Stratigraphy, paleogeography and tectonic evolution of early Paleozoic to Triassic pericratonic strata in the northern Kootenay Arc, southeastern Canadian Cordillera, British Columbia

Abstract

The northern Kootenay Arc in southeastern British Columbia hosts a near-continuous succession of Cambrian to Permian strata that were deposited in overlapping marginal basins outboard of the Canadian Cordilleran miogeocline. The basinal strata of the Late Cambrian and younger Lardeau Group are conformably overlain by a succession of Devonian and Early Mississippian strata herein named the Mount Sproat assemblage. The Mount Sproat assemblage comprises an undated carbonaceous lower unit that passes upward into a lithologically diverse succession of siliciclastic, carbonate and mafic metavolcanic strata that have geochemical signatures recording a transition from non-arc to arc-related volcanism. Calc-alkaline basalt in the Mount Sproat assemblage yielded prismatic zircon dated at 367.2 ± 2 Ma. The Mount Sproat assemblage is truncated by an angular unconformity at the base of the Late Mississippian Milford Group. Lower stratigraphic levels of the Mount Sproat assemblage were deformed penetratively with the Lardeau Group prior to deposition of the Milford Group. The Thompson assemblage (new term) is a localized sedimentary succession between the Viséan Milford Group and the Broadview Formation (Lardeau Group) at Mount Thompson. It was deposited after Early Mississippian deformation of the Lardeau Group and was partially eroded into the overlying the Milford Group. Detrital zircon dates in the intervals 2.8-2.6 Ga and 2.1-1.75 Ga link the Lardeau Group, Mount Sproat assemblage, Thompson assemblage and Milford Group to the Canadian Cordilleran miogeocline. Detrital zircon data also reveal sources of zircon that were not available to the miogeocline. The Broadview Formation is interpreted to record uplift of an outer high of sialic North American crust that contained zircon in the intervals 1.48-1.41 Ga, 1.38-1.32 Ga, and 1.30-0.95 Ga in addition to ages typical of the adjacent miogeocline. Palinspastic restoration of this margin places the edge of attenuated North American cratonic crust near the present day coastline of southern British Columbia. In Late Devonian to Late Triassic time, detrital zircon grains with Neoproterozoic (700-550 Ma) and Silurian (ca. 420 Ma) dates appear in the dataset. These dates are interpreted to originate from an exotic terrane that docked in mid-Devonian time.