Kelly Russell

Palagonite, the first stable alteration product of basaltic glass, is an assemblage of altered glass and authigenic clays and zeolites commonly found in subaqueous volcanic environments. The process of palagonitization transforms volcaniclastic deposits physically, mineralogically, and chemically through the dissolution of basaltic glass and reprecipitation of gels and newly formed mineral phases. Despite its ubiquity in subaqueous volcanic environments, it is as yet poorly understood how palagonite propagates and what conditions drive its formation. We present mineralogical, physical, and chemical data for a set of 134 variably palagonitized cores collected from Cracked Mountain, a subglacial volcano crosscut by an abundant dike complex in the Mount Meager Volcanic Complex, British Columbia. The qualitative and quantitative mineralogy, density, porosity, permeability, uniaxial compressive strength, P-wave velocity, and whole rock geochemistry was studied with the aim of determining what factors control the intensity of palagonitization. Using this data set I investigate the extent to which palagonitization alters the physical, mineralogical, and chemical properties of volcanic deposits. I define three palagonitization environments, determined by their proximity to hot intrusive dikes and pillows. The first environment is defined by a lack of association to a hot intrusive body and allows two possible pathways of alteration determined by the fluid/rock ratio. In lower fluid/rock ratio systems, there is minor smectite growth but minimal changes in physical properties. Higher porosity and permeability areas within the low temperature environment are defined by widespread glass dissolution, which results in extensive, deep orange discoloration. As the degree of alteration increases, analcime forms and increases in abundance in the second and third environments which are defined by their increasing proximity to a hot, intrusive body. Alteration intensity is ultimately defined by the abundance of zeolites, which are representative of the relative duration of time at high temperature. Authigenic minerals cement the deposit, increasing the density and decreasing porosity and permeability. Similar patterns can be seen at other subaqueous volcanic edifices, including in Hawaii and at Surtsey, where mineralogical zones are defined by thermal regimes, and density increases as palagonitization intensity increases.

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Olivine crystals are ubiquitous in kimberlites and record petrologic events through all stages of ascent and eruption. Well-preserved olivine from the Leslie kimberlite, NT, Canada elucidates the sequence of events attending kimberlite emplacement. The Leslie kimberlite mainly hosts a single phase of extrusive coherent kimberlite. We used spatially distributed samples (n = 76) from drill core to develop a 3D representation of the Leslie kimberlite. Petrographic investigation defined four distinct olivine populations: phenocrysts, macrocrysts, dunite “nodules”, and mantle xenolith-hosted olivine. SEM imagery, and major and trace element geochemistry were used to characterize and quantify chemical zonation including xenocrystic cores, mid-zones, magmatically crystallized rims, high-Mg rinds, and/or monticellite coronas. Core compositions (Fo85.7 to Fo93.8, NiO ~0.36 wt%) and other textural evidence show that cores are mantle-derived olivine xenocrysts. Mid-zones developed on cores show a sharp decrease in Fo content (~Fo90.9) across irregular, convoluted, and embayed contacts indicating disequilibrium and partial dissolution of the olivine xenocrysts combined with diffusion-driven replacement. Magmatically crystallized olivine rims can jacket xenocrystic cores and mid-zones. Rims have uniform and distinctive chemical compositions of ~Fo91.8 ± 0.7 (2σ) and NiO concentrations that decrease (~0.4 wt% to ~0.1 wt%) outward. Rinds, thin outer fringes, have a convoluted inner contact and a sawtooth contact with the groundmass. Rind compositions have high ~Fo97.1, low NiO, and elevated Mn and Ca. Rind textural relationships and compositions suggest a non-magmatic, post-emplacement origin. The four olivine populations and chemical zonation (only cores, mid-zones, and rims) are uniformly and consistently distributed throughout the Leslie kimberlite. Thus, olivine within a single phase of kimberlite predominantly records pre-emplacement processes and illustrates that magma batches become well mixed during ascent and emplacement. In contrast, both late-stage high-Mg rinds and monticellite coronas are non-uniformly distributed throughout the pipe. Utilizing olivine geochemistry, coupled with petrographic observations, we reconstruct a sequence of events of the Leslie kimberlite ascent and eruption. Our results indicate olivine entrainment and ascent (within a single kimberlite phase) may be sufficiently characterized by a few samples. Conversely, syn- and post-emplacement processes recorded by olivine require systematic sampling to capture all potential heterogeneities within a given kimberlite phase.

