Depositionai Systems in a Forearc Basin -- Outcrop Examples from the Upper Jurassic Naknek Fm of Southern Alaska
Maurice A. Cucci, William R. Morris, Joseph H. McGowen,
Richard P. Curry, Scot W. Krueger, and David
J. Doherty
Outcrop exposures of the Naknek Fm in Southern Alaska comprise a thick pile of terrigenous clastics that accumulated in a forearc basin. Depositional systems are associated with the Bruin Bay Fault and range from alluviai fans to fan deltas to turbidites. These depositional systems provide excellent examples of stratigraphic cycles where tectonism dominates over eustasy. The Naknek records two types of tectono-stratigraphic cycles. The first cycle was a low frequency event of ~20 Ma duration characterized by early rapid deepening driven by regional cooling following the initiation of subduction. This was succeeded by slow shallowing upward from deep marine to non-marine sedimentation as the forearc was underplated by a growing accretionary prism. Depositional style change from a ramp system in the lower portion of the 20 Ma cycle to a prograding shelf system in the upper portion of this cycle. This overall trend was interrupted by a second type of tectono-stratigraphic cycle comprising short duration uplift events related to changes in the underlying subduction system (plate reorganizations, ridge subduction, segmentation of the down-going slab etc.). Stratigraphic sequences controlled by these high frequency tectonic events are ~5 Ma in duration. An idealized sequence follows a short-lived tectonic uplift event, which typically forms an unconformity. Deposition begins with subsequent subsidence and ends with the next uplift event/unconformity. Each stratigraphic succession was characterized by alluvial fan/braided stream deposits, followed by a transition from coastal to shallow to deep marine sediments, and, where preserved, culminating in a return to shallow water or non-marine sedimentation.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California