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1.
Archean carbonates commonly contain decimetre- to metre-thick beds consisting entirely of fibrous calcite and neomorphosed fibrous aragonite that precipitated in situ on the sea floor. The fact that such thick accumulations of precipitated carbonate are rare in younger marine carbonates suggests an important change in the modes of calcium carbonate precipitation through time. Kinetics of carbonate precipitation depend on the concentration of inhibitors to precipitation that reduce crystallization rates and crystal nuclei formation, leading to kinetic maintenance of supersaturated solutions. Inhibitors also affect carbonate textures by limiting micrite precipitation and promoting growth of older carbonate crystals on the sea floor. Fe2+, a strong calcite-precipitation inhibitor, is thought to have been present at relatively high concentrations in Archean seawater because oxygen concentrations were low. The rise in oxygen concentration at 2.2-1.9 Ga led to the removal of Fe2+ from seawater and resulted in a shift from Archean facies, which commonly include precipitated beds, to Proterozoic facies, which contain more micritic sediment and only rare precipitated beds. 相似文献
2.
S. Anne Reeckmann 《Sedimentary Geology》1988,60(1-4):209-219
Temperate shelf carbonates form in cool marine waters and have skeletal and mineralogical compositions which are different from their tropical counterparts. They commonly lack non-skeletal grains and are often composed of low- and high-magnesium calcite with subordinate aragonite. Many of the aragonitic components found in tropical carbonates, such as corals, ooids, blue-green algae and lime mud, are absent.
Temperate shelf carbonates undergo diagenesis in marine waters with lower carbonate saturation than do tropical carbonates, and are exposed to cool climates with moderate to low rainfall. Marine cementation is rare because of low carbonate saturations in the surrounding waters. However, aragonite and high-magnesium calcite cements have been reported forming under specialized conditions associated with biogenic precipitation, submarine methane and sulphate-reducing bacteria, and more commonly in the intertidal environment where evaporation has increased carbonate concentrations.
In Pleistocene and Tertiary temperate shelf carbonates from southeastern Australia, evidence of marine diagenesis is rare to absent. Diagenetic stabilization of aragonite and high-magnesium calcite has taken from 80,000 y to 1 My, or longer, during subaerial exposure. This is slower than rates reported from tropical climates. A general lack of aragonite in some facies within these temperate carbonates leads to a lack of secondary porosity and only sparse low-magnesium calcite cement, even after prolonged fresh-water diagenesis. However, with lengthy exposure and under the right climatic conditions, karstic solution and calcrete precipitation can occur.
In sequences containing siliciclastic clays, pyrite and glauconite, abundant iron is present in interstitial waters leading to the precipitation of ferroan calcite cements in the phreatic and shallow burial environments, and to the substitution of iron for magnesium in stabilizing high-magnesium calcite skeletal material.
A unique void-filling, micritic internal sediment occurs in discrete layers in many of the Tertiary temperate shelf carbonate sequences in southeastern Australia. This internal sediment is localized as a pore-filling material above permeability barriers such as fine-grained sediments or volcanics, and above paleo-water tables which formed during periods of subaerial exposure. It is a feature of the vadose zone and lithifies to form a dense micritic low-magnesium calcite cement with characteristic pink/brown coloration, often associated with erosion surfaces and nodule beds.
Dolomite is uncommon in the southeastern Australian temperate shelf carbonates. It forms associated with preferential fluid pathways or mixing zones. Ferroan dolomite forms in siliciclastic clay-rich carbonates in the shallow burial environment. The ubiquitous fine, evaporite-related dolomite so common in tropical carbonates is absent. 相似文献
3.
4.
