Browsing by Author "Corlett, Hilary"
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- ItemA geocellular modelling workflow for partially dolomitized remobilized carbonates: an example from the Hammam Faraun Fault block, Gulf of Suez, Egypt(2021) Corlett, Hilary; Hodgetts, David; Hirani, Jesal; Rotevatn, Atle; Taylor, Rochelle; Hollis, CathyConstructing geocellular models of carbonate rocks using standard software is challenging since most of modelling packages are designed, first and foremost, to represent siliciclastic depositional systems, where rock properties are strongly facies-controlled. The distribution and components of carbonate depositional facies vary drastically across the geological timescale as a result of paleoclimate and its effects on carbonate-producing biota. Furthermore, reservoir architecture is less strongly controlled by depositional environment than in clastic settings, and rock physical properties, including fracture networks, are controlled by both primary components and their subsequent diagenetic alteration. This means that rock property distribution is less predictable than in siliciclastic systems, and less well represented by geocellular models that are designed to represent sedimentary architecture. In other words, in carbonate systems, the depositional and diagenetic history needs to be reconstructed in order to successfully model reservoir properties. In this study a geocellular model was created by using a well-characterised outcrop analogue obtained from the Hammam Faraun Fault (HFF) Block, located on the eastern coast of the Gulf of Suez in Sinai, Egypt. This model integrates sedimentological, petrophysical, diagenetic, and structural information into a single database. The workflow utilizes the regional tectonic history, upscaled lithological logs, and two-stage facies modelling (reflecting in and ex situ depositional facies) and resulted in the creation a realistic model of remobilized carbonates that were deposited on the slope of a carbonate platform during a period of tectonic instability. Diagenetic overprinting was achieved using probability functions to reflect the history of burial, rifting, and the spatial relationship of stratabound and non-stratabound dolostone bodies. The study demonstrates a workflow for modelling mass-transport carbonate facies and multistage fault-related diagenesis so that flow controlling facies and diagenetically altered poroperm and fracture networks are accurately represented using commercially available modelling software, and in particular demonstrates how diagenetically controlled geobodies can be captured using simple algorithms.
- ItemAn overview of structurally-controlled dolostone-limestone transitions in the stratigraphic record(2021) Koeshidayatullah, Ardiansyah; Corlett, Hilary; Hollis, CathyIn structurally-controlled dolomitization systems, there is a general consensus that the formation of dolostone-limestone transitions, termed here as “dolomitization fronts”, is governed by either the presence of an ultra-low permeability zone (fluid barrier) or changes in dolomitization potential and kinetics. However, the actual processes controlling the abrupt termination of dolostone bodies, and their corresponding morphology and dimension, are still relatively poorly understood. To address these challenges, we aim to (i) review the different origin and styles of structurally-controlled dolomitization fronts in the stratigraphic record and (ii) provide a standardized framework and quantitative insight to describe and interpret dolomitization fronts. To achieve this, field observations across geologic timescales and geodynamic settings are complimented with published data to document different styles of structurally-controlled dolomitization fronts. The results show that the following morphologies are associated with both tabular and columnar dolostone bodies: (i) lateral contact/bed-perpendicular fronts; (ii) vertical contact/bed-parallel fronts; and (iii) complex-shaped fronts at the distal part of dolostone bodies. This morphological information, when coupled with detailed petrography, mineralogical and geochemical data could help to accurately reveal the governing processes behind the termination of dolostone bodies and their corresponding reaction front geometries. Our review shows that the first front type is primarily controlled by the interplay between intrinsic properties of the host rocks, dolomitizing fluids, and self-organization process. In contrast, the second front type is governed by the presence of laterally continuous depositional, diagenetic, or structural fluid barriers, creating a significant permeability contrast across beds. The formation of complex-shaped fronts is interpreted to be controlled by a combination of original lithological composition and kinetics. This overview provides the first multi-study categorisation of ancient dolomitization fronts and the controls on their formation at a range of scales. This improves our understanding of low temperature metasomatic processes, and their termination, in sedimentary systems. Furthermore, it highlights how accurate interpretation of the origin and styles of dolomitization fronts can improve our understanding of dolomitization processes, paleofluid flow, and distribution of economic resources in dolomitized carbonate platforms, which can be challenging to determine from the dolostone bodies themselves, where they have undergone multiple phases of recrystallization and diagenetic overprinting.
