The Bylot Supergroup comprises 4200 m of Late Proterozoic strata that unconformably overlie Archean-Aphebian gneisses within the North Baffin Rift Zone and infill the component troughs of the Borden Basin. The succession includes three groups separated by intrabasin unconformities. The lower siliciclastic-dominated Eqalulik Group, 600 m to {dollar}\u3e{dollar}2000 m thick, consists mainly of shale, siltstone, sandstone, conglomerate and basalt of the Nauyat, Adams Sound and Arctic Bay Formations. The middle carbonate-dominated Uluksan Group, 440 m to 1550 m thick, contains stromatolitic to calciclastic dolostone and limestone and subordinate subarkose, conglomerate, siltstone and shale of the Society Cliffs, Fabricius Fiord and Victor Bay Formations. The upper siliciclastic-dominated Nunatsiaq Group, 600 m to {dollar}\u3e{dollar}1500 m thick, comprises shale, siltstone, sandstone, pebbly sandstone, conglomerate and minor stromatolitic to calciclastic limestone and dolostone of the Strathcona Sound, Athole Point, Canada Point, Lower Elwin and Upper Elwin Formation. The strata accumulated in shallow marine shelf to deep subtidal basin settings.;Abrupt marginal to basinal facies changes occur in most formations. Major depocentres for the Lower Eqalulik Group were oriented to the northwest, while depocentres for the remaining formations occur in southeastern parts of the basin and marginwards towards major fault zones. Sedimentation patterns were strongly influenced by syndepositional tectonism associated with the fault zones. Strata accumulated in multiple-block fault troughs; major successions are separated by intrabasinal unconformities that mark the transition from periods of regional tectonism to those of general stability. Major intrabasinal boundaries have been termed critical event horizons; at least five occur in the Borden Basin.;Deposition of the Bylot Supergroup is explained by related stages of rift-associated extension and sedimentation. An initial Rift-I Stage (Eqalulik Group) was characterized by faulting and deposition of quartzarenites, siltstones, shales, conglomerates and extrusion of basalt flows. Regional extension was succeeded by a semi-stable Downwarp-I Stage (Uluksan Group) during which carbonates infilled the basin and spread beyond the margins of the component troughs. Active tectonism resumed during the Rift-II Stage (Lower Nunatsiaq Group). The basin was infilled with siltstones, sandstones and conglomerates, with only minor carbonates. The second rift stage was succeeded by another period of regional subsidence and stability, the Downwarp-II Stage (Middle and Upper Nunatsiaq Group). The final infill succession was dominated by sandstones, siltstones and shales
