Sabkha and Burrow-Mediated Dolomitization in the Mississippian Debolt Formation, Northwestern Alberta, Canada (original) (raw)
Related papers
Journal of Sedimentary Research, 2012
The Lonely Bay Formation, located to the west of Great Slave Lake, Northwest Territories, Canada, is a thickbedded limestone succession that includes four facies that are characteristic of a Devonian middle-ramp depositional setting. One facies in the Lonely Bay Formation is intensely bioturbated with some burrows filled with calcite and others with dolomite. The calcite-filled burrows are found close to the paleo-shoreline of the Canadian Shield, whereas the dolomite-filled burrows are found in deeper ramp deposits. In the calcite-filled burrows the parent burrows, each surrounded by a diagenetic halo, are readily apparent, whereas the dolomite-filled burrows are largely devoid of original structures. Each burrow type has its own distinctive geochemical suites of rare-earth-elements (REE), trace-elements, and d 18 O (PDB) and d 13 C (PDB) isotopes. These data indicate that sulfate-reducing bacteria, reducing conditions, and marine organic matter were present in the dolomite-filled burrows. Conversely, geochemical data from the calcite-filled burrows indicate that they remained in suboxic conditions and contained little to no marine organic matter that would have contributed to the formation of early dolomite. For these burrows, continent-derived organic matter may have hindered dolomite formation. The contrast between the two types of burrows clearly shows how the different diagenetic environments influenced the evolution of the carbonate. This study, based on interpretations of various geochemical signatures, highlights the roles that oxygen concentrations and types of organic matter (continental versus marine derived) played in dolomite precipitation.
Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites
Sedimentary Geology, 2000
Several large hydrocarbon accumulations in Alberta, Canada are hosted in dolomitized successions of stacked, thin sabkha-capped cycles of Visean age. Porosity is micro-intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved. Two such fields are here considered, that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures. The dolomite in the upper Debolt Formation of the Dunvegan Field (NW Alberta) forms planar-e or microsucrosic fabrics with crystals in the 1-20 mm range. The dolomite is non-ferroan, Ca-rich (average of 58 mol% CaCO 3 ), and poorly ordered. Its stable isotopic signatures range from Ϫ0.12 to ϩ3.4‰ VPDB for d 18 O (mean ϩ1.3‰) and ϩ0.9 to ϩ4.3‰ VPDB for d 13 C (mean ϩ2.6‰). The average radiogenic 87 Sr/ 86 Sr ratio for this dolomite is 0.7077. Both sets of values are consistent with dolomite precipitation from Mississippian marine or modified marine (evaporated) seawater. These parameters are strongly reminiscent of Holocene protodolomites and hence suggestive of a sabkha dolomitization process (shallow seepage reflux or evaporitive pumping). This dolomite with its high associated porosity (average of 15%, and up to 38%), relatively unaltered mineralogical and chemical signatures, both preserved despite 4 km of burial depth, suggests a very unique set of relatively non-reactive physico-chemical conditions during burial (likely a closed system). In contrast, the dolomite from the Mount Head Formation of the Shell Waterton Field (SW Alberta) has undergone measurable neomorphic alteration in several stages in deeper burial environments (open system). Such alteration has affected its crystal size (range Ͻ10-100 mm), and isotopic chemistry (d 18 O ranges between Ϫ1.5 in the least altered dolomite and Ϫ13.2‰ VPDB; d 13 C ranges from ϩ3.9 to Ϫ1.5‰ VPDB; and 87 Sr/ 86 Sr ranges from 0.7078 to 0.7090). The dolomite has retained a degree of non-stoichiometry with an average Ca content of ϳ55 mol% CaCO 3 . The Mount Head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial, and are in this respect representative of the norm for dolostones in the Western Canada Sedimentary Basin. ᭧
The Formation of Dolomite in Sediments from the Continental Margin of Northeastern Queensland
Proceedings of the Ocean Drilling Program, 1993
