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William R. Morris1, Renee C. Hannon1, Kenneth P. Helmold1, Douglas G. Knock1, Henry W. Posamentier2
(1) ARCO Alaska, Inc, Anchorage, AK
(2) ARCO Indonesia, Inc, Indonesia

Abstract: Jurassic Alpine Sandstone, North Slope, Alaska: An Example of Shoreface Deposition within a Punctuated Transgressive Succession

Alpine field, containing 429 million barrels of oil, is the first producing upper Jurassic sandstone on the North Slope of Alaska. The sandstone represents a nearshore system deposited during periods of short-lived shoreline aggradation punctuating an overall period of shoreline transgression. It consists of several aggradational sandstone deposits buttressed against coastal escarpments and bounded by thin transgressive deposits. Seismic and wire-line data indicate each shoreface succession is approximately 10 km long, 2 km wide, and up to 30 m thick. Regional analyses indicate these sandstone bodies lie parallel to the paleo-shoreline.

The aggradational deposits are interpreted as a lower shoreface environment. They are very fine-grained quartz arenites with little glauconite and are highly burrowed by a robust Cruziana-Skolithos ichnofacies. The transgressive deposits are glauconitic, fine-grained quartz arenites that are also highly bioturbated and occur above an erosive surface burrowed by a glossifungites assemblage. The wireline log expression suggests a lithology that varies from blocky (uniform grain size) to hour-glass (coarser at the top and base).

The sandstones represent aggradational shorefaces that overlie a transgressive surface of erosion. This surface is characterized by angular discordance suggesting a complex multi-genetic surface. A succession of short-lived aggradational deposits are observed to backstep across the study area. Each sandstone unit subsequently is overlain by another transgressive surface of erosion. We refer to this process of aggradation of nearshore deposits imbedded within an overall transgressive succession as punctuated transgression. This model currently is being used and tested in the development of the field.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana