Pressure Solution Seams Research Articles

Reactivation of pressure-solution seams by a strike-slip fault-sequential, dilational jog formation and fluid flow

This work examines how older pressure-solution seams become sheared or reactivated by slip because of movement on a younger fault segment. The reactivation then leads to the creation of secondary structures. These are followed by further changes in the local stress-strain field that result in slip reactivation on the secondary structures and in the creation of third-order structures. This sequence of deformation reflects and reveals transient changes in the stress-strain field along the margins of the fault during active slip on the fault. The reactivation may lead to enhanced rock permeability and/or porosity that allow for temporary periods of fluid movement. Thus, we believe that this serves as an important model to contribute to the understanding of movement of fluids such as oil and gas around active faults.

Influence of depositional setting and sedimentary fabric on mechanical layer evolution in carbonate aquifers

Carbonate aquifers in fold–thrust belt settings often have low-matrix porosity and permeability, and thus groundwater flow pathways depend on high porosity and permeability fracture and fault zones. Methods from sedimentology and structural geology are combined to understand the evolution of fracture controlled flow pathways and determine their spatial distribution. Through this process bed-parallel pressure-solution surfaces (PS1) are identified as a fracture type which influences fragmentation in peritidal and basinal carbonate, and upon shearing provides a major flow pathway in fold–thrust belt carbonate aquifers. Through stratigraphic analysis and fracture mapping, depositional setting is determined to play a critical role in PS1 localization and spacing where peritidal strata have closer spaced and less laterally continuous PS1 than basinal strata. In the peritidal platform facies, units with planar lamination have bed-parallel pressure-solution seams along mudstone laminae. In contrast, burrowed units of peritidal strata have solution seams with irregular and anastamosing geometries. Laminated units with closely spaced bed-parallel solution seams are more fragmented than bioturbated units with anastamosing solution seams. In the deeper-water depositional environment, pelagic settling and turbidity currents are the dominant sedimentation processes, resulting in laterally continuous deposits relative to the peritidal platform environment. To quantify the fracture patterns in the basinal environment, mechanical layer thickness values were measured from regions of low to high bed dip. The results define a trend in which mechanical layer thickness decreases as layer dip increases. A conceptual model is presented that emphasizes the link between sedimentary and structural fabric for the peritidal and basinal environments, where solution seams localize in mud-rich intervals, and the resulting pressure-solution surface geometry is influenced by sedimentary geometry (i.e., stacked fining upward cycles, burrows, planar laminations). In both facies types, laterally continuous PS1 can behave as mechanical layer boundaries. As layer-parallel slip increases to accommodate shear strain in the fold–thrust belt, more PS1 behave as mechanical layer boundaries.

Pressure solution in chalk

Pressure-solution residues in the Cretaceous to Paleocene chalk from the North Sea area were studied to understand the pressure-solution process and fluid flow. Residue seams reach lengths and thickness of more than 800 and 0.15 m (2625 and 0.5 ft), respectively. Seams can present significant barriers to fluid flow because they have nannodarcy permeability and hold back large pressure differentials. Stylolite amplitudes decrease when they merge, making it difficult to quantify volume loss. Measuring amplitudes allows volume-loss estimates using a new dissolution ratio method. The sum of the thicknesses of all residues is multiplied by the maximum amplitude-to-residue thickness ratio. On average, 30 mm (1.2 in.) of chalk had to be dissolved to produce a 1-mm (0.04-in.)-thick residue. The volume loss of chalk from Flamborough averages about 50% and is 60% in the Machar oil field, United Kingdom Central Graben. The minimum fluid volume required to produce the volume loss in 1 unit volume of preserved chalk on the Machar field is estimated to be 8000 unit volumes. This and the stable isotopes of vein carbonates imply that the pressure-solution process involved an open-fluid system. High illite content in mixed layered illite-smectite and high illite crystallinity of clays in the pressure-solution seams in the Machar reservoir indicate high diagenetic maturity, with implied temperatures in the range of 200–360C (392–680F), which is greater than the present-day reservoir temperature of 85C (185F). The anomalously hot temperatures are interpreted to be produced by deep mineralizing fluids transported from the Central Graben. The lack of abundant veining in the chalk and the large fluid volumes involved in the chalk dissolution suggest that this was an open pressure-solution system.

