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1.
Glacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate-rich solutions to produce a strongly indurated calcrete horizon. The low-Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose-zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air-dominated and water-dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation. Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi-arid regions, and suggests that palaeo-calcretes should not be used as absolute palaeoclimatic indicators. The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well-drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.  相似文献   

2.
The Lueders Formation (mid-Permian) in Baylor County, Texas, is an intercalated suite of fluvial siliciclastic, shallow marine siliciclastic, and shallow marine carbonate strata. There are at least two generations of carbonate cements (probably originally composed of calcite) in the fluvial sandstones where fractured grains are observed. These cements represent the initial stages of caliche formation. Cementation is envisioned as a two step process. In the first step, calcite cements form from supersaturated fluids in a freshwater, vadose environment as a meniscus cement at grain contacts. Areas of cement formation are restricted to these sites because fluid distribution is restricted to these sites. Stresses generated by the growth of cements at grain contacts are transmitted through and concentrated at quartz/quartz grain point-contacts until the stress is sufficient to fracture quartz grains, even though the ultimate strength of calcite is less than that of quartz, per unit area. This process occurs too rapidly to be accommodated by pressure solution. In the second phase of cementation, cement nucleation is no longer restricted by vadose conditions. In this phase, calcite growth can no longer result in quartz grain breakage; rather, the quartz grains are dispersed in poikilotopic calcite cement.  相似文献   

3.
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   

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   

5.
A review of the origin and setting of tepees and their associated fabrics   总被引：3，自引：0，他引：3  
Carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal, lacustrine, and subaerial carbonate shelf sediments. These are commonly disrupted and brecciated when the surface area of these crusts increases. In the subtidal environment, megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand. Similar megapolygons form in the supratidal, lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and/or cement also causes the area of the hardgrounds to expand. The arched up antiform margins of expansion megapolygons are known as tepees. The types of tepees found in the geological record include: (1) Submarine tepees which form in shallow carbonate-saturated waters where fractured and bedded marine grainstones are bound by isopachous marine-phreatic acicular and micritic cements. The surfaces of these brecciated crusts have undergone diagenesis and are bored. Unlike tepees listed below they contain no vadose pisolites or gravity cements; (2) Peritidal and lacustrine tepees are formed of crusts characterized by fenestral. pisolitic and laminar algal fabrics. This similarity in fabric makes these tepees of different origins difficult to separate. Peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical. They are associated with fractured and bedded tidal flat carbonates. Their fracture fills contain geopetal asymmetric travertines of marine-vadose origin and/or marine phreatic travertines and/or Terra rossa sediments. The senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material. Lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common. They include groundwater tepees which form over evaporitic ‘boxwork’ carbonates, and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas, but the dominant sediment type is carbonate mud. These latter tepee crusts are coated and crosscut by laminated micrite; the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust. Both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions, in time intervals of days to years. Cement morphologies reflect this and the crusts often contain gravitational, meniscus vadose cements as well as phreatic isopachous cement rinds. (3) Caliche tepees which are developed within soil profiles in a continental setting. They are formed by laminar crusts which contain pisolites, and fractures filled by micritic laminae, microspar, spar and Terra rossa. Most of the cements are gravitational and/or meniscoid. In ancient carbonates, when their cementation and diagenetic fabric can be interpreted, tepee structures can be used as environmental indicators. They can also be used to trace the evolution of the depositional and hydrological setting.  相似文献   

6.
Holocene cemented beach deposits in Belize   总被引：1，自引：0，他引：1  
Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting early diagenesic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle Gays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand Gays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. 14C-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock.

The large-scale distribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays.  相似文献   


7.
Several diagenetic fabrics are described in the uppermost member of the Sarnyéré formation, an Upper Proterozoic dolomite (700-600 Ma). Micronodular micrite, microsparite showing nodulisation processes, orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal (Porostromata) boundstone. Further evolution in a vadose zone creates a network of cavities which are later filled by “cave stromatolites”, micropopcorn micrite and multilayered cements. A rhythmical repetition of facies evidences a polyphased pedogenetic history, leading to a 30 m thick highly complex section. Such numerous and diversified structures are rarely preserved, especially in Proterozoic rocks, but each of them have equivalents in recent examples of caliche soil profiles.  相似文献   

