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1.
Marine diatomaceous siliceous sediments in Neogene sections of northern Japan contrast with the Monterey Shale of California in containing many intercalations of acidic volcaniclastic sediments. Diagenesis of these sediments from deep boreholes and surface sections was investigated. Three diagenetic zones—biogenic opal, opal-CT and quartz zones—are recognized in siliceous sediments, corresponding roughly to amorphous silica, low cristobalite and quartz zones in acidic vitric volcaniclastic sediments. Opal-CT consists almost exclusively of silica and water, while low cristobalite contains appreciable amounts of A1, Ca, Na and K. In subsurface sections, values of d(101) spacing of opal-CT decrease progressively with increasing burial depth. The progressive ordering is not associated with additional silica cementation. In surface sections, the behaviour of d(101) spacing is complicated owing to the modification of the progressive ordering developed during burial diagenesis by later silica cementation during uplift. The cementing opal-CT is probably precipitated from percolating groundwater which dissolves siliceous skeletons in porous diatomaceous mudstones overlying the opal-CT porcellanite. Opaline cherts that form during burial diagenesis are designated as early opaline chert, while those which form during uplift are later opaline chert. The later opaline chert contains two groups of opal-CT; one is progressively ordered opal-CT and the other is additionally cemented opal-CT with higher d(101) spacing than that in the host porcellanite. In diatomaceous siliceous sediments, early opaline chert is scarce. Most, if not all, opaline cherts in surface sections are of later origin.  相似文献   

2.
X-ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal-A), (2) cristobalitic silica (opal-CT), and (3) microcrystalline quartz. Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal-CT zone, the d(101)-spacing of opal-CT decreases in a smooth non-linear fashion from about 4 10 Å to 4-04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal-CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal-CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients. Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal-CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal-CT and quartz, extrapolated experimental quartz-water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal-CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method. Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.  相似文献   

3.
The diagenesis in the organic-rich Cretaceous to Eocene Al Hisa Phosphorite Formation (AHP), Muwaqqar Chalk Marl Formation (MCM) and Umm Rijam Chert-Limestone Formation (URC) formations of Jordan can be linked directly to the fluctuating sedimentary environment of this shelf depositional system in the Middle to Late Eocene, and its influence on the composition of the deposited sediment and the early burial diagenetic environment. Most cementation was early, mostly within the first 10 m of burial, perhaps entirely within the first 100 m of burial. We propose that the siliceous cements are derived from biogenic silica, probably of diatoms, deposited in a shelf of enhanced productivity. Volumetrically, the most important processes were the redistribution of biogenic opal-A (diatoms) and calcite to form pervasive, layered and nodular cements. The formation of the silica and carbonate cements is closely linked through the effects their dissolution and precipitation have on pore fluid chemistry and pH. The chert beds have a biogenic silica origin, formed through replacement of diatoms and radiolaria by opal-CT, and subsequently by quartz. Calcite cement has carbonate derived from microbial diagenesis of organic matter and calcium derived from seawater. The Mg for early dolomite may have been generated by replacement of opal-CT by quartz, ore dissolution of unstable high Mg calcite bioclasts. The silica and carbonate diagenetic processes are both linked to microbial diagenesis of organic matter, and are intimately linked in both time and space, with pH possibly influencing whether a silica or a carbonate mineral precipitates. The paucity of metal cations capable of precipitating as sulphides is crucial to the creation of acidic pore water favourable to silica precipitation, either as opal-CT, chalcedony or quartz. The lack of clay minerals as a sink for the Mg required for opal-CT polymerisation is the principal factor responsible for the remarkably early silica cementation. All the diagenetic processes, with the probable exception of the opal-CT to quartz transition are early, almost certainly within the first 10 m of burial, possibly much less. A paragenetic sequence is presented here based on these two cores that should be tested against a wider core distribution to see whether this diagenetic history can be generalised throughout the basin. Warm bottom water temperatures probably led to silica diagenesis at much shallower burial depths than occurs in many other sedimentary basins. Silicified layers, in turn, commonly host fractures, suggesting that mechanical properties of the strata began to differentiate at a very early stage in the burial cycle. This has wide implications for processes linking diagenesis to deformation.  相似文献   

