Hornby Bay Research Articles

Extent and significance of the Racklan–Forward Orogen in Canada: far-field interior reactivation during Nuna assembly

Abstract Mesoproterozoic orogenesis is well established on the western and southern flanks of Laurentia in the well-known Racklan–Forward and Mazatzal orogens, but its significance within the previously assembled interior of the supercontinent Nuna has not been established. We examine regional isotopic and structural evidence for Mesoproterozoic deformation in the c. 1.7–1.63 Ga Hornby Bay, Elu, Thelon and Athabasca intracontinental basins, and present evidence for Mesoproterozoic reactivation of Paleoproterozoic structures in the Wopmay and Trans-Hudson orogens. The Racklan–Forward Orogeny in the interior of north Laurentia comprises north–south-trending, high-angle, east-vergent folds and thrusts that occur across a region 1660 km wide and over 1000 km long, stretching from the Yukon to near Hudson Bay and from Banks Island to below the Western Canada Sedimentary Basin. The structures progress from ductile amphibolite and greenschist facies in the Racklan type area to sub-greenschist facies and ultimately brittle or brittle-ductile in the far foreland, showing a predominant thick-skinned style typical of many intracontinental orogens. We present compiled low-temperature thermochronological data, including ages of spatially associated uraninite mineralization, to characterize the scope of reactivation of basement structures in the Archean Rae craton in Nuna's interior. We compare the nature of widespread far-field reactivation in the Racklan–Forward Orogen with other orogens of Nuna's assembly to show it is unusual for Nuna's peripheral margin. We suggest that c. 1.6 Ga continent–continent collision of North Australia with NW Laurentia propagated stresses far into the interior as a result of combined favourable pre-existing structural grain and a weak subcontinental lithospheric mantle in the Rae craton due to repeated episodes of refertilization across 500 Ma of accretion and intrusion. Cratons that experience the complex, two-sided collision and protracted upper plate setting during supercontinent assembly noted herein may be particularly susceptible to extensive foreland propagation of peripheral orogens.

Detrital zircon geochronology and provenance of the Mesoproterozoic Husky Creek Formation: a fluvial sandstone recording the waning stages of one of Earth's largest magmatic episodes

The Mesoproterozoic Husky Creek Formation is an interlayered redbed and basalt package that overlies a ca. 2.5 km thick, regionally extensive basaltic plateau linked to the 1.27 Ga Mackenzie Large Igneous Province. This succession provides an opportunity to study the interaction between contemporaneous fluvial sedimentation and flood basalt volcanism. The Husky Creek Formation is approximately 1900 m thick, consisting predominantly of fluvial channel and subordinate floodplain and eolian strata dominated by lithic detritus intercalated with basalt flows. U–Pb dating of detrital zircon grains collected from four stratigraphic levels in the Husky Creek Formation reveals two main age groupings: (1) a 1270 Ma peak attributed to the Mackenzie Large Igneous Province; and (2) Pre-1.6 Ga detrital zircon ages, which reflect sources from a broad region of northwestern Laurentia and are interpreted to be recycled from underlying sedimentary rocks of the Hornby Bay Basin. An upsection decrease in the proportion of ca. 1270 Ma zircon grains, coupled with increasing sandstone compositional maturity, reflects the expansion of the drainage basin during burial of a volcanic pile, with recycling becoming more dominant as the Husky Creek Formation accumulated. The Husky Creek Formation was deposited in the waning stages of the Mackenzie Igneous Event by west-flowing rivers into a geographically restricted basin above an extensive mafic volcanic plateau. This paper provides insight into fluvial basin development during the aftermath of one of Earth's largest igneous events.