Origin of Structures and Textures of some Kurdistan Marbles as Inferred from Sedimentary Ancestor Structures, NE-Iraq

The Quarries of Iraqi Kurdistan metamorphic marbles (true marble) are distributed along a belt near and parallel to the Iranian border in northeastern Iraq. These marbles associated with low-grade pelitic and calc-schist (green schist facies) and phyllite. They are formed by regional metamorphism of different types of sedimentary parent rocks in an environment not exceeded that of green schist facies (400 C and 8kb). The pelitic equivalent of the Kurdistan calcitic marble contain chlorite, albite, muscovite and biotite.In the latter years, their economic value is rapidly increased as their polished slabs extensively used for decoration purpose. The beautiful features of these marbles are consisting of sophisticated mesoscopic structures of different shapes, size and colors originated from plastic flowage and mixing of different composition and impurities reflecting either the original depositional or diageneticallyintroduced materials. Before plastic flowage they suffered from burial and tectonic fracturing which later filled with spary calcite. The origin of the marbles is analyzed according to the stage of paragenesis of the structures such as foliation, banding (layering), flow structure. The field and lab studies proved that they are all retumed to relict of primary and secondary (Nagenetic) sedimentary sttuctures. The marbles suffered from various degrees of tendency toward homogenization as the grade of regional metamorphism progressed, this reflected by mixing of foreground and background together. In addition to diffused boundary of contrasting components. Although they resemble the banded and augen gneisses but no ones formed by metamorphic differentiation. Most of the precursor structures are related to pressure solution seams and stylolites, which are combined with the triangle of Walness which deal with solution seams. Other structures are returned to lamination of the sedimentary parent rocks. Finally the probable stratigraphic unit as the original parent rocks is discussed.

Finite element modelling of the evolution of pressure solution cleavage

Deformation by ‘pressure solution’ (or ‘water-assisted stress-induced diffusion transfer’) is modelled by incorporating volume transfers — driven by variations in mean stress and competency contrast — into a finite element code for viscous deformation. The model assumes a two-component, quartz–mica mixture with a composition-dependent rheology in which the viscosity of an element is solely dependent upon the volume fraction of the mobile quartz component. Both quartz-rich and mica-rich compositions are given a high competency, with a minimum viscosity at some intermediate fraction. The model is able to reproduce many structures observed in natural tectonites: propagation and coalescence of pressure solution seams, formation of microlithons and anastomosing cleavage, and, more generally, tectonic segregation.

Characterization of Opening Mode Fracture Systems in the Austin Chalk

ABSTRACT The Upper Cretaceous Austin Chalk is a low-permeability, fractured reservoir that has been the target of numerous horizontal wells in Texas. For wells to be successful, parameters such as well-bore azimuth, target horizon, and length must be optimized. Furthermore, information on fracture height and the storage volume potentially connected to the well bore is required. Two case studies are presented in which the height, aspect ratio, and vertical persistence of opening-mode fractures in the Austin Chalk have been examined, together with previously known attributes of fracture population attributes of orientation, aperture, effective aperture, spacing, aspect ratio, fracture fill, and fracture permeability. The studies are of an outcrop of the upper Austin Chalk near Waxahachie, North Central Texas (Grove Creek), and of a vertical segment and two horizontal laterals of a core in the Atco Member in Frio County, Pearsall Field (Kinlaw core). Large, potentially open fractures are commonly clustered, the distance between clusters ranging from ~1 to ~50 m. Aperture-size distributions follow power laws, and spacing-size distributions are negative-logarithmic or lognormal. The aperture size at which fractures are open to fluids is variable (0.14 to 11 mm). Fracture permeability, which is scale dependent, has been determined at 7.1 D (for 18 m of lower Austin Chalk core) and 286 D (for 300 m of upper Austin Chalk outcrop). Fractures may terminate at chalk-marl contacts, tip out within the marl, or pass through the marl layers. Fracture height is governed by truncation effects at marl horizons and pressure-solution seams, which in turn partly depend on fracture aperture and thickness of the inhibiting layer.

