Shallow Burial Dolomitization Research Articles

The origin of heterogeneous sucrosic dolomite reservoir of the lower Permian Qixia Formation, NW Sichuan Basin, China

Sucrosic dolomite, an important hydrocarbon reservoir, has long been the focus of carbonate sedimentological and reservoir geological studies. This study investigated a kind of heterogeneous sucrosic dolomite in the Lower Permian Qixia Formation of NW Sichuan Basin, which has recently been the location of giant natural gas discoveries. The heterogeneous sucrosic dolomite is characterized by coexistence of porous euhedral dolomite and tight anhedral dolomite, and it is mainly distributed in the platform-marginal shoal facies with a quasi-layered structure. Further geochemical analysis, including C, O, and Sr isotopes as well as rare earth elements, reveals that the euhedral dolomite and anhedral dolomite have similar geochemical properties to the matrix limestone representing coeval seawater, and they were mainly generated from dolomitization by the closed marine-related fluid (left-leaning REE and δPr < 1) in the shallow burial. The difference in crystal morphology, porosity, and permeability between the euhedral dolomite and anhedral dolomite is mainly related to the compositional and textural heterogeneities of the host rocks. Due to the dissolution of meteoric water (relatively flat REE and low Y/Ho) in the early diagenetic stage caused by high frequent exposures, quasi-layered vugs and caves were formed in the grainstones. In the process of shallow burial dolomitization, the loose-filled carbonate sands formed the porous euhedral dolomite due to sufficient space, while the matrix limestone formed the tight anhedral dolomite due to relatively poor porosity and permeability. Accordingly, the paleogeomorphic highland controlled platform-marginal shoal superimposed by meteoric water dissolution in the early diagenetic stage is the main factor for the formation of Qixia Formation reservoirs, while dolomitization is mainly manifested as the inheritance and adjustment of pre-existing pores in the host rock. Therefore, the exploration direction for dolomite reservoirs in the Qixia Formation in the Sichuan Basin should be shifted to the favorable sedimentary facies-controlled reservoir model, which can also be referential for other cases under similar geological setting.

Source of dolomitizing fluids and dolomitization model of the upper Permian Changxing and Lower Triassic Feixianguan formations, NE Sichuan Basin, China

Dolostone reservoirs have been found in the Changxing (P2c) and Feixianguan (T1f) formations in both the east and west platform edges of the Kaijiang-Liangping trough; they are the main natural gas exploration targets in the NE Sichuan Basin. In this study, the sources of the dolomitizing fluids were determined, and a dolomitization model was established based on petrography, rare earth elements (REEs), major and trace element data, and 87Sr/86Sr ratios. The dolostone and limestone samples with ΣREE values of >5 ppm exhibit relatively flat PAAS-normalized REE patterns; their Zr, Sc, and Th concentrations increase with increasing ΣREE; and their Rb/Sr ratios are generally greater than 0.01, suggesting that these samples were likely contaminated by terrigenous components. Therefore, only the REEs and 87Sr/86Sr ratios of samples with ΣREE values of <5 ppm were selected to constrain the dolomitizing fluids. The PAAS-normalized REE patterns of the P2c–T1f dolostones, which are characterized by LREE depletion, negative Ce anomalies (Ce/Ce* of 0.70–0.85), and relatively high Y/Ho ratios (49.0–152.4), are similar to those of the unaltered coeval limestones, suggesting that the dolomitizing fluid was sourced from fluids similar to coeval seawater. The Ce/Ce* values of the dolostones are lower than those of the unaltered limestone, suggesting that the source of the dolomitizing fluids had more oxidizing conditions and most probably contained evaporated seawater from an evaporite platform. The Eu/Eu* values of the P2c dolostone in the west platform (means of 4.95 and 2.76 for the YB and XLC gas fields, respectively) are notably higher than the Eu/Eu* values of the P2c and T1f dolostones in the east platform (mean 1.29 and 1.39 in the PG gas field, respectively), suggesting that the P2c dolostone in the west platform was formed under more reducing conditions and at a deeper burial depth. The 87Sr/86Sr ratios of the T1f and P2c dolostones in the east platform (such as in the Puguang gas field) are similar to those of the co-existing early T1f limestone, while the 87Sr/86Sr ratios of the P2c dolostone in the west platform (such as in the Yuanba gas field) are higher than those of the coexisting P2c limestone but are close to those of the late T1f dolostone in the east platform. This indicates that the dolomitizing fluids were mainly from the early T1f and late T1f evaporated seawater on the east and west platforms, respectively. Driven by the density difference between the evaporated seawater and normal seawater, the evaporated seawater flowed downward and laterally into the limestone in the high-energy reefs and shoals through the original pores and the early selective dissolution pore system and/or through synsedimentary fractures. The continual seepage of evaporated seawater and the relatively high temperatures (50–60 °C) resulted in the large-scale shallow burial dolomitization in the NE Sichuan Basin.

