Precambrian Dolomite Research Articles

Tracking multiple paleo oil charges in the Precambrian dolomite reservoirs of the central Sichuan Basin, China

In the Precambrian dolomites in the central Sichuan Basin, a substantial paleo-oil system previously existed, which has since evolved into a dry gas system that is currently in place. The genesis of this paleo-oil system remained poorly understood, leaving uncertainties regarding the timing of oil charging and associated source rocks. Integration of Re–Os geochronometry with basin modeling allows temporal-spatial constraints on the ancient petroleum system to be established.Two texturally distinct generations of reservoir bitumen were identified. Late-generation bitumen yielded a geologically meaningful Re–Os age of 189 ± 20 Ma (MSWD = 10.4, 2σ), distinct from the previously documented Re–Os ages such as the 414 ± 44 Ma reported by Shi et al. (2020). This new age, along with the ∼414 Ma, aligns within uncertainties with the two relative timings derived from our independent basin model (ca. 450–390 Ma and 260–180 Ma), revealing two temporally distinct oil generation-migration events. The date of ∼189 Ma also lends support to the previous oil charge timing inferred from fluid inclusions. We evaluated the imprecisions of these two Re–Os ages and considered them to result from a narrow spread in 187Re/188Os, and variations in initial Os isotopic compositions (Osi) due to the giant size of the petroleum system and a long duration of oil generation from source kitchen over a large geographical area in this play. Effects of asphaltene precipitation, due to uplift-induced decompression of the migrating oils, may also have contributed to the scatter. Separating the samples based on Osi produced a higher precision age (184.1 ± 5.7 Ma, Model 1) with an MSWD of 0.97.The first oil charge is relatively weak as shown by the basin model and observed bitumen volumes, and it had not been well preserved due to significant reservoir exhumation by Caledonian tectonism. Consequently, the younger oil charge is considered to be the principal precursor to the current dry gas accumulations. Os isotope histories for four potential source units were constructed using publicly available Re–Os data. Comparisons between bitumen and rock Os compositions tend to support the lower Cambrian Qiongzhusi Formation as the predominant source unit for the two major paleo-oil charges. However, contributions from the Duoshantuo, Dengying, and Maidiping source rocks can not be precluded. Overall, these new observations allow temporal-spatial constraints to be placed on the complex evolution, from paleo-oil generation to destruction and dry gas generation.

Autochthonous Dolomitization and Dissolution in the Microbial Carbonate Rocks of the Fengjiawan Formation in the Ordos Basin

AbstractMany euhedral dolomite crystals and related pores are found in the microbial siliceous stromatolite dolomite and siliceous oolitic dolomite in the Fengjiawan Formation of the Mesoproterozoic Jixian System in the southern Ordos Basin. With the application of the microscope, scanning electron microscope, cathodoluminescence and in‐situ trace element imaging, it can be seen that different from the phase I dolomite that was damaged by silicification, the intact euhedral phase II dolomite occurred through dolomitization after silicification, concentrated mainly in the organic‐rich dark laminae of the stromatolite and the dark spheres and cores of the ooids. A considerable number of phase II dolomite crystals were dissolved, giving rise to mold pores and vugs which constituted the matrix pores and also the major pore space of the Fengjiawan Formation. The formation and dissolution of the dolomite were controlled by the microenvironment favorable, respectively, for carbonate precipitation and dissolution under the influence of microbial biological activities and related biochemical reactions. The driving force and material supply of dolomitization and dissolution were confined to the fabrics enriched with microorganisms, which are highly autochthonous. This mechanism may be a key factor for the development of Precambrian dolomite and related reservoirs, in the context of the domination of microbial rocks.

