Distribution Of Dolomite Research Articles

Dolomites in SE Asia — varied origins and implications for hydrocarbon exploration

Abstract Carbonates in SE Asia range in age from Palaeozoic to Recent, but are most important as reservoirs in the Neogene where they comprise a major target for hydrocarbon exploration (e.g. Batu Raja Formation, South Sumatra, Sunda and Northwest Java basins). Carbonates of pre-Tertiary, Palaeogene and Neogene age all show a strong diagenetic overprint in which dolomite occurs as both cementing and replacive phases associated with variable reservoir quality. This paper reviews published data on the occurrence and types of dolomites in SE Asian carbonates, and considers the models that have been used to explain the distribution and origin of dolomite within these rocks. Pre-Tertiary carbonates form part of the economic basement, and are little studied and poorly understood. Although some, such as in the Manusela Formation of Seram, may form possible hydrocarbon reservoirs, most are not considered to form economic prospects. They are best known from the platform carbonates of the Ratburi and Saraburi groups. in Thailand, and the oolitic grainstones of the Manusela Formation of Seram. The Ratburi Group shows extensive dolomitization with dolomite developed as an early replacive phase and as a late-stage cement. Palaeogene carbonates are widely developed in the region and are most commonly developed as extensive foraminifera-dominated carbonate shelfal systems around the margins of Sundaland (e.g. Tampur Formation, North Sumatra Basin and Tonasa Formation, Sulawesi) and the northern margins of Australia and the Birds Head microcontinent (e.g. Faumai Formation, Salawati Basin). Locally, carbonates of this age may form hydrocarbon reservoirs. Dolomite is variably recorded in these carbonates and the Tampur Formation, for example, contains extensive xenotopic dolomite. Neogene carbonates (e.g. Peutu Formation, North Sumatra) are commonly areally restricted, reef-dominated and developed in mixed carbonate-siliciclastic systems. They most typically show a strong diagenetic overprint with leaching, recrystallization, cementation and dolomitization all widespread. Hydrocarbon reservoirs are highly productive and common in carbonates of this age. Dolomite is variably distributed and its occurrence has been related to facies, karstification, proximity to carbonate margins and faults. The distribution and origin of the dolomite has been attributed to mixing-zone dolomitization (commonly in association with karstic processes), sulphate reduction via organic matter oxidation, and dewatering from the marine mudstones that commonly envelop the carbonate build-up. Dolomite has a variable association with reservoir quality in the region, and when developed as a replacive phase tends to be associated with improved porosity and permeability characteristics. This is particularly the case where it is developed as an early fabric-retentive phase. Cementing dolomite is detrimental to reservoir quality, although the extent of this degradation generally reflects the abundance and distribution of this dolomite. Dolomitization is also inferred to have influenced the distribution of non-hydrocarbon gases. This is best documented in North Sumatra where carbon dioxide occurs in quantities ranging from 0 to 85%. There are a number of possible mechanisms for generating this CO 2 (e.g. mantle degassing), although the most likely source is considered to be the widely dolomitized Eocene Tampur Formation that forms effective basement for much of the basin. High heat flows are suggested to have resulted in the thermogenic decomposition of dolomite with CO 2 produced as a by-product.

Anomalous unradiogenic 87Sr//86Sr ratios in ultrahigh‐pressure crustal carbonates – evidence for fluid infiltration during deep subduction?

AbstractMarbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have 87Sr/86Sr values < 0.7040. These low 87Sr/86Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐87Sr/86Sr source. The introduction of low‐87Sr/86Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P–T–X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.

