Mackenzie Mountains Supergroup Research Articles

The paleoredox context of early eukaryotic evolution: insights from the Tonian Mackenzie Mountains Supergroup, Canada.

Tonian (ca. 1000-720 Ma) marine environments are hypothesised to have experienced major redox changes coinciding with the evolution and diversification of multicellular eukaryotes. In particular, the earliest Tonian stratigraphic record features the colonisation of benthic habitats by multicellular macroscopic algae, which would have been powerful ecosystem engineers that contributed to the oxygenation of the oceans and the reorganisation of biogeochemical cycles. However, the paleoredox context of this expansion of macroalgal habitats in Tonian nearshore marine environments remains uncertain due to limited well-preserved fossils and stratigraphy. As such, the interdependent relationship between early complex life and ocean redox state is unclear. An assemblage of macrofossils including the chlorophyte macroalga Archaeochaeta guncho was recently discovered in the lower Mackenzie Mountains Supergroup in Yukon (Canada), which archives marine sedimentation from ca. 950-775 Ma, permitting investigation into environmental evolution coincident with eukaryotic ecosystem evolution and expansion. Here we present multi-proxy geochemical data from the lower Mackenzie Mountains Supergroup to constrain the paleoredox environment within which these large benthic macroalgae thrived. Two transects show evidence for basin-wide anoxic (ferruginous) oceanic conditions (i.e., high FeHR/FeT, low Fepy/FeHR), with muted redox-sensitive trace metal enrichments and possible seasonal variability. However, the weathering of sulfide minerals in the studied samples may obscure geochemical signatures of euxinic conditions. These results suggest that macroalgae colonized shallow environments in an ocean that remained dominantly anoxic with limited evidence for oxygenation until ca. 850 Ma. Collectively, these geochemical results provide novel insights into the environmental conditions surrounding the evolution and expansion of benthic macroalgae and the eventual dominance of oxygenated oceanic conditions required for the later emergence of animals.

Zircon (U‐Th)/He thermochronology of Neoproterozoic strata from the Mackenzie Mountains, Canada: Implications for the Phanerozoic exhumation and deformation history of the northern Canadian Cordillera

AbstractSedimentary strata of the Neoproterozoic Mackenzie Mountains Supergroup (MMSG) and Windermere Supergroup (WSG) occupy the cores of anticlines in the Mackenzie Mountains of the Canadian Cordilleran Foreland Belt. Stratigraphic and structural evidence suggest that these rocks have undergone several episodes of burial and unroofing relatively intact. We report single‐grain detrital muscovite 40Ar/39Ar and zircon (U‐Th)/He (ZHe) data from a suite of samples across the fold‐thrust belt and the Neoproterozoic stratigraphic record. The strata have not reached high enough temperatures to reset the muscovite 40Ar/39Ar system, and instead our detrital muscovite data refine Tonian‐Cryogenian depositional ages. Single‐crystal ZHe dates range from 432 ± 35 to 46 ± 4 Ma, indicating that MMSG and WSG strata have not been heated sufficiently to fully reset the ZHe system. These factors make the Neoproterozoic strata an attractive natural laboratory to test the utility of the zircon radiation damage and annealing model on the quantification of thermal histories from detrital zircon populations that have accumulated radiation damage over long geologic timescales. Thermal modeling reveals that (1) a substantial sedimentary package was deposited following the Devonian and removed during Permo‐Triassic cooling, and (2) the Cordilleran deformation front propagated through the study area from the Albian to the Paleocene, with a moderate increase in cooling rates between 75–67 Ma in the southwest and 60–55 Ma at the deformation front. Ultimately, relationships between radiation damage and helium diffusion kinetics in zircon explain substantial ZHe date dispersion and elucidate the temperature‐time history of the northern Canadian Cordillera.

