Fabrication Of Cement Research Articles

Characteristics, evolution and distribution of Quaternary channel calcretes, southern Jordan

AbstractCemented and calcretized conglomerates of Quaternary age are found within ancient river channels exposed at various heights (between +1 and +125 m) above the current bed of the Wadi Dana in southern Jordan. In cross‐section, the calcrete deposits are typically lensoid in shape and are found infilling palaeochannels cut into the bedrock. The fossilized channel sediments preserve evidence of past river conditions, sediment loads, source areas, phases of river down‐cutting and sediment accretion, as well as post‐depositional alteration processes.Samples have been analysed in thin section and under the scanning electron microscope in order to determine the calcrete micromorphology, cement types and crystal sizes. The results show that the river deposits have undergone significant displacement and replacement of the detrital grains by CaCO3, and this has resulted in the cementation in places of over 4 m of river material. The cements display an alpha‐type fabric with no evidence of biogenic activity. The nature of the cement fabric within and between deposits differs as a function of two key variables: (i) the specific location within the channel and (ii) height above the modern wadi floor. Firstly, in many of the deposits cement crystal size varies, with sparite dominating at depth within impermeable palaeowadi channels, whereas on the flanks of the channels and near the top of the channels micrite is the main type of cement. These variations are thought to be a result of differences in the length of residence time of calcium carbonate‐rich fluids in different parts of the channel and the spatially varying influence of evaporation. Secondly, distinct changes in cement fabric can be seen in channels of different ages with a reduction in the amount of micrite and an increase in the amount of secondary recrystallized pore‐filling spar over time. Copyright © 2004 John Wiley & Sons, Ltd.

The Relative Roles of Compaction and Early Cementation in the Destruction of Permeability in Carbonate Grainstones: A Case Study from the Paleogene of West-Central Florida, U.S.A.

ABSTRACT How compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented. In this study, the destruction of permeability by compaction relative to early cementation is examined in forty-five shallow-buried (30-440 m) grainstones with a range in permeability of 12 to 5180 md. Compaction and cementation indices calculated from thin-section point-count data indicate a group of grainstones dominated by cementation and another by compaction. Log-linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes. The nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability. As cement abundance increases in the cement-dominated samples, a continuous distribution of cement develops along grain surfaces, which is followed by the development of partial and then complete pore-filling mosaics. Permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats. Permeability loss results from porosity reduction, progressive constriction of pore throats, and increased tortuosity of flow paths. Once permeability is reduced to tens of millidarcies, effective pore radii are all less than 1 µm and many are less than 0.1 µm, but there is no evidence for complete blocking of all pore throats in this suite of samples. In absolute terms, the pore-lining cements reduce permeability by thousands of millidarcies whereas the pore-filling cements reduce permeability only by hundreds of millidarcies. Mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction-dominated samples. Linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases. However, there is no concurrent change in the amount of pore space accessed by large or small throats, indicating that mechanical compaction does not uniformly constrict or seal pore throats. Instead, the data suggest that mechanical compaction reduces permeability by reducing porosity, lengthening pore throats, and increasing tortuosity. At about 500 md the compaction-dominated trend steepens on the log-linear plot of permeability versus porosity, a change that corresponds to the development of grain-to-grain pressure solution. Unlike mechanical compaction, grain-to-grain pressure solution constricts and eliminates pore throats and isolates pore space, resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats. In contrast to cement-dominated samples, the most effective pore radii are still greater than 1 µm at tens of millidarcies of permeability. Such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample.

Influence of gypsum’s mineralogical characteristics on its grinding behaviour applied to cement fabrication

In an industrial process of cement fabrication, the use of mixtures of 95% clinker and 5% gypsum with two different kinds of gypsum (similar chemical composition but different source) results in grinding power consumption differences up to 15%. The mentioned differences are not explained when performing power consumption evaluations according to Bond’s method. On the other hand, studies of both gypsums by means of transparency microscopy show their differences in textural characteristics and lead to the explanation of such a behaviour difference, as much in grinding in the industrial process as in the laboratory tests. The transparency microscopy continues being a recommended tool in the selection of appropriate raw materials in the cement industry.

Attachment of mature oysters ( Saccostrea cucullata ) to natural substrata

It has been suggested that mature oysters attach to their natural substrata by means of a combination of a modification of the prismatic outer-shell layer formed within the periostracum and a “pressing” action of the mantle (Yamaguchi 1994). However, marine surfaces are seldom smooth enough to allow adhesion without the addition of a fluid adhesive to allow electromagnetic interactions to hold the two bodies together. An electron microscope study of the attachment of the oyster Saccostrea cucullata to its natural substrata has confirmed the presence of a crystalline calcareous cement. The cement shows a range of spherulitic and irregular blocky textures that are reminiscent of diagenetic cement fabrics. Their form suggests that the cement crystallises from a calcium-carbonate-saturated liquor trapped between the underside of the shell and the substratum, with crystallites nucleating on all bounding surfaces of the void.

Carbonate petrography as an indicator of climate and sea‐level changes: new data from Bahamian Quaternary units

ABSTRACTCoeval stratigraphic units of similar petrology occur throughout the northern Bahamas islands. The petrographic composition of these limestones provides clues about regional sea level and climate changes during the late Quaternary. At least eight fossil shoreline units, which are linked to transgressive episodes between the middle Pleistocene and the late Holocene, are recognized in the Bahamas. The petrographic composition of these units is either dominated by ooids and peloids or by bioclasts. Sedimentological observations demonstrate that oolitic‐peloidal units were formed when sea level was higher than today, whereas skeletal units were deposited at or below modern ordnance datum. Skeletal units may reflect times of partial, or modest platform flooding, when the bulk of sediments brought to islands originates from bank‐margin reefs. In contrast, oolitic‐peloidal units correspond to major flooding events and active water circulation on the bank top. Cement fabrics further show that the early diagenesis of oolitic units took place during warm and humid climatic conditions, whereas skeletal rock bodies underwent subaerial diagenesis during drier climatic conditions characterized by marked seasonal changes. This example from the Bahamas suggests that compositional analysis of limestone from fossil carbonate platforms could be used for resolving ancient climate and sea‐level changes.

Effects of an Organotitanate Cross-linking Additive on the Processing and Properties of Macro-Defect-Free Cement

We have investigated the effects of an organotitanate cross-linking additive on the processing and moisture sensitivity of macro-defect-free (MDF) cements. The modified samples were formed by a two-part process: first, the as-received calcium aluminate cement powder was coated with the organotitanate compound, followed by the standard MDF cement fabrication procedure. It was estimated that 6 wt% (by weight of the cement powder) of the additive was adsorbed. Rheological measurements showed that cross-linking reactions between the organotitanate additive and the polymer (polyvinyl alcohol-acetate) used to form MDF cement occurred during the high shear mixing process. This led to a 65% reduction in the processing window for the modified pastes as compared to the standard MDF cement pastes. However, the organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100% relative humidity at 22°C. The most significant result was that the modified samples retained nearly 100% of their initial strength after 200 days of exposure to these conditions, whereas the strength of the standard MDF samples was decreased by approximately 45%.

Effects of an organotitanate cross-linking additive on the processing and properties of macro-defect-free cement

Abstract We have investigated the effects of an organotitanate cross-linking additive on the processing and moisture sensitivity of macro-defect-free (MDF) cements. The modified samples were formed by a two-part process: first, the as-received calcium aluminate cement powder was coated with the organotitanate compound, followed by the standard MDF cement fabrication procedure. It was estimated that 6 wt% (by weight of the cement powder) of the additive was adsorbed. Rheological measurements showed that cross-linking reactions between the organotitanate additive and the polymer (polyvinyl alcohol-acetate) used to form MDF cement occurred during the high shear mixing process. This led to a 65% reduction in the processing window for the modified pastes as compared to the standard MDF cement pastes. However, the organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100% relative humidity at 22°C. The most significant result was that the modified samples retained nearly 100% of their initial strength after 200 days of exposure to these conditions, whereas the strength of the standard MDF samples was decreased by approximately 45%.

