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ABSTRACT: Placer Lag Deposits in Submarine Channels in the Gulf of Alaska

Maxwell R. Dobson, Quentin Huggett

GLORIA surveys in the Gulf of Alaska during 1989 have revealed details of sediment transport systems that cross the Slope, Rise, and adjacent abyssal plain. Two systems dominate: channel-levee complexes that promote the construction of major fans, and large single channels with subdued overbank activities which terminate as extended sediment lobes that may coalesce to give sand plains. Both channel types originate from Upper Slope gulley zones developed on rapidly dumped shelf-edge fans associated with major tidewater glaciers that during periods of climatic deterioration and lower sea levels extended across the narrow shelf to the top of the Slope. Thus, the sediment source for these channel systems consists of unsorted rapidly abandoned glacial debris. The nature of ini ial emplacement of unsorted sediments is significant because the Alaskan provenance area is rich in heavy or placer type minerals; particularly those with economic value such as gold and platinum. The reworking of these sediments along submarine channels that morphologically have strong similarities with subaerial systems makes placer prospecting a viable proposition. Surveys using GLORIA, 10 KHz, and 3.5 KHz profilers together with a 140 in.3 airgun array have allowed the identification of prospecting sites and provided the control for the development of predictive models for those processes that ensure heavy mineral concentration in the transport regimes identified for this margin. Importantly, because this margin is an active transform type, individual fans, sourcing as the do from restricted sites along this coastline, are shortlived such that even abandoned fans offer prospects for the surveyor.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990