Thick sulfate evaporite accumulations marking a mid-Neoproterozoic oxygenation event (Ten Stone Formation, Northwest Territories, Canada)

Thick sulfate evaporite accumulations are absent from Proterozoic strata between ca. 2000 and ca. 1000 Ma, and detailed sedimentologic, stratigraphic, and geochemical data for the oldest Neoproterozoic thick marine sulfate evaporite successions are largely lacking. The middle Neoproterozoic Ten Stone Formation (Little Dal Group, Northwest Territories, Canada) consists of ~500 m of pelagic lagoonal gypsite and anhydritite (rocks consisting of the minerals gypsum and anhydrite) deposited shortly before the ca. 811 Ma Bitter Springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of Rodinia. The thickness of regional stratigraphic subdivisions of this formation, defined by subtle silt- and carbonate-bearing intervals, indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest. Deposition of the Ten Stone Formation began with abrupt, tectonically triggered subsidence and restriction, and ended equally abruptly, as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply, with no evidence for hiatus or erosion. Stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom-hugging, dense lagoonal brine from overlying waters. Deposition of the Ten Stone Formation in a basin that experienced intermittent, basin-scale tectonic adjustments, as recorded by details of its stratigraphy, supports the interpretation that the Mackenzie Mountains Supergroup accumulated in an extensional, tectonically active intracratonic basin whose structure resembled a lower-plate extensional system. The absence of halite from the Ten Stone Formation contrasts with its abundance in the stratigraphically lower, gypsum-free Dodo Creek Formation, suggesting that deposition of the lower to middle Little Dal Group spanned a major oxygenation event, during which the sequence of evaporite mineral precipitation from seawater changed from halite-first to sulfate-first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir. The limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid-Neoproterozoic atmosphere, an abundance of sulfate in seawater, and marine oxygenation extending below storm wave base. The mineralogy, sedimentology, stratigraphy, and geochemistry of the Ten Stone Formation are virtually indistinguishable from those of thick, Phanerozoic “deep-water” (below wave-base) evaporite successions, and indicate that the tectonic, climatic, and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca. 811 Ma. Thick sulfate evaporite successions in equivalent stratigraphic positions just below the Bitter Springs carbon isotope excursion elsewhere in Laurentia, as well as on the Congo craton, and in South Australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to Rodinia’s breakup. High relative burial rates of organic matter prevailed before the breakup of Rodinia and led to oxygenation of the atmosphere-ocean system, growth of the seawater sulfate reservoir, and, in association with a warm and arid climate, deposition for the first time in Earth’s history of thick sulfate evaporites in the middle Neoproterozoic, ~100 m.y. before the first Cryogenian glacial episode. The Neoproterozoic Oxygenation Event may have taken place in several steps, the first of which preceded the Bitter Springs anomaly.

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 ∼811Ma. 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.

Detrital zircon lineages of late Neoproterozoic and Cambrian strata, NW Laurentia

The Phanerozoic tectonic evolution of the Arctic is a field of escalating scientific interest. Detrital zircon provenance studies provide vital contributions to clarify the region’s tectonic evolution. Northwest Laurentia exposes a broad expanse of Proterozoic and Paleozoic sedimentary strata for which detrital zircon populations are poorly characterized. Moreover, the significance of sedimentary recycling is becoming better appreciated in light of detrital zircon studies. As more data become available, our understanding of the detrital zircon character of NW Laurentia improves, providing an increasingly reliable baseline at subcontinental resolution against which potentially allochthonous terranes, such as Arctic Alaska, can be assessed. Sandstones of late Neoproterozoic and Cambrian age from NW Canada yield detrital zircon signatures dominated by zircon grains recycled from Proterozoic sedimentary strata. Two Neoproterozoic sandstones from the northern Mackenzie Mountains yield zircon populations sourced from the Mackenzie Mountains Supergroup. Two Lower Cambrian sandstones sourced from the Yukon stable block and deposited in Richardson Trough have zircon populations nearly identical to those of the upper Wernecke Supergroup, locally exposed across the southern Yukon stable block, where they are unconformably overlain by Cambrian strata. Comparisons with similar studies from NW Canada permit generalizations of the patterns of zircon recycling. Four provenance lineages are described that characterize Laurentia-derived successions in NW Canada.

