Eric W. Mountjoy

Chemistry and Environments of Dolomitization —A Reappraisal

Abstract Dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain, preservation, or loss during the replacement process. Experimental data, as well as textures and porosities of natural dolomites, indicate that these reactions must include CO32− and/or HCO3− supplied by the solution to the reaction site. Since dolomite formation is thermodynamically favoured in solutions of (a) low Ca2+/Mg2+ ratios, (b) low Ca2+/CO32− (or Ca2+/HCO3−) ratios, and (c) high temperatures, the thermodynamic stability for the system calcite-dolomite-water is best represented in a diagram with these three parameters as axes. Kinetic considerations favour dolomitization under the same conditions, and additionally at low as well as at high salinities. If thermodynamic and kinetic considerations are combined, the following conditions and environments are considered chemically conducive to dolomitization: (1) environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite (i.e. freshwater/seawater mixing zones, normal saline to hypersaline subtidal environments, hypersaline supratidal environments, schizohaline environments); (2) alkaline environments (i.e. those under the influence of bacterial reduction and/or fermentation processes, or with high input of alkaline continental groundwaters); and (3) many environments with temperatures greater than about 50°C (subsurface and hydrothermal environments). Whether or not massive, replacive dolostones are formed in these environments depends on a sufficient supply of magnesium, and thus on hydrologic parameters. Most massive dolostones, particularly those consisting of shallowing-upward cycles and capped by regional unconformities, have been interpreted to be formed according to either the freshwater/seawater mixing model or the sabkha with reflux model. However, close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive, replacive dolostones commonly inferred to have been formed in these environments. Many shallowing-upward sequences are devoid of dolomite. It is therefore suggested that massive, replacive dolomitization during exposure is rare, if not impossible. Rather, only small quantities of dolomite (cement or replacement) are formed which may act as nuclei for later subsurface dolomitization. Alternatively, large-scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity. The integration of stratigraphic, petrographic, geochemical, and hydrological parameters suggests that the only environments capable of forming massive, replacive dolostones on a large scale are shallow, hypersaline subtidal environments and certain subsurface environments.

Formation of Coarsely Crystalline, Hydrothermal Dolomite Reservoirs in the Presqu'ile Barrier, Western Canada Sedimentary Basin

Late-stage, coarsely crystalline replacement dolomite and associated saddle dolomite cement form a widespread diagenetic facies in the Middle Devonian Presquile barrier that extends southwestward from Pine Point to the subsurface of the Foothills in northeastern British Columbia. These dolomites create hydrocarbon reservoirs in otherwise tight limestones in the subsurface of the Northwest Territories and northeastern British Columbia, and host Mississippi Valley-type deposits at Pine Point. The coarsely crystalline replacement dolomite and saddle dolomite cement are interpreted to have formed during burial because they replace blocky sparry calcite cements, occur continuously across the sub-Watt Mountain unconformity, postdate stylolites and earlier replacement dolomites, and overlap sulfide mineralization. The ^dgr18O values of both replacement and saddle dolomites increase eastward updip along the Presquile barrier from -16^pmil PDB in the deeper subsurface of northeastern British Columbia to -7^pmil PDB at Pine Point, whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92°C. The 87Sr/86Sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the Pres uile barrier from about 0.7106 in the subsurface of northeastern British Columbia to 0.7081 at Pine Point. These geochemical trends suggest a possible basin-scale migration of hot, radiogenic dolomitizing fluids updip eastward along the Presquile barrier. Such large-scale fluid movements probably were related to Western Canada sedimentary basin tectonic compression and sedimentary loading, which occurred at least twice: during early burial between the Late Devonian and Early Carboniferous and during deep burial between the Late Jurassic and early Tertiary.

Namacalathus-Cloudina assemblage in Neoproterozoic Miette Group (Byng Formation), British Columbia: Canada's oldest shelly fossils

The uppermost part of the Miette Group (Windermere Supergroup) in eastern British Columbia has yielded shelly macrofossils in cliff-forming biostromal carbonates (Byng Formation). The biostromes are made up of two principal elements: intergrowths of complex sinuous, plate-like stromatolites (cf. Platella ), and intervening planar to curviplanar pockets filled with packstone and wackestone crowded with Namacalathus and Cloudina , presumed calcified metazoans of uncertain biological affinities. Preservation is best in limestone, and the shells are mostly obliterated where the carbonate is dolomitized. This assemblage was previously known only from the Nama sequence in Namibia. The new find in the antipodal Miette Group in the Canadian Rocky Mountains greatly extends its geographic range, and suggests a more widespread distribution in similar facies in intermediate areas. Both assemblages constitute the earliest occurrences of shelly fossils in their respective regions.