8.
Results obtained from beachrock lying on the north coast of the antique city of Parion in Çanakkale province, NW Turkey, are presented based on field data, petrographic analyses, cement fabrics, and radiocarbon dating. Extending to 20?m offshore at a depth of ?2?m, the studied 50-cm thick beachrock is poorly sorted lithic sandstone. Both exposed and submerged parts are characterized by sequentially precipitated marine phreatic and vadose cements composed of micrite encrustations with micro-organism borings, pseudopeloidal aggregates of high-Mg calcites with scalenohedral habits and meniscus bridges. Radiocarbon ages point to a deposition during the classical period when the sea level was below (between ?1 and ?1.5?m) that of the present. The beachrock witnesses a granule- and pebble-dominated wide beach prior to cementation, suggesting that Parion’s fortification walls were behind the coastline during this lowstand and raises questions concerning the existence of a harbor north of the city.  相似文献   

9.
范嘉松  吴亚生 《地质学报》2004,78(2):149-155
钙结壳是土壤层之下的岩石在出露地表渗流带环境内受大气雨水的溶解,并通过上覆土壤层的淋滤而成的产物。由于雨水的不断的溶解,使原始的岩石或沉积物发生破裂,从而形成各种大小的砾块或砾石。随着雨水不断的溶解,雨水逐渐增加 CaCO_3的含量。这些富含碳酸钙的雨水,当它流到位于潜水面之上的下渗流带时,就在土壤层之下、原来岩石之上沉积了特殊的、层纹状方解石胶结物。它们往往覆盖岩石表面或沿着砾石的外缘、呈包覆状沉积,从而形成典型的层纹状钙结壳。渗滤豆粒的外皮也属于层纹状钙结壳的性质,这些外皮有向下方加厚的趋势,这表明渗流带的含碳酸钙的雨水,受重力作用影响下,具有往下悬垂现象,类似于洞穴内的钟乳石。豆粒之间的相互衔接呈多边形构造,也表明它是受重力作用而形成的构造。  相似文献   

10.
《Sedimentology》2018,65(2):360-399
Sedimentary gaps are a major obstacle in the reconstruction of a carbonate platform's history. In order to improve the understanding of the early diagenesis and the succession of events occurring during the formation of discontinuity surfaces in limestones, secondary ion mass spectrometry was used for the first time to measure the δ 18O and δ 13C signatures of 11 early cement and fabric stages in several discontinuity surfaces from the Jurassic carbonate platform of the Paris Basin, France. Pendant cements show a high variability in δ 18O, which was impossible to detect by the less precise microdrilling method. The morphology of a given cement can be produced in various environments, and dogtooth cements especially can precipitate in marine phreatic and meteoric phreatic to vadose environments. Marine dogtooth cements and micritic microbially induced fabrics precipitated directly as low‐magnesium calcite in marine waters, as attested to by the preservation of their initial δ 18O and δ 13C signals. Five discontinuity types are recognized based on high‐resolution geochemical analyses, and their palaeoenvironmental history can be reconstructed. Two exposure surfaces with non‐ferroan pendant or meniscus cements formed in the oxidizing vadose zone. A hardground displays marine fibrous cements and non‐ferroan dogtooth cements that formed in a subtidal environment in oxidizing water. Two composite surfaces have undergone both marine and subaerial lithification. Composite surface 1 displays non‐luminescent ferroan dogtooth cements that precipitated in reduced conditions in seawater, followed by brown‐luminescent dogtooth cements characteristic of a meteoric phreatic environment. Composite surface 2 exhibits microbially induced fabrics that formed in marine water with abundant organic matter. The latter discontinuity, initially formed in a subtidal environment, was subsequently exposed to meteoric conditions, as evidenced by ferroan geopetal cements. A high‐resolution ion microprobe study is essential to precisely document the successive diagenetic environments that have affected carbonate rocks and discontinuities with a polygenic and intricate history.  相似文献   

