| dc.description.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. | |