Ultra-high-pressure (UHP) marble and eclogite in the Su-Lu UHP terrane, eastern China 总被引：22，自引：0，他引：22
A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su-Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na-bearing garnet (up to 0.2 wt% Na2 O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T =610–660 °C and P =2.5–3.5 GPa (for X CO=0.001). A reaction between dolomite and diopside to form talc under tremolite-unstable conditions indicates a temperature decrease under ultra-high-pressure conditions ( P >2.4 GPa, X CO<0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post-dating the ultra-high-pressure metamorphism. The temperature decrease under ultra-high-pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The P – T conditions of the amphibolitization stage are estimated at <0.9 GPa and <460 °C, and are similar to conditions recorded by the surrounding orthogneisses. 相似文献
5.
Ancient carbonate buildups may contain extraordinarily large amounts of early diagenetic precipitates. In some, host rock lamination may be traced into inclusion bands within the 'cement' crystals, suggesting that the crystals are replacive. By analogy with a Pleistocene speleothem from the Sorrento Peninsula, however, these relationships can be explained differently. In the speleothem, large, repeatedly split and dendritic calcite crystals occur within a laminated carbonate. Lamination consists of sub-mm alternations of micrite and microspar. Micritic laminae pass laterally into inclusion-rich growth bands in the dendritic calcite crystals, and have replaced an aragonitic cement, whereas the microspar laminae were primary calcite cements. Three types of inclusion-rich bands occur in the dendrite crystals: (1) with aragonite relicts, (2) 'ribbon calcite' and (3) with oriented micropores. When aragonite precipitated, the calcite dendrite branches were unable to keep growing as single crystals and split into crystallites (separated by micropores, some forming ribbon calcite), whereas during episodes of calcite lamina precipitation, the larger crystals were regenerated by crystallite coalescence. Calcite crystals are primary: they did not replace a micritic precursor. By analogy with the Italian speleothem, some ancient reefal sparry carbonates may not be replacements of earlier laminated sediments, but may have grown concurrently with them. It is also probable that some ancient laminated sediments were instead sea-floor precipitates, and that stromatolites containing cross-cutting crystal fabrics, and the alternating micrite-microspar laminae typical of Archaeolithoporella , could be largely abiotic crystal growths. 相似文献
6.
Justin Baker Ries 《地学学报》2009,21(5):323-339
Independent lines of geological evidence suggest that fluctuations in the Mg/Ca ratio of seawater between 1.0 and 5.2 have caused the oceans to alternate between favouring the precipitation of the aragonite and high-Mg calcite polymorphs of calcium carbonate ( m Mg/Ca > 2; aragonite seas) and the low-Mg calcite polymorph ( m Mg/Ca < 2; calcite seas) throughout Phanerozoic time. The rise of aragonite-secreting bryopsidalean algae as major producers of carbonate sediments in middle Palaeogene time, a role that they maintained through to the present, has been attributed to a transition from calcite-to-aragonite seas in early Cenozoic time. Recent experiments on the modern, carbonate-sediment-producing bryopsidales Halimeda , Penicillus and Udotea reveal that their rates of calcification, linear extension and primary production decline when reared in experimental calcite seawaters ( m Mg/Ca < 2). These normally aragonite-secreting algae also began producing at least one-quarter of their CaCO3 as calcite under calcite sea conditions, indicating that their biomineralogical control can be partially overridden by ambient seawater chemistry. The observation that primary production and linear extension declined along with calcification in the mineralogically unfavourable seawater suggests that photosynthesis within these algae is enhanced by calcification via liberation of CO2 and/or H+ . Thus, the reduced fitness of these algae associated with their low rates of calcification in calcite seas may have been exacerbated by concomitant reductions in tissue mass and algal height. 相似文献
7.
