Karst and Cave RSS news feed Like us on Facebook! follow us on Twitter!
Community news

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. ...

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. ...

Speleology in Kazakhstan

Shakalov on 11 Jul, 2012
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 vegetation cover is the cover living vegetation on top of the upper soil horizon [16].?

Checkout all 2699 terms in the KarstBase Glossary of Karst and Cave Terms

What is Karstbase?

Search KARSTBASE:

keyword
author

Browse Speleogenesis Issues:

KarstBase a bibliography database in karst and cave science.

Featured articles from Cave & Karst Science Journals
Chemistry and Karst, White, William B.
Engineering challenges in Karst, Stevanović, Zoran; Milanović, Petar
See all featured articles
Featured articles from other Geoscience Journals
Geochemical and mineralogical fingerprints to distinguish the exploited ferruginous mineralisations of Grotta della Monaca (Calabria, Italy), Dimuccio, L.A.; Rodrigues, N.; Larocca, F.; Pratas, J.; Amado, A.M.; Batista de Carvalho, L.A.
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
See all featured articles from other geoscience journals

Search in KarstBase

Your search for fractures (Keyword) returned 335 results for the whole karstbase:
Showing 16 to 30 of 335
Le karst du gypse du centre de la dpression de l'Ebre (Espagne), 1990, Soriano, M. A.
THE GYPSUM KARST OF THE CENTER OF THE EBRE BASSIN (SPAIN) - The central Ebro basin was filled with evaporitic deposits (gypsum and limestones) during the Miocene. During the Quaternary, several alluvial terrace and pediment levels were developed and they overlay the gypsum deposits. A large number of karstic landforms developed on gypsum have been found. The most important reason is its high solubility. We have found different types of microlandforms. The most important are Rillenkarren, solution pits and micro etching. There are also small tumuli. They are active at present. We have differentiated three macrolandforms: paleocollapses, depressions and alluvial dolines. The paleocollapses are very narrow and deep. They are filled with quaternary materials. They are not active and were generated in the Middle or Upper Pleistocene. The depressions were developed by the gypsum dissolution, together with topographical and geomorphologic factors. They do not seem to be active nowadays. The alluvial dolines are developed on the terrace and pediment deposits, which overlay gypsum materials. There is basin, well and pan-shaped dolines and they are especially frequent in the T2 terrace level. From the study of aerial photographs of different years, the variations in the number and size of dolines and their density have been determined. Natural factors (lithology and fractures), together with human activities (irrigation) are the principal causes in their development.

Yates and other Guadalupian (Kazanian) oil fields, U. S. Permian Basin, 1990, Craig Dh,
More than 150 oil and gas fields in west Texas and southeast New Mexico produce from dolomites of Late Permian (Guadalupian [Kazanian]) age. A majority of these fields are situated on platforms or shelves and produce from gentle anticlines or stratigraphic traps sealed beneath a thick sequence of Late Permian evaporites. Many of the productive anticlinal structures are elongate parallel to the strike of depositional facies, are asymmetrical normal to facies strike, and have flank dips of no more than 6{degrees}. They appear to be related primarily to differential compaction over and around bars of skeletal grainstone and packstone. Where the trapping is stratigraphic, it is due to the presence of tight mudstones and wackestones and to secondary cementation by anhydrite and gypsum. The larger of the fields produce from San Andres-Grayburg shelf and shelf margin dolomites. Cumulative production from these fields amounts to more than 12 billion bbl (1.9 x 109 m3) of oil, which is approximately two-thirds of the oil produced from Palaeozoic rocks in the Permian Basin. Eighteen of the fields have produced in the range from 100 million to 1.7 billion bbl (16-271 x 106 m3). Among these large fields is Yates which, since its discovery in October 1926, has produced almost 1.2 billion bbl (192 x 106 m3) out of an estimated original oil-in-place of 4 billion bbl (638 x 106 m3). Flow potentials of 5000 to 20 000 bbl (800 to 3200 m3) per day were not unusual for early Yates wells. The exceptional storage and flow characteristics of the Yates reservoir can be explained in terms of the combined effects of several geologic factors: (1) a vast system of well interconnected pores, including a network of fractures and small caves; (2) oil storage lithologies dominated by porous and permeable bioclastic dolograinstones and dolopackstones; (3) a thick, upper seal of anhydrite and compact dolomite; (4) virtual freedom from the anhydrite cements that occlude much porosity in other fields which are stratigraphic analogues of Yates; (5) unusual structural prominence, which favourably affected diagenetic development of the reservoir and made the field a focus for large volumes of migrating primary and secondary oil; (6) early reservoir pressures considerably above the minimum required to cause wells to flow to the surface, probably related to pressures in a tributary regional aquifer

