Stratigraphic variation across a Middle Devonian to Mississippian rift-basin margin and implications for subsequent fold and thrust geometry, northeastern Brooks Range, Alaska

Abstract

A stratigraphic record from the eastern Brooks Range, Alaska, is interpreted to represent erosion and deposition of syn-rift and post-rift terrigenous clastic rocks across a Middle Devonian - Mississippian rift-basin margin. Middle Devonian - Mississippian terrigenous clastic rocks unconformably overlie complexly deformed Romanzof chert and constrain the age of latest mid-Paleozoic contractional deformation to pre-Middle Devonian time. The succession forms an abruptly southward-thickening basin-margin wedge characterized by abrupt facies changes, local evidence of active tectonism, multiple unconformities merging northward toward the basin margin, locally derived clastic deposits. The oldest deposits of this wedge are Middle - Upper(?) Devonian shallow-marine to alluvial-fan deposits (Ulungarat formation). Algal limestone with intercalated terrigenous clastic deposits and plant fossils (Mangaqtaaq formation) locally overlies the Ulungarat formation. The Ulungarat and Mangaqtaaq formations are interpreted to record syn-rift deposition. Coastal-plain to marine deposits of transgressive Kayak Shale overlie and intertongue with retrogradational Kekiktuk Conglomerate, recording coastal retreat and drowning of low-energy paleoshoreline. Deposits of the retrogradational Kekiktuk fluvial system thin and fine upward and to the north, reflecting depositional onlap of the basin-margin high. Kekiktuk Conglomerate and Kayak Shale are interpreted to overlie the post-rift unconformity and record the beginning of thermal subsidence. This stratigraphic succession provides a spatial and genetic link between structurally separated, stratigraphically distinct rocks of the Endicott Group. Thick, allochthonous rocks to the south record progradation and eventual retrogradation of a basin-filling wedge, whereas thin, autochthonous rocks to the north record transgressive overlap of the basin-margin sediment source area. The structural boundary between the north-central and northeastern Brooks Range coincides with the mid-Paleozoic rift-basin margin. North-vergent duplexes beneath the Kayak Shale consist of horses in the Middle Devonian-Mississippian clastic wedge to the south and thicker horses in pre-Middle Devonian rocks to the north. Above the Kayak Shale, north-vergent thrust-truncated folds are succeeded northward by detachment folds. These structural characteristics reflect the combined influence of abrupt lateral changes in stratigraphy across the rift-basin margin and stratigraphically controlled vertical variations in structural behavior.