- 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.
- ItemBurial dolomitization driven by modified seawater and basal aquifer-sourced brines: Insights from the Middle and Upper Devonian of the Western Canadian Sedimentary Basin(2020) Stacey, Jack; Hollis, Cathy; Corlett, Hilary; Koeshidayatullah, ArdiansyahDolomitization 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.
- ItemCarbonate sedimentology of coral reefs in Sumba Island, Indonesia(2022) Ramos, Cedrick; Corlett, HilaryThe Island of Sumba, Indonesia, comprises several reef terraces formed by changing global sea levels and episodic tectonic activity. Volcanic-derived sediment has been found to have been incorporated into reef fabrics without impacting coral growth. The uninterrupted reef growth shows a conflicting theory that coral reefs do not survive under significant clastic input. Continuous reef growth has been documented in this region known as the Coral Triangle. However, it is unclear whether intervals of increased clastic content affect coral growth strategies or whether the immature nature of the volcaniclastic sediment (larger and angular grain) explains why clastic input does not choke corals. This study employs various thin sections from fossil reefs (late-Pleistocene) to document and characterize the sediment’s size and circularity, which is a measure of sediment maturity. The study involves imaging analysis by taking images of each thin section. The thin sections have all been stained with Alizarin red, a stain that only turns calcite or aragonite grains a shade of red, and all other minerals remain unstained. Then, the images were colour indexed to black and white, with clastic grain appearing as white. The images underwent thresholding to isolate the white grains to measure their circularity and sizes. It was found that the grains of the sediments are sand size and angular, which means that the grains only have some transported involved, but they did not deposit far away from their source. A range of grain sizes in these sections have revealed that some of the grains detected are small enough to enter corallites and pores in the coral skeleton, while others are much larger and would not impact the coral growth.
- ItemClinoform identification and correlation in fine‐grained sediments: a case study using the Triassic Montney Formation(2018) Playter, Tiffany; Corlett, Hilary; Konhauser, Kurt; Robbins, Leslie; Rohais, Sebastien; Crombez, Vincent; MacCormack, Kelsey; Rokosh, Dean; Prenoslo, Donald; Furlong, Carolyn M.; Pawlowicz, John; Gingras, Murray K.; Lalonde, Stefan; Lyster, Steven; Zonneveld, John-PaulStratigraphic correlation of fine-grained successions is not always straightforward. Complicating factors, such as unconformities, structural complexity, subsidence and especially minimal grain-size variation, make the application of traditional correlation methods to fine-grained successions problematic. Alternatively, the analysis of detailed geochemical data can allow for the determination of variations in sediment provenance, mineralogy, detrital flux and hydrothermal input. When compared with modelled clay input over time, these geochemical indicators can be used to determine changes in relative sea-level and palaeoclimate, allowing for the identification of clinoform surfaces. As an example, this study outlines detailed correlations of chemostratigraphic packages within the lower Triassic Montney Formation in Western Canada to demonstrate the effectiveness of chemostratigraphy in defining and correlating fine-grained clinoforms across a sedimentary basin. The data set used includes five wells and one outcrop succession, from which geochemical profiles were generated and tied directly to mineralogical data and well logs. These analyses reveal 13 distinct chemostratigraphic packages that correlate across the basin. Observed elemental and inferred mineralogical changes highlight trends in relative sea-level and palaeoclimate, as well as episodes of inferred hydrothermal input to the Montney basin. Cross-plots of La/Sm and Yb/Sm further suggest hydrothermal input as well as the scavenging of middle rare earth elements by phosphatic fish debris. Additionally, plots of La/Sm versus Yb/Sm, which show volcanic arc input within the Doig Formation, suggest an additional sediment source from the west during the Anisian. Pairing detrital and clay proxies demonstrates changes in relative sea-level and, at the Smithian/Spathian boundary, the lowest relative sea-level in the Montney Formation is observed, corresponding to a change in climate.
