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1.
Holocene carbonate sedimentation in Lake Manitoba, Canada   总被引：1，自引：0，他引：1  
W. M. LAST 《Sedimentology》1982,29(5):691-704
The carbonate mineral suite of the modern offshore bottom sediment of the South Basin of Lake Manitoba consists mainly of high magnesian calcite and dolomite with minor amounts of low-Mg calcite and aragonite. The high-Mg calcite is derived from inorganic precipitation within the water column in response to supersaturation brought about by high levels of organic productivity in the basin. Both dolomite and pure calcite are detrital in origin, derived from erosion of the surrounding carbonate-rich glacial deposits. Aragonite, present only in trace amounts in the offshore sediments, is bioclastic in origin. The upward increase in the amount of magnesian calcite in the post-glacial sediment record is attributed to increasing photosynthetic utilization of CO2 in the lake. Stratigraphic variation in the amount of magnesium incorporated into the calcite lattice is interpreted as reflecting a variable magnesium input to the lake from ground water and surface runoff, and possibly variable calcium removal in the precipitating lake water. The effects of long-term chemical weathering at the source and size segregation explain the changes in dolomite content throughout the section.  相似文献   

2.
Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light 18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in 13C of micrites represent primary secular trends, according to published 13C variations. The 13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.  相似文献   

3.
Progressively younger cycles of Quaternary dune limestones and shallow marine calcarenites infill a coastal embayment near Warrnambool in southeastern Australia. The formations have been dated using U/Th and C14 methods or have had their ages estimated by comparison with adjacent deposits. They provide a well-dated framework for the quantification of diagenetic rates in the fresh-water vadose zone.The sequence as a whole shows progressive increase in diagenetic alteration with age, related to increasing duration of subaerial exposure. Diagenesis includes alteration of high-Mg calcite to low-Mg calcite, dissolution of aragonite and precipitation of increasing amounts of void-filling low-Mg calcite spar. These changes are reflected in the progression of bulk-rock isotope values from the youngest to the oldest strata.The rocks at Warrnambool pass through 3 main stages of diagenetic evolution. The end of stage 1 is marked by the disappearance of high-Mg calcite. This occurs in just less than 90,000 years. The end of stage 2 occurs between 400,000 and 700,000 years and is marked by the disappearance of aragonite. The end of stage 3 and the beginning of stage 4 is marked by the complete infilling of all pore space by low-Mg calcite spar. This is not generally seen in this sequence but is restricted to specific diagenetic sites such as immediately below paleosol horizons.  相似文献   

4.
The uplifted Pleistocene terraces along the coast of southern Sinai exhibit a well developed reef system formed during isotope stage 9, and a younger one formed during isotope stage 5. An intermediate reef corresponding to isotope stage 7 occurs only as an erosional relic in the study area. The sediments comprise reefal framestones, peri-reefal facies, coral rubble, and siliciclastic-dominated beach and aeolian facies. The compositional and textural complexity of the sediments leads to a highly variable spatial distribution of diagenetic features. However, the geometric relationships and elemental analyses allow a reconstruction of the general diagenetic evolution: during the major eustatic sea-level highstand of isotope stage 9, the Older Reef was constructed and cemented with aragonite and high-Mg calcite. Climate was probably semiarid with some rainy periods which permitted the installation of ephemeral freshwater lenses, especially during the minor sea-level lowstand within isotope stage 9. In these lenses, and during the subsequent major sea-level lowstand, some freshwater dissolution occurred. The highstand during isotope stage 7 led to the construction of the Intermediate Reef. In the Older Reef, some high-Mg calcite precipitated at that time. Dolomite cement formed either in marine interstitial waters modified by some freshwater input, or in a hypersaline context. Phreatic-meteoric low-Mg calcite cement covers, and partly replaces, previous marine cements and dolomite, but is still attributed to the major highstand of isotope stage 7 when freshwater lenses could develop during minor sea-level lowstands. The subsequent major sea-level lowstand was dominated by an arid climate, and only a little freshwater corrosion occurred. The Younger Reef formed during the major highstand of isotope stage 5. Aragonite and high-Mg calcite cements, as well as some dolomite, are common within the reef, whereas freshwater cements are limited to beach and aeolian facies. Due to tectonic uplift, only the lower part of the Older Reef was reflooded during isotope stage 5, and only some aragonite crystals precipitated on top of dolomite or low-Mg calcite. The interrelationships between tectonics, sea-level variations of different orders, and climatic changes thus had a profound impact on the diagenetic history of these reef systems.  相似文献   

