Preservation potential of transgressive coastal lithosomes on the U.S. Atlantic shelf

Abstract

Migration of coastal lithosomes across the continental shelf is a response to the latest Quaternary rise in sea level. Variable fractions of the transgressive sequence may be preserved, depending on pre-existing topography, depth of erosion, wave energy, sediment supply, erosion resistance, tidal range, and rate of relative sea-level change. Materials at greater depth in the stratigraphic column are more likely to be preserved. Sediment samples, seismic profiles, and bathymetric observations, indicate better preservation of shoreline elements on the outer shelf and more reworking on the inner shelf. This is hypothesized to be due primarily to the rate of sea-level rise. A relative sea-level curve for Delaware, obtained from radiocarbon dates on basal peats, rises smoothly from 25 m below present sea level 10,000 years B.P. The rate of rise decreases with time. Sea-level rise resulted in rates of coastal retreat of 20 m/yr 10,000 years B.P., 5 m/yr 5000 years B.P., and 1.5 m/yr at present. The long-term average rate of coastal retreat throughout the Holocene was 10 m/yr. In a conceptual model of constant volume of net erosion per unit length of coast, smaller depths of erosion are envisioned during the period of rapid coastal retreat early in the transgression, allowing a greater preservation potential. During the present slower rate of sea-level rise and coastal retreat, depths of erosion are greater, averaging 10 m off Delaware. In this situation there is a decreased potential for preservation of the coastal environmental lithosomes in a continuing transgression. Changes in factors such as wave climate, sediment supply, tidal range, and tectonics affect this simple model.