The occurrence of dolomite in young Pleistocene sediments off the northeastern coast of Australia offers a special opportunity to examine dolomite formation relative to the original mineralogy and the remineralization of organic material through the processes of sulfate reduction and methanogenesis. The four cores examined in this study form a transect from shallow to deep water (Sites 821, 820, 819, and 822). At all sites, dolomite occurs in sediments of Pleistocene age and is more abundant closer to the continental margin (Site 821) than in sediments found in deeper water (Sites 819 and 822). In spite of the fact that the classic diagenetic zones of sulfate reduction and methanogenesis were all well developed at Sites 819, 820, 821, and 822, the δ 13 C of the interstitial dissolved inorganic carbon (DIC) showed no evidence of these diagenetic processes. The dolomites retrieved from sediments showed δ 13 C values similar to that of the DIC. The isotopic composition of the dolomite revealed no evidence of having been formed in either the sulfate reduction zone or the zone of methanogenesis. The Leg 133 sites differ from other anoxic hemipelagic areas in which extensive sulfate reduction and methanogenesis have been documented in that the sediments are dominated by CaCO 3. Recrystallization of calcium carbonate apparently buffers the isotopic composition of the system, masking large isotopic changes that might be induced by additions of isotopically light or heavy CO 2 derived from the processes of sulfate reduction and methanogenesis. The dolomite does not appear to be preferentially located in the portion of the cores in which sulfate is absent. Based on the association between the abundance of high-Mg calcite (HMC) and dolomite, it is thought that dolomites formed in response to the dissolution of HMC.
Chemical Geology, 2015
The early diagenetic formation of dolomite in modern aquatic environments is limited mostly to evaporitic and marine-anoxic, organic-rich sediments dominated by bacterial sulfate reduction (BSR). In such environments, bacterial activity lowers the energy barriers for the nucleation and growth of dolomite and thus promotes the formation of non-stoichiometric, highly disordered and metastable (proto)dolomite. Although the boundary conditions for the formation of modern (proto)dolomites are considered to be generally understood, the role of BSR during limestone dolomitization in ancient marine environments remains questionable. Herein, we present a study about the physicochemical conditions and processes, which led to the formation of partly dolomitized limestone and dolostone in the presence of BSR on a stable carbonate platform during the Upper Jurassic at Oker (Northern German Basin). The dolomite textures, the spatial trace elemental patterns of the dolomite and of the surrounding limestone and the results of δ 18 O and δ 13 C isotope analyses reveal that the Oker dolomite has been formed by the early diagenetic replacement of magnesian calcite precursors at temperatures between 26°C and 37°C. We interpret the mineralizing fluids responsible for dolomitization as pristine-marine to slightly evaporitic and reducing seawater being modified during shallow seepage reflux and/or evaporitic tidal pumping. The elevated δ 34 S CAS values (+17.9 to +19.7‰, V-CDT) of the Oker dolomite, compared to ambient Upper Jurassic seawater, indicate that BSR facilitated dolomite formation. For the first time, we show that a linear anti-correlation exists between decreasing carbonate-associated sulfate (CAS) contents in dolomite and increasing ordering ratio of the dolomite lattice structure, with the degree of cation order in dolomite to be given by: degree of cation order (Dol) : = −0.018•CAS (Dol) + 68.3 (R 2 = 0.98). This correlation implies that the CAS content of sedimentary dolomite can be used as a measure for dolomite maturity. The relationships between the ambient (paleo)environmental controls, the resultant dolomitization pathways and subsequently the structure and the composition of the precipitating dolomite are presented and discussed in relation to the stability of modern and ancient (proto)dolomites throughout burial diagenesis.
