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Speleology in Kazakhstan

Shakalov on 04 Jul, 2018
Hello everyone!   I pleased to invite you to the official site of Central Asian Karstic-Speleological commission ("Kaspeko")   There, we regularly publish reports about our expeditions, articles and reports on speleotopics, lecture course for instructors, photos etc. ...

New publications on hypogene speleogenesis

Klimchouk on 26 Mar, 2012
Dear Colleagues, This is to draw your attention to several recent publications added to KarstBase, relevant to hypogenic karst/speleogenesis: Corrosion of limestone tablets in sulfidic ground-water: measurements and speleogenetic implications Galdenzi,

The deepest terrestrial animal

Klimchouk on 23 Feb, 2012
A recent publication of Spanish researchers describes the biology of Krubera Cave, including the deepest terrestrial animal ever found: Jordana, Rafael; Baquero, Enrique; Reboleira, Sofía and Sendra, Alberto. ...

Caves - landscapes without light

akop on 05 Feb, 2012
Exhibition dedicated to caves is taking place in the Vienna Natural History Museum   The exhibition at the Natural History Museum presents the surprising variety of caves and cave formations such as stalactites and various crystals. ...

Did you know?

That spring, gravity is a spring flowing as a result of gravity [16].?

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Featured articles from Cave & Karst Science Journals
Chemistry and Karst, White, William B.
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Featured articles from other Geoscience Journals
Karst environment, Culver D.C.
Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs, Bontognali, Tomaso R.R.; D’Angeli Ilenia M.; Tisato, Nicola; Vasconcelos, Crisogono; Bernasconi, Stefano M.; Gonzales, Esteban R. G.; De Waele, Jo
Calculating flux to predict future cave radon concentrations, Rowberry, Matt; Marti, Xavi; Frontera, Carlos; Van De Wiel, Marco; Briestensky, Milos
Microbial mediation of complex subterranean mineral structures, Tirato, Nicola; Torriano, Stefano F.F;, Monteux, Sylvain; Sauro, Francesco; De Waele, Jo; Lavagna, Maria Luisa; D’Angeli, Ilenia Maria; Chailloux, Daniel; Renda, Michel; Eglinton, Timothy I.; Bontognali, Tomaso Renzo Rezio
Evidence of a plate-wide tectonic pressure pulse provided by extensometric monitoring in the Balkan Mountains (Bulgaria), Briestensky, Milos; Rowberry, Matt; Stemberk, Josef; Stefanov, Petar; Vozar, Jozef; Sebela, Stanka; Petro, Lubomir; Bella, Pavel; Gaal, Ludovit; Ormukov, Cholponbek;
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Your search for lamina (Keyword) returned 130 results for the whole karstbase:
Showing 16 to 30 of 130
EARLY DEVELOPMENT OF KARST SYSTEMS .2. TURBULENT-FLOW, 1995, Howard A. D. , Groves C. G. ,
A simulation model developed to explore patterns of fracture enlargement within incipient limestone karst aquifers has been extended to turbulent flow. In contrast to the highly selective passage enlargement that occurs early in cave network development under laminar flow, the transition to turbulent flow results in more general passage enlargement, leading to maze networks when initial fractures are large and hydraulic gradients are high. These results support previously published hypotheses for the development of maze patterns, including formation within structural settings that have created initially large fractures or within flow systems periodically inundated by flooding. Maze development is also favored under turbulent flow when passages are entirely water filled, and where the groundwater flow system is long-lived. By contrast, branched patterns are most common when passages become free-surface subterranean streams, because depression of the piezometric surface along main passages, downcutting along main passages, and possible infilling with sediment of side passages limit the sharing of discharge among interconnected fractures or bedding planes that promote maze development

Annually laminated sequences in the internal structure of some Belgian stalagmites - importance for paleoclimatology., 1996, Genty D. , Quinif Y.

Annually laminated sequences in the internal structure of some Belgian stalagmites; importance for paleoclimatology, 1996, Genty D, Quinif Y,

Structure et comportement hydraulique des aquifers karstiques, DSc thesis, 1996, Jeannin, P. Y.

This thesis aims to provide a better knowledge of karst flow systems, from a functional point of view (behaviour with time), as well as from a structural one (behaviour in space). The first part of the thesis deals with the hydrodynamic behaviour of karst systems, and the second part with the geometry of karstic networks, which is a strong conditioning factor for the hydrodynamic behaviour.
Many models have been developed in the past for describing the hydrodynamic behaviour of karst hydrogeological systems. They usually aim to provide a tool to extrapolate, in time and/or space, some characteristics of the flow fields, which can only be measured at a few points. Such models often provide a new understanding of the systems, beyond what can be observed directly in the field. Only special field measurements can verify such hypotheses based on numerical models. This is an significant part of this work. For this purpose, two experimental sites have been equipped and measured: Bure site or Milandrine, Ajoie, Switzerland, and Holloch site, Muotathal, Schwyz, Switzerland. These sites gave us this opportunity of simultaneously observe hydrodynamic parameters within the conduit network and, in drillholes, the "low permeability volumes" (LPV) surrounding the conduits.
These observations clearly show the existence of a flow circulation across the low permeability volumes. This flow may represent about 50% of the infiltrated water in the Bure test-field. The epikarst appears to play an important role into the allotment of the infiltrated waters: Part of the infiltrated water is stored at the bottom of the epikarst and slowly flows through the low permeability volumes (LPV) contributing to base flow. When infiltration is significant enough the other part of the water exceeds the storage capacity and flows quickly into the conduit network (quick flow).
For the phreatic zone, observations and models show that the following scheme is adequate to describe the flow behaviour: a network of high permeability conduits, of tow volume, leading to the spring, is surrounded by a large volume of low permeability fissured rock (LPV), which is hydraulically connected to the conduits. Due to the strong difference in hydraulic conductivity between conduits and LPV, hydraulic heads and their variations in time and space are strongly heterogeneous. This makes the use of piezometric maps in karst very questionable.
Flow in LPV can be considered as similar to flow in fractured rocks (laminar flow within joints and joints intersections). At a catchment scale, they can be effectively considered as an equivalent porous media with a hydraulic conductivity of about 10-6 to 10-7 m/s.
Flow in conduits is turbulent and loss of head has to be calculated with appropriate formulas, if wanting any quantitative results. Our observations permitted us to determine the turbulent hydraulic conductivity of some simple karst conduits (k', turbulent flow), which ranges from 0.2 to 11 m/s. Examples also show that the structure of the conduit network plays a significant role on the spatial distribution of hydraulic heads. Particularity hydraulic transmissivity of the aquifer varies with respect to hydrological conditions, because of the presence of overflow conduits located within the epiphreatic zone. This makes the relation between head and discharge not quadratic as would be expected from a (too) simple model (with only one single conduit). The model applied to the downstream part of Holloch is a good illustration of this phenomena.
The flow velocity strongly varies along the length of karst conduits, as shown by tracer experiments. Also, changes in the conduit cross-section produce changes in the (tow velocity profile. Such heterogeneous flow-field plays a significant role in the shape of the breakthrough curves of tracer experiments. It is empirically demonstrated that conduit enlargements induce retardation of the breakthrough curve. If there are several enlargements one after the other, an increase of the apparent dispersivity will result, although no diffusion with the rock matrix or immobile water is present. This produces a scale effect (increase of the apparent dispersivity with observation scale). Such observations can easily be simulated by deterministic and/or black box models.
The structure of karst conduit networks, especially within the phreatic zone, plays an important role not only on the spatial distribution of the hydraulic heads in the conduits themselves, but in the LPV as well. Study of the network geometry is therefore useful for assessing the shape of the flow systems. We further suggest that any hydrogeological study aiming to assess the major characteristics of a flow system should start with a preliminary estimation of the conduit network geometry. Theories and examples presented show that the geometry of karst conduits mainly depends on boundary conditions and the permeability field at the initial stage of the karst genesis. The most significant boundary conditions are: the geometry of the impervious boundaries, infiltration and exfiltration conditions (spring). The initial permeability field is mainly determined by discontinuities (fractures and bedding planes). Today's knowledge allows us to approximate the geometry of a karst network by studying these parameters (impervious boundaries, infiltration, exfiltration, discontinuity field). Analogs and recently developed numerical models help to qualitatively evaluate the sensitivity of the geometry to these parameters. Within the near future, new numerical tools will be developed and will help more closely to address this difficult problem. This development will only be possible if speleological networks can be sufficiently explored and used to calibrate models. Images provided by speleologists to date are and will for a long time be the only data which can adequately portray the conduit networks in karst systems. This is helpful to hydrogeologists. The reason that we present the example of the Lake Thun karst system is that it illustrates the geometry of such conduits networks. Unfortunately, these networks are three-dimensional and their visualisation on paper (2 dimensions) is very restrictive, when compared to more effective 3-D views we can create with computers. As an alternative to deterministic models of speleogenesis, fractal and/or random walk models could be employed.


