NEW DETAILED (1:63,360-SCALE) MAPPING OF THE BROOKS RANGE NORTHERN FOOTHILLS, SIKSIKPUK RIVER AREA, CENTRAL BROOKS RANGE, ALASKA

New geologic mapping in the Chandler Lake Quadrangle foothills of the Brooks Range fold and thrust belt provides surface data relevant to source, reservoir, and trap characterization of the North Slope. Mapping was augmented by a structural transect and a detailed gravity survey.

The area consists of allochthons to the south and syn- to post-tectonic deposits to the north. Endicott Mountains allochthon (EMA) rocks include Jurassic-Triassic Otuk Formation, the Permian Siksikpuk Formation, and platform carbonates of the Pennsylvanian-Mississippian Lisburne Group. Structurally higher thrust blocks of EMA are exposed to the west, allowing observation of significant Lisburne facies changes.

The structurally overlying Ipnavik River allochthon (IRA) consists of Lower Cretaceous turbidites of the Okpikruak Formation, often containing small mafic bodies and isolated blocks of Jurassic to Pennsylvanian Imnaitchiak chert. An unambiguous sedimentary or structural origin for these disrupted zones is unclear. Olistostromal deposits may be associated with early Okpikruak deposition and their map distribution is likely controlled by both paleotopography and multiple deformational episodes. The Hauterivian-Barremian to Albian Brookian sequence filling the basin to the north contains siliciclastic rocks shed northward during Brooks Range uplift. Coarse proximal deposits of the Fortress Mountain Formation near the range front underlie progradational basin-fill deposits of the marine Torok Formation and shallow marine to fluvial Nanushuk Formation. The Torok Formation displays a zone of south-vergent folds and thrust faults that probably represent a back-thrust. The Torok Formation contains a thick oil-stained sandstone interval. The Desolation Creek fault places allochthonous rocks of the IRA over Torok.