On the Statherian–Calymmian palaeogeography of northwestern Laurentia

The c. 1.75 to 1.27 Ga Hornby Bay intracontinental basin, exposed in northwestern Canada, NE of Great Bear Lake, includes the Big Bear, Mountain Lake and Dismal Lakes groups. The Big Bear group comprises mainly immature clastic rocks deposited by high-energy rivers in restricted fault-bounded basins formed during the late-stage assembly of supercontinent Nuna. Detrital zircon analysis of the Big Bear group suggests provenance from a combination of local sources including recycling of sedimentary rocks, which were originally derived from sources further east, such as the Thelon orogen. The overlying Mountain Lake group was deposited by westerly flowing rivers over a much broader region, with less evidence for syndepositional faulting, suggesting thermal subsidence. It displays provenance via recycling of strata from the proximal Coronation Supergroup and the more distal and correlative Goulburn Supergroup of Kilohigok Basin. Following tectonic uplift and erosion, basal clastic rocks of the Dismal Lakes Group were deposited in fluvial and then shallow-marine to paralic environments. Detrital zircon from these strata displays prominent late Archean peaks demonstrating provenance from adjacent rocks of the Slave craton and recycling of the Mountain Lake group. Detrital zircon geochronology of sandstones from the Hornby Bay Basin supports stratigraphic correlation with the Wernecke Supergroup in the Wernecke Mountains and the Muskwa assemblage, located further southward along the ancient western margin of Laurentia. Our data, along with Nd and C isotopes from corresponding shale and carbonate units, provide robust evidence for a palaeogeographical model whereby these successions represent the terrestrial and marine components of a west-facing, passive-margin clastic wedge, considered the westward extension of an Amazon-scale, braided fluvial system that originated in the foreland of the Trans-Hudson orogen, to the east. Elements of this system have been identified from inliers in northeastern Australia, which was attached to northwestern Laurentia prior to the break-up of supercontinent Nuna. Supplementary material: U–Pb and Hf results, and Hf isotopic compositions of reference zircons are available at https://doi.org/10.6084/m9.figshare.c.5980940

Shale and pyrite Re-Os ages from the Hornby Bay and Amundsen basins provide new chronological markers for Mesoproterozoic stratigraphic successions of northern Canada

Here we present Re-Os geochronological data from two carbonaceous shale units from the Hornby Bay and Amundsen basins that provide important chronological markers for Mesoproterozoic stratigraphic successions within northern Canada. Shale from the basal Escape Rapids Formation yields a robust depositional age of 1067.3 ± 13.5 Ma for the lowermost Shaler Supergroup of Amundsen basin with a remarkably unradiogenic initial 187Os/188Os value (0.15), implying that it derived from weathering of juvenile source material, such as basalt. Sedimentological features, indicative of large basin conditions, suggest deposition within an extensive epeiric sea that was intermittently mixed with waters from an exterior ocean. This age, and the 720 Ma age of the uppermost Shaler Supergroup, constrain deposition of the ∼4 km-thick succession to a period of approximately 350 myr, recording sedimentation through a complete supercontinent cycle - amalgamation through to break-up of Rodinia. Further, it indicates that the Amundsen basin formed at the same time as the Borden, and Fury and Hecla basins on northern Baffin Island, and the Thule basin in northwestern Greenland, and that these basins were perhaps connected as part of a much larger basin situated in the interior of Rodinia. Diagenetic pyrite from sandstone dykelets in carbonaceous shales of the Fort Confidence Formation yielded a Re-Os model age of 1438 ± 8 Ma, providing a minimum age for deposition of the lower Dismal Lakes Group of the Hornby Bay basin. This age strengthens proposed regional correlation of the Hornby Bay basin with the large intracontinental Athabasca and Thelon basins, located on the Rae craton to the southeast, and is evidence for a Laurentia-wide marine flooding episode ca. 1500 Ma. Both new ages place important constraints on newly discovered eukaryotic microfossils from the Shaler Supergroup and Dismal Lakes Group and calibration points for critical chemostratigraphic data.

New constraints on genesis of the polymetallic veins at Port Radium, Great Bear Lake, Northwest Canadian Shield