Compaction in halite-cemented carbonates - the Dawson Bay Formation (Middle Devonian) of Saskatchewan, Canada

Halite-impregnated carbonates in the Dawson Bay Formation of Saskatchewan lie between beds of halite and are buried to a depth of 1 km. They exhibit two different diagenetic styles – some resisted compaction and had high pre-salt porosities; others contain compaction-broken fossils and pressure-solution seams. The uncompacted rocks, together with the difficulty of explaining how halite cement could enter the Dawson Bay after overlying bedded halites were deposited, suggest that halite cementation occurred early with only a few tens of metres of overburden. Early diagenetic compaction is suggested by the presence of unbroken, displacive skeletal halite crystals, which cross-cut compaction structures, and by the difficulty of explaining how (1) later compaction could occur in halite-cemented rocks and (2) how pore-fluids could be expelled after surrounding rocks lost their permeability. The organic-rich nature of many carbonates may explain why compaction was both early and extensive, but this explanation fails to explain how similar compaction developed in horizons with lower organic contents. Chemical compaction may also have been enhanced by aragonite dissolution during seawater evaporation or brine dilution. Early chemical compaction in Dawson Bay carbonates indicates that compaction in other carbonates need not signify deep burial diagenesis; neither can compaction be used indiscriminately to identify other diagenetic events as being of deep burial origin. Early halite cementation, as in the Dawson Bay Formation, preserves carbonates at early diagenetic stages and may thus preserve geochemical information unmodified by later diagenesis.

Pre-Acadian copper mineralization in the English Lake District

The Ordovician sedimentary and igneous rocks of the English Lake District host a widespread suite of epigenetic metalliferous veins dominated by copper sulphides with abundant arsenopyrite, pyrite and accessory galena and sphalerite. New field and microstructural evidence from examples of this suite at Coniston, Wasdale, Honister, Newlands and Borrowdale shows that the veins were strongly cleaved during the Early Devonian (Emsian) Acadian orogenic event. The principal evidence includes the continuity of wall-rock cleavage fabrics with pressure solution seams in the veins and consistently orientated cleavage through enclosed, rotated wall-rock fragments and chloritic mats. There is also widespread complex intracrystalline deformation in quartz, cataclasis of arsenopyrite and pyrite, fracturing and/or buckling of bladed hematite, and growth of quartz or mica-fibre strain fringes. Chalcopyrite was partially or totally remobilized, enabling it to migrate along quartz crystal boundaries, and invade brecciated pyrite. Previous K–Ar Early Devonian age determinations for the mineralization are considered to have been reset. The pre-Acadian age of this mineralization, its style and relationship to the volcanic rocks permits a genetic link with the final phases of Caradoc magmatism.

The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow

Abstract Pressure solution plays a major role in fault initiation and development in carbonate rocks. Strike-slip faults in the Triassic and Jurassic limestones of Somerset, UK, initiated as en echelon extension fractures, which became linked by pressure solution seams. Shear occurred along the pressure solution seams as the bridges between the veins rotated. The linked vein-pressure solution seam systems developed into pull-aparts and eventually into through-going faults. Normal fault planes in the Cretaceous Chalk at Flamborough Head, Yorkshire, UK, commonly have the pitted appearance of slickolites. Phyllosilicates are often concentrated along faults in Chalk, the thickness of the phyllosilicate gouge being proportional to fault displacement. The enrichment of phyllosilicates along the faults is due to pressure solution rather than simply to phyllosilicate smear. Pressure solution can be concentrated at the contractional quadrants of faults, particularly where there is a contractional overstep onto an adjacent fault. Metre-scale oversteps between strike-slip faults in Somerset often have pressure solution seams, while contractional oversteps and bends at Flamborough Head are accommodated by compaction of beds, with pressure solution apparently being important. The concentration of phyllosilicates by pressure solution can hinder fluid flow along and across faults in carbonate rocks. A high density of pressure solution seams between overstepping faults can effectively link these faults, increasing both the effective length of the barrier to fluid flow and the fault seal potential.