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The focus of this thesis is Cracked Mountain Volcano, located in the Garibaldi volcanic belt (GVB), of southwestern British Columbia, Canada. This study investigates the physical and petrochemical evolution of Cracked Mountain. Field mapping, lithostratigraphy, petrography, geochemistry, and thermodynamics were used to model: i) the eruptive sequence, style, duration, and paleoenvironment, ii) the causes of petrochemical variation and the P-T-H2O contents for the Cracked Mountain magmas. The major findings show that Cracked Mountain has a glaciovolcanic origin. We find lapilli tuffs consistent with a phreatomagmatic (i.e. explosive) origin, followed by intrusions (dykes, peperites) and effusions (pillows, sheet-lavas), indicating a transitional eruptive style. Paleomagnetic directions record a single-pole direction (i.e. monogenetic eruption). The edifice morphology indicates syn-eruptive confinement by a paleo-ice sheet that was  850 m thick, capable of holding a ‘leaky’ paleolake system with a maximum of ~0.36 km3 of water. The glaciovolcanic (40Ar/39Ar) age of ~400 ka represents an important record of the Cordilleran Ice sheet (CIS) in southwestern BC during the mid-Pleistocene. Petrographically, Cracked Mountain volcanic rocks comprise two unique suites: i) an olivine-plagioclase phyric (OP) assemblage and ii) an olivine-plagioclase-augite phyric (OPA) assemblage. Thermodynamic modeling based on Rhyolite-MELTS (Gualda and Ghiorso, 2015) reveals that these differences in phenocryst assemblage require distinct storage conditions before the eruption. OP magma must have been stored at depths less than 6 km (
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Kimberlite magmas transport mantle cargo in the form of xenoliths and xenocrysts to the surface of the Earth. Due to the lack of recent kimberlite eruptions and unanswered questions concerning melt composition and magma rheology, the mechanisms supporting efficient ascent of these cargo-rich magmas remains enigmatic. Although olivine is the dominant mineral phase in kimberlite, given the polymineralic nature of mantle xenoliths, xenocrysts are transported as a multi-mineral mixture. Within the ascending dyke, high particle concentrations and high velocities resulting in turbulent flow provides an environment for frequent particle-particle interaction. Xenocryst morphologies, textures and size distributions observed in kimberlite deposits are not reflective of how they occur in xenoliths and are therefore modified during ascent to the surface. The degree of modification of each mineral varies and is a function of its chemical and physical properties. Here I present a series of analogue attrition experiments on the mantle minerals: olivine, orthopyroxene, clinopyroxene, garnet and diamond designed to inform on the ascent of kimberlite magmas. Data is collected on particle size distributions, particle morphologies and particle velocities. Natural xenocrysts extracted from coherent kimberlite reveal remarkably similar surface features and morphologies to that of the experiments suggesting that attrition indeed operates during kimberlite ascent. Kimberlite ascent velocities are estimated by using a scaling analysis of the experiment conditions and by investigating impact pits observed on the surfaces of kimberlitic olivine and garnet xenocrysts. Both methods result in calculated ascent velocities of ~ 4 m s-1. In mineral mixtures, cleavage is shown to be a controlling factor in determining attrition rates whereby minerals with cleavage undergo accelerated breakdown. I suggest that the accelerated breakdown of orthopyroxene increases assimilation rates, contributing to the onset of turbulent ascent.

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A model for the viscosity (η) of melts in the system CaMgSi₂O₆-CaAl₂Si₂O₈-NaAlSi₃O₈(Di-An-Ab) based on data compiled from the literature is presented. The model is calibrated on560 measurements of melt viscosity at 1 bar pressure on 40 individual melt compositions and 303measurements obtained at high pressures on 18 melt compositions. The model uses the Tammann-Vogel- Fulcher equation for temperature (T)- dependence and accounts for variations in melt composition (X) and pressure (P). At ambient pressures, the model spans 15 orders of magnitude of η, a T range of 933-2450 K, and reproduces the datasets well (RMSE = 0.26). The high-P model is calibrated over a T range of 970-2470 K and P range of 50 to 13100 MPa and reproduces the high-pressure data sets to within (larger) experimental uncertainties (RMSE = 0.35). The T-P-X model predicts the glass transition temperatures (Tg), melt fragilities (m) and activation energies (Ea) across the ternary with increasing P. Used in conjunction with chemical proxies for melt structural organization (i.e. SM and NBO/T), the model informs on changes in melt polymerization as functions of T, X, and P. Di-An-Ab melts with SM values > 25 have positive pressure coefficients and show a minimal to substantial increase in viscosity with increasing P. Melts with SM less than 25 show negative P-coefficients indicating a decrease in viscosity with increasing P. The P-effect might be correlated with the degree of polymerization and explained by the increasing coordination numbers of silicate and aluminum cations at elevated pressures. Model values of Tg for Di-An-Ab melts range between 930 K and 1130 K at ambient conditions; whilst at elevated pressures, Tg values are depressed for highly polymerized melts by as much as 150K at 5000 MPa but are higher for more depolymerized melts (i.e. SM > 25). A comparison of the liquidus surface topology for the ternary system to the corresponding isokoms of viscosity shows the relative effects of X and T on melt viscosity to be highly varied.