Dolomitization and hydrotectonic model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift, central Tarim Basin, NW China: Implications from petrography and geochemistry

Abstract The nature of the dolomitizing fluids and genesis of the Furongian-Lower Ordovician dolomites and dolomite cements are interpreted on the basis of petrography, geochemistry (trace elements, REEs, O-C-Sr isotopes) and fluid inclusion microthermometry. These carbonate strata were deposited on a shallow marine platform in the Tazhong Uplift in central Tarim Basin and were extensively dolomitized. Based on field investigations and petrographic examination, two types of fabric-retentive dolomitic textures (dolomicrites (DM) and microbial dolomudstone (DMS)) and three types of fabric-obliterated dolomitic textures (very fine- to fine-crystalline planar-e(s) dolomite (RD1), fine- to medium-crystalline planar-e(s) dolomite (RD2) and medium- to coarse-crystalline nonplanar-a, planar-e(s) dolomite (RD3)) are identified. Two types of dolomite cements (medium- to coarse-crystalline dolomite cement (DC1) and coarse- to very coarse-crystalline saddle dolomite cement (DC2) are identified. The REE patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard. The DC2 dolomite cement and later-stage calcite cement (LSCAL) display positive Europium anomalies (δEu) and positive Cerium anomalies (δCe). Whereas the DM, DMS, RD1, RD2, RD3 and early-stage calcite cement (ESCAL) samples display positive δCe, light REE (LREE) enrichment and negative δEu. The DM and DMS were formed through seawater flux and mesosaline brine derived from evaporative seawater during early-burial seepage-reflux dolomitization and microbial dolomitization, respectively. The preferential occurrence of RD1 along low-amplitude stylolites suggest that their formation may have linked to pressure dissolution by which Mg-ions were probably released for replacement dolomitization during late early-burial to early shallow-burial dolomitization. Moreover, the tightly packed and curved crystal faces of RD2 and RD3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the RD1 and/or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow-burial dolomitization. The average range of the homogenization temperatures (Th) of CD2 is 132.4 °C–198.7 °C; and mostly higher than the estimated ambient temperature (95 °C–150 °C) for the Furongian-Lower Ordovician strata, implying that the growth of DC2 in the study area was the result of hydrothermal dolomitization. The comparable isotopic (δ18O, δ13C and 87Sr/86Sr) values between RD2, RD3, DC1, DC2 dolomites and coeval Neoproterozoic to Early Phanerozoic seawater values, in addition to the relatively high Th and salinity levels in these dolomites, indicates that their dolomitizing fluid were derived from radiogenic 87Sr-enrich coeval seawater during late shallow-burial dolomitization and hydrothermal dolomitization. The hydrotectonic driving model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift resulted from compressional tectonics that affected the Tarim Basin during Cambrian to Middle Ordovician and Late Ordovician period. This model is characterized by fluid flow in response to significant thermal heating, pressure-loading and enhanced fluid expulsion by squeezing, and the resultant higher-temperature Mg-rich connate waters and hydrothermal basinal brines are conveyed by convective-advective flows along permeable horizons, fractures and overthrust fault planes, resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites; forming irregular networks of dolomites and calcite cements in the host carbonate rocks.

Tectonic and geothermal controls on dolomitization and dolomitizing fluid flows in the Cambrian-Lower Ordovician carbonate successions in the western and central Tarim Basin, NW China