The historic Eldorado U-Co-Ni-Cu-Ag veins at Port Radium are hosted by one of the arrays of northeast-trending quartz-filled brittle fault zones in the 1875–1855 Ma Great Bear magmatic zone. The veins formed in a lensoid segment of the fault, which underwent repeated reactivation that created subsidiary fractures, tensional openings, and breccia zones that were favorable for precipitation of multiple stages of ore minerals. The timing of ore precipitation is constrained by two swarms of diabase intrusions. The older ca. 1740 Ma Cleaver dikes predate the polymetallic mineralization and deposition of Mesoproterozoic strata of Hornby Bay Group. The ca. 1590 Ma Western Channel sheets postdate the main ore stages in the veins and intrude the lower strata of the Hornby Bay Group.While diabase intrusions constrain ore precipitation to 1740–1590 Ma, in situ U-Pb analyses of fine-grained uraninite record isotopic resetting at 1453 ± 18 Ma. The δ18O compositions of all uraninite stages range from −35.0 to −27.4 per mil, and reflect an overprint by late meteoric fluids. Chondrite-normalized rare earth element data for uraninite produce bell-shaped patterns centered on Sm-Dy, with slight negative Eu anomalies. The age constraints imposed by the dikes, δ18O values and REE signatures of the early U and Co-Ni sulfarsenide stages indicate that the U and base metal mineralization at Port Radium is akin to the basement-hosted, polymetallic unconformity-related U deposits in the coeval Athabasca and Thelon basins. The 1453 Ma isotopic resetting builds to the growing body of evidence for a major event in Laurentia at 1.4 Ga while the youngest Ag-Bi stage at Port Radium was likely related to intrusion of the Western Channel diabase.

The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China

The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649±14Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900Ma to 1850–1800Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.

Late Paleoproterozoic to Mesoproterozoic rift successions in SW China: Implication for the Yangtze Block–North Australia–Northwest Laurentia connection in the Columbia supercontinent

The late Paleoproterozoic to Mesoproterozoic Dongchuan Group in the southwestern Yangtze Block formed in a series of fault-controlled, rift-related basins associated with the fragmentation of the Columbia supercontinent. The Dongchuan Group is composed, from the base upward, of the Yinmin, Luoxue, Etouchang and Luzhijiang formations. The Yinmin Formation has a basal layer of conglomerate and coarse sandstone, up to 2m thick, marking initiation of rifting. It is a laterally discontinuous layer, indicative of lateral variations in the accommodation space created adjacent to juvenile normal faults in the earliest stages of rifting. Fluvial to intertidal facies sedimentary rocks of the Yinmin Formation are characterized by fining-upward patterns from sandstone to interbedded carbonate and mudstone, representing syn-rifting sedimentation. Enlargement of the basin and establishment of a carbonate platform during sedimentation of the Luoxue Formation suggest that faulting and volcanism were significantly reduced, marking a transition from syn-rifting subsidence to post-rifting subsidence. Black carbonaceous shale of the Etouchang Formation likely formed in a continental slope and deep ocean basin, generated by slow thermal subsidence of heated subcrustal materials during the post-rifting stage. Subsequent shoaling led to re-establishment of a rimmed carbonate shelf, in which massive argillaceous dolostone of the Luzhijiang Formation started to accumulate. Two major igneous events marked the ca. 1710-Ma rifting climax and 1685–1660-Ma waning phase of rifting. The rift sequence in the southwestern Yangtze Block correlates with the late Paleoproterozoic to early Mesoproterozoic sequence in the Mount Isa Basin, North Australia. These successions have identical zircon age patterns and similar igneous and sedimentary assemblages. In addition, the rifting-related 1735–1663Ma Hornby Bay and ~1640–1600Ma Wernecke successions in northwestern Laurentia show similarities with those synchronous successions in the Yangtze Block and North Australia. This correlation suggests that the Yangtze Block was likely connected to northern Australia and northwestern Laurentia during the assembly of Columbia supercontinent but drifted away from these continents before ca. 1590Ma and showed tectonic evolution different from them after that.

Sequence stratigraphy, provenance, C and O isotopic composition, and correlation of the late Paleoproterozoic–early Mesoproterozoic upper Hornby Bay and lower Dismal Lakes groups, NWT and Nunavut

