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ABSTRACT: Mississippian Clastic-to-Carbonate Transition in the Northeastern Brooks Range, Alaska: Depositional Cycles of the Endicott and Lisburne Groups

D. L. Lepain, R. K. Crowden, K. F. Watts

The Ellesmerian sequence in northeastern Alaska consists of a thick succession of Mississippian to Lower Cretaceous platform carbonate and terrigenous clastic rocks. At the base of the Ellesmerian sequence, clastic rocks of the Endicott Group are the lower part of a major transgressive sequence that passes gradationally upward into carbonates of the Lisburne Group. In the Endicott Group, the basal Kekiktuk Conglomerate was deposited in fluvial and marginal marine environments. A broad suite of tidally influenced, shallow-marine environments are recorded in the overlying Kayak Shale. The transition into carbonate platform rocks of the Lisburne Group is recorded in a series of depositional cycles developed within the upper half of the Kayak Shale. In the lower beds of the t ansition, the depositional cycles are multiple upward-thickening and upward-coarsening successions composed of (1) organic-rich siltstone containing flaser-bedded and lenticular-bedded, fine-grained sandstone, (2) fine-grained, ripple-laminated quartzarenite, and (3) an intensely bioturbated horizon of medium- to coarse-grained quartzarenite that contains scattered brachiopods, bryozoa, and crinoids. Each cycle is terminated by a sharp transgressive surface that consists of a thin shale drape. Near the top of the Kayak Shale, the coarse-grained horizons become increasingly replaced by wackestone, grainstone, and coralline boundstone. Despite the lithologic change, the vertical upward-thickening and upward-coarsening cycles continue in the basal limestone of the Lisburne Group. Repeated u ward-shallowing episodes, followed by coastal onlap, are likely mechanisms for this cyclicity and suggests a genetic relation between both the clastic and carbonate depositional cycles.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990