SEQUENCE STRATIGRAPHY OF THE LATE EOCENE DRY BRANCH FORMATION IN THE GEORGIA COASTAL PLAIN

Sequence stratigraphic analysis of 18 sections in the Dry Branch Formation reveals 4 previously undescribed depositional sequences. The lowest sequence boundary (SB) caps the Huber Formation and is characterized by 10 m of erosional incision, rooted paleosols, and burrowed firmgrounds. Sequence A is composed of fluvial fill of the Albion Member of the Clinchfield Sand Formation and contains root structures, gleyed coloration, and iron reduction mottling in its upper portion. The base of sequence B is a transgressive lag of bioturbated coarse sand, quartz pebbles, and clay intraclasts. The TST of sequence B consists of bioturbated, lenticular-bedded, dark grey smectitic clay with plant remains, inclined heterolithic bedding, and well-sorted, bioturbated, sandy channel fills collectively interpreted as a central estuarine environment within the Twiggs Clay Member of the Dry Branch Formation. This is capped by a condensed, bioturbated shell hash with abundant mollusks, phosphatized bone fragments, shark teeth, ray plates, glauconite, phosphate nodules, quartz pebbles, and coarse sand. The HST of sequence B consists of offshore bioturbated, dark grey, smectitic clay of the Twiggs, with rare plant remains and grades upward into heavily bioturbated, well-sorted shoreface sand of the Irwinton Member of the Dry Branch Formation. Sequence C consists of coarse estuarine sands with abundant Ophiomorphia and large-scale cross-bedding within channels incised up to 14 m into underlying deposits. Sequence C is assigned to the Irwinton Member and is capped by a SB locally with rooted kaolin lenses interpreted as interfluve paleosols. Sequence D consists of estuarine coarse sands with Ophiomorphia burrows, large-scale cross-bedding, and abundant large pebbles near its base. Sequence D is found within the Tobacco Road Formation and fills channels incised more than 5 m into sequence C. The presence of major erosion surfaces at SBs, and the tendency for individual formations to straddle SBs and reflect multiple depositional environments, belies the simplicity implied by current lithostratigraphy.