Regional- and outcrop-scale distribution of coarse-grained fluvio-deltaic facies of the Upper Cretaceous Tuluvak Formation, Colville Foreland Basin, northern Alaska

The Upper Cretaceous Tuluvak Formation was deposited in settings ranging from bedload-dominated rivers to deltas and storm-dominated shorefaces. Rivers supplied coarse sediment from the emerging Brooks Range to coastal environments along the margin of the subsiding Colville foreland basin where the formation was deposited in both sandy and gravelly deltas. Within the outcrop belt to the south, exposures consist of relatively thin, laterally extensive, but discontinuous conglomerate. Well data to the north indicate that these coarse-grained intervals form a minor component of the Tuluvak Formation overall.

Outcrop exposures of fluvial deposits consist of poorly sorted, multilateral, channel-fill conglomerate as well as thin- to medium-bedded, tabular sandstone. Discrete channel-fill bodies are bound by erosional surfaces, reflecting deposition in broad, shallow fluvial channels and braidplains. Floodplain deposits are rarely exposed in outcrops, but consist of mudstone, coal, and thin overbank sandstone beds.

These coarse-grained, fluvial systems flowed north and northeastward supplying sediment to coarse-grained deltas and adjacent sandstone-dominated shorefaces. Significantly, an exceptionally well-preserved braidplain Gilbert-type delta provides insight into the architecture and facies of a feature that may have been common during deposition. Distributary channel deposits interfinger with wave-influenced conglomerates and bioturbated sandstones along depositional strike as wave reworking of sediment increased away from the fluvial sediment sources. Steeply dipping (20–23°) delta-front foresets are ~5 m high, and consist of decimeter-thick, pebble conglomerates deposited by mass-flow processes. Delta-front sediment gravity flows contain rare pebbles and are dominantly bioturbated, tabular, fine- to medium-grained sandstone.

The Tuluvak Formation within the study area reaches a maximum thickness of 365 m and contains two conglomeratic intervals, the lower of which is equivalent to the conglomerate beds exposed in the outcrop belt to the south. These intervals reflect progradation of high-energy alluvial environments potentially associated with phases of exhumation in the Brooks Range.