ARCHAEAN SEDIMENTATION: ALLUVIAL FAN AND TURBIDITE DEPOSITS, LITTLE VERMILION LAKE, NORTHWESTERN ONTARIO

Abstract

A sequence of sedimentary rocks within a greenstone belt at Little Vermilion Lake, near Sioux Lookout, in northwestern Ontario, has been mapped in detail, and 5 facies have been established. The lowest 1,700 m of the sedimentary succession rests unconformably on a thick sequence of volcanics, and is composed of a conglomerate facies and an arkose facies. The conglomerates occur in beds up to 20 m in thickness, are laterally impersistent, and comprise pebbles, cobbles and boulders (up to 1.5 m in diameter) of basic to acidic volcanic rocks and granodiorite, with an interstitial matrix of arkose. The conglomerates are commonly massive, and exhibit no cross-stratification or graded beds, but some are imbricated. The arkoses commonly occur as interbeds less than 1 m thick between massive conglomerates, or occur as thick sequences which are commonly massive, but which exhibit some trough cross-bedding, parallel lamination and graded bedding. The arkoses and conglomerates are succeeded by 100 m of felsic tuff, 60 m of greywacke and argillite, 270 m of basic tuff, and a 600 m thick section of graded greywackes and argillites, with individual beds commonly less than 30 cm thick. The arkoses and conglomerates are interpreted as alluvial fan deposits, whilst the greywackes and argillites are interpreted as turbidites. The tuffs mark explosive volcanic activity which accompanied the deepening of the basin- of deposition. The petrographic characteristics of the sediments indicates that the provenance area was composed of basic to acidic volcanic rocks, together with a large e.xtent of granodiorite. The granodiorite may have intruded the volcanics, prior to the deposition of the sediments.