Pressure Solution Features Research Articles

Role of pressure solution in the formation of bedding-parallel calcite veins in an immature shale (Cretaceous, southern UK)

AbstractBedding-parallel fibrous calcite veins in black shales (Cretaceous, southern UK) were investigated using a combined field, stable isotopic geochemistry, petrographic and crystallographic method to examine their formation mechanism. Calcite veins occur in all shale beds and are most abundant in the bituminous shales of the Chief Beef Beds. The calcite fibres in these veins exhibit either an antitaxial fibre growth with curvy stylolites as the median zone, or a predominantly syntaxial, upwards growth. The calcite veins range from –0.49 to 1.78‰ of δ13C values, and –6.53 to –0.03‰ of δ18O values, which are both similar to those of their host shales. Our petrographic observations demonstrate that subhorizontal and interconnecting microstylolite networks commonly occur within the calcite veins. Equant calcite grains in the median zones exhibit indenting, truncating and also interpenetrating grain contacts. It is interpreted that the fibrous calcite veins were sourced by neomorphic calcite from their host shales, with evidence from the δ13C signatures, pressure-solution features (stylolites, microstylolites and grain contact styles) and embedded fossil ghosts within the veins. The diagenetic fluids, from which calcite was precipitated, were a mixing of the original seawaters and18O-depleted meteoric waters. Development of bedding-parallel calcite veins is considered to have been enhanced by pressure solution as a positive feedback mechanism, which was facilitated by the overburden pressure as the maximum principal stress. Calcite fibres, with a predominant subvertical c-axis orientation, exhibit a displacive growth in porous shales and a replacive growth at vein-limestone contacts. This study highlights the critical role of pressure solution in the formation of bedding-parallel calcite veins during burial and diagenesis of immature black shales.

Microstructures and their implications for faulting processes –Insights from DGLab core samples from the Gulf of Corinth

We have examined microstructures, mineralogical composition, geochemical alteration, and texture of four selected fault rock samples from the Deep Geodynamical Laboratory (DGLab) Gulf of Corinth project using optical microscopy, cathodoluminescence microscopy (CL), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron X-ray diffraction measurements. The fault core is composed of red and gray clayey gouge material and surrounded by a damage zone of brecciated limestones. Pressure solution features, calcite veins and calcite clasts in the breccia and gouge material attest the presence of paleo-fluids and fluid-driven mass transfer during deformation. Differences in CL-colors between the matrix and calcite vein cement and inside the vein cement suggest repeated infiltration of fluids with different composition from various sources (formation water and meteoric water). Twin lamellae densities estimated in calcite veins are used as paleo-piezometer. The deduced differential stress is ∼140 ± 70 MPa for the older vein generation and appears to be higher than stress for the youngest veins (45 ± 23 MPa). In spite of the relatively small clay content in both samples, newly formed clay minerals have been observed in gray as well as red clayey gouge material. Differences between gray and red clay gouge material are found in fault rock composition, porosity and clay fabric. The proportion of chlorite in the red gouge is significantly less than that in the gray gouge whereas the initial porosity is significantly higher than in the gray gouge material. The detection of a well-oriented clay fabric in red clay gouge samples is unique in comparison to other major fault zones.

Syn-thrusting polygonal normal faults exposed in the hinge of the Cingoli anticline, northern Apennines, Italy

The Cingoli arcuate anticline is part of the Apennines fold-thrust belt in Italy. The anticline involves sedimentary carbonate strata generally affected by syn-thrusting contractional structures such as bed-normal pressure solution seams, folds, and reverse faults. An exception is constituted by an outcrop in the anticline hinge, where sub-horizontal carbonate and chert beds are affected by joints and intraformational short normal faults. These faults are poorly-systematic and conceivably polygonal in map view. They cut through the carbonate beds while usually stop against the chert layers that are bent and extended along the faults themselves. At the fault tips, the displacement is generally transferred, via a lateral step, to an adjacent similar fault segment. The fault surfaces are often characterized by slickolites, greenish clayey residue, and micro-breccias including chert and carbonate clasts. Fault displacement is partly or largely accommodated by pressure solution. The faults, in effect, are usually accompanied by bed-parallel pressure solution seams in the two contractional quadrants located at the present or past fault tips. The pressure solution features fade away departing from the faults. This evidence and others are analytically explained with fault tip stress distributions. The faults are interpreted as polygonal normal faults syn-tectonically (syn-thrusting) nucleated in response to multi-directional stretching processes occurred at the Cingoli triple-folded anticline extrados. The faults then grew through a four-stage process: (1. stop) the faults stopped at the competent chert beds; (2. shrink) faulting produced shrinkage (pressure solution) of carbonate beds at the fault compressive tips; (3. shrink and step) the faults stepped laterally at the competent chert beds; (4. shatter) the chert beds were shattered along the fault surfaces. The case presented constitutes the first reported one of syn-thrusting non-diagenetic polygonal normal faults.

