Canadian Cordillera Research Articles

Thermal history of the Mackenzie Plain, Northwest Territories, Canada: Insights from low-temperature thermochronology of the Devonian Imperial Formation

Abstract Devonian strata from the Mackenzie Plain, Northern Canadian Cordillera, have undergone two major burial and unroofing events since deposition, providing an excellent natural laboratory to assess the effects of protracted cooling history on low-temperature thermochronometers in sedimentary basins. Apatite and zircon (U-Th)/He (AHe, ZHe) and apatite fission track (AFT) thermochronology data were collected from seven samples across the Mackenzie Plain. AFT single grain ages from six samples span the Cambrian to Miocene with few Neoproterozoic dates. Although there are no correlations between Dpar and AFT date or track length distribution, a relationship exists between grain chemistry and age. We calculate the parameter rmr0 from apatite chemistry and distinguish up to three discrete kinetic populations per sample, with consistent Cambrian–Carboniferous, Triassic–Jurassic, Cretaceous, and Cenozoic pooled ages. Fifteen ZHe dates range from 415 ± 33 Ma to 40 ± 3 Ma, and AHe dates from 53 analyses vary from 225 ± 14 Ma to 3 ± 0.2 Ma. Whereas several samples exhibit correlations between date and radiation damage (eU), all samples demonstrate varying degrees of intra-sample date dispersion. We use chemistry-dependent fission track annealing kinetics to explain dispersion in both our AFT and AHe data sets and detail the thermal history of strata that have experienced a protracted cooling history through the uppermost crust. Thermal history modeling of AFT and AHe samples reveals that the Devonian strata across the Mackenzie Plain reached maximum burial temperatures (∼90 °C–190 °C) prior to Paleozoic to Mesozoic unroofing. Strata were reheated to lower temperatures in the Cretaceous to Paleogene (∼65 °C–110 °C), and have a protracted Cenozoic cooling history, with Paleogene and Neogene cooling pulses. This thermal information is compared with borehole organic thermal maturity profiles to assess the regional burial history. Ultimately, these data reflect the complications, and possibilities, of low-temperature thermochronology in sedimentary rocks where detrital variance results in a broad range of diffusion and annealing kinetics.

Paleo-glacier reconstruction in southwestern British Columbia, Canada: A glaciovolcanic model

Volcanic rocks erupted within glacial environments offer powerful paleoenvironmental insights. These glaciovolcanoes and their deposits serve as proxies that inform on the paleo-presence, distribution and thickness of past glaciers. We investigate the paleoenvironmental implications of three Mid to Late Pleistocene volcanic deposits in the Garibaldi volcanic belt of southwestern British Columbia, Canada. We use these data to inform a simple geometric model that constrains paleo-glacier distributions in the southern Canadian Cordillera. The three volcanoes are used to recover: i) a coalesced mountain ice sheet in late Marine Isotope Stage (MIS) 4, and possibly into MIS 3 (up to ∼49.1 ± 5.5 ka), ii) major and rapid degradation of glaciers during the late, MIS 6 to 5e transition (by 141 ± 12.9 ka), and, iii) a coalesced mountain ice sheet, or continental ice sheet that existed during MIS 15 (598 ± 7.5 ka).

Terrenos tectonoestratigráficos e/ou “Maciços”