The evolution of fluid flow systems prior to, during, and post MVT mineralization in the Givetian-Frasnian carbonates of the Emanuel Range, Western Australia

The Emanuel Range is part of the extensive Devonian Reef complex, which overlies the Lennard Shelf, a section of shallowbasement at the northern end of the Canning Basin in Western Australia. The reef complexes formed in an extensional tectonic environment and host numerous Mississippi Valley-type (MVT) Zn Pb accumulations. The distribution, relative timing, and origin of calcite, dolomite, and MVT sulphide generations in the southeastern Emanuel Range indicate that the fluid flow systems operating during Middle Devonian to Middle Carboniferous burial changed significantly through time. The topographic flow of meteoric water from north to south, driven through alluvial fans shed from the highlands of the Precambrian Kimberley block to the north of the Emanuel Range, was the longest-lived fluid flow system. The topographic flow system operated intermittently during periods of lowstand in the Givetian and early Frasnian and continued throughout subsequent burial and uplift in the Middle Carboniferous. During early to intermediate burial depths (< 500 m), stratigraphically focused compactionally driven fluids of predominantly modified marine origin entered the reefs from a depocentre to the south of the Emanuel Range. During later burial (1–1.5 km) in the Late Devonian and Early Carboniferous, more deeply sourced fluids were driven laterally into the Emanuel Range from the Fitzroy Trough to the south. The introduction of compactionally driven, hot metalliferous hydrocarbon-bearing brines into the sequence occurred episodically, precipitating MVT sulphides. Between hydrothermal pulses, formation waters decreased in temperature and salinity, due to mixing with cooler surface waters. Hydrothermal brines flowed laterally from south to north until encounter with the Cadjebut Fault, which impeded brine flow. Further brine movement was focused through dilatant faults with a NNE orientation (thought to be related to transfer zones; Veamcombe, J.R., Dentith, M., Doerling, S., Reed, A., Cooper, R., Hart, J., Muhling, P., Windrim, D., Woad, G., 1995. Structural controls on Mississippi Valley-type mineralization, the southeastern Lennard Shelf, Western Australia. In: Sangster, D.E. (Ed.), Carbonate-Hosted Lead-Zinc Deposits. Society of Economic Geologists, Special Publication, vol. 4. 1995, pp. 74–94.) and near-vertical hydraulic fractures related to high fluid pressures. Through time, hydrothermal pulses became both less frequent and less intense, and topographically driven flow again dominated.

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

AbstractInterpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ18O = 0·9 ± 1·0‰,N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ18O = 2·1 ± 0·7‰,N = 48) than the hypersaline backreef dolomite (δ18O = 3·6 ± 0·9‰,N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.

Hyperspectral Mapping of Mineral Assemblages Associated with Gold Mineralization in the Central Pilbara, Western Australia

The Indee district, located in the Central Pilbara tectonic zone of the North Pilbara terrain in Western Australia, contains Archean lode gold deposits and epithermal gold systems. Hydrothermal alteration assemblages associated with these deposits contain the minerals pyrophyllite, white mica, chlorite, and tourmaline. Known prospects are surrounded by flat areas of poorly exposed calcretized bedrock, calcretes, rock float, and sandy soils. Two adjacent lines of airborne hyperspectral data (HyMap®) were acquired. To produce mineral abundance maps, these data were processed to correct for atmospheric and illumination conditions and then calibrated to reflectance data using field spectra. This analysis successfully mapped the distribution of pyrophyllite, white mica, Mg chlorite, Fe chlorite, calcite, dolomite, kaolinite, tourmaline, hematite, and goethite. Alteration assemblages associated with the Camel 2 deposit, which is located along the Mallina shear zone, were found to be characterized by pyrophyllite and Al-rich white mica, the latter having an AlOH absorption feature at 2.194 μ m. In contrast, alteration assemblages associated with epithermal deposits south of the Mallina shear zone are characterized by Al-poor white mica with a longer wavelength absorption feature at 2.210 μ m, the shift reflecting the change in Al chemistry (Tschermak substitution). Tourmaline is also present near the epithermal-like deposits, suggesting that it may be an integral part of these systems. Fe chlorite is associated with both the mesothermal and epithermal systems but is also pervasive as a regional metamorphic mineral. Regolith-related carbonates are common throughout the area and their compositions are determined by the hyperspectral processing. Mineral abundance images were verified by PIMA® (portable infrared mineral analyser) analysis of ground samples, and assays of rocks in previously unknown areas of pyrophyllite and white mica confirmed the presence of anomalous gold. These results show that mineral maps and compositional information derived from hyperspectral analysis are extremely valuable for exploration, even in poorly exposed, regolith-dominated districts such as Indee.

