Sedimentology and geochemistry of the Mesoproterozoic Pass Lake and Rossport formations, Sibley group

Abstract

The Sibley Group is an essentially unmetamorphosed, Mesoproterozoic sedimentary succession consisting of a mixture of siliciclastic and chemical sedimentary rocks. This study examined the sedimentology and geochemistry of the Pass Lake and Rossport Formations, the lowermost lithostratigraphic units of the Sibley Group. Lithofacies analysis subdivided the sections studied into sixteen lithofacies associations corresponding to distinct depositional environments. These were divided into four informally defined allostratigraphic units, roughly equivalent to previous lithostratigraphic subdivisions. A lower clastic unit is comprised o f the following lithofacies associations; boulder conglomerate-sandstone-dolocrete (proximal ephemeral braided stream), pebble to cobble conglomerate (ephemeral braided stream), massive cobble conglomerate (transgressive lag, reworking of braided stream deposits during transgression), trough cross-stratified sandstone (braided stream), green sandstonesiltstone (wave and storm influenced fiuvial dominated delta), planar cross-stratified sandstone (nearshore migration of large sandwaves), and thinning-upward sandstone (beach and storm remobilized nearshore sandstone sheets). The lower clastic unit is disconformably to confomably, depending on geographic location, overlain by a mixed siliciclastic-carbonate unit. The mixed siliciclastic-carbonate unit is comprised of the red siltstone (non-saline lake), red siltstone-dolostone (perennial saline lake, distal from clastic source) and red siltstone-dolomitic sandstone (perennial saline lake, proximal to clastic source) lithofacies associations and is sharply overlain by an upper clastic unit. The upper clastic unit consists of the sheet sandstone (ephemeral playa lake (?) or perennial lake with increased sand supply with respect to underlying units) and black chert-carbonate (microbial mats forming at restricted, shallow, subaqeuous shoreline) lithofacies associations. Subaerial exposure features are present at the top of the blackchert- carbonate lithofacies association and include the intraformational conglomerate lithofacies association (subaerial debris flows, intrusive and/or extrusive sedimentary breccias, terra rossa style soils, dissolution collapse breccias). Subaerial exposure features at the top of the upper clastic unit are overlain by the massive dolostone (saline lake), red siltstone-sulfate (wet evaporite-rich mudflats around lake margins) and fine-grained sandstone (dry, evaporite-poor mud and sand flats around lake margins) lithofacies associations of the mixed siliciclastic-carbonate-evaporite unit. During deposition of the lower clastic unit paleocurrents and detrital zircon geochronology suggest a south- to southeast-down paleoslope. A lack o f major thickness changes in stratigraphie units or laterally extensive coarse-grained clastic deposits is consistent with a broad intracratonic sag architecture during initial basin subsidence. Paleocurrents from the upper clastic unit suggest a change to northward oriented paleoslope during its deposition. This corresponds with an apparent thickening of the upper two units towards the north and Black Sturgeon Fault, consistent with the development of a half-graben structure and increased evidence of tectonic activity (e.g. intrusive sedimentary breccias, synsedimentary faulting and debris flows). Carbon, oxygen and sulfur stable isotope analyses, Sr isotope analyses and trace element analyses (in some cases analyses include rare earth elements) were preformed on a variety of distinct carbonate and sulfate lithologies. Sulfur isotope compositions, strontium isotope compositions and rare earth element/yttrium ratios support a nonmarine depositional setting. Low values appear to be a good indicator of diagenetically altered samples. values have typical marine values consistent with lacustrine carbonate precipitated in equilibrium with atmospheric CO2. Stratigraphie changes in C and O isotope compositions in the red siltstone-dolostone lithofacies association were likely driven by evaporation and residence time effects. Stratigraphie variations in sulfur isotope compositions may reflect changes in the composition of sulfides weathering to supply sulfate to the basin. Overall, both the interpretation of physical sedimentologic and various types of geochemical data strongly support a nonmarine lacustrine setting for the deposition of the Pass Lake and Rossport Formations.