Carbonates within a Pleistocene glaciomarine succession, Yakataga Formation, Middleton Island, Alaska

Abstract

Uplifted during the 1964 Alaskan earthquake, extensive intertidal flats around Middleton Island expose 1300 m of late Cenozoic (Early Pleistocene) Yakataga Formation glaciomarine sediments. These outcrops provide a unique window into outer shelf and upper slope strata that are otherwise buried within the southeast Alaska continental shelf prism. The rocks consist of five principal facies in descending order of thickness: (i) extensive pebbly mudstone diamictite containing sparse marine fossils; (ii) proglacial submarine channel conglomerates; (iii) burrowed mudstones with discrete dropstone layers; (iv) boulder pavements whose upper surfaces are truncated, faceted and striated by ice; and (v) carbonates rich in molluscs, bryozoans and brachiopods. The carbonates are decimetre scale in thickness, typically channellized conglomeratic event beds interpreted as resedimented deposits on the palaeoshelf edge and upper slope. Biogenic components originated in a moderately shallow (ca 80 m), relatively sediment-free, mesotrophic, sub-photic setting. These components are a mixture of parautochthonous large pectenids or smaller brachiopods with locally important serpulid worm tubes and robust gastropods augmented by sand-size bryozoan and echinoderm fragments. Ice-rafted debris is present throughout these cold-water carbonates that are thought to have formed during glacial periods of lowered sea-level that allowed coastal ice margins to advance near to the shelf edge. Such carbonates were then stranded during subsequent sea-level rise. Productivity was enabled by attenuation of terrigenous mud deposition during these cold periods via reduced sedimentation together with active wave and tidal-current winnowing near the ice front. Redeposition was the result of intense storms and possibly tsunamis. These sub-arctic mixed siliciclasticcarbonate sediments are an end-member of the Phanerozoic global carbonate depositional realm whose skeletal attributes first appeared during late Palaeozoic southern hemisphere deglaciation.

The trace fossil Diopatrichnus santamariensis nov. isp. – a shell armored tube from Pliocene sediments of Santa Maria Island, Azores (NE Atlantic Ocean)

“…Thalassinoides and Asterosoma (Savrda et al, 2005), as well as from fjord-head deposits (Buatois and Mángano, 2011) and in Pleistocene glaciomarine diamictites of the Yakataga Formation in the Gulf of Alaska, together with Teichichnus, Rhizocorallium, and Zoophycos (James et al, 2009). This ichnogenus was also reported from the Miocene (Badenian) of western Ukraine Diopatraand considered as an (Wysocka et al, 2012), and from middle Pleistocene lower shoreface sediments belonging to the Kongochi Fm.…”