REPLY TO DISCUSSION of H.G. Machel, E.W. Mountjoy, G.D. Jones and B.J. Rostron: Toward a sequence stratigraphic framework for the Frasnian of the Western Canada Basin

We are gratified by the interest that our publication has generated and thank Machel et al. (2002, this issue) for their discussion. However, we do not agree with their assessment of the part of our paper dealing with a mechanism for the formation of replacement dolomite in the Frasnian second-order sequence. We assert that, considering all the data, the most logical explanation for the mechanism of formation for these dolomites is by seepage refluxion with fluids originating at or near the top of the Frasnian sequence. One of the hallmarks of a good dolomitization model is its ability to explain and predict dolomite distribution in space and time. This makes it useful to researchers and to geologists exploring for dolomite reservoirs. The proposed mechanism accomplishes this. We have taken a substantially different approach to understand the mechanism of dolomite formation than that taken by H.G. Machel and his colleagues. Our conclusions about dolomite timing and dolomitizing fluid are based primarily on regional stratigraphic analysis; their conclusions have relied somewhat more heavily on isotopic analyses. We do not apologize for taking a more straightforward, observational approach. Rather, we regard our work as an important contribution to the understanding of these rocks, because it presents a model based on knowledge of the regional stratigraphic distribution of the replacement dolomite. Many dolomitization studies in the Western Canada Sedimentary Basin (WCSB) have been investigations of relatively small areas, have not incorporated all of the relevant stratigraphy, or have lacked the scope to tackle the regional issues. The intent of our paper was to present a regional stratigraphic framework and not to criticize previous work. Rather, we wished to propose an alternative mechanism tied closely to a stratigraphic model. Amthor et al. (1994) also recognize the importance of a predictive model in their assertion …

Alkali carbonate reaction expansion of dolomitic limestone aggregates with porphyrotopic texture

Effects of porphyrotopic textures on expansion of dolomitic limestone aggregates due to alkali–carbonate reaction (ACR) are presented. The dispersed distribution of dolomites in the texture of aggregates can be divided into pophyrotopic-M, psedoporphyrotopic-M and porphyrotopic-S types. It is proved that porphyrotopic textures are more important to ACR expansion than lithologic species of dolomitic limestones. The dolomitic limestones with both porphyrotopic-M or psedoporphyrotopic-M texture and the small size of dolomite crystals are expected to be highly alkali–carbonate reactive. These aggregates can be found not only in dolomitic micritic limestones but also in dolomitic micritic allochemical limestones as well as dolomitic sparry allochemical limestones.

Geology and Production Significance of Dolomite, Arab-D Reservoir, Ghawar Field, Saudi Arabia

