The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) i... more The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth's ocean-atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chalcophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the deposition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O 2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.
Basin-scale processes such as regional fluid flow and organic maturation are commonly associated ... more Basin-scale processes such as regional fluid flow and organic maturation are commonly associated with the precipitation of Pb-Zn sulphide minerals. Despite extensive study, the origin of carbonate-hosted Pb-Zn mineralisation (Mississippi Valley-type and Irish-type) remains controversial, with deposits viewed as exceptional features derived from unusual basin conditions (e.g. regional hydrothermal fluid flux). In order to explore the links between mineralisation and broader-scale basinal processes, we have examined the relationship of base metal sulphides to regional diagenetic phases in the Waulsortian Limestone (Feltrim Formation) of the world-class Irish Midlands Zn-Pb province. Using combined sedimentology, petrography, and trace/rare earth element chemistry of regional calcite cements, we have reconstructed the basinal fluid history before, during and after sulphide mineralisation. The non-bright-dull cathodoluminescence and trace metal composition of calcite cements indicates that the Waulsortian Limestone was subjected to three major chemical environments during progressive burial diagenesis: 1. Near-surface/shallow-burial oxic conditions; 2. subsurface euxinic conditions; and 3. subsurface ferruginous (ferrosulphidic) conditions. Zn-Pb mineralisation occurred under stage 2 euxinic conditions. We suggest that euxinic conditions were initiated when hydrocarbons entered the Waulsortian succession regionally, associated with the introduction of sour gas (H 2 S) accumulations. Mixing between more oxic base metal-bearing and H 2 S-bearing fluids produced ideal conditions for voluminous Zn-Pb sulphide precipitation. Increasing fluid anoxia then led to iron oxide dissolution with consequently increased iron sulphide precipitation, causing eventual depletion of H 2 S, and marked the onset of ferruginous conditions that terminated metal sulphide precipitation. Base metals were potentially derived from a range of sources that include Carboniferous seawater (and the dissolution of associated Mn/Fe oxyhydroxides), hydrocarbons, and basement-interacted basinal brines. The chemical model proposed by this study is compatible with existing data on the Irish Zn-Pb Orefield and indicates that sulphide mineralisation is linked to normal basin-scale processes such as subsidence, aquifer redox evolution and hydrocarbon generation. These results may explain many other global occurrences of carbonate-hosted Zn-Pb mineralisation and provide new insights into the chemical and redox processes that occur during burial diagenesis.
Geochemical data for dolomite and limestone (trace element, rare earth element, carbon and oxygen... more Geochemical data for dolomite and limestone (trace element, rare earth element, carbon and oxygen stable isotope, clumped oxygen isotope, noble gas, fluid inclusion and bulk rock XRD) of the Middle Cambrian Cathedral Formation, Southern Canadian Rocky Mountains.
Dolomitisation of carbonate rocks can significantly modify rock petrophysical properties, and hen... more Dolomitisation of carbonate rocks can significantly modify rock petrophysical properties, and hence hydrocarbon reservoir potential, yet the shape, size and termination of such bodies is often poorly described. This project considers the basin-scale controls on fault-controlled hydrothermal dolomitisation within the Western Canada Sedimentary Basin (WCSB), a mature petroleum province in which dolomitized limestone is one of the most important reservoir-types. This study focuses on Cambrian sections exposed in the Canadian Rocky Mountains to the southwest of Alberta. In this region, the Middle Cambrian carbonate platform is characterised by a series of shallowing-upward cycles known as Grand Cycles (Aitken, 1997; Jeary, 2002). The Cathedral Formation comprises the upper carbonate section of the lowest Grand Cycle, overlying the shales and carbonates of the Mount Whyte Formation. The Mount Whyte and Cathedral formations overlie the coarsegrained clastics of the Gog Group and are capped by the Stephen Formation shales (Jeary, 2002). Outcrops of the Mount Whyte and Cathedral formations are located near Whirlpool Point, northwest of Banff National Park, Alberta, and contain both stratabound and non-stratabound dolostone bodies, tens of metres or more in diameter. Faulting, fracturing, brecciation and zebra dolomite textures are commonly observed features. Geological Framework The Western Canada Sedimentary Basin (WCSB) is a large sedimentary basin underlying most of western Canada. It is a