Abstract

Pliocene–Pleistocene megachannels, as much as 400 m deep and several kilometers wide, are exceptionally well exposed around the Gulf of Alaska coastal margin in glaciomarine sedimentary strata of the Yakataga Formation. In the Icy Bay region, stacked megachannels of the Yakataga Formation are filled predominantly with debris flow diamictites, crudely stratified conglomerates, and thinly bedded sandstone and mudstone turbidites. Detailed sedimentological and micropaleontological analyses of the megachannel deposits identified two distinct facies associations that represent channel margin and central channel fill depositional settings. All facies types indicate deposition by sediment gravity-flow processes and high rates of sediment supply in a tectonically active setting. Foraminiferal biofacies suggest deposition in inner neritic to upper bathyal water depths and vigorous transport of sediment from shallow to deeper water.

The Yakataga Formation megachannels probably formed as slump scars, produced by the retrogressive failure and downslope slumping of rapidly deposited outer shelf–upper slope sediments. The intimate association of megachannels with thrust faults and folds suggests that initial slumping was triggered by tectonic tilting and high-magnitude earthquakes. Megachannels have a relatively immature form and were probably short lived and filled rapidly with sediment gravity-flow deposits. Megachannels of the Yakataga Formation provide a well-exposed analog for megachannels formed in other tectonically active settings or passive margin settings characterized by high sedimentation rates. There is little evidence to suggest that megachannel development is systematically related to episodes of sea-level lowstand, and a sequence stratigraphic context is difficult to identify.

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