4.
Opaline silica (opal-A) has formed in marine, lacustrine and geothermal environments throughout geological time. During diagenesis opal-A normally changes to opal-CT, then opal-C, and finally to quartz. Such changes commonly destroy the original fabrics and any fossils that opal-A contained. The physical changes that accompany the opal-A to opal-CT transition, however, are known poorly. X-ray diffraction analyses, electron microprobe analyses and high-resolution, high-magnification scanning electron microscope imagery of siliceous sinters from the Geysir geothermal area in Iceland show that opal-A is formed of heterometric arrays of randomly packed microspheres (up to 5  μ m diameter) with neighbouring spheres commonly being joined by small connection pads. In contrast, enlarged spheres, lepispheres, inverse opal (two types) and spindle frameworks with hexagonal motifs characterize opal-CT. The textures in opal-CT, which vary on a microscale, reflect the complex interplay between dissolution (e.g. inverse opal) and precipitation (e.g. enlarged spheres, spindle frameworks) that probably was mediated by groundwater in a near-surface environment. The processes deciphered from these young rocks should, however, be applicable to sedimentary opal-A and opal-CT of all ages, irrespective of their origin.  相似文献   

5.
A Tertiary non-marine stratigraphic sequence composed of carbonates(limestone),siliceous carbonates,coaly layers overlain by pyroclastic rocks and lavas,outcrops in the Gavatha area of northwestern Lesvos Island.Pure earbonates eonsist almost completely of calcite,the siliceous carbonate sediments of quartz,opal-CT and calcite,the shales of quartz,opal CT, K-feldspar,smecite-illite and ealcite,and the coaly layers of organic matter,quartz,opal-CT,feldspars and pyrite,Geochemical data indicate that smectite-illite,feldspars and associated elements(La,Zr,Y,Ba,Ce)are the products of alteration of volcanic rocks in a subtropical area A combination of sources in suggested for the formation of silica polymorphs:(a) biogenic or non-biogenic silica(opal-A) that was originally present in the form of diatiom frustules of in the form of inorganically prccipitated silica;(b)transformation o opall-A to opal-CT and quartz opal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics;and(c)opal-CT deposited primarily from hydrothermal solutions.  相似文献   

6.
Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.  相似文献   

7.
Evidence from deep-sea sediments supports the following diagenetic maturation sequence: opal-A (siliceous ooze) → opal-CT (porcelanite) → chalcedony or cryptocrystalline quartz (chert). A solution-redeposition mechanism is involved in the opal-A to opal-CT transformation. Exceptions to the overall maturation sequence are numerous, suggesting that temperature and time are not the only important factors controlling these mineralogical transformations. The rates of the above transformations are strongly affected by the composition of the solution and of the host sediments ; in Mesozoic clayey sediments, opal-CT predominates, while in carbonate sediments quartz is most common.Experiments at 25 and 150°C over a period of one day to six months show that the transformation rate of opal-A to opal-CT is much higher in carbonate than in clay-rich sediments, and that opal-CT lepisphere formation is aided by the precipitation of nuclei with magnesium hydroxide as an important component. The role of carbonate is explained as follows : in carbonate-rich sediments, the dissolution of carbonate provides the necessary alkalinity, and sea water provides the magnesium for the magnesium hydroxide in the nuclei. In contrast, in clay-rich sediments the clay minerals compete with opal-CT formation for the available alkalinity from sea water. As a result, the clays are enriched in Mg, and the rate of opal-CT formation is strongly reduced. This mechanism also bears on the common observation of carbonate replacement by silica.  相似文献   

