| dc.description.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. | |
