Sedimentology and Geochemistry of a 1.4 Ga Continental Playa System, the Lower Sibley Group, Northwestern Ontario: Implications for the Mesoproterozoic Hydrosphere and Atmosphere

English Abstract:

The 900 m thick Sibley Group consists of playa to deltaic to aeolian deposits outcropping north of Lake Superior and east of Thunder Bay. The lowermost 100 m thick succession of highly oxidized siliciclastic rocks and dolostone was deposited in a north-south trending half graben. The sediments can be divided into 15 lithofacies associations representing distinct depositional environments. The lower siliciclastic unit contains: boulder conglomerate-sandstone-dolocrete (proximal ephemeral braided stream), pebble to cobble conglomerate (ephemeral braided stream), trough cross-stratified sandstone (braided stream), green sandstone-siltstone (delta), massive cobble conglomerate (transgressive shoreline lag), planar cross-stratified sandstones (nearshore lacustrine sandwaves), and thinning-upward sandstones (lacustrine storm sand sheets). The overlying mixed siliciclastic-carbonate unit contains: red siltstone (non-saline lake), red siltstone-dolostone or dolomitic sandstone (saline lake), and halite-mudstone (ephemeral salt pans). Next is the upper siliciclastic unit with: sheet sandstones (lake infilling) and stromatolitic dolostone-chert (shoreline). After final desiccation of the lake terra rosa soils, collapse breccias and intraformational conglomerates developed. Paleocurrents, detrital zircon geochronology and sulfur isotopes indicate a change in drainage directions resulting in sand sheets infilling the saline lake. Sr isotopes reflect shallow groundwater circulation and lacustrine dolostone containing significant radiogenic Sr. Carbon and O isotopes are heavier upward in the saline lake deposits, probably due to evaporation and residence time effects. Most interestingly, REE patterns for dolomite in the dolocrete, stromatolitic shoreline deposits and overlying intraformational conglomerates have patterns similar to modern oxygenated groundwater, whereas the saline lake dolomites have hat-shaped patterns resembling modern groundwater draining waterlogged, organic-rich areas.