Detailed Facies Analysis of the Lower Cretaceous Fortress Mountain Formation, Atigun Syncline, Northern Alaska

By

M.A. Wartes and A.R. Carroll (University of Wisconsin, Madison)

The Lower Cretaceous Fortress Mountain Formation marks the first coherent stratigraphic record of the early Colville foreland basin. We investigated one of the most prominent exposures of this interval at Atigun syncline, near Galbraith Lake. Based upon detailed facies analysis of more than 1200 meters of section, we interpret the Fortress Mountain Formation as a large fan-delta complex. The succession is dominated by repeated coarsening-upward packages, recording the progradation of delta-front environments. The base of a typical succession is characterized by low angle cross-bedded, fine-grained sandstone (including rare HCS), interbedded with occasional thin turbidites grading from very-fine grained sandstone to siltstone. Horizontal and vertical burrows are most common in this interval. This is followed by massive to thin-bedded pebbly sandstone and trough cross-stratified sandstone. Bivalve fossil lags are occasionally observed in association with the trough cross-stratified sandstone facies. Conglomerate units fine basinward and are locally shingled, dipping up to 11 degrees seaward. Conglomerate beds often exhibit nonerosive bases, normal to inverse grading, and poor sorting, consistent with amalgamated mass-flow deposition in a mouth bar setting. Individual conglomerate beds are often capped by mega-rippled surfaces that display a unidirectional asymmetry indicating transport to the south. We interpret this sedimentary structure as the record of fair-weather waves impinging on the shoreface. Well-sorted, open-framework conglomerate is less common, but does occur toward the top of some coarsening-upward packages. Based on the textural maturity and seaward dipping clast imbrication, we interpret this facies as foreshore deposits, modified by intense wave action.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.