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.