CLIMATIC AND TECTONIC INFLUENCES ON FAN DELTAS AND WAVE- TO TIDE-CONTROLLED SHOREFACE DEPOSITS - EVIDENCE FROM THE ARCHEAN KESKARRAH FORMATION, SLAVE PROVINCE, CANADA

The 2.6 Ga Keskarrah Formation, located in the central Slave Province, Northwest Territories, Canada, is a late-orogenic, tectonically contr olled sedimentary sequence that developed under unusual climatic and d epositional conditions. The formation is adjacent to the crustal-scale , north-trending Beniah Lake Fault and overlies the 3.15 Ga Augustus G ranite, the 2.69-2.7 Ga mafic volcanic Peltier Formation and the turbi ditic Contwoyto Formation unconformably. Principal lithofacies in the Keskarrah Formation include conglomerate, sandstone and siltstone-sand stone. The conglomerate lithofacies represents coalescing gravelly str eamflow-dominated fan deltas adjacent to topographic highs. Up-section quartz-rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow-water shoreface deposits influenced by wave action and tides. The overlying feldspathic litharenites of the s iltstone-sandstone lithofacies are consistent with a lower shoreface t o proximal offshore environment dominated by wave and tide interaction . Tidal influence in both sandstone-dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross-beds, as well as from abundant reactivation surfaces with lo cal mudstone drapes. Intense chemical weathering during the Archaean, resulting from elevated atmospheric CO2 levels, higher temperatures an d moist climatic conditions, played an important role in the developme nt of quartz-rich arenites that appear to be first-cycle deposits. Few lithic fragments and feldspar grains are preserved due to in-situ hos t rock weathering, chemical weathering during transport and wave and t ide action. Hydraulic sorting and abrasion in the shoreface environmen t contributed to the continued breakdown and transport of labile miner als. Increased proportions of lithic fragments in sandstone beds of th e conglomerate lithofacies are the result of shorter transport distanc es from source areas to the depositional environment. Abundant conglom erate with up to 4-m large granitic boulders derived from the adjacent Augustus Granite and mafic clasts from the Peltier Formation indicate high relief and fault-related uplift and subsidence. The intimate ass ociation of fan deltas and wave- and tide-influenced shallow-marine de posits in association with quartz-rich sandstones forming in a high-re lief area make the Keskarrah Formation remarkable in the rock record. (C) 1998 Elsevier Science B.V. All rights reserved.