Rock Joints, 1990,


Modeling of regional groundwater flow in fractured rock aquifers, PhD Thesis, 1990, Kraemer, S. R.

The regional movement of shallow groundwater in the fractured rock aquifer is examined through a conceptual-deterministic modeling approach. The computer program FRACNET represents the fracture zones as straight laminar flow conductors in connection to regional constant head boundaries within an impermeable rock matrix. Regional scale fracture zones are projected onto the horizontal plane, invoking the Dupuit-Forchheimer assumption for flow. The steady state flow solution for the two dimensional case is achieved by requiring nodal flow balances using a Gauss-Seidel iteration. Computer experiments based on statistically generated fracture networks demonstrate the emergence of preferred flow paths due to connectivity of fractures to sources or sinks of water, even in networks of uniformly distributed fractures of constant length and aperture. The implication is that discrete flow, often associated with the local scale, may maintain itself even at a regional scale. The distribution of uniform areal recharge is computed using the Analytic Element Method, and then coupled to the network flow solver to complete the regional water balance. The areal recharge weakens the development of preferential flow pathways. The possible replacement of a discrete fracture network by an equivalent porous medium is also investigated. A Mohr's circle analysis is presented to characterize the tensor relationship between the discharge vector and the piezometric gradient vector, even at scales below the representative elementary volume (REV). A consistent permeability tensor is sought in order to establish the REV scale and justify replacement of the discrete fracture network by an equivalent porous medium. Finally, hydrological factors influencing the chemical dissolution and initiation of conduits in carbonate (karst) terrain are examined. Based on hydrological considerations, and given the appropriate geochemical and hydrogeological conditions, the preferred flow paths are expected to develop with time into caves.


IMPACT OF PAST SEDIMENT ECOLOGY ON ROCK FRACTURATION AND DISTRIBUTION OF CURRENT ECOSYSTEMS (JURA, FRANCE), 1991, Gaiffe M, Bruckert S,
Differences in the fracture type of limestone rocks have resulted in the formation of several main plant soil ecosystems in the montane and subalpine zones of the Jura (800-1 700 m). The sites were on stable landscape with slope < 5%. Locations were chosen to reflect the variation in physical properties of the bedrock and lithic contact. The rock fractures (densities and size), the shape and size of the fragments and the hydraulic conductivities were described and analyzed to characterize the 3 main bedrocks in the area studied (table 1): 1), lapiaz, ie, large rock fragments separated from each other by wide fractures (figs 1-2), 'broken' rocks traversed by numerous fine fractures (fig 2-3), paving-stones crossed by infrequent narrow fractures (fig 3). The effects of rock fracturing on vegetation (table II) and soil formation were significant in reference to porosity and permeability relationships (figs 6-7). Under similar precipitation, meteoric waters flow through the soil and porosity is relative to fracture systems (figs 4, 5). The weathering of cobbles in the soil profiles and along the lithic contacts maintains different soil solution Ca levels and is an important variable in soil and ecosystem formation (table III). Regarding the regional orogenic phases and the tectonic origin of the fractures, we postulate that the different types of fracturation originated from the different chemical and mineralogic composition of the rocks. Significant differences exist in both the calcite and dolomite content, in the insoluble residue content (table IV) and in the percentage of organic matter of the carbonate-free residues (table V, fig 8). The results indicate that the differences in rock composition arose early at about the period of sedimentation. The origin of the differentiation might be due to the sedimentation conditions and environment (fig 9). It is concluded that the present-day plant soil ecosystems may be related to the marine sediment environments of the Jurassic period (fig 10)

