Glass fission-track ages, composition, and origin of tephra beds in the Upper Cretaceous Kanguk Formation, Banks Island, Arctic Canada

Abstract

More than 50 conspicuous tephra beds occur in the Kanguk Formation on the southwestern coast of Banks Island. Their glass shards are remarkably well preserved and permit comprehensive characterization, offering the potential for reliable, precise correlation of Upper Cretaceous sedimentary rocks across the three major depocentres of the Arctic Archipelago and adjacent northern continental margin: Sverdrup, Banks, and the Beaufort–Mackenzie basins. Twenty-one tephra beds were analyzed; all have a high-K, peraluminous, rhyolitic composition, with quartz, plagioclase, ilmenite, biotite, and zircon as the dominant minerals. Trace-element concentrations, especially low Nb and Ta, show that the parental magmas formed in a continental-margin subduction environment. Glass fission-track ages range from 100 Ma to younger than 60 Ma, and indicate a very low sedimentation rate giving a very condensed sedimentary sequence on southwestern Banks Island — a sequence that may well contain the K–Pg transition. Source calderas are unknown but most likely are situated in east-central Alaska and the central and northern Kuskokwim volcanic belt, some 1000 to 1500 km distant from southwestern Banks Island. It is also possible that some of the very thin tephra beds come from the Okhotsk–Chukotka volcanogenic belt in northeastern Russia.