11.
A laterally extensive calcrete profile has been identified in the Late Asbian (Lower Carboniferous) shallow marine shelf limestones of the Llangollen area, North Wales. The upper surface of the profile is defined by a laterally discontinuous palaeokarstic surface and by laminated calcareous crusts which developed within the underlying limestone. The profile contains a unique series of early pore-filling vadose cements which only occur down to 1 m below the palaeokarstic surface. Cathodoluminescence reveals that these cements pre-date the late pore-filling meteoric phreatic cements which occur throughout local Asbian lithologies. A spar cement stratigraphy has been established for the calcrete profile. Subaerial vadose cements comprise two generations of non-luminescent cement, followed by a brightly luminescent generation which occasionally shows an acicular habit. This needle-fibre calcite represents the final stage of vadose cementation. Precipitation of vadose cements was contemporary with subaerial alteration and micritization of the limestone. Textures, visible only with cathodoluminescence, provide evidence of recurrent periods of fabric dissolution. The most extensive phase of dissolution occurred immediately after the precipitation of the non-luminescent subaerial vadose cements. Several different textures have been recorded, each reflecting the morphology of a partially dissolved substrate. Dissolution textures are generally confined to the walls of the larger pores and to early brecciation fractures. These probably acted as fluid pathways in the calcrete during early subaerial diagenesis. Much of the non-marine micrite in the calcrete profile appears as needle-fibre calcite under cathodoluminescence. This acicular calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near-surface evaporation. Since needle-fibre luminescence is strongly variable, these ambient conditions are not believed to have directly controlled the activator ion concentrations of cementing pore waters. Needle-fibre calcite is considered to be a cement precipitate which has almost completely recrystallized to micrite, probably during the late stages of subaerial diagenesis. Two generations of subaerial micrite which define a ‘micrite stratigraphy’, have been distinguished under cathodoluminescence. Reconstructing the diagenetic history of this ancient calcrete profile has revealed that subaerial alteration was multistaged, with many diagenetic processes acting simultaneously during a single phase of emergence.  相似文献   

12.
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   

13.
Well-rounded pebbles and cobbles of reef rock, which in bio- and lithofacies closely resembles the Recent algal cup reefs, occur in the basal conglomerate of the Pleistocene Devonshire Formation cropping out along Grape Bay on the south shore of Bermuda. The framebuilders of this rock include coralline red algae, vermetid gastropods, and barnacles. Study by petrographic microscope, electron microprobe, and SEM revealed aragonite-needle and spherulitic cements, high-Mg calcite micrite and palisade cements including calcified algal filaments. Two or three generations of these may occur in a given pore. By analogy with the Recent cup-reef material, on the basis of their composition (high strontium content of aragonite, high magnesium content of calcite) and of circumstantial evidence, these are considered submarine cements.In addition, a vadose blocky low-Mg calcite cement occurs, which never precedes any, but frequently succeedes one, two, or three of the above cements. Its fabrics vary: it may exhibit an increase in crystal size toward the center or not, it may or may not be bounded by the meniscus outline characteristic of the vadose diagenetic environment. The limitation of cement precipitation by a meniscus gives rise to a large diversity of forms determined by respective combinations of solution supply, permeability of the micro-environment, and substrate morphology. To illustrate the diversity, the meniscus cement is shown in relation to sediment particles, aragonite-needle cement, and calcified filaments.From the blocky low-Mg cement, selective neomorphism extends into some aragonitic gastropod shells; it is controlled, in part, by original shell structure. The resulting low-Mg calcite mosaic is characterized by relic structures and plane crystalline boundaries. Aragonite needles between blocky calcite cement and neomorphic calcite were not affected by the process of mineralogic alteration.This study shows that cements recognized earlier in Recent Bermuda cup reefs can be traced into the past and that additions to the diagenetic sequence can be distinguished. Knowledge of these features and their sequences should help decipher the diagenetic history of other fossil reefs.  相似文献   

14.
The most ubiquitous syn-sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO3 concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation. Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg-calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg-calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro-voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg-calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg-calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non-marine water flow, probably in the vadose environment. Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement-like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre-existing Mg-calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg-calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ186 by about 3‰ PDB within calcite-dolomite pseudo-alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization.  相似文献   

15.
Sequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo-environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world. Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive-grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns. Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO3 and SiO3 from the soil and carbonate bedrock. Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.  相似文献   