Barite-hyalophane sulfidic ores at Rozna, Bohemian Massif, Czech Republic; metamorphosed black shale-hosted submarine exhalative mineralization

Strata-bound barite-hyalophane sulfidic orebodies recently discovered at Rozna (in the Moldanubian zone of the Bohemian Massif, Czech Republic) are hosted within the high-grade metamorphosed complex composed of migmatitic biotite or sillimanite-biotite gneiss and amphibolite, with rare intercalations of marble and calcsilicate gneiss. The orebodies and the host rocks have a complex metamorphic history. The last Variscan amphibolite facies metamorphic event is recorded in the quartz-biotite oxygen isotope fractionation in the host gneiss, which indicates temperatures from 600 degrees to 700 degrees C. Metamorphic textures in the orebodies include granoblastic annealing-recrystallization of barite and sulfides, pressure solution seams, and local development of shear zones near the margins of the orebodies. Despite intense metamorphism, the ore is generally strata bound, with sharp host-rock contacts. At a few places, tectonically modified offsets (usually enriched in sulfides) extend up to 2 m across the foliation of the surrounding rocks.Based on ore mineralogy and texture, the orebodies can be divided into three types: (1) massive baritehyalophane sulfidic rock, (2) calc-silicate gneiss with minor barite and sulfides, and (3) disseminated sulfides in marble. Massive barite ore, which is the only type of mineralization of economic interest (with estimated reserves of over 2 million tons [Mt]), contains an average of 62.2 percent barite, 3.2 percent Zn, and 1.8 percent Pb. The relatively simple mineral assemblage of massive ore includes barite, hyalophane, calcite, quartz, pyrite, pyrrhotite, sphalerite, and galena, as well as minor amounts of pyrargyrite, Ag tetrahedrite, fluorite, pyrophanite, and ilmenite. Calc-silicate gneiss with sulfides and barite usually forms the outer parts of the barite orebodies hosted in gneiss. It consists mainly of pyroxene, hyalophane, and quartz, with minor garnet, calcite, forsterite, minerals of the zoisite group, and sulfides. Disseminated Pb-Zn mineralization in marbles occurs only near the contact of marbles with massive barite orebodies.The delta 34 S values of sulfide minerals are heavy in all mineralization types (7.2-14.8ppm), probably indicating their formation in an anoxic, restricted marine environment, where the isotopic composition of sulfate and sulfide gradually shifted to more positive values as a result of the fractionation of S isotopes during bacterial reduction of sulfate to sulfide and the removal of isotopically light S by sulfide sedimentation. The restricted character of the sedimentary environment during barite formation is supported by the elevated content of graphite in the host rocks (0.01-2.0%) and by very heavy delta 34 S values of pyrite and anhydrite in a small (up to 1 m thick) anhydrite lens that occurs in a host-rock complex. The 87 Sr/ 86 Sr ratios in barite ore (0.7077-0.7079) are close to those for anhydrite (0.7080). Because the anhydrite probably represents a meta-evaporite, the isotopic composition of both barite and anhydrite strontium is close to that of contemporaneous seawater. The isotopic composition of lead in galenas from massive barite and calc-silicate ore is very homogeneous ( 206 Pb/ 204 Pb from 17.65 to 17.66, 207 Pb/ 204 Pb from 5.47 to 15.51, and 206 Pb/ 204 Pb from 37.36 to 37.45), and lead isotope ratios prove the pre-Variscan age of the orebodies.It is proposed that the Ron strata-bound mineralization formed by mixing of the hydrothermal solutions carrying Ba, Zn, and Pb with seawater in a restricted oceanic or intra-arc trough filled with clastic sediments derived mostly from contemporaneous volcanic rocks. The restricted nature of the trough resulted in low oxygen contents in the bottom waters and low recycling rates for the organic matter. During the subsequent thermal events, the mineralization and its host rocks were deformed and metamorphosed to the upper amphibolite grade and the carbonate-enriched orebodies were transformed to calc-silicate ore. Following deformation and retrograde metamorphic events at rather superficial tectonic levels caused retrograde shearing at the margins of the orebodies. At a waning stage of the metamorphism, the ore and host rocks were metamorphosed under greenschist facies conditions.