Two detrital zircon signatures for the Cambrian passive margin of northern Laurentia highlighted by new U-Pb results from northern Canada

In time, Laurentia was bounded on all sides by passive margins, providing an opportunity to establish a Cambrian of detrital zircon provenance for the craton during a relatively quiescent tectonic stage. U-Pb detrital zircon ages from laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) analysis of seven sandstone samples from northern Northwest Territories are consistent with generally local provenance. Samples from Mackenzie Mountains were most likely derived from the Proterozoic Mackenzie Mountains Supergroup and are dominated by detrital zircon ages of ca. 1020–1516 Ma, but also contain populations of ca. 1615–1692 Ma and ca. 2699–2752 Ma. sandstones from south of Great Bear Lake have a complementary set of detrital zircon age populations dominated by ca. 1810–2010 Ma, with smaller relative probability peaks at 2100 Ma, 2300 Ma, and 2600 Ma, which are consistent with derivation from the Hottah terrane, Great Bear magmatic zone, and Slave craton. Comparison of the signatures from Mackenzie Mountains and Great Bear Lake south suggests that there were two distinct sources of detrital zircon available to the northern Northwest Territories during the Cambrian. Detrital zircon ages from strata of Victoria Island appear to be a mixture of Shaler Supergroup, Slave craton–like, Taltson-Thelon orogen, and Wopmay orogen sources. Following the dichotomy of detrital zircon age spectra presented by samples from Mackenzie Mountains and Great Bear Lake south allows subdivision of other published detrital zircon ages from across northern Laurentia into two groups: Laurentia type 1 suite reflects the Archean cratons and Paleoproterozoic orogens of the Precambrian Shield. Laurentia type 2 suite is dominated by Grenvillian and older ages (1000–1500 Ma) and includes relative probability peaks at 1600–1700 Ma and 2700–2750 Ma. We find, therefore, that this baseline mirrors the Precambrian tectonic fabric of Laurentia from Archean cratonization and Paleoproterozoic through Mesoproterozoic orogenesis, culminating in assembly of the supercontinent Rodinia. Furthermore, the Neoproterozoic breakup of Rodinia and establishment of passive margins surrounding Laurentia provided little new zircon available to the sedimentary system. The detrital zircon spectra for the of northern Laurentia are intended to provide a reference to test the affinity of suspect terranes and potentially exotic detrital zircon within Paleozoic foreland basins.

Regional and global chemostratigraphic correlation of the early Neoproterozoic Shaler Supergroup, Victoria Island, Northwestern Canada

High resolution inorganic carbon isotope stratigraphy provides a new tool for correlating the Neoproterozoic Shaler Supergroup with the Mackenzie Mountains Supergroup. Previous stratigraphic studies established a widely accepted basis for correlation between the two supergroups. This contribution introduces a revised, testable hypothesis for the correlation of the Shaler and Mackenzie Mountains supergroups, based on matching distinctive features in the Shaler Supergroup δ 13 C data with the previously published δ 13 C curve for the Mackenzie Mountains Supergroup. The hypothesis suggests that some lithologically similar units in the two basins were deposited diachronously, and that a significant disconformity may exist within the Wynniatt Formation of the Shaler Supergroup. An important implication of the newly proposed correlation is that the flora and fauna of the Wynniatt Formation predate the “Bitter Springs” isotopic stage, placing a new constraint on the chronology of the Neoproterozoic fossil record.

Basin architecture and syndepositional fault activity during deposition of the Neoproterozoic Mackenzie Mountains supergroup, Northwest Territories, CanadaNorthwest Territories Geoscience Office Contribution 0040.