The Stromatolites of the Cenote Lakes of the Lower South East of South Australia, 1996, Thurgate, Mia E.

Stromatolite are lithified, laminated, organosedimentary deposits. Preliminary examination of eight cenote lakes near Mt. Gambier has revealed the presence of tens of thousands of actively - forming stromatolites. Based on the external morphology, 14 different types of stromatolites have been identified, columnar growth forms are most common. Three genus of Diatom and three genus of Cyanobacteria are the most likely responsible for stromatolite development.


Symposium Abstract: Annual Laminations in Speleothems - Recent Palaeoclimatic interpretations and future prospects, 1997, Baker A. , Barnes W. L. , Genty D. , Mockler N. J.

Hydrogeological study and discharge features of the Niksar karst springs (Tokat-Turkey), 1997, Syed M. A. , Afsin M. , Celik M. ,
The exposed Paleozoic and Recent units in the study area have various hydrogeological characteristics such as pervious, semipervious, and impervious. Pervious limestones and associated impervious formations that were not influenced by tectonic movement are connected to produce karst springs. This paper presents the relationship between the discharge coefficient and other aquifer properties by using the hydrograph analyses of the karst springs. The magnitude of the discharge of the spring apparently controls the character of flow (such as laminar) and conduit in the aquifer. The correlation analysis shows a positive relation between Q(0)-Q(t), Q(0)-storage capacity, Q(t)-storage capacity, and alpha-discharge change, These results enhances the properties of the karst springs. Both monthly and annual rainfall contribute to spring discharge. All karst springwaters are suitable for household and food industry uses

The Lower Triassic Montney Formation, west-central Alberta, 1997, Davies Gr, Moslow Tf, Sherwin Md,
The Lower Triassic Montney Formation was deposited in a west-facing, arcuate extensional basin, designated the Peace River Basin, on the northwestern margin of the Supercontinent Pangea, centred at about 30 degrees N paleolatitude. At least seasonally arid climatic conditions, dominance of northeast trade winds, minimum fluvial influx, offshore coastal upwelling, and north to south longshore sediment transport affected Montney sedimentation. Paleostructure, particularly highs over underlying Upper Devonian Leduc reefs and lows associated with graben trends in the Peace River area, strongly influenced Montney depositional and downslope mass-wasting processes. A wide range of depositional environments in the Montney is recorded by facies ranging from mid to upper shoreface sandstones, to middle and lower shoreface HCS sandstones and coarse siltstones, to finely laminated lower shoreface sand and offshore siltstones. and to turbidites. Dolomitized coquinal facies occur at seven stratigraphic horizons in the Montney. Some coquinas are capped by karst breccias and coarse-grained aeolian deflation lag sand residues indicating subaerial exposure. The Montney has been divided into three informal members that have been dated by palynology and compared with global Early Triassic sequences. The subdivisions are: the Lower member, of Griesbachian to Dienerian age, correlated with a third-order cycle; the Coquinal Dolomite Middle member, of mixed Dienerian and Smithian ages; and the Upper member, of Smithian to Spathian age, correlative with two, shorter-duration third-order cycles. A forced regressive wedge systems tract model is adopted for deposition of the Coquinal Dolomite Middle member and for turbidites in the Valhalla-La Glace area of west-central Alberta. With this model, coquinas and turbidites accumulated during falling base level to lowstand, with a basal surface of forced regression at the base of the coquina and a sequence boundary at the top of the coquinal member. This is supported by the evidence for subaerial exposure and maximum lowstand at the top of the coquina. Very limited grain size distribution in the Montney, dominantly siltstone to very fine-grained sandstone, but often very well sorted, is interpreted to reflect an aeolian influence on sediment source and transport, High detrital feldspar and detrital dolomite in the Montney are consistent with (but not proof of) aeolian source from an arid interior, as is high detrital mica content in finer size grades. Extensive and often pervasive dolomitization, and early anhydrite cementation within the Montney, are also consistent with an arid climatic imprint. As new exploratory drilling continues to reveal the wide range of facies in the Montney, it adds to both the complexity and potential of this relatively unique formation in western Canada