Sedimentary — diagenetic interpretation and reservoir characteristics of the Middle Jurassic (Araej Formation) in the southern Arabian Gulf

Oil and gas condensates discovered so far in the Middle Jurassic Araej Formation occur in several structures in western and central offshore Abu Dhabi and Qatar. The source for the oil is thought to have migrated updip primarily from the Hanifa/Diyab Formation (Upper Jurassic), whereas the source of the gas condensate is believed to be from Middle Triassic-Lower Jurassic formations. The Araej Formation is described from well data and thin sections from wells drilled in the south and south-west Arabian Gulf. It is a carbonate sequence that has a thickness ranging from 180 to 300 m (590 to 980 ft) and is divided into three members: Lower Araej, Uwainat and Upper Araej. The Lower Araej Member is made up of slightly argillaceous lime mudstones and wackestones with subordinate interbeds of peloidal/bioclastic packstones and grainstones. The Uwainat Member consists of peloidal bioclastic packstones and grainstones with subordinate wackestones and lime mudstones. The Upper Araej Member consists predominantly of variably cemented grainstones and packstones with minor wackestones. The sediments of the Araej Formation were deposited as a shallowing upwards sequence in quiet shelf waters grading to a shallow to very shallow marine shelf. Lithological variation within each member suggests higher order shallowing upwards sequences produced by sea-level fluctuations. Petrographic analysis of the Araej suggests a diagenetic path which includes neomorphism and recrystallization of allochems and marine cements to low Mg-calcite, dissolution and partial dolomitization of muddy lithologies. Pressure solution seams and stylolitization improved the permeability and these compactional features are often filled by bitumen. Some seams are filled by coarse mosaic calcite crystals or coarse rhombic dolomite, which is believed to be a late diagenetic cement. Porosity is highly variable, but never exceeds 16%. Intercrystalline, interparticle, mouldic and vuggy porosities are the dominant pore types.

Bedding parallel veins and their relationship to folding

Laminated bedding parallel veins hosted in turbiditic sandstone shale sequences from central Victoria, Australia, consist of stacked, millimetre thick, sub-parallel sheets of quartz separated by micaceous layers, wall rock slivers and pressure solution seams. They have very high length to thickness ratios, are laterally continuous over tens to hundreds of metres, and have relatively uniform thickness compared to other vein types. They are intimately associated with and folded by chevron folds, and the quartz grain shape elongation lineation is commonly orthogonal to mesoscopic and macroscopic fold hinge lines. The bedding parallel veins have two morphological forms. Type I are thin (commonly 5–10 cm) laminated veins which have complex microstructures dominated by phyllosilicate inclusion surfaces, related to oblique opening along bedding with varying rates of deposition (opening) relative to shear displacement (slip) along the bedding surfaces. More common are Type II, thicker (generally <20 cm), banded veins of alternating milky-white quartz with wall rock inclusion laminae (formerly fragments) bounded by stylolitic partings parallel to both bedding and the vein margins. The inclusion surfaces in Type I veins track the opening direction during vein formation. Vein opening-sense criteria suggest cyclical pore fluid pressure fluctuations which predate the amplification and propagation of the host chevron folds; i.e. prior to attainment of significant limb dip. Different layer parallel shortening and amplification rates for individual layers within the sedimentary sequence may lead to bedding parallel veins with an opening sense unrelated to the flexural slip folds which eventually follow.

