Environmental geology of Harrison Bay, northern Alaska

Abstract

The surficial and shallow subsurface geology of Harrison Bay on the Beaufort Sea coast was mapped as part of the U.S. Geological Survey's prelease evaluation for Outer Continental Shelf (OCS) Oil and Gas Lease Sale 71. During the 1980 summer season, approximately 1600 km of multisensored, high-resolution geophysical profile data were collected along a rectangular grid with 4.8 km line spacing. Interpretation of these data is presented on five maps showing bathymetry, sea-floor microrelief, ice-gouge characteristics, Holocene sediment thickness, and geologic structure to depths of approximately 1000 m. On a broad scale, the seafloor is shallow and almost flat, although microrelief features produced by sediment transport and ice-gouge processes typically vary up to several meters in amplitude. Microrelief bedforms related to hydraulic processes are predominant in water depths less than 12 m. Microrelief caused by ice gouging generally increases with water depth, reaching a maximum of 2 m or more in water depths beyond the 20 m isobath. This intensely gouged area lies beneath the shear zone between the seasonal landfast ice and the mobile polar ice pack. The thickness of recent (Holocene) sediment increases offshore, from 2 m near the Colville River delta to 30 m or more on the outer shelf. The thin Holocene layer is underlain by a complex horizon interpreted to be the upper surface of a Pleistocene deposit similar in composition to the present Arctic Coastal Plain. The base of the inferred Pleistocene section is interpreted to be a low-angle unconformity 100 m below sea level. Beneath this Tertiary-Quaternary unconformity, strata are interpreted to be alluvial fan-delta plain deposits corresponding to the Colville Group and younger formations of Late Cretaceous to Tertiary age. Numerous high-angle faults downthrown to the north trend across the survey area. With few exceptions, these faults terminate at or below the 100 m unconformity, suggesting that most tectonism occurred before Quaternary time. Acoustic anomalies suggesting gas accumulation are rare, and where identified typically occur adjacent to faults. A laterally continuous zone of poor seismic data occurs in the nearshore area and is interpreted to be caused by subsea permafrost. This report describes these geologic conditions in Harrison Bay and discusses potential hazards that they may pose for future oil and gas operations in Sale 71 and adjacent Beaufort Sea shelf areas.