Now showing 1 - 5 of 268
- ItemShortwave infrared hyperspectral imaging as a novel method to elucidate multi-phase dolomitization, recrystallization, and cementation in carbonate sedimentary rocks(2021) McCormick, Cole A.; Corlett, Hilary; Stacey, Jack; Hollis, Cathy; Feng, Jilu; Rivard, Benoit; Omma, Jenny E.Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitation, dissolution, or recrystallisation (diagenesis). Unravelling the sequence of these events is time-consuming, expensive, and relies on destructive analytical techniques, yet such characterization is essential to understand their post-depositional history for mineral and energy exploitation and carbon storage. Conversely, hyperspectral imaging offers a rapid, non-destructive method to determine mineralogy, while also providing compositional and textural information. It is commonly employed to differentiate lithology, but it has never been used to discern complex diagenetic phases in a largely monomineralic succession. Using spatial-spectral endmember extraction, we explore the efficacy and limitations of hyperspectral imaging to elucidate multi-phase dolomitization and cementation in the Cathedral Formation (Western Canadian Sedimentary Basin). Spectral endmembers include limestone, two replacement dolomite phases, and three saddle dolomite phases. Endmember distributions were mapped using Spectral Angle Mapper, then sampled and analyzed to investigate the controls on their spectral signatures. The absorption-band position of each phase reveals changes in %Ca (molar Ca/(Ca + Mg)) and trace element substitution, whereas the spectral contrast correlates with texture. The ensuing mineral distribution maps provide meter-scale spatial information on the diagenetic history of the succession that can be used independently and to design a rigorous sampling protocol.
- ItemBasin scale evolution of zebra textures in fault-controlled, hydrothermal dolomite bodies: insights from the Western Canadian Sedimentary Basin(2023) McCormick, Cole A.; Corlett, Hilary; Clog, Matthieu; Boyce, Adrian; Tartèse, Romain; Steele-MacInnis, Matthew; Hollis, CathyStructurally controlled dolomitization typically involves the interaction of high-pressure (P), high-temperature (T) fluids with the surrounding host rock. Such reactions are often accompanied by cementation and recrystallization, with the resulting hydrothermal dolomite (HTD) bodies including several ‘diagnostic’ rock textures. Zebra textures, associated with boxwork textures and dolomite breccias, are widely considered to reflect these elevated P/T conditions. Although a range of conceptual models have been proposed to explain the genesis of these rock textures, the processes that control their spatial and temporal evolution are still poorly understood. Through the detailed petrographical and geochemical analysis of HTD bodies, hosted in the Middle Cambrian strata in the Western Canadian Sedimentary Basin, this study demonstrates that a single genetic model cannot be applied to all the characteristics of these rock textures. Instead, a wide array of sedimentological, tectonic and metasomatic processes contribute to their formation; each of which is spatially and temporally variable at the basin scale. Distal to the fluid source, dolomitization is largely stratabound, comprising replacement dolomite, bedding-parallel zebra textures and rare dolomite breccias (non-stratabound, located only proximal to faults). Dolomitization is increasingly non-stratabound with proximity to the fluid source, comprising bedding-inclined zebra textures, boxwork textures and dolomite breccias that have been affected by recrystallization. Petrographical and geochemical evidence suggests that these rock textures were initiated due to dilatational fracturing, brecciation and precipitation of saddle dolomite as a cement, but significant recrystallization occurred during the later stages of dolomitization. These rock textures are closely associated with faults and carbonate-hosted ore deposits (e.g. magnesite, rare earth element and Mississippi Valley–type mineralization), thus providing invaluable information regarding fluid flux and carbonate metasomatism under elevated P/T conditions.
- ItemMapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification(2023) Corlett, Hilary; Feng, Jilu; Playter, Tiffany; Rivard, BenoitSilica cycling in the world’s oceans is not straightforward to evaluate on a geological time scale. With the rise of radiolarians and sponges from the early Cambrian onward, silica can have two depositional origins, continental weathering, and biogenic silica. It is critical to have a reliable method of differentiating amorphous silica and crystalline silica to truly understand biogeochemical and inorganic silica cycling. In this study, opal-A is mapped across the Western Canada Sedimentary Basin in the Late Devonian Duvernay Formation shales using longwave hyperspectral imaging alongside geochemical proxies that differentiate between crystalline and amorphous SiO2, during the expansion of the world’s early forests. Signaled by several carbon isotope excursions in the Frasnian, the punctata Event corresponds to the expansion of forests when vascular land plants develop seeds and deeper root networks, likely resulting in increased pedogenesis. Nutrients from thicker soil horizons entering the marine realm are linked to higher levels of primary productivity in oceans and subsequent oxygen starvation in deeper waters at this time. The results of this study reveal, for the first time, the spatial distribution of amorphous SiO2 across a sedimentary basin during this major shift in the terrestrial realm when forests expand and develop deeper root networks.
- ItemFar-ultraviolet dust extinction and molecular hydrogen in the diffuse Milky Way interstellar medium(2023) Putte, Dries Van De; Cartledge, Stefan; Gordon, Karl; Geoffrey, Chiekwero; Roman-Duval, JuliaWe aim to compare variations in the full-UV dust extinction curve (912–3000 Å), with the H I/H2/total H content along diffuse Milky Way sightlines, to investigate possible connections between ISM conditions and dust properties. We combine an existing sample of 75 UV extinction curves based on IUE and FUSE data, with atomic and molecular column densities measured through UV absorption. The H2 column density data are based on existing Lyman–Werner absorption band models from earlier work on the extinction curves. Literature values for the H I column density were compiled, and improved for 23 stars by fitting a Lyα profile to archived spectra. We discover a strong correlation between the H2 column and the far-UV extinction, and the underlying cause is a linear relationship between H2 and the strength of the far-UV rise feature. This extinction does not scale with H I, and the total H column scales best with A(V) instead. The carrier of the far-UV rise therefore coincides with molecular gas, and further connections are shown by comparing the UV extinction features to the molecular fraction. Variations in the gas-to-extinction ratio N (H) A(V) correlate with the UV-to-optical extinction ratio, and we speculate this could be due to coagulation or shattering effects. Based on the H2 temperature, the strongest far-UV rise strengths are found to appear in colder and denser sightlines.
- ItemPermafrost, geomorphic, and hydroclimatic controls on mercury, methylmercury, and lead concentrations and exports in Old Crow River, arctic western Canada(2022) Staniszewska, Kasia J.; Reyes, Alberto V.; Cooke, Colin A.; Miller, Brooklyn S.; Woywitka, RobinPermafrost degradation has been implicated as a dominant control on riverine mercury fluxes in arctic watersheds. However, the importance of permafrost thaw on fluxes of mercury, methylmercury, and trace metals such as lead—relative to other geomorphic and hydroclimatic controls—remains unclear. To investigate these controls, we conducted ~weekly water chemistry sampling at the mouth of the Old Crow River, a pristine, 13,900 km2 watershed in arctic Canada underlain entirely by continuous permafrost. Mercury, methylmercury, and lead concentrations were low on average (~ 2 ng/L, 0.04 ng/L, 0.8 μg/L, respectively), and peaked during the freshet (< 7 ng/L, 0.11 ng/L, 11 μg/L, respectively). The trace elements had strong positive association with suspended sediment, and were mobilized during periods of high discharge (freshet and rainfall). Summer time sampling of major tributaries and at thaw slumps revealed that trace element concentrations were not elevated downstream of thaw slumps or thermokarst lakes across the watershed. Ubiquitous thermokarst in the Old Crow basin did not result in anomalously high catchment-scale concentrations, fluxes, and yields of mercury, methylmercury, nor lead. Rather, warming-driven increases in precipitation and elevated discharge during freshet and rainfall promoted permafrost and talik river bank erosion. This erosion, which was controlled by landscape and geomorphic factors, supplied short-lived increases in particle-bound trace element flux.