5.
华南地区栖霞组菊花状天青石的交代及其地质意义   总被引：4，自引：0，他引：4  
本文在收集大量菊花石标本的基础上，通过岩石薄片镜下鉴定及X射线衍射分析，识别了华南地区栖霞组菊花石假象内矿物的成因类型，建立了矿物交代序列，并通过对菊花石假象内各种矿物成分的电子探针分析和对交代方解石，脉方解石及其围岩的氧，碳同位素分析，探讨了菊花石交代过程中的成岩环境,合理地解释了菊花石内正延性玉髓和栖霞组内海泡石的成因。  相似文献   

6.
The Kopet-Dagh basin of northeastern Iran was formed during the Middle Triassic orogeny. From Jurassic through Miocene time, sedimentation was relatively continuous in this basin. The Shurijeh Formation (Neocomian), which consists of red bed siliciclastic sediments that were deposited in fluvial depositional settings, crops out in the southeastern part of the Kopet-Dagh basin. In addition to clastic lithofacies, non-clastic facies in the form of calcrete paleosols, were identified in this formation. The calcrete host rocks are mainly sandstone, pebbly sandstone. The calcrete in middle unit in the Shurijeh Formation consists of, from bottom to top: incipient calcrete, nodular calcrete, massive calcrete horizons. The maturity pattern of these calcrete gradationally increases from bottom to top in this unit. Lack of organo-sedimentary structure (mainly plant roots), diversity of calcite fabric, suggest that the studied calcretes have a multi-phase development: a short vadose phase followed by a long phreatic phase. These calcretes are neither pedogenic nor groundwater calcretes. Petrographic studies show that they are composed of micritic textures with a variety of calcite fabrics, microsparitic/sparitic veins, displacive, replacive fabrics, quartz, hematite grains. Cathodoluminescence images, trace elemental analysis (Fe, Mn increased, Na, Sr decreased) of calcrete samples show the effects of meteoric waters during the calcrete formation when water tables were variable. In this study, we conclude that evaporation, degassing of carbon dioxide are the two main factors in the formation of non-pedogenic or groundwater calcrete. The sources of carbonate were probably parent materials, surface waters, ground waters, eolian dusts, numerous outcrops of limestones that have been exposed in the source area during Neocomian time.  相似文献   

7.
沙庆安 《地质科学》1977,12(2):172-178
引言研究标本由我所碳酸岩组采自海南岛南部的一个地点。在那里,沿岸覆于原生礁之上有一套由生物骨屑(主要是珊瑚)砂、砾所组成的、已被碳酸盐胶结的岩层,其厚约3米左右(图1)。此岩层一般称为“次生礁”。它是由当时原生礁盘上的生物被海浪击碎并冲带到礁盘上沉积下来的,这里属连岛砂堤沉积,它现已完全暴露地表。次生礁在它抬升过程中,下部被海蚀形成海蚀龛。  相似文献   

