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A tidewater glaciomarine depositional model is proposed for the upper Yakataga Formation in the Robinson Mountains, eastern Gulf of Alaska. The upper 2,000 to 3,000 m of the Yakataga Formation is characterized by major channels (depth > 90 m) cut into and filled with both fluvial and/or marine rocks. The channels are elongate and steep walled, and appear to be confined within or parallel with paleotopographically low areas. Detailed studies of five channels show maximum dimensions of 430 m in depth and about 3 km in width. Channel length was not determinable.
Channel margins are in places grooved and striated. Channel-base conglomerates exhibit foreset bedding inclining down the channel axis, and are interpreted to be subglacial melt-water deposits. The channel fill is dominated by interbedded siltstones and pebbly siltstones (diamictites). The siltstones contain abundant in-situ marine fossils, commonly encrusting the tops of dropstones. These deposits are interpreted to be proglacial marine sediments and ice-rafted glacial erratics. Thin sandstones interbedded within the siltstones are graded and exhibit traction features. These are believed to originate as gravity-flow units from oversteepened clastic wedges deposited by subglacial melt-water discharge at the grounded terminus of a glacier. Diamictites are of two major types: (1) poorly sorted sandy siltstone units with patchy distribution of angular clasts (nonmarine tillite); and (2) moderately sorted, muddy siltstone units with in-situ fossils and evenly dispersed, slightly rounded clasts (marine tillite). Some of the diamictites are highly contorted, particularly those underlying younger channel bases. The contorted character is probably the result of loading sediments by an advancing ice lobe.
Nonmarine sandstones and conglomerates are present as interbedded lenticular packages, and are interpreted as fluvial units deposited within a braided glacial-stream complex. These units occur both within channels and as clastic wedges within marine-shelf sequences. The channels are interpreted as fiords, and the modern fiords of the Yakataga area (Icy and Yakutat Bays) serve as modern depositional analogs.
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