A Model for Striated Boulder Pavement Formation on Glaciated, Shallow-Marine Shelves: An Example from the Yakataga Formation, Alaska

Abstract

ABSTRACT Laterally extensive, striated boulder pavements crop out within glaciomarine diamicts of the early Pleistocene Yakataga Formation on Middleton Island, Alaska. Boulder pavements consist of planes of one clast thickness, with faceted upper surfaces showing consistently oriented striation directions, cropping out over several kilometers along strike. Clasts do not have the preferred long-axis alignment nor the characteristic bullet shape of boulders transported at a glacier base and deposited by lodgement processes. In most cases, pavements identify the planar-bounding surfaces of thick (up to 100 m), tabular units of fossiliferous, muddy diamict deposited by the settling of fine-grained sands and muds from suspension, together with ice-rafted debris, in water depths between 20 and 250 m. Diamict facies both underlying and overlying the pavements contain abundant unabraded macro-and microfossils and show no evidence of deformation by subglacial shear processes. Pavements appear to have originated as boulder lag surfaces produced by wave and tidal-current winnowing of diamict on relatively shallow bank areas during episodes of relative sea-level lowering. Lags were subsequently overridden and abraded by a grounding ice shelf. The glacially abraded pavements are thought to record the repeated expansion of a partially floating ice shelf to the edge of the Gulf of Alaska continental shelf during the early Pleistocene. Recognition of boulder pavements formed in this manner may help establish the former presence of ice shelves whose depositional record remains enigmatic.