The Tertiary Northeastern Brooks Range Fold-Thrust Belt: An Onshore-Offshore Synthesis

By

T.E. Moore, C.J. Potter, A. Grantz (U.S. Geological Survey), and P.B. O’Sullivan (Syracuse University)

The eastern coastal plain of ANWR is part of an extensive north-vergent fold-thrust belt that extends from the northeastern Brooks Range across the Arctic margin into the Canada basin. An integrated view of the fold-thrust belt is depicted in a balanced cross section constructed from field and map data and onshore and offshore seismic data. Beneath the present Beaufort Sea coast, the basal detachment lies above the Lower Cretaceous (breakup) unconformity and descends northward with stratigraphy from a depth of about 7 km onshore to about 10 km under the modern continental rise. Large-scale structures offshore are interpreted as faulted detachment folds cored by syn- and post-rift strata, including the pebble shale. Near the coast, structures are thin-skinned and comprise a frontal triangle zone composed of a duplex of Cretaceous and Paleocene units beneath a roof thrust at the base of Eocene strata. Southward (rearward) from the coastal area, the basal detachment drops stepwise to a depth of >15 km in pre-Mississippian basement under the Brooks Range. Large subsurface antiforms under the coastal plain (Niguanak, Aurora) are modeled as fault-bend folds of basement as are major antiforms exposed in the Brooks Range. Fission-track data and stratigraphic relations indicate that folding and thrusting in the southern part of the profile began in the latest Cretaceous and propagated northward to the frontal triangle zone in the Oligocene. Thrusting in basement antiforms under the coastal plain fed detachment folds offshore in the Miocene to Holocene. About 72 km (43%) total shortening is required to restore pre-Mississippian basement.

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.