Pressure solution inhibition in a limestone–chert composite multilayer: Implications for the seismic cycle and fluid flow

Pressure solution seams (PSSs) are frequent features in carbonate rocks undergoing tectonic shortening. In particular, pervasive, anticline-axis-parallel, bed-normal PSSs are known to develop during layer-parallel-shortening of (marly) carbonate rocks in fold-thrust belts. These pressure solution features can impact subsequent fracture development, fluid circulation, and strain localization including the seismic cycle. It is here demonstrated that the occurrence of frequent and continuous chert layers may strengthen a limestone sequence and inhibit pressure solution under layer-parallel-shortening. Field observations and laboratory determinations are reported from marly limestone with continuous chert layers of the Scaglia Fm. (Cingoli anticline, northern Apennines, Italy) exhumed from a depth of c. 1km. In these outcrops, bed-normal solution seams do not occur or they occur only where infrequent chert layers have been shortened by small thrusts. In analogy with laminae-reinforced composite materials, a model is developed explaining the field observations with the strengthening effect of chert in the chert–limestone composite multilayer. During layer-parallel-shortening, the composite multilayer deforms under equal strain boundary conditions. In this situation, the tectonic load is mostly supported by the stiff and frequent chert layers and the strain of the whole chert–limestone composite remains in the elastic field, so that pressure solution seam development is prevented in the limestone beds. Our model may be applied down to a depth of a few kilometers in the upper crust that is relevant for the seismic cycle and fluid flow.

Diagenetic Implications of Stylolitization In Pelagic Carbonates, Canterbury Basin, Offshore New Zealand

Abstract Stylolites are irregular discontinuity surfaces that are thought to result from localized stress-induced dissolution during burial or tectonic compression. The genesis of stylolites and the controls on stylolitization are still debated, and the interplay between stylolitization, generation of carbonate-rich fluids, diagenetic fluid flow within fractures and matrix, cementation, and porosity modifications is complex. All of these processes have important diagenetic effects potentially altering the intrinsic properties of the host rock, with implications for hydrocarbon exploration and water resources in aquifers. We investigate the process of stylolitization by a macroscopic, petrographic, and geochemical study of pressure-solution features in Eocene to early Oligocene limestones in cores from the Integrated Ocean Drilling Program (IODP) Hole 317-U1352C (Canterbury Basin). The results indicate correlations among stylolite amplitude, stylolite density, and siliciclastic content of the host rock. These relationships are interpreted to suggest that siliciclastic content in carbonate rocks increases heterogeneity, which in turn impacts stylolite nucleation. Moreover, the geochemical data support that clay along the stylolite is not authigenic, but a relict from impurities in the limestone host rock. The statistical approach of stylolite spacing used in this study, which is different than previous studies, reinforces the model of random occurrence of stylolites in (carbonate) lithologic units. This study shows that local stresses in tectonically passive areas may allow the formation of rare oblique stylolites with peaks perpendicular to the stylolite plane. Estimation of the amount of limestone dissolved during stylolitization (minimum 7% to 12% of the depositional limestone) and the volume of sparite in the host rock (up to 1.6% of the compacted limestone) suggests that the pressure-solution fluids cemented the micropores, reducing porosity to about 10% (from a common porosity of about 40% in mechanically compacted chalk). This study thus highlights the importance of microporosity as a sink for burial cements, an observation difficult to make in thin-section. Analysis of stylolite infills at Site U1352 shows no evidence that the stylolites acted as conduits for diagenetic fluids, unless if the fluid was host-rock buffered and thus its chemistry indistinguishable from that of the host rock. The link between siliciclastic content in carbonate rocks and the spacing and amplitude of stylolites improves understanding of burial compaction processes. Predicting the morphology and spacing of stylolites in the subsurface can be used to identify baffles to cross-stylolite fluid flow or cementation (of microporosity) influencing heterogeneity of petrophysical properties in (subsurface reservoir) host rocks.

