Stratigraphic comparison of the Lardeau and Covada groups: implications for revision of stratigraphic relations in the Kootenay Arc

The Lardeau Group is a heterogeneous assemblage of lower Paleozoic, outer continental margin strata present in the Kootenay Arc in southeastern British Columbia. From east to west, structurally lowest to highest, and what has been previously interpreted as stratigraphically lowest to highest, it consists of green and grey phyllite, argillite, limestone, and rare pillow flows (Index Formation); siliceous argillite and phyllite (Triune Formation); grey massive quartzite (Ajax Formation); siliceous argillite and phyllite (Sharon Creek Formation); alkalic(?) pillow basalt, breccia, and tuff (Jowett Formation); and quartzo-feldspathic wacke and phyllite (Broadview Formation).We propose a correlation between the Lardeau Group and the Covada Group and Bradeen Hill assemblage, both in north eastern Washington. The latter contain the same stratigraphic elements, in the same structural order, as those of the Lardeau Group. These include, from east to west, black and grey argillite and slate, chert, chert–quartz sandstone, limestone, and rare tuff, pillow flows, and quartz arenite (Bradeen Hill assemblage); alkalic(?) pillow basalt, breccia, tuff, and limestone (Butcher Mountain Formation); and quartzo-feldspathic wacke and slate (Daisy Formation). However, the sense of facing, and hence the stratigraphie sequence in the Covada Group and Bradeen Hill assemblage, is reversed in relation to the Lardeau Group, with the quartzo-feldspathic wacke unit the oldest and slate and argillite the youngest. Because the degree of preservation (and consequently the evidence for facing and age) of the units in northeastern Washington is superior to that of the Lardeau Group, we suggest that (1) the Lardeau Group may be inverted relative to the sequence as originally defined; (2)the Lardeau Group may range from Late Cambrian (Broadview Formation) to Devonian (Index Formation) in age; and (3)further work is warranted to test this hypothesis. This correlation unites lower Paleozoic stratigraphic units along several hundred kilometres of the ancient continental margin.