Palaeoenvironmental reconstruction of a condensed hardground-type depositional sequence at the Cretaceous-Tertiary contact in the Parnassus-Ghiona zone, central Greece

Phosphatic hardgrounds at the K/T boundary in the Parnassus—Ghiona zone reflect a major stratigraphic break. The hardgrounds developed on top of the uppermost Maastrichtian beds, which represent a condensed pelagic carbonate sequence of seamount type. Synsedimentary lithification and hardground formation took place on shallow, periodically emerged, seafloors. Phosphate cementation includes fillings of intra- and inter-particle porosity and occludes passive framework open spaces, as well as rim cement fabrics, developed through mineralization by non-phototropic microbial communities. Within the hardground superstructure a highly condensed phosphate sediment was precipitated, post-dating the hardground formation. Phosphate cementation was developed under well-oxygenated conditions, involved in a “low-productivity” phosphogenic system. Phosphate ions were released into pore waters through bacterial decomposition of organic matter. Glauconite developed due to prolonged contact of lithified chalk with seawater. The irregular, bored, impermeable and phosphate-cemented pavements are covered by stratiform and columnar laminated phosphatic stromatolitic crusts trapping Early Paleocene pelagic sediments, deposited during the subsequent Danian transgression. The source of phosphorus was external, either marine or continental. Phosphatization occurred during the bacterial degradation of the organic sheaths. Pyrite crystals within the stromatolitic laminae suggest a sulphidic environment beneath the mat-forming communities. Successive phases of intraformational reworking resulted in “mature” hardground development. Lags from eroded subjacent hardgrounds were accumulated in neighbouring submarine depressions. Marly material rich in quartz has been concentrated along discontinuities. On former topographic heights both the phosphatic hardground and stromatolites have been assimilated by a cryptocrystalline phosphatic material, rich in clay-iron oxides and quartz, considered to originate from subaerial weathering (phoscretization).

Holocene marine cement coatings on beach-rocks of the Abu Dhabi coastline (UAE); Analogs for cement fabrics in ancient limestones

Marine carbonate cements, which are superficially like travertines from meteoric caves, are coating and binding some intertidal sedimentary rock surfaces occurring in coastal Abu Dhabi, the United Arab Emirates, (UAE). Near Jebel Dhana these surficial cements can be up to 3 cm thick and envelope beach rock surfaces and fossils. They are also present both as thin coats and a fracture-fill cement in the intertidal hard grounds associated with the Khor Al Bazam algal flats.

Diagenesis of Pleistocene reef-associated sediments from the Red Sea coastal plain, Egypt

Pleistocene reef-associated sediments from the Red Sea coastal plain of Egypt consist of carbonate to mixed carbonate-siliciclastic sediments. The recognized facies are calcareous sandstone, skeletal rudstone, grainstone, wackestone and micritic crusts. Evidence from cement fabrics (aragonite, high-Mg calcite and gypsum) suggests that cementation mainly occurred in the intertidal zone under marine vadose conditions. However, the interaction of these sediments with meteoric waters resulted in the modification and stabilization of the metastable carbonates, the replacement of detrital feldspars by carbonates and the silicification of carbonates.

Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays

Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock. The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites. Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples. Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface. Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO 2 and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Norman's Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry. Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.