Este trabalho é uma síntese acerca do conceito de terrenos tectônico-estratigráficos.O retrospecto histórico do tema é particularmente importante para o entendimento real do conceito; implica o reconhecimento de diferentes fases de evolução do conhecimento geológico:(a) Fase preliminar e longa de discriminação e reconhecimento destes elementos tectônicos por aqueles geólogos chamados de geossinclinalistas / fixistas[~ 1900-1990], todos eles apoiados somente em argumentos descritivos.(b) Fase curta [~1970-1990] de proposição dos primeiros modelos de Tectônica de Placas, quando foram omitidas as muitas ocorrências reconhecidas dos terrenos/“maciços”, porque não podiam se ajustar aos esquemas simplistas dos primeiros modelos, ditos plaquistas.(c) Amplo desenvolvimento do conceito, desde as observações verificadas inicialmente na cordilheira norte-americana ocidental pelos geólogos do United States Geological Survey, USGS (especialmente de 1990-1995) até o presente, pari passu com o marcante enriquecimento dos diferentes e novos suportes alcançados, os quais culminaram com a (atual)admissão de uma Tectônica Global. Como consequência da última fase, quando o reconhecimento da presença de terrenos foi feito praticamente em todos os continentes e em diferentes colagens orogênicas, o conceito deve estar presente como tema obrigatório e usual dos artigos científicos com suporte geológico.

Seismic evidence for a mantle suture and implications for the origin of the Canadian Cordillera

The origin of the North American Cordillera and its affinity with the bounding craton are subjects of contentious debate. The mechanisms of orogenesis are rooted in two competing hypotheses known as the accretionary and collisional models. The former model attributes the Cordillera to an archetypal accretionary orogen comprising a collage of exotic terranes. The latter, less popular view argues that the Cordillera is a collisional product between an allochthonous ribbon microcontinent and cratonic North America. Here we present new seismic evidence of a sharp and structurally complex Cordillera–craton boundary in the uppermost mantle beneath the southern Canadian Cordillera, which can be interpreted as either a reshaped craton margin or a Late Cretaceous collisional boundary based on the respective hypotheses. This boundary dips steeply westward underneath a proposed (cryptic) suture in the foreland, consisent with the predicted location and geometry of the mantle suture, thus favoring a collisional origin.

Tectonics of the northern Canadian Cordillera imaged using modern magnetotelluric analysis

Magnetotelluric data from LITHOPROBE SNORCLE Corridor 2 in western Canada crossing the Rocky Mountain Trench-Tintina Fault between 56° and 60°N have been analyzed using modern MT methods to provide new insights into the tectonics of the North American Cordillera and Ancestral North America. Resistivity images of the crust were obtained using two independent sets of unconstrained 2-D inversions and constrained 2-D inversions incorporating seismic constraints. Structure to the west of the Tintina Fault includes uniform, moderate, mid to lower crustal resistivity in a seismically-defined westward-tapering wedge of Ancestral North American rocks. To the east, between the Tintina Fault and the Liard line, a transfer zone in asymmetric Neoproterozoic rifting, the wedge includes a major synformal conductor that parallels seismic reflections. The large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting. The synformal conductor is attributed to graphitic rocks from the Aida Formation of the Mesoproterozoic Muskwa Assemblage that were re-distributed in a sag basin on the lower plate margin in the Blackwater Block. Spatially-complex upper crustal resistivity in the Cordillera can be interpreted in terms of lithological variations and alteration processes. An east-dipping conductor in Stikinia is attributed to east-dipping Jurassic and relatively high conductivity beneath the Eskay rift may be explained by crustal fertilization. A conductor in the Cache Creek terrane is attributed to listwanite alteration of ultramafic bodies on the King Salmon Fault, and high conductivity in the Slide Mountain terrane within the Sylvester Allochthon to localized carbonaceous alteration. A series of conductors above North American Basement is attributed to deformation and metamorphic processes associated with Mesozoic thrust faulting. To the west of the Tintina Fault the source material for these conductors is likely from the Stikinia terrane, and to the east it is likely from the Muskwa Assemblage.

Accelerating exhumation in the Eocene North American Cordilleran hinterland: Implications from detrital zircon (U-Th)/(He-Pb) double dating

AbstractBasins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve records of deformation that are unavailable in footwall rocks. Following prolonged late Mesozoic–early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth’s best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105–740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4–6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45–43 Ma strata and to <10 m.y. in 43–41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1–20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains–East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere.

Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: New zircon U-Pb age and whole-rock Nd- and Hf-isotope and lithogeochemical results from the Kechika group, Yukon, Canada

AbstractPost-breakup magmatic rocks are recognized features of modern and ancient passive margin successions around the globe, but their timing and significance to non-plume-related rift evolution is generally uncertain. Along the Cordilleran margin of western North America, several competing rift models have been proposed to explain the origins of post-breakup igneous rocks that crop out from Yukon to Nevada. New zircon U-Pb age and whole-rock geochemical studies were conducted on the lower Paleozoic Kechika group, south-central Yukon, to test these rift models and constrain the timing, mantle source, and tectonic setting of post-breakup magmatism in the Canadian Cordillera. The Kechika group contains vent-proximal facies and sediment-sill complexes within the Cassiar platform, a linear paleogeographic high that developed outboard of continental shelf and trough basins. Chemical abrasion (CA-TIMS) U-Pb dates indicate that Kechika group mafic rocks were generated during the late Cambrian (488–483 Ma) and Early Ordovician (473 Ma). Whole-rock trace-element and Nd- and Hf-isotope results are consistent with the low-degree partial melting of an enriched lithospheric mantle source during margin-scale extension. Equivalent continental shelf and trough rocks along western North America are spatially associated with transfer-transform zones and faults that were episodically reactivated during Cordilleran rift evolution. Post-breakup rocks emplaced along the magma-poor North Atlantic margins, including those near the Orphan Knoll and Galicia Bank continental ribbons, are proposed modern analogues for the Kechika group. This scenario calls for the release of in-plane tensile stresses and off-axis, post-breakup magmatism along the nascent plate boundary prior to the onset of seafloor spreading.

The thermochronologic record of erosion and magmatism in the Canyonlands region of the Colorado Plateau

The Cenozoic history of crustal deformation, surface uplift, and erosion of the central Colorado Plateau (southwestern USA) is distinct from and relatively poorly understood compared with surrounding regions in the North American Cordillera, including the Grand Canyon region and the southwestern Plateau margin. Here, we present new apatite thermochronologic results from Paleozoic-Mesozoic sandstones sampled in the Canyonlands region—the interior of the Colorado Plateau in southeastern Utah. The apatite (U-Th-Sm)/He (He) ages are highly dispersed, with both positive-slope and negative-slope age-effective U (eU) trends. Samples with apatite He results suitable for thermal history modeling are from the Abajo and La Sal mountains intrusive complexes and the Permian, Triassic, and Jurassic rocks sampled near the Colorado River at Lees Ferry, Arizona, and Hite, Utah. Samples with the richest thermal history information have positive-slope apatite He age-[eU] trends, with ages ca. 10 to 5 Ma at [eU] 60 ppm. Modeled thermal histories that best predict this pattern require two periods of rock cooling: one during the middle Cenozoic ca. 30 to 20 Ma and the other since the latest Miocene ca. 6 Ma. The most recent cooling documents the transition from a slowly eroding Miocene landscape to recent rapid erosion that likely postdates 6 Ma and the integration of the modern Colorado River. Middle Cenozoic rock cooling can be attributed to either ∼1 km of erosion between ca. 25 and 15 Ma, as previous workers have suggested in other parts of the Colorado Plateau region, or relaxation of a geothermal gradient transiently doubled by magmatism associated with the vigorous magmatic flare-up that swept through the region ca. 34 to 20 Ma. The ambiguity of using middle Cenozoic rock cooling as a proxy for erosion in this region means that this event should be described using a nongenetic term: the Middle Cenozoic Cooling Event.

