Stratigraphy and Geologic History of Quaternary Sediments in Lower West Passage, Narragansett Bay, Rhode Island

Seismic stratigraphy, sedimentology, lithostratigraphy, biological content, sediment geochemistry and radiocarbon dating were used to investigate the origin and history of Quaternary sediments infilling the lower West Passage bedrock valley, Narragansett Bay. Sediment samples from 133 bridge borings provided the primary data set. When glacial ice retreated from lower Narragansett Bay, a late Pleistocene glacial depositional sequence was deposited in the West Passage bedrock valley. A discontinuous till, 2–6 m thick and a gravelly sand glaciolacustrine fan deposit, up to 16 m thick, were the initial accumulations. With further ice retreat, up to 38 m of glaciolacustrine lakefloor varved silt and fine sand was deposited, interfingering westward with and overlain by up to 25 m of glaciolacustrine delta front and 5 m of glaciofluvial delta plain gravelly sand. After the glacial lake drained, perhaps by 14,000 yr BP, the lake floor was fluvially eroded. A 9733–11,975 yr BP upper age was estimated for the deepest fluvially-cut valleys based upon radiocarbon dates. As the sea advanced into lower Narragansett Bay, three Holocene depositional sequences accumulated. In the fluvially-cut tributary valleys a paralic depositional sequence, up to 9 m thick was laid down. Subsequently the landward surf zone migration eroded the paralic sequence and interfluves with the resulting ravinement being covered by an estuarine sediment sequence, up to 8 m thick. However, in the trunk valley, tidal current flow limited estuarine accumulation to 2 m. In lower West Passage sea level rose at a rate of 2.3 mm/yr between 2000 and 7000 yr BP. During the last 9400 yr BP, the net sedimentation rate varied between 0.9 and 1.3 mm/yr.