Cement Fabrics of the Bahamian Platform and Its Margin Near Lee Stocking Island

Consolidated to firable carbonate rock samples were collected on and around Lee Stocking Island to determine the distribution, fabric, and mineralogy of their cements. The rocks include: (1) beachrock rimming the islands, (2) shallow-water hardgrounds, (3) reef rock, (4) channel stromatolites, and (5) Pleistocene bedrock. Analyses by SEM, microprobe, X-ray diffraction, and petrographic microscope have revealed ten different cement fabrics. Five of these cements are varieties of fibers, all of which are aragonitic except the whisker' fibers that form coarse networks of intertwining high-Mg calcite in a Pleistocene cave sample. Acicular fan-druse and square-tipped coarse fibers cement the beachrock, while an isopachous, needle-fiber rim is found only in the hardgrounds. A radial fibrous cement occurs in several ooids and biogenic grains, representing a replacement fabric of aragonite that has inverted to high-Mg calcite. Two types of blades are present: a stubby variety with a length:width ratio of 2:1 and an elongated 5:1 variety, both of which are high-Mg calcite. While the 2:1 variety is rather common, the 5:1 variety only occurred in one sample. Aragonitic micrite envelopes often surround grains in beachrock and hardgrounds, but only in association with fibrous cement. An aragonitic lime mud matrix cements the crusted mudmore » beds and low-Mg calcite equant spar cements the Pleistocene samples and occurs as void-fill in beachrock and hardgrounds. The most common marine cementation is associated with the aragonitic fibers found in the discontinuous hardgrounds and beachrocks. The more widespread cements are the low Mg-calcite spar associated with meteoric diagenesis and cementation of the Pleistocene surface.« less

Diagenesis and Cement Fabric of Gas Reservoirs in the Oligocene Vicksburg Formation, McAllen Ranch Field, Hidalgo County, Texas: ABSTRACT

ABSTRACT McAllen Ranch field produces natural gas from 12 deep, overpressured sandstone reservoirs, each interpreted to be the deposit of a prograding shelf-edge delta. Correlation of petrographic and sedimentologic features reveals a predictable pattern of sandstone cementation. The sandstones are feldspathic litharenites containing subequal proportions of volcanic rock fragments (VRF), feldspar, and quartz grains. Grain size ranges from very fine to coarse sand. Porosity is mostly secondary, having formed through dissolution of VRF and feldspar grains. Four major diagenetic facies in cored reservoir rock can be grouped by the predominance of one diagenetic cement type and by appearance in hand specimen: (1) calcite-cemented; (2) chlorite-cemented, tight; (3) chlorite-cemented, porous; and (4) quartz overgrowths, porous. The calcite-cemented facies predominates in very fine-grained sandstones and siltstones and encroaches into adjoining sandstones irrespective of grain size. Core permeabilities are generally less than 0.01 md, and porosities range from 7% to 15%. Authigenic chlorite generally cements sandstones intermediate in grain size between those cemented by calcite and those cemented by quartz. Two types of diagenetic chlorite fabric are interbedded, forming distinct alternating bands 0.1 in (0.25 mm) to 3 ft (1 m) thick. In the tightly chlorite-cemented facies, permeabilities are less than 0.3 md, and porosities range from 8% to 16%. In the porous chlorite-cemented facies, dissolution of framework grains and early yellow chlorite cement increased porosity, and a second coarsely crystalline chlorite cement was precipitated. Core permeability ranges from 0.1 to 1 md, and porosities range from 15% to 20%. The quartz overgrowth facies occurs in 1- to 2-ft-thick (0.3- 0.6-m) zones within the coarsest grained sandstones. Although these overgrowths constitute only 5% of the rock volume, they are the predominant cement. A relative abundance of intergranular pores results in permeabilities ranging from 1 to 20 md. Porosities range from 15% to 20%. Three depositional facies are present in McAllen Ranch reservoirs: (1) mid-delta-front deposits that are characterized by thin upward-fining sequences; (2) massive upper delta-front sandstones; and (3) distributary channel-fill sandstones. The diagenetic facies are localized into a distinct pattern within each depositional element. Mid-delta-front upward-fining sequences 0.5 to 5 ft thick (0.2 to 1.6 m) are cemented by calcite at the base and top. Porous and tightly cemented chlorite bands occur in the middle of the upward-fining sequences, with the proportion of porous bands increasing toward the top. Within massive delta-front sandstones, porous and tightly chlorite-cemented bands are commonly parallel to sedimentary structures. Distributary channel fills consist of thicker (10 to 20 ft), poorly sorted, upward-fining sandstones. The basal 1 to 2 ft (0.3 to 0.6 m) of the distributary channel fills, where underlain by shales, are tightly cemented with calcite. Porous, quartz-cemented diagenetic facies form 1- to 2-ft-thick (0.3- to 0.6-m-) intervals in the coarsest grained sandstones. The rest of the coarse channel fill is dominated by porous, chlorite-cemented facies. The upper, finer grained portions of the channel fills are tightly cemented with chlorite and calcite. A predictable relationship between depositional facies and diagenetic facies allows extrapolation of the distribution of cements and the resultant reservoir characteristics to uncored wells and intervals with the objective of maximizing recovery of natural gas.

