Sedimentology and structural geology of the Endicott Mountains allochthon, central Brooks Range, Alaska
Handschy, James William
Oldow, John S.
Doctor of Philosophy
The Endicott Mountains allochthon is an east-west striking stack of north-northwest vergent thrust sheets which were emplaced during late Mesozoic and Cenozoic (Brookian) orogenesis. Thrust sheets in the allochthon are composed of clastic and carbonate rocks which track the progressive evolution of a Late Devonian and Early Carboniferous continental margin. Sedimentary facies in lower Upper Devonian rocks of the Beaucoup Formation delimit a volcanically active depositional basin. Volcaniclastic sediments within the Beaucoup were apparently derived from the south; whereas siliciclastic sediments were derived from the north. By the late Late Devonian, the Beaucoup depositional basin had developed into a south-facing continental margin. Southwestward progradation of the Kanayut-Hunt Fork delta system deposited thick conglomerates, sandstones, and shales on the margin and created a lithofacies pattern in which the Kanayut Conglomerate is thicker in the north and the Hunt Fork Shale is thicker in the south. Transgression of the Lower Mississippian Kayak Shale over the Kanayut Conglomerate occurred as sea level rose during the Early Mississippian. Subsequent transgressive-regressive cycles in carbonates of the Lisburne Group indicate that the margin had evolved into a stable passive margin by the middle Mississippian. The style of Brookian structures in the Endicott Mountains allochthon changes from imbricate thrust sheets and large single-phase folds in the north to a thick, variably strained thrust nappe in the south. Strain variation in the southern nappe is evidenced by a progressive change from single-phase folds at the top of the nappe to polyphase folds at the bottom. First phase fold axes change from strike-parallel at the top of the nappe to dip-parallel at the bottom, and the angle between first phase axial planes and the basal thrust decreases with depth. The change from thrust imbrication in the north to heterogeneous intranappe strain in the south apparently was controlled by the distribution of sedimentary facies and the extent of tectonic burial. The greater thickness of Kanayut Conglomerate and lack of a superjacent thrust sheet favored thrust imbrication in the north; whereas the greater proportion of shale and tectonic burial beneath the Skajit allochthon favored heterogeneous intranappe deformation in the south. Changes in fold orientation, the number of superposed fold phases, and measured strain in the southern nappe indicate that deformation was facilitated by a combination of layer parallel shortening and simple shear in a collapsing shear zone.