16.
Many fabrics in Corallian (Upper Jurassic) carbonates in England, France and Switzerland are bigenetic, forming by solution/precipitation and recrystallization processes. Early precipitated cements are non‐ferroan, whereas those formed later are decidedly ferroan. Mossbauer spectroscopy has shown that the iron in the carbonates is largely divalent, substituting for calcium in the calcite lattice. The cements may be subdivided into those forming in an oxidizing environment, and those forming in a reducing environment. Fabric evidence indicates that a solution period separates the two cement phases. The diagenetic history is linked with the sedimentation pattern. Thus, under progressive build up of sediments into shallow‐water or supratidal conditions, the first‐phase cement probably precipitated from non‐marine, partially oxygenated vadose water. Subsequent subsidence and sedimentation resulted in the depression of partially cemented sediments into zones within the sedimentary pile conducive first to pyrite precipitation, and then to ferroan calcite precipitation. Fabrics can therefore be used to interpret the diagenetic environment of regressive sedimentary associations.  相似文献   

17.
Two oolites in the Dinantian (Mississippian/Lower Carboniferous) of Glamorgan, SW Britain, were deposited in similar depositional environments but have contrasting diagenetic histories. The Brofiscin and Gully Oolites occur in the upper parts of shallowing-upward sequences, formed through strandplain progradation and sand shoal and barrier growth upon a southward-dipping carbonate ramp. The Brofiscin Oolite is characterized by a first-generation cement of equant calcite spar, preferentially located at grain-contacts and forming non-isopachous fringes around grains, interpreted as meteoric vadose and phreatic in origin. Isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons. Burial compaction was not important and porosity was occluded by poikilotopic calcite spar. Fitted grain-grain contacts locally occur and could be the result of near-surface vadose dissolution-compaction. Syntaxial overgrowths on echinoderm debris are common. Pre-compaction overgrowths are cloudy (inclusion-rich) and probably of meteoric origin, and post-compaction overgrowths are inclusion-free. By contrast, the Gully Oolite has little first-generation cement. However, marine fibrous calcite is common in oolitic intraclasts, as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment; and early equant, drusy calcite spar occurs in the uppermost part of the Gully, beneath a prominent palaeokarst where pedogenic cements also occur. The major feature of Gully diagenesis is burial compaction, resulting in extensive grain-grain dissolution and microstylolitic grain contacts, and post-compaction poikilotopic spar occluded remaining porosity. The Brofiscin Oolite is pervasively dolomitized up-dip but the Gully Oolite for the most part only contains scattered pre-compaction dolomite rhombs and late veins of baroque dolomite, with less pervasive dolomitization. The difference in diagenetic style of the two Dinantian oolites is attributed to prevailing climate. The paucity of early meteoric cements in the Gully is a result of an arid climate, and this is supported by the nature of the capping palaeokarst. The abundant meteoric cements in the Brofiscin reflect a more humid climate, and effective meteoric recharge also resulted in up-dip pervasive mixing-zone dolomitization. The style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis: in the Brofiscin, the early cements inhibited grain-grain dissolution and pressure solution, while these processes operated extensively in the Gully Oolite. Thus, prevailing climate can influence a limestone's diagenetic history from near-surface through into deep burial.  相似文献   

18.
Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

19.
The Permian Yellow Sands are preserved as nine aeolian ridges in north-east England. The sands are soft and friable and locally contain large amounts of interstitial detrital clays, which are interpreted as the product of early mechanical clay infiltration. Features within the sands considered to be due to mechanical clay infiltration include grain coatings, meniscus and geopetal fabrics. The infiltrated clays mask crystal nucleation sites on grain surfaces and often amalgamate into lenses or form continuous clay-rich layers. Calcite and quartz cement growth has consequently been limited and often confined to the coarser, generally clay-free horizons. The meniscus and geopetal fabrics were formed early during accretion of the dunes and helped to bind and stabilize the sediment, which restricted reworking during the Zechstein transgression to the uppermost few decimeters.  相似文献   

20.
Cement-porosity relationships are described from the Lower Triassic Sherwood Sandstone Group and the Middle Jurassic Ravenscar Group in the United Kingdom. Calcite cemented sandstones display a variety of replacement textures, with preferential replacement of grains and of overgrowth faces with high free-surface energy. Dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates. Examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains. These range from small ‘v’-shaped notches and pits, through regular and irregular shaped embayments, into large depressions. These textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones, and are interpreted to have been formed by the dissolution of pore-filling and grain replacive authigenic carbonates. This is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones. These textures indicate that part of the intergranular porosity in these sediments is secondary in origin, and has been generated by the dissolution of carbonate cements. The identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface.  相似文献   

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