Ibrahim M. Ghandour Hamad A. Al-Washmi Rashad A. Bantan Mohamed M. Gadallah 《Arabian Journal of Geosciences》2014,7(1):355-365
The present study describes the petrographic and petrophysical characteristics of three stratigraphically asynchronous beachrocks (A, B, and C) along the Red Sea coast at Al-Shoaiba area, Saudi Arabia, to identify the compositional, depositional, and diagenetic controls on beachrock formation and their petrophysical properties. The beachrocks at the three locations consist basically of calcareous skeletal remains of different types and grain size reflecting the composition of adjacent beach sediments and the depositional conditions. They were cemented by aragonite and high-Mg calcite (HMC) in intertidal zone. The cement exhibits three major fabrics: (1) micritic coatings, (2) isopachous to asymmetric aragonite rim, and (3) cryptocrystalline pore-fillings HMC. In addition, some intergranular and intragranular pores were partially filled with infiltrated silt-sized carbonate and siliciclastic sediments. The cementation was accomplished by the combination of inorganic and organic processes, and the former was dominant through evaporation of pore-filling seawater under hot and dry climatic conditions. Cementation in the present beachrocks is selective; carbonate grains are cemented, whereas siliciclastic grains are cement free. The cement nucleation on carbonate grains was easier than on siliciclastic grains which do not provide good "seed crystals" for carbonate cement growth. The petrophysical measurements are in agreement with petrographic studies. The degree of cementation is the major control on petrophysical characteristics where the lowest and highest porosity and permeability values were obtained from the least cemented beachrocks of location A and the highest cemented beachrocks of location B, respectively. The thickness of cement seems to be controlled by cement precipitation rate, grain size, and size and shape of pore spaces. The coarse sediments of beachrocks at location B were deposited under higher depositional energy than those at locations A and C, and therefore, their primary pore spaces were large enough to be filled with seawaters. Under hot and dry climate, cement precipitation increases leading to partial to complete occlusion of pore spaces. 相似文献
8.
显生宙非骨屑碳酸盐矿物经历了文石海和方解石海的交替,主要造礁生物和沉积物生产者的骨骼矿物与非骨屑碳酸盐矿物具有同步变化的趋势。这种长期的变化趋势可以用海水化学Mg/Ca摩尔比的变化来解释。流体包裹体、同位素和微量元素等证据也证实了海水化学在地质历史中经历过剧烈的变化。虽然生物诱导矿化和生物控制矿化的相对重要性一直存在争议,但古生物地层记录和人工海水养殖实验结果都表明,海水化学演化对生物矿化有重要的影响,体现在造礁生物群落的兴衰、生物起源时对骨骼矿物类型的选择以及微生物碳酸盐岩在地质历史中的分布等。这些为研究前寒武纪海水化学演化、古气候和古环境的重建、同位素地层对比以及碳酸盐的沉积和成岩等问题提供了新的思路。 相似文献
9.
L. Zhang D. J. Ellis R. J. Arculus W. Jiang C. Wei 《Journal of Metamorphic Geology》2003,21(6):523-529
The solid‐state reaction magnesite (MgCO3) + calcite (aragonite) (CaCO3) = dolomite (CaMg(CO3)2) has been identified in metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane‐chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560–600 °C, 4.95–5.07 GPa based on the calcite–dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (< 3.7 °C km?1) and is within what was previously considered a ‘forbidden’ condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO2 to the overlying mantle wedge. 相似文献
10.
Dr. G. Niedermayr Dr. E. Scheriau-Niedermayr Dr. A. Beran 《International Journal of Earth Sciences》1979,68(3):979-995
Dolomite, magnesite, calcite and aragonite are described from sandstones and conglomerates of the Grödener Schichten of the Dobratsch in the Gailtal Alps, Carinthia — Austria. The carbonates occur as early and late diagenetic cements, as recrystallized matrix of more or less laminated carbonate layers (up to 0.5 m thick), as concretions, as fillings of early diagenetic shrinkage cracks and late diagenetic fissures and as crystals in open cavities and clefts. The formation of dolomite and magnesite may have been caused by pore solutions highly enriched in magnesium, which are likely to have formed by evaporation in a hypersaline environment. Magnesite and dolomite most probably have been formed diagenetically from aragonite and/or calcite during progressive evaporation cycles. Because of magnesite is a common constituent in Permo-Scythian sediments of the Eastern Alps, its presence is of importance for the evaluation of the environment. Additionally the wide-spread occurrence of magnesite within these post variscian sediments has important implications regarding the paleogeographical reconstruction and the evaluation of diagenetic processes. 相似文献
11.