8.
Diatomite associated with the Kolubara Coal Basin was studied to better understand early stage silica diagenesis of shallow water deposits. The Kolubara Basin consists of Neogene siliciclastic rocks, diatomite, marlstone and rare carbonates. Palaeozoic metamorphic and Mesozoic sedimentary and igneous basement rocks are transgressively overlain by Upper Miocene sandstone, siltstone, shale and mudstone. This Upper Miocene section is transgressively overlain by the Pontian section, which contains diatomite and coal beds. White and grey diatomite forms beds 0.7-2.2 m thick that are continuous over an area of about 2 km2. Siliceous rocks vary in composition from diatomite (81-89 per cent SiO2) to diatom-bearing shale (58-60 per cent SiO2). Siliceous deposits are laminated in places, with the laminae defined by variations in clay minerals, organic matter and diatoms. Diatomite shows only incipient diagenesis characterized by the fragmentation of diatom frustules, the minor to moderate corrosion of frustules and the formation of minor amounts of opal-A' (X-ray amorphous inorganic opal) cement. The low degree of diagenesis results from the young age of the deposits, low burial temperatures and possibly also from the presence of abundant organic matter and the dissolution of kaolinite. The presence of only weak diagenesis is also reflected by the characteristically poor consolidation of the rocks and low rank of the associated coal.  相似文献   

9.
苏皖坡缕石粘土中蛋白石特征及其成因意义   总被引：1，自引：0，他引：1  
TEM和XRD研究表明，在苏皖坡缕石粘土矿床的蛋白石坡缕石层中存在0pal-A和0pal-CT两种结构的蛋白石。蛋白石坡缕石层主要含坡缕石、opal-A、opal-CT，以及少量白云石和其它矿物，由富opal-A层和富opal-CT层互层组成，交互层的厚度在几个厘米左右。蛋白石坡缕石层中的矿物基本是自生矿物，从蒸发湖水中化学沉淀形成。矿物组成特征研究表明，蛋白石坡缕石层的矿物组分(Si、Mg、Al)来源于盆地周围玄武岩淋滤的浅层地下水。根据Opal-A和Opal-CT溶解度图解和城缕石、白云石形成物理化学条件图解，当湖水具有高浓度溶解SiO2和Mg^2 时，有利于opal-A和坡缕石形成，当湖水具有低浓度溶解SiO2时，有利于opal-CT结晶。因此，沉淀SiO2的结构状态取决于地下水补给的湖水溶解SiO2浓度。富opal-A和富opal-CT交互层的形成是古气候、古水文周期性变化的指示。富opal-CT层指示高地下水补给流入量，低蒸发量，湖水低盐度和溶解组分，代表湿润气候时期；而富opal-A层代表低地下水补给流入量，高蒸发量，高溶解组分浓度，代表干旱气候时期。  相似文献   

10.
The Coniacian Arnager Limestone Formation is exposed on the Danish island of Bornholm in the Baltic Sea. It is composed of mound-bedded siliceous chalk, and X-ray diffraction and scanning electron microscopy indicate a content of 30–70% insoluble minerals, including authigenic opal-CT, quartz, clinoptilolite, feldspars, calcite, dolomite, and barite. Opal-CT and clinoptilolite are the most common and constitute 16–53% and 2–9%, respectively. The content of insoluble minerals varies laterally both within the mounds and in planar beds, and the opal-CT content varies by up to 10% vertically. The mounds consist of two microfacies, spiculitic wackestone and bioturbated spiculitic wackestone, containing 10–22% and 7–12% moulds after spicules, respectively.Subsequent to deposition and shallow burial, dissolution of siliceous sponge spicules increased the silica activity of the pore water and initiated precipitation of opal-CT. The opal-CT formed at temperatures around 17 °C, the precipitation lowered the silica activity and the Si/Al ratio of the pore water, resulting in precipitation of clinoptilolite, feldspar and smectite. Calcite formed synchronously with the latest clinoptilolite. Minor amounts of quartz precipitated in pore water with low silica activity during maximum burial, probably to depths of 200–250 m. The dissolution of sponge spicules and decomposition of the sponge tissue also resulted in the release of Ba2+, Sr2+, Mg2+, Ca2+ and CO32?, facilitating precipitation of barite and dolomite. Precipitation of especially opal-CT reduced the porosity to an average of 40% and cemented the limestone. The study highlights the diagenetic pathways of bio-siliceous chalk and the effects on preservation of porosity and permeability.  相似文献   

