PROVENANCE ANALYSIS OF THE LOWER CRETACEOUS COLVILLE FORELAND BASIN; NEW CONSTRAINTS ON THE UNROOFING HISTORY OF THE BROOKS RANGE, NORTHERN ALASKA

The Fortress Mountain Formation forms a >3000 m clastic wedge along the southern margin of the Colville foreland basin, that has been interpreted to record either the latest contraction of the Brooks Range, or early post-contractional fill of a residual foredeep. Conglomerate clast-counts (~1700 clasts) at Fortress and Castle Mountains document a distinctive erosional history related to unroofing of specific structural levels within the orogen. The lowest conglomerate is dominated by chert and siliceous mudstone clasts but also contains up to 31% mafic to intermediate volcanic clasts, interpreted to derive from the structurally high Copter Peak and Misheguk Mountain allochthons. Chert clasts increase to a maximum of ~90% at the top of the Fortress Mountain and at the base of stratigraphically higher Castle Mountain. Locally enhanced concentrations of distinctive tasmanite clasts indicate erosion of the Ipnavik River allochthon, and the Picnic Creek allochton may also have been eroded at this time. Chert clasts at Castle Mountain then gradually decrease in abundance upward as contributions from lower structural elements (i.e. Endicott Mountains allochthon) increase. Limestone clasts (up to 16%) that contain crinoids and rugose corals mark unroofing of the Lisburne Group. Quartzite clasts, interpreted to represent the underlying Kanayut Conglomerate and associated clastic units, appear next and rapidly replace the limestone. The top of the succession is characterized by an increasing abundance of granitic clasts, (up to19%). Their most likely sources are Devonian plutonic complexes in the core of the Brooks Range, suggesting a large component of vertical uplift. This uplift may be related to large scale coeval(?) extension documented in the hinterland, or to isostatic adjustment of the orogen following detachment of a dense subducting slab.