RECONSTRUCTING EOCENE TOPOGRAPHY AND VOLCANISM IN THE NORTH AMERICAN CORDILLERAN HINTERLAND, SOUTHERN BRITISH COLUMBIA

In southern British Columbia, Eocene basins contain fluvial and lacustrine deposits with interbedded volcanic tuffs and ignimbrites. The basins provide a potential record of topographic evolution and volcanism. Previous Eocene paleoelevation estimates in the region, based on paleofloral and δ¹⁸O data, range from 1.6-4.2 km. Our current research aims to quantify paleotopography; constrain the timing of the onset of extension; and provide evidence for the geodynamic mechanisms driving basin formation in southern British Columbia.

Deposition of regional basin sediments occurred in nonmarine environments and includes pebble to boulder conglomerate, sandstone, and finely laminated organic-rich claystone to siltstone. Hydrogen stable isotope analyses of meteoric waters preserved in volcanic glass from interbedded tuffs provide insights into the evolution of paleotopography and climate. Here we present preliminary results of volcanic glass stable isotope analyses and stratigraphy from McAbee, BC and Princeton, BC.

McAbee sedimentary units include organic-rich, finely laminated claystones to sandstones, with interbedded volcanic tuffs representing episodic distal volcanic ash deposition in a fluvial-lacustrine setting. Clast-supported pebble-cobble volcaniclastic conglomerates represent proximal volcanic deposition in a fluvial setting. Stable isotope analyses of the interbedded volcanic tuffs show a decrease in δD_glass values with movement up section from -109‰ to -115‰ (± 3.5‰ for all but one sample). This may be a response to decreased evaporation, increased inflow, or a response to tectonics. An ignimbrite sample from Princeton showed a δD_glass value of -134‰ (± 2‰). This sample is recording deposition of an ignimbrite in a fluvial-floodplain setting and likely represents the composition of Eocene precipitation. Further analyses of Eocene sedimentary and volcanic units, including detailed stratigraphy, glass TREE analyses, volcanic glass paleoaltimetry, and ⁴⁰Ar/³⁹Ar radiometric dating, across the region are needed to correlate strata between basins, reconstruct the paleotopography, and constrain the timing and magnitude of volcanism. These analyses will test proposed regional tectonic, geodynamic, and climatic models during the Eocene.