NORTH AMERICAN SEQUENCE STRATIGRAPHY IN THE TRICORNER OF WASHINGTON, BRITISH COLUMBIA, AND IDAHO

Most of the Vendian and Paleozoic formations of the Tricorner have maximum thicknesses > 1 km. All are North American; yet, sequence stratigraphy has languished. Most stratigraphic discontinuities and unusually thin stratigraphic intervals have been attributed to faulting. Discontinuities marked by one lithologic unit overlying two or more older units are recognized here as unconformities.

Most of the Vendian Windermere Group is an unconformity-bounded sequence (UBS) on the Belt Supergroup. Formations in this UBS, from the base up, are Shedroof (with tillites), Leola (greenstone), and Monk (with tillites). Unconformities (intra- and inter-formational and the sub-Sauk) eliminate much of this UBS. Thus, the isopachs and distribution of the Windermere do not outline its original depositional basin.

The Sauk UBS is unconformable on Belt and Windermere. Formations of the Sauk, from the base up, are Three Sisters (feldspathic quartzite of the Windermere Group), Gypsy (quartzite), Maitlen (phyllite), and Metaline (limestone and dolostone). This UBS is Vendian to mid-Cambrian; its formations are thicker and mostly older than the Sauk of ID and MT. The absence of the Maitlen near Chewelah implies an intra-Sauk unconformity at the base of the Metaline.

The mid-Ordovician Ledbetter Slate rests on formations as old as Gypsy. This unconformity, not paleogeography, limits the northern extent of the Metaline. Presumably, the Ledbetter marks the base of the Tippecanoe UBS. Other authors have shown that in two places the top of the Metaline is mid-Ordovician and that it is conformably overlain, with little or no hiatus, by Ledbetter.

In a few small areas, Devonian to Mississippian strata are in contact with Belt to Ledbetter. These contacts likely are the basal unconformity of the Kaskaskia UBS.