Early Carboniferous Transgression on a Passive Continental Margin: Deposition of the Kekiktuk Conglomerate, Northeastern Brooks Range, Alaska

Abstract

The base of the Ellesmerian sequence in the northeastern Brooks Range is a widespread terrigenous clastic succession assigned to the Kekiktuk Conglomerate (late Tournaisian-Visean). The Kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin. Six depositional units have been recognized in the Kekiktuk Conglomerate. Unit A consists of clast-supported and rare matrix-supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone. It records deposition at the base of incised paleovalleys from high-sediment-concentration flows and debris flows. Units B, C, and D consist of multistory fining-upward cycles that record deposition within incised paleovalleys from low-sinuosity bed-load-dominated streams, low to moderate-sinuosity bed-load-dominated streams, and high-sinuosity mixed-load streams, respectively. Unit E consists of multistory fining-upward cycles and laminated sandstone with thin conglomerate stringers. It records deposition in the distal regions of incised paleovalleys in a variety of marginal-mari e settings, including fluvial channel, channel-mouth bar, beach ridge, and coastal swamp environments. Unit F consists of horizontally bedded sandstone, interbedded sandstone, and organic-rich mudstone. It records deposition over paleotopographic highs on the sub-Mississippian unconformity from sheetfloods and in associated coastal swamp and mud-flat settings. Relative sea level rise is thought to have initiated fluvial deposition in the northeastern Brooks Range by the latest Tournaisian-earliest Visean. Coal-bearing fluvial channel-fill and flood-basin successions and palynologic data suggest that this part of Arctic Alaska was situated in a humid climatic zone during the Early Carboniferous. The stratigraphic and sedimentologic organization of the Kekiktuk Conglomerate indicates that once base-level rise began to affect the northeastern Brooks Range, local and regional variations in paleotopography along the sub-Mississippian unconformity exerted primary control on accommodation and consequent distribution of Kekiktuk depositional environments. The stratigraphic position of the Kekiktuk Conglomerate above a regional angular unconformity nd below marginal-marine and shallow-marine shales, combined with its limited thickness, widespread distribution, and suite of paleoenvironments, suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin. Thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in Arctic Alaska and the resulting regional transgression. End_Page 679------------------------------