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SECURITY CLASSIFICATION OF THIS PAGEfWhan Data Bnlend) 



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 vvith .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. 

 Tlie 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 gran- 

 ular 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 poten- 

 tial 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. 



UNCLASSIFIED 



SECURITY CLASSIFICATION OF THIS PAGECWTien Data Entered) 