Anisotropic scaling of tectonic stylolites: A fossilized signature of the stress field?

Vertical stylolites are pressure solution features, which are considered to be caused by horizontal tectonic loading, with the largest principal compressive stress being (sub‐)parallel to the Earth's surface. In the present study we analyze the roughness of such tectonic stylolites from two tectonic settings in southern Germany and northeastern Spain, aiming to investigate their scaling properties with respect to the stress during formation. High‐resolution laser profilometry was carried out on opened stylolite surfaces of nine samples. These data sets were then analyzed using one‐ and two‐dimensional Fourier power spectral approaches. We found that tectonic stylolites show two self‐affine scaling regimes separated by a distinct crossover length (L), as known for bedding parallel stylolites. In addition, tectonic stylolites exhibit a clear in‐plane scaling anisotropy that modifies L. Since the largest and smallest crossover lengths are oriented with the sample vertical and horizontal directions (i.e., σ2 and σ3) and L is a function of the stress field during formation as analytically predicted, we conclude that the scaling anisotropy of tectonic stylolites is possibly a function of the stress field. Knowledge of this crossover‐length anisotropy would enable the reconstruction of the full three‐dimensional stress tensor if independent constraints of the depth of formation can be obtained.

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Microfabric analyses applied to the Witwatersrand gold- and uranium-bearing conglomerates: constraints on the provenance and post-depositional modification of rock and ore components

Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle–ductile crystal–plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.

Origin of Stylolites in Upper Permian Zechstein Anhydrite (Gorleben Salt Dome, Germany)

ABSTRACT Stylolites in evaporites are very uncommon. In the Gorleben Zechstein salt dome, stylolite horizons have been found in the so-called Banderanhydrit (z3HA11) at the top of the Zechstein Hauptanhydrit (Main Anhydrite). These pressure-solution features seems to be related to the presence of thin magnesite-clay interlayers. Five principal stylolite morphologies can be distinguished in the core sections studied. The orientation of the sutured seams is generally bedding-parallel with the stylolite axes vertical to the bedding surface. This demonstrates that pressure-solution features clearly predate the formation of the salt dome and that there is no relation between their formation and halokinesis. Solution rates of up to about 26% have been calculated by measuring maximum stylolite amplitudes in certain core intervals. Considering the bedding-parallel occurrence of the stylolites and the fact that the stylolite seams are crosscut by anhydrite crystals that we interpret as pseudomorphs after gypsum, it must be concluded that the stylolites formed before the dehydration of gypsum to anhydrite was completed. The lower parts of the Hauptanhydrit were excluded from these processes because they had already been converted to anhydrite by the overburden of overlying Zechstein units and by ascending compaction fluids of the underlying Zechstein 2 Salt. We assume that the dehydration of gypsum to anhydrite was completed before the end of Zechstein sedimentation and that the formation of stylolites thus must be attributed to the same time interval.

The Diagenetic Role of Brittle Deformation in Compaction and Pressure Solution, Etjo Sandstone, Namibia: A Discussion

Previous articleNext article No AccessDiscussion and ReplyThe Diagenetic Role of Brittle Deformation in Compaction and Pressure Solution, Etjo Sandstone, Namibia: A DiscussionRussell J. MaharajRussell J. Maharaj Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 104, Number 4Jul., 1996 Article DOIhttps://doi.org/10.1086/629845 Views: 2Total views on this site Copyright 1996 The University of ChicagoPDF download Crossref reports no articles citing this article.

Models of Porosity Formation and Their Impact on Reservoir Description, Lisburne Field, Prudhoe Bay, Alaska