- ItemControls on the formation of stratabound dolostone bodies, Hammam Faraun Fault block, Gulf of Suez(2018) Hirani, Jesal; Bastesen, Eivind; Boyce, Adrian; Corlett, Hilary; Gawthorpe, Rob; Hollis, Cathy; Cédric, John M.; Robertson, Hamish; Rotevatn, Atle; Whitaker, FionaDolomitization is commonly associated with crustal-scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass-imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north-south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1.43 concentration) seawater at less than ca 80 degrees C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface-breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0.7 m year (super -1) into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal-scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.
- ItemEcological controls on Devonian stromatoporoiddominated and coral-dominated reef growth in the Mackenzie Basin, Northwest Territories, Canada(2011) Corlett, Hilary; Jones, BrianThe Horn Plateau Formation, composed of isolated reefs, is part of the Devonian strata that formed in the Mackenzie Basin in the Northwest Territories, Canada. The reefs stretch over a 350 m northeast-southwest trend and are dominated by tabulate and rugose corals in the northeast, near exposed Canadian Shield rocks, and stromatoporoids further out in the basin. Detailed facies analyses of each reef type shows distinct differences in their biological makeup, energy regimes, and carbonate sedimentation rates. Geochemical analyses (stable isotopes and rare-earth elements) set against established paleogeography in the Mackenzie Basin reveal that the coral-dominated and stromatoporoid-dominated reefs grew under different ecological conditions. Separations in the data imply that the coral-dominated reefs grew in waters that were relatively enriched in nutrients and the stromatoporoid-dominated reefs, further down the ramp, were in oligotrophic conditions. With no current established method to directly measure Paleozoic nutrient levels or to detect where they were sourced from, it is unclear why the coral-dominated reefs experienced higher nutrient levels. The paleogeography of the Mackenzie Basin could have affected the apparent stratification of nutrients on the carbonate ramp. Possible nutrient sources in the area are from coastal upwelling from the open ocean northwest of the ramp, or locally sourced nutrients from runoff on the adjacent exposed Canadian Shield rocks.
- ItemEpiphyte communities on Thalassia testudinum from Grand Cayman, British West Indies; their composition, structure, and contribution to lagoonal sediments(2007) Corlett, Hilary; Jones, BrianThalassia testudinum, the most common seagrass found in lagoons around Grand Cayman, influences sedimentation by baffling currents, binding sediment on the seafloor, and providing substrates for a diverse epiphytic biota. About 85% of the epiphytic biota is formed of at least 3 species of coralline algae, 72 species of foraminifera, and 61 species of diatoms. The rest of the biota is formed of sponges, gastropods, ostracods, coccoliths, dinoflagellates, brown algae, and worms. The epiphytes are organized in three communities that are part of an organized tripartite community succession. The basal diatom community is overlain by the coralline algae community, which is then overlain by a community composed of a variety of taxa. The coralline algae community, which is the most extensive, typically covers ∼ 75% of the leaf's surface. Potentially, the skeletons of these epiphytes can make a significant contribution to the fine-grained sediment budget of these lagoons. Surprisingly, only a few of the epiphytes were found in the lagoonal sediment. It appears, therefore, that the epiphytes are lost through skeletal dissolution or transported out of the lagoon following storms. Irrespective of the cause, the epiphytes do not form a significant part of the lagoon sediment in Grand Cayman.
- ItemEvaluating new fault-controlled hydrothermal dolomitization models: insights from the Cambrian Dolomite, Western Canadian Sedimentary Basin(2020) Koeshidayatullah, Ardiansyah; Corlett, Hilary; Stacey, Jack; Swart, Peter K.; Boyce, Adrian; Robertson, Hamish; Whitaker, Fiona; Hollis, CathyFault-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 δ18Owater and 87Sr/86Sr 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 CO2-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.