The deeply buried Cambrian-Lower Ordovician carbonates in the western and central Tarim Basin were extensively dolomitized in the course of their diagenetic history. Petrographic, geochemical (O-C-Sr-isotopes and trace elements) and fluid inclusion studies have been used in this study to elucidate the origins of the ancient massive dolomites. Based on detailed examination, dolomicrite and microbial (or algal)-laminated dolomites (DML) and three types of crystalline dolomites (fine-crystalline, nonplanar-a(s), dolomite (RD1), fine- to medium-crystalline, planar-e (s) dolomite (RD2), and medium- to coarse-crystalline, nonplanar-a dolomite (RD3)) are identified. Nonplanar-a dolomite (saddle) cement (CD), and early and later-stage calcite cement as vug/fracture infills are identified. Most samples of DML still possess relics of algal stromatolitic and anhydritc sediment fabrics, common in Sabkha dolomites formed during penecontemporaneous supratidal dolomitization. The occurrence of RD1 and RD2 along low-amplitude stylolites in most samples indicates that their formation was somewhat linked with pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during early shallow-burial dolomitization. Moreover, the curved crystal faces and the tightly packed crystals of RD2 and RD3 with irregular overgrowth rims and increasing crystal sizes may have resulted from the recrystallization and replacement upon the RD1 and/or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow-burial dolomitization. The average range of the homogenization temperatures (Th) of CD dolomites is 87.1 °C to 161.5 °C, mostly lower than the estimated ambient temperature (60 °C to 240 °C) for the Cambrian-Lower Ordovician strata, implying that the growth of the CD dolomites in the study area was of ‘geothermal’ origin during late shallow to deep-burial dolomitization. The large overlap of δ18O, δ13C and 87Sr/86Sr values of RD2, RD3 and CD dolomite suggest that the dolomitizing fluids that precipitated the dolomite cement apparently inherited the composition of the remnant saline connate seawater preserved in host limestones/dolomites and siliciclastics in the Terreneuvian to Cambrian Series 3 strata through fluid–rock interaction. The conceptual model to explain tectonic influence on dolomitizing fluid flows show that faults, fractures and permeable horizons which evolved from a strong tensional-compressional tectonic during the Late Hercynian orogeny in this basin effectively provided conduits through which extensive cross-formational and ascending higher-temperature saline-rich dolomitizing fluids were driven by thermal convective-advective circulation due to geothermal variation, forming irregular networks of dolomites, CD dolomite and calcite cements in the host carbonate rocks.

Uranium isotope variations in a dolomitized Jurassic carbonate platform (Tithonian; Franconian Alb, Southern Germany)

The uranium (U) isotopic composition (δ238U) of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean. However, many limestones have undergone significant dolomitization during diagenesis. To assess the potential impact of diagenetic changes on the U isotope composition of dolomitized rocks, we examined the uranium isotopic composition of a Jurassic carbonate platform of the Franconian Alb in Southern Germany. This platform underwent dolomitization during shallow burial due to the supply of magnesium-rich fluids in the form of slightly modified seawater. This type of dolomitization is common in the geologic record. The carbonate platform of the Franconian Alb can therefore serve as an example for many dolomitized carbonate platforms.A positive correlation between the concentrations of redox sensitive elements (e.g., Re and V) and δ238U confirms that the authigenic uptake of U under reducing conditions preferentially incorporates U enriched in 238U. This positive correlation between redox sensitive elements and δ238U does not change within the dolomitized interval, suggesting that the δ238U is not altered during shallow burial dolomitization of limestones. Therefore, our results indicate that dolostones can retain information about secular variations in seawater δ238U.The diagenetic uptake of U shifts δ238U of bulk carbonate sediments to values heavier than contemporaneous seawater. The magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set. Samples with high Re and V concentrations and high Re/Mo ratios are the most enriched in 238U. This positive correlation could be used to constrain the amount of diagenetic offset of bulk δ238U values from seawater. This is particularly important in carbonate systems that had a primary calcitic mineralogy. Primary calcite precipitates have low U concentrations and so small amounts of 238U uptake during diagenesis can have a significant impact on the bulk composition.The samples with the most depleted δ238U values have high Mo concentrations and low Re/Mo ratios. As carbonate sediments are generally Mo poor, the high Mo concentrations suggests that the depleted δ238U values might be linked to a manganese cycle that operated across the sediment-water interface during the deposition of this Jurassic carbonate platform.

Origins and Geochemistry of Oolitic Dolomite of the Feixianguan Formation from the Yudongzi Outcrop, Northwest Sichuan Basin, China