POST JURASSIC BRITTLE TECTONICS OF THE HAMMAM ZRIBA MINE (NORTHEASTERN TUNISIA) AND RELATED KARST AND FLUORINE, BARYTINE AND CELESTITE OCCURRENCES IN CARBONATE ENVIRONMENTS, 1991, Melki F. , Zargouni F. ,
The Hammam Zriba mine is located in a lenticular horst structure, of varying width (0.3 to 1 km) and NNW-SSE strike over about 3 kms. The mineralization is strata-bound at the top of massive Portlandian limestones and is overlain by embedded Middle to Upper Campanian limestones with marl intercalations. This horst has formed during the late Jurassic as an emerged block bound by major faults that were remobilized later during various deformation stages. These facts are clearly documented by field observations and tectonic analysis essentially along the N160-N180 trending faults in the Portlandian lithofacies. These fractures have also controlled the palaeomorphological framework of the uppermost part of the Portlandian massive facies. The overlying Campanian unit exhibits onlap structures that rest on the irregular eroded karstified and mineralized surface which forms a screen surface for the upward channelized fluids and mineral formations in karst and graben. Fluids were apparently channelized by faults trending N070-N090 and N160-180, a few hundred metres long, that have facilitated karst, dissolution and mineral deposition during tectonic events

LATE-STAGE DOLOMITIZATION OF THE LOWER ORDOVICIAN ELLENBURGER GROUP, WEST TEXAS, 1991, Kupecz J. A. , Land L. S. ,
Petrography of the Lower Ordovician Ellenburger Group, both in deeply-buried subsurface cores and in outcrops which have never been deeply buried, documents five generations of dolomite, three generations of microquartz chert, and one generation of megaquartz. Regional periods of karstification serve to subdivide the dolomite into 'early-stage', which predates pre-Middle Ordovician karstification, and 'late-stage', which postdates pre-Middle Ordovician karstification and predates pre-Permian karstification. Approximately 10% of the dolomite in the Ellenburger Group is 'late-stage'. The earliest generation of late-stage dolomite, Dolomite-L1, is interpreted as a precursor to regional Dolomite-L2. L1 has been replaced by L2 and has similar trace element, O, C, and Sr isotopic signatures, and similar cathodoluminescence and backscattered electron images. It is possible to differentiate L1 from L2 only where cross-cutting relationships with chert are observed. Replacement Dolomite-L2 is associated with the grainstone, subarkose, and mixed carbonate-siliciclastic facies, and with karst breccias. The distribution of L2 is related to porosity and permeability which focused the flow of reactive fluids within the Ellenburger. Fluid inclusion data from megaquartz, interpreted to be cogenetic with Dolomite-L2, yield a mean temperature of homogenization of 85 6-degrees-C. On the basis of temperature/delta-O-18-water plots, temperatures of dolomitization ranged from approximately 60 to 110-degrees-C. Given estimates of maximum burial of the Ellenburger Group, these temperatures cannot be due to burial alone and are interpreted to be the result of migration of hot fluids into the area. A contour map of delta-O-18 from replacement Dolomite-L2 suggests a regional trend consistent with derivation of fluids from the Ouachita Orogenic Belt. The timing and direction of fluid migration associated with the Ouachita Orogeny are consistent with the timing and distribution of late-stage dolomite. Post-dating Dolomite-L2 are two generations of dolomite cement (C1 and C2) that are most abundant in karst breccias and are also associated with fractures, subarkoses and grainstones. Sr-87/Sr-86 data from L2, C1, and C2 suggest rock-buffering relative to Sr within Dolomite-L2 (and a retention of a Lower Ordovician seawater signature), while cements C1 and C2 became increasingly radiogenic. It is hypothesized that reactive fluids were Pennsylvanian pore fluids derived from basinal siliciclastics. The precipitating fluid evolved relative to Sr-87/Sr-86 from an initial Pennsylvanian seawater signature to radiogenic values; this evolution is due to increasing temperature and a concomitant evolution in pore-water geochemistry in the dominantly siliciclastic Pennsylvanian section. A possible source of Mg for late-stage dolomite is interpreted to be from the dissolution of early-stage dolomite by reactive basinal fluids