Late Paleoproterozoic basins of Canada's north, including the Thelon, Athabasca, and Hornby Bay basins, can be broadly correlated based on similarities in age, sequence stratigraphy, paleocurrents, and detrital zircon provenance, but their detailed evolution is poorly understood and their age and lateral equivalencies are poorly constrained. This study contributes to the understanding of sedimentary basins during a critical interval of time, representing the latest stages of amalgamation of the supercontinent Laurentia/Nuna, through a detailed study of the Hornby Bay Basin. The Hornby Bay Basin strata have been divided into four unconformity-bounded subsequences, A1, A2, A3, A4, that belong to the first of three unconformity-bounded post-Hudsonian Proterozoic stratigraphic packages in northwestern Canada. Subsequences A2 (>1600Ma) and A3 (<1600–1270Ma) of the Hornby Bay Basin were intersected during diamond drilling for uranium exploration. Sequence stratigraphic interpretations reveal that the subsequence A2–A3 boundary lies below the LeRoux Formation, which forms the base of the Dismal Lakes Group, a critical relationship that was previously disputed. Stratigraphic patterns in the late stages of development of the Hornby Bay Basin are considered to have been largely controlled by the Paleoproterozoic Forward Orogeny (∼1600Ma), rather than by global eustasy.Analysis of Sm–Nd isotopes on mudrocks yielded isotopic compositions in the range −8.2 to −2.3 (ɛNd(1600Ma)) in subsequence A2 and −9.8 to −5.0 (ɛNd(1400Ma)) in subsequence A3. An ɛNd versus time graph illustrates two distinct groupings that plot within known isotopic evolution fields of the Taltson-Thelon Tectonic Zone, and the Great Bear Magmatic Zone. Sediment provenance was strongly influenced by the Paleoproterozoic Forward Orogeny, but there are no significant changes in sedimentary provenance across the A2–A3 sequence boundary. Stable isotope analyses of C and O in the dolomitic East River Formation of subsequence A2 reveal a relatively monotonous signal with an average of −0.5‰ δ13C and −8.76‰ δ18O, which is consistent with global values of other late Paleoproterozoic carbonate successions. Enriched Mn values and depleted Sr values, however, suggest that the East River carbonate rocks experienced some degree of meteoric alteration; it is unknown how much this alteration affected stable isotope compositions.Based on similarities in sedimentary provenance, age, and sequence stratigraphy, subsequence A2 of the Hornby Bay Group is correlated with the Wernecke Supergroup (Yukon). The latter is interpreted to represent the deep-water distal equivalent of the upper Hornby Bay Group in a westward-deepening basin. Deposition of subsequence A2 is recorded across North America and is mirrored by deep-water sedimentation along the eastern margin of the North Australian craton. The tectonic evolution of subsequence A2 in Laurentia is therefore remarkably similar to that of the North Australian craton up to ∼1600Ma, the age of the Forward Orogeny, and this has implications for interpretation of the latest stages of accretion of the supercontinent Nuna.

U–Pb geochronology of the Western Channel Diabase, northwestern Laurentia: Implications for a large 1.59 Ga magmatic province, Laurentia's APWP and paleocontinental reconstructions of Laurentia, Baltica and Gawler craton of southern Australia

U–Pb baddeleyite ages of 1592 ± 3 and 1590 ± 4 Ma are reported for paleomagnetic sites in sheets and dykes of Western Channel Diabase (WCD) that intrude Proterozoic rocks of the flat-lying Hornby Bay Group in the Hornby Bay basin and the deformed volcanic-plutonic Great Bear Magmatic Zone of Wopmay orogen of northwestern Laurentia. A published WCD paleomagnetic pole at 9°N, 115°W ( A 95 = 6°) has been demonstrated primary. The new ages indicate that the WCD pole falls midway in time between poles for the 1.74 Ga Cleaver dykes and 1.48–1.42 Ga Elsonian-aged plutons, filling an important gap in the Proterozoic apparent polar wander path (APWP) for Laurentia. The WCD pole can be compared with poles reported from similar-aged magmatic units on other cratons in order to test paleocontinental reconstructions. A comparison of the Laurentian WCD pole with primary ca. 1.63 Ga and ca. 1.575 Ga poles for Baltica, along with an earlier comparison of precisely dated 1.27–1.255 Ga poles for Laurentia and Baltica, suggests that the two cratonic blocks drifted as a single entity with Baltica adjacent to eastern Greenland during the ca. 1.59–1.27 Ga interval. On the basis of less well constrained ca. 1.84–1.83 Ga poles from Laurentia and Baltica, it is possible that this reconstruction existed as early as ca. 1.83 Ga. The WCD is the same age as Wernecke breccias of the Wernecke and Ogilvie Mountains of northwestern Laurentia and bimodal Gawler Range Volcanics (GRV) and related Olympic Dam breccias of the Gawler craton. It has been proposed by others that the Gawler craton lay adjacent to northwestern Laurentia at 1.59 Ga, with the Olympic Dam and Wernecke breccias forming a large hydrothermal province. The primary WCD pole provides an opportunity to test Laurentia–Gawler craton reconstructions at 1.59 Ga. A paleopole has been reported for the GRV, although its primary or secondary nature is open to interpretation. If primary, or if acquired as an overprint during the later stages of 1.60–1.58 Ga Hiltaba-GRV magmatism, then a position for the Gawler craton adjacent to northwestern Laurentia is permitted. If the GRV pole is a later secondary overprint then a reliable comparison with Laurentian poles cannot be made.

