Transgressive sedimentation in rift-flank region: Deposition of the Endicott Group (early Carboniferous), northeastern Brooks Range, Alaska

Abstract

In the range-front region of the northeastern Brooks Range, the Endicott Group overlies a regional angular unconformity (sub-Mississippian unconformity) and consists of a relatively well-exposed transgressive fluvial succession at its base (Kekiktuk Conglomerate) and a gradational terrigenous clastic-to-carbonate transition at its top (Kayak Shale). Thirty-one stratigraphic sections were measured to characterize the sedimentology, paleogeography, and tectonic setting of the Endicott Group. Just prior to latest Tournaisian-earliest Visean time, the range-front region was characterized by fluvial incision. Fluvial incision throughout the region was followed in latest Tournaisian-earliest Visean time by fluvial deposition in incised paleovalleys, which were gradually filled and superseded by marginal- and shallow-marine environments (Kayak Shale) as transgression progressed. The limited thickness, widespread distribution, and organization of the Kekiktuk Conglomerate, combined with its stratigraphic position above an angular unconformity and below marginal- and shallow-marine shales, suggests deposition in an upland, rift-flank region landward of the tectonic hinge zone on a passive continental margin. Widespread but volumetrically minor coal in the Kekiktuk Conglomerate combined with plant spores and abundant plant fragments in the Kayak Shale suggest that the coastal zone and some valley bottoms were heavily vegetated and that the range-front region was in a humid climatic zone during latest Tournaisian-Visean time. Large volumes of terrestrial organic material were subsequently transported into shallow-marine environments and resulted in a widespread oxygen-depleted bottom-water layer. Regional stratigraphic studies indicate that widespread carbonate sedimentation (Lisburne Group) began south and southwest of the range-front region in late Tournaisian time, which suggests conditions of restricted circulation probably existed in marine environments toward the north, in the range-front region, and promoted oxygen-deficient conditions. The upper Kayak Shale records a gradational transition from terrigenous clastic-dominated environments below to carbonate-dominated environments above (Lisburne Group). Superimposed on this transition are small-scale terrigenous clastic-to-carbonate transitions recorded in meter-scale parasequences and acyclic successions. The stratigraphic and geographic distribution of parasequences and acyclic successions reflects their paleogeographic position with respect to the strand line and terrigenous clastic sources. The humid climate, low paleolatitude, and tectonic setting (upland rift-flank region) were fist-order controls on the terrigenous clastic-to-carbonate transition recorded in the upper Kayak Shale.