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1.
The Maastrichtian chalk of the southern Central Graben, Danish North Sea, is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea, which covered large parts of northern Europe. The sediment displays a pronounced cyclicity marked by decimetre‐thick bioturbated beds alternating with slightly thinner non‐bioturbated, mainly laminated beds. The laminated half‐cycles consist of alternating millimetre‐thick, graded, high‐porosity laminae and non‐graded, low‐porosity laminae. The cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds, with the latter reflecting random and local resedimentation processes. Based on textural and structural analysis, the millimetre‐scale, non‐graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production. The graded laminae were deposited from small‐volume, low‐density turbidity currents and suspension clouds. The sedimentation rates of the cyclical chalk are similar to those known elsewhere, and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor. Bioturbated–laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a Milankovitch scale. A direct implication of this interpretation is that the cycles are areally widespread, probably extending throughout the southern Central Graben area and may be useful for correlation and high‐resolution cyclostratigraphy in the chalk fields of the Danish North sea. If the laminated half‐cycles represent a few rapid resedimentation events, with a high sedimentation rate as suggested by most workers, then the sediment would not be truly cyclic, but would represent event sedimentation within a pelagic background represented by the bioturbated beds. In this case, the cycles would have very limited potential for correlation.  相似文献   

2.
Although it is a pelagic sediment, fine-grained calcareous ooze may be mobilized prior to general lithification and redeposited as allochthonous units. Numerous occurrences of allochthonous chalk have been reported in recent years, having been recognized by large-scale bedding features seen in outcrop. Smaller-scale internal features, such as contorted laminae, and larger features, such as smeared burrows and imbricated flint nodules, attest to a significant amount of soft-sediment deformation and synsedimentary slumping in European chalk sections of Late Cretaceous age. Truly autochthonous chalks contain complex, tiered ichnofabrics and in some cases exhibit a diagenetic nodular fabric that is undisturbed by transport. In some situations, such as stagnant water conditions, autochthonous chalks may exhibit primary lamination, although this is very uncommon in European chalk sequences. Different types of redepositional processes produce an array of varied allochthonous fabrics. Glide and slump units, for example, contain internal deformational features produced during sliding. Ooze flow causes plastic deformation of chalk units, internally as well as externally. Resuspension and fluid flow of chalk sediment produces a deposit having a totally new fabric, such as a conglomerate composed of detrital chalk clasts. In this paper, typical macroscopic, sedimentary fabric types are illustrated, and the means of identifying them are discussed in terms of bioturbation features, in situ diagenetic nodules versus detrital clasts, physical deformation structures and development of flints.  相似文献   

3.
 During the Cenozoic, in the western Paris Basin, atmospheric weathering of the chalks with flints of the Upper Cretaceous led to the creation of clay with flints. A reconstitution of the chalks lost to dissolution is proposed and is based on the determination of the age of the parent chalks of the clay with flints and the quantification of the thickness of dissolved chalk. The chalks affected by weathering range in age from Turonian to Maastrichtian, thus confirming the deposition of calcareous sediments in the western Paris Basin up to the Maastrichtian. Chalk weathering took place in situ, as indicated by the preservation of the stratigraphic succession of the chalk in the clay with flints profiles. Weathering led to the dissolution of 20–200 m of chalk, with regional variations. The weathering rate varies between 2.1 and 14.5 m/Ma. Received: 20 July 1998 / Accepted: 1 July 1999  相似文献   

4.
Diagenetic history of a North Sea chalk   总被引：1，自引：0，他引：1  
A study of the petrofabrics of Danian and uppermost Maastrichtian chalk from the North Sea was undertaken to investigate its particulate components and diagenetic history. Danian and Maastrichtian chalks are intensely mottled and burrowed globi-gerinid lime mudstones. The Danian chalk matrix is composed of coccolith and thoracosphaerid debris, whereas the Maastrichtian chalk matrix contains mainly coccoliths. The lower part of the Danian is often argillaceous. Three modes of lithification are evident—a spot-welding of adjacent grains (important in Danian chalk), selective overgrowths (prolific in Maastrichtian chalk), and a sparry calcite pore filling associated with Maastrichtian stylolitization. Not only does the scant cementation of chalk stem from an inadequate source of metastable calcium carbonate in the form of aragonite, but also indirectly in that extensive pressure-solution is impeded by certain pore fluid compositions. Pressure-solution can occur only at point contacts where a threshold linear pressure is exceeded and so allows an increase in calcite solubility. It is proposed that through the formation of spot-welds an initial rigid intergranular framework is constructed in chalk relatively early during diagenesis. Subsequent increases in overburden eventually permit extensive stylolitization and the late diagenetic reprecipitation of a sparry calcite pore filling adjacent to stylolites. The time and genesis of selective overgrowths is less clear.  相似文献   