Stratigraphy, sedimentology, and structural geology of gypsum caves in east central New Mexico, 1997, Forbes J, Nance R,
Hundreds of solution caves have developed in evaporites and carbonates of the Permian San Andres Formation where it crops out between Vaughn and Roswell, New Mexico, USA. Several of the caves are over 3.2 km (2 miles) in length, and the deepest has a vertical extent of over 120 m (400 feet). These gypsum caves afford an extraordinary opportunity to examine the evaporite rocks in which they are developed. We have examined interbedded gypsum and dolostone strata exposed in the walls of 11 of these caves, and show stratigraphic sections on two geologic cross sections. Gypsum textures exposed in the caves include massive, nodular, and laminar types. While we refer to them as ''gypsum caves,'' gypsum is not the only lithology exposed. Some cave passages and rooms are developed in thick dolostone units intercalated with or overlain by gypsum beds. Correlation of beds exposed in two or more caves has allowed us to infer the local geologic structure. The sedimentary sequence penetrated by a cave exerts a profound effect on the geometry and passage cross-section of the cave. Many cave passages have gypsum walls and a dolostone or limestone floor. Although many of the cave passages flood completely during major storm events, the stairstep profile of most of the caves is indicative of speleogenesis that has occurred predominantly within the vadose zone

Mixed transport reaction control of gypsum dissolution kinetics in aqueous solutions and initiation of gypsum karst, 1997, Raines M. A. , Dewers T. A. ,
Experiments with gypsum in aqueous solutions at 25 degrees C, low ionic strengths, and a range of saturation states indicate a mixed surface reaction and diffusional transport control of gypsum dissolution kinetics. Dissolution rates were determined in a mixed flow/rotating disc reactor operating under steady-state conditions, in which polished gypsum discs were rotated at constant speed and reactant solutions were continuously fed into the reactor. Rates increase with velocity of spin under laminar conditions (low rates of spin), but increase asymptotically to a constant rate as turbulent conditions develop with increasing spin velocity, experiencing a small jump in magnitude across the laminar-turbulent transition. A Linear dependence of rates on the square root of spin velocity in the laminar regime is consistent with rates being limited by transport through a hydrodynamic boundary layer. The increase in rate with onset of turbulence accompanies a near discontinuous drop in hydrodynamic boundary layer thickness across the transition. A relative independence of rates on spinning velocity in the turbulent regime plus a nonlinear dependence of rates on saturation state are factors consistent with surface reaction control. Together these behaviors implicate a 'mixed' transport and reaction control of gypsum dissolution kinetics. A rate law which combines both kinetic mechanisms and can reproduce experimental results under laminar flow conditions is proposed as follows: R = k(t) {1 - Omega(b)() zeta [1 - (1 2(1 - Omega(b)())(1/2)]} where k(t) is the rate coefficient for transport control, and Omega(b)() is the mean ionic saturation state of the bulk fluid. The dimensionless parameter zeta(=Dm(eq)()/2 delta k() where m(eq)() = mean ionic molal equilibrium concentration, D is the diffusion coefficient through the hydrodynamic boundary layer, delta equals the boundary layer thickness and k() is the rate constant for surface reaction control) indicates which process, transport or surface reaction, dominates, and is sensitive to the hydrodynamic conditions in the reactor. For the range of conditions used in our experiments, zeta varies from about 1.4 to 4.5. Rates of gypsum dissolution were also determined in situ in a cavern system in the Permian Blaine Formation, southwestern Oklahoma. Although the flow conditions in the caverns were not determinable, there is good agreement between lab- and field-determined rates in that field rate magnitudes lie within a range of rates determined experimentally under zero to low spin velocities A numerical model coupling fluid flow and gypsum reaction in an idealized circular conduit is used to estimate the distance which undersaturated solutions will travel into small incipient conduits before saturation is achieved. Simulations of conduit wall dissolution showed-member behaviors of conduit formation and surface denudation that depend on flow boundary conditions (constant discharge or constant hydraulic gradient and initial conduit radius. Surface-control of dissolution rates. which becomes more influential with higher fluid flow velocity, has the effect that rate decrease more slowly as saturation is approached than otherwise would occur if rates were controlled by transport alone. This has the effect that reactive solutions can penetrate much farther into gypsum-bearing karst conduits than heretofore thought possible, influencing timing and mechanism of karst development as well as stability of engineered structures above karst terrain

Comparison of annual luminescent and visible laminae in stalagmites, 1997, Genty D, Baker A, Barnes W,
Polished sections of stalagmites from France and Belgium were examined using UV excitation and natural light in order to examine their annual growth laminae. The most prominent luminescence occurs in the dark compact laminae; this would signify that both types form at the end of Autumn or the beginning of Winter when soil organic matter is flushed into the cave. Under UV excitation, and at a weaker magnification, white porous calcite fabric is found to be much more luminescent than the dark compact one. These results could be due either to optical effects associated with porosity or to the higher luminescence of dark compact laminae

