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

The 0.2-1 km thick Archean Beaulieu Rapids Formation is a late-orogenic, te ctonically-controlled sedimentary sequence overlying the Beaulieu River vol canic belt unconformably on the west and bordering the major, N-trending Be niah Lake fault on the east. Local porphyry stocks, characteristic of late- orogenic successions, were emplaced along strike of the fault. Four lithofa cies comprise the Beaulieu Rapids Formation: (1) the 20-320 m thick conglom erate 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. Lithofa cies stacking displays two distinct, large-scale, fining-upward sequences r epresented by the 300-375 m thick conglomerate I and sandstone-siltstone (s equence A), and the 300-400 m thick conglomerate II and quartz-rich sandsto ne (sequence B). Fining-upward sequences, a bounding unconformity, and the Beniah Lake fault are compelling indicators of tectonic influence on sedime ntation. Conglomerate I was deposited by debris flows, traction currents, a nd sheetfloods on the proximal, medial, and distal portions of alluvial fan s or fan deltas, respectively. The overlying siltstone-sandstone contains t wo distinct lithological units. A sandstone-dominated unit formed from fluv ial dune migration and overbank deposition in a sandy braidplain setting wi th scattered ponds, whereas sedimentary structures in the siltstone-dominat ed unit are consistent with lacustrine deposition. A second phase of tecton ic 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 braidpla in. Mineralogy and textures reflect high sediment influx, limited transport distance, local sources, and climatic influence. Clast populations in cong lomerate indicate a prevalent mafic volcanic source, whereas sandstones con tain 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 sto cks. The humid, weathering-aggressive Archean climate was responsible for d estroying labile minerals and lithic fragments in proximal source rocks ric h in plagioclase, accounting for the high quartz component in the sandstone s. The Beaulieu Rapids Formation is similar to modern strike-slip basins th at form along major fault zones in terms of basin geometry, faulted and unc onformable contacts, lithological units, and sedimentary facies stacking an d organization. Fault-controlled basins of this nature commonly develop dur ing the last stage of terrane accretion on Archean cratons. (C) 1999 Elsevi er Science B.V. All rights reserved.