Mapping of Wilcox Group and lower Claiborne Group strata in western Georgia and eastern Alabama has shown significant facies changes within the Hatchetigbee Formation. In updip areas, the Hatchetigbee (upper part of the Wilcox) is composed of cross-bedded and massive quartz sand and carbonaceous laminated silt and clay that were deposited in lagoonal through very shallow marine (lower shoreface) environments. Sections 30 mi (48 km) downdip consist of shelly glauconitic silt and very fine-grained sand, commonly including a thin sequence of laminated silt and clay at the top. These strata represent inner-shelf deposits succeeded by a regressive, more nearshore or lagoonal phase. The shelly beds downdip are coeval with and lithologically similar to the Bashi Marl Member of t e Hatchetigbee in western Alabama.

The Hatchetigbee is thinner downdip, 20 to 35 ft (6 to 11 m) thick than updip, 55 ft (17 m) in our study area. On the basis of measured sections and biostratigrahic data in eastern Alabama and biostratigraphic data from western Alabama, the Bashi Member in the study area is a downdip equivalent of the entire updip Hatchetigbee Formation. Calcareous nannofossils and planktonic foraminifers place both the transgressive and regressive events represented by the Hatchetigbee Formation entirely within the first 500,000 years of the Eocene (lower part of calcareous nannofossil Zone NP10 and middle of planktonic foraminiferal Zone P6). The overlying Tallahatta Formation of the Claiborne Group contains calcareous nannofossils diagnostic of Zones NP13-16; thus, the lowest part of the Claiborne roup is of latest early Eocene age. The absence of Zones NP11 and 12 indicates a gap of at least 2.6 m.y. between the top of the Wilcox and bottom of the Claiborne in this area. This unconformity suggests a major coastal offlap in the eastern Gulf coastal plain during the early Eocene at a time when worldwide eustatic sea-level curves indicate a major highstand of sea level.

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