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Fluvial Facies Models
Proglacial Braided Outwash: A Model for Humid Alluvial-Fan Deposits
The recent proglacial outwash fans of the Northeast Gulf of Alaska and the southern coast of Iceland exhibit similar depositional patterns and suites of sedimentary structures along the paths of individual braided-stream systems. Each system has a regional gradient ranging from 6 - 17 m/km in the proximal zone to 2 - 3 m/km at the distal margin. Maximum clast size (long-axis) decreases regularly downfan from 50 cm (proximal) to sand (distal). Proximal bars are longitudinal; distal bars are linguoid.
Individual fans have different types of distal margins, and may also be different laterally, in the areas between active streams. Sandy distal facies are best developed on Icelandic fans and are absent on Alaskan fans less than 8 - 10 km in length. A meandering-stream facies is sometimes present at the distal margin of Alaskan fans. Inactive stream areas support extensive dune fields and wind-tidal flats in Iceland, but are marsh-and-swamp covered in Alaska.
Fan lengths vary from 3 - 30 km. A standard proximal to distal (0 - 15 km) succession of bar and channel sedimentary structures exists for all fans studied and allows the identification of the individual or “core” braided-stream system. The addition of: 1) suites of sedimentary structures of the various distal margins (15 - 30 km), and 2) the lateral, “overbank and floodplain” facies assemblages, then leads to the construction of a multivariate model that characterizes the deposits of these humid alluvial fans.
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