Chilcotin Group Research Articles

The Miocene Red Lake macroflora of the Deadman River Formation (Chilcotin Group), Interior Plateau, British Columbia, Canada

Despite early interest in Neogene floras, primarily Miocene sites associated with Mio–Pliocene volcanic deposits of the Interior Plateau of British Columbia, few systematic accounts of the Miocene macrofloras of British Columbia – or elsewhere in non-Arctic Canada – have been published since the pioneering studies of J.W. Dawson and his contemporaries in the late 19th century. In this report, the Red Lake macroflora from sediments of the middle Miocene Deadman River Formation exposed in the Red Lake diatomite mine north of Kamloops, British Columbia, is illustrated, and a preliminary assessment presented, along with a brief review of Miocene floras from British Columbia and the U.S. Pacific Northwest. The Red Lake macroflora contains rare Ginkgo leaves, shoots of Cupressaceae (Cupressinocladus, Metasequoia, Taxodium) and shoots and seeds of Pinaceae (Pseudotsuga, Tsuga), maple (Acer) seeds and leaves, Liquidambar (fruit), Trochodendraceae (Zizyphoides auriculata leaves, Nordenskioeldia interglacialis fruits), leaves of 4 species of red and white oaks (Quercus columbiana, Q. prelobata, Q. pseudolyrata, Quercus sp.), leaves of an alder (Alnus harneyana) and birch (Betula thor), chestnut (Castanea spokanensis), beech (Fagus pacifica), sycamore (Platanus dissecta), elm (Ulmus speciosa), leaves of unidentified taxa, fruits of Tilia pedunculata (Malvaceae) and fruits and inflorescences of other unidentified taxa, and leaves of a reed or rush (indet. monocot). The Red Lake middle Miocene climate reconstructed from leaf physiognomy was temperate and mesic, with mean annual temperature ~11–13°C, mild winters (coldest month mean temperature ~3°C), mean annual precipitation 170 −51/+73 cm/yr, and growing season precipitation ~92 cm, with moderate seasonality of precipitation (three wettest months ~51 cm vs. three driest months ~25 cm). The Red Lake flora shows similarities to middle to late Miocene floras from the U.S. Pacific Northwest (i.e., richness in oaks) but is of much lower diversity and lacks key elements common to many of the contemporaneous U.S. Miocene floras (e.g., foliage of Pinaceae esp. Pinus), and is missing taxa detected in the microflora, a pattern likely due to sampling effectiveness at the Red Lake Mine and sampling of different lithofacies for macro- and microfloras.

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.

The Rock Physical Property Database of British Columbia, and the distinct petrophysical signature of the Chilcotin basalts,

Rock physical properties provide the link between geophysical surveys and their geological interpretation. The British Columbia rock physical properties database, compiled by the Geological Survey of Canada, now comprises 3876 values of density, 930 values of electric resistivity, 12 356 values of magnetic susceptibility, and 2576 values of magnetic remanence and Koenigsberger ratio. The measurements are linked to sample locations, lithologies, and geological formation or unit. Maps, histograms, and biplots are used to demonstrate useful links between lithology and physical properties, and serve as a background for future rock physical properties studies. As expected, density and resistivity are controlled mostly by porosity and mineralogy. Magnetic susceptibility has a bimodal distribution (maxima at 4 × 10−4 and 2 × 10−2 SI) controlled by magnetite concentration. Magnetic remanence is shown to be more important than usually considered in magnetic survey analysis, with Koenigsberger ratios greater than unity in 42% of the samples. A case study of the Chilcotin Group basalts is highlighted, as they form a significant barrier to mineral exploration in central British Columbia. These rocks are magnetically distinct from other basalts in British Columbia, distinguished by magnetic susceptibilities having a range of values concentrated around 3 × 10−3 SI and falling in the valley between the two susceptibility modes. These basalts are also characterized by very high Koenigsberger ratios (96% above unity), probably caused by a preponderance of fine-grained single-domain magnetite. The database provides a wealth of petrophysical properties that can help constrain analysis of several types of geophysical surveys and, in particular, modelling of anomalies in the quest to determine the three-dimensional distribution of rock units.

Interpretation of structures in the southeastern Nechako Basin, British Columbia, from seismic reflection, well log, and potential field data1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.2Geological Survey of Canada Contribution 20100002.

The structure and stratigraphy of the southeast Nechako Basin, which are poorly understood primarily because of substantial volcanic cover, are investigated in an analysis of seismic reflection, well, and potential field data. Formation and development of the SE Nechako Basin resulted in sub-basins containing Cretaceous and Eocene rocks. Interpretation reveals that dextral transtension in the Early to Middle Eocene created NNW-trending, en echelon, strike-slip faults linked by pull-apart basins, which locally contain a thickness of Eocene volcaniclastic rocks of &gt;3 km. This structural pattern is consistent with regional observations that suggest the transfer of slip from the Yalakom fault to the north via a series of en echelon strike-slip faults. In the Middle to Late Eocene, faults associated with a change in the direction of stress, echoed by the north-trending right-lateral Fraser fault, reactivated and cut earlier structures. A simple model agrees with local observations, that northeast-directed compression was subparallel to the relic Cretaceous grain. Cretaceous rocks are discontinuous throughout the basin and may be remnants of a broader basin, or a number of contemporaneous basins, formed in a regional transpressional tectonic setting that caused northeast-directed thrusting along the eastern side of the Coast Plutonic Complex. Results suggest that thrusting affected most of the SE Nechako Basin, as observed across the Intermontane Belt to the northwest and southeast. The pattern of deposition of Neogene volcanic rocks of the Chilcotin Group was in part controlled by the Eocene structural grain, but we find no evidence of Neogene deformation.