Hydrological Responses of Headwater Basins to Monthly Perturbed Climate in the North American Cordillera

Abstract How mountain hydrology at different elevations will respond to climate change is a challenging question of great importance to assessing changing water resources. Here, three North American Cordilleran snow-dominated basins—Wolf Creek, Yukon; Marmot Creek, Alberta; and Reynolds Mountain East, Idaho—each with good meteorological and hydrological records, were modeled using the physically based, spatially distributed Cold Regions Hydrological Model. Model performance was verified using field observations and found adequate for diagnostic analysis. To diagnose the effects of future climate, the monthly temperature and precipitation changes projected for the future by 11 regional climate models for the mid-twenty-first century were added to the observed meteorological time series. The modeled future was warmer and wetter, increasing the rainfall fraction of precipitation and shifting all three basins toward rainfall–runoff hydrology. This shift was largest at lower elevations and in the relatively warmer Reynolds Mountain East. In the warmer future, there was decreased blowing snow transport, snow interception and sublimation, peak snow accumulation, and melt rates, and increased evapotranspiration and the duration of the snow-free season. Annual runoff in these basins did not change despite precipitation increases, warming, and an increased prominence of rainfall over snowfall. Reduced snow sublimation offset reduced snowfall amounts, and increased evapotranspiration offset increased rainfall amounts. The hydrological uncertainty due to variation among climate models was greater than the predicted hydrological changes. While the results of this study can be used to assess the vulnerability and resiliency of water resources that are dependent on mountain snow, stakeholders and water managers must make decisions under considerable uncertainty, which this paper illustrates.

Mapping Curie Depth Across Western Canada From a Wavelet Analysis of Magnetic Anomaly Data

AbstractIn western Canada, geophysical studies infer an abrupt change in crustal temperatures between the Canadian Cordillera and the adjacent North American craton, with important implications for the tectonics and geodynamics of the area. We use a wavelet analysis of magnetic anomaly data in western Canada to map the depth to the bottom of the magnetic source, or Curie depth. This depth corresponds to the point at which crustal rocks reach their Curie temperature, thus providing an estimate of geothermal gradient. Our model is defined by a fractal distribution of magnetization characterized by the parameter β, as well as the depths to the top (zt) and bottom (zb) of the magnetized layer. Synthetic tests reveal the increased accuracy of the estimated zb when values for zt and β are fixed prior to the inversion. We set zt to the thickness of sedimentary rocks overlying the magnetic bedrock and use various values of β to estimate zb. We determine β a posteriori by comparing Monte Carlo simulations of predicted heat flow values (assuming a Curie temperature of 580 °C) with observed heat flow in various regions. Our results suggest a β value of 2.5 for the Canadian Cordillera and Slave craton, and 2 for the remaining North American craton. The Curie depths resolve geological domains and important structural features, with estimates for zb averaging 15 ± 1 km in the Cordillera, 32 ± 3 km in the Slave craton, and 34 ± 3 km in the North American craton to the south.

Evaluation of mineral exploration targets defined by airborne gravity gradiometry through gravity and magnetic modelling: vicinity of the Iron Range Fault, Purcell anticlinorium, southern Canadian Cordillera

An airborne gravity gradiometer survey was recently flown over the Iron Range Fault in the Purcell anticlinorium, southern Canadian Cordillera. The fault is commonly associated with iron oxide mineralization having characteristics similar to those of iron oxide Au ± Cu deposits. Drilling near the fault has revealed Au ± Cu–Pb–Zn mineralization. Prominent positive vertical gravity gradient (VGG) anomalies defined by the survey were identified as targets for follow-up exploration. Possible sources of the target anomalies were investigated by modelling gravity, VGG, and magnetic data along several profiles. Modelling of regional-scale profiles of the vertical component of gravity crossing exploration targets provides a regional perspective on the regional geological setting, dominated by the broad Goat River anticline, whose axis closely follows the Iron Range Fault. Modelling indicates that several VGG anomalies are related to Moyie sills, although one anomaly is modelled as a narrow vertical body (120 m wide, 1000 m vertical extent, 40 m deep) just west of the Iron Range Fault. Its apparent high density of 3500 kg/m3 suggests metallic content, making it a choice candidate for follow-up investigation. Drilling at the southern end of this geophysical target intersected a Moyie intrusion, but untested geochemical anomalies in the vicinity encourage follow-up exploration. The densities of modelled units derived from VGG profiles across two other specific targets indicate that Moyie sills represent one target and iron oxide mineralization the other, as supported by magnetic modelling, which also delineated vertical zones of significantly magnetic material along the Iron Range Fault.

