SAUK SEQUENCE STRATIGAPHY IN THE TRICORNER OF WASHINGTON, BRITISH COLUMBIA, AND IDAHO

Neoproterozoic and Paleozoic strata in the Tricorner are the western limb of the Purcell anticlinorium. Because most formations are more than a kilometer thick and appear to be conformable, sequence stratigraphy languishes in this former trailing margin of North America.

The uppermost formation of the Neoproterozoic Windermere Group is the feldspathic quartzitic Three Sisters (TS). Previous authors (PA) noted that TS is unconformable on the next lowest Windermere formation and is conformable with the overlying Addy Quartzite and Maitlen Phyllite. Because the latter contain Nevadella and archaeocyathids, respectively, TS, Addy, and Maitlen are Sauk I. In WA, PA showed that the sub-Sauk unconformity progressively eliminates the Windermere Group southwestward.

PA included in the Addy a quartzite that is only a few hundred meters thick. Near Chewelah, WA, this thinner quartzite is unconformable on Mesoproterozoic Belt Supergroup and the Addy. Near Addy, WA, limstone of the lower Metaline Fm. overlies the thinner quartzite. Near Salmo, BC, this quartzite, associated pelite, and the Reeves Limestone are above (not below) the Maitlen; they are overlain by the Active Fm., which is lithologically similar to the lower Metaline. The thinner quartzite occurs at least as far west as the Columbia River. It correlates with the Flathead of the craton. Thus, this quartzite and overlying rocks are Sauk II.

The unfossiliferous, peritidal, dolomitic middle portion of the Metaline probably is the base of the Tippecanoe. PA showed that the upper limestone of the Metaline grades upward lithologically and paleontologically into the lower Ordovician (Arenigian) Ledbetter Slate. Furthermore, along the International Border, Sauk II and older sequencs are in greenschist facies, but the dolomite and younger rocks are not. The absence of upper Cambrian fossils in the Tricorner suggests that an unconformity below the dolomite eliminated any Suk III.

The Eocene Flume Creek and Leadpoint are major normal faults in WA. Stratigraphic revisions reveal that these faults continue northeasterly into BC. Thus the Black Bluff, Day, and Black Canyon faults are offsets of a major southeasterly dipping thrust. This thrust explains the previously puzzling differences in the geology of adjacent parts of BC and WA.