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Figure 1. Location of National Oceanic and Atmospheric Administration (NOAA) survey H11322 study area (red polygon) in Rhode Island Sound, southeast of Point Judith, Rhode Island.

Figure 2. Sidescan-sonar image of National Oceanic and Atmospheric Administration survey H11322 in western Rhode Island Sound.

The U.S. Geological Survey (USGS) has been working with the National Oceanic and Atmospheric Administration (NOAA) to interpret areas of the sea floor along the northeastern coast of the United States. In July 2004, NOAA completed hydrographic survey H11322 in Rhode Island Sound, about 8 km southeast of Point Judith, Rhode Island, and 12 km east of Block Island (fig. 1). Sidescan-sonar and bathymetric data obtained during the survey were used to delineate sea-floor features and sedimentary environments. This report documents the interpretation of 60 km² of sea floor covered by NOAA survey H11322. This information is useful for studying benthic habitats and provides a framework for future studies.

Prior work in this area includes studies of the sea-floor characteristics and stratigraphy to determine active sediment processes and geologic history. McMaster (1960) mapped the sediment distribution of Narragansett Bay and Rhode Island Sound. He classified and mapped the surficial sediments in the current study area as mostly sand, with silty sand in the northwest and gravel scattered throughout the west (McMaster, 1960). McMaster and others (1968) outlined the northern boundary of Cretaceous Coastal Plain sediments and mapped buried stream valleys. Knebel and others (1982) used sidescan sonar to map features of the sea floor in Rhode Island Sound, such as areas of sand waves and sediment accumulation. Needell and others (1983a) identified sedimentary units in the stratigraphic framework of Rhode Island Sound. Four sediment samples from the National Ocean Service (NOS) Hydrographic Database and one sample from the Hathaway 71 data set fall within the boundary of NOAA survey H11322: three samples of sand or fine sediment are from the east and one sample of mud and one of gravel are from the central part of the study area (fig. 2; Poppe and others, 2003).

Two recent studies that interpret NOAA hydrographic surveys in Rhode Island Sound were conducted east of this study area (fig. 1; McMullen and others, 2007; McMullen and others, 2008). These studies delineate sea-floor features including rocky areas, sand waves, a hummocky recessional moraine, glaciolacustrine erosional outliers, and a series of scarps and benches interpreted to be paleoshorelines. The eastern border of the present study area overlaps the western boundary of these studies (McMullen and others, 2008).