5.
Santonian-Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment-ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high- and low-productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well-oxygenated, current-swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate-fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment-seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post-oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo-Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.  相似文献   

6.
Danian marine sedimentation in the Paris Basin occurred between two major erosional phases. The earlier was responsible for the stripping of presumably deposited Maastrichtian sediments and of a variable thickness of Campanian chalk. The later occurred during the late Palaeocene and resulted in the erosion of almost all Danian deposits, which are now limited to small and scattered outcrops. One of these outcrops corresponds to reefal and peri‐reefal limestones of middle to late Danian age, exposed in the quarries of Vigny (NW of Paris). Danian deposits here show intricate relations with the surrounding Campanian chalk. Danian sedimentation was contemporaneous with faulting, which generated signifiant sea‐floor relief and resulted in contrasting depositional areas: topographic highs with coralgal reefs, and depressions where calcirudite channel fill accumulated. Normal faulting occurred along WNW–ESE master faults. The generation of submarine fault scarps gave rise to various types of gravity‐driven phenomena, including the sliding and slumping of large blocks of reefal limestone and the deposition of carbonate debris flows. Along with the redeposition of the Danian carbonates, flows of fluidized and reworked Campanian chalk resulted from the peculiar physical properties of the undercompacted chalks. Erosion and faulting occurred predominantly during the Palaeocene and represent a major episode in the physiographic evolution of the Paris Basin.  相似文献   

7.
Detailed facies analysis of a 350 m long core of upper Campanian–Maastrichtian chalk at Stevns Peninsula, eastern Denmark, shows that four mudstone and wackestone chalk facies account for close to 95% of the succession, and that bioturbated mudstone chalk alone accounts for nearly 55% of the sediment. Sedimentation took place in deep water, below the photic zone and storm‐wave base, and is characterized by decimetre to metre‐scale variations in facies and trace fossil assemblages indicating repeated shifts in depositional environment. Integration of facies with published data on sea‐surface temperature and accumulation rates suggests that sea‐surface temperature is the most important parameter in controlling stratification of the water column and thereby, indirectly, the observed variations in depositional facies. However, bioturbated mudstone chalk occurs in all stratigraphic levels independent of accumulation rates and sea temperatures and is interpreted to represent a very broad set of deep water environmental conditions with an ample supply of calcareous nannofossil debris and intense bioturbation. Longer term shifts in deposition are best expressed by distribution of clay, flint and bioturbated micro‐wackestone, bioturbated wackestone and laminated mudstone chalk facies, whereas the trace fossil assemblages appear less useful. The data set indicates overall shallowing over time with two distinctive events of clay influx to the basin during the late Campanian–earliest Maastrichtian and late Maastrichtian.  相似文献   

8.
The Corallian Group of the Dorset coast is reinterpreted as representing four regressive–transgressive cycles in contrast to previous interpretations which recognized four asymmetric regressive cycles. Each regressive sequence is separated at its top from each transgressive sequence by a sharp erosional contact. Overall, the regressive, coarsening-up sequences are characterized by relatively low faunal diversity, dominantly vertical burrows, relatively coarse sediments with cross-bedding and swaley cross-stratification, and the diagenesis predominantly influenced by meteoric water. In comparison, the transgressive, fining-up sequences are generally characterized by relatively high faunal diversity, dominantly horizontal burrows, basal lag deposits (1–2m in thickness) overlying a sharp, erosional surface, followed by clay or nodular wackestone and dominantly deep burial diagenesis. The consistence of the four regressive-transgressive cycles with the eustatic sea-level curve suggests that the sedimentary cycles of the Corallian outcrops may have been mainly produced by the eustatic sea-level changes. Tectonic movements may have been important in generating localized unconformity, relatively coarse clastic sediment influxes and rapid lateral facies variations. The recognition of storm- and tidally-related sedimentary features suggests that the Corallian Group of the Dorset coast may have been deposited in a storm- and tidally-influenced shallow epicontinental shelf.  相似文献   

9.
Chalk is a variable material, the properties of which are dependent upon its composition, textural features and diagenetic history. With the exception of certain horizons in the Lower Chalk that contain appreciable amounts of clayey material, the English Chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions. The latter comprises 70–80% of chalk. Cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures. Hence, high values of porosity were retained. Chalk varies appreciably in density and hardness. The harder chalks are the result of diagenetic processes and bioturbation that brought about densification. In soft chalks the grains are only bound together at the points of contact by thin films of calcite.

