Petrography and Genesis of Recent Sediments in Narragansett Bay and Rhode Island Sound, Rhode Island

Abstract

ABSTRACT The assemblages and general distribution of heavy minerals have been determined for the sand fraction of bottom surface sediments in the Narragansett Bay system and from Rhode Island Sound and its approaches. From these data distinct heavy mineral associations have been idetnified: an amphibole association with some staurolite, epidote, and clinozoisite-zoisite in upper Narragansett Bay; a rock fragment association in Mount Hope Bay and upper Sakonnet Passage; and an opaque mineral association with variable amounts of amphibole, garnet, and tourmaline on the inner shelf. Pre-Cambrian (?) greenstones and metamorphosed sedimentary rocks are believed to be the sources of the amphibole assemblage; Pennsylvanian sedimentary rocks, the source of the rock fragment association; and coastal ro ks of southern Rhode Island and southeastern Massachusetts, the principal source of the opaque mineral suite. Present mineral composition of inner shelf sediments is primarily dependent on the nature of immediate source materials and not on the rigors of the reworking or redistribution processes. In the Narragansett Bay system, East Passage acts as the main dispersal route for the distribution of upper Bay amphibole sands as well as the rock fragment assemblage of Mount Hope Bay. On the inner shelf, bay sediments are deposited primarily along a winding belt near Point Judith Neck and also appear to be dispersed south of Narragansett Bay proper. From Buzzards Bay sediments are apparently carried seaward through the submarine valley which crosses the Sound. From these data it is inferred that as an inner shelf sedimentary environment, Rhode Island Sound represents an early stage in a newly developed depositional cycle under present marine conditions.