Seismic Evidence for Lithospheric Thinning and Heat in the northern Canadian Cordillera

AbstractGeophysical and geological data indicate that the lithosphere in the northern Canadian Cordillera (NCC) is thin and hot. However, lack of direct constraints on mantle structure is fueling debates on the origin and nature of this lithosphere. Here we image the crust and uppermost mantle beneath the NCC and resolve the Moho at ~35‐km depth, the lithosphere‐asthenosphere boundary ~15 km deeper, and a westward dipping structure in the mantle below the eastern NCC. We examine end‐member tectonic models where the thin lithosphere is either long‐lived or has been rejuvenated by gravitational removal. In the long‐lived model, steady state thermal calculations indicate a Moho at ~1,000 °C; alternatively, time‐dependent calculations suggest that a removal event 5–25 Myr ago reproduces the seismic constraints. Combined with geologic evidence, these results suggest that the mantle lithosphere beneath the NCC is either exotic or the result of progressive thickening and cooling since the Miocene.

Ammonoid biostratigraphy and paleobiogeography of Lower Jurassic sections from Sonora (northwest Mexico) and their tectonic implications

The Antimonio terrane is a tectonostratigraphic unit known from the northwestern part of Sonora state of Mexico, where it is developed on the Proterozoic to Permian basement of the Caborca block. The Upper Permian to Lower Jurassic El Antimonio Group, which includes the Lower Jurassic Sierra de Santa Rosa Formation, is its most extensive stratigraphic unit. Biostratigraphic dating of the thick Lower Jurassic sedimentary succession is crucial for the reconstruction of the basin evolution and tectonic history. However, the Early Jurassic ammonite faunas have only been partially treated in previous works; therefore a comprehensive paleontological and biostratigraphical study was carried out.Ammonoids were collected from sections exposing the Sierra de Santa Rosa Formation, including Sierra del Álamo (type locality of the El Antimonio Group), Sierra de Santa Rosa, Pozos de Serna, and Sierra la Jojoba. From 455 moderately preserved specimens, a total of 49 taxa were distinguished, 29 of them identified at the species level, which belong to 22 genera.The ammonoid assemblages represent three stages of the Lower Jurassic, from the Upper Hettangian through the Sinemurian to the Lower Pliensbachian. The provincialism of the fauna tests the recently developed North American regional ammonoid zonation: out of eleven relevant zones, eight were recognized and documented in the studied sections. Stratigraphically from oldest to youngest, these include the Upper Hettangian Rursicostatum Zone, the Lower Sinemurian Involutum and Leslei Zones, and the Upper Sinemurian Carinatum Zone that are identified in Sierra del Álamo. Paltechioceras-dominated assemblages of the uppermost Sinemurian Harbledownense Zone also occur, besides Sierra del Álamo, in the sections at Pozos de Serna and Sierra de Santa Rosa, whereas the lowermost Pliensbachian Imlayi Zone is only represented at Pozos de Serna. The Lower Pliensbachian Whiteavesi and Freboldi Zones occur in sections at Sierra de Santa Rosa and at Sierra la Jojoba. In the latter zone, Fuciniceras perplicatum permits correlation between local sections and represents the first appearance of hildoceratids in North America, only known from the Upper Pliensbachian elsewhere.Although the original proximity of the Antimonio terrane and localities in Nevada is predicted by the Mojave-Sonora megashear hypothesis, paleobiogeographic analyses using multivariate methods revealed only modest similarity of the ammonoid faunas and lend more support for the alternative tectonic model of earlier, pre-Jurassic dextral displacement along the California-Coahuila transform. The early appearance of the hildoceratid Fuciniceras at Sonora and the coeval absence at Nevada also supports our findings. Similarities with coeval faunal from the Insular and Intermontane superterrane of the Canadian Cordillera strengthen arguments for their origin at much lower paleolatitudes and large scale northward displacement as postulated by the Baja BC hypothesis.