The Lisburne field at Prudhoe Bay, Alaska, produces from shelfal carbonates in the Pennsylvanian Wahoo formation. Four major factors control reservoir behavior: (1) depositional stratification, (2) a fractured, permeable subunconformity alteration zone (SAZ), (3) multiple episodes of porosity formation, and (4) faulting. This paper is the first written description of facies, cyclicity, and diagenetic processes as they apply to porosity formation and reservoir modeling in the Lisburne field. Successive depositional cycles in the Wahoo formation pass from ooid/skeletal grainstones deposited in shoal complexes to oncolitic packstones and skeletal/peloidal wackestones formed in restricted lagoonal environments. Geochemical data and crosscutting relationships between porosity and unconformities, pressure solution features, fractures, and faults provide evidence of three distinct episodes of porosity formation. Earliest porosity is probably due to periodic, localized exposure during the Pennsylvanian. A second stage of porosity is associated with shallow-burial dolomitization that probably began during Permian-Triassic subaerial exposure. Reservoir quality in dolomites varies with the degree of neomorphic recrystallization. The third stage began in the Cretaceous and lasted in o the Tertiary, and is associated with final burial and hydrocarbon maturation. This burial dissolution event also opened existing fault systems, creating a complex reservoir. Most Lisburne field effective porosity is of late-burial origin due to either dolomitization or dissolution. Faults and the SAZ act as giant collectors of oil from low-permeability matrix, and they have reduced the number of wells needed for field development. Faults also complicate waterflood implementation and maintenance of uniform reservoir pressure. Field studies such as these demonstrate the importance understanding the diagenetic history of a reservoir can have for field management and development planning.

Compaction and Porosity Prediction in Chert-Rich Sandstones and Conglomerates

Chert-rich sandstones and conglomerates are volumetrically important oil and gas reservoir units in western and northern Canada and in Alaska. Unlike quartzofeldspathic sandstones, chert-rich sandstones are highly susceptible to intergranular pressure solution resulting in dramatic loss of intergranular volume (IGV) and porosity with burial. Thin-section petrographic data were used to develop a suite of IGV-decline curves for chert-rich sandstones and conglomerates as the basis for porosity prediction algorithms. The samples studied range in composition from chert-quartz litharenites (Viking Formation, Paddy-Cadotte and Falher members, western Canada basin) to chert-rich litharenites containing appreciable feldspars and volcanic grains (Clearwater and Belly River formations, western Canada basin, and Reindeer Sequence, Mackenzie Delta). Physical characteristics of chert, such as the degree of microcrystallinity or the presence of impurities and micropores, in part, determine chert's susceptibility to dissolution. Where chert and quartz grains are compacted together, chert is typically penetrated by quartz. Pressure-solution features of chert, such as pocked or dimpled clasts and microstylolitic textures, can be observed in core and cuttings, thin sections, and SEM samples. Early cementation in chert-rich sandstones can limit the effects of compaction. For example, quartz overgrowths in the Viking Formation inhibit later intergranular pressure solution. Formation inhibit later intergranular pressuremore » solution. In chert-rich sandstones and conglomerates, porosity and permeability prediction is complicated by cement timing, cement type and distribution, degree of leaching, and grain textural variety.« less

Effects of Pressure Solution on Petroleum Migration in the Miocene Monterey Formation, Southern San Joaquin Basin, California: ABSTRACT

Pressure-solution features are common in the Monterey Formation and occur as bedding-parallel stylolites and solution cleavages at high angle to bedding. Both structures are visible in hand specimen, but they occur abundantly at a microscale. Petrographic study of siliceous rocks of the Monterey Formation shows that stylolites and solution cleavages provide horizontal (bedding parallel) and vertical avenues for petroleum migration respectively. Together with tectonic fractures, they are major migration pathways of petroleum. Pressure solution is a dissolution process that involves volume reduction of the rock and concentration of insoluble rock components, including organic matter, along pressure-solution features. Previous geochemical studies have shown the concentration of organic matter along stylolites, but no previous work has demonstrated it petrographically. Petrographic examination of siliceous rocks in the Monterey Formation under transmitted, reflected, and fluorescent light, shows the occurrence of liptinite and inertinite along stylolites. Petrographic evidence suggests that removal of rock mass by pressure solution can concentrate organic matter and help the development of a three-dimensional organic network believed to play an important role in primary migration.

Significance of Stylolite Development in Hydrocarbon Reservoirs with an Emphasis on the Lower Cretaceous of the Middle East

