Abstract

Box core X-ray radiographs from estuarine and closely adjacent nearshore shelf sediments of Georgia were used to study interrelationships among detrital valve orientations, sediment textures, and physical and biogenic sedimentary structures. The cores represent preserved sediment sequences and thus should be more indicative of the rock record than shell orientations observed on substrate surfaces. With little bioturbation, physical processes mainly govern shell configurations. Horizontal concave-down valves predominate in large- and small-scale crossbedded sands, laminated sands, and thin shell beds; inclined concave-down valves also are common, together with lesser numbers of horizontal concave-up valves. Vertical and inclined concave-up valves predominate in thick shell beds. shell-filled burrows, and thin shell beds consisting of shell-ornamented Diopatra tube fragments; horizontal concave-up valves also are common in each of these, and inclined concave-down valves are abundant in thick shell beds. With progressively more intense bioturbation, as is commonplace in estuarine and nearshore shelf sediments, proportions of horizontal and inclined concave-down valves decrease as those of vertical and inclined concave-up valves increase, the latter two becoming dominant. The abundance of horizontal concave-up valves increases initially but decreases between 90 and 100 percent bioturbation. Some of these results, especially the predominance of vertical and inclined concave-up valves, differ from those of previous workers. Small sizes of dominant Georgia bivalves probably account for much of the difference. Because of pronounced local variations, large numbers of measurements also are imperative.

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