Physical and biogenic aspects of backbarrier sedimentary sequences, Georgia Coast, U.S.A.

Abstract

Configuration of the present Georgia coast, with its broad expanses of salt marsh and intricate tidal drainages behind Pleistocene—Holocene barrier islands, implies the filling of an extensive backbarrier lagoonal system. However, riverine and salt marsh estuaries dominate the modern depositional setting and reconstructed vertical sequences suggest that backbarrier filling was accomplished by estuarine processes. Backbarrier sediments exhibit considerable diversity in grain size and physical and biogenic sedimentary structures. Principal sediment textures include clean to muddy coarse and fine sand, mud and, to a lesser extent, shells, pebbles, and mud and bedrock clasts. Physical sedimentary structures, many modified by bioturbation, include large-scale cross-bedding (trough and planar) with graded sets, small-scale crossbedding (ripple lamination and flaser, wavy and lenticular bedding), laminated to interbedded sand and mud, and rare pebble or shell beds. Ebb-oriented structures predominate over flood-oriented ones. Except in upper reaches of riverine estuaries, biogenic sedimentary structures typically are diverse and abundant. characteristic lebensspuren include polychaete dwelling tubes, decapod and polychaete burrows, ray holes, and bioturbate textures imparted by amphipods, ophiuroids, bivalves and anemones. Patterns of bioturbation are controlled to some extent by sediment type, salinity levels and rates of deposition and physical reworking of sediments. Although intensity of bioturbation generally increases seaward, bioturbate textures are more readily preservable in the lower-energy, middle reaches of estuaries. Because of numerous local Pleistocene sediment sources and local variations in hydrographic regimes, the stratigraphic record of salt marsh estuaries differs relatively little from that of riverine estuaries. The basic sequence, in ascending order, typically consists of: (1) channel-lag and (or) channel-fill deposits; (2) estuarine accretionary beds; (3) point bar, tidal flat and shoal sediments; and (4) channel margin and salt marsh deposits.