Hydrocarbon Generation and Expulsion in Shale Vs. Carbonate Source Rocks

For a number of commercially important source rocks of shale and of carbonate lithologies, which were studied by geochemical, microscopical, and petrophysical techniques, a systematic comparison was made of the processes on how hydrocarbon generation and migration proceed with maturity progress. In this way, several fundamental differences between both types of source rocks were recognized, which are related to differences of sedimentary facies and, more importantly, of diagenetic processes responsible for lithification. Whereas siliciclastic sediments lithify mainly by mechanical compaction, carbonate muds get converted into lithified rocks predominantly by chemical diagenesis. With respect to their role as hydrocarbon source rocks, pressure solution processes appear to be key elements. During modest burial stages and prior to the onset of hydrocarbon generation reactions by thermal decomposition of kerogen, pressure solution seams and stylolites. These offer favorable conditions for hydrocarbon generation and expulsion-a three-dimensional kerogen network and high organic-matter concentrations that lead to effective saturation of the internal pore fluid system once hydrocarbon generation has started. As a consequence, within such zones pore fluids get overpressured, leading ultimately to fracturing. Petroleum expulsion can then occur at high efficiencies and in an explosive fashion, whereby clay minerals and residual kerogen particles are squeezed inmore » a toothpaste-like fashion into newly created fractures. In order to elucidate several of the above outlined steps of hydrocarbon generation and migration processes, open-system hydrous pyrolysis experiments were performed. This approach permits one to monitor changes in yield and composition of hydrocarbon products generated and expelled at 10[degrees]C temperature increments over temperature range, which mimics in the laboratory the conditions prevailing in nature over the entire liquid window interval.« less

Depositional Texture-Dependent and Independent Diagenetic Control of Petrophysical Properties, Norphlet Sandstone, Onshore and Offshore Alabama

ABSTRACT Diagenetic factors influencing reservoir heterogeneity vary significantly throughout the region of Norphlet hydrocarbon production. Distribution of some diagenetic components in these eolian reservoirs is controlled by depositional texture, including preferential carbonate cementation of coarser-grained laminae, concentration of pyrobitumen in finer-grained laminae, localization of pressure solution seams, and concentration of cements in deposits of specific eolian subenvironments, such as interdunes. The distribution of these diagenetic components, which create local to widespread barriers and baffles to fluid flow, can be determined by depositional modeling. However, the distribution of other diagenetic components in Norphlet reservoirs, including quartz, clay minerals, and pyrobitumen, is independent of depositional texture and cannot be determined by similar modeling. Factors controlling the distribution of texture-independent diagenetic components include the availability of chemical constituents from external sources, past and present positions of hydrocarbon-water contacts, and the time available for diagenetic reactions to proceed. In onshore fields, such as Hatter's Pond field, the position of fluid contacts influences reservoir quality. Permeability is highest above the hydrocarbon-water contact where authigenic illite is less abundant. The opposite relationship occurs in offshore fields in Alabama coastal waters and Federal OCS, areas where sandstone below paleo or present hydrocarbon-water contacts has the highest reservoir quality. Up to four diagenetic zones may occur stratigraphically. In descending order they are: (1) the dominantly quartz-cemented tight zone at the top of the Norphlet; (2) an interval above paleo or present fluid contacts in which pyrobitumen grain coats reduce pore volume and constrict pore throats; (3) an interval between paleo and present fluid contacts which lacks pyrobitumen and has the highest reservoir quality; and (4) an interval similar to interval 3 that lies below the present gas-water contact. Delineation of controls on the distribution of these intervals is critical to evaluation of gas reserves in offshore areas. End_of_Record - Last_Page 520-------

Pervasive burial dissolution of early and late dolomites in Devonian pools, Alberta, western Canada

Middle Devonian Keg River and Muskeg pools along the northern margin of Rainbow basin in northern Alberta produce mainly from secondary porosity created by dissolution of replacement dolomites and cements, including saddle dolomites. Resultant pores are predominantly vug and moldic and often microporous. Coarse vuggy porosity is also associated with Zebra dolomites. Several relationships confirm the burial dissolution of dolomites in these sequences. First, dolomites which replaced stylolitic sediment, including saddle dolomites, are leached, as shown by fluorescence microscopy. Second, high amounts of secondary porosity are preserved in highly stylolitic dolomites, often directly adjacent to pressure solution seams. Third, pressure solution seams terminate directly into secondary porosity. And fourth, fractures which cut stylolites also commonly feed directly into secondary pores. These petrographic relationships are consistent only if dolomite dissolution and secondary porosity development were contemporaneous with or postdated pressure solution. The spatial juxtaposition of fractures and stylolites with secondary porosity implies these diagenetic fabrics served as pathways for burial fluids promoting dolomite dissolution. Given the regional extent of pervasive dolomite dissolution, dolomite stability in these sequences is not a function of the type or relative age of dolomite crystals precipitated. Rather, pore-fluid chemistry appears to be the major control. Calcium-richmore » burial fluids, responsible for emplacement of burial anhydrites and fluorite cements, are thought to promote the deep-burial dolomite dissolution and secondary porosity development in these pools.« less