11.
During diagenetic stages, the aragonitic skeletons and the inter/intra-corallite cement of the upper Jurassic corals of Hanifa Formation either dissolved or subjected to diagenetic alterations including cementation, micritization, recrystallization, silicification, dolomitization and dedolomitization. The proposed sequence of diagenetic stages is as follows: early marine diagenesis, early meteoric and mixing zone diagenesis, late meteoric diagenesis, and shallow burial diagenesis. Each stage is characterized by certain diagenetic processes. The source of sulfate solutions for dedolomitization in the studied corals is the dissolved anhydrite deposits of the Arab–Hith Formations, sometime before their erosion. A possible source of silica, needed for the formation of chert and chalcedony, is the sponge spicules dispersed in many carbonates of the Hanifa Formation.  相似文献   

12.
Pleistocene red soil horizons were exposed in different areas of the Barind Tract in north-west Bangladesh. X-ray diffractions of twenty seven samples from different depths of these soil horizons revealed that the soil horizons consisted of kaolinite, illite and chrysotile with significant amount of opal-CT. Samples from Maddhapara, Bogra, and Nachole contain kaolinite, illite, quartz and opal-CT, and the samples from Kantabari contain chrysotile instead of kaolinite. Clay mineral compositions of different soil horizons indicated two different types of clay assemblages, viz. (a) illitekaolinite and (b) illite-chrysotile. In the village of Kantabari, illite-chrysotile clay mineral assemblage indicate that soil horizons were formed under low temperatures with alkaline and reducing conditions. However, other soil horizons of illite-kaolinite clay mineral assemblage indicate that soils were possibly formed under humid, temperate and welldrained conditions. These two soil horizons were formed under different geochemical, geomorphological and climatic conditions from different parent materials. Scanning Electron Microscopy photographs showing the presence of glass shards and no opal-A were found using XRD, suggesting that the opal-A might not be a precursor to opal-CT in the red soil horizon of the study area. This opal-CT along with the general lack of fossils and presence of glass shards was indicative of a volcanogenic rather than biogenic origin for the Opal-CT in the study area, and X-ray fluorescence data reveals higher percentages of silica which is comparable to the Toba Ash of Toba Caldera, Indonesia of about 75,000 B.P.  相似文献   

13.
Siliceous hot spring deposits from Steamboat Springs, Nevada, U.S.A., record a complex interplay of multiple, changing, primary environmental conditions, fluid overprinting and diagenesis. Consequently these deposits reflect dynamic geologic and geothermal processes. Two surface sinters were examined—the high terrace, and the distal apron-slope, as well as 13.11 m (43 ft) of core material from drill hole SNLG 87-29. The high terrace sinter consists of vitreous and massive-mottled silica horizons, while the distal deposit and core comprise dominantly porous, indurated fragmental sinters. Collectively, the three sinter deposits archive a complete sequence of silica phase diagenetic minerals from opal-A to quartz. X-ray powder diffraction analyses and infrared spectroscopy of the sinters indicate that the distal apron-slope consists of opal-A and opal-A/CT mineralogy; the core yielded opal-A/CT and opal-CT with minor opal-A; and the high terrace constitutes opal-C, moganite, and quartz. Mineralogical maturation of the deposit produced alternating nano–micro–nano-sized silica particle changes. Based on filament diameters of microbial fossils preserved within the sinter, discharging thermal outflows fluctuated between low-temperatures (< 35 °C, coarse filaments) and mid-temperatures ( 35–60 °C, fine filaments). Despite transformation to quartz, primary coarse and fine filaments were preserved in the high terrace sinter. AMS 14C dating of pollen from three horizons within core SNLG 87-29, from depths of 8.13 to 8.21 m (26′8″ to 26′11″), 10.13 to 10.21 m (33′3″ to 33′6″), and 14.81 to 14.88 m (48′7″ to 48′10″), yielded dates of 8684 ± 64 years, 11,493 ± 70 years and 6283 ±60 years, respectively. In the upper section of the core, the stratigraphically out-of-sequence age likely reflects physical mixing of younger sinter with quartzose sinter fragments derived from the high terrace. Within single horizons, mineralogical and morphological components of the sinter matrix were spatially patchy. Overall, the deposit was modified by sub-surface flow of alkali-chloride thermal fluids depositing a second generation of silica, and periodically, by acidic steam condensate formed during periods when the water table was low. Local faulting produced considerable fracturing of the sinter. Hence, the Steamboat Springs sinter experienced a complex history of primary and secondary hydrothermal, geologic and diagenetic events, and their inter-relationships and effects are locked within the physical, chemical and biological signatures of the deposit.  相似文献   