Ancient acid rains in the Ediacaran period – An alternative story for sulfate sedimentation

Vast deposits of anhydrite and magnesite are widely distributed in the Ediacaran strata of East Siberia near the Riphean unconformity. Anhydrite-rich rocks are likely not of evaporitic origin and mostly occur in the forms of nodules, the layers of chicken-wire structures otherwise disseminated as tiny sulfatic forms amongst the terrigenous rocks. Here, we propose an alternative point of view for anhydrite origin: the enrichment and increase of sulfur content in the Ediacaran atmosphere is due to high volcanic activity. It is suggested that the ancient Earth's atmosphere could have also been influenced by powerful sulfuric acid rains that eroded the Precambrian dolomites, causing their aggressive degradation. Chemical reactions with dolomite and sulfuric acid showed that an unstable phase of bassanite occurred in the early stages, which later stabilised as anhydrite after heating as an analogue of ageing. Aggressive acids have caused global dolomite karstification of the Siberian craton with the appearance of Ediacaran strata in addition to sulfate phases, including magnesite and sulfurous phases of pyrite and barite.

Dissolved silica-catalyzed disordered dolomite precipitation

AbstractThere is a great abundance of sedimentary dolomite in the Proterozoic and Lower Paleozoic, but examples of primary dolomite are scarce in the Cenozoic. This discrepancy suggests a poorly understood but dramatic shift in the geochemical system that inhibited dolomite formation. Previous research on microbial-mediated dolomite formation demonstrated that microbial activity could promote disordered dolomite precipitation through the catalytic role of polysaccharides. However, the microbial-mediated model cannot explain some of the Precambrian dolomite for which there is no evidence of microbial origin. Here, we present an abiotic mechanism with dissolved silica catalyzed dolomite precipitation that provides new insight into this long-lasting “dolomite problem.” In this study, we demonstrate that the presence of 1–2 mM of aqueous Si(OH)4 in high Mg:Ca ratio solutions at room temperature will promote disordered dolomite precipitation (with up to 48.7 mol% MgCO3) and inhibit aragonite formation. Dissolved silica in solution also promotes Mg incorporation into the Ca-Mg carbonates. Dissolved silica possesses low-dipole moment and dielectric constant similar to hydrogen sulfide, dioxane, polysaccharide, and exopolymeric substances (EPS), which are catalysts in previously established room-temperature dolomite synthesis. The molecules with low-dipole moment adsorbed on the dolomite surface can lower the dehydration energy barrier of a surface Mg2+-water complex and promote dolomite nucleation and growth. This study provides a new model for abiotic sedimentary dolomite formation, which is likely to be responsible for the significant amount of primary dolomite in Earth history.

Dating and characterizing primary gas accumulation in Precambrian dolomite reservoirs, Central Sichuan Basin, China: Insights from pyrobitumen Re-Os and dolomite U-Pb geochronology

Primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the Precambrian Dengying Formation of the Gaoshiti-Moxi block, Central Sichuan Basin, China. In this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the Dengying dolomite reservoirs. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ U-Pb dating, we obtained a high-precision absolute age of 259.4 ± 3.0 Ma for the saddle dolomite cementation, consistent in timing with the eruption of Emeishan flood basalts (262–251 Ma). Combined with high homogenization temperatures (Th) and salinities of primary fluid inclusion, as well as rare earth element (REE) compositions, we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the Emeishan mantle plume.The identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo-oil reservoirs. These bitumens are characterized by high equivalent vitrinite reflectance (Requ), no fluorescence, and deformed fibrous textures, typical of highly mature pyrobitumens. Gas geochemistry data indicates that only minor thermochemical sulfate reduction (TSR) has occurred. Thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo-oils, and its associated Re-Os isochron age of 154 ± 21 Ma, representing the timing of dry gas formation. Petrographical evidence suggests that the paleo-oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation. This, combined with coeval aqueous inclusion Th data and published thermal-burial models, constrains the time of paleo-oil emplacement to ca. 222–205 Ma, suggesting an oil-filling event related to the Indosinian Orogeny. This study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world.