The topic of dolomite formation has long presented a challenge to researchers. In this study, the origin of widely occurring oolitic dolomites from the Yudongzi outcrop in the lower Triassic Feixianguan formation in northwest Sichuan, China, was investigated through petrographic observations, and mineralogical and geochemical analysis. Analytical methods used include cathodoluminescence, X-ray diffraction, stable isotopes, and electronic microprobe characterization. The dolomites were categorized into three major genetic types according to their textural and structural characteristics, which reflect their various origins. The first genetic type of these dolomites, seepage reflux dolomitization, occurs in marly to microcrystalline dolomite during the penecontemporaneous stage, and displays negatively skewed δ18Ο (−2.83‰ Pee Dee Belemnite (PDB)), positively skewed δ13C (2.71‰ PDB), a low degree of order (0.48), and 87Sr/86Sr ratios of 0.707509–0.707634, indicating involvement of a Mg-rich brine fluid in an open evaporative environment. The second type, shallow burial dolomitization, is the most significant genetic type of dolomite reservoir in this area. This process produced dominantly silty to fine crystalline dolomite in a platform-margin oolitic beach facies with negatively skewed δ18Ο (−3.26‰ PDB), positively skewed δ13C (1.88‰ PDB), a high degree of order (0.70), and 87Sr/86Sr ratios of 0.707318–0.707661, which are related to seawater-derived fluids in a shallow burial environment. The third type is moderate to deep burial dolomitization, and is the main process responsible for zoned dolomite and dolomite with cloudy cores and clear rims (CCCR dolomite), which have the most strongly negatively skewed δ18Ο (−7.32‰ PDB), positively skewed δ13C (3.02‰ PDB), and 87Sr/86Sr ratios of 0.707217–0.707855, representing diagenetic alteration and fluid flow in a closed environment. These findings indicate that dolomite was likely affected by various degrees of burial and related marine-derived fluids, which will aid exploration efforts in high-quality hydrocarbon reservoirs in the Sichuan Basin.

Shallow burial dolomitization of an Eocene carbonate platform, southeast Zagros Basin, Iran

ABSTRACT Here, a case example of a dolomitized Eocene ramp setting from the southeastern Zagros Basin is documented and discussed in the context of published work. This is of significance as well-documented case examples of Eocene dolomitized inner platforms are comparably rare. The same is true for detailed diagenetic studies from the Zagros Basin in general. Three measured field sections were combined with detailed petrographic and geochemical analyses and four main dolomite types were defined. The most significant dolomite type is present in the form of a volumetrically significant occurrence of meter-thick beds of strata-bound dolostones. These dolomites are characterized by near-stoichiometric composition, fabric-retentive and fabric-destructive textures, subhedral to anhedral in shape and most being in the tens-of-microns range. Dolomite δ18O (averaging -2.6‰) values are depleted relative to that expected for precipitation from Eocene seawater (averaging 0‰), while δ13C (averaging -0.1‰) values are within the range of Eocene seawater values (averaging 0.5‰). Dolomite Type II and III 87Sr/86S values from 0.7079 to 0.7086 are somewhat elevated with respect to Eocene seawater (0.7077 and 0.7078). Based on these data, it is suggested that moderately evaporated seawater, via shallow seepage reflux, acted as agent for the initial dolomitization process. Subsequently, early diagenetic dolomites were recrystallized during shallow burial to variable degrees. The absence of volumetrically significant evaporitic deposits indicates that the salinity of porewater during dolomitization was beneath the threshold limit for gypsum precipitation. In addition, ascending saline fluids from deep-seated salt diapirs might have affected dolomitizing fluids.

Similarities and differences in the dolomitization history of two coeval Middle Triassic carbonate platforms, Balaton Highland, Hungary

Dolomitization of platform carbonates is commonly the result of multiphase processes. Documentation of the complex dolomitization history is difficult if completely dolomitized sections are studied. Two Middle Anisian sections representing two coeval carbonate platforms were investigated and compared in the present study. Both sections are made up of meter-scale peritidal–lagoonal cycles with significant pedogenic overprint. One of the sections contains non-dolomitized, partially dolomitized, and completely dolomitized intervals, whereas the other is completely dolomitized. Based on investigations of the partially dolomitized section, penecontemporaneous dolomite formation and/or very early post-depositional dolomitization were identified in various lithofacies types. In shallow subtidal facies, porphyrotopic dolomite was found preferentially in microbial micritic fabrics. Microbially induced dolomite precipitation and/or progressive replacement of carbonate sediments could be interpreted for stromatolites. Cryptocrystalline to very finely crystalline dolomite, probably of pedogenic origin, was encountered in paleosoil horizons. Fabric-destructive dolomite commonly found below these horizons was likely formed via reflux of evaporated seawater. As a result of the different paleogeographic settings of the two platforms, their shallow-burial conditions were significantly different. One of the studied sections was located at the basinward platform margin where pervasive fabric-retentive dolomitization took place in a shallow-burial setting, probably via thermal convection. In contrast, in the area of the other, smaller platform shallow-water carbonates were covered by basinal deposits, preventing fluid circulation and accordingly pervasive shallow-burial dolomitization. In the intermediate to deep burial zone, recrystallization of partially dolomitized limestone and occlusion of newly opened fractures and pores by coarsely crystalline dolomite took place.