Sedimentary Geology, 2011
The Denault Formation (2.1-1.9 Ga) crops out in the Labrador Trough, northeastern Québec and western Labrador. Rocks surrounding the town of Schefferville, Quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm-influenced shallow ramp. Mid-ramp facies consist of intraclastic grainstones with hummocky cross-stratification (HCS), swaley cross-stratification (SCS), current ripples, and graded event beds. Further outboard, grainstones grade into deeper-water laminites that are composed of even, mm-scale couplets of flat-lying organic and dolomudstone laminae. Scours within the laminites suggest periodic storm activity. Laminites gradually grade into outer ramp deep-water shales. An isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km (present day). This succession can be interpreted as the remnant of a near-continuous margin or may simply represent an isolated accumulation that developed on a pre-existing topographic high. The presence of gypsum pseudomorphs in all lithofacies indicates that the Denault margin was restricted and evaporitic. Four paragenetic stages are recognized in the diagenetic evolution of the Denault Formation: (1) carbonate deposition, contemporaneous marine cementation, authigenic gypsum growth, and precipitation of authigenic chert; (2) synsedimentary mimetic dolomite precipitation; (3) pore-rimming and pore-occluding shallow burial dolomite cement; and (4) fabric destructive, sutured, anhedral burial dolomite. Gypsum crystals occur in all lithofacies, form the nuclei of interstitial dolomite rhombs, average 10 μm in length, and often display swallowtail twinning. Paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum. Formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water Mg/Ca ratios and lowering its dissolved sulfate concentrations. Such an interpretation may explain why there is an abundance of synsedimentary dolostone in the Precambrian and the relative paucity of Phanerozoic analogs.
Applied Geochemistry, 1992
Dolomites from the Middle and Upper Devonian of five regions in the Western Canada Sedimentary Basin were studied: Middle Devonian Winnipegosis Formation (outcrop), Keg River Formation reefs in the Rainbow field and Presqu'ile barrier, and the Upper Devonian Miette buildup and the Wabamun Group of northern Alberta. Data from the Upper-Middle Devonian Swan Hills Formation in the Rosevear field and Nisku Formation reefs were also included. In these areas of western Canada there are at least three dolomitizing events: 1. Early dolomites in 87 86 Winnipegosis reefs, and the subsurfacepart of the Presqu'ile barrier have Sr/ Sr values near 0.7080, Wabamun sabkha dolomites have 87Sr/~°Sr values near 0.7083, and all fall on their respective parts of the seawater curve; thus the sources of dolomitizing fluids were Devonian seawaters; 2. Secondary matrix dolomites from the Presqu'ile barrier, Rainbow, Rosevear, Miette, Nisku and Wabamun all have slightly higher 87Sr/a6Sr values (0.7081-0.7094) than the corresponding Devonian seawater. Dolomitizing fluids were probably modified from Devonian seawater with small amounts of radiogenic STSr being added during early compaction from adjacent or underlying clastics, or older carbonates; 3. Late-stage saddle dolomites from Presqu'ile, Rainbow, Rosevear, Miette, Nisku and Wabamun are all highly radiogenic (>0.7100 uj~ to 0.7151), except for most Pine Point saddle dolomites, indicating significant input of radiogenic °'Sr, with likely sources from the underlying Cambrian and Precambrian clastics and/or the Precambrian basement. At Pine Point, some Presqu'ile saddle dolomites have Sr isotopic values similar to matrix dolomites, suggesting a different fluid source, possibly from subsurface brines that may have evolved from pressure solution of Devonian strata. Alternatively, formation waters derived from the deeper part of the basin may have been progressively diluted with low 87 Sr/S6Sr shallow formation waters as they moved updip along the Presqu'ile barrier. The STSr/86Sr values support and refine earlier conclusions based on textural evidence, C and O isotopes, geochemistry and fluid inclusions. They considerably limit the sources of the dolomitizing fluids in open diagenetic systems. However, the timing of the widespread secondary matrix dolomites can only be broadly constrained to sometime during the Late Paleozoic. The similarity of the O and Sr isotopes in the replacement dolomites from widely separated locations geographically and stratigraphically tentatively suggests that these dolomites may have precipitated from similar formation waters on a basin-wide scale. The late-stage saddle dolomites, and probably the coarsely crystalline dolomites, could only have formed near maximum burial conditions in the Late Cretaceous to Early Tertiary from warm hydrothermal brines that moved updip from more deeply buried portions of the basin. Later coarse-crystalline calcites precipitated from highly saline brines and are generally more radiogenic than saddle dolomites. They apparently formed from more radiogenic formation waters than have currently been reported from the basin.
