MIDDLE DEVONIAN TO EARLY MISSISSIPPIAN PALEOGEOGRAPHY AND ITS INFLUENCE ON STRUCTURAL STYLE, EASTERN BROOKS RANGE

We interpret a stratigraphic record from the eastern Brooks Range, Alaska to represent erosion and deposition across a Middle Devonian to Mississippian rift-basin margin. Deposits of the rift-basin margin are juxtaposed across a south-dipping thrust fault that marks the structural boundary between the eastern and northeastern Brooks Range. In the footwall, deposition of thin, laterally discontinuous Lower Mississippian Kekiktuk Conglomerate was controlled by topography on an unconformity on lower Paleozoic rocks. In the hangingwall, an abruptly southward-thickening basin-margin wedge records complex interfingering of environments. The oldest deposits are Middle to Upper(?) Devonian shallow-marine to alluvial-fan deposits (Ulungarat formation). Lacustrine(?) algal limestone with intercalated terrigenous clastic deposits and plant fossils (Mangaqtaaq formation) locally overlies the Ulungarat formation. Braided fluvial deposits of the Lower Mississippian Kekiktuk Conglomerate unconformably overlie both the lacustrine(?) deposits and the Devonian clastic succession. Three tongues of Kekiktuk Conglomerate progressively backstep northward toward the basin margin and are overlain by marine Kayak Shale. To the south, argillaceous limestone near the base of the Kayak is probably coeval with the middle Kekiktuk tongue to the north.

All contractional structures within the rift-margin succession are Cretaceous(?) to Tertiary (late Brookian). Shortening beneath the Kayak Shale is accommodated in north-vergent duplexes. To the south, imbricate thrust faults internally deformed the basin-margin sedimentary wedge. To the north, thicker horses in pre-Middle Devonian polydeformed rocks define major east- trending, doubly plunging anticlinoria. These different structural characteristics from south to north reflect the influence of abrupt lateral changes in mechanical stratigraphy across the rift-basin margin: The thick, mechanically layered clastic wedge to the south deformed internally, while the thin clastic veneer to the north deformed along with underlying basement.