Diagenetic fluorapatite and aluminum phosphate–sulphate in the Paleoproterozoic Thelon Formation and Hornby Bay Group, northwestern Canadian Shield

In the northwestern part of the Canadian Shield, fluorapatite and a rare-earth element-bearing hydrated aluminum phosphate–sulphate mineral (APS) occur as cements in continental successions near the base of the Paleoproterozoic Thelon Formation (Thelon Basin) and Hornby Bay Group (Hornby Bay Basin). These minerals occupy interstitial sites, form euhedral crystals, display micro-scale zonation, make up part of an unmetamorphosed paragenetic assemblage, and are distributed in correlative units across thousands of square kilometres, suggesting a diagenetic origin. Stratigraphy, geochronology, and other lines of evidence suggest that the Thelon Formation and Hornby Bay Group containing these phosphatic cements, as well as the Ellice Formation and Athabasca Group, are correlative and may have been originally interconnected. The evidence suggests that the basal Thelon Formation and the Hornby Bay Group underwent similar, and approximately coeval, diagenetic mineral paragenesis. Furthermore, the diagenetic fluids in these different locations must have been remarkably similar, especially those that produced the delicate APS mineral. Compared to phosphatic cements in the Hornby Bay and Thelon basins, unmineralized sandstone in the Athabasca Basin contains "crandallite group" and fluorapatite cements higher in the basin fill sequence (Wolverine Point Formation) in tuffaceous sandstone and as relatively early cement in the paragenetic sequence.

Revisions to the Paleoproterozoic Sequence A, based on reflection seismic data across the western plains of the Northwest Territories, Canada

Abstract New insights into Proterozoic Sequence A were gained from analysis of reflection seismic data acquired across the western plains of the Northwest Territories, Canada, a re-examination of geological maps of the Coppermine Homocline, and recently published studies in the Cordillera. Results show that the sequence contains four unconformity-bounded sub-sequences and should be, therefore, subdivided. They are informally named here, from oldest to youngest, as A1, A2, A3 and A4. Sequence A1 is laterally restricted and consists of the Bigbear and Fault River formations of the Hornby Bay Group on Coppermine Homocline, seismic Sequence A1 in the subsurface, and is potentially equivalent to the very thick Wernecke Supergroup of the Cordillera. Sequence A2 on Coppermine Homocline consists of the clastic Lady Nye Formation, carbonates of the East River Formation, and the syntectonic clastics of the Kaertok and LeRoux formations of the Hornby Bay Group. Sequence A2 in the subsurface consists of the Basinal, Platformal, and Syntectonic seismic units and does not correlate with strata of the Cordillera. A2 strata in the subsurface and on the homocline were deformed by the intracratonic style Forward Orogeny (ca. 1.66 Ga), which is likely an expression of the Racklan Orogeny, described from the Cordillera. Sequence A3 consists of the Dismal Lakes Group on Coppermine Homocline, seismic Sequence A3 in the subsurface and the Pinguicula Group in the Cordillera. It overlies a regional unconformity that bevelled the underlying Forward Orogeny and Racklan Orogeny structures and completely removed Sequence A2 strata from the region of the present Cordillera. Sequence A4 is made up of the Coppermine River Group on the homocline and the Tweed Lake basalts in the subsurface. No correlative strata have been reported in the Cordillera.