Dolomite: occurrence, evolution and economically important associations
Earth-science Reviews, 2000
Dolomite is not a simple mineral; it can form as a primary precipitate, a diagenetic replacement, or as a hydrothermal/metamorphic phase, all that it requires is permeability, a mechanism that facilitates fluid flow, and a sufficient supply of magnesium. Dolomite can form in lakes, on or beneath the shallow seafloor, in zones of brine reflux, and in early to late burial settings. It may form from seawater, from continental waters, from the mixing of basinal brines, the mixing of hypersaline brine with seawater, or the mixing of seawater with meteoric water, or via the cooling of basinal brines. Bacterial metabolism may aid the process of precipitation in settings where sulfate-reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings. Dolomite is a metastable mineral, early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism. Each new phase is formed by the partial or complete dissolution of an earlier dolomite. This continual re-equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures. Because subsurface dolomite evolves via dissolution and reprecipitation, a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart. Dolomitization also creates new crystals, with new rhomb growth following the dissolution of less stable precursors. Repetition of this process, without complete pore cementation, can generate intercrystalline porosity a number of times in the rock's burial history. Intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage. The fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings. The same association of evaporites (sulfate source) and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals. This tends to occur at higher temperatures (>60°C--80°C) and so the resulting dolomites tend to be ferroan and consist of saddle-shaped crystals.
Testing the preservation potential of early diagenetic dolomites as geochemical archives
Sedimentology
Early marine diagenetic dolomite is a rather thermodynamically-stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post-depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable d 13 C and d 18 O isotopes, 87 Sr/ 86 Sr ratios, and U-Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike-slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric-preservation, and marine dolomitization of precursor aragonite and calcite during the Middle-Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto-Atlantic opening stress regime (T ca 250°C) during the Early-Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high-grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late-stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 10 8 or more years. Evidence for this preservation includes preserved Late Triassic seawater d 13 C DIC values and primary fluid inclusion data. Data also indicate that oversimplified statements 849
Geobiology, 2004
The formation of dolomite is generally explained using models that reflect larger-scale processes that describe the relationship between the supply and transport of Mg, and geochemical conditions that are amenable to the formation of dolomite. However, heterogeneities in the substrate, such as those made by bioturbating infauna, may play a more important role in dolomitization than has been previously considered. Burrow-facilitated dolomitization is evident in the Ordovician Tyndall Stone (Red River Group, Selkirk Formation) of central Canada. The diagenetic fabrics present are attributed to dolomitizing fluids that both flowed through and evolved within burrow networks. Petrographic analysis suggests that two phases of dolomite formation took place. The first formed a fine-grained, fabric-destructive type that probably accompanied early burial; the second is a fine-to medium-grained, locally sucrosic dolomite that is interpreted to have precipitated during later burial. Isotopic analysis supports the proposed paragenetic history: (1) an apparent linking of the stable isotopes 13 C and 18 O strongly suggests that the micrite matrix formed during very early diagenesis and was derived from seawater;
Sedimentology, 2020
In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow-water carbonate sediments the δ 34 S, 47 , 13 C, 18 O, 44/40 Ca and 26 Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ 34 S values similar to those found in contemporaneous seawater and 44/40 Ca, 26 Mg values, Sr contents and 47 temperatures (25 to 30 o C) indicating relatively shallow dolomitization in a fluid-buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ 34 S values indicating dolomitization in a more sediment-buffered diagenetic system where bacterial sulphate reduction enriches the residual SO 4 2in 34 S, consistent with high sediment Sr concentrations and low 44/40 Ca and high 26 Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ 34 S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive 44/40 Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid-buffered or closed sediment-buffered diagenetic system have similar 26 Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ 47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow-water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.