PETROGRAPHIC AND GEOCHEMICAL CHARACTERIZATION OF EOCENE VOLCANIC ROCKS, BRITISH COLUMBIA, AND THEIR APPLICATION TO ARCHAEOLOGICAL TOOLSTONE SOURCING

The geologic history of Interior British Columbia was characterized by episodic volcanic events associated with accretion of the Intermontane and Insular terranes to the North American craton starting in the Jurassic (~170 Ma) and continuing into the late Eocene (~ 35 Ma). Fine-grained basaltic to rhyolitic products from the younger volcanic events surround the Bridge River archaeological site, a large First Nations pithouse village intensively occupied during four main phases from ca. 1800 cal B.P. to the historic Fur Trade period. Geochemical analysis of fine-grained volcanic rocks (FGV) using hand-held portable x-ray fluorescence (HHpXRF) technology provides elemental characterization that enables matching of FGV artifacts with their original toolstone source locations.

Over the course of this study, 280 FGV artifacts recovered from Housepit 54 at the Bridge River site and volcanic outcrop samples from eleven potential source locations were analyzed with HHpXRF. Additionally, twenty two petrographic thin sections were prepared of the different outcrop materials to better understand the mineralogy of the locally available toolstone material. The outcrops targeted by the inhabitants of Housepit 54 were extremely fine-grained, nearly aphyric, trachydacites and rhyolites.

Trace element ratios (e.g. Nb/Sr, Rb/Sr) of the artifacts revealed four chemically distinct groups. Three of these chemical groups were matched to local lava flows less than 55 km from the Bridge River site; the fourth group is chemically distinct from any of the outcrops sampled. Overall, 56% of the artifacts analyzed matched with lava flows sampled, indicating that the majority of toolstone material was obtained from these sources. Geochemical characterization of toolstone material illustrates how FGV toolstone sourcing can provide important insights into local / non-local procurement patterns.