Petrological and geochemical characteristics of the botryoidal dolomite of Dengying Formation in the Yangtze Craton, South China: Constraints on terminal Ediacaran “dolomite seas”

Recent studies suggest that the massive Precambrian dolomite successions were likely the products of specific seawater conditions known as “dolomite seas”. However, whether these so-called “dolomite seas” existed in the terminal Ediacaran ocean has not yet been constrained. The widespread botryoidal dolomite with well-preserved sedimentary fabrics from the Upper Ediacaran Dengying Formation (ca. 551 to 545 Ma), South China can be potentially an ideal target to constrain this issue. Results showed that within the botryoidal dolomite, fibrous dolomite cements are the dominant constituent part, and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites, implying a syn-sedimentary diagenesis. Two different types of fibrous cements are newly redefined: fascicular and radial slow dolomite. Well-preserved chemical and cathodoluminescent zonations and length-slow optical characters are direct evidence for syn-sedimentary primary dolomite precipitation, different with the previous alternative interpretations of mimetic dolomitization of aragonite or high-Mg calcite precursors. In contrast, microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics, reveling an early mimetic dolomitization of aragonitic precursors. The dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater-type fluids as indicated by comparable δ13C and 87Sr/86Sr data with global changes. In particular, the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific “dolomite seas”, associated with anoxic or even euxinic shallow water conditions (inferred by redox-sensitive trace elements and REEs) and increased microbial activity, also prevailed in the terminal Ediacaran ocean. However, the occurrence of “dolomite seas” was not one occasional event, but common even if not persistent in the early to terminal Neoproterozoic oceans. This study helps to improve the understanding of “dolomite problem” and ubiquitous distribution of Precambrian dolomite worldwide.

Controls on formation and alteration of early diagenetic dolomite: A multi-proxy δ44/40Ca, δ26Mg, δ18O and δ13C approach

The full potential of the dolomite Ca isotope proxy only unfolds when combined with data of the other main elements (C, O, Mg) in the crystal lattice of Mg-carbonates. Data presented here reveal the level of complexity inherent to dolomite precipitation and alteration environments and add new constraints to the understanding of early diagenetic dolomite formation. Well-constrained Precambrian to Pleistocene dolomites were investigated, representing three characteristic formation and alteration environments: (i) sabkha, (ii) altered marine and (iii) lacustrine/palustrine dolomites. Primary sabkha dolomites with typically low cation ordering degree (COD), high δ13C and low δ44/40Ca values contrast with recrystallized sabkha dolomites with relatively high COD, low δ13C and high δ44/40Ca values. Both δ13C and δ44/40Ca values of sabkha dolomite bear witness to the relative effects of kinetic and equilibrium Ca isotope fractionation conditions. Primary sabkha dolomites display Δ44/40Cadolomite-fluid ranging from −0.4 to −1.3‰, whereas recrystallized dolomite is approaching isotopic equilibrium (Δ44/40Cadolomite-fluid ∼ 0‰). Using the fractionation factor deduced from the sabkha dolomite data set, recrystallized Precambrian dolomite points to a δ44/40Caseawater/pore fluid of about 1.2‰ (SRM 915a), a value that is distinctively lower compared to previously suggested ones and modern seawater. Altered marine dolomites display evidence for meteoric overprint, indicated by δ18O values as low as −4.62‰. Both Mg and Ca isotope signatures correlate with δ18O values of altered marine dolomites, whereas δ13C values lack correlation with the other isotope systems. We propose that freshwater circulated through silicate aquifers prior to reaching the dolostone units and Ca, Mg and O isotopes of altered marine dolomites reflect variable degrees of this meteoric overprint. Lacustrine/palustrine dolomites display a correlation between the isotope values of C, Mg and Ca. These dolomites are formed during pulses of marine ingression in swamp, playa and lake environments and are thus characterized by water-logged conditions (anaerobic) and saline, sulfate-rich fluids. Bacterial-sulfate reduction induces dolomite formation and leads to lower δ13C and δ44/40Ca as well as to higher δ26Mg values. The Ca isotope proxy acts as a benchmark against which other proxy data can be calibrated or processes tested. Taking COD and dolomite stoichiometry into consideration, the here documented multi-proxy isotope approach is promising and provides benchmarks against which proxy signals can be calibrated and tested. Interestingly, even the limited data sets shown here point to patterns that can be interpreted in a meaningful manner that is of relevance for dolomite research.

