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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 564

Last Page: 565

Title: Deposition and Preservation of Supratidal and Intertidal Shell Deposits in a Back-Barrier Environment, Wassaw Sound, Georgia: ABSTRACT

Author(s): Victoria A. Verross

Article Type: Meeting abstract


Extensive supratidal and intertidal shell deposits occur along the margins of Wassaw Sound and major tidal rivers of the Georgia coast. Shell material, primarily from oysters, is derived from the sound, salt marsh, tidal channels, and tidal flats. The deposition and accumulation of supratidal and intertidal shell facies depends on five factors: (1) fetch, (2) storm frequency, (3) wind direction, (4) supply of shell material, and (5) shoreline stability.

Shell berms and shell aprons, the major accumulations of shell in the area, are deposited above mean high tide on the sound and river margins by storm tides and waves. Shell berms are elongate ridges, 15 cm to 2 m (6 in. to 6.5 ft) high and 25 cm to 30 m (10 in. to 98 ft) wide, of accumulated oyster and other shell, marsh float, sand, and organic matter occurring in varying proportions. Shell aprons are lobate deposits, 2 to 30 m (6.5 to 98 ft) wide and up to 2 m (6.5 ft) thick, which are comprised of shell material with or without a sand matrix. These supratidal deposits will be preserved if quickly covered by the relatively impermeable tidal flat and marsh muds to depths below the redox potential discontinuity. Intertidal shell pavements, the result of storm and tidal action on oyst r reefs, often have a distinct fabric of vertically packed, tightly wedged shells which are stable under all but storm conditions. Pavements and other intertidal shell deposits are widespread on the tidal flats of Wassaw Sound and are the most easily preserved shell facies. Rapid progradation of the upper tidal flat accompanied by vertical accretion of salt-marsh sediments will result in the rapid burial and consequent preservation of shell aprons and intertidal shell facies. Preserved shell deposits occur on and under the salt-marsh surface in the study area, and are interpreted as oyster reefs, shell aprons, and shell pavements based on their shell orientation, shell body geometry, stratigraphic position, and matrix material.

Preserved supratidal shell aprons, intertidal shell pavements, intertidal and subtidal oyster reefs, and subtidal tidal-channel lag deposits can be differentiated in outcrop by the distinctive geometry, fabric, structure, shell condition, and stratigraphic position of each. The presence of preserved shell berms, aprons, and pavements in outcrop is a reliable environmental indicator of a storm-influenced, estuarine or lagoonal coastline, and of the relative

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position of mean high tide and the lagoon or channel margin at the time of deposition.

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