NEW CONSTRAINTS ON THE TIMING, THICKNESS AND DISTRIBUTION OF PAST INCARNATIONS OF THE CORDILLERAN ICE SHEET FROM NORTHWESTERN BRITISH COLUMBIA, CANADA

The timing of global Quaternary glaciations is largely estimated from d¹⁸O records in marine sediment (Lisecki and Raymo, 2005). The oceans, however, provide no direct evidence for the areal distribution or thickness of continental ice sheets over time, and terrestrial records for waxing and waning of global ice are only locally abundant. Here we present nineteen new ⁴⁰Ar/³⁹Ar ages from basaltic glaciovolcanoes that help to delineate the behavior of the northern Cordilleran ice sheet (CIS) in western North America during the last 2.8 Ma. Based on this new data a minimum of 13 periods of inferred continental-scale ice cover occurred between 2.80 and 0.06 Ma, several of which are previously undocumented. Additional ⁴⁰Ar/³⁹Ar ages of subaerial basaltic eruptions distinguish four periods of ice-free conditions (4.1 Ma, 2.2 Ma, 1.6 Ma, 0.3 Ma) thereby documenting periodic ice sheet decay.

All of the deposits occur in the northern interior of British Columbia, which is not presently glacierized, but which is postulated to have been covered by incarnations of the CIS multiple times. The studied deposits form part of the Tuya-Kawdy volcanic field within the alkaline northern Cordilleran volcanic province (NCVP), which is related to transtension along the Pacific-North America plate boundary. The ages roughly cluster into four groups: Group 1 (2.8 Ma), Group 2 (2.2-1.6 Ma), Group 3 (1.1-0.5 Ma), and Group 4 (less than 0.24 Ma) and show some geographic control. The two groups with the oldest ages are furthest west. Groups 3 and 4 are further east and their locations are broadly interspersed with each other. Of the 19 newly dated glaciovolcanic deposits, 8 are flat-topped or complex tuyas, for which passage zones demarcate minimal ice thicknesses. The other 11 likely formed beneath ice that was above the elevations of present exposure. The deposits are distinguished from subaerial volcanic deposits by the presence of some or all of the following: pillow lava, complex cooling joints, and extensive palagonitization of poorly-bedded tephra.

We believe that, given the physiography surrounding most of the newly dated tuya deposits, these new ages represent periods of regional/global glaciations. If true, this conflicts with recent studies suggesting that global glaciations were not extensive until the latter half of the Pleistocene.