NORTH ALASKA UPPER CRETACEOUS TEPHRA: EURASIAN OR NORTH AMERICAN SOURCE CALDERAS?

Cretaceous tephra are important components of clastic sedimentary sequences in North Alaska, forming aggregate thicknesses of over 25 to 200 m in a variety of Formations (eg., Seabee, Schrader Bluff, Prince Creek, Hue Shale) from the Delong Mountains to the Arctic National Wildlife Refuge. These tephra deposits occur over a wide area (>100x500 km) in marine and non-marine sequences of diverse depositional environments. This talk summarizes their nature, distribution, stratigraphy, age, geochemistry, and petrology, and speculates on likely source calderas in the Circum-Arctic region.

Cretaceous tuffs and bentonites are largely composed of K-bentonite, an interlayered type of illite-smectite clay, illite, and kaolinite (all largely alteration products of volcanic glass), with trace to minor amounts of biotite, sanidine, plagioclase, quartz, Fe-Ti-oxides, zircon and apatite. These tephra represent distal pyroclastic fall deposits of dacite to rhyolite composition, Plinian-style caldera eruptions from Circum-Arctic volcanoes located 500->1000 km from the North Slope. Primary glass is abundant in the Maestrichtian group of tuffs at Ocean Point, where several 10-80 cm-thick, glass shard-rich tuffs overlie dinosaur bone-bearing non-marine mudstones and siltstones of the Prince Creek Formation. Biotite-rich bentonites over 1 m thick occur along the Colville River upstream of Umiat within shelf deposits of the Seabee Formation.

K-Ar, 40Ar/39Ar, and fission track ages on a variety of phases (biotite, sanidine, glass, apatite, zircon), as well as a variety of radiogenic isotope (Sr-Nd), major and trace element geochemistry, and X-ray diffraction data place firm constraints on the absolute age (66-95 Ma), compositional diversity, and source character of these tephra beds. Based on their composition, age, and proximity, the Cretaceous tephra deposits preserved in North Alaska sequences were most likely derived from caldera eruptions in NE Asia of the Okhotsk–Chukotka volcanogenic belt. Sources in East Central Alaska or the Canadian Arctic Islands are possible candidates, but less likely based on inferred paleo-wind direction and other data.