Comparaison entre les lamines luminescentes et les lamines visibles annuelles de stalagmites, 1997, Genty D, Baker A, Barnes W,
ResumeLes sections polies de plusieurs stalagmites provenant de France et de Belgique ont ete observees apres une excitation en lumiere ultraviolette (LTV) et en lumiere naturelle pour etudier les lamines annuelles luminescentes et visibles. La luminescence la plus forte se trouve dans les lamines sombres et compactes, ce qui signifierait que ces deux types de lamines se formeraient a la fin de l'automne et au debut de l'hiver, lorsqu'il y a une brusque augmentation du debit et un apport important de matiere organique. Sous excitation laser UV, la calcite blanche et poreuse est beaucoup plus luminescente que la calcite sombre et compacte. Cette luminescence est soit, liee a la porosite, soit a la presence de lamines sombres et compactes plus luminescentes.AbstractPolished sections of stalagmites from France and Belgium were examined using UV excitation and natural light in order to examine their annual growth laminae. The most prominent luminescence occurs in the dark compact laminae, this would signify that both types form at the end of Autumn or the beginning of Winter when soil organic matter is flushed into the cave. Under UV excitation, and at a weaker magnification, white porous calcite fabric is found to be much more luminescent than the dark compact one. These results could be due either to optical effects associated with porosity or to the higher luminescence of dark compact laminae

Risk assessment methodology for karst aquifers .2. Solute-transport modeling, 1997, Field Ms,
Ground-water flow and solute-transport simulation modeling are major components of most exposure and risk assessments of contaminated aquifers. Model simulations provide information on the spatial and temporal distributions of contaminants in subsurface media but are difficult to apply to karst aquifers in which conduit flow is important. Ground-water flow and solute transport in karst conduits typically display rapid-flow velocities, turbulent-flow regimes, concentrated pollutant-mass discharge, and exhibit open-channel or closed-conduit how Conventional groundwater models, dependent on the applicability of Darcy's law, are inappropriate when applied to karst aquifers because of the (1) nonapplicability of Darcian-flow parameters, (2) typically nonlaminar flow regime, and (3) inability to locate the karst conduits through which most flow and contaminant transport occurs. Surface-water flow and solute-transport models conditioned on a set of parameters determined empirically from quantitative ground-water tracing studies may be effectively used to render fate-and-transport values of contaminants in karst conduits. Hydraulic-flow and geometric parameters developed in a companion paper were used in the surface-water model, TOXI5, to simulate hypothetical slug and continuous-source releases of ethylbenzene in a karst conduit. TOXI5 simulation results showed considerable improvement for predicted ethylbenzene-transport rates and concentrations over qualitative tracing and analytical ground-water model results. Ethylbenzene concentrations predicted by TOXI5 simulations were evaluated in exposure and risk assessment models

Precipitation kinetics of calcite in the system CaCO3-H2O-CO2: The conversion to CO2 by the slow process H?->CO2? as a rate limiting step, 1997, Dreybrodt W, Eisenlohr L, Madry B, Ringer S,
Precipitation rates of CaCO3 from supersaturated solutions in the H2O - CO2 - CaCO3 system are controlled by three rate-determining processes: the kinetics of precipitation at the mineral surface, mass transport of the reaction species involved to and from the mineral surface, and the slow kinetics of the overall reaction HCO3- H --> CO2 H2O. A theoretical model by Buhmann and Dreybrodt (1985a,b) taking these processes into account predicts that, due to the slow kinetics of this reaction, precipitation rates to the surface of CaCO3 minerals depend critically on the ratio V/A of the volume V of the solution to the surface area A of the mineral in contact with it, for both laminar and turbulent flow. We have performed measurements of precipitation rates in a porous medium of sized particles of marble, limestone, and synthetic calcite, with V/A ratios ranging from 3.10(-4) to 1.2-10(-2) cm at 10 degrees C. Calcite was precipitated from supersaturated solutions with [Ca2] approximate to 4 mmol/L and an initial P-CO2 of 5.10(-3) or 1.10(-3) atm, respectively, using experimental conditions which prevented exchange of CO2 with the atmosphere, i.e., closed system. The results are in qualitative agreement with the theoretical predictions. Agreement with the observed data, however, is obtained by modifying the rate law of Plummer et al. (1978) to take into account surface-controlled inhibition effects. Experiments with supersaturated solutions containing carbonic anhydrase, an enzyme which enhances the conversion of HCO3- into CO2, yield rates increased by a factor of up to 15. This provides for the first time unambiguous experimental evidence that this reaction is rate limiting. We have also measured precipitation rates in batch experiments, stirring sized mineral particles in a solution with V/A ranging from 0.03 to 0.75 cm. These experiments also give clear evidence on the importance of the conversion of HCO3- into CO2 as rate limiting step. Taken together our experiments provide evidence that the theoretical model of Buhmann and Dreybrodt (1985a,b) can be used to predict reliable rates from the composition of CaHCO3- solutions with low ionic strength in many geologically relevant situations. Copyright (C) 1997 Elsevier Science Ltd

