Holocene stratigraphy and depositional history of the Narragansett Bay System, Rhode Island, U.S.A.

Abstract

An extensive seismic reflection survey has been used to gain further knowledge of Holocene stratigraphy and depositional history in the Narragansett Bay System (NBS). The early Holocene stream‐dissected surface beneath the NBS is interpreted as having been flooded by the Holocene sea in a manner suggested by Oldale & O'Hara's (1980) sea‐level‐rise curve. The sea initially is believed to have penetrated the pre‐NBS East Passage trunk valley about 9000 yr BP and subsequently spread landward via the trunk valley and its branches. The Holocene sediments display stratigraphic relationships that differ spatially. At passage mouths, the basal unconformity is inferred to be covered over with some 3 m of paralic and 5 m of marine sands and silts separated, by a transgressive unconformity. In contrast the interior sequences reveal (a) a valley section up to more than 15 m thick in which the regressional unconformity is overlain by probable lower fluvial and/or estuarine sand‐silt facies that commonly grades upward to an estuarine silt‐clay facies and (b) an interfluve section in which a basal transgressive unconformity is blanketed by an estuarine nearshore sand‐silt facies that locally may change upward to a silt‐clay facies. Primarily Holocene silt‐clay accretion, produced by sedimentation processes associated with net non‐tidal estuarine circulation, infilled the evolving NBS. Depositional bodies, lenticular in shape and comprised of 12 m or more of sediment, developed in lowlands near Gould Island (˜9000 yr BP), in Upper Narragansett Bay (˜7500 yr BP), around Hope Island (˜7500 yr BP) and in Mt Hope Bay (˜6250 yr BP) with an average minimal sedimentation rate of 1.6–2.2 mm yr‐1. Silt‐clay deposition, commonly gas‐bearing, has buried the basal relief in most of the NBS upper and middle portions except for middle East Passage. A comparison of NBS sedimentation with that of Chesapeake, Delaware and Hudson Estuaries shows that the estuaries to the south have accumulated more sediment over a slightly longer period yet, with the exception of the higher rate in the Hudson Estuary, the sedimentation rates appear to be similar.

Reconstructed topography of Southern New England prior to isostatic rebound with implications of total isostatic depression and relative sea level

“…The isostatic rebound model presented here supports a transgression history in southern New England with inundation of Block Island Sound and Narragansett Bay during latest Pleistocene and Holocene time (Oldale and O'Hara, 1980;McMaster, 1984). This differs from the interpretation of a marine incursion into Rhode Island, Block Island, and Long Island Sounds prior to isostatic rebound (Lewis and Stone, 1991;Stone et al, 2005).…”

“…The topography below present sea level was not corrected to reflect post-glacial deposition. The thickness of postglacial sediment is less than 6 m across most of the study area, except in deep channels in the East Passage of Narragansett Bay, Rhode Island Sound and Block Island Sound (O'Hara and Neddell et al, 1983;McMaster, 1984;Needell and Lewis, 1984;Boothroyd and Oakley, 2005). Due primarily to large uncertainties in the amount of sediment eroded during the transgression, there was no attempt to account for sediment lost to erosion since deglaciation, and it is assumed that the post-glacial deposition is in the same order of magnitude of post-glacial erosion.…”

Extensive estuarine sedimentary storage of plastics from city to sea: Narragansett Bay, Rhode Island, USA

“…NB is carved out of the Narragansett Basin sedimentary strata and its morphology and stratigraphy have been heavily influenced by glacial processes 35 , 36 . Fine sediment input to the system is very low due to the low-sediment yield rivers draining to the estuary, and many fluvial sources are dammed upstream 35 .…”

“…Fine sediment input to the system is very low due to the low-sediment yield rivers draining to the estuary, and many fluvial sources are dammed upstream 35 . Sediment accumulation in NB is controlled by microtidal estuarine sedimentation processes which deposit fine sediment in deep, sheltered areas in the upper and middle Bay, and sandy sediment dominates the mouth of the Bay due to ocean transport by waves and near-bed currents 36 . Modern day net sediment accumulation rates in NB are low, generally < 3 mm year −1 23 , 37 – 39 .…”