Stratigraphy And Sedimentation Of The Middle Proterozoic Sibley Group, Thunder Bay District, Ontario

The Middle Proterozoic Sibley Group is a mixed clastic - carbonate, red bed sequence located in the Thunder Bay - Nipigon area on the north shore of Lake Superior. The lowest unit, the Pass Lake Formation, consists of a basal paraconglomerate member, of probable alluvial debris flow origin, overlain by 20 to 80 metres of plane-bedded and cross-bedded quartz arenites which were probably deposited by sheetfloods and eolian processes on alluvial outwash sandflats. The overlying Rossport Formation is dominated by red and buff dolomicritic mudstone. The association of these mudstones with relatively pure massive carbonate beds and sheetflood sandstone units is strongly suggestive of a playa lake depositional environment. Fluctuations in playa lake levels, due to rainfall variations, may have resulted in oscillations between carbonate-dominated and clastic-dominated sedimentation. The upper unit, the Kama Hill Formation, consists of horizontally-laminated purple shales and ripple cross-laminated buff siltstone to fine sandstones. The presence of stacked "powering-down" sequences and abundant desiccation features is suggestive of sheetflood deposition on a distal alluvial floodplain.;The sequence of depositional environments suggests that the Sibley Basin formed in a broad, slowly subsiding intracratonic basin. The extensional regime responsible for the subsidence may have been a precursor to the main period of volcanic activity along the Keweenawan Midcontinent Rift Zone. It is more probable, however, that the period of extension accompanied Elsonian anorogenic magmatism and continental rifting approximately 300 million years prior to the Keweenawan event.