Abstract

High-resolution subbottom profiles have been used to identify and map the pre-Holocene erosional surface in Delaware Bay and in the Atlantic Ocean off the Delaware coast. Vibracoring has revealed the nature and age of the various seismic layers. Age identifications based on radiocarbon dating, on oxidation zones, and on lithologic characteristics confirm the partial retention of Holocene coastal environments under the inner shelf. The Holocene transgression across the ancient Rehoboth and Indian River lagoons has involved erosion through the Holocene lagoonal sediments in some areas, leaving pre-Holocene ridges exposed on the inner-shelf sea floor where once there may have been headlands. Where the ancient lagoonal deposits are thickest over depressions in the pre-Holocene surface, Holocene coastal sediments were retained. The presence of marsh peat, dated as ∼7,500 yr B.P., and overlying very fine lagoonal mud proves that lagoonal conditions existed east of Delaware even then. The position of the barrier complex related to these earlier Holocene lagoons is projected on a paleographic reconstruction to be at least 12 km (7 n mi) east of the present coast. This yields a minimum average coastline retreat rate of 1.6 horizontal m per yr. (6 ft per yr).

This study of the shallow subsurface geology also revealed that the bottom morphological features of the inner shelf off Delaware are related to dynamic molding of hydraulic bedforms in equilibrium with the present marine environment. Although one ridge form was found to be a subcrop of Pleistocene sand and gravel, most of the ridge and channel topography was found to be formed within the upper Holocene sediments. This is especially true of the shore-face-connected ridges off Rehoboth Bay and Bethany Beach, the inlet-associated shoals near the mouth of Delaware Bay, and the Delaware shelf-valley flood channel.

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