ABSTRACT At least five distinct types of dolomite occur in the Arab-D Reservoir in Ghawar field, Saudi Arabia – one of which appears to be responsible for high flow or ‘super-k’. These dolomite types are distinct petrographically, geochemically and stratigraphically: a finely-crystalline non-fabric-preserving (NFP) variety of dolomite in the lower Arab-D (Zone 3) with low oxygen isotope values and generally poor reservoir quality;a medium-crystalline NFP dolomite with high oxygen isotope values and very poor reservoir quality in the upper Arab-D (Zone 2);a medium to coarsely-crystalline NFP dolomite with low oxygen isotopic values and very good reservoir quality (‘super-k’) occurring in Zone 2; anda finely-crystalline fabric-preserving (FP) dolomite in the uppermost Arab-D (Zone 1) that contains high oxygen isotope values and has generally fair to poor reservoir quality. Previous studies have documented a rare fifth type of dolomite, baroque or ‘saddle’ dolomite, that occurs locally in the reservoir as well. This study also quantified and mapped the abundance and distribution of dolomite across the field, using all available core and log data. Analysis of dolomite distribution map patterns reveals that dolomite occurs in Ghawar as a series of linear trends extending for tens’s of kilometers. These map pattern trends are best-developed in Zone 2B, but are also visible in Zones 2A and 3A as well. Baroque dolomite appears to be limited to a few areas of vertically pervasive dolomite occurring on the same trends of high dolomite content. The linearity of these dolomite trends strongly suggests that some structural element is responsible for controlling their orientation. We interpret these linear patterns to have formed in response to a series of fracturing and/or faulting events that allowed dolomitizing fluids to move up into the reservoir from below, and preferentially dolomitize there. Both a qualitative and a quantitative analysis of performance data (flowmeters) in southern Ghawar (Haradh) indicate that these trends of high dolomite have a profound influence on fluid flow in the reservoir. A qualitative analysis of occurrences of ‘super-k’ in the Arab-D in Haradh suggests that most ‘super-k’ zones (seen as ‘spikes’ or step profiles on the flowmeter) occur in the high dolomite trend in Haradh. A quantitative analysis of flowmeter data and a comparison of this analysis with dolomite map patterns indicate that most reservoir flow occurs where dolomite is abundant, and suggests that there is a direct relationship between patterns of high flow and high dolomite.

ABSTRACT: The Sequence Stratigraphic and Paleogeographic Distribution of Reservoir-Quality Dolomite, Madison Formation, Wyoming and Montana

ABSTRACT: The Sequence Stratigraphic and Paleogeographic Distribution of Reservoir-Quality Dolomite, Madison Formation, Wyoming and Montana

Numerical modelling of geothermal and reflux circulation in Enewetak Atoll: implications for dolomitization

Two types of regional-scale seawater circulation have been proposed to explain the formation of Enewetak Atoll dolomites: geothermal and reflux circulation. We have used a finite element groundwater flow model to examine the pattern, magnitude and dynamic interaction of these two different circulation mechanisms in Enewetak Atoll. Geothermal circulation is concentrated around the atoll-margin whereas refluxing mesosaline brines flow from the atoll interior towards the margin to restrict and eventually shut off geothermal circulation. Refluxing brines of 36–80‰ can account for the salinity signature recorded in dolomite fluid inclusions. Distributions of fluid flux and Mg mass-balance calculations suggest that both geothermal and reflux circulation mechanisms could account for the observed distribution of dolomite in Enewetak Atoll. Furthermore, the atoll interior may be extensively dolomitized as observed in other atolls.

Abstract: Sequence Stratigraphic Distribution of Reservoir-Quality Dolomite, Madison Formation (Mississippian), Wyoming and Montana

Abstract: Sequence Stratigraphic Distribution of Reservoir-Quality Dolomite, Madison Formation (Mississippian), Wyoming and Montana

Climate-controlled early dolomite, Late Triassic cyclic platform carbonates, Hungary

ABSTRACT The 2 km thick Late Triassic Hungarian carbonate platform has a completely dolomitized lower part and a limestone-dominated upper part. The platform succession is made up of meter-scale, possibly precessional ( 20 ky) carbonate cycles. In the lower platform (Main Dolomite), cycles are completely dolomitized and capped by caliche laminites and pisolites. In the upper platform (Dachstein Limestone), dolomite is confined to laminite caps of cycles that are bounded by clayey paleosols. Most of the dolomitization in the platform interior occurred early in tidal-flat settings during each high-frequency cycle. Subtidal dolomites are slightly coarser grained, are low in Fe2+ and Mn2+, and have the heaviest 18O signature. This indicates they formed from evaporative, oxidizing brines sourced from supratidal flats. Intertidal-supratidal dolomites are fine grained, commonly Fe2+ and/or Mn2+ rich, and slightly enriched in 18O compared to the marine calcite cement. They formed from marine water that was weakly to moderately reducing. Dachstein paleosols have light 18O and 13C signatures reflecting meteoric soil waters. Repeated emergence stabilized the dolomites to low Sr2+ and Na+ dolomite similar to Cenozoic platform dolomites. In contrast to these early cyclic dolomites, coarse-grained platform-margin dolomites with very low Mn2+ and Fe2+ and light 18O signatures formed as thermally driven, warm, oxidizing marine water associated with Jurassic rifting of the Neo-Tethys Pennini Ocean circulated through the margin. Early dolomitization of cycles may have been controlled by high-frequency sea-level changes, but the overall vertical distribution of early dolomite on the platform does not reflect long-term eustasy. Rather, the intense dolomitization of the lower platform reflects a semiarid, hot subtropical, seasonal setting and megamonsoonal climate. Global cooling and increased humidity toward the latest Triassic and Early Jurassic inhibited pervasive early dolomitization, leaving the upper platform largely as limestone.