Large-scale fluid flow in the Middle Devonian Presqu'ile barrier, Western Canada Sedimentary Basin

Saddle dolomite cements are regionally extensive in the Middle Devonian Presqu9ile (or Keg River) barrier and are spatially associated with Mississippi Valley-type mineralization at Pine Point (Northwest Territories). From northeastern British Columbia to Pine Point over a lateral distance of 400 km, these dolomite cements show general trends of decreasing 87 Sr/ 86 Sr ratios (0.7106 to 0.7081) and homogenization temperatures (178 to 92 °C), with some increase in δ 18 O values (-16‰ to -7‰ PDB). These regional trends suggest that hotter and more radiogenic basinal fluids moved eastward updip along the Presqu9ile barrier and mixed with cooler ambient formation waters. These movements of basinal fluids were probably related to tectonic thrusting and compression, sedimentary loading, and tectonic uplift on the western margin of the Western Canada Sedimentary Basin, which began to form during Late Jurassic to Early Cretaceous time and climaxed during the Late Cretaceous to Paleocene. The Middle Devonian Presqu9ile barrier appears to have acted as a deeply buried regional conduit system that played an important role in focusing and channeling these basinal fluids. These regional fluid flows appear to have been responsible for extensive burial dolomitization, secondary migration of hydrocarbons, and local Mississippi Valley-type mineralization in the Western Canada Sedimentary Basin.

An Early Wisconsin Reef Terrace at Barbados, West Indies, and Its Climatic Implications

A discontinuous reef terrace, 0 to 4.5 m above sea level dated at approximately 60,000 yrs B.P. by thorium-230 and protactinium-231, occurs along the northwest coast of Barbados. This terrace developed as a narrow fringing reef during the last relatively warm period (St. Pierre interstadial), prior to sea-level lowering and extensive glaciation of the “Early” Wisconsin. Interstadial terraces like this one were formed about 28,000, 45,000, and 65,000 yrs ago and are exposed today only in areas such as Barbados that have undergone subsequent uplift.

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

ABSTRACT Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

Isotopic composition and diagenetic history of carbonate cements in Devonian Golden Spike reef, Alberta, Canada

Extensive cementation in the margin of the Golden Spike (Upper Devonian, Alberta) reef complex is due largely to formation of carbonate cements in a marine environment. Subaerial cements are absent in the reef margin, but they occur in shallow-water reef interior facies. Middle and late burial calcite cements are less abundant but occur vertically and laterally throughout Golden Spike. The δ 13 C and δ 18 O values of selected carbonate cements and sediments, mainly from the reef margin, show: (1) a narrow range (+4.0‰ to +1.6‰) of δ 13 C values for all samples; (2) the most positive (X = +3.8‰) δ 13 C values occur in lime mudstones; (3) radiaxial calcite cements (submarine) and associated (often inter-layered) marine internal sediments have similar δ 13 C (+3.5‰ to + 1.6‰) and δ 18 O (−5.8‰ to −7.7‰) values; (4) submarine radiaxial and radial fibrous calcites (X = −6.7‰), middle burial non-ferroan calcites (X = −8.8‰), and late burial ferroan calcites (X = −13.0‰) contain progressively more negative δ 18 O values. Textural evidence (that is, cement fabrics, their distribution, superposition, and relationships with associated sediments) and isotopic compositions considered in light of the Golden Spike burial history indicate the following. Submarine cements of the reef margin apparently have not been significantly modified by meteoric waters ( 18 O, 13 C depleted) throughout their diagenetic history. Submarine cements and associated sediments have δ 13 C values similar to modern marine cements and sediments, and any diagenetic re-equilibrations occurred in a “closed system” with little or no addition of 12 C. δ 13 C values in middle and late burial stage cement are the result of calcite precipitation, probably in equilibrium with subsurface fluids. The 18 O/ 16 O ratios for marine sediments, fossils, and submarine cements are probably due to isotopic re-equilibration of originally enriched (marine) 18 O/ 16 O at elevated burial temperatures before the end of the Mississippian. On the basis of burial history, middle and late burial calcite cements are post-Mississippian to pre-Late Cretaceous; the Late Cretaceous is the most likely time of major petroleum entrapment.

Multistage dolomitization in Rainbow buildups, Middle Devonian Keg River Formation, Alberta, Canada