8.
The Mg/Ca ratio of seawater has varied significantly throughout the Phanerozoic Eon, primarily as a function of the rate of ocean crust production. Specimens of the crustose coralline alga Neogoniolithon sp. were grown in artificial seawaters encompassing the range of Mg/Ca ratios shown to have existed throughout the Phanerozoic. Significantly, the coralline algae’s skeletal Mg/Ca ratio varied in lockstep with the Mg/Ca ratio of the artificial seawater. Specimens grown in seawater treatments formulated with identical Mg/Ca ratios but differing absolute concentrations of Mg and Ca exhibited no significant differences in skeletal Mg/Ca ratios, thereby emphasizing the importance of the ambient Mg/Ca ratio, and not the absolute concentration of Mg, in determining the Mg/Ca ratio of coralline algal calcite. Specimens grown in seawater of the lowest molar Mg/Ca ratio (mMg/Ca = 1.0) actually changed their skeletal mineralogy from high-Mg (skeletal mMg/Ca > 0.04) to low-Mg calcite (skeletal mMg/Ca < 0.04), suggesting that ancient calcitic red algae, which exhibit morphologies and modes of calcification comparable to Neogoniolithon sp., would have produced low-Mg calcite from the middle Cambrian to middle Mississippian and during the middle to Late Cretaceous, when oceanic mMg/Ca approached unity. By influencing the original Mg content of carbonate facies in which these algae have been ubiquitous, this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the Phanerozoic. The crustose coralline algae’s precipitation of high-Mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of CaCO3. However, the algae’s nearly abiotic pattern of Mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal Mg incorporation. Therefore, the Mg/Ca ratio of well-preserved fossils of crustose coralline algae, when corrected for the effect of seawater temperature, may be an archive of oceanic Mg/Ca throughout the Phanerozoic. Magnesium fractionation algorithms that model algal skeletal Mg/Ca as a function of seawater Mg/Ca and temperature are presented herein. The results of this study support the empirical fossil evidence that secular variation of oceanic Mg/Ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time.  相似文献   

9.
At Tylicz, near Krynica Spa (Polish Carpathians), spelean deposits fill fissures and caverns in Eocene flysch rocks. They occur as: (1) clastic cave sediments transformed into hard crusts due to cementation by finely crystalline low-Mg calcite, (2) drusy calcite that covers crust surfaces and fills voids in the crust and (3) colloform calcite. Two varieties of drusy calcite are distinguished: acicular and columnar. The acicular calcite is built up of crystallites forming spherulitic fans or cones. In places it is syntaxially covered with colloform calcite. The drusy calcite is low-Mg ferroan calcite with non-ferroan subzones, whereas the colloform calcite is a low-Mg non-ferroan variety. The columnar calcite crystals form fan-like bundles. Cross-sections cut perpendicular to the c-axes of columnar crystals are equilateral triangular in shape, although some have slightly curved edges. The columnar crystals have steep rhombic terminations and most have curved triangular faces, i.e. gothic-arch calcite. Saddle crystals have also been observed. The columnar crystals are composed of radially orientated crystallites whose long dimension is parallel to the c-axis. The curved crystal faces of such polycrystals are interpreted as a result of differential growth rates of the crystallites. The spelean calcites precipitated from CO2-saturated water. The high rate of CaCO3 precipitation is thought to be responsible for the formation of radial structures. Finely crystalline calcite formed within pore spaces of clastic sediments close to the water-air interface, drusy calcite crystallized beneath the water-air interface, and colloform calcite precipitated from thin films of water.  相似文献   

10.
Carbonate nodules and slabs in late Holocene shelly terrigenous deposits of the modern Fraser River delta (～49°N) are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high-Mg (～ 10–20 mol% MgCO3) calcite cement as coalescing isopachous crusts on grains. Previous reports that the cement is low-Mg calcite are not supported by this study. Highly negative δ13C values of ? 7 to ? 59‰ for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial. In particular, the production of biogenic methane (CH4) by anaerobic bacterial fermentation, its upward migration, chemical or biological oxidation to CO2 and neutralization in the near-surface sediment, and diffusion to microenvironments relatively enriched in organic components, are a possible set of conditions influencing the process and sites of carbonate cementation. Methane-derived Mg-calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow-water terrigenous sands and muds at non-tropical latitudes.  相似文献   

11.
我国海南岛及南海诸岛沿岸,广泛发育海滩岩。1980年我队赴海南岛考察现代沉积时,对海滩岩的分布、岩性特征及其与周围环境的关系进行了观察和采样。样品采自崖县鹿回头三亚湾水尾岭海蚀崖、西洲岛、小东海、东瑁岛、西瑁岛、天涯海角,乐东县莺歌海,文昌县渔业等地(图1)。有关的地质、地貌及岩性特征等,已有许多描述,对海滩岩的岩石学及成岩作用也有许多研究。本文侧重探讨海滩岩的胶结作用及与成岩环境的关系。  相似文献   

12.
The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ13C and δ18O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO2 at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ13C and δ18O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ18O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ18O and highly variable δ13C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.  相似文献   

