Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer
Abstract: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 occur… Show more
“…The role of stylolites as conduits for diagenetic fluids in very different diagenetic settings has been documented in the literature. Stacey et al (2021) describe low-amplitude stylolites cross-cutting (and thus post-dating) four phases of hydrothermal replacive dolomite in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin, and how they are filled with a late dolomite cement. As in the case of the Benicàssim case study, these authors relate this cement to hydrobreccias related to fluid volume expansion.…”
Section: Stylolites As Diagenetic Fluid Flow Conduitsmentioning
There is an ongoing debate on whether stylolites act as barriers or conduits for fluids, or even play no role in terms of fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study (Maestrat Basin, E Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during the geological evolution of a basin. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and calcitization processes. Anastomosing stylolites, which pre-date burial dolomitization, likely acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front weaves up and down following consecutive anastomosing stylolites, which are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Contrarily, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers again when the burial and stress conditions changed. Stylolites within dolostones close to faults are found corroded and filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent calcitization reactions and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.
“…The role of stylolites as conduits for diagenetic fluids in very different diagenetic settings has been documented in the literature. Stacey et al (2021) describe low-amplitude stylolites cross-cutting (and thus post-dating) four phases of hydrothermal replacive dolomite in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin, and how they are filled with a late dolomite cement. As in the case of the Benicàssim case study, these authors relate this cement to hydrobreccias related to fluid volume expansion.…”
Section: Stylolites As Diagenetic Fluid Flow Conduitsmentioning
There is an ongoing debate on whether stylolites act as barriers or conduits for fluids, or even play no role in terms of fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study (Maestrat Basin, E Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during the geological evolution of a basin. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and calcitization processes. Anastomosing stylolites, which pre-date burial dolomitization, likely acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front weaves up and down following consecutive anastomosing stylolites, which are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Contrarily, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers again when the burial and stress conditions changed. Stylolites within dolostones close to faults are found corroded and filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent calcitization reactions and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.
“…(3) Lower Cambrian (541–509 Ma) to Triassic strata were deposited on a passive margin. Episodic basement reactivation and renewed rifting in the Cambrian gave rise to regional thermal subsidence with evidence that heat flow and tectonic activity remained high 26 , 30 , 31 . (4) Jurassic to Paleocene strata were deposited in a foreland basin that developed during the Columbian (Jurassic to Early Cretaceous) and Laramide (Late Cretaceous to Paleocene) orogenies.…”
Section: Geological Settingmentioning
confidence: 99%
“… Figure 1 ( a ) Location of the study area in Alberta, Canada showing the major thrust faults and the Cambrian platform margin (modified from Stacey et al 26 ; based on Vandeginste et al 37 ). ( b ) Geological map of the study area (modified from Stacey et al 26 ; based on the Alberta Geological Survey Interactive Minerals Map: https://ags.aer.ca/publication/iam-001 ). ( c ) Stratigraphy of the southern Rocky Mountains (drafted by Dr. J. Stacey, based on Aitken 32 , 34 ).…”
Section: Geological Settingmentioning
confidence: 99%
“…Non-stratabound dolomite bodies originate from normal-to-transtensional faults and include several diagenetic mineral phases that have distinct compositions and textures. Given that the timing and mechanism of dolomitization are well-constrained 23 – 26 , this succession is ideal to test whether hyperspectral imaging can be used to identify and map multiple, visibly indistinguishable, phases of dolomite in outcrop. In particular, this methodological study investigates the extent to which dolomite stoichiometry and texture can be determined by hyperspectral imaging.…”
Section: Introductionmentioning
confidence: 99%
“…In particular, this methodological study investigates the extent to which dolomite stoichiometry and texture can be determined by hyperspectral imaging. Consequently, the mineral distribution map products facilitate the validation and/or revision of existing fault-controlled dolomitization models 23 – 26 .…”
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.
The Derbyshire Platform is a Mississippian aged flat-topped, steep sided platform that forms the westernmost expression of the Derbyshire-East Midlands Platform. On the south-east platform margin, 60 km 2 of Visean limestone has been dolomitised, forming two distinct bodies. One of these bodies forms along a major NW-SE trending basement fault and smaller, associated, N-S trending faults and fractures. This study uses outcrop, petrographic and geochemical analysis to better constrain the timing and mechanism for this fault-controlled dolomitisation. Field relationships demonstrate dolomitisation was multi-phase and initiated after the main phase of matrix pore-occluding calcite cementation on the Derbyshire Platform and terminated prior to the main phase of mineralisation. Fluids are interpreted to have fluxed from adjacent basins, primarily along strike-slip crustal faults that were reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded. The study demonstrates the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults that connect the carbonate platform to basin compartments. This ultimately controlled the position of dolomite geobodies along faults and provides a record of fluid flow during the transition from thermal subsidence to post-rift basin inversion. The findings have implications for the exploration of both minerals and hydrocarbon within dolomitised host rocks and can inform studies of fluid transfer and reaction on carbonate platforms within the burial realm.
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