Ground-water silicifications in the calcareous facies of the Tertiary piedmont deposits of the Atlas Mountain (Hamada du Guir, Morocco), 1997, Thiry M. , Benbrahim M. ,
The Tertiary piedmont deposits (Hamada Formations), on the southern edge of the Haut-Atlas mountains, form extensive tablelands in the Boudenib area. They consist of two main sedimentary sequences, the Hamada de Boudenib and the Hamada du Guir, of Eocene and Miocene age. Both sequences show elastic facies at their base (conglomerates, calcareous sandstones, silty clays) and end with thick lacustrine limestones and pedogenic calcretes are characterised by rather confined facies, palygorskite-rich, with some gypsum in the second sequence. The recent evolution of the region is marked by the dissection of the tableland that is lined with high cliffs. The water flaw is mainly through wide karst features as there is no major river on the tableland. Silicifications which affect the different facies, form pods of various shape and size, and show an erratic spatial distribution. In the calcareous sandstones, there are irregularly shaped tubules of about 5 cm in diameter, more planar bodies from 5 to 50 cm thick, which frequently display voids lined with translucent silica concretions. The conglomerates display relatively few silicifications, the more characteristic ones consist of a silica cortex on some Limestone pebble and silica plates fitting closely the base of the pebbles. The lacustrine limestones and the calcretes from the upper part of the formation show frequently well developed silicifications. These show very variable shapes; horizontally stretching layers, interconnected or isolated amoeba-like bodies, thin slabs, karst micro-breccia, with frequent concretionnary structures, and quartz crystallisations. Limestone nodules remain often included in these silicifications. The more argillaceous facies display either small tubules or thin plates formed of translucent concretionnary silica. As a rule, the importance of the voids and related structures (concretions, drusy crystals) has to be noticed in all these silicifications, sometimes they are also linked with fractures or karst pipes. Petrography of the silica minerals, their relation with the primary structures. their distribution and their succession, give invaluable information on the silicification processes. Microcrystalline and fibrous quartz are the most common silica minerals, including minor amounts of opal and euhedral quartz. But micrographic arrangements show clearly that primary opal deposits have been more extensive and have recrystallized into chalcedony, microcrystalline quartz, or even ''flame-like'' quartz. Silica deposits in voids make up an important part of the silica pods. The tubules and thin plates of translucent silica of the argillaceous facies are formed of laminar chalcedony deposited around voids. Silica deposits in voids are also particularly obvious in the sandstones. The pores between the quartz grains are then cemented by fibrous quartz and little opal. Some samples show very large cemented voids that cannot be related to the primary porosity of the sandstone. These large voids correspond to the dissolution of the primary calcareous cement, which even led to the collapse of the sandstone fabric. In the limestones, there are silicified micro-karst breccia with a very high primary porosity cemented by quartz crystals, and even in the large microcrystalline quartz zones there are numerous void fillings, the primary porosity often exceeding 50%. There is obviously the alternation of silica deposits and calcite dissolution. Beside the void filling, silicifications comprise also matrix epigenesis, that is replacement of the carbonate by silica with preservation of most of the limestone structures, without development of voids. Nevertheless, the epigenesis of the limestone matrix is restricted to the vicinity of the voids. The silicifications relate to diagenetic processes. The main part of the silica is formed of void deposits and matrix replacement (epigenesis) on the edge of the voids. These void deposits give evidence of the feeding solutions. The regularity of the deposits all around the voids point out to a hydrologic regime characterised by a ground-water our now. Silica originates most probably from alteration of the magnesian clay minerals along the ground-water path. Regarding the low solubility of silica in surficial waters, high flows are needed in order to renew continuously the silica precipitated from solution. This points to a relatively humid climate at time of silicification, and to relief and incised landscapes to bring about these high flows

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