Regional structural framework and petroleum assessment of the Brooks Range foothills and southern coastal plain, National Petroleum Reserve, Alaska

New interpretations of the frontal part of the Brooks Range orogen beneath the foothills and coastal plain in the National Petroleum Reserve-Alaska (NPRA) are based on reprocessed regional seismic reflection data, recent geologic field observations, and new apatite fission-track analyses. Three long north-south transects illustrate the configuration of thrust faulting above a basal detachment that, within the southern part of NPRA, steps up from the Triassic Shublik Formation, to the Jurassic Kingak Shale, and finally into Cretaceous Torok mudstones. This thrust system represents the youngest recognized pulse of major shortening, about 60 Ma.

The transects, along with other seismic-reflection examples, illustrate four play concepts being used in the deformed area for the 2002 U.S. Geological Survey oil and gas assessment of the National Petroleum Reserve-Alaska (NPRA). The Brookian topset structural play includes broad west-northwest-trending anticlines in the Cretaceous Nanushuk Group, developed above structurally thickened Torok mudstones in the incipiently-deformed, most northerly part of the thrust system. The Torok structural play includes prominent anticlines affecting deep-basin sandstones, many of which are detached from folds exposed at the surface. The Ellesmerian structural play includes closures developed in the clastic part of the Ellesmerian sequence, mainly above a detachment in the Shublik Formation. The thrust belt play includes antiformal stacks of allochthonous Endicott Group clastic rocks and Lisburne Group carbonates; these stacks were assembled at about 120 Ma, and were transported to their present positions in the foothills at about 60 Ma.