摘要:

ABSTRACTWidespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock‐dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature.All stages of calcite cement pre‐date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late‐stage diagenetic origin is also indicated within the sandstones, with dolomite post‐dating the development of quartz overgrowths.Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space‐filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO3. Generally the dolomites have isotopic compositions depleted in δ18O compared to the host limestone, with similar or lighter δ13C values.Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ18O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements.Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00785.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub‐basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%oto +3.00%oδ13C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near‐surface drilled sections fall in the range from —0.50%oto — 2.00%oδ18O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub‐basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea.The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature‐induced isotopic re‐equilibration in the water‐rock system. In these sequences the oxygen isotope values range from − 1.50%oto — 7.50%oδ18O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure‐dissolution, recrystallization, reprecipitation and ion‐exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re‐equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore‐water controlled isotopic re‐equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re‐equilibration process increasingly becomes influenced by the rock‐introduced isotopic change in

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00786.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTCarbonate 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 andsteinkernsof 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 Gub

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00787.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTIn the French Southern Alps, alternating pelagic limestones and marls of Hauterivian age contain a dense network of burrows. Differences between the original and preserved forms of the burrows provide evidence of sediment deformation. (a) Total compaction, from early dewatering of sediment to deep burial compression, caused flattening of burrows parallel to bedding. Its relation to carbonate is measured, and a tentative mathematical relation is proposed, (b) Synsedimentary slumping is shown by total disordering of the bioturbation network in the displaced layers. (c) A more characterized deformation is occasionally visible only on the upper part of some limestone beds. The sliding is indicated by the dip of burrows in the same direction as the flow. (d) Similar embryonic flows occur in the middle of rare limestone beds. It is thus suggested that this can account for some of the double beds that occur in pelagic alternations of calcareous and marly beds.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00788.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTHydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO2partial‐pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO2partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO3as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate de

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00789.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTIn Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones ofSaccocomaand peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00790.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTEuhedral quartz and albite crystals are common in Devonian (Givetian‐Frasnian) shallow‐marine shelf carbonates from the Belgian Ardennes. Several features such as morphology, the presence of carbonate inclusions, inversion temperatures and occurrence in the insoluble residues of stylolitic surfaces indicate that these crystals have developed authigenically. Oxygen isotope ratios point to an intermediate deep burial realm of origin at temperatures of 60–90°C. The predominance of illite and the almost total absence of smectite clay minerals is interpreted as an indication that illitization produced the silica needed for authigenesis. The mineral composition of inclusions indicates that the carbonate host rock must have consisted of low‐Mg calcite already at the time of authigenesis. These inclusions represent an earlier diagenetic stage than the present carbonate rock, since they were protected from further diagenetic alteration by the surrounding quartz. The calcite inclusions display a higher Sr/Ca and Mg/Ca ratio than the carbonate host‐rock. Because neomorphic diagenesis of the carbonate continued after silicate authigenesis, the contents of Mg and Sr in the calcite of the host carbonate are even lower. The authigenic feldspar mineralogy seems to be determined by the composition of the hos

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00791.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTUltra‐fine structures of the Negev high‐grade phosphorites provide valuable clues to unravelling the genesis of these rocks, the question of their differential areal distribution, and the biosedimentary mechanisms involved in their considerable enrichment. Peculiar to these phosphorites is an intergranular phosphate matrix, for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro‐organizations. The phosphate particles fixed by this filamentous meshwork also consists of internally organized and non‐organized packed microbial remains of different types.Analogies between the fabrics of the matrix and the corpuscles lead to conception of a two‐stage depositional scheme for these phosphorites, based on rhythmical repetition of two sedimentary mini‐events, in a slightly oscillating, very shallow marine system—(1) a low‐energy event of microbial colonization of the Mishash bottoms, followed by early phosphatization of the organic structures mainly in marginal situations; and (2) a higher‐energy event which broke up the phosphatic mats into debris, redepositing them as clastic layers in nearby basinward sites, while becoming bound by a new meshwork of filamentous microphytes. The differential phosphatization of the intergranular microbial binder, again occurring mostly in marginal localities, produced highly enriched phosphorites. Minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks. Examining the structural and textural features, the validity of the sedimentary mechanisms of Recent phosphorite formation for the Campanian Negev r

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00792.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe Lower Jurassic Aztec Sandstone is an aeolian‐deposited quartzose sandstone that represents the western margin of the southerly‐migrating Navajo‐Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments.In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue‐shaped aeolian sand bodies interstratified with fluvially‐deposited coarse sandstone and mudstone. Above, evaporite‐rich fine sandstone and mudstone are overlain by thick, cross‐stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west.Thin aeolian sequences are interbedded with volcanic flow rocks, ash‐flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian‐deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc‐derived sediment.A two‐part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross‐strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed thefore‐erg,is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian‐deposited sediment encased in fluvial and sabkha deposits. Continued sand‐sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thickcentral‐ergsediments buried the fore‐erg deposits. The trailing, upwind margin of the erg is represented byback‐e

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00793.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTReliable field data obtained by directly measuring bed‐load transport of fine‐ to coarse‐grained bed material are extremely scarce, mainly because of the difficulty of sampling accurately. Therefore, the verification of bed‐load transport formulae is largely based on flume experiments, which refer to unrealistic shallow‐water conditions. In this study, some bed‐load transport formulae were tested against data from natural environments. As an alternative to ascertaining the bed‐load transport rate by sampling the bed‐load, the transport rate was deduced from data on bedform height and bedform celerity. For this purpose, 43 sets of data from rivers, representing a wide range of bed material, bedform dimensions and hydraulic conditions were collected as were some sets of data from tidal settings. Two formulae were used for the prediction of the bed‐load transport: the formula of Van Rijn (1981) and the Kalinske (1947) formula as approximated by Elzerman&Frijlink (1951) (and, in the present study, slightly modified for application to tidal waters). Both the bed‐load function of Van Rijn and the modified formula of Kalinske‐Frijlink require data which are easily obtained and that can be measured accurately.At those stages of the flow when bed‐load transport was high the Van Rijn function tended to overestimate that transport. For flow stages when bed‐load transport was low the opposite was true. The modified Kalinske‐Frijlink function gave consistently good results: 86% of the transport rates predicted using the river data were within 0·5–2·0 tim

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00794.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY