Dissolution Cavities Research Articles

Hydrochemistry and sediment characteristics of polar periglacial lacustrine environments on Fisher Island and Broknes Peninsula, East Antarctica

Fisher Island and Broknes Peninsula in the Larsemann Hills constitute part of a polar lowland periglacial environment between marine and glacial ecosystems. The landscape is characterized by gently rolling hills and broad valleys interspersed with lakes formed in glacially scoured basins. We analyzed the physicochemical parameters and the ionic constituents of water samples from 10 lakes in each of these two locations. Our results showed considerable differences between the two regions and demonstrated the influence of lithology and processes including weathering, evaporation, and atmospheric precipitation. All major cations and anions in the lake waters showed positive correlations indicating balanced ionic concentrations. Unconsolidated sediments were sparsely distributed and scattered over glacial deposits, valley fills, and occasional moraine ridges. The type and rate of sedimentation was mainly controlled by surface run-off and aeolian influx. The sediment samples from lake beds and the catchment area on Fisher Island were immature and poorly to very poorly sorted, consisting of gravelly sand with negligible silt and finer fractions. Sediments had a polymodal grain size distribution with the two major populations lying between -2 and -1 phi and between 0 and 1.5 phi. The sediments were lithic arenite to arkosic in composition and the microtextures imprinted over quartz grains were dominated by mechanical textures resulting from several stages of glacial crushing and grinding. The presence of deep dissolution cavities, cryptocrystalline precipitation, and euhedral crystal growth signified the effect of chemical activity after the deposition of grains in the lacustrine environment.

Unusual polygenetic void and cave development in dolomitized Miocene chalks on Barbados, West Indies

Barbados provides an unusual case of polygenetic cave development within dolomitized chalks and marls of the Miocene Oceanics Group. These diagenetic processes are driven by a succession and interplay of tectonic uplift, fracturing, hypogene fluid injection, overprinting by mixing zone diagenesis, and mechanical and biological erosion in the current littoral zone. The significance of the voids and caves within the chalks on Barbados are: 1) these appear to be the first dissolution caves documented in dolomitized chalk, and 2) these features show a polygenetic origin documenting the diagenetic changes in lithology that allowed the development and preservation of these cave types.

The Long Canyon Deposit: Anatomy of a New Off-Trend Sedimentary Rock-Hosted Gold Discovery in Northeastern Nevada

Abstract The Long Canyon deposit is a new sedimentary rock-hosted gold discovery located in northeastern Nevada, over 150 km east of the well-explored Carlin trend. Unlike most examples of this type, the deposit is hosted in a sequence of Cambro-Ordovician carbonate rocks deposited in a periodically emergent, platform to shelf-edge environment. The strata were strongly deformed and weakly metamorphosed during a mid-Mesozoic orogenic event that segmented and extended a brittle, 80-m-thick dolomite layer into a series of elongate, N- to NE-trending boudin blocks, while the enclosing limestone underwent ductile deformation. This unusual structural architecture focused subsequent contractional deformation, as well as Tertiary extensional brittle deformation, forming pathways for meteoric and hydrothermal fluids, which formed and enlarged dissolution cavities and dissolution breccias along boudin margins, boudin necks, and normal faults. Gold occurs primarily in zones of polyphase dissolution breccias that are localized in minor faults and fold hinges in the structurally complex areas along and adjacent to dolomite block margins. Two general phases of brecciation are present, including calcareous breccias and later, Fe ± As oxide-rich breccias, which carry the highest gold grades. Breccias are accompanied by variable pervasive silicification. Alteration assemblages include zoned dickite + white mica + hematite in the cores of mineralized zones, with peripheral kaolinite + sericite and goethite + hematite and, ultimately, more distal illite + smectite. Local preservation of fine-grained sulfides encapsulated in calcite and silica suggests that the iron oxide assemblages may represent oxidation of fine-grained pyrite + marcasite + arsenopyrite assemblages. As currently defined, the Long Canyon deposit extends for 3 km in a northeasterly direction and consists of several subparallel zones of mineralization focused on boudin block margins, boudin necks, and incipient boudins. It contains a combined (measured, indicated, and inferred) estimated resource comprising 2.2 million ounces of gold, at an average grade of ~2.3 g/t gold. Metallurgical work indicates that the deposit is highly amenable to cyanidation by heap leaching or milling. The Long Canyon deposit is open to further exploration and resource additions, and is shallowly buried, relatively high grade, and thoroughly oxidized. The stratigraphic position of the Long Canyon deposit on the Paleozoic shelf and its location well east of the majority of identified major trends indicate significant potential for new discoveries in this underexplored area of eastern Nevada.

Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits

This study provides insights into the diagenesis of a SE Asian Tertiary carbonate platform and the variations in alteration that occur across platform top, margin, slope and adjacent basinal deposits. A petrographic, cathodoluminescent and stable isotopic study allowed evaluation of the diagenesis of the Kedango Limestone of Borneo. The Kedango Carbonate Platform developed as a predominantly low to moderate energy system in the semi-enclosed marine embayment of the Kutai Basin during the Eocene to Miocene. Early alteration through pervasive micritisation and minor isopachous fringing, syntaxial overgrowth and bladed to radiaxial cements was due to marine diagenesis on the basis of limited, relatively enriched stable-isotope compositions and non-luminescent cathodoluminescent (CL) characters. Minor early marine cementation formed just in the shallowest-platform top and margin deposits, and was likely promoted in high energy portions of the platform due to wave and/or current activity. Dissolution cavities, banded calcite cements with variable CL characteristics and depleted stable-isotope compositions, micritic sediment infiltration and alveolar textures are minor features, but together are indicative of localised subaerial emergence and meteoric diagenesis. The most prevalent and pervasive diagenetic feature of the deposits is the neomorphic alteration and replacement of metastable bioclasts and micritic matrix, together with contiguous calcitisation of pore spaces. Burial fluids with a marine character are inferred as the parent diagenetic fluid, since stable-isotope compositions for the neomorphic spar is largely consistent with precipitation from SE Asian Oligocene–Miocene seawater in a burial environment. Tectonic subsidence, the protected semi-enclosed marine embayment setting, and probably terrestrial runoff, together with component variability reflecting local environmental variations, are inferred to have been major controlling influences on the diagenesis of the Kedango Platform. Variations in diagenesis across bathyal, slope and shallow-platform deposits of the Kedango Limestone are not as marked as in other platforms from the region.

Caves of the Nile Valley (Governorate of Assiut, Middle Egypt): a long-term interaction between human societies and their environment

In the Governorate of Assiut, Egypt caves are one of the most important morphological features of the Nile Valley landscape. They can be divided into three types: dissolution caves, caves of rock-cut tombs dated from the dynastic period and caves derived from ancient quarries. The variation in the functional role of caves in the area throughout the centuries provides a good example of the interrelationship between man and his environment. The geographic nature of the Nile valley, geological characteristics, the morphology of the caves, and the political and social conditions explain why natural caves have not been exploited since Pharaonic times and why man-made caves, especially caves of rock-cut tombs, were used instead. The characteristics of natural caves could not qualify them to perform new functional roles posed by the ambition of ancient Egyptians to build a great civilisation. Their morphology is 'unengineered' and their sites are 'not selected', compared with the caves of rock-cut tombs whose design takes into consideration the principles of engineering design.

The origin of sulfate mineralization and the nature of the BaSO4–SrSO4 solid-solution series in the Ain Allega and El Aguiba ore deposits, Northern Tunisia

Ain Allega and El Aguiba are among the largest barite–celestite deposits attributed to the Triassic period in Tunisia. They are located in the flysch zone on the eastern edge of the Triassic diapir of Jebel Hamra. The ore body consists of dolomite intensely brecciated and surrounded by marls, clay, gypsum and dolomite which forms the hanging wall of the deposit, and rimmed by Paleocene marls. The ore minerals show a cap-rock type mineralization with various forms and types, in particular impregnation in dolomite, breccia cement, replacement of carbonate cap-rock and open space-filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite and pyrite. Principal gangue minerals are barite, celestite, calcite, dolomite and quartz. The ore minerals are hosted by the Triassic carbonate rocks, which show hydrothermal alteration, dissolution and brecciation.The study of barite–celestite solid-solution shows a bimodal distribution in which the composition is not continuous. Some intermediate compositions are missing; 20–80% SrSO4 for the El Aguiba ore deposit and 50–70% SrSO4 for the Ain Allega ore deposit.X-ray diffraction peaks in the BaSO4–SrSO4 series display considerable broadening at intermediate compositions. The unit cell volume varies from 346.97Å3 in barite (100% BaSO4) to 308.29Å3 in celestite (100% SrSO4). This variation in cell parameters is attributed to the degree of substitution of barium by strontium since there is less variation between ionic radii of these elements. Consequently, these substitutions caused variation in morphologies, position and d-spacing of peaks (200, 011, 113, 312,122, 112, and 111). Some barite crystals have small-scale, strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during deposition. The degree of supersaturation in Ba2+ and Sr2+ controls the growth mechanism and consequently the surface structure, which in turn controls the mechanism of adsorption and incorporation of minor and trace elements into the growing crystal. For a solid solution, the transitional supersaturation for different growth mechanisms can be significantly different for the two end members. One of the most interesting and intriguing phenomena observed in natural crystals of the (Ba,Sr)SO4 solid solution from this study area is the development of compositional oscillatory zoning, consisting of alternating Ba-rich and Sr-rich layers.The sulfur isotope analyses show significant variability for barite and celestite (from 16.2 to 23‰). These values are interpreted as the result of the mixing of two sulfur end-members in the mineralizing fluids, corresponding to thermochemical sulfate reduction of Messinian seawater, together with Triassic sulfate, as sulfur sources.Fluid inclusion studies of celestite show that the BaSO4–SrSO4 solid-solution in both deposits was precipitated from hot saline solution (Th=190±20°C; 16.37wt.% NaCl equivalent in Ain Allega and 8.2wt.% NaCl equivalent in El Aguiba). Consequently, the precipitation could be from mixing of basinal brines with magmatic–meteoric fluid.

The theory and application of DEM-Gassmann rock physics model for complex carbonate reservoirs

The storage spaces of carbonate reservoirs in the Tarim Basin are dominated by secondary pore porosity such as dissolution caves, holes, and cracks (Song et al., 2001). Research has shown that pore shapes can significantly influence the velocities of elastic waves (Eshelby, 1957; Kuster and Toksöz, 1974; Xu and White, 1995; Yan et al., 2002; Sun et al., 2004). Empirical [Formula: see text] relationships such as Castagna's (1985) mudrock line, Han's relations (1986), and Greenberg-Castagna's (1992) relationship that ignore the effect of pore geometry are not applicable to carbonates (Xu and White, 1995; Wang et al., 2009). Cheng and Toksöz (1979) imaged various pore space structures using SEM, and proposed a pore aspect ratio spectrum that can be used to explain velocity prediction. Han (2004) measured velocities of 52 different carbonate rocks, indicating that caves influence seismic velocities due to high rigidity while micropores or cracks significantly decrease velocities of rocks due to the low rigidity. Regarding carbonate reservoirs that are dominated by secondary pore porosity, Eberli et al. (2003) discussed the velocity-porosity relationship for microporosity rocks, moldic-porosity rocks, interparticle porosity rocks, and densely cemented rocks. Wang et al. (2009) analyzed the practicality of three published models (Wyllie time-average equation, Gassmann's equation, and Kuster-Toksöz model) on velocity prediction for carbonate reservoirs in the Tarim Basin, proving that Kuster-Toksöz model is the best one. Meanwhile, they also pointed out that the Kuster-Toksöz model is a very high-frequency model, and is limited to dilute concentrations of the pores. Based on effective medium theories of Norris et al. (1985), Berryman (1992) proposed a new differential effective medium (DEM) model. The matrix in this model begins as phase 1 and is updated at each step when a new increment of phase 2 (inclusions) is added; the process continues until the expected proportion of the inclusions is reached. In this process, the former incremental inclusion (phase 2) and the matrix phase are taken as a new matrix, which makes the inclusion added at each step “dilute.” Unfortunately, DEM is also a high-frequency model. At high frequencies, there is not enough time for wave-induced pore pressure to equilibrate, so it is not applicable to consider the effect of pore fluids on elastic properties. Wu's (1992) arbitrary aspect ratio and Berryman's (1995) 3D special pores are introduced into Berryman's (1992) DEM model to calculate the elastic moduli of dry rock in this paper. Spheres, needles, and penny-shaped cracks are used to represent dissolution vugs (small-sized caves), holes (needle-shaped), and cracks that have developed in the Tarim Basin. These “special” pores are incrementally added into the matrix of carbonate rocks so that the inclusion added at each step is dilute. Then, Gassmann's equation is employed to calculate the elastic properties of the saturated rocks. This rock physics model in this paper is called the DEM-Gassmann's model.

