Influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the Archean Beaulieu Rapids Formation, Northwest Territories, Canada

Abstract

The 0.2–1 km thick Archean Beaulieu Rapids Formation is a late-orogenic, tectonically-controlled sedimentary sequence overlying the Beaulieu River volcanic belt unconformably on the west and bordering the major, N-trending Beniah Lake fault on the east. Local porphyry stocks, characteristic of late-orogenic successions, were emplaced along strike of the fault. Four lithofacies comprise the Beaulieu Rapids Formation: (1) the 20–320 m thick conglomerate I; (2) the 10–305 m thick siltstone–sandstone; (3) the 20–120 m thick conglomerate II; and (4) the 40–320 m thick quartz-rich sandstone. Lithofacies stacking displays two distinct, large-scale, fining-upward sequences represented by the 300–375 m thick conglomerate I and sandstone–siltstone (sequence A), and the 300–400 m thick conglomerate II and quartz-rich sandstone (sequence B). Fining-upward sequences, a bounding unconformity, and the Beniah Lake fault are compelling indicators of tectonic influence on sedimentation. Conglomerate I was deposited by debris flows, traction currents, and sheetfloods on the proximal, medial, and distal portions of alluvial fans or fan deltas, respectively. The overlying siltstone–sandstone contains two distinct lithological units. A sandstone-dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds, whereas sedimentary structures in the siltstone-dominated unit are consistent with lacustrine deposition. A second phase of tectonic uplift produced sequence B. Conglomerate II was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars. The quartz-rich sandstone developed from dune migration on a sandy braidplain. Mineralogy and textures reflect high sediment influx, limited transport distance, local sources, and climatic influence. Clast populations in conglomerate indicate a prevalent mafic volcanic source, whereas sandstones contain prominent quartz (av. 76%) with subordinate feldspar (13%) and lithic fragments (11%). The disparity between clast and sandstone composition is a function of chemical weathering, typical of a humid climate. Source areas are restricted to the mafic-dominated Beaulieu River volcanic belt, the >2.8 Ga gneisso-plutonic Sleepy Dragon Complex, and fault-related porphyry stocks. The humid, weathering-aggressive Archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase, accounting for the high quartz component in the sandstones. The Beaulieu Rapids Formation is similar to modern strike–slip basins that form along major fault zones in terms of basin geometry, faulted and unconformable contacts, lithological units, and sedimentary facies stacking and organization. Fault-controlled basins of this nature commonly develop during the last stage of terrane accretion on Archean cratons.