The early Neoproterozoic Mackenzie Mountains supergroup (MMSG; ∼4 km thick, <1083 Ma, >779 Ma) was deposited in a large but poorly understood basin. Analysis of new and existing stratigraphic data reveals unexpected, abrupt changes in thickness and lithofacies patterns in formation scale and smaller units over short distances parallel and perpendicular to depositional strike. These isopach and lithofacies patterns depict irregularly subsiding syndepositional sub-basins. These basins may have developed during basin extension in a lower-plate segment of an evolving passive margin, as the result of episodic, excessive subsidence along growth faults parallel to the basin margin, and crustal-scale (transfer) faults at a high angle to the margin. The extent of the lower-plate area is inferred to correspond to the distribution of the MMSG (∼500 km strike length); upper-plate segments are inferred to flank the MMSG embayment to the northwest and southeast. A lower-plate model for the arcuate MMSG embayment has important implications for the Mackenzie Mountains zinc district. The present-day regional distribution of known carbonate-hosted base-metal showings suggests that the configuration of the thinned, faulted Neoproterozoic lower-plate segment, together with later reactivation of basement-rooted transfer faults during Neoproterozoic–Paleozoic rifting or Laramide compression, may have been important in focussing metalliferous fluids into overlying units.

Early and Middle Proterozoic evolution of Yukon, Canada

This paper provides a comprehensive synthesis of virtually all units and events of Early and Middle Proterozoic age in the Yukon, spanning ~1 Ga. Early and Middle Proterozoic time was dominated by a series of extensional-basin-forming events punctuated by orogenesis, magmatism, and hydrothermal activity. Basinal deposits include the Wernecke Supergroup (>1.71 Ga), Pinguicula Group (~1.38 Ga), and Mackenzie Mountains Supergroup (1.00–0.78 Ga). Igneous rocks include the Bonnet Plume River Intrusions (1.71 Ga), Slab volcanics (≥1.6 Ga), Hart River sills and volcanics (1.38 Ga), and Bear River (Mackenzie) dykes (1.27 Ga). A voluminous hydrothermal event generated the widespread Wernecke breccias at 1.60 Ga. The Racklan orogeny deformed the Wernecke Supergroup prior to emplacement of the Wernecke Breccia. The Corn Creek orogeny deformed Mackenzie Mountains Supergroup and older rocks prior to deposition of the Windermere Supergroup (<0.78 Ga). Long intervals with scanty rock records extended for as much as 300 Ma and appear to represent periods of crustal stability and subaerial conditions. By the time of Windermere rifting (<0.78 Ga), the supracrust of northwestern Laurentia was a mature, largely denuded orogenic belt with a composite sedimentary–metamorphic–igneous character. New isotopic data include Nd depleted mantle model ages for the Wernecke Supergroup (2.28–2.69 Ga) and Wernecke Breccia (2.36–2.96 Ga), a U–Pb zircon age for a Hart River sill 1381.9+5.3-3.7 (Ma), detrital U–Pb zircon ages from the basal Pinguicula Group (1841–3078 Ma), detrital muscovite ages from the Mackenzie Mountains Supergroup (1037–2473 Ma), and muscovite 40Ar/39Ar cooling ages from the Wernecke Supergroup (788 ± 8 and 980 ± 4 Ma).

The Imperial anticline, a fault-bend fold above a bedding-parallel thrust ramp, Northwest Territories, CanadaGeological Survey of Canada contribution no. 1997169.

