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Cenozoic syntectonic sedimentation and strike-slip basin development along the Denali fault system, Yukon Territory
This is a multidisciplinary study which combines sedimentology and sedimentary petrology, palynology, organic petrology and structural analysis of the middle Cenozoic Amphitheatre Formation in the St. Elias Mountains, and documents syntectonic deposition in strike-slip basins along the Denali fault system. The outcrops of the Amphitheatre Formation can be divided into two discrete basins:: the northern Burwash basin and the southern Bates Lake basin. Sedimentological analysis of the Burwash basin identified fault-controlled depocentres which allowed the development of several different types of non-marine depositional environments in close proximity. Palynology and organic petrology data indicate that the Amphitheatre Formation is diachronous and spans the Eocene-Oligocene boundary. Light-mineral provenance studies of sandstones, clast-counts in conglomerates and paleocurrent analyses suggest that the Wrangellia and Yukon Crystalline Terranes were sources for the Amphitheatre Formation in the Burwash basin. In contrast, preliminary work suggests that Wrangellia and possibly the Gravina-Nutzotin Terrane may have been important sources for the Amphitheatre Formation in the Bates Lake basin. Structural data combined with geologic mapping indicate a predominance of strike-slip deformation during and after deposition of the Amphitheatre Formation in the Burwash basin. The presence of syndepositional faults with subhoriziontal slickenside indicates that strike-slip deformation occurred during deposition of the Amphitheatre sediments. The Burwash basin contains structures indicative of both contractional and strike-slip deformation, whereas the Bates Lake basin contains structures indicative of both extensional and strike-slip deformation. The change in structural style between the two basins suggests that the Amphitheatre Formation may have been deposited in transpressional as well as transtensional tectonic settings along individual segments of the Denali fault system during the middle Cenozoic.
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Geology of the Dezadeash Range and adjacent northern Coast Mountains (115A), southwestern Yukon: Re-examination of a terrane boundary
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Granodiorite of the Coast Plutonic Complex intruded metasedimentary rocks in the Dezadeash Range of the northern Coast Belt in the late Mesozoic. Graphitic staurolite-biotite schist, associated with the Kluane Metamorphic Assemblage, underlies the western Dezadeash Range, whereas cordierite-biotite gneiss, previously correlated with the Late Proterozoic - Paleozoic Nisling Assemblage, is exposed in the eastern and southern regions. A terrane boundary was placed in the central Dezadeash Range. Recent petrographic studies reveal a southeastward increase in metamorphic grade. Prograde appearance of cordierite partly obliterated an older schistosity and caused a fabric change near the postulated terrane boundary. Furthermore, typical continental margin rocks, such as marble and quartzite, are not observed. This suggests that all metamorphic rocks in the Dezadeash Range can be correlated with the Kluane Metamorphic Assemblage, whereas Nisling Assemblage rocks occur in the Coast Mountains to the east. Therefore, the terrane boundary is located in the Dezadeash River valley, further southeast than previously thought.
Bedrock geology at the boundary between Yukon-Tanana and Cassiar terranes, Truitt Creek map area (NTS 105L/1), south-central Yukon
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The Tummel fault zone, a northwest-trending belt of rocks of uncertain age and/or tectonic affinity, separates Paleozoic miogeoclinal strata of Cassiar Terrane from Yukon-Tanana Terrane metavolcanic and metasedimentary rocks. Northeast of the fault, Cassiar Terrane comprises pelitic and semipelitic rocks with rare amphibolite, which are correlated with the Kechika Group. These are overlain by carbonate correlated with the Askin Group. Southwest of the fault, in Yukon-Tanana Terrane, Devono-Mississippian siliciclastic rocks are overlain by Mississippian arc volcanic rocks. Granodiorite and diorite of the Telegraph Plutonic Suite (348-350 Ma) intrude the siliciclastic rocks. Foliated greenstone, leucogabbro intrusions, serpentinite and chert occur in the Tummel fault zone. The Early Cretaceous Glenlyon Batholith intrudes strata of Cassiar Terrane. Contact metamorphism recognized across the Tummel fault zone is interpreted to have been imposed by the Glenlyon Batholith. If correct, this interpretation requires that post-mid-Cretaceous displacement across the Tummel fault zone has been minimal (~5 km).
Stratigraphy, geochemistry and source rock potential of the Boundary Creek Formation, North Slope, Yukon and a description of its burning shale locality
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The Cenomanian-Turonian (Upper Cretaceous) Boundary Creek Formation is a mudstone, shale and silty shale unit that is exposed in river and creek cuts on Yukon’s North Slope. As part of the CASE-15 expedition, co-led by the Yukon Geological Survey and the German Federal Institute for Geosciences and Natural Resources, fieldwork in July 2013 involved measuring and sampling Boundary Creek Formation strata in two locations, and investigating a burning shale exposure near the confluence of Boundary Creek and the Big Fish River. Shale and mudstone samples were analyzed for XRF lithogeochemistry and organic matter quantity, along with thermal maturity and type using RockEval/TOC and vitrinite reflectance techniques. The Boundary Creek Formation is interpreted to have been deposited by turbidity currents moving through an outer shelf to slope environment in the distal part of the foreland basin, outboard of the Cordilleran orogeny. Lithogeochemical data suggest that at times throughout the deposition of Boundary Creek Formation shale, ocean water may have been depleted in oxygen, resulting in anoxic conditions that would have been favourable for organic matter preservation. Analyses of surface samples suggest that some areas have poor to no petroleum potential and are thermally overmature with respect to oil generation. In others, good to very good petroleum potential exists and the shale is oil to oil and gas prone and thermally mature with respect to oil generation. In these latter areas, specifically in the vicinity of the type section on Boundary Creek, the shale has the necessary components for spontaneous combustion: pyrite, organic matter and a fresh supply of oxygen provided by a landslide. Although burning shale is not unknown in northern Canada, the outcrop of burning shale on Yukon’s North Slope is the first observed in shale of the Upper Cretaceous Boundary Creek Formation.