P-wave fracture prediction algorithm using data with limited azimuthal distribution

Many carbonate reservoirs are present in China's Tarim Basin. The storage spaces are mostly secondary and about 99% of the porosity is from fractures and dissolution caves or holes. Fractures represent both the storage space and migration channels of hydrocarbon fluids. Therefore predicting fracture development is a challenge that confronts us in this area.

Key issues and strategies for processing complex carbonate reservoir data in China

The surface structure of carbonate reservoirs in China is rather complicated. The reservoirs are deeply buried, and dominated by secondary dissolution caves, holes, and fractures, which cause strong heterogeneity and anisotropy. Such geological settings make it extremely challenging to accurately image the fracture-cave unit, and this thus becomes the key issue in data processing.

Origin and architecture of fractured-cavernous carbonate reservoirs and their influences on seismic amplitudes

Ordovician carbonate is important for petroleum exploration and production in the Tarim Basin. The matrix porosity of the carbonate reservoirs is usually less than 2% because of their age-old sedimentary history and depth (deeper than 5500 m). The storage spaces are secondary dissolution pores and fractures which are dominated by visible dissolution caves, holes, and fractures (Zhou et al., 2006). The reservoirs are therefore called fractured-cavernous reservoirs. Good quality reservoir usually appears as a strong reflection (called “bead-like reflection” or “BR”) on poststack seismic sections, and the corresponding reservoir is called a fracture-cave body (FCB) which is the main drilling target of current E&P in Tarim Basin. The FCBs are unevenly distributed in carbonate strata, and the storage spaces inside an FCB are also unevenly distributed. Therefore, the reservoirs have both macroscopic and microscopic heterogeneities, which bring great challenges to petroleum exploration.

Studying heterogeneity and anisotropy via numerical and physical modeling

Palaeozoic carbonate rocks are widely distributed in China. The storage spaces of these carbonate reservoirs are mainly secondary and include dissolution caves, holes, and fractures. The original porosity is less than 1% in most formations. Hydrocarbon distribution is controlled by the systems of fractures and caves, which have a quasi-layered shape. The reservoir media are heterogeneous (caves and holes) and have varying degrees of anisotropy degree (fractures). So it is important to evaluate the heterogeneity and to check the accuracy of anisotropy inversion.

A new method of formation evaluation for fractured and caved carbonate reservoirs: A case study from the Lundong area, Tarim Basin, China