The Upper Devonian Aley carbonatite, NE British Columbia: a product of Antler orogenesis in the western Foreland Belt of the Canadian Cordillera

Paleozoic continental margin strata in the western Foreland Belt of the Canadian Cordillera are characterized in part by alkaline volcanic sequences, carbonatite intrusions, coarse clastic sedimentary units, and erosional unconformities. These strata also contain a record of mid-Paleozoic contractional deformation unseen in coeval passive margin strata in the eastern Foreland Belt. In order to test potential genetic links between Paleozoic alkaline igneous activity, active margin sedimentation, and deformation in the western Foreland Belt, and better understand their implications for the evolution of the Foreland Belt as a whole, we have undertaken a detailed mapping and structural study of the Aley carbonatite intrusion and its host strata in the western Foreland Belt of NE British Columbia. Our work demonstrates that carbonatite emplacement was coeval with a Late Devonian contractional nappe-forming tectonic event. Interpreting tectonism as associated with continental collision along a long-lived active margin provides the best explanation for our structural and stratigraphic observations, and suggests that the western Foreland Belt is far-travelled and exotic relative to coeval passive margin strata in the eastern Foreland Belt. Deformed alkaline-carbonatite intrusions that characterize continental suture zones in Africa may provide an analogue for the Aley carbonatite and correlative alkaline-carbonatite complexes in the western Foreland Belt.

Distribución y patrones espaciales del impacto de un ingeniero de los ecosistemas exótico e invasor, Castor canadensis, en Tierra del Fuego, Argentina

El castor americano (Castor canadensis) es una especie ingeniera de los ecosistemas introducida en 1946 en Tierra del Fuego, en el extremo austral de America del Sur. En la actualidad, esta presente en los diferentes ambientes del archipielago fueguino, donde transforma notablemente la estructura y el funcionamiento de los ecosistemas, en particular al inundar ambientes riparios mediante la construccion de diques. El objetivo de este trabajo fue determinar, por primera vez a partir de un relevamiento exhaustivo, los patrones espaciales de la distribucion y el numero de diques construidos por esta especie en la porcion argentina de la Isla Grande de Tierra del Fuego. Para esto se elaboro una base de datos geografica con el registro de diques mediante la interpretacion visual de imagenes satelitales de acceso libre y de alta resolucion espacial. Se identificaron 70682 diques, cuya longitud acumulada supera los 2300 km y la superficie que inundan es de, al menos, 87 km2. La mayor densidad de diques se observo en la region cordillerana, donde alcanzo valores maximos de 123 diques/km2 al SE del lago Fagnano. En la porcion sur de la isla, el 84% del territorio se encuentra a menos de 1 km de un dique. La informacion generada en este trabajo es de uso libre para fines de investigacion y divulgacion. Se la podra usar para poner a prueba hipotesis sobre los controles biofisicos y humanos de la invasion, para identificar areas prioritarias para investigacion y desarrollar estrategias de manejo, asi como para monitorear la invasion en el espacio y en el tiempo, detectar tempranamente nuevas areas invadidas y entrenar metodos automaticos de deteccion de areas impactadas por el castor.https://doi.org/10.25260/EA.19.29.1.0.736

A multivariate statistical approach identifying the areas underlain by potential porphyry-style Cu mineralization, south-central British Columbia, Canada