The latest classification of chalk is based on an assessment of intact dry density, discontinuity aperture and discontinuity spacing. Chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation. Under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel-shaped failure occurs indicating plastic deformation and textural disaggregation. Similarly, at low loading, chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded.

Chalk undergoes dissolution and so solution features are found throughout its outcrop.

Mineworkings in the Chalk extend back into the distant past, the most ancient being those excavated in the Neolithic Age for flint. Several types of workings exist. Collapse of old mineworkings, most of which are unrecorded, is difficult to predict. The potential for subsidence, caused by the collapse of both mineworkings and dissolution features, affects development and its occurrence can lead to the abandonment of property or, worse, the loss of lives.  相似文献   


10.
New lithological, ichnological, and isotope data are given for the rhythm from the section of the Middle Cenomanian rhythmic sequence at Mount Selbukhra. The intense rock bioturbation has distinct multistage structure that is typical of writing chalk sections and corresponds to the Cruziana ichnofacies that are characteristic of the soft grounds of the open shelf. The values of the ??13C and ??18O ratios in ichnofossils contradicts the model of the farming structures for the Chondrites and Zoophycos and indicate the passive infilling of Thalassinoides burrows by sediments. It was established that organic-rich marlstones accumulated under higher bioproductivity and the bottom anoxia were augmented by dissolution cycles. Based on our data, it is not possible to relate the rhythmicity to the precession cycles with confidence.  相似文献   

11.
The geochemical cycles of iron and sulphur in marine sediments are strongly intertwined and give rise to a complex network of redox and precipitation reactions. Bioturbation refers to all modes of transport of particles and solutes induced by larger organisms, and in the present-day seafloor, bioturbation is one of the most important factors controlling the biogeochemical cycling of iron and sulphur. To better understand how bioturbation controls Fe and S cycling, we developed reactive transport model of a coastal sediment impacted by faunal activity. Subsequently, we performed a model sensitivity analysis, separately investigating the two different transport modes of bioturbation, i.e. bio-mixing (solid particle transport) and bio-irrigation (enhanced solute transport). This analysis reveals that bio-mixing and bio-irrigation have distinct—and largely opposing effects on both the iron and sulphur cycles. Bio-mixing enhances transport between the oxic and suboxic zones, thus promoting the reduction of oxidised species (e.g. iron oxyhydroxides) and the oxidation of reduced species (e.g. iron sulphides). Through the re-oxidation of iron sulphides, bio-mixing strongly enhances the recycling of Fe and S between their reduced and oxidised states. Bio-irrigation on the other hand removes reduced solutes, i.e. ferrous iron and free sulphide, from the sediment pore water. These reduced species are then reoxidised in the overlying water and not recycled within the sediment column, which leads to a decrease in Fe and S recycling. Overall, our results demonstrate that the ecology of the macrofauna (inducing bio-mixing or bio-irrigation, or both) matters when assessing their impact on sediment geochemistry. This finding seems particularly relevant for sedimentary cycling across Cambrian transition, when benthic fauna started colonizing and reworking the seafloor.  相似文献   

12.
Middle and Upper Eocene biogenic sediments in the Willunga Embayment along the eastern margin of the St Vincent Basin are a series of warm‐temperate limestones, marls and spiculites. The Middle Eocene Tortachilla Limestone is a thin, coarse grained, quartzose, biofragmental, bryozoan–mollusc calcarenite of stacked metre‐scale depositional cycles with hardground caps. Lithification, aragonite dissolution and the filling of moulds by sediment and cement characterize early marine‐meteoric diagenesis. Further meteoric diagenesis at the end of Tortachilla deposition resulted in dissolution, Fe‐oxide precipitation and calcite cementation. The Upper Eocene Blanche Point Formation is composed of coccolith and spiculite marl and spiculite, all locally rich in glauconite, turritellid gastropods and sponges. Decimetre‐scale units, locally capped by firmgrounds, have fossiliferous lower parts and relatively barren upper parts. Carbonate diagenesis is minor, with much aragonite still present, but early silicification is extensive, except in the spiculite, which is still opal‐A. All depositional environments are interpreted as relatively shallow water: high energy during the Middle Eocene and low energy during the Upper Eocene, reflecting the variable importance of a basin‐entrance archipelago of carbonate highs. Marls and spiculites are interpreted to have formed under an overall estuarine circulation system in a humid climate. Basinal waters, although well mixed, were turbid and rich in land‐derived nutrients, yet subphotic near the sea floor. These low‐energy, inner‐shelf biosiliceous sediments occur in coeval environments across other parts of Australia and elsewhere in the rock record, suggesting that they are a recurring element of the cool‐water, carbonate shelf depositional system. Thus, spiculites and spiculitic carbonates in the rock record need be neither deep basinal nor polar in origin. The paradox of a shallow‐water carbonate–spiculite association may be more common in geological history than generally realized and may reflect a characteristic mid‐latitude, humid climate, temperate water, palaeoenvironmental association.  相似文献   

