OCEANOGRAPHIC SIGNIFICANCE OF AN EXTREME COLD-WATER CARBONATE ENVIRONMENT: GLACIOMARINE SEDIMENTS OF THE PLEISTOCENE YAKATAGA FORMATION, MIDDLETON ISLAND, ALASKA

Uplifted during the 1964 Alaskan earthquake, extensive intertidal flats of Middleton Island expose 1.3km of late Cenozoic (Early Pleistocene) Yakataga Formation glaciomarine sediments. These exposures provide a unique window into outer shelf and upper slope strata that are otherwise buried within the SE Alaska continental shelf prism. The rocks consist of five principal facies in descending order of thickness; (1) extensive pebbly mudstone diamictite containing sparse marine fossils, (2) proglacial submarine channel conglomerates, (3) burrowed mudstones with discrete dropstone layers, (4) boulder pavements whose upper surfaces are truncated, faceted, and striated by ice, (5) carbonates rich in mollusks and brachiopods. The carbonates are dm-scale, typically channelized conglomeratic beds interpreted as resedimented storm deposits on the paleoshelf edge and upper slope. Biogenic components are a mixture of parautothonous large pectenids or smaller brachiopods with locally important serpulid worm tubes and robust gastropods augmented by allochthonous sand-size bryozoan and echinoderm fragments. Ice rafted debris is present throughout these seafloor-cemented cold-water carbonates which are thought to have formed during periods of lowered sea level that allowed coastal ice margins to advance near to the shelf edge. These relatively shallow water environments (~ 50 m) are associated with biogenic productivity related to ice-front upwelling. As such these sub-arctic mixed siliciclastic-carbonate sediments are an extreme end-member of the Phanerozoic global carbonate depositional realm.