Multiple episodes of dolomitization and dolomite recrystallization during shallow burial in Upper Jurassic shelf carbonates: eastern Swabian Alb, southern Germany

Abstract The Upper Jurassic of the eastern Swabian Alb is composed of oolitic platform sands with associated microbe-siliceous sponge mounds at the platform margins. They are surrounded by argillaceous or calcareous mudstones and marl–limestone alternations, deposited in adjacent marl basins. Partial to complete dolomitization is predominantly confined to the mound facies. Six types of dolomite, as well as one type of ankerite, document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization. The earliest massive matrix dolomitization is Ca-rich, has slightly depleted oxygen isotope values relative to Late Jurassic seawater, and carbon isotopic values in equilibrium with Late Jurassic seawater. This initial massive matrix dolomitization occurred during latest Jurassic to earliest Cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50°C. Hydrologic conditions and mass-balance calculations indicate that burial compaction provided sufficient fluids for dolomitization. Mg is derived from negligibly modified seawater, that was expelled from the adjacent off-reef strata into the mound facies. Position of the mounds along the platform margins controlled the distribution of the shallow-burial dolomite. Covariant trends between textural modification, increasing stoichiometry, partial changes in trace element content (Mn, Fe, Sr) and depletion in stable isotopes as well as distinctive CL pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures. Strong Sr enrichment of the second phase of recrystallized dolomite is ascribed to Sr-rich meteoric waters descending from overlying aragonite-bearing reef limestones or evaporite-bearing peritidal carbonates. Late-stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial. Ankerite, associated with sulphide and sulphate minerals, and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during Late Cretaceous to Tertiary Alpine orogenesis.

Origin and modification of early dolomites in cyclic shallow platform carbonates, Yeongheung Formation (middle Ordovician), Korea

The middle Ordovician Yeongheung Formation, Korea, consists of numerous metre-scale, shallowing-upward cycles, which were deposited on a shallow-marine carbonate platform. Depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology, whereas those developed on the outer platform were partially dolomitized. The amount of replacement dolomite within a metre-scale, shallowing-upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences. The cross-platform distribution of dolomites and their stratigraphic position in the sequences suggest that the Yeongheung dolomite formed syndepositionally. The common existence of pseudomorphs of evaporite minerals and nodules in the Yeongheung Formation indicates that dolomitization of the Yeongheung Formation occurred in a sabkha-type tidal-flat environment. It also occurred during each cycle of deposition influenced by high-frequency sea-level oscillations. The present-day Yeongheung replacement dolomites are characterized by near-stoichiometric to stoichiometric composition (49.9 to 52.1 CaCO 3 mole%), and low strontium concentration (52 to 133 ppm). The δ 18O value of the replacement dolomites ranges from −7.7 to −5.2‰ (PDB); this is much depleted relative to that of contemporary sabkha-type dolomites. These geochemical characteristics of Yeongheung dolomite indicate significant post-depositional modification. The common existence of oomolds, leached skeletal grains and dissolved evaporites in the Yeongheung Formation illustrates invasion of meteoric water into the sediments. Recrystallization of Yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter δ 18O values and lower strontium concentration.

