DETRITAL ZIRCON PROVENANCE OF EOCENE STRATA IN THE CORDILLERA OF SOUTHERN BRITISH COLUMBIA AND NORTHERN WASHINGTON: PRELIMINARY RESULTS

Eocene-age sedimentary strata in southern British Columbia and northern Washington record the topographic and tectonic conditions within the interior of the southern Canadian Cordillera during initial stages of crustal extension. Sediments were deposited in a variety of nonmarine environments and units in the region include clast- and matrix-supported cobble and pebble conglomerates, sandstones, coal, and finely laminated siltstones and claystones. The overall depositional settings are interpreted as fluvial, lacustrine, and palustrine environments. Here, we present preliminary uranium-lead detrital zircon data from Eocene strata exposed in southern British Columbia and northern Washington state. Detrital zircons from sandstones in Princeton, British Columbia have ages ranging from early Cenozoic to Paleozoic, with the greatest population of grains having Middle Jurassic ages (~170 Ma). We tentatively interpret the Jurassic zircons to be derived from surrounding plutons in southern British Columbia. Detrital zircons from age-equivalent strata exposed near Republic, Washington have ages ranging from early Cenozoic to Proterozoic. The largest age population of detrital zircons from the Republic region are Paleoproterozoic in age (~1800 Ma). The Paleoproterozoic zircons are likely to have been recycled from eroded sedimentary and metasedimentary strata that were exposed in nearby regions during the Eocene. The distinct difference in detrital zircon age populations between the two areas suggests sediment provenance in the interior of the Cordillera was strongly influenced by local geology. Additional sampling and analyses are needed to determine if these sedimentary units represent deposition in one or more intermontane basins.