The carbonate reservoirs in the Tarim Basin are characterized by low matrix-porosity, heterogeneity and anisotropy, which make it difficult to predict and evaluate these reservoirs. The reservoir formations in Lundong area experienced a series of diagenesis and tectonic evolution stages. And secondary storage spaces such as fractures and dissolution caves were developed while nearly all the primary pores have disappeared. Based on a summary of different types of storage spaces and their responses in conventional logs, FMI and full waveform sonic logs which are sensitive to different reservoirs, the comprehensive probability index (CPI) method is applied to evaluating the reservoirs and a standard of reservoir classification is established. By comparing the evaluation results with actual well-logging results, the method has proven to be practical for formation evaluation of carbonate reservoirs, especially for the fractured carbonate reservoirs. In reservoir fluid identification, the multivariate stepwise discriminant analysis (MSDA) method is introduced. Combining the CPI method and MSDA method, comprehensive formation evaluation has been performed for fractured and caved carbonate reservoirs in the Tarim Basin. Additionally, on the basis of secondary pore inversion results, another new method of formation evaluation is also proposed in the discussion part of this paper. Through detailed application result analysis, the method shows a promising capability for formation evaluation of complex carbonate reservoirs dominated by various secondary pores such as holes, caves, and cracks.

Pre-stack inversion for caved carbonate reservoir prediction: A case study from Tarim Basin, China

The major storage space types in the carbonate reservoir in the Ordovician in the TZ45 area are secondary dissolution caves. For the prediction of caved carbonate reservoir, post-stack methods are commonly used in the oilfield at present since pre-stack inversion is always limited by poor seismic data quality and insufficient logging data. In this paper, based on amplitude preserved seismic data processing and rock-physics analysis, pre-stack inversion is employed to predict the caved carbonate reservoir in TZ45 area by seriously controlling the quality of inversion procedures. These procedures mainly include angle-gather conversion, partial stack, wavelet estimation, low-frequency model building and inversion residual analysis. The amplitude-preserved data processing method can achieve high quality data based on the principle that they are very consistent with the synthetics. Besides, the foundation of pre-stack inversion and reservoir prediction criterion can be established by the connection between reservoir property and seismic reflection through rock-physics analysis. Finally, the inversion result is consistent with drilling wells in most cases. It is concluded that integrated with amplitude-preserved processing and rock-physics, pre-stack inversion can be effectively applied in the caved carbonate reservoir prediction.

Characteristics and significance of the burial dissolution of dolomite reservoirs: Taking the Lower Paleozoic in eastern Tarim Basin as an example

Abstract Based on the characteristics of core, thin-section, physical properties and carbon, oxygen and strontium isotope, the characteristics of Lower Paleozoic dolomite reservoirs, dissolution features and mechanism in eastern Tarim Basin were studied. Massive dissolution pore-cave dolomite is the main reservoir type in eastern Tarim Basin. The dissolution pores and caves are mainly medium to small with dense distribution. The fillings in dissolution pores and cracks commonly show negative δ 18 O, generally far lower than the surrounding rock; slightly negative δ 13 C, partially positive; slightly high 87 Sr/ 86 Sr. Proportion of CH 4 and CO 2 is high in fluid inclusions and some of them contain high proportion of H 2 S, which demonstrated that the organic matter was at the high evolution stage when the inclusions were captured, suggesting deep burial circumstances. Silicification and high homogeneous temperature also indicate the presence of hydrothermal fluids. Dissolution includes organic acid dissolution and hydrothermal water dissolution. Acid fluid mainly includes organic acid, CO 2 and H 2 S, which were mainly generated in the process of evolution of organic matter, with a little H 2 S from sulfate reduction, a little CO 2 from volcanic activity. As the carrier of acid fluid, hydrothermal fluids overcome kinetic obstruction of dissolution and dissolve the deep dolomite. Major faults and associated faults provide channels for hydrothermal fluids, the acid fluid was further dispersed in dolomites through numerous inter-crystal pores caused by dolomitization and micro-cracks caused by compaction to complete dissolution for dolomite reservoirs.

Xenotime–hematite aggregates on opaline filaments: evidence for biomineralization in weathered siliciclastic rocks, Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilatero Ferrifero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime–hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology.

