Hazard Analysis on the Mid-Atlantic Continental Slope, DCS Lease Sale 59 Area

Abstract

A multi-parameter high-resolution seismic survey covering 253 offshore lease blocks was undertaken for analysis of critical structural and depositional features and a suite of piston cores was examined for geotechnical properties on the Mid-Atlantic continental slope in the OCS Lease Sale 59 area. The analysis of this data revealed complex interrelationships between a number of buried structural and depositional features indicating the existence of a variety of slope environments in the proposed lease sale area. The relationship these depositional features have to fault scarps and other topographic irregularities is critical to hazards assessment in this area. Southwest of the Hudson Canyon area, a major slump complex was partially delineated and numerous drape structures, which in some cases appear to have developed into contemporaneous down-to-the-basin faults, are associated with topographic irregularities. Southwest of the Baltimore Canyon area, slumps may be a result of the formation of mud diapers. These diapers? are the first reported in the Mid-Atlantic continental slope. Piston cores were collected at selected locations to provide information on geotechnical strength parameters of slumps, slides, and undisturbed sediments. These data indicate that localized areas of under consolidated sediments are found on valley walls and ridges of the upper slope. These zones may represent discrete areas where either mass movement has occurred or the potential for mass movement may exist. Many of the mass-movement features identified in the OCS Lease Sale 59 area may be Pleistocene in age and are related to' conditions prevailing during low sea stands. Since these conditions are presently absent, the potential for sediment mass movement does not appear to present a major problem to oil and gas operations within the proposed OCS Lease Sale 59 area. Introduction A recently completed study by the U.S. Geological Survey assessed the existence of potential geologic hazards and constraints which might adversely affect oil and gas exploration and development in the Mid-Atlantic OCS Lease Sale 59 area. The 253 blocks in the study area total some 1,440,376 acres, and are located in the Baltimore Canyon Trough in water depths of 85 m to 2,455 m (figure 1). Seventy of these blocks were previously surveyed for geohazards in connection with OCS Lease Sales 40 and 49. It was not considered cost effective or necessary to resurvey these blocks, but tie lines were run into these blocks at approximately 8,000-m intervals in order to facilitate merging the different data sets. The assessment of geologic hazards was partially based on the interpretation of high-resolution seismic- reflection profiles collected by Inter sea Research Corporation under contract to the U.S. Geological Survey. A total of 7,574 km of multi-spectral high-resolution acoustic data were collected from June to September 1980 over 183 blocks aboard the RV Albert using an 800 by 2,400-m grid. The data were collected using a 24-kHz narrow-beam (100 cone) fathometer, a single-channel (l/Z-ms sampling rate) digital seismic profiler, and a broad-band analog acquisition system. The analog data were played back through a 450-Hz low-cut filter. A single 15cubic inch water gun was used as the sound source for the seismic system.

Timing of occurrence of large submarine landslides on the Atlantic Ocean margin

“…The mid-Atlantic slope between roughly the entrance to Chesapeake Bay and Hudson Canyon has been studied intensively for the locations and mechanics of submarine landslides (Embley and Jacobi, 1977;McGregor and Bennett, 1977;McGregor, 1977;Malahoff et al, 1980;Cardinell et al, 1982;Robb et al, 1982). This is mainly because the area was a focus for possible Outer Continental Shelf (OCS) energy resource development in the late 1970s and early 1980s prior to moratoria on offshore drilling in the area passed in the 1980s.…”