The Bonaparte Lake area in southern part of the Quesnellia terrane of the Canadian Cordillera is proximal (<50 km) to several large porphyry CuAu and CuMo deposits. This area has potential for finding more porphyry type Cu deposits, but is overlain by a thick and nearly continuous cover of glacial sediments, which adds difficulty in finding mineral deposits in underlying bedrocks. This study uses geochemical data following an aqua regia leach on the clay fraction (<2 μm) of basal till (n = 726) to assess the mineral potential in the study area. Principal component analysis (PCA) captures four PCs containing large fractions of Cu variance: PC2 (31.2%), PC6 (17%), PC3 (16.8%) and PC16 (12.6%). Copper is associated with Au-Mo-Ag-As-Hg-Pb-Sb-Zn-Ni-Co-Mn in PC2; Au-Pb-Mn-W in PC6, Cr-V-Ti in PC3 and Pb-Se-Hg in PC16. The elemental assemblages in PC2, PC6 and PC16 may reflect the geochemical signature of underlying porphyry-style polymetallic mineralization, whereas the element association in PC3 may be originated from mafic volcanic rocks of the Chilcotin Group. Partitioning around medoids based on the dissimilarity matrix produced by unsupervised random forest using elements associated with Cu yields two groups, Group A and Group B, with minor overlap. Group A likely represents till samples with a composition influenced by a mineralized provenance because the contents of elements (Cu, Au, As, Mo, Pb and Zn) are much higher than samples in Group B. The Group A samples are mostly located in the areas underlain by the Nicola Group volcanic rocks and the northern boundary of the Thuya Batholith. Group A till samples are dominantly located close to or down-ice from known mineral occurrences, suggesting that more porphyry-Cu mineralization and other multi-elemental enrichments occur in these areas, such as Au, Mo, Pb and Zn. This study demonstrates the usefulness of multivariate analysis and unsupervised machine learning in detecting potential Cu mineralization in bedrocks based on the composition of basal till. The statistical methods used in this study may be applicable to large scale geochemical data of sediments to evaluate multi-element assemblages associated with a variety of mineralization in bedrocks.

Protolith age of the Altar and Carnero complexes and latest Cretaceous–Miocene deformation in the Caborca–Altar region of northwestern Sonora, Mexico

In the Caborca–Altar area of northwest Sonora, variably deformed and metamorphosed sedimentary and volcanic rocks crop out in a northwest-southeast–trending belt (El Batamote belt) at least 70 km long. We obtained detrital zircon U-Pb ages from two distinctive components of the belt near Altar, here termed the Altar complex and Carnero complex. Zircon ages for metasandstone and metaconglomerate matrix from the Altar complex indicate a Late Cretaceous maximum age of sedimentation, with at least part of the complex no older than 77.5 ± 2.5 (2σ). Pre-Cretaceous detrital zircons in the complex were derived largely from local sources, including Proterozoic basement, the Neoproterozoic–Cambrian miogeocline and the Jurassic arc. The detrital zircon ages and lithologic character of the Altar complex suggest correlation with the Escalante Formation, the uppermost unit of the Upper Cretaceous El Chanate Group. In contrast, one sample from the Carnero complex yielded a Middle Jurassic maximum depositional age and a detrital zircon age distribution like that of the Jurassic eolianites of the North American Cordillera. The Carnero complex may correlate with the Middle Jurassic Rancho San Martín Formation but could also be a metamorphosed equivalent of the Upper Jurassic Cucurpe Formation, Upper Jurassic to Lower Cretaceous Bisbee Group, or El Chanate Group derived by recycling of Jurassic erg sandstones. The Late Cretaceous age for the Altar complex protolith contradicts models that relate deposition of the entire El Batamote protolith to a basin formed by oblique slip along the Late Jurassic Mojave-Sonora megashear. Instead, the belt is best explained as an assemblage of Middle Jurassic to Upper Cretaceous formations deformed and locally metamorphosed beneath a northeast-directed Laramide thrust complex. Potassium-argon and 40Ar/39Ar ages confirm previous inferences that deformation of El Batamote belt occurred between the Late Cretaceous and late Eocene. A second phase of deformation, involving low-angle normal faults, occurred during and/or after intrusion of the ~22-21 Ma Rancho Herradura granodiorite.