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Mantle-derived peridotite xenoliths represent a valuable source of information about the structure, chemistry, origin, and evolution of the mantle lithosphere underlying various tectonic environments. Xenolith-bearing volcanic suites at Mt. Timothy and Summit Lake in south-central British Columbia are two particularly intriguing sites for evaluating the nature of the lithosphere beneath the southern Canadian Cordillera. The peridotite xenoliths at Mt. Timothy are texturally and mineralogically homogenous, while the xenoliths from Summit Lake contain a greater variety of textures and rock types, both mantle-derived and crustal. These peridotites are similar geochemically to other peridotite datasets from the Canadian Cordillera, indicating a geochemically homogenous mantle lithosphere and evolution from a fertile peridotite source. Enrichments of MREEs and LREEs in certain samples may indicate that these rocks were subject to a small degree of metasomatism. The geochemical signatures of the pyroxenes in these xenoliths are indicative of an orogenic lithospheric mantle. Temperatures and depths of equilibration for these peridotites have been determined by combining a two-pyroxene geothermometer with a geotherm constructed from several regional geophysical parameters. The Mt. Timothy peridotites sample an extensive window of mantle lithosphere ranging from the Moho (813 °C; 33 km) to relatively deep within the mantle lithosphere (1091 °C; 50.9 km), while the Summit Lake peridotites sample a very deep and narrow window of mantle lithosphere (1061 °C to 1119 °C; 49.0 km to 52.7 km). The window sampled by Mt. Timothy is most similar to a number of other xenolith suites scattered across the southern Cordillera, while the comparatively unique sampling range of the Summit Lake suite shares certain mineralogical characteristics with other xenolith suites straddling the boundary between the Cordillera and the North American craton.

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Kima’Kho is one of several englacially erupted volcanoes located on the Tuya-Kawdy plateau in northwestern British Columbia. Evidence that Kima’Kho erupted into ice is recorded by lithofacies and stratigraphical features indicating deposition into a sustained but fluctuating englacial lake. Mapping of the edifice alongside analysis of an extensive sample suite, has elucidated the first in-depth model of physical growth for this glaciovolcano within the context of a reconstructed englacial lake. Four major stratigraphic units were mapped: lapilli tuffs (Lt₁/Lt₂), altered pillow bearing tuff breccias (Tb₁/Tb₂), basalt lavas (L₂/L₃/L₄), including pillow basalts (L₁), and basalt intrusions (IDS). This work also introduces new geochronological data and integrates stratigraphical relationships alongside geochemical data, to reconstruct volcanological evolution through time.⁴⁰Ar/³⁹Ar data indicate Kima’Kho formed ~1980 ka with an unexpected late phase of intrusive activity. This is demonstrated by cross-cutting relationships, variation in petrography and a chemical evolution describing an outlying group of evolved dykes, distinct and separate from other stratigraphical units. Mapping of Kima’Kho has defined six points in stratigraphic time where the englacial lake level can be established during the growth of the volcanic edifice. The first lake level recorded is associated with the initial explosive phase of eruption which formed a pyroclastic tephra cone that hosts a ‘passage zone’ – an inferred surface delineating the transition between sub-aqueous and sub-aerial deposition. Dynamic fluctuations in lake level are recorded by two later passage zones exposed at lower elevations on the flanks of the cone and also within effusive units. Effusive eruption is represented by basaltic pillow tuff breccias and subaerial lavas. The three lattermost passage zones are marked by the contact between these tuff breccias and lavas and record a second significant decline in lake level followed by relative stability, at which point the effusive lavas were able to cap the brecciated deposits. The decryption of the stratigraphy to this complex ancient glaciovolcano provides many insights that cannot be recovered from modern glaciovolcanoes, which are obscured by their present-day ice fields. This study demonstrates the feasibility of reconstructing glacial conditions through the proxy of englacial lake fluctuations recorded in the glaciovolcanic deposits.