The intracratonic Paleoproterozoic Forward orogeny, and implications for regional correlations, Northwest Territories, Canada

Reflection seismic data from the Colville Hills and Anderson Plain document an intracratonic compressional event, herein named the Forward orogeny, which affected strata considered equivalent to the &gt;1663 Ma Hornby Bay Group outcropping on Coppermine Homocline. Forward orogeny structures were peneplaned and unconformably overlain by strata considered equivalent to the &gt;1267 Ma Dismal Lakes Group. A comparable tectonic history is recorded in the exposed rocks of Coppermine Homocline to the east. Structural orientations indicate a general northwest–southeast direction of maximum compression during an early phase and a more west–east direction in a later phase. Regional sequence A of G.M. Young and co-workers is subdivided into preorogenic sequence A1 (Hornby Bay and equivalents), and postorogenic sequences A2 (Dismal Lakes and equivalents) and A3 (Coppermine River group and equivalents). The Forward orogeny, dated as approximately 1663 Ma (the age of the syntectonic Kaertok Formation on Coppermine Homocline), is not related to the 1.84–1.9 Ga Wopmay Orogen. Relationships between the Forward orogeny and the Racklan orogeny, in the Wernecke and Ogilvie mountains, remain unresolved because the age of the Racklan is uncertain.

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.

Seismic reflection trace synthesized from Proterozoic outcrop and its correlation to seismic profiles in northwestern Canada

Geological descriptions of the Proterozoic Dismal Lakes and Hornby Bay Group strata northeast of Great Bear Lake in Canada's Northwest Territories were used to compute a synthetic seismic reflection trace that was then correlated with reflection data recorded in the Colville Hills region to the west. Reflectivity of the synthetic trace shows a striking similarity to reflections on the recorded profiles at travel times appropriate for the stratigraphic level of the Dismal Lakes and Hornby Bay groups. The similarity suggests that the method of synthesizing traces from outcrop information can be a valuable tool in areas where well control or other direct ties are lacking. Calibration of the Colville Hills seismic profiles with the synthetic trace indicates that the reflections are best interpreted in terms of the observed regional stratigraphy and, along with geometries observed on more structurally complex reflection profiles immediately west of the Colville Hills, affirms an interpretation of Proterozoic layer-parallel thrust faulting in the area. From this information it follows that a deep detachment may occur near the top of the Hornby Bay Group, in either the Kaertok or Lady Nye formations and that the autochthonous basement (probably 1.8–2.0 Ga Hudsonian) is situated about 1.0–1.5 s (about 3.0–4.5 km) beneath the lowest observed detachment in the Colville Hills region.

Cool climate bauxite

The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.

Eruptive style and construction of shallow marine mafic tuff cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Northwest Territories, Canada)

Abstract The Early Proterozoic (1663 Ma) Narakay Volcanic Complex, exposed in Great Bear Lake (Northwest Territories, Canada), is a bimodal suite of basalt and rhyolite erupted in a continental setting and consisting largely of pyroclastic rocks interlayered with shallow marine sedimentary rocks of the Hornby Bay Group. Mafic pyroclastic rocks consist of lapilli tuff, tuff, tuff breccia and agglomerate that represent the remnants of small subaerial tuff cones (0.5 to 2 km in diameter) that in most cases have subsided into the volcanic conduit. Stratification styles, sedimentary structures and grain morphologies in pyroclastic rocks reflect variations in the water:magma ratio during eruptions and have been used to help elucidate eruptive mechanisms and reconstruct volcanic edifices. Basaltic pyroclasts are commonly bounded by fracture surfaces and are morphologically similar to modern pyroclasts produced by thermal quench fragmentation or steam-blast disruption of magma. Most fragments have low vesicularity and scoria is only locally abundant which indicates that eruptive energy was supplied mostly by water—melt interaction rather than exsolution of magmatic gases. Cored bombs and lapilli, fusiform bombs, and pyroclasts similar in texture to those of Strombolian cinder and agglutinate spatter, are uncommon but are stratigraphically widespread and imply the occurrence of Strombolian eruptions, presumably when water access to the vent was impeded. Massive bedding is typical of the tuffs and, in addition to the poorly sorted ash-rich nature of the tuffs, implies deposition from water- and/or steam-rich hydrovolcanic eruption clouds and cypressoid jets by airfall and dense pyroclastic flows. Uncommon well-stratified and sorted ash and lapilli tuff record airfall and pyroclastic flow(?) deposition from eruption clouds rich in magmatic gases. Base surge deposits are uncommon and occur only in the subaerial portion of a sequence of tuffs inferred to record the progradation of a cone-margin surge platform into standing water. Few of the tuff cone deposits display a systematic vertical sequence of stratification styles, structures and grain morphologies. This indicates that either the eruptive style varied irregularly between hydrovolcanic and Strombolian and/or that pyroclasts of different origin were mixed during eruptions.

