The Kootenay Arc in northeastern Washington and southeastern British Columbia contains the transition between autochthonous Upper Proterozoic to lower Paleozoic miogeoclinal strata and outboard Paleozoic to Mesozoic eugeoclinal terranes of uncertain paleogeographic affinity. To better understand the nature of this transition, U–Pb detrital zircon geochronologic studies were carried out on Upper Proterozic and lower Paleozoic sedimentary units in the Kootenay Arc, including miogeoclinal strata of the Horsethief Creek and Hamill groups and eugeoclinal strata of the Broadview and Ajax formations (Lardeau Group) and Daisy Formation (Covada Group). The results indicate that all units sampled are derived from source terranes of ~1.7 to 2.7 Ga, with notable populations of 1.76–1.85, 1.9–2.1, and 2.5–2.7 Ga zircons. These results are consistent with derivation of both the miogeoclinal and lower Paleozoic eugeoclinal units from adjacent portions of the southern Alberta craton. This extends the western limit of supracrustal rocks of known North American affinity, formed approximately in situ in the southern Canadian Cordillera, to outboard of the lower Paleozoic eugeoclinal strata in the Kootenay Arc.