Diagenetic modification of dolomite in middle Ordovician carbonates, Taiyuan City area, China

Patchy dolomite occurs in the middle Ordovician limestones of the Taiyuan City area, China. The dolomite is finely-crystalline, subhedral to euhedral and is considered to have formed initially by syndepositional, subtidal dolomitization in evaporative or modified marine water, as indicated by the close association of supratidal gypsum deposits with subtidal dolomite, by the widespread distribution of dolomite and by the close association of dolomite with burrows. The much depleted δ 18O and δ 13C values of both dolomite and coexisting limestone relative to middle Ordovician marine carbonates suggest intensive diagenetic modifications. The average δ 18O and δ 13C values of present-day dolomite are −6.0% and −1.2%, enriched by about 3% and 1%, respectively, when compared with the coexisting limestone (−8.8% δ 18O and −2.4% δ 13C on average). This is consistent with the known fractionation between dolomite and calcite and may indicate that both the dolomite and coexisting limestone were finally modified, or stabilized, in the same diagenetic fluid. The trace element (Sr, Fe and Mn) concentration difference between the dolomite and coexisting limestone also supports a simultaneous modification or stabilization. Although the diagenetic modification of both dolomite and coexisting limestone by seawater, modified seawater or basin-derived water is possible, the simultaneous modification by meteoric water seems most likely.

Reinterpretation of the peninsular Florida Oligocene: an integrated stratigraphic approach

A very thick (> 300 m) nearly continuous Oligocene section exists in southern peninsular Florida, as revealed by lithostratigraphic, biostratigraphic (mollusks and dinocysts), chronostratigraphic (Sr isotopes) and petrographic analyses of twelve cores and two quarries. The Oligocene deposits in the subsurface of southern Florida are the thickest documented in the southeastern U.S., and they also may represent the most complete record of Oligocene deposition in this region. No major unconformities within the Oligocene section are detected in the southern portion of the peninsula; hiatuses at the Eocene-Oligocene boundary, the early Oligocene-late Oligocene boundary, and the late Oligocene-Miocene boundary, are of limited duration if they exist at all. No significant disconformity is recognized between the Suwannee Limestone and the Arcadia Formation in southern Florida. However, on the coast of Florida a hiatus of more than 12 m.y., spanning from at least the middle of the early Oligocene to early Miocene is present. The Suwannee Limestone was deposited during the early Oligocene. The top of the Suwannee Limestone appears to be diachronous across the platform. The ‘Suwannee’ Limestone, previously identified incorrectly as a late Oligocene unit, is herein documented to be early Oligocene and is encompassed in the lower Oligocene Suwannee Limestone. An unnamed limestone, found on the east coast of the peninsula is, at least in part, correlative with the Suwannee Limestone. The Arcadia Formation, basal Hawthorn Group, accounts for a large portion of the Oligocene deposition in southern Florida, spanning the interval from the middle of the early Oligocene to at least the early Miocene. Comparisons of the depositional patterns, and the distribution of dolomite and phosphate within the Suwannee Limestone and the Arcadia Formation, suggest fluctuating sea levels and that the paleo-Gulf Stream played a role in determining the nature and extent of Oligocene deposition in peninsular Florida.