The Imperial anticline, about 50 km west of Norman Wells, Northwest Territories, Canada is a fault-bend fold which overlies an unusual thrust ramp, totally dictated by stratigraphy and not at all like those in text books. The lower-flat/ramp/upper-flat geometry of the underlying thrust fault is decreed by the stratigraphic configuration, which is that of a southward dipping, gently flexed, Neoproterozoic Mackenzie Mountains Supergroup (MMS) succession truncated unconformably by a flat-lying Phanerozoic succession. The lower flat and ramp are controlled by a bedding-parallel glide zone in the flexed MMS, whereas the upper flat is controlled by another bedding-parallel glide zone in salt beds in the lower part of the flat-lying Phanerozoic. This study provides new insight into a type of stratigraphically dictated thrust ramp that may well be present in other thrust belts. Angular truncations at unconformities are to be expected at craton margins and stratigraphic configurations similar to the one documented here may be the cause for other large, low-angle thrust ramps. Understanding the Imperial anticline and its underlying thrust permits a logical linkage of an inferred deep detachment in Proterozoic strata beneath the Mackenzie Mountains with the much shallower detachment in Upper Cambrian salt beds beneath the northern Franklin Mountains. Represented on the seismic lines are at least 1.5 km of basal MMS strata that do not outcrop in Mackenzie Mountains. They probably correlate with the Pinguicula Group that is exposed in the Wernecke Mountains about 250 km to the west.

Geochemistry of the Little Dal basalts: continental tholeiites from the Mackenzie Mountains, Northwest Territories, Canada

Analyses of the Little Dal lavas and the 779 Ma Tsezotene sills, both of the Mackenzie Mountains Supergroup, Northwest Territories, Canada, show them to be continental tholeiites that are geochemically related. The plagioclase- and clinopyroxene-phyric lavas are geochemically evolved and enriched in iron (up to 20.2 wt.% as Fe2O3). Two magmatic lineages are identifiable and may represent different degrees of partial melting in the same source region, but cannot be related by fractional crystallization. Within each lineage, geochemical variation can be explained by fractional crystallization involving up to 60% crystallization of the original magma. The most evolved lavas occur at the base of the pile; less fractionated lavas occur toward the top of the sequence. The Nd isotopic composition of the Little Dal lavas averages ε = 1.4 at 780 Ma. Trace element and isotopic compositions are permissive of contamination by continental crust, but do not require a crustal component. The preserved volume of the Little Dal basalts is anomalously low (−100 km3) compared with other Proterozoic continental tholeiites. There is considerable basaltic detritus in the sedimentary rocks of the overlying Coates Lake and Rapitan groups, and much of the original lava sequence may have been eroded. The Little Dal magmatic event is interpreted to be an early manifestation of rifting of North America from Australia.

The early Neoproterozoic sedimentary Succession B of northwestern Laurentia: Correlations and paleogeographic significance

The Mesoproterozoic–Neoproterozoic stratigraphic record of ancestral North America (Laurentia) comprises three, unconformity-bounded sedimentary successions that are termed, from oldest to youngest, A, B, and C. Recent and ongoing detailed stratigraphic studies of Succession B, along with improved geochronology, allow extension and refinement of existing correlation schemes for northwestern Canada and Alaska. Succession B strata include the Shaler Supergroup of the Amundsen Basin, Mackenzie Mountains supergroup of the Mackenzie Mountains fold belt, Pinguicula group of the Wernecke Mountains inlier, Fifteenmile group of the Ogilvie Mountains inliers, and the lower Tindir Group of Tatonduk inlier. The Katakturuk Dolomite, in the northeast Brooks Range of Alaska, is included with Succession B on the basis of platformal character, geochronology, and inferred paleogeographic affinity. The framework for regional lithostratigraphic correlation of Succession B is built on recognition of four distinctive lithostratigraphic assemblages: two thick stromatolitic platformal carbonate assemblages separated by two largely subaerial siliciclastic assemblages. The correlation is supported by geochronology of detrital zircons from the upper quartzarenite assemblage, which indicates a maximum age of ca. 1000 Ma for the lower part of Succession B. These rocks are interpreted to be remnants of a northwesterly trending (present coordinates) early Neoproterozoic basin-margin promontory (Amundsen-Ogilvie-Mackenzie platform) that developed within an intracratonic basin on the northwest margin of Laurentia. The Neoproterozoic stratigraphic record of northwestern North America bears striking similarity to contemporaneous stratigraphy on other continents, particularly in the Amadeus Basin and Adelaide fold belt of central and southern Australia. Reconstructions of the Neoproterozoic supercontinent juxtapose the eastern margin of ancestral Australia against the western margin of Laurentia during the time these strata were being deposited. The Amundsen-Ogilvie-Mackenzie platform consequently may represent a segment (of the margin) of a large intracratonic basin that rifted apart with the breakup of the supercontinent during the latest Proterozoic. This hypothesis provides a template for future sequence stratigraphic, chemostratigraphic, biostratigraphic, paleomagnetic, and geochronologic comparisons and has implications for predictive economic geology in both areas. Capitalization of group and supergroup indicates formalization according to the International Stratigraphic Code. Noncapitalization indicates that the names have not been formalized.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows ( Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.