Reconstructing basin-scale drainage dynamics with regional subsurface mapping and channel-bar scaling, Aptian, Western Canada Foreland Basin

An extensive subsurface dataset from the Lower Cretaceous McMurray Formation of northeastern Alberta, Canada, is leveraged to provide new insights into the Aptian drainage arrangement of the Western Canada Foreland Basin. Regional-scale stratigraphic mapping is supplemented with paleodrainage area estimates obtained using quantitative relationships between fluvial bankfull flow depth and drainage area in order to reconstruct paleocatchments, including those of a continental-scale river system and its tributaries. Using channel geometries delineated in high-resolution 3D seismic time slices, we test for evidence of a tidal prism within Aptian-aged drainage systems in order to assess the applicability of fluvial scaling relationships to the McMurray Formation.Refined paleogeographic reconstructions incorporate catchment estimates for the southerly-sourced trunk drainage system of the McMurray Formation, which corroborate the claim that the unit was associated with a continental-scale drainage network. Drainage area estimates for southwesterly-sourced tributary systems are, however, up to an order of magnitude smaller. These tributaries likely had headwaters in the North American Cordillera. This work demonstrates the applicability of combining regional mapping, sedimentological analysis, and fluvial scaling relationships to reconstruct sediment-routing systems from source-to-sink in the deep time record.

Upper Windermere Supergroup and the transition from rifting to continent-margin sedimentation, Nadaleen River area, northern Canadian Cordillera

AbstractNeoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin.

Sedimentological and geochemical evolution of an Ediacaran mixed carbonate-siliciclastic continental slope system, Windermere Supergroup, southern Canadian Cordillera, British Columbia, Canada

In this study a 165–265 m-thick siliciclastic-carbonate (mixed) base-of-slope succession within the Ediacaran Isaac Formation (hereafter termed FIC) in the deep-marine Windermere Supergroup (WSG), southern Canadian Cordillera was analyzed. Excellent outcrop exposure at three study areas (Castle Creek, Hill Section and Milk River), spaced over a distance of 20 km parallel to depositional dip provided the opportunity to observe the stratigraphic and inorganic carbon (δ13Ccarb) isotopic evolution of an ancient mixed, passive margin deep-water system. Based on detailed sedimentological analysis six stratal elements make-up the FIC (channel complexes, levees, siliciclastic-to-carbonate tripartite deposits, scour dominated units, debrites and slides). Within these strata the most voluminous carbonate phase (C1) represents neomorphism of primary aragonite on the slope and during early burial, and thus preserves the primary shallow-marine isotopic signature even after downslope transport. At the base of the FIC δ13Ccarb is negative (−5.2‰) and then progressively increases to 2–3‰ in the middle of the succession, after which it plateaus for about 65 m before declining to a nadir of −6.3‰ at its top. Coincident with these isotopic changes are systematic changes in the architecture and mineralogical composition of the component stratal elements that makeup the FIC, which collectively suggest long-term transgressive followed by regressive conditions that quite possibly are tied to rising and then falling sea-level, and accordingly the state of the continental shelf carbonate factory. Additionally, the upward change in δ13Ccarb closely resembles the trend reported in the upper June beds-Gametrail Formation of the WSG in the northern Canadian Cordillera, but the interpreted association of the latter with the Shuram-Wonoka anomaly (570–551 Ma) makes its correlation with the FIC unlikely. Instead, the FIC excursion potentially correlates with the EN2 excursion in the Doushantuo Formation of South China (and its potential global correlatives), which has been interpreted to be associated with the Gaskiers glaciation (580 Ma). This suggests that the FIC might be a stratigraphic and δ13Ccarb record of conditions that prevailed in the deep sea preceding the Gaskiers, which previously has been unreported from strata of the WSG.