mature hydrocarbon province that comprises both conventional and unconventional resources. The basin is situated within a complex tectonic region, influenced by the Foreland Fold and Thrust Belt of the Canadian Rocky Mountains to the west and several transform faults. The sedimentation history of this basin can be divided into two phases associated with distinct tectonic settings (Wright et al., 1994; Ross et al., 1994). The Palaeozoic to Jurassic was mostly dominated by a succession of carbonate platforms with minor siliciclastic incursions, deposited in a relatively stable cratonic setting. The basin originally developed as a passive margin (Price, 1994), persisting from the Precambrian to Late Devonian (Bradley, 2008), when the Antler Orogeny is thought to have changed the basin configuration to that of a back-arc setting (Nelson et al., 2002). Root (2001) proposed that the WCSB was a distal foreland basin during the Devonian and Mississippian, and used sedimentological and structural evidence to support the existence of this event. Although the exact impact of the Antler Orogeny on the WCSB is poorly understood, certain fault-related dolomites have been dated as Devonian in age using 87Sr/86Sr data (Duggan et al., 2001; Lonnee and Machel, 2006; Machel and Buschkuehle, 2008), suggesting that fault reactivation and hydrothermal fluid flux occurred from the Late Devonian to Mississippian. The back-arc setting established in the Late Devonian persisted until the Middle Jurassic, when a foreland basin stage was initiated by the Columbian and Laramide Orogenies (Pana et al., 2001). The Mount Whyte Formation (Figure 1) represents the first carbonate unit deposited during the first cratonic transgression in the WCSB (Slind et al., 1994). It predominantly consists of interbedded limestone-dolostone and shale, which in places, can be up to 176 m thick. Aitken (1989) concluded that the Mount Whyte Formation comprises the earliest part and base of the first Grand Cycle, with the Cathedral Formation representing the ‘cap carbonate unit’. The Cathedral Formation represents the first major cliff forming carbonate unit in the Canadian Rockies and is comprised of limestone and dolostone with a maximum thickness of ~360 m in the Main Ranges. Methodology A total 105.9 m section was logged and described in the field and 78 representative carbonate samples were collected from both the Mount Whyte and Cathedral formations at the Whirlpool Point locality, northwest of Banff National Park. Thin sections were studied using standard petrographic techniques and stained with Alizarin Red S (Dickson, 1966) to qualitatively discriminate between calcite and dolomite. Cathodoluminescence (CL) analysis was performed to determine different dolomite phases and cements. In situ XRF analysis was also conducted along the logged section.
Determination of Basin-Scale Fluid Flux to Understand the Processes Controlling Regional Dolomiti... more Determination of Basin-Scale Fluid Flux to Understand the Processes Controlling Regional Dolomitization in the Western Canada Sedimentary Basin I declare that no portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentary and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that fine-grained deposition in deep-water environments corresponds to positive δ 13 C excursions and eustatic highstands, while coarse-grained deposition corresponds to negative δ13 C excursions and eustatic lowstands during the earliest Oligocene. While alternative explanations cannot be ruled out on the basis of this dataset alone, our results suggest that eustatic fluctuations across the EOT and into the early Oligocene influenced sediment supply to deep-water environments.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentological and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that submarine fan retreat in the Alpine foreland basin corresponds with positive δ 13 C excursions related to major global perturbations of the carbon cycle and cooling in the earliest Oligocene. Submarine fan retreat is suggested to be influenced by this cooling through enhanced aridity and reduced subaerial runoff from the Corsica-Sardinia hinterland. The influence of aridity was periodically overwhelmed by local environmental factors, such as hinterland uplift, which increased sediment supply to deep-water during arid periods. These results highlight that: 1) hinterland climate may play a greater role than sealevel in dictating sediment supply to deep-water and, 2) submarine fan evolution occurs through a complex interplay between climate, eustasy and tectonics, which makes robust interpretations of paleoenvironmental change from their stratigraphic record, without multi-proxy records, difficult.
This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral For... more This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases...
Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitati... more 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...
Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurrin... more Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO 3) is replaced by dolomite (CaMg (CO 3) 2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO 3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ 18 O water values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body. These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
The work contained in this paper forms a section of a PhD study undertaken as part of the Natural... more The work contained in this paper forms a section of a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil & Gas (grant number NE/M00578X/1) under its Extending the Life of Mature Basins research theme. It is fully funded by NERC, with additional funds from the American Association of Petroleum Geologists (AAPG) Grants-in-Aid General Fund, whose support is gratefully acknowledged. The analyses and interpretations of this work were primarily undertaken in the Microanalysis Facility and the Advanced Isotope Geochemistry and Cosmochemistry Suite at the University of Manchester, for which the University of Manchester is thanked for its support of the labs and facilities. Stable isotope analyses were conducted in the Liverpool Isotope Facility for Environmental Research at the University of Liverpool and Dr Stephen Crowley is thanked for his help. We acknowledge the www.ags.aer.ca website administered by the Alberta Geological Survey (AGS), for access to digital well data and maps invaluable to this study. The AGS is also thanked for its provision of data and access to drill cores used in this study.
Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any dept... more Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including 'deep-seated', 'crustal' or 'basinal' brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large-scale dolostone bodies are fabric-destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross-cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the d 18 O water and 87 Sr/ 86 Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO 2-rich fluids. This study highlights that large-scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.
The term hydrothermal dolomitization (HTD) has been widely applied to describe fault-controlled d... more The term hydrothermal dolomitization (HTD) has been widely applied to describe fault-controlled dolomitization. HTD can occur at any depth from a range of different fluids, even though it is often associated with the burial realm. To date, many studies have failed to identify a source of fluids for HTD, often invoking “deep-seated brines”. Consequently, there is a gap in our knowledge as to the likely source of water and magnesium for the formation of hydrothermal dolostone. The Middle Cambrian Mount Whyte Formation displays exceptionally preserved m- to km-wide non-stratabound dolostone bodies with stratabound terminations within a bioturbated mudstone facies. Dolomitization is fabric-destructive and shows a wide range of crystal sizes (40-800 µm). Texturally, dolomite crystals can be subdivided into different phases that are all post-dated by bedding-parallel stylolites. This cross-cutting relationship suggests a shallow depth of dolomitization (< 1 km). All the dolomite types ...
This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral For... more This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230°C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/ km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite cemented breccias at much shallower depths than previously realized.
Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, produci... more Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta-analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium-rich formation water data published over 30 years, with new petrographic, X-ray diffraction, stable isotope and rare-earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross-cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross- cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr-rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/ albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium-rich formation waters suggest basinal brines originated as residual evapo-concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo-concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentary and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that fine-grained deposition in deep-water environments corresponds to positive δ 13 C excursions and eustatic highstands, while coarse-grained deposition corresponds to negative δ13 C excursions and eustatic lowstands during the earliest Oligocene. While alternative explanations cannot be ruled out on the basis of this dataset alone, our results suggest that eustatic fluctuations across the EOT and into the early Oligocene influenced sediment supply to deep-water environments.
Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitati... more 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.
Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurrin... more Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO3) is replaced by dolomite (CaMg (CO3)2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ18Owater values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body.
These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) i... more The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth's ocean-atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chalcophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the deposition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O 2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.