13.
The benthic foraminifer Quinqueloculina impressa Reuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty-clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water-sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze-like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze-like structures are living burrows. Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedimentation.  相似文献   

14.
Chalks originate as Cretaceous to Recent pelagic or hemipelagic calcareous ooze, which indurate via burial diagenesis to chalk and limestone. Because they accumulate in pelagic settings with high environmental continuity, chalks may form thick formations and even groups. For this reason, and because chalks have a simple mineralogy (low magnesium calcite, silica and clays), they are ideal for the study of diagenetic processes including the depth-related decrease of porosity. It is the aim of this study to illustrate how the evaluation of in situ elastic strain can help in understanding these processes including the interplay between stress-controlled diagenetic processes and processes furthered by thermal energy. Petrophysical core and well data can be used for analyses of how porosity reduction via pore collapse and pressure dissolution is related to in situ elastic strain. The data in question are: depth, density of overburden, pore pressure, ultrasonic P-wave velocity and dry density/porosity. The analysis reveals that the transition from ooze to chalk is associated with high elastic strain and consequent pressure dissolution at calcite–particle contacts causing contact cementation. The transition from chalk to limestone is also associated with high elastic strain, especially at clay–calcite interphases causing development of stylolites via pressure dissolution, and consequent pore-filling cementation. Following each transformation the elastic strain drops rapidly. The observation of this diagenesis-related pattern in elastic strain of the sedimentary rock is novel and should not only be helpful in understanding the porosity development in sedimentary basins, but also add basic scientific insight.  相似文献   

15.
The arrangement of sediment couplets preserved in Thalassinoides shafts suggests that tides regulated the passive filling of these trace fossils and, thus, represent tubular tidalites. The thickness variation in individual layers and couplets implies a mixed diurnal, semi‐diurnal tidal signature where packages of either thick‐layered or thin‐layered couplets alternate. Calcarenitic sediment accumulated when tidal current velocity was too high to allow deposition of mud, whereas a marly mud layer is interpreted to have formed during more tranquil times of a tidal cycle (in particular, low‐tide slack water). The tidal record within the burrows covers a few weeks and the corresponding spring–neap cycles. The fill of the Thalassinoides shafts is the only known record to decipher the tidal signature from otherwise totally bioturbated sediments. These deposits accumulated in a lower‐shoreface to upper‐offshore setting during the late Miocene on a shallow shelf extending from the Atlantic Ocean to the west into northern Patagonia. The fill of all investigated burrows started around spring tide and, thus, the behaviour of the burrow producers – probably crustaceans – is speculated to have been affected by tides or the high water level because all studied burrows became abandoned around the same period of a tidal cycle.  相似文献   

16.
Aptian Lower Greensand Group exposures in the cliffs of the Isle of Wight (southern England) display a consistent coarsening-up cyclicity on the scale of centimetres to tens of metres that reflects the bed, bed-set, parasequence, parasequence set and sequence hierarchy. These coarsening-up cycles are most commonly recognized at the scale of parasequences (20 cm to 10 m thick), genetically related groups of which form parasequence sets. Both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development. Numerous episodes of erosion, deposition and colonization are recorded, reflecting multiple erosion/bypass events. The increase in mean grain-size through each cycle is reflected by changes in physical sedimentary structures; ichnofauna or bioturbational fabric; fossil fauna and diagenesis. Interbedded mudstones, siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low-angle, hummocky, or tabular cross-strata, sandstone-filled erosional gutters and planar lamination. The cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble/plant debris-filled scours preserved. Bioturbational fabrics change upward through each cycle from small, subhorizontal, mud- or sandstone-filled burrows to large, branching, clay-filled or cemented burrow systems. The top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions. Concentrations of bivalves, brachiopods, bryozoa, crinoids and corals are preferentially cemented by iron oxide, carbonate or phosphate. Such cements were early and thus utilized by firm or hard substrate dwellers. This fossiliferous, cemented sandstone is overlain by a flooding surface marked by the mudstone and silt-rich sandstones at the base of the next cycle. Together, the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions, culminating in the diverse firmground fauna preserved at the top of each cycle. The fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding. High species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces. Increasing faunal maturity allows recognition of a hierarchy of hiatuses. This hierarchy is analogous to the parasequence–parasequence set division. The stratigraphic condensation of firmgrounds can be used to empirically define the condensed section, the thickness of sediment between firmgrounds being a function of sediment supply and water depth (accommodation space).  相似文献   