14.
The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate grains. Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filled with widespread, very coarse, blocky calcite spar. These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement and pore-filling calcites have δ13C values ranging from 0·5 to -25·3%, and δ18O values of -16·1 to -24·30 (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements. The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement. The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.  相似文献   

15.
Nodular chert from the middle and upper Arbuckle Group (Early Ordovician) in the Slick Hills, SW Oklahoma, was formed by selective replacement of grainstones, burrow fillings, algal structures, and evaporite nodules. Chert nodules are dominantly microquartz with minor fibrous quartz (both quartzine and chalcedony), megaquartz, and microflamboyant quartz. Lepisphere textures of an opal-CT precursor are preserved in many (especially in finely-crystalline) chert nodules. The δ18O values of microquartz chert range from +23.4 to + 28.80/00 (SMOW), significantly lower than those of Cenozoic and Mesozoic microquartz chert formed both in the deep sea and from near-surface sea water. The δ18O values of chert decrease with increasing quartz crystal size. Silicification in the Arbuckle Group occurred during early diagenesis, with the timing constrained by the relative temporal relationships among silicification, burial compaction, and early dolomite stabilization. Silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica-enriched sea water. Chert nucleation appears to have been controlled by the porosity, permeability, and organic matter content of precursor sediments. This conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter. Growth of chert probably occurred by a maturation process from opal-A(?), to opal-CT, to quartz, as indicated by the presence of opal-CT precursor textures in many chert nodules. Although field and petrographic evidence argues for an early marine origin for chert in the Arbuckle Group, the light δ18O values are inconsistent with this origin. Meteoric resetting of the δ18O values of the chert during exposure of the carbonate platform best explains the light δ18O values because: (i) the δ18O values of chert nodules decrease with decreasing δ18O values of host limestones, and (ii) chert nodules from early dolomite, which underwent more extensive meteoric modification than associated limestones, have lighter δ18O values than chert nodules from limestones. Increasing recrystallization of chert nodules by meteoric water resulted in progressive 18O depletion and (quartz) crystal enlargement.  相似文献   

16.
Examination of hydrothermally transformed silica from controlled experiments reveals that amorphous silica changes to quartz through an intermediate phase of opal-CT and that the d(101) spacing of cristobalite progressively decreases from 4.10 Å to 4.05 Å. The rate of spacing decrease is definitely dependent on the reaction temperature, being faster at higher temperatures. This spacing change represents ordering of opal-CT crystals with the passage of time.The relationship between thermal history and degree of ordering suggests that stratigraphic boundaries are usually parallel to isopleths of d (101) spacings, but do not always coincide with them. The isopleths should be more or less discordant to the stratigraphic boundaries where the strata have been folded. This discordancy can be ascribed to the difference of ordering, chiefly controlled by the thermal history during the burial and folding process.  相似文献   