Geochronology of the Narakay Volcanic Complex: implications for the age of the Coppermine Homocline and Mackenzie igneous events

U–Pb isotope analysis of four zircon separates from a rhyolite porphyry that is part of the bimodal Narakay Volcanic Complex (Dease Arm, Great Bear Lake) yields an age of 1663 ± 8 Ma. East-trending feldspar-phyric dykes that cut the complex yield zircons that are, within analytical error, the same age as the porphyry. The Narakay Volcanic Complex occurs in the upper part of the Hornby Bay Group, the lowermost group of the Coppermine Homocline, and the stratigraphic position of the dated rocks indicates that 1663 Ma is the depositional age of the upper Hornby Bay Group.This is the first U–Pb zircon age determined from rocks of the Coppermine Homocline and has important implications for previous geochronology and extrabasinal correlation. Earlier Rb–Sr dates from the conformably overlying Coppermine River Group basalts of ca, 1200 Ma, when taken together with the new data, suggest that the Dismal Lakes Group, which conformably overlies the Hornby Bay Group, was deposited during a period of more than 400 Ma. Given that the Dismal Lakes Group is a relatively thin, transgressive shallow platformal succession, this amount of time for deposition is interpreted to be excessive. If this interpretation is correct, then it follows that either there are many unrecognized diastems in the Dismal Lakes Group, or presently available dates for rocks of the Mackenzie magmatic event in the Coppermine Homocline region may be too young by several hundred million years.

Microfossils from the Early Proterozoic Hornby Bay Group, District of Mackenzie, Northwest Territories, Canada

Spheroidal and filamentous organic-walled microfossils have been detected in ca. 1.7 Ga old cherts of the Hornby Bay Group, Northwest Territories, Canada. The majority of the spheroidal forms range from 1 to 4 μm in diameter, are referable to the genus Sphaerophycus, and probably represent the preserved sheaths of chroococcacean cyanophytes. A single, robust-walled, 27 μm diameter, spheroidal microfossil of undetermined affinity is also present. The filamentous forms are tubular, unbranched, and range from 1 to 8 μm in diameter. They appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria. The filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic-walled microfossils. Where they occur in silica the filaments are preserved as three-dimensional tubular micro-structures, which are readily recognized as microfossils. In contrast, where they extend from silica into adjacent dolomite they are highly compressed and not readily discernable as microfossils. This compression appears to have been caused by aggrading neomorphism and pressure dissolution of the carbonate minerals, and it illustrates the contribution of diagenesis, in addition to decomposition of organic material, in causing the paucity of microfossils in Precambrian carbonate rocks.

Geochemical exploration for unconformity-type uranium deposits in permafrost terrain, Hornby bay basin, Northwest territories, Canada

The Hornby Bay sandstone basin of the Northwest Territories represents one of three areas in Canada in which the geological environment is favourable for the occurrence of unconformity-type uranium deposits. The project area lies within the zone of continuous permafrost and is associated with relatively thin and locally derived tills and abundant outcrops. Topographic relief is normally in the order of 100 m but may locally exceed 300 m/km in more rugged areas. Geochemical case histories for soil and lake-sediment surveys illustrate: identification of regional anomaly, anomaly follow-up, and detailed investigations. Success in locating uranium occurrences using geochemical surveys is high. Orientation studies trace anomalous dispersion trains 10 to 200 m downslope of recognized sources. Anomaly generation is favoured by active hydromorphic processes in geochemically homogeneous overburden containing an average background of 0.5 ppm uranium (using a 4 N nitric acid leach). Anomaly contrasts are typically 2 X to 3 X this value, and anomalous values exceeding 1.5 ppm uranium are highly reproducible. Copper and radium anomalies act as pathfinders for uranium. Lead is present in anomalous concentrations in specimens from the uranium occurrences but lead enrichment in overburden is not sufficiently great to permit its use as a pathfinder element. Preliminary work based on radon and 210Po has not led to identification of anomalies other than those recognized by more routine geochemical surveys. Association of geochemical anomalies with faults within the sandstone basin constitutes one of four conditions elevating an area to drill target status. Methods for drill testing of geochemical soil anomalies depend on the relationship between the geochemical anomaly and an identifiable fault zone.