- ItemFault-controlled dolomitization in a rift basin(2017) Hollis, Cathy; Bastesen, Eivind; Boyce, Adrian; Corlett, Hilary; Gawthorpe, Rob; Hirani, Jesal; Rotevatn, Atle; Whitaker, FionaThere are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.
- ItemFaults and associated karst collapse suggest conduits for fluid flow that influence hydraulic fracturing-induced seismicity(2018) Galloway, Elwyn; Hauck, Tyler E.; Corlett, Hilary; Pena, Dinu; Schultz, RyanDuring December 2011, a swarm of moderate-magnitude earthquakes was induced by hydraulic fracturing (HF) near Cardston, Alberta. Despite seismological associations linking these two processes, the hydrological and tectonic mechanisms involved remain unclear. In this study, we interpret a 3D reflection-seismic survey to delve into the geological factors related to these earthquakes. First, we document a basement-rooted fault on which the earthquake rupture occurred that extends above the targeted reservoir. Second, at the reservoir’s stratigraphic level, anomalous subcircular features are recognized along the fault and are interpreted as resulting from fault-associated karst processes. These observations have implications for HF-induced seismicity, as they suggest hydraulic communication over a large (vertical) distance, reconciling the discrepancy between the culprit well trajectory and earthquake hypocenters. We speculate on how these newly identified geological factors could drive the sporadic appearance of induced seismicity and thus be utilized to avoid earthquake hazards.
- ItemLinking fossil reefs with earthquakes: geologic insight to where induced seismicity occurs in Alberta(2016) Schultz, Ryan; Corlett, Hilary; Haug, Kristine; Kocon, Ken; MacCormack, Kelsey; Stern, Virginia; Shipman, ToddRecently, a significant increase in North American, midcontinent earthquakes has been associated with contemporaneous development of petroleum resources. Despite the proliferation of drilling throughout sedimentary basins worldwide, earthquakes are only induced at a small fraction of wells. In this study, we focus on cases of induced seismicity where high-resolution data are available in the central Western Canada Sedimentary Basin. Our regional comparison of induced earthquake depths suggests basement-controlled tectonics. Complementary to these findings, hypocenters of induced seismicity clusters coincide with the margins of Devonian carbonate reefs. We interpret this spatial correspondence as the result of geographically biased activation potential, possibly as a consequence of reef nucleation preference to paleobathymetric highs associated with Precambrian basement tectonics. This finding demonstrates the importance of geologic/tectonic factors to earthquake induction, in addition to industrial operational parameters. In fact, the observation of induced seismicity silhouetting deep fossil reef systems may be a useful tool to identify future regions with increased seismogenic potential.
- 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.
- ItemMeteoric diagenesis and dedolomite fabrics in precursor primary dolomicrite in a mixed carbonate–evaporite system(2018) Hauck, Tyler E.; Corlett, Hilary; Grobe, Matthias; Walton, Erin L.; Sansjofre, PierreDedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate-evaporite successions. In these settings, elevated Ca (super 2+) /Mg (super 2+) ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north-eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non-pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north-eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative delta (super 18) O and low delta (super 13) C values (averages of -13.6 ppm (sub VPDB) and 0.5 ppm (sub VPDB) , respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of -6.0 ppm (sub VPDB) and 1.2 ppm (sub VPDB) , respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non-pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs.
- ItemMixing of brine with oil triggered sphalerite deposition at Pine Point, Northwest Territories, Canada(2021) Szmihelsky, Marko; Steele-MacInnis, Matthew; Bain, Wyatt M.; Falck, Hendrik; Adair, Robin; Campbell, Brandon; Dufrane, S. Andrew; Went, Ashley; Corlett, HilaryHydrocarbons are commonly invoked as triggers for the precipitation of sphalerite in carbonate-hosted Pb-Zn deposits, but direct evidence for the presence of petroleum during sulfide mineralization is rarely documented. Here, we report evidence of fluid mixing between basinal brines and oil during deposition of coarse sphalerite at a classic carbonate-hosted Pb-Zn district, Pine Point, Northwest Territories, Canada. The brines contain high concentrations of Pb, detectable aqueous sulfate, and hydrocarbons that attest to chemical interaction with oil. The oil inclusions in sphalerite contain much less Pb relative to the brines and no evident H2S, suggesting that the principal role of hydrocarbons was as a reductant. Mixing of brine with oil enabled the conversion of aqueous sulfate to sulfide, and thereby triggered sphalerite deposition.