Dolomitization and recrystallization of middle Silurian reefs and platformal carbonates of the Guelph Formation, Michigan Basin, southwestern Ontario

ABSTRACT Regional study of the Guelph Formation (middle Silurian) in the subsurface of southwestern Ontario highlights the role of recrystallization during burial diagenesis in producing massive dolomites with relatively uniform petrographic features but complicated geochemistry. The dolomite is composed mainly of replacive, unimodal to polymodal mosaics characterized by uniform, dull to weak red cathodoluminescence. Whereas patch reefs on the platform and most pinnacle reefs on the upper basin slope are pervasively dolomitized, reefs on more basinward parts of the slope may be partially dolomitized, usually in their lower portions, or they remain limestone. The lithology of overlying carbonates in the lower Salina Group mimics closely that of the underlying Guelph Formation. Initial shallow burial dolomitization occurred in the Late Silurian in response to isolation and hydrological drawdown of the Michigan Basin. The basinward-decreasing dolomite trend resulted from continuous seawater flux from the middle (back reef) platform downward through Guelph limestones to the discharge areas on the slope and beyond. Reefs at the distal end of the regional paleoflow system, or those that were poorly connected to the system remained limestone. Both dolomite 13C values, which range from +1.1 to +5.0 ‰ (PDB), and 87Sr/86Sr ratios, which range from 0.70845 to 0.70910, reflect a Silurian seawater source. Values for 18O range from -5.2 to -9.7‰ (PDB) but cannot be explained by the above mechanism alone. Recrystallization likely occurred during the progressive burial of these rocks to depths of up to 2 km, and may at least in part explain the lack of correlation among dolomite crystal size, cathodoluminescence characteristics and the various geochemical parameters measured in this study. The reason for the wide spread in 18O values both geographically and stratigraphically reflects varying degrees of recrystallization as a function of macro- to micro-scale permeability and porosity differences in the precursor lithology and their control on fluid-rock interactions during recrystallization. The cause of the apparent decrease in dolomite 18O values from patch reefs on the platform (average 18O = ~ -6‰) to the lower slope (average 18O = ~ -8.5‰) is unknown, but may have resulted from a combination of a basinward increase in burial depth and a basinward increase in heat-flow, which are now only subtly preserved through the noise of multiple recrystallizations. End_Page 177------------------------

Shallow Burial Dolomitization and Dedolomitization of Cenozoic Cool-Water Limestones, Southern Australia: Geochemistry and Origin

ABSTRACT The Gambier Limestone, a bryozoan-rich, cool-water carbonate of Late Eocene to Early Miocene age in the Otway basin of southeastern Australia, is about one-third variably dolomitized limestone and dolostone. Individual dolomite crystals range from 10 to 500 µm and are of three types: (1) silt-size crystals (4-64 µm diameters) having a brightly luminescent core (10-20 µm) of nearly stoichiometric dolomite with 1 mol% MnCO3 and SiO2 but no Fe, surrounded by a thin, dull or alternating dark and bright CL cortex that is nonstoichiometric with 44-46 mol % MgCO3 but sharp to diffuse zoning in Mn and Fe, (2) sand-size crystals, which also have a brightly luminescent and stoichiometric core and dark and bright non-stoichiometric thin inner cortex, but are mostly a thick outer cortex (at ca. > 50 µm diameter) that is nonluminescent and nonstoichiometric with some Fe but no Mn, and (3) identical sand-size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement. These three crystal populations represent two distinct dolomites and their alteration product. The smallest of the silt-size crystals, which are rarely preserved, are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition. The 18O values of dolomite rhombs (-1.6 to 2.9 ) correlate positively with 13C values (0.7 to 3.0), with the lowest values from silt-size or small sand-size crystals and the highest values from large sand-size crystals with leached, unfilled cores. Low and variable 18O and 13C values in conjunction with high and variable Mn and Fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater. Higher values for the outer cortex in sand-size crystals are typical of those expected for precipitation from normal or near-normal seawater. Low Sr, Mn, and Fe contents in the largest crystals, in conjunction with fluid inclusions of seawater salinity in the outer cortex, are also consistent with the bulk of the large crystals having precipitated from seawater. Using the Sr-isotope evolution curve for Cenozoic seawater, Sr-isotope ratios of the dolomites can be divided into four distinct groups, which correspond to times of major transgressions. This Cenozoic example, in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression-regression, offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained.