Pervasive Burial Dissolution of Early and Late Dolomites in Devonian Pools, Alberta, Western Canada

Middle Devonian Keg River and Muskeg pools along the northern margin of Rainbow basin in northern Alberta produce mainly from secondary porosity created by dissolution of replacement dolomites and cements, including saddle dolomites. Resultant pores are predominantly vug and moldic and often microporous. Coarse vuggy porosity is also associated with Zebra dolomites. Several relationships confirm the burial dissolution of dolomites in these sequences. First, dolomites which replaced stylolitic sediment, including saddle dolomites, are leached, as shown by fluorescence microscopy. Second, high amounts of secondary porosity are preserved in highly stylolitic dolomites, often directly adjacent to pressure solution seams. Third, pressure solution seams terminate directly into secondary porosity. And fourth, fractures which cut stylolites also commonly feed directly into secondary pores. These petrographic relationships are consistent only if dolomite dissolution and secondary porosity development were contemporaneous with or postdated pressure solution. The spatial juxtaposition of fractures and stylolites with secondary porosity implies these diagenetic fabrics served as pathways for burial fluids promoting dolomite dissolution. Given the regional extent of pervasive dolomite dissolution, dolomite stability in these sequences is not a function of the type or relative age of dolomite crystals precipitated. Rather, pore-fluid chemistry appears to be the major control. Calcium-richmore » burial fluids, responsible for emplacement of burial anhydrites and fluorite cements, are thought to promote the deep-burial dolomite dissolution and secondary porosity development in these pools.« less

Paleomagnetic Dating of Depositional and Diagenetic Events in Upper Cambrian Rocks of Central Texas: ABSTRACT

Hematite, magnetite, and goethite are present in samples from the Riley and Wilberns Formations in central Texas. Samples containing abundant goethite possess characteristic paleomagnetic directions that are Tertiary to recent in age. The goethite component is easily removed by thermal demagnetization temperatures greater than 150/degrees/C. A minority of samples from the Wilberns Formation contains magnetite as the dominant magnetic component. These samples display shallow easterly characteristic directions that the authors interpret as a true detrital remanent magnetization. Most samples from both units possess hematite as the dominant carrier of magnetic remanence. Samples in which hematite occurs as ooids and as a replacement for carbonate allochems possess shallow easterly declinations similar to magnetite-dominant samples. This generation of hematite probably formed during or shortly after deposition. The paleopole for samples in which hematite occurs as a replacement for glauconite grains is similar to previously published Ordovician paleopoles. This indicated Ordovician age is corroborated by a previously published Late Ordovician radiometric age for glauconite from the Riley Formation. Samples in which hematite occurs as an infilling within fractures and pressure solution seams display characteristic paleomagnetic directions that indicate a late Paleozoic age. This age corresponds to tectonic activity along the Ouachita tectonic beltmore » to the east. Such an age may reflect hematization from meteoric fluids during structural uplift of the area, or it might reflect precipitation during mixing of tectonically driven basin-derived fluid and meteoric fluid.« less

The Orlock Bridge Fault: a major Late Caledonian sinistral fault in the Southern Uplands terrane, British Isles