DOUBLE FOURIER-SERIES ANALYSIS OF COCKPIT AND DOLINE KARST NEAR BROWNS TOWN, JAMAICA, 1991, Brook Ga, Hanson M,
Double Fourier series models of two doline and two cockpit landscapes in northern Jamaica, each 2 x 2 km in area, explained 92% and 90%, and 73% and 58% of the variance in the topographic data. The ten most significant waves accounted for 74% and 76%, and 61% and 58% of the variance in each model. The greater importance of frequency pairs of longer wavelength in the doline karst models suggests that there are fewer horizontally and vertically persistent bedrock fractures controlling topographic development in the doline areas than in the cockpit terrains-a fact confirmed by fracture-trace mapping. Frequency pair orientations and powers indicate that northwest- and northeast-trending fractures exert a major influence on topographic development in the cockpit terrains and that east-trending fractures are relatively more important in the doline areas. Further studies are needed to determine if Fourier models of the doline, cockpit, and tower karst styles differ in a consistent fashion and to find out to what extent these differences are related to the bedding, fracture, and relief characteristics of the karst bedrock

DOLOMITE-ROCK TEXTURES AND SECONDARY POROSITY DEVELOPMENT IN ELLENBURGER GROUP CARBONATES (LOWER ORDOVICIAN), WEST TEXAS AND SOUTHEASTERN NEW-MEXICO, 1991, Amthor Je, Friedman Gm,
Pervasive early- to late-stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present-day burial depths of 1.5-7.0 km. Seven dolomite-rock textures are recognized and classified according to crystal-size distribution and crystal-boundary shape. Unimodal and polymodal planar-s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void-filling cement. Planar-e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non-planar-a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non-planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space. Dolomitization is closely associated with the development of secondary porosity; dolomitization pre-and post-dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non-fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late-diagenetic coarse-crystalline dolomite, whereas the non-porous intervals are composed of dense mosaics of early-diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late-stage dolomitization in a deep burial environment, resulting in coarse-crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites

Pertes du Gave d'Ossau et naissance du Neez (Pyr.-Atl., Fr.), 1992, Bauer J. , Oller G. , Sabrier R.
SINKS OF THE OSSAU GAVE AND SOURCE OF THE NEEZ RIVER (PYRENEES) - A Gave is a mountain torrent in the Pyrenees. The water supply of the city of Pau is depending almost entirely on a karstic spring called Oeil du Neez (the Neez Eye, which means spring of the Neez). LOeil du Neez is the main resurgence - among several others - of water sinks in the Gave bed across the urgonian limestone ridge of Arudy. Development of this karstic system through the complex structural setting of a frontal thrust has been favoured by the geomorphic evolution of the Arudy basin, dammed to the north by a morainic loop, but also by the presence of highly suitable hydrodynamic and tectonic conditions (water head and fractures network). Water quality of the Oeil du Neez is closely depending on Gave water quality. However, the catchment area proper, of this resurgence, exerts its own chemical influence.

