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Title: Geomorphology, shallow subbottom structure and sediments of the Atlantic Inner Continental Shelf off Long Island, New York
Authors: Williams, S. Jeffress
Keywords: Atlantic Inner Continental Shelf
Long Island
New York
Beach nourishment
Seismic reflection
Issue Date: Mar-1976
Publisher: Coastal Engineering Research Center (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical paper (Coastal Engineering Research Center (U.S.)) ; no.76-2.
Description: Technical Paper
Abstract: About 800 square miles of the Atlantic Inner Continental Shelf off Long Island, New York, were studied by CERC to obtain information on the sea floor morphology, sediment distribution, and shallow subbottom stratigraphy and structure. This information is used for delineating sand and gravel resources and deciphering shelf geologic history. Basic survey data by CERC consist of 735 miles of high-resolution continuous seismic profiles and 70 vibratory cores; additional data were available from 82 sediment cores and 225 miles of seismic records. Data coverage extends from Atlantic Beach east to Montauk and in Gardiners Bay; and from the shoreface seaward about 10 miles to water depths of 105 feet. Three primary acoustic horizons are evident on the seismic profiles and have been identified by correlation with cores, land borings, and surface exposures of the reflectors. Granitic bedrock is the oldest and deepest horizon underlying Long Island, but its recognition on the seismic records, due to limited subbottom penetration, is confined to northern Gardiners Bay. The bedrock surface slopes southeast and exhibits considerable relief where glacial ice has enlarged pre-Pleistocene drainage channels. Upper Cretaceous and Tertiary semiconsolidated clastic sediments overlie the bedrock and dip and thicken to the southeast. The surfaces of these strata, which are present throughout the study area and project north under Long Island, are the second major horizon. The third seismic horizon is a Pleistocene erosion surface cut by fluvial and glacial agents into the older rock units. Depth of this surface varies from -50 to -300 feet MSL off the western and eastern Long Island shelf to sea floor outcropping in parts of the central Long Island inner shelf. Pleistocene detritus consists primarily of blanketlike deposits of outwash sand and gravel; however, radiocarbon dates show that Holocene-age barrier-lagoonal sequences and estuarine sediments cover parts of the Long Island shelf. Surficial sediments on the inner shelf are primarily fine to medium quartz sand with secondary occurrences of coarse sand and pea gravel on the Atlantic shelf and silt-clay mixtures in the Gardiners Bay region. The granular facies are relict outwash detritus, carried onto the shelf by ancient rivers and washed and sorted by marine processes since the Holocene rise of sea level. Fine-grained sediments on the shelf originated in early Holocene back-barrier or lacustrine environments; however, those in Gardiners Bay are estuarine or lacustrine deposits from Pleistocene lakes which occupied that region. Glauconitic sands, restricted to a zone off Fire Island Inlet, appear to be residual from the underlying Monmouth Group which, along with other Cretaceous strata, form a cuesta where strata are truncated by the sea floor. Numerous major buried ancestral drainage channels transect Long Island mainland in a north-south orientation and continue south across the shelf. Thalweg depths of the channels range from -100 to -550 feet MSL and channel widths are often several miles. Many channels on the north shore of Long Island underlie reentrant bays and most were significantly enlarged by Pleistocene glacial ice and later filled with sediment. Much of the surficial sand on the inner shelf is suitable as fill for beach restoration, except for that of the shoreface region (0 to -30 feet MSL) which contains fine sand and that of major parts of Gardiners Bay which contain organic-rich silt and clay. Topographic highs on the sea floor in the form of linear shoals, and broad deltalike platforms in eastern Long Island appear most suitable for sand recovery. The sea floor in most potential borrow areas is flat and sand occurs as blanket deposits. Potential sand reserves within about 12 feet of the sea floor in the region are estimated to be more than 8 billion cubic yards.
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