Denali Fault Zone Field, Microvein, and Microbeam Data, Kluane Ranges, Yukon, Canada
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Detailed geologic mapping and sample collection was conducted in rare bedrock exposures of the Denali fault zone during the summer months of 2013, 2014, and 2015 by the U.S. Geological Survey. Data include measurements of structural geologic orientations and documentation of fault zone and host rock characteristics. Representative samples were selected for measurements of fault vein attributes and electron backscatter diffraction studies to characterize deformation mechanisms. All station locations, pertinent sample information, and associated data are included in this data release.
Denali Fault Zone Field, Microvein, and Microbeam Data, Kluane Ranges, Yukon, Canada
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Detailed geologic mapping and sample collection was conducted in rare bedrock exposures of the Denali fault zone during the summer months of 2013, 2014, and 2015 by the U.S. Geological Survey. Data include measurements of structural geologic orientations and documentation of fault zone and host rock characteristics. Representative samples were selected for measurements of fault vein attributes and electron backscatter diffraction studies to characterize deformation mechanisms. All station locations, pertinent sample information, and associated data are included in this data release.
Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia
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for a copy of this paper please contact the Yukon Geological Survey; geology@yukon.ca.
Late Cambrian and Ordovician Alkalic Magmatism Along the Dawson Fault, Yukon: Evidence for Multiple Reactivations of a Rift‐Transfer Fault in the Northern Canadian Cordillera
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Lower Paleozoic continental margin rocks of the North American Cordillera, from Yukon to Nevada, include coeval platformal carbonate and basinal clastic strata that are offset along rift‐transfer faults, including the Liard, St. Mary‐Moyie, and Snake River structures. The Dawson fault is a prominent east‐weststriking structure in central Yukon that is interpreted herein to have been active as a rift‐transfer fault by late Cambrian time. This hypothesis is supported by new zircon U‐Pb dates that range from 501.98 ± 0.17 Ma to 497.57 ± 0.70 Ma from alkaline mafic volcanic rocks concentrated along the Dawson fault. The development of a sub‐Jiangshanian unconformity immediately post‐dates this alkaline magmatism and indicates that final continental breakup and establishment of the northern Cordilleran margin occurred by the late Miaolinginan. Alkaline magmatism caused by local decompression partial melting of the mantle may have been triggered by the release of in‐plane tensile stresses during lithospheric rupture and edge‐driven mantle convection. Upper Ordovician alkaline mafic volcanic and plutonic rocks that occur along a northwest‐southeast striking segment of the Dawson fault erupted ∼50 Myr after breakup and represent an example of post‐rift magmatism along a rift‐transfer fault. New bedrock mapping, and geochronological, paleontological, and petrological results from Upper Ordovician rocks indicate that there was localized basin development and punctuated volcanism along the Dawson fault from 453 to 447 Ma. Late Ordovician extension and post‐breakup magmatism in central Yukon is compatible with dextral strike‐slip reactivation of the Dawson rift‐transfer fault associated with counterclockwise rotation of Laurentia.
Geochemistry and geochronology of Cambrian and Upper Ordovician mafic igneous rocks along the Dawson fault zone, Yukon
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Neoproterozoic to lower Paleozoic basin and platform strata that formed during and after rifting along the western Laurentian margin are preserved in the northern Cordillera. Several pulses of magmatism occur within margin strata and are concentrated along the Dawson fault in central Yukon. Magmatism is dated as late Cambrian and Late Ordovician using: 1) U-Pb zircon geochronology of volcaniclastic rocks; and 2) fossil ages from strata interbedded with, and enclosing, volcanic rocks. Volcanic rocks from both pulses are predominantly alkaline and basic and erupted in subaqueous environments. The trace element geochemical compositions of the rocks suggest that they formed from partial melting of enriched lithosphere from the garnet stability field.
Geochemistry and U-Pb zircon geochronology of mid-Cretaceous tay river suite intrusions in southeast Yukon
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Reconnaissance geological mapping in the Coal River map area of southeastern Yukon investigated several small mid-Cretaceous plutons. The intrusions are composed of unfoliated or incipiently foliated, fine to coarse-grained, equigranular and porphyritic, biotite ± hornblende quartz monzodiorite to granodiorite. They are metaluminous to peraluminous and have reduced to oxidized geochemical characteristics. The composition of selected samples is consistent with magma formation from partial melting of infracrustal source rocks.U-Pb ages were obtained for nine plutons from five or six zircon single-grain analyses by the isotope dilution thermal ionization mass spectrometry method with chemical abrasion (CA-TIMS). All interpreted ages are concordant within statistical uncertainty. The plutons range in age from 99.80 ± 0.03 to 97.70 ± 0.03 Ma. Given the primarily unfoliated nature of the plutons, contractional, fabric-forming deformation within the Cordilleran orogeny must therefore have largely ceased at the present level of exposure in the Coal River area by the time of intrusion (ca. 98 Ma).The ages and compositions of the plutons in Coal River map area are consistent with their being part of the Tay River plutonic suite, a northwest-trending belt of coeval and compositionally similar plutons and local volcanic rocks (South Fork volcanic suite) that, when augmented by the addition of the Coal River plutons, extends approximately 465 km with a width of up to 150 km.
Continental fragments, mid-Paleozoic arcs and overlapping late Paleozoic arc and Triassic sedimentation in the Yukon-Tanana terrane of northern British Columbia and southern Yukon
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for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.
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