some groups but not for others. Essential continuity is indicated by relatively high indices, computed by the range method, for all groups, separately and combined.
The major faunal discontinuity, occurring between the Pachuta Marl and the basal marly phase of the Shubuta Clay, is here regarded as the Jacksonian-Vicksburgian boundary at the locality studied.
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