Source terranes for Proterozoic sedimentary rocks in southern British Columbia: Nd isotopic and petrographic evidence

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Abstract
Quartz arenites of the Fort Steele Formation, the lowest exposed stratigraphic unit of the Purcell Supergroup, have Nd crustal residence times (Tcr) of 2.61–2.63 Ga. These overlap the low end of the Tcr range of the polymetamorphic Kenoran–Hudsonian basement of southern Alberta and southwestern Saskatchewan. The Tcr values of overlying fine-grained Purcell rocks average 2.0 Ga.Near Lake Windermere, the basal Toby conglomerates of the Windermere Supergroup, apparently derived from underlying Purcell strata, have Tcr values of 1.98 and 2.03 Ga. Stratigraphically higher argillite and arkoses range from 2.35 to 2.54 Ga. This higher range indicates contributions from a source terrane different from the main body of the Purcell Supergroup.Our interpretation of events in the East Kootenay area is that early Purcell sedimentation (Fort Steele Formation) was derived from reworking of the crystalline basement of the Alberta shelf as the Beltian Sea transgressed. As the basin deepened, it was filled by fine-grained turbidites from a distal southern or southeastern source. Later Purcell strata have more complex paleocurrent direction patterns but relatively uniform Tcr values similar to those of the earlier turbidites. Following the East Kootenay orogeny, the basal conglomerate of the Windermere Supergroup (Toby Formation) consisted dominantly of locally derived clasts. These Purcell-derived strata grade upward into arkosic rocks with an admixture of Archean detrital material, probably from a proximal eastern source.

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