Analyse des conditions de dveloppement de la karstification profonde, 1994, Fourneaux, J. C.
Karstification is a dissolving process which enlarges some channels clefts and fractures and eventually creates caves. The phenomenon Is only possible in deep areas located under the base level, if water flows easily. The analysis of the physical and chemical data accumulated at the beginning of the flood shows hotter and more mineralised waters once the flow starts accelerating. The hydrodynamic study of the phenomenon allows to build a model that explains the deep karstification process. The deep karstification process occurs when a very heterogenous distribution of pressures briefly takes place in the aquiferous system at the beginning of the recharge. This is due to the fact that the waters reach the karstic conduits at different times and therefore the refill and the eviction of waters do not occur uniformly in the saturated zone. Actually, the very mineralised waters located under the base level in the caves, conduits and other holes are evicted first. Then, these waters are replaced by aggressive waters, which are often with a high C02 concentration. As a result, the limestones dissolution process starts again in the area under the outlet point and the splits and bed ding joints keep on enlarging. The heterogeneous distribution of pressures also opens new splits through a corner effect and leads to the development in depth of the karstification process.

RELATIONSHIP BETWEEN FRACTURES AND KARSTIFICATION - THE OIL-BEARING PALEOKARST OF ROSPO MARE (ITALY), 1994, Soudet H. J. , Sorriaux P. , Rolando J. P. ,
The Rospo Mare oil field is located in the Adriatic Sea, 20 km off the Italian coast. The reservoir lies at a depth of 1300 m and consists of a paleokarst oi Oligocene to Miocene age which developed within Cretaceous limestones, now covered by 1200 m of Mio-Pliocene sequences. The oil column is about 140 m 8 high. The karstic nature of the reservoir was identified through vertical, cored drill holes which allowed us to analyse the various solution features and the sedimentary infilling (speleothems, terra rossa, marine clays), as well as their vertical distribution. Erosion morphology at the top of the karst is highly irregular, including in particular paleovalleys as well as many pit-shaped sink holes. Detailed geophysical knowledge of that morphology helped to optimize the development of the field through horizontal drilling. Observations concerning the upper part of the reservoir were compared to a palaeokarst of the same age, outcropping widely onshore, in quarries located nearby. The Rospo Mare paleokarst is an integral part of the ante Miocene paleokarst assemblages of the periphery of the Mediterranean which were formed in tropical conditions. Only the fractures enhanced by meteoric water during the formation of the karat are important for reservoir connectivity. During the formation of the karst there were several phases of dissolution and infilling which modified the geometry of the open fissures and only these fractures play an important role in the reservoir drainage. Vertically we can distinguish three very different zones from top to bottom: at the top the epikarst (0-35 m) in a zone of extension. All the fractures have been enlarged by dissolution but the amount of infilling by clay is substantial. The clays are derived either from alteration of the karat fabric or by deposition during the Miocene transgression; the percolation zone (15-45 m) is characterized by its network of large fractures vertically enlarged by dissolution which corresponds to the relict absorption zones in the paleokarst. These fractures, which usually have a pluridecametric spacing, connect the epi-karst with the former sub-horizontal river system. This zone has been intersected by the horizontal wells during the field development. In this zone there are local, horizontal barriers oi impermeable clay which can block vertical transmissibility. In these low permeability zones the vertical fractures have not been enlarged due to dissolution hence the horizontal barrier; the zone of underground rivers (35-70 m) is characterized by numerous horizontal galleries which housed the subterranean ground water circulation. When these fissures are plurimetric in extent this can lead to gallery collapse with the associated fill by rock fall breccia. This can partly block the river system but always leaves a higher zone of free circulation with high permeabilities of several hundreds of Darcys. These galleries form along the natural fracture system relative to the paleohydraulic gradient which in some cases has been preserved. The zone below permanent ground water level with no circulation of fluids is characterized by dissolution limited to non-connected vugs. Very locally these fissures can be enlarged by tectonic fractures which are non-connected and unimportant for reservoir drainage. Laterally, only the uppermost zone can be resolved by seismic imaging linked with horizontal well data (the wells are located at the top of the percolation zone). The Rospo Mare reservoir shows three distinct horizontal zones: a relict paleokarst plateau with a high index of open connected fractures, (area around the A and B platforms); a zone bordering the plateau (to the north-east of the plateau zone) very karstified but intensely infilled by cap rock shales (Miocene - Oligocene age); a zone of intensely disturbed and irregular karst paleotopography which has been totally infilled by shales. The performance of the production wells is dependent on their position with respect to the three zones noted above and their distance from local irregularities in the karst paleotopography (dolines, paleovalleys)