Sedimentology and early diagenesis of the Lower Jurassic, Applecross, Wester Ross

Synopsis The Jurassic strata at Applecross comprise 60 m of mixed siliciclastics and carbonates deposited in a variety of shallow marine environments including shallow carbonate shelf with patch reefs, oolitic shoal, restricted lagoon, beach and siliciclastic mudflats. Vadose cement fabrics, blackened carbonate allochems, evaporite pseudomorphs and plant remains indicate several phases of sediment emergence during deposition. Changes in the source of siliciclastic sediment during deposition are also evident. The complex nature of the sequence is related to interplay of a regional marine transgression across the Hebridean area and localised uplift immediately north of Applecross. Differential offshore subsidence continually modified the topography of the depositional basin altering the hydraulic regime in the Applecross area.

Migration of orogenic fluids through the Siluro-Devonian Helderberg Group during late Paleozoic deformation: constraints on fluid sources and implications for thermal histories of sedimentary basins

Fluid inclusion data and fracture-filling cement fabrics in deformed, but non-overthrusted, carbonate and siliciclastic rocks of the Siluro-Devonian Helderberg Group, central Appalachians, indicate post-cementation, late Paleozoic migration of high pressure, high temperature fluids. Void-filling quartz and calcite cements contain secondary, two-phase fluid inclusions (liquid + vapor) which occur in annealed microfractures. Most melting temperatures (last occurrence of ice) are −20° to −25° C, which correspond to salinities of greater than 22 equivalent wt.% NaCl. Pressure-corrected liquid-vapor homogenization temperatures are 200° to 300 + °C and greatly exceed maximum paleotemperatures (≈150°C) given by conodont CAI values, vitrinite reflectance, illite crystallinity, or temperatures calculated from known sedimentary overburden. Completely cemented sandstone and limestone also have multiple crosscutting trains of secondary hydrocarbon inclusions. Hydrocarbon inclusions occur along deformation microstructures in cements. Some hydrocarbon inclusion trains crosscut cement-filled fractures. The nature of occurrence of the secondary inclusions suggests that fluids moved along intracrystalline deformation microstructures either during or after late Paleozoic deformation. Rare fractures contain transported skeletal grains, “exotic” clasts (lithologies different from wall rock), recemented clasts of fracture-filling cement, and mud. Cement clasts contain solid inclusions of mud and skeletal fragments, and indicate several episodes of particle transport, cementation, and refracturing prior to final fracture filling and cementation. These fracture fills locally crosscut stylolites. Coarse-grained, poorly sorted, “clastic fracture fills” suggest migration of rapidly moving fluids which were capable of transporting clasts through fracture conduits under deep burial conditions. Conduction calculations place upper limits on size and duration of thermal anomalies which were caused by extrabasinal fluids. These calculations indicate that hot fluid migration events most likely were short-lived. Heat transfer by extraformational fluids moving along faults and fractures may have been the mechanism by which thermal anomalies developed in the Helderberg Group. Stratigraphic constraints suggest that likely sources for the hot brines are overthrusted terranes to the east of the study area. Possible stratigraphic horizons which were sources for the overpressured, hot fluids may have been thick Cambrian shales which are important detachment horizons for major, late Paleozoic thrust sheets in the eastern Valley and Ridge province (east of the study area). Fluids may have migrated during thrusting, along thrust faults which ramp upwards from east to west into stratigraphically higher units; thrust faults ultimately die out in Devonian units. Some of the dissolved ions in the secondary two-phase inclusions may have been incorporated when shale-derived fluids moved through Silurian evaporites or Cambro-Ordovician carbonates which contain evidence of cryptic evaporites. Potential source rocks for secondary hydrocarbon inclusions may have been Lower Silurian through Lower Devonian strata, based on Lopatin modeling and assuming that migration occurred during late Paleozoic deformation. This study may help to explain many apparently anomalous geologic phenomena in deformed foreland basin sediments. It also has implications for tracking migration pathways for fluids which facilitate regional scale deformation in fold-and-thrust belts and how “tectonic fluids” may affect the rocks through which they move.