Models of Porosity Formation and Their Impact on Reservoir Description, Lisburne Field, Prudhoe Bay, Alaska

The Lisburne field at Prudhoe Bay, Alaska, produces from shelfal carbonates in the Pennsylvanian Wahoo formation. Four major factors control reservoir behavior: (1) depositional stratification, (2) a fractured, permeable subunconformity alteration zone (SAZ), (3) multiple episodes of porosity formation, and (4) faulting. This paper is the first written description of facies, cyclicity, and diagenetic processes as they apply to porosity formation and reservoir modeling in the Lisburne field. Successive depositional cycles in the Wahoo formation pass from ooid/skeletal grainstones deposited in shoal complexes to oncolitic packstones and skeletal/peloidal wackestones formed in restricted lagoonal environments. Geochemical data and crosscutting relationships between porosity and unconformities, pressure solution features, fractures, and faults provide evidence of three distinct episodes of porosity formation. Earliest porosity is probably due to periodic, localized exposure during the Pennsylvanian. A second stage of porosity is associated with shallow-burial dolomitization that probably began during Permian-Triassic subaerial exposure. Reservoir quality in dolomites varies with the degree of neomorphic recrystallization. The third stage began in the Cretaceous and lasted in o the Tertiary, and is associated with final burial and hydrocarbon maturation. This burial dissolution event also opened existing fault systems, creating a complex reservoir. Most Lisburne field effective porosity is of late-burial origin due to either dolomitization or dissolution. Faults and the SAZ act as giant collectors of oil from low-permeability matrix, and they have reduced the number of wells needed for field development. Faults also complicate waterflood implementation and maintenance of uniform reservoir pressure. Field studies such as these demonstrate the importance understanding the diagenetic history of a reservoir can have for field management and development planning.

Shallow burial dolomitization and dedolomitization of Mid-Cenozoic, cool-water, calcitic, deep-shelf limestones, southern Australia

ABSTRACT Oligocene to mid-Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably altered to gray to orange, Ca-rich, medium-crystalline, sucrosic dolomite. Degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent. The locale and timing of dolomitization are tightly constrained to shallow burial and mid- to late Miocene. Dolostone varies from friable to dense and well lithified. Dolomitization is fabric specific; muddy sediments are preferentially replaced; calcite bryozoans and brachiopods form biomolds. Geochemistry suggests that dolomitization was predominantly by seawater-limestone interaction, but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements. Sr isotopes confirm a mid-Miocene age for the dolomite, if precipitated from seawater with no inherited limestone values. Dolomite crystals have undergone variable degrees of dissolution, and meteoric calcite cement has locally filled the resultant rhombic voids. Dissolution began in the crystal cores, implying some sort of metastability, and expanded outward until, in some ins ances, the whole dolomite crystal was dissolved, leaving dolomolds in otherwise unaffected limestone. This dolomite occurrence, well constrained by regional geology, shows that widespread, porous and permeable, fabric-destructive, medium-crystalline, sucrosic dolostone can form in the shallow subsurface soon after sediment deposition. Such dolomite, however, may be metastable in the presence of variably mixed continental-marine waters, and newly formed crystals can undergo dedolomitization soon after precipitation.

Shallow Burial Dolomitization of Mid-Cenozoic, Cool-Water, Calcitic, Deep-Shelf Limestones, Southern Australia

Oligocene to middle Miocene, deep-shelf, bryozoan-rich limestones across southern Australia are variably replaced by green to orange, Ca-rich, zoned, medium-crystalline, sucrosic dolomite. The degree of replacement varies from scattered rhombs in limestone to complete dolostones a few tens of meters in thickness and a few kilometers in lateral extent. Dolostone texture ranges from dense and well lithified to completely unlithified, resembling a loose sand of dolomite rhombs. Dolomitization is fabric specific; calcite and Mg-calcite bryozoans are either the last components to be replaced or are molds. The timing and locale of dolomitization are tightly constrained; Sr isotopes indicate a middle to late Miocene age while clasts of dolostone in overlying Pliocene limestones above a regional unconformity confirm a shallow-burial, pre-Pliocene origin. {delta}{sup 13}C and {delta}{sup 18}O values support a marine source for the carbonate; the influence of meteoric fluids appears to have been negligible. Quaternary exposure has resulted in local dedolomitization and/or subaerial erosion, especially in the Murray basin. These rocks are excellent analogues for localized, lenticular dolostone bodies in calcite-rich Paleozoic platform carbonates.