Stratigraphy, Sedimentology And Tectonic Setting Of The Upper Shaler Group, Victoria Island, Northwest Territories

The stratigraphy and sedimentology of the uppermost late Proterozoic Shaler Group (Kilian and Kuujjua formations) in the Minto Inlier, Victoria Island, was investigated in order to better understand conditions of deposition and to place these strata in a regional tectonic context so that they might be compared with models developed for similar strata in adjacent and formerly adjacent areas.;Detailed stratigraphic section measurement, regional correlation and lithofacies analysis reveals that much of the upper Shaler Group was deposited on a tectonically stable, low gradient, shallow marine platform, perhaps within a passive margin or intracratonic seaway (Amundsen-Mackenzie Basin or Embayment). Subtidal to supratidal alternations (cycles) within the Kilian Formation can be correlated for at least 200 km and are attributed to two scales of glacio- and/or tectono-eustatic sea-level fluctuations. Marine deposition and cyclicity was abruptly terminated by deposition of the overlying Kuujjua Formation, a mature quartzarenite interpreted as a low sinuosity fluvial braidplain deposit, which flowed into the Amundsen Basin from the southeast. Intercalated evaporite deposits in both the Kilian and Kuujjua Formations are indicative of a prevailing arid climate.;Unimodal northwesterly paleocurrents, detrital quartz petrography, detrital zircon geochronology and mudstone geochemistry indicate that the river(s) which deposited the Kuujjua Formation may have incorporated detritus from as far away as the Grenville Province, presently 3000 km to the southeast. It also suggests that the extremely high compositional maturity of the quartzarenite may in part be due to extreme humid climate-related weathering in the source area.;Differential tectonic uplift which increased in intensity from southwest to northeast, resulting in a decrease in the rate of basin subsidence and eventual net uplift is recorded by: (1) The abrupt marine to terrestrial transition at the Kilian-Kuujjua contact, (2) pinch out of the uppermost Kilian and Kuujjua formations to the northeast, (3) block faulting and erosional preclusion of some of the uppermost Kilian Formation in the northeast part of the Minto Inlier. Uplift is considered to be a consequence of thermal upwelling which accompanied the arrival of a mantle plume at the base of the lithosphere, prior to the eruption of flood basalts of the Natkusiak Formation, uppermost stratigraphic unit of the Shaler Group

Sequence and tectonostratigraphy of the Neoproterozoic (Tonian-Cryogenian) Amundsen Basin prior to supercontinent (Rodinia) breakup

Intracontinental basins that lack obvious compartmentalization and extensional faults may lie inboard of, and have the same timing as, rifted continental margins. Neoproterozoic successions of northwest Laurentia are an example where rift and intracontinental basins are spatially and temporally related. This study describes Tonian-Cryogenian pre-rift strata of the upper Shaler Supergroup, deposited in the Amundsen Basin (Victoria Island, Canada), in which five transgressive-regressive (T-R) cycles are identified. The pre-breakup succession in the Amundsen Basin has stratigraphic architecture that differs from adjacent, fault-bound rift basins. There is little evidence for extensive progradation, which resulted in broad, layer-cake stratigraphy where shallow-water facies predominate, deposited on a storm-dominated ramp. Correlation between the Amundsen and Fifteenmile (Yukon) basins is complicated by differing rates and regimes of subsidence, with the exception of a basin-deepening event that occurred in both basins and correlates with the global Bitter Springs isotope stage, initiating sometime after ~811 Ma. Contrary to previous correlations, we propose that the upper Shaler Supergroup and Little Dal Group of the Mackenzie Mountains Supergroup (Mackenzie Basin) are equivalent to the entire Fifteenmile Group. The identification of cycles and subsidence patterns in the Amundsen Basin prior to Rodinia break-up has implications for understanding the stratigraphic architecture of other intracontinental sag basins. We recognize three tectonostratigraphic units for the upper Shaler Supergroup that record an initial sag basin, followed by early extension and thermal doming, and finally rifting of the Amundsen Basin. Subsidence possibly was related to multiple cycles of intra-plate extension that complemented coeval fault-controlled subsidence. Analysis of pre-rift strata in the Amundsen Basin supports multi-phase, non-correlative break-up of Rodinia along the northwest margin of Laurentia

Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis

Although the geological record indicates that eukaryotes evolved by 1.9–1.4 Ga, their early evolution is poorly resolved taxonomically and chronologically. The fossil red alga Bangiomorpha pubescens is the only recognized crown-group eukaryote older than ca. 0.8 Ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis. Because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast, B. pubescens is uniquely important for calibrating eukaryotic evolution. However, molecular clock estimates for the divergence between the red and green algae are highly variable, and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1.2 Ga for B. pubescens. As a result, many molecular clock studies reject this fossil ex post facto. Here we present new Re-Os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of B. pubescens in the Bylot Supergroup of Baffin Island and revise its first appearance to 1.047 +0.013/–0.017 Ga. This date is 150 m.y. younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution. Using cross-calibrated molecular clock analyses with the new fossil age, we calculate that photosynthesis within the Eukarya emerged ca. 1.25 Ga. This date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the Proterozoic biosphere

Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis

Although the geological record indicates that eukaryotes evolved by 1.9–1.4 Ga, their early evolution is poorly resolved taxonomically and chronologically. The fossil red alga Bangiomorpha pubescens is the only recognized crown-group eukaryote older than ca. 0.8 Ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis. Because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast, B. pubescens is uniquely important for calibrating eukaryotic evolution. However, molecular clock estimates for the divergence between the red and green algae are highly variable, and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1.2 Ga for B. pubescens. As a result, many molecular clock studies reject this fossil ex post facto. Here we present new Re-Os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of B. pubescens in the Bylot Supergroup of Baffin Island and revise its first appearance to 1.047 +0.013/–0.017 Ga. This date is 150 m.y. younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution. Using cross-calibrated molecular clock analyses with the new fossil age, we calculate that photosynthesis within the Eukarya emerged ca. 1.25 Ga. This date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the Proterozoic biosphere

Geology of Elu Inlet and Melville Sound, Nunavut, Arctic Canada

We present the results of helicopter- and field-based geological mapping of Elu Inlet and Melville Sound, Kitikmeot Region of Nunavut, Arctic Canada. The area includes a ∼150 km-wide belt of Proterozoic sedimentary rocks that unconformably overlie the Archean Slave Province of the Canadian Shield and are cross-cut by Neoproterozoic mafic rocks and covered by early Palaeozoic deposits. This work introduces an updated sedimentologic, stratigraphic, and structural framework for the area and is corroborated by geophysical analysis of natural radioactivity. Three Proterozoic sedimentary sequences have been identified, spanning in age from ∼1.9 to ∼1.2 Ga, and including fluvial–aeolian sandstone and shallow-marine carbonate rocks. Mass-spectrometric analyses identified above-baseline concentrations of uranium along the unconformities underlying the two oldest Proterozoic sequences. Proterozoic deposits display weak deformation, related to syn-orogenic foreland and intracratonic-sag stages of the Kilohigok and Elu basins, respectively