Inversion of the Proterozoic Wernecke basin during tectonic development of the Racklan Orogen, northwest Canada

New deep seismic reflection data coupled with regional stratigraphic correlations, drill-hole information, and potential field data are interpreted to provide images of Middle Proterozoic Wernecke Supergroup (meta-)sedimentary layers that were uplifted during tectonic development of the ca. 0.9–1.3 Ga Racklan Orogen in Canada's western Northwest Territories. The reflection data are located at the eastern front of the Mackenzie Mountains portion of the Canadian Cordillera and on the western flank of the Fort Simpson structural trend that is a prominent Proterozoic structure in the subsurface throughout the region. Along three parallel profiles, layers that are correlated with thick Wernecke Supergroup sedimentary rocks produce prominent reflections between about 3.0 and 9.0 s (about 7.5 and 23 km) that were arched prior to deposition of younger Proterozoic (probably Mackenzie Mountains Supergroup) and Phanerozoic sedimentary rocks. The strata are considered to be Wernecke basin sedimentary rocks that were uplifted during deformation associated with the development of the Racklan Orogen.

Racklan Orogen

The term Racklan Orogen is applied to an extensive region of deformed rocks that underlies northwestern Canada. At least one period of deformation predated extrusion of the ca. 1267–1270 Ma basalt of the Coppermine River Group, and at least two periods of deformation postdated extrusion of the Coppermine basalt. Limits on the age of deformation can be confidently placed between 1663 Ma (the age of the Narakay volcanic complex that postdates uppermost strata of the Hornby Bay Group) and 1100–800 Ma (the maximum age of the Mackenzie Mountains Supergroup).

The Proterozoic Fort Simpson structural trend in northwestern Canada

The Fort Simpson structural trend is a series of north-northwest-striking Middle Proterozoic anticlinal culminations buried beneath Paleozoic and Mesozoic cover in northwestern Canada. Interpretations of seismic reflection profiles coupled with drilling data and outcrop and potential-field observations show that the culminations are cored by ∼1.85 Ga crystalline rocks and include overlying sedimentary rocks of the ∼1.7-1.3 Ga Hornby Bay and Dismal Lakes Groups. Formation of the structures probably occurred along west-dipping thrust faults following deposition of the Dismal Lakes Group strata and prior to deposition of the ∼0.8-0.7 Ga Mackenzie Mountains Supergroup strata that unconformably overlie the Dismal Lakes rocks. This time interval coincides with major east-southeast-directed thrusting in the same rocks beneath the Anderson Plains to the north.

Strontium isotopic variations of Neoproterozoic seawater: Implications for crustal evolution

We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively welldated Mackenzie Mountains Supergroup constrain Shaler deposition to ~770–880 Ma, a range corroborated by 723 ± 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb 86Sr ratios (< 0.01) were selected for Sr isotopic analysis. δ 18 O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn Sr (≥2) and variable δ 18 O; most are dolomites. The data indicate that between ca. 790–850 Ma the 87Sr 86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr 86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr 86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (≈900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr 86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790–850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows us to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500–900 Ma. The results indicate that the hydrothermal flux reached a maximum value at ca. 830 Ma. In contrast, a large peak in erosion rate is indicated at ca. 570 Ma. The peaks in hydrothermal flux and erosion rate are most likely related to developments in the Pan-African and related orogenic events, whose initial development is characterized by production of juvenile crust during supercontinental break up and rifting. The time ca. 570 Ma is characterized by continent-continent collision and production of recycled crust. Sr isotope data from Proterozoic carbonates offer a valuable resource for understanding large-scale crust dynamics.