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This thesis is a high-temperature, low-pressure experimental study that quantifies the temperature-dependence of water (H₂O) solubility in a rhyolitic melt at atmospheric pressure, and assesses the sensitivity of the water exsolution and bubble growth processes to thermodynamic and kinetic parameters. In the investigation of H₂O solubility I defined the magnitude of retrograde solubility (-7.1x10-³ wt% H₂O per 100°C) and estimated the enthalpy and entropy of the H₂O exsolution reaction (ΔH⁰ = +17.8 kJ mol-¹, ΔS⁰ = 107 J K-¹ mol-¹). I also modelled the implications of retrograde solubility for the glass transition temperature (Tg) and outline the potential effect on cooling volcanic bodies at surface- and conduit-relevant pressures if cooling is slow enough to facilitate H2O resorption. In my investigation of bubble growth dynamics and the vesiculation process in my experimental products I recalibrated the estimates of H₂O exsolution enthalpy and entropy (ΔH⁰ = +18.5 kJ mol-¹, ΔS⁰ = 108 J K-¹ mol-¹). Additionally I identified the viscosity (η) dependence of average volumetric growth rate (dV/dt) (log dV/dt = -0.79 log η + 4.95) and have calculated the time to develop 60% porosity for melts of varying viscosities at conduit-relevant pressures that are up to 15% oversaturated with H₂O. By dismantling a complex system and individually investigating the behaviours of dissolved and exsolved H₂O I have developed models that can be used to study volcanic hazards past, present and future.

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Columnar joints form as a brittle relaxation response to tensile stresses within cooling lava flows and magma bodies, and are found in lavas that vary greatly in chemistry and outcrop geometry. However, columnar joints do not form in all cooling igneous rocks, and the specific conditions under which columnar joints form are unknown. In this study, outcrops containing columns in the Cheakamus Valley basalt flows near Whistler, BC are studied, and the size, orientation, and distribution of columns is recorded. Forward numerical models using the finite element method are created with Matlab using the Partial Differential Equation Toolbox to model the outcrops in the Whistler field area, and determine the cooling rates (∂T/∂t) and thermal gradients (∂T/∂x) experienced by the lava flows during their formation. High temperature experimentation involving basalt rock samples is then used to determine the cooling rates and thermal gradients present during the cooling of these samples under a variety of naturally occurring conditions. This study finds that noticeable differences in the distribution of columns within an outcrop occur only when there are large differences in cooling rates between the upper and lower outcrop surfaces. Modeling shows that the cooling rates must differ by approximately an order of magnitude. High temperature experiments show that extremely high cooling rates (especially in the small sample sizes used in this study) between approximately 700 to 800 ˚C are necessary for the formation of columnar joints.

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Results of a rock deformation study designed to investigate the energy budgets of glass fragmentation under triaxial conditions are presented. This work comprises a series of room-temperature experiments designed to explore the fundamental mechanical behaviour of natural (obsidian) and synthetic glasses (Pyrex) under confining pressures of 0.1 - 100 MPa and at displacement rates of 40μm/s. The results quantify the amount of energy stored in the samples prior to failure, and establish a relationship between grain-size distributions of experimental-products (D-values) and the stress drop at failure. The relationship found for compressive fragmentation is significantly different from the relationship between D-values and energy densities established by previous authors for decompressive fragmentation. Furthermore, I show that natural glasses have less potential to store elastic energy after fragmentation than synthetic glasses. However, the stress storage capacity of natural glass can be enhanced (approaching synthetic glasses) through heat-treatment. The evolution of the physical properties (strain, porosity, permeability and ultrasonic wave velocities) of conduit breccia deposits during compaction is addressed. Compaction produces strongly anisotropic materials, and the measured physical properties are controlled by this anisotropy. Measurements of permeability are up to two orders of magnitude higher and seismic wave velocities up to twice as fast along the direction of elongation. Measurements of physical properties are combined with models describing the timescales of porosity loss and from that, the timescales of permeability reduction and re-pressurization of the edifice are discussed.

