Geology

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The Okhotsk-Chukotka Volcanogenic Belt (OCVB)

The Okhotsk–Chukotka Volcanogenic Belt (OCVB) is approximately 3250 km long (from the mouth of the Uda River in Khabarovsk krai to the Chaplino settlement in the eastern Chukchi Peninsula) and 100–350 km wide. Belyi (1977, 1994) estimated the total volume of erupted material in the OCVB as more than 1 × 106 km3 but this is a minimum because it ignores eroded and removed phreatic materials and the original volume could be twice as large based on data relating to modern volcanism in Kamchatka (e.g., Frolova et al., 1985). Within the OCVB is the world’s largest andesite association, up to 370000 km3 in volume (Belyi, 1994).

		Latest Albian – Late Cretaceous paleobotanical-paleogeographical subregions of the North Pacific Region (A); modern outline of North-Eastern Asia is shown for the Coniacian (after Smith et al., 1981): 1 – the Verkhoyansk–Chukotka Subregion, 2 – the Okhotsk–Chukotka Subregion, 3 – the Anadyr–Koryak Subregion (modified from: Herman, 2013); and position of the Okhotsk-Chukotka volcanogenic belt (B) (Belyi, 1977); modified from Shczepetov and Herman (2013).

The huge volume of extrusive rocks and the catastrophic character of volcanism in some OCVB calderas and supervolcanoes must have had significant impacts on the Northern Pacific Late Cretaceous climate and biosphere. Volcanic ashes are a significant component of Late Cretaceous sediments in northern Alaska and were likely derived from the OCVB (Kelley et al., 1999). This would suggest large quantities of ash and aerosols would have been injected into the polar stratosphere with inevitable consequences for changes in atmospheric chemistry and solar radiation balance. The extent of environmental impact depends not just on the volume of erupted material but when and over what time period the eruptions occurred.

The determination of the age of the OCVB has long been a topic of debate. Early age constraints were based on phytostratigraphy or using archived K–Ar and Rb–Sr isotopic ages for bulk rocks (Belyi, 1977, 1994; Lebedev, 1987; Filatova, 1988; Kotlyar et al., 2001; Kotlyar and Rusakova, 2004) leading to an age span for OCVB activity of Neocomian–Paleogene (Ustiev, 1959; Umitbaev, 1986), Albian–Cenomanian (Belyi, 1977; Kotlyar and Rusakova, 2004; Zhulanova et al., 2007), Albian–Paleogene (Filatova, 1988), and middle Albian–Santonian (Decision of..., 2009; Belyi, 2008). The absence of a consensus was related to the insufficient resolution of the geochronological methods used. Here the first 40Ar/39Ar results of Kelley et al. (1999) led to a reappraisal of the age of key floras such as the Chauna Flora, a revision of the phytostratigraphy, and stimulated further isotopic investigation.

In their major summary of OCVB geochemistry and geochronology Akinin and Miller (2011) conclude that in general OCVB magmatism occurred from the middle Albian to the early Campanian (106–77 Ma) in a discontinuous and laterally asynchronous fashion. There were several peaks of volcanism with modes at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian–Santonian peaks correspond to the most extensive stages of the middle and late cycles of felsic volcanism. Activity seems to have decreased towards a hiatus at the end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78 Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension in zones normal to the slip direction. Catastrophic eruptions occurred within narrow (<2 Myr) intervals based on new reliable age estimates for some individual large calderas. The accumulation rate of volcanic materials in such structures was up to 0.15– 0.36 km3/yr and even higher.

Some bentonites on the North Slope of Alaska have slightly younger dates than the youngest age (76 Ma) of the OCVB terminal plateau basalts suggesting ongoing explosive volcanism not directly associated with the OCVB. Of particular note are those of the Prince Creek Formation dated at 71-68 Ma (Conrad et al., 1990).