17.
Isotopic and mineralogic data from an 8500-m thick section of the Great Valley sequence, northern California, indicate that changes in the δ18O values of authigenic minerals resulted from the conversion of smectite to a 10 Å clay-mineral as temperature increased with burial in the Jurassic- Cretaceous outer-arc basin. The clay-mineral assemblage in mudstone is characterized by a proportional increase of the 10 Å clay-mineral with increasing stratigraphic depth, and by a depletion in the δ18O value of the mixed-layer smectite/10 Å clay-mineral with descending stratigraphic position from +21.9 to + 15.5%. SMOW. Modeling of the oxygen isotopic data from authigenic phases, based on equilibrium fractionation during clay-mineral diagenesis, indicates that δ18O values of calcite in mudstones and of calcite cements in sandstone precipitated along a temperature gradient of about 25°C/km during maximum burial to about 6–7 km. δD values of the mixed-layer smectite/10 Å clay-mineral range between ?69 to ?44%. SMOW. Using temperatures calculated from the oxygen isotopic data, the deuterium and oxygen isotopic data indicate that the smectite underwent late-stage dehydration and probably buffered the composition of formation waters from sea water values to isotopic compositions of δ18O ≈ +8%. SMOW and δD ≈ ?25%. SMOW. The δ13C values of calcite from mudstone and sandstone imply that crystallization of authigenic calcite was linked to organic diagenesis during which dissolved HCOt-3 was continuously enriched in 13C as temperature increased with burial. At the base of the sequence and immediately overlying the ophiolitic basement rocks, several hundred meters of strata were altered by more oxygen-depleted (δ18O ? +4 to +5%.) hydrothermal fluids emanating from the ophiolitic rocks, probably at maximum burial depth.  相似文献   

18.
《Sedimentology》2018,65(3):745-774
This paper explores little investigated diagenesis of spicule‐dominated sediments, based on Permian spiculites and cool‐water carbonates of the Tempelfjorden Group in central Spitsbergen. Field observations, petrography, stable isotope geochemistry, and mineralogical and chemical analyses reveal that the strata have been subjected to multistage diagenesis as the result of silica phase transitions at medium burial depths and deep‐burial overprinting. The growth of silica concretions occurred during the opal‐A/opal‐CT conversion and was controlled by the content and distribution of clay and spicules in the sediment, resulting in a variety of megascopic silica fabrics. Opal‐CT was subsequently dissolved, and all silica is now in a stable quartz stage. Petrographically, the rocks are characterized by a variety of chalcedony and quartz cements which perfectly preserve precursor textures. Most cements precipitated from silica‐oversaturated fluids, and their shapes reflect the silica saturation state and geometry of the pore space. Some microquartz and cryptoquartz also formed by a solid–solid inversion (recrystallization) of chalcedony. The cements have δ 18O values between +30‰ and +20‰ Standard Mean Ocean Water and display a systematic depletion in 18O from the first to the last crystallized, interpreted to reflect a gradual increase in temperature during burial. The precipitation of quartz cements started in the Middle Triassic when the strata passed the 19°C isotherm at burial depths of ca 600 m, and was completed in the mid‐Cretaceous, 2·3 km beneath the sea floor at temperatures of 75°C. Late diagenetic overprinting of the chert includes fracturing, brecciation and cementation with carbonate cements having δ 18O values between +2‰ and −30‰ Pee Dee Belemnite and δ 13C values between +4‰ and −14‰ Pee Dee Belemnite; they are linked to hot solutions introduced during Cretaceous volcanism or Palaeogene tectonism. This study illustrates the diagenetic pathway during burial of spicule‐rich sediments in a closed system and thereby provides a baseline for studies of more complexly altered chert deposits.  相似文献   