Basin-scale processes such as regional fluid flow and organic maturation are commonly associated ... more Basin-scale processes such as regional fluid flow and organic maturation are commonly associated with the precipitation of Pb-Zn sulphide minerals. Despite extensive study, the origin of carbonate-hosted Pb-Zn mineralisation (Mississippi Valley-type and Irish-type) remains controversial, with deposits viewed as exceptional features derived from unusual basin conditions (e.g. regional hydrothermal fluid flux). In order to explore the links between mineralisation and broader-scale basinal processes, we have examined the relationship of base metal sulphides to regional diagenetic phases in the Waulsortian Limestone (Feltrim Formation) of the world-class Irish Midlands Zn-Pb province. Using combined sedimentology, petrography, and trace/rare earth element chemistry of regional calcite cements, we have reconstructed the basinal fluid history before, during and after sulphide mineralisation. The non-bright-dull cathodoluminescence and trace metal composition of calcite cements indicates that the Waulsortian Limestone was subjected to three major chemical environments during progressive burial diagenesis: 1. Near-surface/shallow-burial oxic conditions; 2. subsurface euxinic conditions; and 3. subsurface ferruginous (ferrosulphidic) conditions. Zn-Pb mineralisation occurred under stage 2 euxinic conditions. We suggest that euxinic conditions were initiated when hydrocarbons entered the Waulsortian succession regionally, associated with the introduction of sour gas (H 2 S) accumulations. Mixing between more oxic base metal-bearing and H 2 S-bearing fluids produced ideal conditions for voluminous Zn-Pb sulphide precipitation. Increasing fluid anoxia then led to iron oxide dissolution with consequently increased iron sulphide precipitation, causing eventual depletion of H 2 S, and marked the onset of ferruginous conditions that terminated metal sulphide precipitation. Base metals were potentially derived from a range of sources that include Carboniferous seawater (and the dissolution of associated Mn/Fe oxyhydroxides), hydrocarbons, and basement-interacted basinal brines. The chemical model proposed by this study is compatible with existing data on the Irish Zn-Pb Orefield and indicates that sulphide mineralisation is linked to normal basin-scale processes such as subsidence, aquifer redox evolution and hydrocarbon generation. These results may explain many other global occurrences of carbonate-hosted Zn-Pb mineralisation and provide new insights into the chemical and redox processes that occur during burial diagenesis.
Geochemical data for dolomite and limestone (trace element, rare earth element, carbon and oxygen... more Geochemical data for dolomite and limestone (trace element, rare earth element, carbon and oxygen stable isotope, clumped oxygen isotope, noble gas, fluid inclusion and bulk rock XRD) of the Middle Cambrian Cathedral Formation, Southern Canadian Rocky Mountains.
Dolomitisation of carbonate rocks can significantly modify rock petrophysical properties, and hen... more Dolomitisation of carbonate rocks can significantly modify rock petrophysical properties, and hence hydrocarbon reservoir potential, yet the shape, size and termination of such bodies is often poorly described. This project considers the basin-scale controls on fault-controlled hydrothermal dolomitisation within the Western Canada Sedimentary Basin (WCSB), a mature petroleum province in which dolomitized limestone is one of the most important reservoir-types. This study focuses on Cambrian sections exposed in the Canadian Rocky Mountains to the southwest of Alberta. In this region, the Middle Cambrian carbonate platform is characterised by a series of shallowing-upward cycles known as Grand Cycles (Aitken, 1997; Jeary, 2002). The Cathedral Formation comprises the upper carbonate section of the lowest Grand Cycle, overlying the shales and carbonates of the Mount Whyte Formation. The Mount Whyte and Cathedral formations overlie the coarsegrained clastics of the Gog Group and are capped by the Stephen Formation shales (Jeary, 2002). Outcrops of the Mount Whyte and Cathedral formations are located near Whirlpool Point, northwest of Banff National Park, Alberta, and contain both stratabound and non-stratabound dolostone bodies, tens of metres or more in diameter. Faulting, fracturing, brecciation and zebra dolomite textures are commonly observed features. Geological Framework The Western Canada Sedimentary Basin (WCSB) is a large sedimentary basin underlying most of western Canada. It is a mature hydrocarbon province that comprises both conventional and unconventional resources. The basin is situated within a complex tectonic region, influenced by the Foreland Fold and Thrust Belt of the Canadian Rocky Mountains to the west and several transform faults. The sedimentation history of this basin can be divided into two phases associated with distinct tectonic settings (Wright et al., 1994; Ross et al., 1994). The Palaeozoic to Jurassic was mostly dominated by a succession of carbonate platforms with minor siliciclastic incursions, deposited in a relatively stable cratonic setting. The basin originally developed as a passive margin (Price, 1994), persisting from the Precambrian to Late Devonian (Bradley, 2008), when the Antler Orogeny is thought to have changed the basin configuration to that of a back-arc setting (Nelson et al., 2002). Root (2001) proposed that the WCSB was a distal foreland basin during the Devonian and Mississippian, and used sedimentological and structural evidence to support the existence of this event. Although the exact impact of the Antler Orogeny on the WCSB is poorly understood, certain fault-related dolomites have been dated as Devonian in age using 87Sr/86Sr data (Duggan et al., 2001; Lonnee and Machel, 2006; Machel and Buschkuehle, 2008), suggesting that fault reactivation and hydrothermal fluid flux occurred from the Late Devonian to Mississippian. The back-arc setting established in the Late Devonian persisted until the Middle Jurassic, when a foreland basin stage was initiated by the Columbian and Laramide Orogenies (Pana et al., 2001). The Mount Whyte Formation (Figure 1) represents the first carbonate unit deposited during the first cratonic transgression in the WCSB (Slind et al., 1994). It predominantly consists of interbedded limestone-dolostone and shale, which in places, can be up to 176 m thick. Aitken (1989) concluded that the Mount Whyte Formation comprises the earliest part and base of the first Grand Cycle, with the Cathedral Formation representing the ‘cap carbonate unit’. The Cathedral Formation represents the first major cliff forming carbonate unit in the Canadian Rockies and is comprised of limestone and dolostone with a maximum thickness of ~360 m in the Main Ranges. Methodology A total 105.9 m section was logged and described in the field and 78 representative carbonate samples were collected from both the Mount Whyte and Cathedral formations at the Whirlpool Point locality, northwest of Banff National Park. Thin sections were studied using standard petrographic techniques and stained with Alizarin Red S (Dickson, 1966) to qualitatively discriminate between calcite and dolomite. Cathodoluminescence (CL) analysis was performed to determine different dolomite phases and cements. In situ XRF analysis was also conducted along the logged section.
Determination of Basin-Scale Fluid Flux to Understand the Processes Controlling Regional Dolomiti... more Determination of Basin-Scale Fluid Flux to Understand the Processes Controlling Regional Dolomitization in the Western Canada Sedimentary Basin I declare that no portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentary and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that fine-grained deposition in deep-water environments corresponds to positive δ 13 C excursions and eustatic highstands, while coarse-grained deposition corresponds to negative δ13 C excursions and eustatic lowstands during the earliest Oligocene. While alternative explanations cannot be ruled out on the basis of this dataset alone, our results suggest that eustatic fluctuations across the EOT and into the early Oligocene influenced sediment supply to deep-water environments.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentological and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that submarine fan retreat in the Alpine foreland basin corresponds with positive δ 13 C excursions related to major global perturbations of the carbon cycle and cooling in the earliest Oligocene. Submarine fan retreat is suggested to be influenced by this cooling through enhanced aridity and reduced subaerial runoff from the Corsica-Sardinia hinterland. The influence of aridity was periodically overwhelmed by local environmental factors, such as hinterland uplift, which increased sediment supply to deep-water during arid periods. These results highlight that: 1) hinterland climate may play a greater role than sealevel in dictating sediment supply to deep-water and, 2) submarine fan evolution occurs through a complex interplay between climate, eustasy and tectonics, which makes robust interpretations of paleoenvironmental change from their stratigraphic record, without multi-proxy records, difficult.
This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral For... more This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases...
Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitati... more 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...
Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurrin... more Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO 3) is replaced by dolomite (CaMg (CO 3) 2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO 3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ 18 O water values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body. These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
The work contained in this paper forms a section of a PhD study undertaken as part of the Natural... more The work contained in this paper forms a section of a PhD study undertaken as part of the Natural Environment Research Council (NERC) Centre for Doctoral Training (CDT) in Oil & Gas (grant number NE/M00578X/1) under its Extending the Life of Mature Basins research theme. It is fully funded by NERC, with additional funds from the American Association of Petroleum Geologists (AAPG) Grants-in-Aid General Fund, whose support is gratefully acknowledged. The analyses and interpretations of this work were primarily undertaken in the Microanalysis Facility and the Advanced Isotope Geochemistry and Cosmochemistry Suite at the University of Manchester, for which the University of Manchester is thanked for its support of the labs and facilities. Stable isotope analyses were conducted in the Liverpool Isotope Facility for Environmental Research at the University of Liverpool and Dr Stephen Crowley is thanked for his help. We acknowledge the www.ags.aer.ca website administered by the Alberta Geological Survey (AGS), for access to digital well data and maps invaluable to this study. The AGS is also thanked for its provision of data and access to drill cores used in this study.
Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any dept... more Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including 'deep-seated', 'crustal' or 'basinal' brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large-scale dolostone bodies are fabric-destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross-cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the d 18 O water and 87 Sr/ 86 Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO 2-rich fluids. This study highlights that large-scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.
