DOES AN INTERNATIONAL BOUNDARY FAULT OCCUR IN THE KOOTENAY ARC BETWEEN THE SALMO DISTRICT, BRITISH COLUMBIA AND THE METALINE DISTRICT, WASHINGTON?

For nearly five decades, the stratigraphy and structure of North American Paleozoic strata have been difficult to match between Salmo, BC, and Metaline, WA. We (2003) recognized that (1) the quartzite at Salmo is the quartzite of Chewelah (QOC is mid-Cambrian, Flathead-equivalent), not the much thicker, lower Cambrian Addy Quartzite, and (2) the pelitic and phyllitic Active Formation at Salmo is not the Ordovician Ledbetter Slate of Metaline. Thus, we can piece together structures mapped by others into a southwesterly dipping, Jurassic or older, thrust system cut by northerly tending Eocene normal faults.

The uppermost thrust is the Black Bluff in BC (over the Active Formation) and the Russian Creek in WA (locally over bedded-barite bearing, presumably late Devonian pelite at Deep Lake). Neoproterozoic to Devonian strata occur in the upper plate, but QOC and associated Reeves Limestone do not. Ordovician to Devonian strata are unfoliated; folds are mostly upright. The lower Lime Creek Mountain plate (LCMP) has mid-Cambrian to Devonian strata, including QOC and the Reeves Limestone, some of which are variably foliated. LCMP is an anticline overturned to the northwest.

The Black Canyon is the lower bounding thrust of LCMP; it emerges southwestward below the Russian Creek thrust. Below LCMP are mostly pelitic rocks similar to the Ordovician to Devonian Covada/Lardeau Group, and they are on strike with the Active Formation at Salmo.

Below the Black Canyon thrust and the portion of the Russian Creek thrust east of it are slices of QOC with Reeves Limestone and associated phyllites. The Day thrust separates these from the underlying Active Formation and Covada-like rocks of the Salmo plate.

The previously mapped Argillite fault of the Salmo District probably is an unconformity at the base of the Active (on QOC and Reeves Limestone). If so, on a regional scale, but not on a mine scale, rocks in the Salmo plate are upright. The Salmo plate is the structurally lowest, and it has the youngest (latest Devonian? Active Formation) and most foliated rocks.

The three thrusts converge east of the Pend Oreille River, and they create the apparent mismatch along the International Border. Furthermore, the Salmo River (BC)/Flume Creek (WA) and Leadpoint (WA and BC) Eocene normal faults displace the thrusts enough to have prevented previous recognition of their continuity.