Growth of stylolites and related pressure-solution features may adversely affect the continuity of carbonate reservoirs by producing barriers or impedences to fluid flow. The impact of stylolite development on reservoir performance, however, may vary greatly from one part of a reservoir to another. Therefore, it is important to know the distribution and the permeability of stylolites for development of an effective program of reservoir management. The variable impact of stylolite development is illustrated through analysis of a reservoir case history. In this example, stylolite frequency and cumulative stylolite amplitude increase from the crest to the flanks of the anticlinal closure which constitutes the field. These trends are largely controlled by: 1) the pattern of stress concentration developed during growth of the anticlinal structure and by 2) the inhibition of pressure solution by hydrocarbons along the anticlinal crest. Although stylolites occur throughout the reservoir, three principal zones of stylolite development are of concern with regard to reservoir management. Of the three, only the uppermost zone (D 1) is a significant barrier to fluid flow. Variation in permeability among the main stylolite zones probably is related to the timing of stylolite formation with respect to hydrocarbon entrapment. The impermeable stylolite zone (Dl) formed largely before hydrocabon entrapment. In this case, abundant crystal nucleation sites are present immediately adjacent to the pressure-solution feature allowing local precipitation of calcium carbonate derived from dissolution along the pressure-solution surface. The remaining stylolite zones (02 and 03) largely developed during or after hydrocarbon entrapment. Hydrocarbons inhibit precipitation of calcium carbonate by coating many favorable nucleation sites within the reservoir. Thus, the bulk of the calcium carbonate dissolved at pressure-solution surfaces can be trailsported away from the stylolite zone prior to precipitation. Localized cementation and permeability reduction, therefore, will not occur adjacent to the stylolite zone. Consequently, stylolites developed largely after hydrocarbon entrapment, in this example, do not constitute significant barriers to vertical fluid flow.

Distribution and Permeability of Stylolite-Bearing Horizons Within a Lower Cretaceous Carbonate Reservoir in the Middle East

Summary Stylolites and related pressure-solution features occur in many carbonate reservoirs. They are seams of insoluble residue that record the removal of rock by pressure dissolution. Stylolites may limit reservoir continuity by forming barriers or impedances to fluid flow. This core-based study documents both the distribution of stylolites and the variable impact of stylolite-bearing horizons on reservoir continuity for a Lower Cretaceous carbonate reservoir in the Middle East.

Paleomagnetic and oxygen isotopic evidence for long term diagenesis in radiolarian chert, Pindos Mountains, Greece

Paleomagnetics was used in an attempt to improve chronostratigraphy in the Middle and Upper Jurassic radiolarian chert and siliceous mudstone of the Pindos Zone, Greece. Remanent magnetism studies showed strong magnetic intensity but scattered orientation. Orientation of some pressure solution features in the radiolarities indicates their formation under horizontal pressure, a condition which in Pindos geologic history would only have occurred during early Cretaceous or Cenozoic tectonics, indicating very late diagenesis in these parts of the section. From time of deposition to later time at which diagenesis can be documented, the Earth's magnetic field would have reversed many times. Remagnetization during solution-precipitation steps of silica diagenesis could complicate the rock magnetics. Oxygen isotopic and major element analyses of radiolarite lithologies show a systematic variation of rate of silica diagenesis in different host lithologies, thus solution-precipitation would occur at widely differing times throughout the section lithologies. If the dissolution of the silica cement were physically to free magnetic material from an earlier orientation, the result could be a partial shift toward alignment with the ambient magnetic field. Alternatively, complete reorientation of particles could have occurred at varying times in different parts of the section as a function of host lithology. During the northward movement and clockwise rotation of the Apulian subplate (including Pindos) these different lithologies could completely reorient during different stages of silica diagenesis, locking the orientation of iron magnetic moments into alignment with the ambient magnetic field at time of precipitation to result in a strong intensity but scattered orientation of Pindos rock magnetics.

Diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon, Nevada, U.S.A.

A model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper Paleozoic Golconda allochthon of Nevada, U.S.A. Numerous thrust faults slice the cherts into packets, each with a unique set of internal structures. Fold geometries, boudin profiles, pressure-solution features, etc. vary from packet-to-packet and layer-to-layer. Analogous variations in the diagenetically mixed siliceous sediments of the Miocene Monterey Formation. California, suggest the cherts of the Golconda allochthon were similarly mixed when they were deformed. Radiolarian sediments composed of biogenic silica (opal-A) developed ductile structures and pressure-solution features because of their high porosity, weak lithification and the high solubility of disordered silica. Lowporosity, strongly-lithified, relatively-insoluble quartz cherts, the final product of silica diagenesis, deformed in a brittle fashion and developed fewer pressure-solution features. Diagenetically intermediate CT-cherts and CT-porcelanites exhibited transitional behavior. Mixed diagenetic zones produced structures with layer-by-layer changes in structural style. The dehydration of opal-A and opal-CT helped create the excess pore fluid pressures responsible for thrust faults, hydraulic fractures, dilation breccias and elastic intrusions. The presence of ductile structures and numerous pressure solution features in the oldest cherts of the Havallah sequence suggests that these cherts were deformed while diagenetically immature, possibly within a long-lived upper Paleozoic accretionary prism.

