Sediments of Narragansett Bay System and Rhode Island Sound, Rhode Island

Abstract

ABSTRACT Gravel, sand, silt, and clay content was determined for 942 samples from the bays and adjacent inner shelf to a distance of some 20 miles off the Rhode Island coast. In the Narragansett Bay system clayey silt and sand-silt-clay are the most abundant sediments although sand is important locally. Neither the Bay system nor the inner shelf contain a predominant clay type of sediment. Primarily, sediments are derived from the unconsolidated subaerial and subaqueous glacial and post-glacial deposits. It is believed that the sediment available for deposition is composed of some fine and very fine sand, silt, and clay, but the quantity is very small. During the period in which present environmental conditions have been effective clayey silt and sand-silt-clay have accumulated for the most part in the more protected middle and upper areas of the Bay system. The depositional sites for the finest of these sediments may be the result of the peculiarities of urface tidal current flow. Areas which show marked gradational changes in texture probably indicate significant local variations in current activity along the bottom interface. In general, toward the lower reaches of the Bay passages the sediments show a progressive change to coarser textures. On the inner shelf the predominant sediment type is clean, well-sorted sand. Apparently gravel is concentrated along two major trends which are generally associated with the two submarine elevation features that cross the area. A well-defined depositional zone of sandy silt and silty sand, believed to be the result of non-tidal drift, begins near the entrances to Narragansett Bay and follows the trend of a winding submarine slope toward the southwest and Block Island Sound. A tongue of sand which lies adjacent to the Rhode Island mainland from Point Judith into lower West Passage owes its origin to the northerly moving longshore current which pushes into the Bay from Block Island Sound. In the Bay system and certain areas on the inner shelf unburied glacial or post-glacial lag deposits juxtapose with recently accumulated fine sediments. Present environmental forces are not responsible for the creation of the major submarine topographic features of the region, but in the Bay depositional processes of recent environmental conditions have had considerable success in burying sedimentary deposits of previous environments.