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Sedimentation on Gravel Outwash Fans, Malaspina Glacier Foreland, Alaska
Thomas C. Gustavson (3)
Three depositional facies occur on the outwash fans of Fountain Stream and Alder Stream, which presently drain portions of the Malaspina Glacier along the northeastern Gulf of Alaska. Deposition of longitudinal bars occurs during high-flow stage and consists of plane-bedded, imbricate, very poorly sorted pebble and cobble gravels. The sediment of the bars decreases in mean size downstream. Some bars terminate in avalanche slip faces of poorly sorted, silty, medium to coarse sand. Paleocurrent directions suggest that stream flow diverges across bar surfaces.
Deposition in channels occurs in two morphologically distinct areas, riffles and pools. Sediment deposited in pools consists primarily of ripple cross-laminated, poorly sorted, silty, medium to coarse sand, commonly capped by draped lamination. Ripple crests vary from straight to cuspate. Sediment in riffles consists of gravel laid down under upper flow regime conditions as transverse ribs and stone cells. Both apparently are relict antidune bedforms. During late-stage flow, thin patches of horizontally bedded sand are deposited between transverse ribs and adjacent stone-cell borders.
The longitudinal bar facies and channel pool facies are common in cutbank sections. The channel riffle facies was not recognized in cutbank sections.
Scour pits, commonly observed on some bar surfaces, are produced by currents scouring around grounded ice blocks. Ice-block trails are produced by ice blocks dragged through soft sediment. Neither of these sedimentary structures have been reported in the literature. If found in ancient sediments, both structures would indicate a nearby source of ice.
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