A Palaeoproterozoic dolomite (Vempalle Formation, Cuddapah Basin, India) showing Phanerozoic-type dolomitisation

The Palaeoproterozoic Vempalle Formation of the Cuddapah Basin, India, significantly adds to our understanding of the evolution of Precambrian marine carbonate systems and the redox state of the Earth’s early oceans. A facies-microfacies-diagenetic-geochemical examination of samples from a ∼1000-m long exposure in a freshly-cut canal section shows that 10–15% of precursor limestone is still preserved in the Vempalle Formation in the form of remnant patches of calcimicrite and ooids with calcite spar cement. The ooids, preserving primary radial and concentric fabrics and radial fractures, are considered to have been originally precipitated as calcite, which may have been low-Mg. In places the preserved calcite spar, that is partially replaced by fabric-destructive dolomite, shows Type I calcite twin lamellae. Petrographic observations demonstrate that Vempalle Formation dolomite formed through very early precipitation, which in stromatolites preserved microbial filaments, as well as through fabric-destructive dolomitization during shallow to moderate burial. Vempalle Formation dolomite is characterized by micritic dolomite crystals which suggest rapid early dolomitization of lime mud and micritic calcite from a supersaturated Mg-Ca-rich solution, probably near-surface or during shallow burial. Depletion of Na and Sr contents of Vempalle Formation dolomite along with negative δ18O values indicate dolomite recrystallisation during burial and further replacement. Dolomite δ13C values of −0.5 to 2‰ are likely inherited original marine values. Geochemical proxies (trace elements and rare earths) imply that Cuddapah Basin seawater and dolomitizing fluids were anoxic and ferruginous but not euxinic. Geochemical analyses also indicate that the burial diagenetic fluids evolved from Eu-enriched seawater that probably resulted from continental rifting around 1.9–2.0 Ga. This probable ocean chemistry is in contrast with the anoxic, ferruginous and extremely high Mg/Ca “dolomite oceans” that prevailed during Proterozoic time. The Vempalle dolomite shows more similarities with dolomitised Phanerozoic platform carbonates than typical Precambrian dolomite with its well-preserved textures and burial dolospar cements.

A Model of Hydrothermal Dolomite Reservoir Facies in Precambrian Dolomite, Central Sichuan Basin, SW China and its Geochemical Characteristics

AbstractHydrothermal mineral assemblages and related hydrothermally enhanced fracturing are common in the Precambrian Dengying Formation of Central Sichuan Basin. Petrographic and geochemical analyses of core samples show that the hydrothermal dolomite reservoirs of Dengying Formation consist of four main types of pores in the reservoir facies. These include: 1) hydrothermal dissolution vug (or pore), 2) intercrystalline pore, 3) residual inter‐breccia vug (or pore), and 4) enlarged dissolved‐fracture. There are three different fabrics dolomite in hydrothermal dolomite reservoirs, namely, saddle dolomite, fine‐medium dolomite and micritic dolomite. Micritic dolomite is the original lithology of host rock. Saddle dolomite with curved or irregular crystal faces was directly crystallized from hydrothermal fluids (average temperature 192°C). Fine‐medium dolomites are the products of recrystallization of micritic dolomite, resulting in abnormal geochemical characteristics, such as slight depletion of δ18O, significant enrichment of Mn‐Fe and 87Sr/86Sr, and positive Eu anomaly. A model for the distribution of various hydrothermal dolomite reservoir facies is proposed here, which incorporates three fundamental geological controls: 1) extensional tectonics and tectono‐hydrothermal events (i.e., the Xingkai Taphrogenesis of Late Sinian‐Early Cambrian, and Emei Taphrogenesis of Late Permian), 2) hydrothermal fluid storage in clastic rocks with large thickness (e.g., Nanhua System of Chengjiang Formation and part of Doushantuo Formation), and 3) confining bed for hydrothermal fluids (such as, the shale in Qiongzhusi Formation). The supply of hydrothermal fluid is critical. Large basement‐rooted faults and associated grid‐like fracture system may function as the channels for upward migration of hydrothermal fluid flow. The intersection of the above‐mentioned faults (including the conversion fault), especially transtensional sags above negative flower structures on wrench faults can serve as a key target for future hydrocarbon exploration.