The term hydrothermal dolomitization (HTD) has been widely applied to describe fault-controlled d... more The term hydrothermal dolomitization (HTD) has been widely applied to describe fault-controlled dolomitization. HTD can occur at any depth from a range of different fluids, even though it is often associated with the burial realm. To date, many studies have failed to identify a source of fluids for HTD, often invoking “deep-seated brines”. Consequently, there is a gap in our knowledge as to the likely source of water and magnesium for the formation of hydrothermal dolostone. The Middle Cambrian Mount Whyte Formation displays exceptionally preserved m- to km-wide non-stratabound dolostone bodies with stratabound terminations within a bioturbated mudstone facies. Dolomitization is fabric-destructive and shows a wide range of crystal sizes (40-800 µm). Texturally, dolomite crystals can be subdivided into different phases that are all post-dated by bedding-parallel stylolites. This cross-cutting relationship suggests a shallow depth of dolomitization (< 1 km). All the dolomite types ...
This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral For... more This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometer-scale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230°C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/ km, due to rift-related basin extension, fluids likely convected along faults that extended to ∼6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regional-scale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomite cemented breccias at much shallower depths than previously realized.
Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, produci... more Dolomitization in the Western Canadian Sedimentary Basin has been extensively researched, producing vast geochemical datasets. This provides a unique opportunity to assess the regional sources and flux of dolomitizing fluids on a larger scale than previous studies. A meta-analysis was conducted on stable isotope, strontium isotope (87Sr/86Sr), fluid inclusion and lithium-rich formation water data published over 30 years, with new petrographic, X-ray diffraction, stable isotope and rare-earth element (REE+Y) data. The Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group contain replacement dolomite (RD) cross-cut by stylolites, suggesting replacement dolomitization occurred during shallow burial. Stable isotope, REE+Y and 87Sr/86Sr data indicate RD formed from Devonian seawater, then recrystallized during burial. Apart from the Wabamun Group of the Peace River Arch (PRA), saddle dolomite cement (SDC) is more δ18O(PDB) depleted than RD, and cross- cuts stylolites, suggesting precipitation during deep burial. SDC 87Sr/86Sr data indicate contributions from 87Sr-rich basinal brines in the West Shale Basin (WSB) and PRA, and authigenic quartz/albite suggests basinal brines interacted with underlying clastic aquifers before ascending faults into carbonate strata. The absence of quartz/ albite within dolomites of the East Shale Basin (ESB) suggests dolomitizing fluids only interacted with carbonate strata. We conclude that replacement dolomitization resulted from connate Devonian seawater circulating through aquifers and faults during shallow burial. SDC precipitated during deep burial from basinal brines sourced from basal carbonates (ESB) and clastic aquifers (WSB, PRA). Lithium-rich formation waters suggest basinal brines originated as residual evapo-concentrated Middle Devonian seawater that interacted with basal aquifers and ascended faults during the Antler and Laramide Orogenies. These results corroborate those of previous studies but are verified by new integrated analysis of multiple datasets. New insights emphasize the importance of basal aquifers and residual evapo-concentrated seawater in dolomitization, which is potentially applicable to other regionally dolomitized basins.
The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifyi... more The Eocene-Oligocene transition (EOT) was a period of considerable environmental change, signifying the transition from Paleocene greenhouse to Oligocene icehouse conditions. Preservation of the sedimentary signal of such an environmental change is most likely in net-depositional environments, such as submarine fans, which are the terminal parts of sedimentary systems. Here, using sedimentary and stable isotope data from the Alpine foreland basin, we assess whether this major climatic transition influenced the stratigraphic evolution of submarine fans. Results indicate that fine-grained deposition in deep-water environments corresponds to positive δ 13 C excursions and eustatic highstands, while coarse-grained deposition corresponds to negative δ13 C excursions and eustatic lowstands during the earliest Oligocene. While alternative explanations cannot be ruled out on the basis of this dataset alone, our results suggest that eustatic fluctuations across the EOT and into the early Oligocene influenced sediment supply to deep-water environments.
Carbonate rocks undergo low-temperature, post-depositional changes, including mineral precipitati... more 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.
Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurrin... more Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO3) is replaced by dolomite (CaMg (CO3)2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ18Owater values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body.
These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
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These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.