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Pyroclastic deposits resulting from explosive volcanic eruptions commonly contain clasts originating from the conduit wall rocks, which were entrained into the rapidly ascending stream of erupting material. These particles are termed accessory lithic clasts. Previous studies of the relative abundances and compositions of accessory lithic clasts have been used to identify the subsurface wall rocks of the volcanic conduit, to document variations in eruptive intensity, or to track changes in conduit or vent geometry over the course of the eruption. However, the morphological properties of accessory lithic clasts are largely ignored and offer an unused means of studying the processes operating in the conduit during explosive eruptions. During a volcanic eruption, wall rocks are violently fragmented to produce clasts that are incorporated into a hot, high velocity, particle-laden gas jet. There the clasts are subjected to elevated temperatures, blasting by volcanic ash, and occasional collisions with other large particles or with the conduit walls. The resultant morphologies of the accessory lithic clasts will be influenced by 1) the intrinsic physical properties of the clasts in question; 2) the specific physical and thermal processes to which the clasts were subjected within the conduit; and 3) the residence times of the clasts within the conduit. The 2360 B.P. Pebble Creek Formation of the Mount Meager Volcanic Complex in SW British Columbia is the product of the most recent explosive eruption in Canada. This formation includes a widespread pumice fallout deposit containing anomalously rounded and smoothed monzogranite accessory lithic clasts. In this study, I seek to explain the unusual shapes and surface textures of these clasts through detailed field work, analysis of sample morphology, and the computation of likely conditions within the conduit. My aim is to produce a comprehensive, mechanistic model of how these lithic clasts were reshaped within the volcanic conduit.

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The enigmatic Eastgate-Whipsaw metamorphic belt (EWm belt) is located on the western margin of the Late Triassic to Early Jurassic Nicola Group’s southernmost exposure in the Princeton area of south-central B.C. (NTS 092H/SE). Previously affiliated to the Nicola Group, new Early Permian ages of 281.3 ± 3.3 Ma and 282.7 ± 3.7 Ma show the EWm belt to be significantly older and is likely a southern manifestation of the Late Paleozoic Harper Ranch Group. This suggests a previously unrecognized potential in the Harper Ranch for hosting VHMS-style mineralization.The belt comprises metamorphically deformed, upper greenschist to lower amphibolite facies volcanic and volcanic-derived sedimentary rocks as well as pre- to syn-deformational intrusive phases. These rock types are divided into: 1) mafic volcanic amphibole-rich schists defining the western margin of the belt; 2) a thick, south-central package of quartzofeldspathic volcanic-derived sedimentary schists that transition northward into 3) a series of intercalated volcanic-derived sedimentary schists; 4) mafic volcanic amphibole (chlorite)-epidote schists on the south-east margin of the EWm belt; 5) a north-eastern package of felsic volcanic and volcanically-derived sedimentary schists that contain relict quartz and plagioclase phenocrysts, and 6) consanguinous meta-gabbro and foliated plagioclase porphyry intrusions of unknown ages. The EWm belt originated within an intra-arc or back-arc marine setting, as two geochemically distinct volcanic types. The eastern type 1 volcanic rocks comprise calkalkaline and tholeiitic compositions from LREE-enriched island arc basalts. The western type 2 volcanic rocks comprise a suite of back (or intra)-arc basin basalts. A complex set of foliations, the result of strong metamorphic deformation, is preserved within minor rock types of the EWm belt. The foliation set includes: 1) a dominant N-S, steeply westward dipping S₂ foliation that is predominantly continuous but also develops into differentiated crenulation cleavages, 2) a shallow east-dipping S₁ that is crenulated and progressively dextrally rotated by S₂-associated deformation, 3) a poorly-developed S₃ fabric that shallows the dominant S₂ foliation on the eastern side of the EWm belt. Equal metamorphic temperatures, equivalent to the greenschist to amphibolite facies transition, were established across the width of the EWm belt.