13.
Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica   总被引：3，自引：0，他引：3  
Wholesale removal of the unstable carbonate phases aragonite and Mg-calcite, and precipitation of calcite and dolomite is currently taking place where phreatic waters (the modern water table) invade 120,000-year-old Pleistocene biolithites (Falmouth Formation), North Jamaica. Pleistocene rocks presently in the vadose zone are relatively unaltered, and consist of mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a ‘water table cave’ is commonly encountered. Below the water table the rocks are invariably more highly altered than those above. Mg-calcites are very rare, and considerable dissolution of aragonite has commonly occurred. Dolomite occurs as 8–25 μm, subhedral to euhedral crystals replacing micrite, or precipitated as void linings. The isotopic composition of the dolomite (δO18=-1·0 %0, δC13=-8·4 %0), and its high strontium content (3000 p.p.m.) suggest precipitation as CO2-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve Mg-calcites and Sr-rich aragonites, and de-gas. Because some dolomitized rocks are enriched in magnesium relative to unaltered biolithites, addition of magnesium to the system is necessitated, and is probably derived from sea water in the mixing zone. Phreatic meteoric diagenesis is thus demonstrated to be a rapid process, and to be capable of dolomitization.  相似文献   

14.
In laboratory experiments, the precipitation of dolomite at ambient temperature is virtually impossible due to strong solvation shells of magnesium ions in aqueous media and probably also due to the existence of a more intrinsic crystallization barrier that prevents the formation of long-range ordered crystallographic structures at ambient surface conditions. Conversely, dolomite can easily form at high temperature (>100 °C), but its precipitation and growth requires several days or weeks depending on experimental conditions. In the present study, experiments were performed to assess how a single heat-ageing step promotes the formation of dolomite under high-carbonate alkaline conditions via dissolution-precipitation reactions. This reaction pathway is relevant for the so-called hydrothermal dolomite frequently observed in carbonate platforms, but still ill-defined and understood. Our precipitation route is summarized by two main sequential reactions: (1) precipitation of Mg-calcite at low temperature (∼20 °C) by aqueous carbonation of synthetic portlandite (Ca(OH)2) in a highly alkaline medium (1 M of NaOH and 1 M of MgCl2), leading to precipitation of oriented nanoparticles of low- and high-Mg calcite (∼79 wt%) coexisting with aragonite (∼18 wt%) and brucite (∼3 wt%) after 24 h; (2) fast dolomitization process starting from 1 h of reaction by a single heat-ageing step from ∼20 to 200, 250 and 300 °C. Here, the Mg-calcite acts as a precursor that lowers the overall kinetics barrier for dolomite formation. Moreover, it is an important component in some bio-minerals (e.g. corals and seashells). Quantitative Rietveld refinements of XRD patterns, FESEM observations and FTIR measurements on the sequentially collected samples suggest fast dolomite precipitation coupled with dissolution of transient mineral phases such as low-Mg calcite (Mg < 4 mol%), high-Mg calcite (Mg > 4 mol%), proto-dolomite (or disordered dolomite; Mg > 40 mol%) and Ca-magnesite. In this case, the dolomite formation rate and the time-dependent mineral composition strongly depend on reaction temperature. For example, high-purity dolomitic material (87 wt% of dolomite mixed with 13 wt% of magnesite) was obtained at 300 °C after 48 h of reaction. Conversely, a lower proportion of dolomite (37 wt%), mixed with proto-dolomite (43 wt%), Ca-magnesite (16 wt%) and high-Mg calcite (4 wt%), was obtained at 200 °C after 72 h. The present experiments provide an additional mechanism for the massive dolomite formation in sedimentary environments (ex. deep sea organic-rich carbonate-sediments) if such sediments are subjected to significant temperature variations, for example by hot fluid circulations related to volcanic activity. In such systems, organic degradation increases the carbonate alkalinity (HCO3−) necessary to induce the dolomitization process at low and high temperature.  相似文献   

15.
Geochemical controls on a calcite precipitating spring   总被引：2，自引：0，他引：2  
A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km2 basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca2+/Mg2+ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO2 degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO2 degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr2+ between the water and calcite.  相似文献   