Limestone–marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals. However, uncertainty in such interpretations stems from the differential diagenesis that most limestone–marl alternations have undergone. Differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters. For an unequivocal interpretation of the origin of rhythmic alternations, diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required. The present study uses a multiproxy approach (independent biotic, sedimentary and geochemical parameters) in order to unravel the palaeoenvironmental signal recorded in Valanginian (Early Cretaceous) limestone–marl alternations from the Blake‐Bahama Basin (DSDP site 391). Using this approach, terrestrial and marine influences can be distinguished, changes in nutrient levels estimated and prediagenetic differences in the non‐carbonate fraction constrained. Surprisingly, no systematic variations in any of these parameters were observed between limestone and marl layers, implying that none of these was directly responsible for the formation of the rhythmic alternation. Hence, none of the current models of sedimentary formation of limestone–marl rhythmites is applicable here. Calcareous nannofossils are equally well preserved in limestone and marl layers, ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds. Sr values of 700–900 p.p.m. indicate that aragonite may have been present in the original, pelagic sediment. The assumption of fine‐grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites, including the source of the calcite cement observed in the limestones, the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers. The study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results. Clearly, careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions. 相似文献
12.
Small (5–30 μm) aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at Rapolano Terme in Tuscany, Italy. The aggregates are mainly concentrated in irregular, wispy and dark laminae which cross-cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit; they also occur less commonly scattered irregularly through the calcite layers. The aragonite needle aggregates are in the form of crosses, fascicles (sheaf shaped bundles, or dumbbell shaped), rosettes and spherulites. Locally, irregular masses of needles also occur. The fascicles, rosettes and spherulites have hollow centres which resemble microbial components (?fungal spores, bacterial colonies and pollen), suggesting that the aragonite crystals are biotically nucleated. The lamination is interpreted to reflect diurnal control. Stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification. Calcite feather crystals, although traversed by the aragonite aggregate laminae, have a clear appearance under the light microscope. They form more or less continuously through the diurnal cycle by abiotic precipitation. The constant association of aragonite with organic nuclei, irrespective of whether the latter are in laminae or scattered through the calcite layers, supports a biotic control on aragonite formation. Lamination in Pleistocene travertines is superficially similar to that in the present day deposits, but is diagenetically altered. In the Pleistocene deposits, the calcite feathers appear dark under the light microscope and the aragonite aggregates, where they are not altered to dark calcite, are dissolved, together with parts of the adjacent spar calcite, and therefore appear light coloured. 相似文献
13.
Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis? 总被引：4，自引：0，他引：4
Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. 'Meniscus-type cements' (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure. 相似文献
14.
New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy 总被引：2，自引：0，他引：2
PHILIP A. SANDBERG 《Sedimentology》1975,22(4):497-537
Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time. 相似文献
15.
ALESSANDRA SPADAFORA EDOARDO PERRI† JUDITH A. MCKENZIE CRISÓGONO VASCONCELOS Associate Editor: Gert-Jan Reichart 《Sedimentology》2010,57(1):27-40
Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high‐Mg calcite and/or Ca dolomite formed by an iso‐oriented assemblage of sub‐micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as sub‐micron flat and filamentous mucus‐like structures inside the interspaces of the high‐Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0·2 to 4 μm in diameter, sub‐spherical, rod‐like and filamentous forms, isolated or in colony‐like clusters. The co‐existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub‐micron polyhedral crystal formation of high‐Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub‐micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro‐environmental water conditions around the peloids change. 相似文献
16.
Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas) 总被引：1，自引：0，他引：1
CHRISTINA GLUNK CHRISTOPHE DUPRAZ OLIVIER BRAISSANT KIMBERLEY L. GALLAGHER ERIC P. VERRECCHIA PIETER T. VISSCHER 《Sedimentology》2011,58(3):720-736
Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments. 相似文献
17.