19.
Inorganic minerals in mudstone are composed of clay minerals,carbonate and detrital minerals.Detrital minerals(such as quartz and feldspar)are mainly original deposit.However,clay minerals(kaolinite,illite,and chlorite)and carbonate(calcite and dolomite)are mostly diagenetic minerals.Furthermore,conversion of the four kinds of clay minerals are common.The formation of clay minerals and carbonate is controlled by temperature,pressure,p H,Eh and type of cations during diagenesis.Therefore mineral assemblage can indicate the characteristics and change of diagenetic environment.In addition to inorganic minerals,there are also organic matter of different sources and chemical properties in mudstone.Traditionally,it is considered that evolution of organic matter is controlled by thermal effect.Now studies show that inorganic and organic matter can interact with each other and form clay-organic complexes.This suggest that attention should be paid to the influence of diagenetic mineral assemblage and diagenetic environment on the evolution of organic matter* Samples of mudstone from 1500-4500m of the Palaeogene in the Dongying Depression,China,were collected to investigate the changes of mudstone diagenetic environment.XRD,thin section and SEM were used to detect diagenetic minerals and assemblage characteristics.Results showed that content of detrital minerals,which are floating in mud matrix or preserved as silt laminae,is basically unchanged from shallow to deep strata.Clay minerals which are gathered as argillaceous matrix or preserved as argillaceous laminae have growth and decline relation to carbonate which mainly appear as micropoikilitic ferriferous calcite and ferriferous dolomite.All these characteristics indicate that detrital minerals are exogenetic,whereas carbonate is diagenetic minerals.Based on the SEM analysis of the clay minerals,it was found that smectite present honeycomb and reticulate structure,while illite present filiform and schistose structure and there are growth and decline relationship between them.Nevertheless,hexagonal tabular and stratified kaolinite has the highest content from 2400m to3300m.Rosette and stratified chlorite shows increase trend when the burial depth is deeper than 3300m.These characteristics indicated that clay minerals are diagenetic minerals and there are conversions among the four types.Therefore form shallow to deep,three diagenetic mineral assemblage zones can be divided based on the characteristics of carbonate and clay minerals in mudstone.Namely,smectite+illite/smectite zone in the depth of 2000-2500m;kaolinite+illite/smectite zone in the depth of 2500-3300m and illite+chlorite+carbonate zone below 3300m.Previous studies showed that kaolinite is stable under acidic conditions,while other clay minerals and carbonate are stable under alkaline conditions.Hence according to mineral assemblages feature,it was inferred that diagenetic environment of mudstonehasundergonethechangeof alkaline-acid-alkaline.For the organic matter with different chemical properties in mudstone,the hydrocarbon generation will be different in the acidic and alkaline diagenetic environment even if the conditions of temperature and pressure are the same.Therefore,for hydrocarbon generation we should not only focus on thermal effect,but also pay more attention to the differences of diagenetic environment which have great significance for the understanding of hydrocarbon generation,hydrocarbon expulsion and reservoir formation in mudstone.  相似文献   

20.
Cementation of bryozoan-echinoid-benthic foraminiferal temperate shelf carbonates of the Oligocene Te Kuiti Group, North Island, New Zealand, occurred mainly during subsurface burial. The calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan (given an iron supply) and, compared to the skeletal material with normal marine δ18O values from +2 to −1‰, more depleted in 18O with depth of burial, the δ18O composition of bulk cement samples ranging from −1 to −7‰. These trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures (20–50°C), respectively. The δ13C values (0 to +3‰) of the cements remain the same as the host marine shells, suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution.

Two gradational burial diagenetic environments influenced by marine-derived porewaters are arbitrarily distinguished: shallow burial phase and moderate burial phase. During the shallow burial phase, down to 500–600 m sub-bottom depth, the carbonates lost at least 25% of their original porosity by mechanical compaction and were selectively cemented by non-ferroan or usually ferroan, variably luminescent, slightly 18O-depleted sparry calcite cement (δ18O −2 to −4‰), mainly as syntaxial rims about echinoid grains. These shallow-burial cements form less than about 10% of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction. During the moderate burial phase, between about 600 and 1100 m sub-bottom depth, porosity loss continued (typically to about 70% of its original value) as a result of pressure-solution of calcitic bioclasts associated with more advanced stages of chemical compaction. This involved development of a wide variety of non-sutured and microstylolitic solution seams, including both single and composite, wispy or continuous, bedding-parallel types and non-parallel reticulate forms. The released carbonate was precipitated as ferroan (or non-ferroan where iron supply was negligible), dull luminescent, strongly 18O-depleted (δ18O −4 to −7‰), mainly equant calcite spar cement, occluding available pore space in the limestones.  相似文献   


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