Unravelling sources of solutes in groundwater of an ancient landscape in NW Australia using stable Sr, H and O isotopes

The Precambrian meta-sedimentary fractured rock aquifers of the Hamersley Basin in northwest Australia are some of the oldest water-bearing formations on the planet and host enormous iron ore deposits. Groundwater is the only permanent source of water in the basin, therefore understanding the hydrological processes that effect water quality and quantity is a pre-requisite for sustainable water management. We used a combination of major dissolved ion concentrations, including Sr and Ca, in combination with δ2H, δ18O and δ87Sr in flood water and groundwater as tracers to constrain the processes affecting groundwater chemistry. The δ87Sr composition of groundwater in three major aquifer types ranges from 11.8‰ to 40.6‰ and reflects the mineralogy of altered Precambrian dolomite (15.1‰ to 55.4‰) rather than the host iron ore formations (22.5‰ to 46.5‰ >95% iron oxides) or highly radiogenic shale bands and clay minerals (200‰ to 2322.5‰). Groundwater in the terminal Fortescue Marsh wetland of the basin has a rather constant δ87Sr signature of 36.6±1.4‰ irrespective of variations in TDS, δ18O and Sr concentration. This groundwater is considered to be mature in a geochemical sense, representing the final stage of water evolution on a basin scale. Mixing calculations utilising δ87Sr and Ca/Sr data demonstrate contributions of salts from three major sources: on average >92% from precipitation, ~7% from carbonate rocks and <1% from rocks with highly radiogenic signatures (shales and clays). These results demonstrate groundwater evolution from a recharge area to discharge area at the regional scale, but more importantly that water quality in the terminal wetland is primarily driven by rainfall chemistry in floodwaters rather than water–rock interactions in the catchment.

Co-variation of Mg and C isotopes in late Precambrian carbonates of the Siberian Platform: A new tool for tracing the change in weathering regime?

This study reports significant Mg isotopic variations in 32 samples of late Precambrian dolomites from various parts of the Siberian Platform exhibiting a range of δ 13C from − 10‰ to + 6‰ (V-PDB). In the Ediacaran dolomites from the N and NE part of the Patom Paleobasin, a systematic increase of δ 26Mg from − 2.3 ± 0.2‰ to − 0.8 ± 0.3‰ (DSM3) along with a δ 13C decrease from + 6‰ to − 10‰ (δ 26Mg = − 0.072 × δ 13C − 1.83) with a correlation coefficient of R 2 = 0.73 (n = 20) are observed. Within the stratigraphic sequence of Ediacaran and Ediacaran–Lower Cambrian transition successions, all positive C-isotope excursions co-vary with negative Mg-isotope shifts and vice versa. Other samples from the Siberian Platform lay on this regression with R 2 = 0.66 (n = 28) except for 4 samples, probably diagenetic dolostones, that exhibit a positive δ 26Mg–δ 13C correlation. The enrichment of sedimentary dolomite in heavy carbon and light Mg isotopes may be interpreted as a result of a complex interplay of contrasting sedimentation conditions (sea regression versus sea transgression regime) yielding different weathering regimes and relative contributions of carbonate versus silicate rocks. The enrichment of both oceanic water and carbonate sediments by 26Mg could occur during transgression and would coincide with sedimentation of low- 13C carbonates, whereas sedimentation of the depleted in 26Mg, and enriched in 13C carbonates could happen under regression regime when exposed carbonate platforms and coastal planes are subjected to intense weathering.

