Sedimentology and Tectonics of Devonian Nation River Formation, Alaska, Part of Yet Another Allochthonous Terrane

David G. Howell, Richard W. Murray, Thomas J. Wiley, Susan Boundy-Sanders, Linden Kauffman-Linam, David L. Jones

Sandwiched between terra incognito of the Yukon Flats, Alaska, and the disrupted cratonal sequences of Yukon Territory, Canada, is a complex array of Proterozoic and Phanerozoic rock units composing a poorly defined group of tectonostratigraphic terranes. The Nation River Formation (NRF) is a conspicuous siliciclastic submarine fan complex interbedded in a Paleozoic sequence characterized by deep-water cherts, siliceous shales, and platform to basin-plain carbonates.

The NRF ranges from 500 to 2,000 m thick. Where the basal part is exposed, NRF overlies the Devonian McCann Hill Chert, a deep-water radiolarian chert sequence. Above the NRF is either another radiolarian chert sequence, the Mississippian Ford Lake Shale, or Permian shallow-water Tahkandit Limestone or Step Conglomerate. NRF lithologies include fine-grained to pebbly turbidites assembled in both thinning- and fining-upward and thickening- and coarsening-upward cycles typical of middle to outer fan settings.

Compositionally the grains are principally chert (green, gray, white, black, and rarely red) with minor amounts of vein quartz and quartz sandstone. Most of the chert seems to be replacement chert from a carbonate terrane, though some pebbles yield an Ordovician radiolarian assemblage.

Paleocurrent flow directions based on thousands of bottom features (flutes, prods, and grooves) indicate, in present-day coordinates, flow toward the east. Individual azimuth directions are throughout the two easterly quadrants, but 60% of these data indicate flow between 045 and 150°. This spread of data is consistent from outcrop to outcrop, indicating that there are no localized block rotations. Easterly flow has also been determined for the overlying Cretaceous units of the Kandik basin (Biederman Argillite and Kathul Graywacke).

The lack of an obvious source terrain and the anomalous cratonward paleoflow suggests these rocks are structurally allochthonous; the extent of allochthoneity, however, is not known.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.