17.
The carbonate compaction law: a new tool   总被引：2，自引：0，他引：2  
Carbonate content, compaction, and porosity are evaluated from a large number of samples using micritic marl-limestone alternations from Germany, France, and Italy. Compaction is measured directly by utilizing both deformed, originally cylindrical bioturbation tubes and steinkerns of ammonites. Additionally, a method is developed to indirectly evaluate the total compaction of the rock matrix by using early, selectively cemented burrows and fossils. Plots representing measurements of compaction versus carbonate content display distinctly non-linear relationships which show increasing compaction with decreasing carbonate content. These relationships are found to clearly correspond with the carbonate compaction law. The compaction law is introduced as a theoretical derivation for sediments and rocks which calculates the carbonate and non-carbonate content, compaction, and porosity. It is based on the assumption that the non-carbonate fraction standardized to the primary sediment-volume remains constant during carbonate diagenesis. The compaction law is used to calculate the most commonly found sediment/rock transformations (e.g. mechanical compaction, cementation, and chemical compaction) and to simulate the diagenetic processes of given examples. Decompaction formulas are developed to evaluate the porosity and carbonate content of the primary sediment. An example of calculating decompaction and determining the original composition of the sediment is given utilizing carbonates spanning the Cretaceous-Tertiary boundary at Gubbio (Italy).  相似文献   

18.
Trace fossils from an upper Maastrichtian cyclic chalk-marl succession, the Rørdal Member, exposed in the Rørdal quarry, Denmark, are analysed in order to test whether the changes in substrate lithology exerted any control over the ichnodiversity, tiering complexity, and density of the infauna. The cyclicity is interpreted as caused by orbital changes within the Milankovitch frequency band. The carbonate content varies between 71 and 82 weight% in the marl and 82-92 weight% in the chalk beds. The material is based on 19 samples collected from six chalk and marl beds. The investigated bedding-normal sample surfaces vary in area between 29 and 155 cm2. Eight ichnogenera and two undetermined ichnogenera are recognised. The member is characterised by three ichnofabrics (A, B and C). The ichnofabric analysis is based on texture and internal structure of the sediments resulting from bioturbation. Ichnofabric A is found only in chalk samples and shows a poor preservation of trace fossils, whereas ichnofabric C is found in a few chalk and all marl samples and comprises a very dense, diverse and well preserved ichnofauna representing a high tiering complexity. Ichnofabric B represents an intermediate situation between ichnofabrics A and C and occurs in chalk samples immediately adjacent to marl beds. The observed changes in ichnofabrics between chalk and marl are related to the amount of clay in the samples and the differences in the occurrence of trace fossils are interpreted as due to differences in the visibility of traces between chalk and marl and not due to differences in ecological stress upon the endobenthic community of the two lithologies. The study thus provides an excellent example of how the effect of taphonomic factors may give a misleading and biased impression of apparent differences in the endobenthic community between chalk and marl.  相似文献   

19.
Two types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ18O values (?5‰ to ?8‰, PDB) and the δ13C values (+2‰ to ?3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ18O values (?3‰ to ?1‰ PDB) and δ13C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.  相似文献   

20.
为深化潟湖相碳酸盐岩储集层非均质性认识,以伊拉克M油田白垩系Mishrif组为例,基于岩心观察、物性分析数据、铸体薄片及压汞实验,对潟湖相储集层特征及成因机理开展研究.结果显示:研究区潟湖环境岩石类型复杂,生物碎屑具有多样性,储集层以低渗、特低渗为主,孔隙度分布范围宽,发育大量的基质微孔、铸模孔和晶间孔.储集层强非均质性是复杂沉积作用和差异成岩作用的结果.潟湖沉积物泥质含量高,发育大量基质微孔,与生物扰动作用相伴生的埋藏白云化作用可形成晶间孔,易溶型生物碎屑被选择性溶蚀形成大量的铸模孔.研究认为:潟湖沉积物原始物性较低,后期成岩作用可改善储集层物性.生物扰动期次、扰动强度、充填物类型、环境的封闭性及外来流体性质等因素控制了潜穴中充填物的改造趋势;生屑类型、成岩序列和成岩环境等影响了沉积物中铸模孔的发育和保存.综合潜穴充填物类型、生屑类型、成岩环境和成岩作用等因素,建立了生物扰动成因孔隙模式和铸模孔发育模式.   相似文献   

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