Molybdenum isotopic records across the Precambrian-Cambrian boundary

Molybdenum (Mo) isotopic data from early Cambrian formations in southern China demonstrate the importance of non-euxinic sediments as a promising archive for coeval seawater Mo records. For the first time, in analogue to noneuxinic, nonskeletal carbonates, we report that the pristine phosphorites also can preserve the ambient fluid, which make the pristine phosphorites a new potential fingerprinting tool to record the coeval paleoseawater. Pristine phosphorite and dolomite samples of two early Cambrian formations in southern China suggest that early Cambrian seawater may have had δ 97/95 Mo values of at least 1.4‰, similar to modern oceans. Low δ 97/95 Mo (as low as −0.5‰) values were obtained from reworked phosphorite, but evidence suggests that these are reflecting Mo related to Fe oxyhydroxide minerals rather than the coeval seawater. In contrast, late Precambrian dolomite just below the transition to the Cambrian would indicate a δ 97/95 Mo of coeval seawater slightly heavier than that of the Mesoproterozoic (0.9‰–1‰ versus 0.8‰), suggesting that oceanic circulation patterns may have been thoroughly reorganized by that time, and may have triggered biological diversification from the Ediacaran to the early Cambrian.

Stratigraphic results of diamond drilling of the Hunterston Dome, Tasmania: implications for palaeogeography and hydrocarbon potential

The structure known as the Hunterston Dome, in central Tasmania, was drilled to a depth of 1324 m, through Jurassic dolerite, Lower Parmeener Supergroup and into Precambrian dolomite basement. The base of the Lower Parmeener Supergroup does not outcrop in the area, and drilling revealed the absence of the extensive glacial diamictites present elsewhere in the Tasmania Basin. A conglomeratic facies is found in place of the Bundella Mudstone and Woody Island Siltstone. Basement to the Lower Parmeener Supergroup is shown to be deformed Precambrian dolomite, of similar lithology to the relatively undeformed Black River Dolomite of northwestern TlSInania. Significant hydrocarbons were not encountered during drilling, but stratigraphic drilling proved the maturity of potential source beds in the region and the nature of potential reservoir rocks, where they are found in close association with a dolerite intrusion.

Airborne Mineral Fibers and Quartz Dust in Precambrian Metamorphic Limestone and Dolomite Mines in Finland

Abstract The airborne concentrations of mineral fiber and quartz dust to which miners are exposed are reported here for four limestone and five dolomite mines. The air samples were collected at fixed monitoring points located in the main work areas. The material comprises 46 samples analyzed by phase contrast microscope (PCM), 90 samples analyzed by scanning electron microscope (SEM), and 50 samples of dust analyzed for quartz content. Fibers (length >5 μm, diameter < 3 μm, and aspect ratio ≥ 3:1) were counted and silicate minerals were identified with an energy-dispersive spectrometer. On the basis of morphology the fibers were classified as perfect fibers, fibrous cleavage fragments, or cleavage fragments. In various work stages the mean fiber concentrations were 0.05 to 0.71 fibers/cc as measured with PCM and 0.06 to 0.34 fibers/cc as measured with SEM. The mean concentrations of the respirable quartz dust (<5 μm) were 0.07 to 0.96 mg/m3. The predominant fibrous silicates were tremolite and wollastonit...

Systematic decrease of high δ 13C values with burial in late Archaean (2.8 Ga) diagenetic dolomite: evidence for methanogenesis from the Crixás Greenstone Belt, Brazil

