THE BROOKS RANGE FORELAND BASIN—WHERE IS THE PROVENANCE FOR BROOKIAN SEDIMENTATION?

The main phase of the thin-skinned Brooks Range foreland fold-thrust-belt emplaced allochthons of Devonian to Jurassic distal passive-margin deposits and mafic rocks onto autochthonous strata of the North Slope during north-vergent arc-continent collision in the latest Jurassic and Neocomian, culminating in filling of the western Colville foreland basin in the mid-Cretaceous. In order to characterize the foreland basin, ~1200 detrital zircon grains from 13 samples from the western part of the foreland basin were dated using LA-ICPMS and SHRIMP techniques. Eleven samples are from lithic sandstones shed from the orogenic belt (Brookian sequence) and two are from quartz-rich sandstones derived from the authochthon (Beaufortian sequence). Seven samples are from Brookian sandstones collected at various ages and structural levels in the thrust belt. Neocomian samples consist of predominately Phanerozoic zircons dominated by peaks at 250-350 Ma and/or 140-150 Ma. Two samples previously reported as latest Jurassic (the oldest foreland basin deposits in the Brooks Range) were found to consist almost entirely of 140-150 and 110-100 Ma zircons that demonstrate a Late Cretaceous or younger depositional age. Three mid-Cretaceous Colville basin samples also yield ages mainly at 250-350 Ma, but only one contains significant numbers of 140-150 Ma zircons. A quartz-rich Hauterivian Beaufortian sandstone from the Tunalik well in contrast, contains largely Precambrian zircons with multiple peaks between 1.0 and 1.8 Ga and 2.2 to 2.8 Ga. This sample represents externally derived detritus shed from the autochthon during thrusting. Two samples from Tingmerkpuk Mountain at the Brooks Range mountain front record the transition from autochthon-derived sandstone dominated by Precambrian zircons to Brookian sandstones dominated by 250-350 Ma zircons between the Valanginian and Barremian. The samples are interpreted in the context of a foreland basin system with sources in the autochthon and in the orogenic belt. The succession at Tingmerkpuk Mountain may represent forebulge deposition or, more likely, blind thrusting in advance of the emergent thrust front. However, the source rocks for the 250-350 and 140-150 Ma zircons in Brookian sandstones are not known in the Brooks Range orogen and probably lie in Chukotka and interior Alaska.