The thickness of Neogene and Quaternary cover across the central Interior Plateau, British Columbia: analysis of water-well drill records and implications for mineral exploration potential1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia. 2Geological

Analysis of over 10 000 water-well records has been used to produce new depth-to-bedrock maps for areas around five cities on the central Interior Plateau of central British Columbia: 100 Mile House, Prince George, Quesnel, Vanderhoof, and Williams Lake. Hitherto, exploration for mineral and hydrocarbon resources has been hampered by a lack of basic knowledge of the thickness of Neogene and Quaternary lithologies. Interpretation of these new maps provides first-order constraints on the localization of thick drift in pre-Late Wisconsinan bedrock paleovalleys, some of which are now buried. Basalt lavas of the Chilcotin Group are restricted to erosional remnants of previously extensive sheets emplaced onto an older peneplain. Our results confirm that the Neogene and Quaternary cover is primarily controlled by paleotopography and is generally thin and patchy across much of the region. Increased understanding of the three-dimensional distribution of cover produces a corresponding increase in the utility of geological, geochemical, and geophysical exploration techniques, and a reduction in the risk for future mineral exploration activities, especially when combined with more sophisticated data sets.

Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography

Geological investigation of the near-surface in the southeastern Nechako Basin is difficult. Shallow seismic reflection imaging is poor due in part to an extensive cover of Eocene and Neogene volcanic rocks. Outcrops of these volcanic rocks, and the primarily Cretaceous bedrock, are commonly obscured by Quaternary deposits and vegetation. Estimates of near-surface P-wave velocity are derived from the tomographic inversion of seismic first-arrivals, an effective tool when seismic imaging is poor. Tomographic model velocities are in agreement with sonic logs and laboratory samples, except for those from the Neogene Chilcotin Group. Cretaceous sedimentary rocks have velocities of ∼2800–4200 ms–1. The Eocene Endako and Ootsa Lake groups, which have velocities of ∼3000–4200 ms–1, are not distinguishable based on velocity. The velocity, the character (density, focus, and penetration depth) of rays, and ties with well and surface geology constrain the subsurface extent of the Endako Group adjacent to well b-82-C. The Chilcotin Group typically exhibits velocities (∼2400–3000 ms–1) lower than corresponding velocities from sonic logs (4500–5200 ms–1) and laboratory measurements (5000–5200 ms–1). These low model velocities may be due to the presence of high porosity, brecciated rocks near to the surface, in comparison with the other measurements that have focussed on lower porosity massive lavas. The lowest mean velocities, located to the southeast, are related to anomalously thick, high porosity, breccia-rich deposits of Chilcotin Group. This conclusion is consistent with the interpretation that the Chilcotin Group is thicker in paleo river valleys.

The Cheslatta Lake suite: Miocene mafic, alkaline magmatism in central British Columbia

New mapping, mineralogical, and geochemical studies help characterize late Tertiary primitive, alkaline, sodic basanite, alkali olivine basalt, transitional basalt, and diabase in the Nechako River, Whitesail Lake, and McLeod Lake map areas of central British Columbia and distinguish the Miocene Cheslatta Lake suite. The suite encompasses scattered erosional remnants of topographically distinct, columnar-jointed, olivine-phyric basalt and diabase volcanic necks, dykes, and associated lava flows north of the Anahim volcanic belt and west of the Pinchi Fault. Volcanic centres at Alasla Mountain and at Cutoff Creek, near Cheslatta Lake, are proposed as type areas. Olivine, plagioclase, and pyroxene phenocrysts, megacrysts, and (or) xenocrysts; common ultramafic xenoliths; and rare but significant plutonic and metamorphic xenoliths are characteristic. Basanite, transitional basalt, and alkali olivine basalt groundmass contain plagioclase, clinopyroxene, Fe-Ti oxides, feldspathoid, olivine, and apatite. The Cheslatta Lake suite is characterized by its alkaline character, olivine-rich (&gt;10 wt.%) normative mineralogy, and silica-undersaturated nature (&gt;1 wt.% normative nepheline; hypersthene-normative rocks are uncommon). Mg numbers vary between 72–42. Some samples encompass near-primitive mantle melt compositions. Cheslatta Lake suite rocks in the Nechako River area are distinguished from the underlying Eocene Endako and stratigraphically higher Neogene Chilcotin groups basaltic andesite lavas within the study area, and from the Chilcotin Group basalt in the type area south of the Anahim volcanic belt, by form, preserved thickness, phenocryst–xenocryst mineralogy, amygdule abundance, included xenoliths, isotopic age, and major and incompatible, high field strength, and rare-earth trace element contents.

