Abstract

The coastline of the Northeast Gulf of Alaska is characterized by a series of sandur plains (e.g., Copper River Delta area, Malaspina Foreland). Presently active braided-stream systems on each plain are building individual fans that exhibit a systematic variation in gradient (S), clast size (C), morphology, channel pattern, and suites of sedimentary structures from glacier terminus to ocean.

The upper fan (S>5m/km) usually has a single channel incised in coarse gravel (C>10cm long axis). The central fan (S<5> 2m/km) is occupied by a network of streams exhibiting a coarse-braided pattern but flowing on fine gravel (C>10 long axis). Streams on the sandy lower fan (S<2m/km) flow in a fine-braided pattern. A marsh or swamp area with meandering streams may be present at the fan terminus. Bar morphology changed downstream from longitudinal bars (gravel), to linguoid bars (sand). Lateral and point bars are found in meandering streams.

A proximal to distal succession of bar and channel sedimentary structures is: (1) well-imbricated, poorly-sorted, coarse gravel with rudimentary bedding; (2) interbedded fine gravel and plane laminated sand, together with some large-scale planar cross-lamination; (3) interbedded planartangential cross-lamination and sandy ripple drift cross-lamination and (4) a repeated sequence of large-scale trough cross-lamination, overlain by sandy-ripple drift capped by a silt drape. The preceding succession fines downstream but does not fine upward. However, finer overbank deposition is present as sandy ripple-drift, drape lamination, and silty-clay laminations.

Sandur lengths vary from 3 to 30 km. Successive facies develop in order downstream, but distal facies may be absent due to variation in stage of sandur development. Each sandur plain may be viewed as a regressive series of coalescing fan deltas.