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Mount St. Helens reawakened 24 years after erupting in the 1980’s. This effusive eruption produced 95 million cubic meters of dacite in the form of 7 discrete, competent spines or domes of lava between September 2004- June 2008. The spines comprise low-porosity dacite that is inferred to have crystallized at a depth of about 1 km and are enveloped by a 1-3 meter carapace of fault gouge. The rate of linear extrusion of the spines peaked at 11 m/day in November 2004 and subsequently slowed to
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Accessory minerals such as carbonates and sulfides are important indicators of metasomatic enrichment events in the mantle lithosphere and ultimately control mantle melting. However, these phases are rarely expressed modally in mantle-derived peridotitic xenoliths. Here we report on a unique occurrence of primary mantle-derived carbonate preserved in spinel peridotite xenoliths within a 19 Ma basanite dike. The dike, defined as part of the Cheslatta Lake suite, intrudes Jurassic volcaniclastic rocks near the Intermontane - Coast Belt boundary in western British Columbia, and contains primary magmatic calcite. The peridotite xenoliths are concentrated in a 4 metre interval where the dike is narrowest and are dominated by lherzolite, with less abundant harzburgite, dunite, and websterite. Thermometry (Taylor, 1998) produces paleoequilibration temperatures of 792 to 1044 °C, corresponding to approximately 32 to 49 km depth on an average geotherm for warm, thin Cordilleran-style lithosphere. Magnesian calcite occurs in all of the 51 xenoliths examined. The Mg-calcite appears as intergranular grains that appear to have been in textural equilibrium with neighbouring minerals, as inclusions, and as fracture-filling veins, commonly in association with pentlandite and chalcopyrite. Carbon-oxygen isotopic compositions of carbonates from the dike, wall rock, and xenoliths indicates that each have distinct compositions. Oxygen isotopic compositions of the xenoliths’ carbonate indicates that, consistent with textural observations, the carbonates have equilibrated with the host mantle, having δ¹⁸O compositions less than 3 ‰ greater than compositions interpreted for primary mantle-derived carbonatites. In addition, carbonates from the mantle xenoliths fall into two separate groups identified by different δ¹³C and ⁸⁷Sr/⁸⁶Sr isotopic compositions, indicating different sources for carbonate in the two xenolith groups, and thus multiple enrichment events. Strontium isotopic ratios also show that the dike scavenged carbonate from disaggregated mantle xenoliths, becoming enriched in CO₂. Timing of enrichment is unconstrained, and no relationships between the host magma and mantle-derived xenoliths is required, contrasting with other studies. A plausible metasomatic agent is a carbonate melt with associated monosulfide solution, derived from the subduction of oceanic crust beneath North America during Coast Plutonic Belt magmatism, when Mt. Preston was in an arc to back arc position.

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The Chilcotin Group basalt (CGB) of south-central British Columbia, Canada defines a medium-sized igneous province (ca. 17, 000 km²), characterized by basaltic lavas, volcaniclastic deposits, and paleosols with minor ash deposits. The CGB has previously been mapped only at reconnasissance scale (1:250 000), and most studies concentrated on geochemical and petrological studies; no stratigraphic relationships or volcanological models were attempted. Chasm canyon exposes one of the thickest successions of the CGB. Here, I explicate the volcanic facies architecture at Chasm to reconstruct the emplacement history and volcanism in the Neogene using geological mapping, cross-sections, and graphic logs. Specifically, seven discrete facies are recognized. The coherent facies are: i) vesicular/amygdaloidal pahoehoe lobes; ii) columnar-jointed, sheet-like lava; and iii) intact basaltic pillow lava. The clastic facies are: iv) paleosols; v) pillow-fragment breccia; vi) hyaloclastite; and vii) lacustrine sandstone. Facies are grouped into broad facies associations including the subaerial facies and interstratified subaqueous and subaerial facies. The subaqueous facies are a minor component in the canyon stratigraphy. The geometry of the lavas is indicative of the eruptive style of volcanism at Chasm, which defines the volcanic facies architecture. Four architectural elements have been observed: i) tabular-classic (TC), which represents a steady continuous supply of subaerial effusive basaltic lavas; ii) compound-braided (CB), which is typical of a shield volcano where anastomosing, branching flow fields result; iii) transitional-mixed, a combination of TC- and CB-type suggestive of bimodal emplacement, perhaps sourced from coalesced shield volcanoes and flank fissures; and iv) foreset-bedded indicative of subaqueous lavas. The exposed rocks record the evolution of CGB volcanism through ten distinct eruptive episodes and intermittent lakes, with periods of quiescence characterized by the paleosol development. Whole-rock Ar-Ar dates were obtained; the duration of volcanism is calculated as 1.28 ± 0.61 m.y. Emplacement is suggestive of shield volcanoes and small fissure eruptions with a northerly flow direction. Laterally extensive paleosols, classified as Brunisolic soils, were examined closely and display a range of morphological features suggestive of the paleo-environment. Lateral variability amongst paleosols have been mapped over a distance of more than 8 km, including a subqueous to subaerial transition.

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