Characterizing and Modelling Fractures and Karst in Carbonate Units - The Porocarste Project (original) (raw)
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Porosity and permeability along fractured zones in carbonates could be significantly enhanced by ascending fluid flow, resulting in hypogene karst development. This work presents a detailed structural analysis of the longest cave system in South America to investigate the relationship between patterns of karst conduits and regional deformation. Our study area encompasses the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves, which are ca. 107 km and 34 km long, respectively. This cave system occurs in Neoproterozoic carbonates of the Salitre Formation in the northern part of the São Francisco Craton, Brazil. The fold belts that are around and at the craton edges were deformed in a compressive setting during the Brasiliano orogeny between 750 and 540 Ma. Based on the integrated analysis of the folds and brittle deformation in the caves and in outcrops of the surrounding region, we show the following: (1) The caves occur in a tectonic transpressive corridor along a regional thrust belt; (2) major cave passages, at the middle storey of the system, considering both length and frequency, developed laterally along mainly (a) NE–SW to E–W and (b) N to S oriented anticline hinges; (3) conduits were formed by dissolutional enlargement of subvertical joints, which present a high concentration along anticline hinges due to folding of competent grainstone layers; (4) the first folding event F1 was previously documented in the region and corresponds with NW–SE-to N–S-trending compression, whereas the second event F2, documented for the first time in the present study, is related to E–W compression; and (5) both folding events occurred during the Brasiliano orogeny. We conclude that fluid flow and related dissolution pathways have a close relationship with regional deformation events, thus enhancing our ability to predict karst patterns in layered carbonates.
Geomorphology, 2020
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Polyphased karst systems in sandstones and quartzites of Minas Gerais, Brazil
2005
The state of Minas Gerais (Brazil) exhibits several major karst areas located in sandstone and quartzite terrains, that display a complex suite of underground and surface karstic forms. In the Espinhaço Ridge, central Minas Gerais, several caves, up to a few hundred metres long, occur in the surroundings of the town of Diamantina. Some of these caves, such as Salitre, represent swallow-holes and show dome pits. Other horizontal caves are characterized by corrosion forms generated into the phreatic zone. In some places, such as in the Rio Preto area, these phreatic forms have been overprinted by ceiling tubes, suggesting a polyphase karst evolution, prior to the draining of the cave. Relicts of passages, with circular cross section up to a metre in diametre, can be found amidst the residual tower-like surface landforms, which constitute a typical scenery in the landscape. Their dissection is due to a generalised karstification in the area, resulting in closed canyons, megakarrens and kamenitzas. In southern Minas Gerais, close to the Mantiqueira Ridge, the caves of the state park of Ibitipoca can extent 2 km in length. These caves are associated with a very large hanging geological syncline. Several of these caves contain active streams, that flow for hundreds of metres before disappearing in sand-choked passages. Keyhole cross sections characterize steeply descending passages in these caves, indicating a change from slow phreatic flow towards a faster vadose flow responsible for the vertical incision of the passage. Such change is probably related to base level lowering and/or to turn in the direction of the water flow. Several generations of wall-pockets, from a few centimetres to over a metre long, occur into the caves. These features are good indicators of the initial phase of speleogenesis, generating the initial conduits by their coalescence. This mechanism is also responsible for cut-off meanders. The main river in the area, which flows along the syncline axis, cuts through a rock barrier, generating a tunnel-like passage. This cave drains, through resurgences in its walls, part of the water that flows in other caves located in the flank of the syncline. The non-carbonate karst features observed in the state of Minas Gerais demonstrate the complex organisation of polyphase karst systems due to the linkage of underground and surface forms not previously connected. As in carbonate areas, these systems may play an important hydrological role. Fig. 1: Localisation of the studied areas
Near Surface Geophysics, 2015
Collapsed paleocave systems are carbonate reservoirs with high internal spatial complexity that are the result of several stages of karst processes. Paleocave-related reservoirs can be spread over large areas with significant thicknesses that are favourable for hydrocarbon exploration. Nevertheless, few studies have provided a detailed understanding of the strong lateral heterogeneity of these reservoirs and their complex karst-controlled development using modern karst terrain analogues. To elucidate this issue, the internal architecture of a collapsed paleocave system has been mapped accurately at the western border of the Potiguar Basin in Northeastern Brazil. The collapsed paleocaves outcrop in an escarpment that delimits the carbonate platform from the transgressive phase of the basin. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) sections were acquired parallel and perpendicular to a road cut and served to parameterize the geophysical signatures of the collapsed paleocaves and the host rocks. The collapsed paleocaves were mapped by identifying high-resistivity zones and high-amplitude ground-penetrating radar reflectors. In contrast, the host rocks are marked by low to intermediate resistivity and ground-penetrating radar reflections that range from low amplitude to almost transparent. The resistivity data and the ground-penetrating radar attribute of the rootsquare energy enabled the mapping of the complex spatial distribution of the collapsed paleocaves system. At depths of approximately 20 m, the paleocaves are more spread out and eventually become isolated while sometimes being connected vertically through shafts. However, at shallower levels, the paleocaves are interconnected by ducts or coalesce into a system of paleocaves that are hundreds of metres long over an area of 12000 m 2 . The results of the study show the detailed internal geometry of this paleocave system at a subseismic scale, which enables the identification of the connectivity pattern among these karst features and the porosity and total volume of the reservoir. This system could serve as an outcrop analogue for other collapsed paleocave carbonate reservoirs worldwide.
