Seafloor Geologic Hazards in OCS Lease Area 55, Eastern Gulf of Alaska

Abstract

ABSTRACT Seafloor geologic hazards in the region of lease-sale area 55 must be identified in order for exploration and development to be conducted on tracts that are free of significant hazards. Seafloor hazards that have been discovered in and near the lease area include shallow faults, gas-charged sediment, submarine slides and flows, and buried channels. Numerous seismic events have been recorded in or near the lease area. Seafloor fault scarps, with reliefs of 15-20 m, have been mapped indicating the active nature of the fault traces, especially those associated with the Fairweather fault. Gas charged sediments are common near the Alsek and Dangerous Rivers and also occur elsewhere in the region. These rivers are major sources of sediment and may be transporting to their fullest during times of glacial-lake "breakouts." Buried channels are most prevalent seaward of these rivers and off the Fairweather and Grand Plateau Glaciers. Submarine slides and sediment gravity flows are prominent in two parts of the OCS lease area. Submarine slides and flows are detectable on seismic profiles in the nearshore zone off the Dangerous River and in the heads of Yakutat and Alsek Sea Valleys. Extensive slumps and slides are evident on most seismic profiles from the upper part of the continental slope. The slope is an exceeding active and unstable area, especially in the tectonically active eastern Gulf of Alaska, and probably poses the largest challenge to successful exploration and development. INTRODUCTION The continuing push for energy independence by the United States includes exploration of new segments of the outer continental shelf (OCS). In October 1980, a second lease sale is planned for the eastern Gulf of Alaska. The first OCS lease sale (39), held in 1976, included the area between Prince William Sound and Yakutat Bay, and the second (55) is centered on the continental margin between Icy Bay and Cross Sound (Fig. 1). In this region of frequent earthquakes and numerous destructive storms, any seafloor developments must be carefully sited and engineered to avoid potential seafloor hazards. The purpose of this paper is to delineate, describe, and illustrate geologic hazards on the seafloor in the eastern Gulf of Alaska between Pamplona Ridge and Cross Sound Sea Valley that must be considered in the design of offshore facilities. GEOLOGIC SETTING The eastern Gulf of Alaska is an area of high seismicity and continuing tectonism because of its proximity to the intersection of the Pacific and North American crustal plates. To the north and west of this area, the Pacific plate is being subducted beneath the North American plate along the Aleutian Trench; to the east, a strike-slip motion persists between the two plates. The leasesale area lies in the transition zone between the two tectonic regimes. (Plafker, 1971). The result is a complex series of faulted and folded structures in Tertiary rocks underlying the continental shelf west of Yakutat Sea Valley and simpler structures to the east (Bruns, 1979).