Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas: A comparison

Abstract

In the North Spirit Lake greenstone belt within the Sachigo geological subprovince epiclastic metasediments interlayered with mafic and intermediate to felsic metavolcanics unconformably to disconformably overlie a sequence of ultramafic to felsic metavolcanics and quartz-magnetite iron-formation. Polymictic conglomerate of mixed volcanic and plutonic provenance overlies the unconformity. In the northern part of the belt it is followed by cross-bedded sandstone, mudstone and marble: alluvial fan, braided fluvial, and lacustrine deposits. In the southern part of the belt the basal conglomerate is overlain by channelled submarine fan deposits: inverse to normally graded and normally graded conglomerate; sandstone showing dish structure, Bouma cycles, and flame structures; mudstone and quartz-magnetite iron-formation. The submarine fan deposits are separated from the alluvial fan deposits by an area of non-deposition. In the Wabigoon geological subprovince near Rainy Lake polymictic conglomerate of volcanic provenance unconformably overlies volcanic rocks, a gabbro-anorthosite body, and a granitic subvolcanic intrusion. This alluvial fan deposit is followed upwards by braided river deposits: parallel laminated or low-angle cross-stratified sandstone and trough cross-bedded sandstone. Siltstone and magnetite iron-formation associated with the sandstone may be lacustrine deposits. To the south in the adjacent Quetico subprovince are medium-grade submarine fan sandstones. They are separated from the alluvial fan deposits to the north by a fault. Within the context of present tectonic concepts the North Spirit Lake area is comparable to a marginal basin whereas the Rainy Lake area shows characteristics more compatible with an arc-continent interface.