Geochemical signature and seismic stratigraphic setting of Coppermine basalts drilled beneath the Anderson Plains in northwest Canada

Basalts drilled in the Petro-Canada – Canterra Tweed Lake M-47 well in the Anderson Plains of northwestern Canada have geochemical characteristics, including of major, trace, and rare earth elements, that are similar to those of the most enriched Coppermine lavas but significantly different from those of the younger Proterozoic volcanics, such as the Natkusiak basalts. This result provides a geological tie and timing constraint for structures observed on seismic reflection data in this area. Correlation of stratigraphic data from the well to seismic data shows that the lavas are within a sequence of layered reflections that onlap, and are thus younger than, easterly verging structures, and that were themselves probably uplifted prior to deposition of the Mackenzie Mountains Supergroup. These relationships thus show that at least two stages of Proterozoic compressional deformation, one that predated and one that postdated the basalts, produced structures beneath the Anderson Plains.

Paleomagnetism and the origin of the Mackenzie Arc of northwestern Canada

Analysis of paleomagnetic data obtained from 1966 alternating-field treatment and from recent thermal demagnetization of the same samples of Late Proterozoic (770 Ma) diabase sills and dykes distributed about the Mackenzie Arc from northeastern British Columbia to the Alaskan border has revealed a primary magnetization in seven sites that is similar to existing data from 10 sites confined to the central Mackenzie Mountains region (N = 17 site poles; 222.2°W, 01.6°N; R = 16.73; K = 60; A95 = 5°). The diabases are confined to the dominantly clastic Late Proterozoic Tsezotene Formation and Katherine Group of the Mackenzie Mountains Supergroup. Tests of Carey's orocline hypothesis for the arc using linear regression and a plan-view application of the fold test suggest, in line with earlier structural studies, that the arc is largely nonrotational and that it is not an orocline resulting from the Cretaceous and early Tertiary Laramide Orogeny. Rather, it conforms to the arcuate foreland margin predating deposition of the Late Proterozoic Mackenzie Mountains Supergroup.

Late Proterozoic stratigraphy, U–Pb zircon ages, and rift tectonics, Mackenzie Mountains, northwestern Canada

Stratigraphic evidence suggests sporadic rifting began during deposition of the mainly platformal Mackenzie Mountains Supergroup: minor magnetite iron-formation in shale basins, lead–zinc in karsted and brecciated carbonates, red-bed and evaporite wedges, and basalts at the top. In the unconformably overlying Coates Lake Group similar climates and definite rifting are recorded by thin orthoconglomerates with thick red-bed and evaporite wedges containing stratiform copper deposits in paleovalleys. Unconformably above this, basal Windermere Supergroup records major climatic change and more emphatic rifting, with thick orthoconglomerates next to fault scarps overlain by glaciomarine deposits with volcanics and hydrothermal iron-formation.A quartz diorite plug, here dated by the U–Pb zircon method at [Formula: see text], is bounded by faults but is contained in a thrust panel together with Mackenzie Mountains Supergroup and is chemically similar to diabase sills previously dated at 766–769 ± 27 Ma (Rb–Sr). A diatreme intruding Coates Lake Group contains clasts of granite and gneiss from inferred basement. U–Pb systematics from a granite clast indicate inherited zircons about 1.6 ± 0.25 Ga in age and crystallization between 1100 and 1175 Ma, a maximum age for Mackenzie Mountains Supergroup.