Abstract

The Zodiac fan, a large body of upper Eocene through at least lower Oligocene turbidities, underlies the Aleutian Abyssal Plain and lies just south of the Aleutian Trench and the Alaskan Peninsula in the northeastern north Pacific. The fan deposits cover an area of more than 1 × 106 km2 and contain at least 280,000 km3 of terrigenous sediment. The most striking feature of the fan is its well-developed channel distributary system, which persists nearly to the plain limits. Levee overbank deposits associated with the channels are the dominant sedimentary style, a process believed to predominate on fans receiving primarily fine-grained detritus. Four major channels have been identified on the fan with the following relative age relation (> = older than): Sagittarius > Aquarius > Taurus, and Sagittarius >Seamap. Recently obtained seismic information implies that Seamap Channel may be older than Aquarius Channel. If Seamap is the youngest channel, a depositional interval of 8 m.y. (40−32 m.y. B.P.) is indicated for the fan as a whole. If Taurus is the youngest, as we believe, the interval of deposition is greater than 8 m.y. but is probably less than 16 m.y. (40−24 m.y. B.P.). Nanno-flora, pollen, and spores obtained from Deep Sea Drilling Site 183, near the northern margin of the fan, indicate that the fan formed in a nontropical (northern) environment, and that the source terrane had a climate similar to or slightly cooler than that recorded from coeval onshore deposits in the eastern Gulf of Alaska. On the basis of the interval of deposition, the volume of the fan, and sediment yield from climatically similar modern drainages, a minimum drainage of 500,000 km2 is believed necessary to have supplied the sediment to form the fan, assuming no sediment was deposited before it reached the fan. Tertiary plate-motion models requiring large amounts of relative convergence along the Aleutian Trench are judged unworkable, as such reconstructions require the Zodiac fan to have formed 1,500 to 3,000 km from the nearest continental landmass and separated from it by topographic barriers, requiring that the drainage be inflated in size manyfold to overcome anticipated sediment losses during such a lengthy transport. As the minimum drainage (500,000 km2) is already equal to one-half of the State of Alaska, any major expansion is judged unreasonable. This requires that relative convergence at the Aleutian Trench be limited to less than ∼500 km from 40 m.y. B.P. to present. A possible method by which this limitation can be satisfied is to allow a significant portion of southern Alaska to move in concert with the Pacific plate since the upper Eocene.

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