Early Burial Quartz Authigenesis in Silurian Platform Carbonates, New Brunswick, Canada

ABSTRACT Several types of quartz cement and replacive quartz fabrics occur in the Silurian platform carbonates of northern New Brunswick. Petrographic relationships with calcite, pyrite, dolomite, and intraformational erosional clast surfaces indicate that this quartz formed during early burial at depths ranging from the upper few meters to perhaps tens of meters. The quartz cement microtextures suggest that it is a primary precipitate. A close spatial association of fossils and authigenic quartz suggests some kind of organic control on silica precipitation and that the quartz authigenesis was controlled more by molecular diffusion than fluid flow. Several lines of evidence point to bacterial sulfate reduction coupled with oxidation of organic matter as a key process leading to quartz authigenesis, including: 1) documented dominance of this kind of reaction in shallow buried marine sediments, 2) close spatial association of authigenic quartz and pyrite, 3) quartz replacement of gypsum, and 4) experimental evidence that sulfate-reducing bacteria mediate silica precipitation (Birnbaum 1984).

Magnesian calcite cements and their diagenesis: dissolution and dolomitization, Mururoa Atoll

ABSTRACTThe 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.

A Spherulitic Fabric in Selectively Dolomitized Siliciclastic Crustacean Burrows, Northern Kuwait

ABSTRACT In a Quaternary siliciclastic tidal-bar deposit, crustacean burrows have been selectively dolomitized, giving two distinct zones of cement fabrics. The inner, central part of the burrow has a microdolomite fabric. In contrast, the outer wall shows a variety of coarse dolomites ranging from zoned rhombic or quasi-rhombic crystal fabrics to rounded, spherulitic fabrics comprising concentric layers and radial fibrous crystallites. The latter fabric is derived from other, much simpler arrangements, but evidence of replacive features is lacking. The boundary between the two fabric zones is sharp. All evidence points to a primary origin for the spherulitic dolomite. The composition of the dolomite varies from Ca52 Mg48 to Ca57 Mg43, and both fabric zones in a given burrow show variable dolomite compositions along its length. These observations suggest that the composition of the dolomitizing fluids and the kinetics of crystal growth were not uniform during the growth of these cements. It is surmised that fluid-flow conditions and organic matter (very likely related to bacterial activity) within the burrows were probably significant controls of selective dolomitization and cement fabrics.

