Crystalline Bedrock / Coastal Plain Sediment and Fluvial Unconformity. Differentiating coastal plain sediment from underlying crystalline bedrock is not critical for the purposes of identifying offshore sand and gravel deposits. Crystalline bedrock does not outcrop at or near the seafloor and holds no potential for sand or gravel resources. Coastal plain sediment also underlies glacial deposits within the study area, making it inaccessible for dredging. For this reason, the interpreted figures do not explicitly differentiate these seismic units. A brief description of regional crystalline bedrock and coastal plain sediment is provided below. Early subsurface investigations from Block Island Sound (x.x. XxXxxxxx et al., 1968; Xxxxxxxx, 1970) report crystalline bedrock as the deepest regional seismic reflector. The seismic characteristics have been described as a south-southeast dipping surface with steepening of the slope occurring several kilometers north of Block Island, RI. The bedrock surface has been described as an irregular, continuous surface with prominent channels forming a south trending bedrock drainage surface (McMaster and Ashraf, 1973a). The age of the rocks has been constrained as pre-Mesozoic based on southern New England bedrock studies. Much like the onshore bedrock geology of southeastern New England, the bedrock geology underlying Block Island Sound is inferred to consist of gneisses and granites associated with the Paleozoic accretion of the microcontinent of Avalonia (Xxxxx and Xxxxx, 1991). Many regional geological studies have reported the presence of unconsolidated to semi- consolidated sediment underlying Quaternary glacial deposits in the mid-Atlantic and southern New England (e.g. McMaster et al., 1968; Xxxxxxx and Xxxxx, 1984). Referred to as coastal plain sediment of the Atlantic coastal margin, these sediment are composed of both non-marine and marine interbedded sands, gravels and clays, deposited with the outbuilding of the passive margin since continental rifting formed the proto-Atlantic Ocean. These sediments have been constrained to Late Cretaceous to Tertiary in age. In seismic stratigraphy, the coastal plain dips to the southeast and is deeply incised by north draining channels. This prominent fluvial unconformity truncates the coastal plain, and represents a long period of fluvial erosion due to subaerial exposure. In Block Island Sound, the northern extent of the remnant coastal plain is demarcated by an irregular, north facing cuesta (Needell and Xxxxx, 1984). The absence of coastal p...