16.
The early Pliocene Shirahama Limestone is a grainstone-packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore-filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (?2 to ? 3.5%0 PDB) and a light oxygen group (?5.5 to ? 7.5%0 PDB). Calcite associated with heavy oxygen dolomite has δ18O of ? 6.5 to ?8.5%0 PDB, whereas those associated with light oxygen dolomite have a wide range from ?7.5 to ?14%0 PDB. Calcite in dolomite-free limestone has an oxygen isotopic composition of ?2 to ?8.5%0 PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric-marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C. Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric-marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric-hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug some of the remaining pores (Stage IV).  相似文献   

17.
The Pennsylvanian phylloid algal mounds exposed in the Cervatina Limestone of the Cantabrian Zone (NW Spain) developed during the highstands of high-frequency shallowing-upward cycles and lack evidence of subaerial exposure at their tops. Mound core facies are composed of massive bafflestones with variable amounts of calcite cements and anchicodiacean phylloid algae with cyathiform thalli preserved in growth position. Through standard petrographic, isotopic (δ18O and δ13C), major and trace element (Ca, Mg, Fe, Mn, Sr) and cathodoluminescence analyses, five calcite cement phases (cement 1 (C1)–cement 5 (C5)) have been identified filling primary and secondary pores. Early marine diagenesis is represented by micritization and non-luminescent to mottled-dull luminescent high-Mg calcite fibrous marine cement (C1). A dissolution phase then occurred and created vuggy and moldic pores. Based on the absence of field or petrographical or geochemical evidence of exposure, it is inferred that dissolution occurred in near-surface undersaturated marine waters with respect to aragonite related to progressive organic matter oxidation. Secondary porosity was subsequently filled by dull-bright-dull bladed high-Mg calcite (C2), which precipitated in the early shallow burial from marine-derived pore waters. Remaining porosity was occluded by shallow-burial precipitates consisting of non-luminescent scalenohedral low-Mg calcite (C3) followed by non-ferroan dull luminescent calcite spar (C4). Latter phases of calcite spar exhibiting non- and dull luminescence (C5) are associated with burial calcite veins. Low δ18O values (around ?8‰), moderately depleted δ13C values (around 0.5‰) and the homogeneity of trace element contents of carbonate matrix, cements and vein-filling calcites suggest burial isotopic re-equilibration and recrystallization, probably in Early Permian times during post-thrusting orocline formation.  相似文献   

18.
Reservoir fluid compositions have been assessed from analytical data on water samples collected from thermal and cold waters in Balçova geothermal field. The results of mineral equilibrium modelling indicate that the waters, with some exceptions, are systematically supersaturated with respect to calcite, aragonite, dolomite, chalcedony and quartz, but undersaturated with respect to amorphous silica, celestite, anhydrite and gypsum and undersaturated or supersaturated with respect to barite, low-albite, K-feldspar, gibbsite and Fe(OH)3(a). Calculation of mineral saturation states and geochemical analyses of scale and field observations show that carbonate minerals (calcite, aragonite and dolomite) are most likely to be precipitated as a scale type. Besides carbonates, scale formation risk of amorphous silica, Fe(OH)3(a), anhydrite, barite and celestite minerals should be taken into account in some wells and surface equipment. Most of the waters, with some exceptions, have carbonate scaling risk at all temperatures, whereas the other scaling risks only exist over a limited temperature range. While silica, Fe(OH)3(a) and barite show a scaling tendency at low temperatures, anhydrite and celestite scaling occurs at higher temperatures.  相似文献   

19.
The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red-brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non-luminescent to dull luminescent, whereas later spar cements exhibit bright yellow-orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow-orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn2+ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe2+ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 13C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO2. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO2; the use of nodule compositions may result in overestimation of PCO2 by as much as 30%.  相似文献   

20.
Ana M. 《Earth》2003,60(3-4):261-298
Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates.

The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates. Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. More indications of climatic conditions may be obtained through the analysis of the δ18O content of both calcretes and palustrine carbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also important in the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the δ13C signal of the primary carbonates, which, in pedogenic carbonates, has also been used to estimate atmospheric pCO2 during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or accommodation rates, and therefore as indicators of different tectonic regimes.

Even though the study of these carbonates has notably increased in recent years, much less is known about them than about marine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins, with a need to include the carbonates analysed in this paper.  相似文献   


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