Permian and Triassic Sedimentation in the Northeastern Brooks Range, Alaska: Deposition of the Sadlerochit Group (1)

Abstract

Permian and Triassic rocks of the Sadlerochit Group crop out extensively in the northeastern Brooks Range adjacent to the coastal plain of the Arctic National Wildlife Refuge. Terrigenous clastic rocks of the Sadlerochit Group are separated from underlying carbonates of the Lisburne Group by a regional unconformity marking the Pennsylvanian-Permian systemic boundary. Lithologic and diagenetic patterns of rocks above and below the sub-Sadlerochit unconformity vary systematically with substantial paleotopography along the erosional surface. Within the Sadlerochit Group, nonmarine conglomerate of the basal Echooka Formation fills erosional channels and valleys and grades upsection into a retrogradational succession of tempestites deposited during episodic storm-surge in shal ow-marine environments. The overlying Ivishak Formation records an abrupt reversal of environmental migration pattern and the beginning of southerly progradation of a broad suite of deltaic environments. Depositional units in the Ivishak are strongly progradational and cyclic at a variety of scales, reflecting alternating episodes of progradation and transgression inherent in delta construction. The basal Kavik Member of the Ivishak Formation is an upward-coarsening depositional assemblage recording initial migration of prodelta environments and evolution of an extensive subaqueous delta platform. The overlying Ledge Sandstone Member is organized into three depositional assemblages recording the evolution of delta-fringe, distributary-channel, and crevasse-splay environments of the lower delta plain. The Fire Creek Siltstone Member is an aggradational and transgressive assemblage deposited in shallow-marine environments that reworked sediment originally deposited on the lower delta plain. The depositional assemblage records the final large-scale subsidence and drowning of the Ivishak delta platform. The most favorable reservoir strata are developed immediately above and below the sub-Sadlerochit unconformity and in distributary-mouth bar and distributary-channel deposits of the Ledge Sandstone Member.