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This paper is a continuation of previously published work that dealt with the biostratigraphy and paleoclimatology of late middle Eocene to early Oligocene sporomorph assemblages from the eastern Gulf Coast. Here, the emphasis is on data from 48 new samples obtained from two continuous sections and two composite sections, from the base of the Eocene to the upper middle Eocene, of eastern Mississippi, Alabama, and western Georgia. One hundred sixty-four operational taxonomic units of sporomorphs were tabulated in this study. Of these, eight are new species of pollen grains that are formally named and described. Fourteen new taxonomic combinations are also proposed. Graphic correlation was used to tie together the four stratigraphic sections. The sporomorph data support previously determined positions of formation contacts in the four sections. However, sporomorph range data suggest that the uppermost part of the Kosciusko Formation of eastern Mississippi correlates with the lower part of the Cubitostrea sellaeformis Zone at Little Stave Creek (in western Alabama) rather than with the middle part of this zone, as suggested by some authors. I also tried graphic correlation of the sections I had previously studied, of the Jacksonian Stage and adjacent strata, but so few sporomorph range bases and tops are present in this part of the Eocene of this region that no conclusion could be reached about the thorny problem of correlating members of the Yazoo Clay across Mississippi. In this paper, I drew the line of correlation on the graphic correlation charts on the basis of previously known formation contacts and used the sporomorph biostratigraphic events (range bases and tops) to check whether the positions of the formation boundaries appeared reasonable. In all the graphic correlation plots in this paper, drawing the line of correlation using all possible range bases and tops proved impossible because of the very wide scattering of data points. It was found much more useful to assign even a crude ranking of quality to each event and to plot only the higher quality events. Because most high rankings are assigned to range bases and tops of abundantly represented, relatively long-ranging taxa, I found that if a tradeoff must be made between numbers of samples to be examined and numbers of taxa to be tabulated, it is better to examine more samples but fewer taxa. As a result of graphic correlation of the four new sections, together with my previous data on the Jacksonian Stage and adjacent strata, I have produced a composite standard section for the entire Eocene of the eastern Gulf Coast that includes ranges of 141 sporomorph operational taxonomic units. The value of this composite standard section is enhanced because the formations making up the section have previously been assigned to calcareous nannofossil zones, which have been related elsewhere to a geochronologic (time) scale. Thus, the geochronologic ages of the sporomorph range bases and tops in the eastern Gulf Coast are much better known than the ages in most previously published Eocene sporomorph-stratigraphic sections. The exact position of the Eocene-Oligocene boundary in the Gulf Coast is uncertain, but, on the basis of previous planktic foraminiferal and calcareous nannofossil work, it undoubtedly falls within the Shubuta Member of the Yazoo Clay rather than at the top of the Shubuta. Thus, my composite standard section, which includes the whole Yazoo Clay, includes the lowermost Oligocene as well as the Eocene. On the basis of the stratigraphic and geochronologic data in the composite standard section, I have drawn a second section in which the primary scale is geologic time. One column of this geochronologic section shows inferred rates of deposition of the Eocene sediments in the composite standard section; the data in this column support the concept that a disconformity is likely to be present between the Tallahatta and Lisbon Formations. The Cocoa Sand Member of the Yazoo Clay may be associated with a disconformity caused by an apparent eustatic drop in sea level at about the end of NP17 time. Additional columns of the geochronologic section show times and rates of changes of the angiosperm floras during Eocene time in the eastern Gulf Coast. These data indicate the existence of an early Eocene event, a rapid turnover of the earliest Eocene angiosperm flora reflecting the extinction of some Paleocene holdovers and the appearance (due to evolution and immigration) of new forms in the region. The rate of new operational taxonomic unit appearances is the highest such rate known for the Eocene of the eastern Gulf Coast, and I interpret this high rate to be due in large measure to rapid immigration of angiosperm taxa from Europe over the North Atlantic land bridge near the beginning of the Eocene. Accompanying the rapid change in the flora was a general increase in diversity during the first part of early Eocene time. After this early Eocene event, the angiosperm taxon diversity in the region remained rather constant and was accompanied by low rates of taxon turnover for about 9 million years. Then, at about 46 Ma, a middle Eocene event began that lasted about 4.5 million years, to about 41.5 Ma, and included in succession (1) a concentration of first appearances, (2) a concentration of last appearances, and (3) a second concentration of first appearances. This sequence of turnover pulses was accompanied by a rapid rise in diversity to a peak at about 45 Ma, followed by a long, more or less steady decline in diversity to a minimum at about 42-41.5 Ma. The middle Eocene event was followed by a period of steady diversity, evidenced by a lack of detectable turnover of angiosperm taxa, that lasted until the end of the Eocene. The Oligocene deterioration of Wolfe (=the terminal Eocene event of Wolfe) is manifested in the eastern Gulf Coast by two kinds of rapid changes in the pollen assemblages, a rise in relative frequency of an oaklike pollen species and a simultaneous concentration of angiosperm taxon last appearances leading to a decrease in taxon diversity; these changes probably did not commence until the very beginning of the Oligocene. Most turnover events affecting angiosperm pollen taxa in the Eocene and earliest Oligocene of the eastern Gulf Coast are difficult to interpret in terms of particular climates or openings and closings of migration routes, but the pattern seems clear that times of rapid climatic change and migration were interspaced with periods of gradual or no climatic change or migration. The apparent 5-million-year stasis in evolution of angiosperm taxa from the late middle Eocene to the end of the Eocene suggests that the Stationary model of evolution has more applicability to angiosperms than the Red Queen model.

Sporomorph biostratigraphy, floral changes, and paleoclimatology, Eocene and earliest Oligocene of the eastern Gulf Coast