Abstract In the eastern Gulf Coastal Plain, four, third-order unconformity-bounded depositional sequences are recognized for upper Paleocene and lower Eocene marginal marine to marine shelf strata of the Wilcox Group. In this area, the Wilcox Group includes the Nanafalia Formation (TAGC-2.1 depositional sequence), the Tuscahoma Sand (in part, the TAGC-2.1 and the TAGC-2.2 and TAGC-2.3 sequences), and the Hatchetigbee Formation (TAGC-2.4 depositional sequence). These cycles are interpreted to result from changes in sea level and coastal onlap during the late Paleocene and early Eocene epochs. The Nanafalia Formation and lower part of the Tuscahoma Sand are within the upper Paleocene (Selandian Stage) Planorotalites pseudomenardii Range Zone, whereas the middle and upper parts of the Tuscahoma Sand have been assigned to the upper Paleocene (Selandian Stage) Morozovella velascoensis Interval Zone. The Hatchetigbee Formation contains planktonic foraminifera diagnostic of the lower Eocene (Ypresian Stage) Morozovella subbotinae Interval Zone. The Paleocene-Eocene Epoch boundary (ca. 54–55 Ma) occurs in the Wilcox Group and coincides with the lithostratigraphic contact of the upper Paleocene Tuscahoma Sand with the lower Eocene Hatchetigbee Formation. Palynological studies of the marginal marine strata of the uppermost clay and silt beds of the Tuscahoma Sand and the lowermost sand beds of the Hatchetigbee Formation indicate late Paleocene and early Eocene ages, respectively, for these formations. Planktonic foraminiferal zone boundaries generally occur at depositional sequence boundaries dividing the upper Paleocene and lower Eocene marginal marine to marine shelf strata in the eastern Gulf Coastal Plain. Factors that affect the stratigraphic position of biozone boundaries in these strata include the presence of unconformities and associated biostratigraphic discontinuities resulting from nondeposition or erosion, the presence or absence of lowstand systems tract strata, the differential amounts and rates of sedimentation associated with paleobathymetry and/or distance from the shoreline at various depositional sites, differential subsidence within the depositional basin, and paleoenvironmental conditions. The development of a major fluvial-dominated delta complex in this area during Paleocene and early Eocene time had a significant influence on depositional conditions. An integrated geochronologic, biostratigraphic, and sequence stratigraphic approach provides a useful mechanism for correlation of marginal marine and marine shelf strata of the Wilcox Group in the eastern Gulf Coastal Plain. The use of physical surfaces associated with depositional sequences, such as sequence bounding unconformities, transgressive surfaces, and surfaces of maximum transgression/sediment starvation, in conjunction with the first and last occurrences of age diagnostic microfossils and radiometric data, provides a framework for local, regional, and worldwide correlation.

