Mixed local and ultra-distal volcanic ash deposition within the Upper Cretaceous Kanguk Formation, Sverdrup Basin, Canadian Arctic Islands

摘要

The Upper Cretaceous Kanguk Formation of the Sverdrup Basin, Canadian Arctic Islands, contains numerous diagenetically altered volcanic ash layers (bentonites). Eleven bentonites were sampled from an outcrop section on Ellesmere Island for U-Pb zircon secondary ion mass spectrometry dating and whole-rock geochemical analysis. Two distinct types of bentonite are identified from the geochemical data. Relatively thick (0.1 to 5 m) peralkaline rhyolitic to trachytic bentonites erupted in an intraplate tectonic setting. These occur throughout the upper Turonian to lower Campanian (c. 92-83 Ma) outcrop section and are likely associated with the alkaline phase of the High Arctic Large Igneous Province. Two thinner (<5 cm) subalkaline dacitic to rhyolitic bentonites of late Turonian to early Coniacian age (c. 90-88 Ma) are also identified. The geochemistry of these bentonites is consistent with derivation from volcanoes within an active continental margin tectonic setting. The lack of nearby potential sources of subalkaline magmatism, together with the thinner bed thickness of the subalkaline bentonites and the small size of zircon phenocrysts therein (typically 50-80 mu m in length) are consistent with a more distal source area. The zircon U-Pb age and whole-rock geochemistry of these two subalkaline bentonites correlate with an interval of intense volcanism in the Okhotsk-Chukotka Volcanic Belt, Russia. It is proposed that during late Turonian to early Coniacian times intense volcanism within the Okhotsk-Chukotka Volcanic Belt resulted in widespread volcanic ash dispersal across Arctic Alaska and Canada, reaching as far east as the Sverdrup Basin, more than 3000 km away.