- ItemOrigin and evolution of fault-controlled hydrothermal dolomitization fronts: a new insight(2020) Koeshidayatullah, Ardiansyah; Corlett, Hilary; Stacey, Jack; Swart, Peter K.; Boyce, Adrian; Hollis, CathyDolomitization 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 Owater 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.
- ItemOrigin, dimensions, and distribution of remobilized carbonate deposits in a tectonically active zone, Eocene Thebes Formation, Sinai, Egypt(2018) Corlett, Hilary; Bastesen, Eivind; Gawthorpe, Rob; Hirani, Jesal; Hodgetts, David; Hollis, Cathy; Rotevatn, AtleDetermination of the distribution and mechanism for carbonate-dominated mass transport sediments is often compromised by the scale and access to exposures. Consequently, many studies lack the resolution to capture the heterogeneity and dimensions of mass transport deposits. This study documents the size, shape, and stratal assemblage of remobilized carbonates in the Eocene Thebes Formation in the Hammam Faraun Fault Block (HFFB) of western Sinai, revealing the complexities of carbonate mass transport deposits at sub-seismic scale. Present day pseudo three-dimensional exposure of the Thebes Formation in a large fault block, formed during the opening of the Gulf of Suez, allowed for lateral and down-dip measurement of slope and basinal facies in the field and from photos. Remobilized facies were digitized in the photos and evaluated using image analysis software, a technique with a wide range of applications to outcrop studies of sedimentary architecture. Debris flow deposits in the lower section of the Thebes Formation comprises clasts with differing fossil assemblages. A relative sea level rise at the start of upper Thebes Formation deposition resulted in basinal sediments comprising periodic incursions of high-density turbidite grainstones encased within a background of planktonic foraminiferal wackestones. Foraminiferal assemblages of remobilized facies imply multiple sources on the carbonate platform, demonstrating the effect of short-lived tectonism on slope instability and deposition of mass transport deposits. The results of the study confirm that tectonism associated with the Syrian Arc Fold Belt, which altered the style of basin sedimentation between Egypt and Syria, persisted into the Eocene at least as far south as Wadi Araba in the western desert and Hammam Faraun in Sinai. In addition, the shape, size, and extent of the two dominant remobilized facies, debris flows and grainstone turbidites are influenced by their mud-rich versus grainy compositions.
- ItemQuantification of depositional and diagenetic geobody geometries for reservoir modelling, Hammam Fauran Fault Block, Sinai Peninsula, Egypt(2013) Hollis, Cathy; Corlett, Hilary; Hirani, Jesal; Hodgetts, David; Gawthorpe, Rob; Rotevatn, Atle; Bastesen, EivindOutcrop data has traditionally been used to constrain conceptual models during subsurface reservoir characterisation and geocellular modeling, but published data of depositional and diagenetic geobodies in carbonate systems is lacking. Furthermore, few studies address how these diagenetic bodies, which often cross-cut sedimentary bedding, can be captured in reservoir models, even though diagenetic modification is likely to impart a significant influence on flow behaviour. This paper presents a case study from the differentially dolomitised pre-rift Eocene Thebes Formation on the Sinai Peninsula. It documents the size of depositional and diagenetic geobodies and demonstrates how these data have been incorporated into a 3D geocellular model. The results can be used as input parameters or templates for reservoirs in which fault/fracture controlled dolomite bodies have been described, whilst the workflow could have broader applications to other carbonate reservoirs.
- ItemRegional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer(2021) Stacey, Jack; Corlett, Hilary; Holland, Greg; Koeshidayatullah, Ardiansyah; Cao, Chunhui; Swart, Peter K.; Crowley, Stephen; Hollis, CathyThis 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.