Abstract

The Nation River Formation of east-central Alaska, now considered to be late Devonian in age, is 2,000–4,000 feet (600–1300 m) thick and consists of intermixed chert-pebble conglomerate, chert-quartz arenite, siltstone, mudstone, shale, and pebbly mudstone. It has long been considered to be a nonmarine deposit because of its coarse-grained character, its lack of marine fossils versus abundance of vascular plant fragments, the local presence of fossil spores, and its presumed affinities to other Devonian nonmarine strata in Alaska, the Cordilleran geosyncline, the Canadian Arctic Islands, and North Atlantic regions. Situated within a thick and widespread sequence of Paleozoic graptolitic shale, chert, and limestone, this formation provides sedimentary evidence for significant Late Devonian orogenic activity. Our stratigraphic and sedimentologic studies suggest that the Nation River Formation was deposited by turbidity currents and related processes in a deep marine basin. The assemblage of sedimentary structures, lithofacies, paleocurrents, and stratigraphic relations indicate deposition as a deep-sea fan complex.

Sedimentary structures and features include: graded bedding; a variety of different Bouma sequences; thick, massive, and ungraded sandstone and conglomerate beds; abundant sole markings, including flute casts, groove casts, load casts, and a variety of smaller markings; abundant small-scale cross-stratification, convolute lamination, and current ripple markings; uncommon medium- to large-scale cross stratification; primary current lineation; oriented carbonaceous plant fragments; slump structures and sandstone dikes; local trace fossils; possible dish structures; channeling at the bases of sandstone and conglomerate beds; and amalgamated bedding.

Paleocurrents indicate transport of sediments toward the west. The thickly bedded conglomerate and sandstone appear to have been deposited in channels on a deep-sea fan by turbidity currents, grain flows, and fluidized sediment flows. The graded finer grained sandstone and shale sequences appear to have been deposited in interchannel and fan fringe areas by turbidity currents, overbank spilling, and vertical settling of pelagic detritus. The pebbly mudstone appears to have been deposited by debris flows from basin margin slopes. Paleogeographic reconstructions suggest that the source area was located generally toward the northeast and contained abundant chert and quartz.