Geochemistry of the Miocene Mount Noel Volcanic Complex, British Columbia and comparison with the Columbia River basalt

The 19.6 Ma Mount Noel Volcanic Complex (MNVC) is located about 150 km east of the Garibaldi Volcanic Belt and adjacent to the Chilcotin plateau basalts in southwestern British Columbia. The MNVC unconformably overlies Jurassic to Cretaceous metasedimentary rocks and Lower Cretaceous Coast Plutonic rocks. The complex consists of flat-lying, columnar-jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks. Basaltic flows have olivine (Fo 60 to Fo 85) and plagioclase (An 60 to An 80) phenocrysts with clinopyroxene microphenocrysts. Major, trace and rare earth element analyses of the MNVC indicate the flows can be divided into two groups. The largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt. Rare earth element patterns are slightly enriched in the light-rare elements in the transitional group and more strongly enriched in the alkaline basalts. In tectonic discriminant diagrams, all basalts plot in the within-plate fields or in overlap areas typical of continental flood basalts. Alkaline basalts, found only at the base of the thick sequence of tholeiitic to transitional basalts, are characterized by high P 2O 5, Sr and Ba contents. The MNVC tholeiitic to transitional basalts are comparable to the Chilcotin Group basalts and to the lowermost basalts of the Columbia River Group. The origin of MNVC basalts is best interpreted as volcanism behind the Pemberton volcanic arc.

Neogene Chilcotin basalts in south-central British Columbia: geology, ages, and geomorphic history

An extensive but generally thin mantle of basalt flows, the Chilcotin Group, covers much of the Interior Plateau of south-central British Columbia. It provides material for dating and for reconstructing the original form of the paleosurface on which it was deposited. K–Ar whole-rock dates demonstrate that several ages of basalt are represented, from Early Miocene (or even Late Oligocene?) to Early Pleistocene, with particularly abundant eruptions about 14–16, 9–6, and 1–3 Ma ago.Basalts of Middle Miocene and later ages, if not the Early Miocene relics as well, clearly rest on land surfaces of low local relief. In places the low-relief surfaces had been incised to depths of 100–200 m and the valleys backfilled with mid-or late Cenozoic sediments prior to burial by the basalts. The low-relief surfaces throughout the area are believed to have been developed close to a common base level, and regional differences in their present elevation are thus largely a product of post-basalt deformation. This is recorded by Miocene or later uplift of the southern Coast Mountains and gentle flexing in parts of the Interior Plateau.Major stream incision to depths of up to 1000 m following uplift provides a convenient, but not infallible, means of distinguishing Chilcotin basalts from mid-Pleistocene and younger "valley basalts."The Chilcotin Group is, for the most part, a small-scale counterpart of the roughly contemporaneous Columbia River basalt group of Washington and Oregon.

Implications of Chemical and Isotopic Composition for Petrogenesis of Chilcotin Group Basalts, British Columbia

Thin, flat-lying basalt flows of the Mio-Pliocene Chilcotin Group, erupted from central vents, coalesce to form a 50,000 km2 lava plateau in south-central British Columbia. Olivine-bearing transitional basalt is the predominant rock type; alkali basalt and quartz tholeiite are also present. Sr and Pb isotopic ratios indicate a mantle source for the magmas. Based on Mg values of 45–65, and Ni contents of <250 ppm, Chilcotin basalts are differentiated magmas. The genesis of Chilcotin basalt magma can be modelled in two steps, (1) partial melting of spinel lherzolite to produce picritic basalt, and (2) fractionation of olivine from picritic magma to produce transitional basalt. Residual peridotite left over from the partial melting event is similar in mode and composition to ultramafic nodules found in many alkali basalt flows from western North America. Magma was not trapped in the crust long enough to react with it or to produce silicic differentiates. Individual flows of Chilcotin basalt from different vents are isotopically distinct and cannot be related to each other by any reasonable crystal fractionation model, suggesting that the mantle source for Chilcotin basalt is heterogenous on the scale of the basalt plateau, and that many batches of slightly different primary magma were parental to the basalts that make up the volcanic field.

Regional stratigraphy and age of Chilcotin Group basalts, south-central British Columbia

Mio-Pliocene Chilcotin Group basalt flows form a 50 000 km2 lava plateau in south-central British Columbia. Two periods of eruptive activity at 2–3 and 6–10 Ma are indicated by a compilation of available age data, including 10 new K–Ar age determinations, and basalts from these two periods are chemically indistinguishable. The Chilcotin Group consists of thin, crudely columnar-jointed pahoehoe flows, some thick, tiered flows, pillow lava and pillow breccia, and rare silicic tephra layers. The presence of many vesicle sheets and cylinders and collapsed pahoehoe toes suggests that the basalts were volatile-rich. Known vents (gabbro and basalt plugs) for the basalt flows form a northwest trend along the axis of the lava plateau. The plateau appears to have formed from the overlap of many low-profile shield volcanoes and is similar in morphology to other plains basalts such as the Snake River Plain and parts of Iceland. Glaciation has stripped off an unknown volume of the flat-lying basalt flows.