Remnants of Fe-poor (2700–4400 ppm) dolomite crystals, crystal aggregates, and microconcretions are cryptically distributed within stratiform bodies (tens of metres across) of metamorphic Fe-rich (3–6 wt% Fe) dolomite that occur in graphitic schists of the 2.8 Ga Ribeira˜o das Antas Formation in the Crixás Greenstone Belt, Brazil. Fe-poor dolomite is a diagenetic carbonate formed during burial of organic-bearing, muddy sediments. Using cathodoluminescence microscopy, a consistent sequence of dolomite growth stages is well defined within individual crystals, crystal aggregates, and microconcretions. From comparison with similar textures in Phanerozoic carbonates, we suggest that the early dolomite formed during progressive burial, aggregates having developed from mergent growth of individual crystals during mechanical and then chemical compaction. The δ 13C dolomite values of crystals and concretion cores are very high, 12–14‰ (PDB), and a negative shift of about 7‰ occurs over millimetres in subsequent dolomite generations. Early diagenetic Fe-poor dolomite and late-stage Fe-rich dolomite show overlapping ranges of initial 87Sr 86Sr ratios, between 0.7080 and 0.7164, and δ 18O values, −14 to −16‰ (PDB). These similarities may indicate isotopic exchange between the early dolomite and subsequent metamorphic fluids, yet the well preserved growth zonation of the Fe-poor dolomite suggests that such exchange did not follow a process of recrystallization. The decrease in the δ 13C of dolomite is correlated with crystal and concretion growth during burial. This behaviour is interpreted to indicate a change in the δ 13C of pore-water bicarbonate resulting from bacterially mediated organic diagenesis within the lower part of the zone of methanogenesis. The Crixás dolomite represents direct evidence that methanogenic bacteria were important contributors in shallow subsurface geochemical systems in the early Precambrian. The pattern of δ 13C values found for the Crixás dolomite is also documented in other early to middle Precambrian dolomites and limestones. Previous interpretations have considered positive δ 13C values to be the result of primary (oceanographic) controls, whereas variability in δ 13C is commonly linked to metamorphic alteration. We suggest that the role played by organic diagenesis in controlling the δ 13C values of diagenetic carbonates has been underestimated.

Diagenesis, Geochemistry, and Origin of a Precambrian Dolomite: the Beck Spring Dolomite of Eastern California

The Beck Spring Dolomite is a platform carbonate sequence dominated by cryptalgal laminites with some stromatolites and grainstones and is inferred to have been deposited in shallow subtidal to intertidal environments. Cavity structures, where the cements occur, include birdseyes, laminoid fenestrae, stromatactis, sheet cracks, and neptunian dikes. Four types of synsedimentary-early diagnetic fibrous dolomite are distinguished. Type A forms isopachous fringes consisting of length-slow acicular to columnar pseudopleochroic crystals with fine, substrate-parallel color banding. The dolomite is mostly nonluminescent with some bright blotches, and inclusion trains, define acicular crystallites within the columnar crystals. Type B consists of length-slow columnar crystals with distinctive bladed relic structures, defined by inclusions and nonluminescence, surrounded by inclusion-free and brightly luminescent dolomite. Type C consists of bladed to wedge-shape pseudopleochroic crystals with undulose extinction (mostly fascicular optic); inclusion trains define acicular crystallites within crystals, and forms mammillated crusts and botryoids. Type D fibrous dolomite forms thin layers of acicular crystals, with a distinctive geometry of the cement layers thickening into the corners of cavities. Types B and C contain relic structures indicating replacement, but types A and D have fabrics which could be primary. The depositional components (pisolites and micrite) and some fibrous dolomites have oxygen isotopic values around -2 per mil versus PDB, compatible with a marine origin. However, Na (around 220 ppm) and Sr (around 65 ppm) contents are very low, suggesting precipitation from dilute waters. Meteoric-marine mixing-zone dolomitization of CaCO 3 sediments and early cements. Or primary marine precipitation of poorly ordered calcian dolomite and early diagenetic stabilization to stoichiometric dolomite. The dolomite spar has very negative values of delta 18 O (-9.6 per mil versus PBD) and is more depleted in Na (34 ppm) and Sr (47 ppm), reflecting precipitation from meteoric waters of very low ionic strength during burial diagenesis. A replacement origin for the dolomite of the depositional components and fibrous cements can be reconciled with the lack of obliterative replacement fabrics invoking an original high-Mg calcite mineralogy.--Modified journal abstract.

