Stratigraphy of the western Lake St. Joseph greenstone terrain, Northwestern Ontario

Abstract

The western Lake St. Joseph area in Northwestern Ontario is underlain by Archean metavolcanic and metasedimentary rocks. The strata, referred to as the Lake St. Joseph Group comprise three formations representative of mafic to felsic volcanic cyclicity. The Blackstone Formation, stratigraphically the lowest, contains a lower member composed of predominantly high-Mg, low-K tholeiitic pillowed basalts and an upper member composed of rhyoliticflows and pyroclastic rocks. The overlying Western Lake St. Joseph Formation contains a lower member composed of predominantly calc-alkaline massive and pillowed basalts and an upper member composed of dacitic pyroclastic and epiclastic rocks. The overlying Carling Formation contains a volcanic member and a sedimentary member. The volcanic member is composed predominantly of high-Fe, low-K tholeiitic pillow breccias and dacitic to rhyolitic epiclastic and pyroclastic rocks. The sedimentary member consists of basal greywacke turbidites and laminated iron formation of economic potential overlain by chloritic classical turbidites. These sedimentary rocks are overlain by massive and cross-bedded arkosic greywackes which are in turn overlain by conglomerate and pebbly sandstone. The deformation of the rocks is expressed by isoclinal folding, most evident on Eagle Island, and development of the regional Lake St. Joseph Fault. Contact and regional metamorphism (lower to middle greenschist facies) have also affected the rocks. The primary structures, vertical and lateral variations in the felsic volcanic rocks of the Blackstone and Western Lake St. Joseph Formations indicate deposition on a subaqueous paleoslope. Similarly the felsic volcanic rocks of the Carling Formation indicate subaqueous deposition on a different paleoslope. The lithologies, primary structures, vertical and lateral variations of the units in the sedimentary member of the Carling Formation indicate deposition on a prograding submarine fan. Meyn and Palonen (1980) support this interpretation. Reconstruction of the paleoenvironment envisages three stages of evolution. In stage 1 the Blackstone and Western Lake St. Joseph Formations are extruded and deposited on the flank of a volcanic edifice. In stage 2 the volcanic member of the Carling Formation is extruded from a separate vent and in part deposited upon the degradation products of stage 1. In stage 3 laminated iron formation is deposited with clastic sediments in a submarine fan-basin plain system to form the sedimentary member of the Carling Formation.