Tectonic Speleogenesis of Devils Hole, Nevada, and Implications for Hydrogeology and the Development of Long, Continuous Paleoenvironmental Records, 1994, Riggs Alan C. , Carr W. J. , Kolesar Peter T. , Hoffman Ray J. ,
Devils Hole, in southern Nevada, is a surface collapse into a deep, planar, steeply dipping fault-controlled fissure in Cambrian limestone and dolostone. The collapse intersects the water table about 15 m below land surface and the fissure extends at least 130 m deeper. Below water, most of the fissure is lined with a >30-cm-thick layer of dense maxillary calcite that precipitated continuously from groundwater for >500,000 yr. The thick mammillary calcite coat implies a long history of calcite-supersaturated groundwaters, which, combined with the absence of dissolutional morphologies, suggests that Devils Hole was not formed by karst processes. Devils Hole is located in a region of active extension; its tectonic origin is shown by evidence of spreading of its planar opening along a fault and by the orientation of its opening and others nearby, perpendicular to the northwest-southeast minimum principal stress direction of the region. Most Quaternary tectonic activity in the area, including seismicity and Quaternary faults and fractures, occurs on or parallel to northeast-striking structures. The hydrogeologic implications of this primarily structural origin are that fracture networks and caves opened by extensional tectonism can act as groundwater flowpaths functionally similar to those developed by karst processes and that, during active extension, transmissivity can be maintained despite infilling by mineral precipitation. Such extensional environments can provide conditions favorable for accumulation of deposits preserving long, continuous paleoenvironmental records. The precipitates in Devils Hole store chronologies of flow system water-level fluctuations, hydrochemistry, a half-million-yr proxy paleoclimate record, evidence of Devils Hole's tectonic origin, and probably atmospheric circulation

EARLY DEVELOPMENT OF KARST SYSTEMS .1. PREFERENTIAL FLOW PATH ENLARGEMENT UNDER LAMINAR-FLOW, 1994, Groves C. G. , Howard A. D. ,
Modeling of flow and solutional processes within networks of interconnected conduits in limestone aquifers indicates that enlargement occurs very selectively during the early stages of karst aquifer development under laminar flow. If initial flow paths are uniform in size, almost all enlargement occurs along a single set of connected conduits that lie along a direct path between recharge and discharge locations and are aligned along the hydraulic gradient. With a sufficiently large variation in initial aperture widths, enlargement occurs along the flow path offering the least resistance to flow, but since flow rates in laminar flow are proportional to the fourth power of diameter but only linearly proportional to hydraulic gradient, the preferentially enlarged set of fractures may follow an indirect path. Results disfavor earlier suggestions that nonselective cave patterns result from artesian flows (at least under laminar flow conditions) and that all passages should be competitive until the onset of turbulent flow

Karst in siliceous rocks; Karst landforms and caves in the Auyn-Tepui Massif (Est. Bolivar, Venezuela)., 1995, Piccini Leonardo
During the expedition Tepuy 93'. six caves were explored in the precambrian quartzites of Roraima Group, in the Auyan-tepui massif. One of this caves reaches the depth of 370 m and a development of almost 3 km; its name is "Sima Auyan-tepui Noroeste" and it is currently the deepest cave in the world discovered in siliceous rocks The geological and morphological study of this cave has underlined again the importance of deep solutional weathering, along the network of fractures for the formation of caves in siliceous rocks. The different formation stages of the big superficial shafts, called "simas" were observed in some vertical collapse caves explored during the expedition, while galleries with phreatic forms were observed in the deep network of caves. All these deep forms involve karst processes of solution at least in the initial stage.

Results 16 to 30 of 335
You probably didn't submit anything to search for