Oil-Bearing Glacial Deposits from Permo-Carboniferous Haushi Group, Oman: ABSTRACT

A sequence of glacial and related deposits up to 700 m (2,296 ft) thick in the lower part of the Haushi Group contains significant quantities of oil in several fields in south and central Oman and is the main exploration target along the eastern flank of the South Oman salt basin. More than 300 well penetrations demonstrate that the glacial deposits extend over more than 60,000 km2 (23,166 mi2) in outcrop and subsurface. The Late Carboniferous to Early Permian Haushi Group (dated through palynology) unconformably overlies Precambrian to Devonian rocks of the Huqf and Haima Groups with, locally, a considerable paleorelief. The glacial deposits are overlain by fluvial and shallow marine deposits of the upper Haushi Group which include a fossiliferous l mestone dated as Early Permain (probably Sakmarian). The glacial origin of units within the lower part of the Haushi Group is established from the outcrop area on the west flank of Huqf massif where striated pavements of Precambrian dolomite have been found. One pavement is directly overlain by a 4 m (13 ft) thick diamictite interpreted as a basal till. In nearby exposures striated boulders have been found. Both in the outcrops and in the subsurface diamictites with abundant far-traveled material (granite and volcanic boulders) occur. They are considered to be mostly debris-flow deposits of glacially transported material. Their common interbedding with shales with dropstones is diagnostic of a glaciolacustrine setting. The association of varved shales and diamictites is commonly of sealing quality. Reservoirs in the glacial sequence are formed by sandstones and clast to matrix-supported sandy conglomerates which are interpreted as deltaic and fluvial deposits. Depositional models and paleogeographic reconstructions have been made with the help of core studies and correlation of facies units using wireline logs. These correlations demonstrate the limited lateral predictability of reservoirs which is not unexpected in view of the geometrical complexity of many modern glacial deposits. The geometry of facies units is, however, also complicated by syndepositional subsidence, due to both progressive dissolution of Precambrian salt, and to differential erosion at the contact between the glacial sediments and the underlying, unconsolidated sands of the Haima Group. End_of_Article - Last_Page 429------------

Late Palaeozoic Gondwana glaciation in Oman

Late Carboniferous–early Permian sediments belonging to the Haushi Group in Oman have been considered to be partly of glacial origin1–3. However, conclusive evidence of a continental glaciation has been lacking, and the extension of the late Palaeozoic Gondwana glaciation into the Arabian Peninsula has not been generally accepted4. During recent visits to outcrops of the Haushi Group in the Al Khlata area of the Huqf Massif (Fig. 1), we have found good exposures of striated pavements of Precambrian dolomite, which are directly overlain by diamictites of the Haushi Group (Fig. 2). This occurrence forms the strongest evidence so far for the presence of a continental glaciation in Oman during the late Palaeozoic.

Comments and Reply on ‘Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites’

Research Article| December 01, 1982 Comments and Reply on ‘Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites’: COMMENT B. D. Ricketts B. D. Ricketts 1Geological Survey of Canada, 3303 33rd Street, N. W., Calgary, Alberta T2L 2A7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information B. D. Ricketts 1Geological Survey of Canada, 3303 33rd Street, N. W., Calgary, Alberta T2L 2A7, Canada Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1982) 10 (12): 663. https://doi.org/10.1130/0091-7613(1982)10<663a:CAROPD>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation B. D. Ricketts; Comments and Reply on ‘Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites’: COMMENT. Geology 1982;; 10 (12): 663. doi: https://doi.org/10.1130/0091-7613(1982)10<663a:CAROPD>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Precambrian dolomites: Petrographic and isotopic evidence that they differ from Phanerozoic dolomites

Dolomites are more common in Precambrian than in Phanerozoic rocks and, from a petrographic-isotopic study of the middle to upper Proterozoic Beck Spring Dolomite of eastern California, it is suggested that Precambrian dolomites are different from those of the Phanerozoic. Petrographically, all fabric details of Precambrian dolomites studied are preserved, just as if they were limestones, and isotopically, they show the same trend as limestones: increasingly negative δ I8 O and to a lesser extent increasingly light δ I8 C, from depositional components, through early fibrous dolomite cements to later dolomite spar cements. The data suggest that in Precambrian time dolomite was the principal carbonate mineral precipitated from seawater and during diagenesis, and this implies that Precambrian seawater was different from that of the Phanerozoic.