Stratigraphy and sedimentology of the Great Oolite Group in the Humbly Grove Oilfield, Hampshire

The main reservoir of the Humbly Grove Oilfield comprises variably dolomitic grainstones and packstones representing the Bathonian Great Oolite Group. The Bathonian sequence commences in Lower Fuller's Earth claystones which coarsen upwards into oncolitic claystones and skeletal packstones probably equivalent to the Fuller's Earth Rock. Above is a variable succession of wackestones and thin packstones which have a distinctive sandstone at their base. This sequence is named here the Hester's Copse Formation. The succeeding Great Oolite Limestone is predominantly oolitic and cross-bedded on a variety of scales. It exhibits both coarsening and fining sequences which have locally well-developed capping hardgrounds and burrowed horizons. The Great Oolite Limestone is subdivided into three Members: the lowest (the Humbly Grove Member), and the highest (the Herriard Member) begin with massive shoal oolite deposition, but each then pass upward into more interbedded sequences representing a more transgressive environment. The middle member (the Hoddington) is a thin but widely correlatable wackestone. The overlying Forest Marble commences abruptly in claystones, but there is an upward increase in both the incidence and thickness of discrete oolitic limestones. Both the Great Oolite Limestone and Forest Marble were affected by early fresh-water dissolution and cementation in addition to the localized development of submarine cements. The top of the Great Oolite Group is represented by the Cornbrash. The Lower Cornbrash is a thin micritic limestone while the Upper Cornbrash is a calcareous claystone which passes upwards into the Kellaways Clay. The Bathonian sequence overlies the dolomitic limestones of the Inferior Oolite, the Lower Fuller's Earth claystones being interpreted as a basinal marine mudstone sequence, marking a substantial deepening and transgressive phase at the opening of the Bathonian. These mudstones shoal upwards into the quiet, but photic, water deposits of the Fuller's Earth Rock. The Hester's Copse Formation represents the temporary development of wave-dominated terrigenous shoreface and lagoonal conditions. Renewed transgression established a high-energy, tide-dominated, carbonate shelf upon which the Great Oolite Limestone was deposited as a series of shoal oolites, channels, tidal deltas and spill-overs. Periodic exposure of the carbonate sand-bodies led to the production of early dissolutional and cementation fabrics that post-date (and largely obliterate) submarine cements. The Forest Marble opened with a further phase of deepening, and the temporary establishment of muddy facies. Subsequently discrete tide-dominated ridges and linear channelized oolitic sands prograded into the area. The latest Bathonian is marked by subsidence of the carbonate ramp to the south of the London Platform, the Cornbrash-Kellaways Clay sequence accumulating under progressively deepening waters.

Freshwater-Phreatic Calcite Cementation, Schooner Cays, Bahamas: ABSTRACT

Freshwater-phreatic calcite cementation is an active process on 700 and 2,700 yr-old ooid-sand islands in the Schooner Cays, Bahamas. Cement fabrics and textures indicate a general, four-stage model of pore infilling. (1) The precipitation of isolated, decimicron-sized, rhombohedrons of calcite on grain surfaces forms an incipient circumgranular cement. (2) Continued precipitation enlarges crystal sizes and forms new rhombohedral crystals, resulting in a continuous circumgranular rim of cement. (3) Additional cementation quickly masks the circumgranular fabric, producing a partial pore-filling mosaic. (4) The remaining pore space is occluded with a mosaic of calcite cement. Petrographic evidence for the earlier circumgranular rim of cement is not necessarily apparent afte the last stage of cementation. Empty pores and all four stages of phreatic-zone cementation were observed in the diagenetically immature 700 yr-old rocks, but only stages 2 through 4 were observed in the diagenetically more mature 2,700 yr-old phreatic-zone samples. Cements are distributed homogeneously within each pore at every stage, yet because each pore may proceed through the four stages at different rates, each pore can be at a different stage of infilling. This results in an inhomogeneous distribution of cement between pores during the initial stages of cementation. Recognition of a cement stratigraphy similar to that described here should aid in the identification of freshwater-phreatic diagenesis in ancient carbonate rock sequences. Variability in the amount of freshwater-phreatic cement between pores should be expected and not interpreted as the product of different paragenetic sequences. End_of_Article - Last_Page 241------------