Stratigraphic Patterns of Deep-water Dolomite, Northeast Australia

ABSTRACT In ODP Hole 815A, located along the edge of the Townsville Trough and marginal to the Marion Plateau of northeast Australia, dolomite forms about 5-20 wt % of carbonate within a distal part of a muddy contourite succession of Pliocene (2.6-4.0 Ma) age. Integration of seismic, lithic, and biostratigraphic data identifies a history of current-controlled episodic deposition subsequently shut down during rapid subsidence by ~2.6 Ma. Overlying pelagic-periplatform sediment contains no or trace amounts of dolomite. High-order, rhythmic (1-5 m thick) variation in dolomite, quartz, and carbonate content within the contourite succession defines a past, high-order stratigraphic variation in diagenetic potential for dolomite. d13C values of dolomite reflect incorporation of at least two sources of bicarbonate mixed with seawater: (1) in more carbonate-rich sediment, d13C values (1 to -2.0%) indicate likely contribution from dissolution of metastable carbonate; (2) in more siliciclastic-rich sediment, with locally elevated total organic content, d13C values (-4 to -6%) reflect contribution from organic-matter diagenesis. The majority of d18O values of dolomite over the entire contourite succession vary between 3.2 and 2%, and are interpreted to reflect near-surface to shallow (tens of meters) subsurface, marine-derived diagenesis. A few dolomites with lower (2 to 0%) values are restricted to two stratigraphic intervals, and reflect one or some combination of the following processes: a second stage of dolomitization at greater depths; near-surface diagenesis with much warmer bottom waters; or burial diagenesis with mixture of meteoric- and marine-derived pore fluids. Magnesian calcite is absent today in the contourite succession. However, the depositional and diagenetic frameworks of the contourite succession suggest that metastability of aragonite and magnesian calcite, variable current activity, and seawater diffusion across the sediment-water interface all promoted dolomitization. Compilation of dolomite stratigraphy along slopes off northeast Australia illustrates that dolomite distribution in Hole 815A is one of four intervals (D1-D4) in strata younger than 4 Ma: D1, 0.3-0.8 Ma; D2, 1-1.3 Ma; D3 (D3a, ~ 1.8 and D3b, 2.0-2.4 Ma); and D4, 2.6-4.0 Ma. The stratigraphy is linked to temporal variation in supply of shelf- and pelagic-derived metastable carbonate regulated by changes in paleoceanography, eustasy, and tectonics. D1, D3b, and D4 are in sediments with ages similar to previously documented dolomitization stages in shallow-water platforms elsewhere in the world, and suggest a coincidence between elevated diagenetic potential for deep-water dolomitization in platform margins and substantive changes in ocean c volume transport linked to eustatic change in sea level.

Tentative kinetic model for dolomite precipitation rate and its application to dolomite distribution

The ‘dolomite problem’ has a long history and remains one of the most intensely studied and debated topics in geology. Major amounts of dolomite are not directly forming today from seawater. This observation has led many investigators to develop geochemical/hydrologic models for dolomite formation in diagenetic environments. A fundamental limitation of the current models for the growth of sedimentary dolomite is the dearth of kinetic information for this phase, in contrast to that available for calcite and aragonite. We present a simple kinetic model describing dolomite growth as a function of supersaturation using data from published high temperature synthesis experiments and our own experimental results. This model is similar in form to empirical models used to describe precipitation and dissolution rates of other carbonate minerals. Despite the considerable uncertainties and assumptions implicit in this approach, the model satisfies a basic expectation of classical precipitation theory, i.e., that the distance from equilibrium is a basic driving force for reaction rate. The calculated reaction order is high (~ 3), and the combined effect of high order and large activation energy produces a very strong dependence of the rate on temperature and the degree of supersaturation of aqueous solutions with respect to this phase. Using the calculated parameters, we applied the model to well-documented case studies of sabkha dolomite at Abu Dhabi (Persian Gulf), and organogenic dolomite from the Gulf of California. Growth rates calculated from the model agree with independent estimates of the age of these dolomites to well within an order of magnitude. A comparison of precipitation rates in seawater also shows the rate of dolomite precipitation to converge strongly with that of calcite with increasing temperature. If correct, this result implies that dolomite may respond to relatively modest warming of surface environments by substantial increases in accumulation rate, and suggests that the distribution of sedimentary dolomite in the rock record may be to some extent a temperature signal.

