Comparison of Pleistocene and Holocene barrier island beach-to-offshore sequences, Georgia and northeast Florida coasts, U.S.A.

Abstract

Well-exposed vertical sequences of Pleistocene shoreline deposits are rare on the coastal plain of the southeastern United States. An important exception is an exposure along the St. Mary's River on the Georgia-Florida state line. This outcrop contains an excellent depositional strike section of a prograding offshore, shoreface, foreshore and backshore sequence with well-preserved physical and biogenic sedimentary structures. Offshore sediments are composed of highly bioturbated, muddy fine sand. Distinct burrows and bedding become progressively more abundant upward through a transition zone into shoreface deposits. The shoreface is dominated by the trace fossil Ophiomorpha nodosa and physical sedimentary structures are poorly preserved. Foreshore sediments contain low-angle seaward dipping beds, high-angle landward dipping beds and ripple laminae. Heavy-mineral accumulations in the backshore accentuate bedding and biogenic structures such as ghost crab and insect burrows and bioturbation by amphipods. Direct correlation of most primary physical and biogenic sedimentary structures and textures can be made between the Pleistocene and Holocene beach-to-offshore facies assemblages. However, our studies of the Pleistocene indicate that the existing Holocene vertical sequence model for the Georgia coast needs to be modified to account for biogenically produced post-depositional effects.