Sedimentation patterns and facies geometries on a temperate glacially-influenced continental shelf: the Yakataga Formation, Middleton Island, Alaska

Abstract The 5 km thick Yakataga Formation of the Gulf of Alaska provides the longest and most accessible record of late Cenozoic glaciation in the world. The formation consists of interbedded marine and glacimarine sediments of late Miocene to Holocene age and is well exposed on offshore islands and in the coastal mountain ranges as a result of the convergent margin setting of the Gulf. About 1.3 km of early Pleistocene Yakataga Formation sediments are exposed on extensive intertidal platforms around Middleton Island which is sited close to the modern continental shelf edge. Variably graded gravels and sands at the base of the Middleton Island succession were deposited by sediment gravity flows and represent the fill of a large submarine channel. The remainder of the succession is dominated by thick (up to 100 m) planar tabular units of fossiliferous diamict containing a series of striated boulder pavements and shell-rich beds (coquinas). Diamict facies formed predominantly by the settling out of fines from suspended sediment plumes together with ice-rafting of coarser debris; foraminiferal biofacies data indicate palaeobathymetries of between 20 and 250 m. Boulder pavements are lag surfaces which developed in relatively shallow water and were subsequently faceted and striated by a grounding ice sheet extending to the continental shelf edge; coquinas record episodes of current winnowing in shallow water (20–50 m) but also contain ice-rafted debris. Extensive mud units formed in relatively deep water during periods of reduced ice volume and are similar to ‘mud blankets’ accumulating on the modern shelf. The Yakataga Formation on Middleton Island contains a rich record of changing palaeobathymetry and ice volumes in the Gulf of Alaska.