Geological controls on mineralization and dolomitization of the Lisheen Zn-Pb-Ag deposit, Co. Tipperary, Ireland

Abstract The Lisheen deposit is a stratabound Zn-Pb-Ag deposit hosted in dolomitized Waulsortian limestone of Courceyan age. The deposit was discovered in April 1990 and the current resource is estimated to be 22 million tonnes at 11.5% Zn, 2% Pb and 16% Fe, with 26 g/t Ag. Lisheen is located along the Rathdowney Trend, a NE-SW trending, structurally controlled belt of carbonate rocks. Sulphides are present in the hanging wall of two major E to NE-trending normal faults, the Killoran and Derryville faults. These structures are large, northerly-dipping normal faults with displacements of approximately 200 m. The Killoran and Derryville structures form short fault segments of a major relay fault system that extends along the Rathdowney Trend. Faults are observed to have influenced sedimentation, and exercized a major control on the distribution of mineralization, hydrothermal alteration and dolomitization. Several phases of dolomitization, which affect Chadian rocks and temporally overlapped mineralization, help to constrain the timing of this mineralization to no earlier than the late Chadian.

Multiple Episodes of Dolomitization in the Arbuckle Group, Arbuckle Mountains, South-Central Oklahoma: Field, Petrographic, and Geochemical Evidence

ABSTRACT The Cambro-Ordovician Arbuckle Group in the Arbuckle Mountains, south-central Oklahoma, had a complex history of dolomitization that resulted in two different geometries of dolomite bodies: stratal dolomite of stratigraphically consistent, widespread distribution, and non-stratal dolomite of stratigraphically inconsistent, local occurrence. Stratal dolomite includes the Royer and Butterly units in the lower Arbuckle Group. Most stratal dolomite samples are coarsely crystalline and have 87Sr/86Sr ratios (0.70890-0.70919) similar to Late Cambrian limestone and coeval seawater (0.7089-0.7092). All stratal dolomite and Arbuckle limestone samples have low 18O values (- 5.3 to - 11.9). The cross-platform distribution of stratal dolomite, coupled with dominant 87Sr/86Sr ratios similar to coeval seawater, suggests that stratal dolomite formed during early diagenesis. However, the low 18O values indicate that all stratal dolomite and Arbuckle limestone samples suffered postdepositional modification, probably as the result of infiltration of meteoric water during development of the Ordovician unconformities. Nonstratal dolomite is present in two areas: the Tishomingo Anticline and the Arbuckle Anticline. In the Tishomingo Anticline area, massive bodies (> 10 km2) of nonstratal dolomite are present in a paleokarst system of pre-Middle Ordovician age. Some of the nonstratal dolomite bodies were displaced by Pennsylvanian faults, whereas many dolomite bodies are related to the faults. Petrographically, nonstratal dolomite is finely to medium crystalline and has replaced recrystallized limestone. The dolomite has lower 87Sr/86Sr ratios (0.70772-0.70900; average 0.70834) and higher 18O values (+0.6 to -5.8; average -2.9), relative to associated (Lower Ordovician) limestone (87Sr/86Sr, 0.70878-0.70898, average 0.70887; 18O, -6.1 to -8.9, average -7.4). The petrographic and isotopic characteristics suggest that the nonstratal dolomite probably resulted from dolomitization of recrystallized limestone by post-Early Ordovician seawater. The relation of the nonstratal dolomite bodies to Pennsylvanian faults may indicate that the dolomite originated from multiple times of seawater circulatio through a paleokarst system. In the Arbuckle Anticline area, nonstratal dolomite is present as small (< 1 km2) irregular bodies that are related to Pennsylvanian faults and are associated with the margins of stratal Butterly dolomite. The nonstratal dolomite, medium to coarsely crystalline and brightly luminescent, is characterized by high (radiogenic) 87Sr/86Sr ratios (0.70951-0.70974), 18O) values (average -1.1), and Fe and Mn concentrations (average 1.32 mol % and 0.65 mol %, respectively), relative to all Arbuckle carbonates. Such compositions suggest that this type of dolomite probably originated from fluids that were derived from the adjacent basin(s) duri g late Paleozoic time.