ABSTRACT Condensed section deposits in carbonates and siliciclastics are generally fine-grained rocks that commonly contain relatively high concentrations of organic matter; therefore, these rocks may have the potential to be petroleum source rocks if buried under conditions favorable for thermogenic hydrocarbon generation. Of the condensed section deposits in the Mesozoic strata of southwest Alabama, only the Upper Jurassic Smackover Formation carbonate mudstones from the condensed section of the lower Zuni A Gulf Coast-4.1 depositional cycle have sufficient organic carbon and were subjected to burial and thermal conditions in which this potential has been realized. These condensed section and transgressive carbonate mudstones contain total organic carbon contents of algal and amorphous kerogen of as much as 2.19% and exhibit thermal alteration indices of 2− to 3+. The laminated carbonate mudstones of the Smackover Formation have apparently served as the hydrocarbon source for the majority of Mesozoic reservoirs throughout southwest Alabama. The Upper Cretaceous Tuscaloosa Group marine claystones from the condensed section of the upper Zuni A Gulf Coast-2.5 depositional cycle are rich (total organic carbon values of as much as 2.91%) in herbaceous and amorphous organic matter but have not been subjected to burial and thermal conditions favorable for thermogenic hydrocarbon generation. These claystones exhibit thermal alteration indices of 1 + to 2 − . The Jurassic Norphlet shales of the condensed section of the lower Zuni A Gulf Coast-3.1 depositional cycle are low in total organic carbon content (0.1%). These rocks have experienced burial and thermal conditions favorable for thermogenic hydrocarbon generation, but depositional conditions have limited their potential as source rocks because of the paucity of organic carbon preserved in these deposits. No well-developed condensed sections are recognized in the Upper Jurassic Haynesville lower Zuni A Gulf Coast-4.2 depositional cycle or the Upper Cretaceous Tuscaloosa upper Zuni A Gulf Coast-2.3 and upper Zuni A Gulf Coast-2.4 depositional cycles. Although condensed sections within depositional sequences, in general, should have the highest source rock potential, specific environmental, preservational and/or burial and thermal history conditions within a particular basin dictate whether or not this potential is realized. This relationship is shown by the condensed sections of the Mesozoic depositional sequences in southwest Alabama. Therefore, petroleum geologists can use sequence stratigraphy as a tool to help identify stratigraphic intervals that might have potential to contain hydrocarbon source rocks; however, only through geochemical analyses can the actual source rock potential be determined.

ABSTRACT Micropaleontologists using planktonic foraminifera for biostratigraphic age correlation differ on the placement of the Eocene-Oligocene boundary from those workers utilizing calcareous nannoplankton. Employing the stratigraphic distribution of the planktonic foraminifera recovered from the upper Eocene Yazoo Clay and lower Oligocene Red Bluff Clay/Bumpnose Limestone, Forest Hill Sand and Mint Spring Marl Member of the Marianna Limestone in southeastern Mississippi and southwestern Alabama, the epoch boundary is recognized to be at or near the top of the Yazoo Clay. The extinctions of the key calcareous nannoplankton species occur at a lower stratigraphic level. The dilemma regarding the placement of the Eocene-Oligocene boundary exists probably because changes in faunal and floral assemblages were gradual during the late Eocene through the early Oligocene and because this epoch boundary represents a stratigraphically condensed section of a Type 2 depositional sequence. No dramatic faunal or floral changes should be expected at the Eocene-Oligocene boundary because of the absence of a major drop in sea level and because no sudden climatic changes occurred at this time. In southeastern Mississippi and southwestern Alabama, the Cocoa Sand, Pachuta Marl and Shubuta Clay Members of the Yazoo Clay, the Red Bluff Clay, the Bumpnose Limestone, and the Forest Hill Sand comprise an unconformity-bounded, Type 2 depositional sequence that accumulated during the TE3.3 coastal onlap cycle of the Td supercycle. The contact of the Cocoa/Pachuta with the underlying North Twistwood Creek Clay Member of the Yazoo Clay (highstand regressive deposits of the underlying depositional sequence) is a Type 2 unconformity. The transgressive deposits of the TE3.3 sequence consist of the Cocoa and Pachuta. The condensed section of the sequence includes the Shubuta (lower condensed section deposits) and Red Bluff/Bumpnose (upper condensed section deposits). The Shubuta-Red Bluff/Bumpnose contact, which approximates the Eocene-Oligocene boundary, is a surface of maximum starvation associated with the greatest landward transgression of the coastline during a relative rise in sea level. The Forest Hill Sand overlies the Red Bluff/ Bumpnose and comprises the highstand regressive deposits of the sequence. The contact of the Forest Hill with the overlying Mint Spring is a Type 2 unconformity. The Mint Spring represents the transgressive deposits of the overlying sequence. The use of stratigraphically condensed sections, sequence boundaries and genetic depositional sequences has the potential to be a useful correlation tool for resolving stratigraphic and depositional problems in the Gulf Coastal Plain. The age of a condensed section within a specific depositional sequence should be synchronous worldwide.