Hundreds of karstic features (n = 266) and gullies (n = 80) are indentified in an area (56.3 km 2 ) of non-carbonate rocks which include pure quartzite (Q), impure quartzite (Qi) and gneiss (G) within the Santana Basin (286 km 2 ), a part of Middle Paraiba do Sul River Valley, southeastern Brazil. Karstic features include caves (n = 32), collapsed dolines (n = 23), solution dolines (n = 209), Sinkholes (n = 02) and resurgences (n = 02). Gullies include active gullies (n = 46) and stabilized gullies (n = 34). Other general features as knickpoints (n = 06) and captured valleys (n = 03) were also indentified. We used GIS to study correlations between the presence of features, precise lithology and relief to indentify possible controls. The results suggest a lithological control on the karstic features with higher frequency of caves, collapsed dolines, sinkholes and resurgences in pure quartzite (Q) and in areas with high relief, but there is no lithological control on solution dolines. Gullies, knickpoints and captured valleys are also not controlled by lithology or relief. The results suggest a fluvial control of gullies and depositional features in valley bottoms caused by headward erosion, after underground captures or knickpoint disruption.
Paleokarst features in the Aptian carbonates of the Barra Velha Formation, Santos Basin, Brazil
Brazilian Journal of Geology
Seismic data, borehole image logs, and conventional well logs were used to investigate the distribution and characteristics of paleokarst features in the Aptian carbonates of the Barra Velha Formation in a pilot area of the Santos Basin, Brazil. Multiple seismic attributes were used to enhance details on the seismic data and highlight key seismic parameters including strata deformation and geometry, continuity of seismic events, and fault patterns. The study found that karst structures are controlled by faults and fractures along structural highs, which served as a conduit for the flow of dynamic fluids that dissolved the carbonate materials. Several closed, circular depressions and bright spots identified in the northeastern portion of the study area represent possible sinkhole structures. Epigenic and hypogenic processes due to the action of meteoric water, hydrothermal activity, and intra-formation acidity along regional unconformities in the Barremian-Aptian may have been responsible for the dissolution. Limitations of this study are related to the difficulty of integrating multiple datasets with various scales. However, the higher confidence for the occurrence of the karst features is provided by borehole images at the sub-seismic scale. The findings of this study hold significant relevance for the strategic planning of energy development and carbon sequestration initiatives in the Brazilian continental margins, thereby aiding in informed decision-making.
The salitre cave karst in the quartzite rocks of diamantina, Minas Gerais, Brazil
This work presents the main morphological characteristics of the Salitre Cave, located in the municipal district of Diamantina -MG and formed in quartzite rocks of the Espinhaço Supergroup, Sopa-Brumadinho Formation as an example of a well-developed karst system. This system now supported by two to three independent small seasonal rivers and displays well-developed dissolution and breakdown structures, as a result of intense intemperance activity. This karst formed, probably before the beginning of Quaternary taking advantage of a system of fractures, normal and thrust faults caused by Espinhaço and Brasiliano events in the Middle to Late-Cambrian. This work attempts at presenting an integrated view of the investigated area and at emphasizing its importance for the understanding of the processes, which occurred in the in carbonatic, quartzitic and silico-carbonatic rocks of Rio San Francisco and Rio Jequitinhonha Basins among the chain of the Espinhaço Range on east and the river valley of San Francisco on the west.
Hypogenic Karst Dissolution in Carbonate Rocks has Implications for Karstified Carbonate Reservoirs
Dissolution along zones of preferential flow enhances anisotropy in geological media and increases its complexity. Many karst models tend to be descriptive and fail to predict localization and structure of karst systems. Conduit architecture is difficult to derive from borehole data, and it can be hardly mapped even with the advent of 3D seismic techniques. Therefore, analysis of the geometry of karst conduit systems remains elusive where direct access to karst conduits is not possible.The present study investigates the relationship between lithofacies, tectonics and karstification in the Neoproterozoic Salitre Formation, located in the central-eastern Brazil. This unit comprises several systems of caves including the Toca da Boa Vista and da Barriguda hypogenic caves, the largest in South America, and focus of this study. We focused on cave mapping and morphogenetic analysis, determination of petrophysical properties, thin-section description, micro-tomography, and isotopic analysi...
Geomorphological units in Arcos-pains karst region, Minas gerais, Brazil
Acta Carsologica, 2023
Geomorphological units in Arcos-pains karst region, Minas gerais, Brazil The Arcos-Pains Karst Region is approximately 850 km 2 , formed by massive carbonate rocks that host hundreds of caves, rock shelters and shafts. Many fossil discoveries exist in the region; one can also find rock paintings and other archaeological remains. A ruiniform landscape, weathered caves and continuous or isolated outcrops characterize the karst. They are cut by diaclasis, fractures and fissures, separated by flattened relief with dolines, uvalas, ponors and blind valleys. The evolution of karst morphology in the region reflects pluviometric and paleoclimatic variations, with faults and fractures from neotectonics events exerting control over water flows. One can observe four geomorphological compartments in this region, each with specific characteristics. The region is an outstanding example of the Brazilian intertropical karst and one of the most important Brazilian mineral deposits of limestone and dolomite. So, delimitating its geomorphological units can facilitate understanding the speleological heritage and guide strategic decisions for sustainable use.