Khalid Al-Ramadan 《Arabian Journal of Geosciences》2014,7(11):4933-4942
Beachrocks occur in present-day intertidal zones of the Arabian Gulf and the Gulf of Aqaba, on the eastern and northwestern coasts, respectively, of Saudi Arabia. The beachrocks occur as linear patches within beach deposits, which have variable grain size and detrital compositions. The Arabian Gulf beachrocks are composed of sand-sized bioclasts and siliciclastic grains, whereas the Gulf of Aqaba beachrocks are composed of sand- to pebble-size grains, which are dominated by igneous rock fragments and small amounts of skeletal carbonate grains. The cement includes micritic high-magnesian calcite and isopachous acicular/bladed aragonite. In addition to cements, intergranular pores are locally filled by a lime–mud matrix. Radiocarbon dating of beachrock samples from the Arabian Gulf yielded ages from ca. 655 to 2185 year bp, whereas the Gulf of Aqaba samples range in age between 2745 and 5075 year bp. 相似文献
18.
Several publications on the environmental control of strontium in carbonate rocks have stimulated this investigation. Samples from three lithologically different units of the marine Upper Jurassic in the Northeastern Alps are analysed for strontium (x-ray fluorescence, atomic absorption, microprobe). Reported data on Sr in Recent carbonate secreting organisms, Recent and fossil carbonate sediments, on partition between calcite, aragonite and seawater have been used for comparison with those of the Upper Jurassic carbonate rocks. Reef sediments predominantly consisting of aragonite and high magnesium calcite lost extreme portions of their original Sr during diagenesis. Some layers (Oberalmer Schichten, St. Koloman) of probable bathypelagic origin are still comparable in strontium with primary marine calcite (about 0,1 % Sr).The pattern of the Sr distribution in the investigated Upper Jurassic limestones is mainly due to different courses of diagenetic change from different primary carbonates into low magnesium calcite. The change in Sr concentrations of diagenetic solutions is a function of the type of the supporting carbonate in an closed or open system of carbonates with porous solutions and is a function of seawater- or freshwater influence on the latter. A small strontium portion of these rocks occurs in apatite crystals with abnormal sulfur contents which could be identified as fragments of fish teeth.
Wir danken Herrn Dr. A. Schkbidbb, (Göttingen) für zahlreiche Mikrosondenuntersuchungen und Herrn A. FENMNGER (Graz) für die quantitative Bestimmung der Tonrückstände in den Kalken. Für einen Teil der analytischen Arbeiten wurde ein Röntgenspektrometer der Deutschen Forschungsgemeinschaft benutzt. 相似文献
Wir danken Herrn Dr. A. Schkbidbb, (Göttingen) für zahlreiche Mikrosondenuntersuchungen und Herrn A. FENMNGER (Graz) für die quantitative Bestimmung der Tonrückstände in den Kalken. Für einen Teil der analytischen Arbeiten wurde ein Röntgenspektrometer der Deutschen Forschungsgemeinschaft benutzt. 相似文献
19.
The middle Permian Chihsia Formation of south China accumulated on a shallow shelf, and consists mainly of black to dark grey micritic limestone rich in chert nodules and organic matter. A unique type of nodular crystal cluster is distributed widely in the carbonate succession. Most crystal clusters consist of calcite. Some, however, are composed of celestite, and geochemical, microscopic and crystal morphological data suggest that celestite was the precursor of the calcite. The celestite developed displacively within the sediments during early diagenesis, before compaction and before local dolomitization of the host rock. Similar strontium isotopic values were obtained from the celestite clusters, replacement calcite, vein calcite and host rock. The values are within the range of middle Permian sea water. The strontium in the celestite was furnished chiefly by either diagenetic alteration of strontium‐rich marine aragonite to strontium‐poor calcite, or aragonite dissolution induced by aerobic oxidation of organic matter, or both. The sulphur isotopic values of the celestite are about 6–11‰ heavier than the sulphur isotopic value of sulphate in coeval sea water. Based on geological context, this difference is attributed to microbial reduction of porewater sulphate in the Chihsia sediments. 相似文献