ABSTRACT More than 80 partially to completely dolomitized buildups of Middle Devonian Keg River Formation occur in the Rainbow sub-basin of northwestern Alberta. In six buildups (A, B, E, F, G, and Tehze), four petrographic types of dolomites are identified: gray, fine crystalline dolomite; floating dolomite rhombs/patches; matrix dolomite; and saddle dolomite. The petrographic and geochemical data indicate that these dolomites probably formed in three principle stages of dolomitization during progressive burial. Gray, fine crystalline dolomite (less than 1% of total dolomite by volume) is interpreted as penecontemporaneously dolomitized lime muds by either normal marine waters or evaporitic brines during early exposure. The following evidence suggests that this dolomite probably formed early: 1) it usually occurs in fractures and breccias presumably related to the early exposure of the buildups; 2) it is absent in rugs and molds that formed in the subsurface environment; and 3) locally, clasts of gray, fine crystalline dolomite are embedded in marine limestones. Rhomb/patch dolomite is the major type of dolomite in dolomitic limestones and is widespread throughout the limestone buildups. Matrix dolomite is fabric destructive and is most abundant in the dolostone buildups. Both these dolomites probably formed during shallow to intermediate burial because they 1) postdate stylolites, 2) are widespread throughout the buildups and crosscut various facies, and 3) have light oxygen isotope compositions and low Sr but high Fe and Mn concentrations. Dolomitizing fluids for dolomite rhombs/patches and matrix dolomites were possibly derived from 1) mechanical compaction, 2) dehydration of the adjacent Muskeg gypsum at relatively shallow burial depths, and 3) intermediate-depth basinal fluids conducted updip along porous and permeable conduits. During D vonian and carly Carboniferous times, compaction fluids were probably responsible for the early dolomite rhombs and patches. However, the main phase of matrix dolomitization took place later, probably during Late Mississippian to Jurassic time when the east side of the basin was intermittently tilted and uplifted, causing updip flow of intermediate-depth basinal brines. Saddle dolomite occurs as late-stage vug and fracture fillings. It is most depleted in 18O but most enriched in Mn and Fe, suggesting precipitation during deeper burial from warm basinal fluids. These fluids were expelled during Upper Cretaceous sedimentary and tectonic loading of the western side of the basin and then moved up the east side of the basin along existing platform, fault, and fracture conduits. The source of the Mg is uncertain. Some or most of the Mg may have resulted from chemical compaction of the earlier matrix dolomite.

Nubrigyn algal reefs (Devonian), eastern Australia: Allochthonous blocks and megabreccias

The widely known Lower Devonian “algal reef ” limestones of the Nubrigyn Formation, New South Wales, are enormous allochthonous blocks contained within a 400-m interval of interbedded mudstones, allodapic carbonates, and megabreccias that form part of a 5,000-m succession of Lower Devonian volcanics and flysch. Previous workers have interpreted these massive limestone bodies to be algal bioherms that developed in sublittoral to littoral environments around volcanic pedestals on a “Nubrigyn shelf.” The allochthonous nature of the limestone bodies is clearly indicated by (1) occurrence of a wide range of clast sizes, as much as 1 km across; (2) presence of a wide range of clast types and sizes in close juxtaposition; (3) discordance between stratigraphic facing of the large limestone bodies and stratification in surrounding beds; (4) lack of distinctive and regular facies changes within the limestone bodies, particularly near their margins; (5) abrupt and random truncation of internal fabrics at block margins; (6) lack of an autochthonous volcanic foundation for the “reefs”; and (7) anomalous lithofacies association of the massive bodies of shoal-water limestone with enclosing flysch. The limestones initially formed in a shoal-water carbonate complex to the west upon a geologically persistent volcanic archipelago, the Molong Arch, where source rocks for the Nubrigyn megaclasts and megabreccias crop out in the Lower Devonian Garra Formation and Cuga Burga Volcanics. The Nubrigyn megaclasts were transported eastward as debris flows into the adjacent and relatively deep water Hill End Trough after dislodgement from the eastern margin of the Garra shelf. Megaclasts isolated within hemipelagic mudstones and flysch were presumably transported by sliding or rolling. The loci of accumulation of the debris flows and exotic blocks occupy a meridional basin-margin position between the Molong Arch to the west and the predominantly turbidite-filled Hill End Trough to the east. Other debris-flow megabreccias, many previously unrecognized as having been transported and deposited in this manner, occur in the Paleozoic rocks of the Tasman mobile belt of eastern Australia.

Strontium isotopic composition of Devonian dolomites, Western Canada Sedimentary Basin: significance of sources of dolomitizing fluids

Abstract Dolomites from the Middle and Upper Devonian of five regions in the Western Canada Sedimentary Basin wwere studied: Middle Devonian Winnipegosis Formation (outcrop), Keg River Formation reefs in the Rainbow field and Presquile 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 Winnipegosis reefs, and the subsurface part of the Presquile barrier have 87Sr/86Sr values near 0.7080, Wabamun sabkha dolomites have 87Sr/86Sr 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 Presquile barrier, Rainbow, Rosevear, Miette, Nisku and Wabamun all have slightly higher 87Sr/86Sr values (0.7081–0.7094) than the correspnding Devonian seawater. Dolomitizing fluids were probably modified from Devonian seawater with small amounts of radiogenic 87Sr being added during early compaction from adjacent or underlying clastics, or older carbonates; 3. Late-stage saddle dolomites from Presquile, Rainbow, Rosevear, Miette, Nisku and Wabamum are all highly radiogenic ( > 0.7100 up to 0.7151), except for most Pine Point saddle dolomites, indicating significant input of radiogenic 87Sr, 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 87Sr/86Sr shallow formation waters as they moved updip along the Presuile barrier. The 87Sr/86Sr values support and refine earlier conclusions based on textual 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 curretly been reported from the basin.