Petrography and carbonate chemistry of some Dalradian dolomitic metasediments: preservation of diagenetic textures

The Bonahaven Formation of northern Islay, Scotland, despite having undergone high-pressure greenschist facies metamorphism, contains many relics of its diagenetic history. Early diagenetic dolomite textures, calcite and clay mineral cements are well preserved locally. Burial diagenetic compactional and pressure solution features are recognizable together with quartz and feldspar overgrowths and variable styles of replacement by and recrystallization of dolomite, calcite, pyrite, quartz and albite. The imprint of deformation and metamorphism is variable, probably as a result of differences in the degree of pressure solution and the CO 2 , content of pore fluid. Typically, some solution-transfer phenomena occur, accompanied by growth of cleavage-defining phengite and sporadic phlogopite. Locally, abundant phlogopite or biotite is present in lithologies with fully developed metamorphic textures. Dolomite shows good preservation of pre-metamorphic textures and minor element zonation, but calcite does not. Metamorphic re-equilibration was insufficient to allow the derivation of maximum temperatures by calcite–dolomite geo-thermometry.

Petrology of Middle Jurassic Twin Creek Limestone, Lincoln and Sublette Counties, Southwestern Wyoming: ABSTRACT

End_Page 298------------------------------Cluster analysis was used to delineate lithofacies in the Twin Creek Limestone of Middle Jurassic age in the Tunp, Salt River, and Wyoming Ranges in southwestern Wyoming. Subjective appraisal of the petrographic data produced lithofacies similar to that created by cluster analysis. Modern carbonate environments and their ancient analogs were compared with information obtained from field study and petrographic analysis of samples of the Twin Creek Limestone to delineate environments of deposition, paleogeography, and diagenetic history. Six major lithofacies were recognized: (1) carbonate mudstone, (2) packstone-grainstone, (3) fossiliferous wackestone, (4) terrigenous mudstone, (5) sandstone, and (6) carbonate mudstone breccia. These lithofacies were deposited in a variety of environments, including outer shelf platforms (carbonate mudstone and fossiliferous wackestone), oolitic sand belts (packstone-grainstone), open to restricted lagoons (carbonate mudstone, fossiliferous wackestone, and terrigenous mudstone), tidal flats, and supratidal environments (terrigenous mudstone, sandstone, and carbonate mudstone breccia). The Twin Creek epeiric seaway experienced two major transgressions (early Bajocian and late Bathonian-early Callovian) and two regressions (early Bathonian and middle Callovian). Lateral migration of t e adjacent facies occurred in response to these changes in sea level. Eogenetic diagenetic features include minor compaction, micritization, coarse fibrous rim cementation, granular cementation, syntaxial rim cementation, and silicification of carbonates. These features were produced in environments ranging from freshwater phreatic to marine phreatic. Mesogenetic diagenesis was characterized by pressure-solution features and neomorphism. Telogenetic features are limited to calcite vein fillings and oxidation coating on carbonate and detrital grains. End_of_Article - Last_Page 299------------

Crystallographic Influences on Pressure Solution in a Quartzose Sandstone: ABSTRACT

The solubility of quartz differs with crystallographic direction. A universal stage was used to measure the orientations of the optic axes and contact planes of 160 pairs of quartz grains in the Bromide Formation (Simpson Group) of Oklahoma. These quartz grains exhibit long, sutured, and concave-convex contacts. Results indicate that the geometry of a pressolved contact is independent of the crystallographic orientation of opposing grains. However, given a concave-convex contact, the optic axis of the concave grain tends to lie at a higher angle to the contact plane than the optic axis of the convex grain. We conclude that the extent and type of pressure-solution contacts in quartzose sandstones are not significantly influenced by crystallographic orientation. Other facto s, such as grain size and clay content, are probably more important in controlling the pressure-solution features. Geometric etch pits, which form at the point of emergence of crystal defects, were produced by hydrothermally etching quartz crystals, quartz sand, and quartzose sandstones. The abundance, nature, and distribution pattern of crystal defects inherited from source rocks might be more important factors in affecting pressure solution of quartz grains than differences in quartz solubilities arising solely from variations in Si-O bond strengths. The extent of etch-pit formation on quartz cement may also serve as a qualitative indicator of the dissolved silica saturation in pore fluids. End_of_Article - Last_Page 265------------