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

Abstract

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.