ABSTRACTA detailed examination of the boundary between Peach and Home's (1899) Northern and Central Belts, in the Lower Palaeozoic rocks of the Southern Uplands of Scotland and their Irish continuation in Longford–Down, demonstrates it to be a major Caledonian sinistral wrench fault. Maps and descriptions of outcrops of the fault at Slieve Glah in County Cavan, at Orlock Bridge on the NE coast of Down, at Cairngarroch on the Rhinns of Galloway and at Garvald, 11 km SW of Dunbar, are presented. A distinctive fault fabric, clearly the result of repeated sinistral slip at the first three localities, characterises these outcrops along the 400 km trace. The fabric elements include: (1) lenticular shearing of arenites in the fault protolith, (2) a fault-associated phyllonitic fabric, with abundant pressure-solution seams, overprinting the regional S1, (3) numerous foliation-parallel quartz segregations, (4) refolding of the regional S1 cleavage, segregation veins and the phyllonitic fabric in at least two generations of steeply plunging, sinistrally-verging folds, (5) a non-penetrative crenulation cleavage, (6) a locally developed sinistral S–C fabric, and (7) typical protomylonite, mylonite and ultramylonite textures in thin sections of the fault rocks. The zone of fault-associated deformation varies from a few metres across at Garvald to over 1 km at Slieve Glah. The fault thus differs from other tract-defining faults in the Southern Uplands, particularly in its clear evidence of ductile, quasi-plastic, deformation at depth, in its lack of associated imbrication, in its significant refolding and overprinting of the regional S1 cleavage and in its uniquely large stratigraphic effects.The main fabric-generating movement postdates the regional, accretion-related, S1 cleavage but predates minor Caledonian (c. 400 Ma) intrusions. For its entire length the fault separates the Northern Belt, with proof only of Upper Ordovician turbidites, and the Central Belt, where turbidite deposition began in the Silurian. The age of the base of the turbidites decreases southward by some six graptolite zones at the fault trace. The fault thus either (a) excises the equivalent of four tracts from a single accretionary-prism terrane, or (b) juxtaposes two distinct terranes, one of late Ordovician and the other of Silurian age. In either event the sinistral slip is probably in excess of 400 km.

Diagenetic Growth of Euhedral Megaquartz in the Skeleton of a Stromatoporoid

ABSTRACT Megaquartz, typically as individual crystals showing a number of euhedral faces, inclusion-rich or inclusion-poor, is disseminated throughout the interior of a large specimen of the stromatoporoid Hermatoporoidea from the Middle Devonian Burdekin Formation, northeastern Australia. It represents a replacing, late diagenetic phase occurring after the occlusion of pore space within the skeleton by carbonate spar and was contemporary with, and concentrated near, microscopic pressure-solution seams. Skeletal tissue rather than carbonate spar sponsored megaquartz growth that occurred while an intimate contact between growing quartz crystal faces and the carbonate it replaced was maintained. Spherulitic quartzine, the common type of silicification in limestone of the Burdekin Formatio , replaced the exterior surfaces of carbonate skeletal debris in a similar manner. Diffusion on intergranular films is considered to be the mechanism by which both megaquartz and quartzine were emplaced, and the two modes are considered to reflect different replacement rates. Rapid replacement, associated with short diffusional pathways, is attributed to quartzine, whereas megaquartz is thought to represent slow replacement associated with long, diffusional pathways.

Two-dimensional analysis of shape-fabric using projections of digitized lines in a plane

A method of two-dimensional shape-fabric analysis is presented which is based on the projection of lines or ellipses on the x-axis. If a set of lines or ellipses displays a preferred orientation, the average length of projection of the lines or ellipses on the x-axis changes with direction in the x-y plane. The proposed method involves use of Fortran programs which evaluate the average length of projection of lines (e.g., outlines of ooides, pressure solution seams) while the fabric is rotated by small increments in a counterclockwise sense about the origin of the x - y plane. Data acquisition is by digitizing the lines on an enlarged photograph. The sets of x-y coordinates that are thus obtained serve as input for the programs. The basic equations of the method are explained and application to a sedimentary fabric is demonstrated. The proposed method is both fast and sensitive. It is especially useful if the particle outlines are closely spherical or if the fabric is only weakly developed. Yet, the method is not restricted to weak fabrics only. Also, if the fabric is due to deformation this method yields the axes of the two-dimensional strain ellipse.