Void development on carbonate coasts: creation of anchialine habitats

Anchialine caves in coastal locations develop in two ways: by pseudokarst processes that form talus caves, sea caves, tafoni, fissure caves and lava tubes, and by karst dissolutional processes that form stream caves, flank margin caves, and blue holes. Pseudokarst caves are of minor importance in anchialine cave habitat development, with some lava tubes being notable exceptions. Dissolution caves provide the most extensive, variable, and long-term environments for anchialine habitats. The Carbonate Island Karst Model (CIKM) allows dissolutional cave development in carbonate coasts to be understood as the interplay between freshwater and marine water mixing, sea-level change, rock maturity, and interaction with adjacent non-carbonate rocks. Glacioeustatic sea-level changes of the Quaternary have moved all coastal anchialine cave environments repeatedly through a vertical range of over 100 m, and modern anchialine environments could not develop at their current elevations until ~4,000 years ago when sea level reached its present position. Blue holes form by a variety of mechanisms, but the most common is upward stoping and collapse from deep dissolutional voids. As a result, they provide vertical connection between different levels of horizontal cave development produced by a variety of earlier sea-level positions. Blue holes are overprinted by successive sea-level fluctuations; each sea-level event adds complexity to the habitats within blue holes and the cave systems they connect. Blue holes can reach depths below the deepest glacioeustatic sea-level lowstand, and thereby provide a refugia for anchialine species when cave passages above are drained by Quaternary sea-level fall. Blue holes represent the most significant anchialine cave environment in the world, and may provide clues to anchialine cave species colonization and speciation events.

Provenance of groundwater in the crystalline aquifer of Johannesburg area, South Africa

Recharge dynamics, possible flow path and human impact within the crystalline aquifer of the Johannesburg area have been described based on the hydrogeochemical characteristics and environmental isotopes. The dominant circulation zone has also been identified in the study. Understanding of the groundwater dynamics is very important for strategic management of the resource as the country is dependent on groundwater for its different activities. It has also been observed that mixing of fresh and polluted groundwater takes place at shallow zones within the weathered crystalline rocks and dissolution cavities in dolomites where the rocks are characterized by the lithological and structural complexities. From the compositional features of the groundwater in the Johannesburg area, it follows that the solute composition of the water basically results from two basic contributions: The reaction of meteoric water with weathered zone in the near surface area and direct infiltration through dolomite dissolution cavities. The results further indicate that the main constituent movement mechanism within the crystalline aquifer is through a diffusive process, which could be due to few fractures with depth and possible sealing by calcite precipitation. Therefore, the provenance of groundwater circulation is limited to the depth of 40 m. The natural water quality is found to be highly impacted by acid mine drainage that is derived from gold mining centers distributed in the upper Witwatersrand basin. Key words: Crystalline aquifer, diffusive circulation, dissolution cavities, Johannesburg, water supply.

Groundwater Resource in the Crystalline Rocks of the Johannesburg Area, South Africa

Understanding the groundwater dynamics is very important for strategic management of the water resource as urbanization and population growth put South Africa’s current water supply under tremendous stress. The groundwater resources potential and quality characteristics of the greater Johannesburg area, was thoroughly assessed using hydrogeochemical and environmental isotope techniques. The results of the geochemical and stable isotope investigations help understand the groundwater setting. Mixing process of fresh and polluted water is taking place at shallow zones within the weathered crystalline rocks and dissolution cavities in dolomites, where the rocks are characterized by the lithological and structural complexities. Furthermore, it was confirmed that the solute composition of the water results from two basic contributions: the reaction of meteoric water with weathered zones in the near surface area and direct infiltration through dolomite dissolution cavities. The results further indicate that the dominant movement mechanism of ions is through a diffusive process, which could be due to the limited availability of fractures with depth and its possible sealing by calcite precipitation. Consequently, the provenance of groundwater circulation is limited to shallow depths, however, dissolution cavities, fractured and sheared zones allow deeper circulation of groundwater. Moreover, the natural water quality is found to be highly impacted by acid mine drainage which is derived from the gold mines in the Witwatersrand basin as it has been observed in previous studies.

Identification of Underground Karst Features using Ground‐Penetrating Radar in Northern Yucatán, México

Northern Yucatán is a dry tropical area where limestone karst terrain supports a subdeciduous forest that is critical for sustaining the local economy of Mayan people. The 5‐ to 10‐m‐deep vadose zone is characterized by shallow soils (<30 cm thick) with frequent rock outcrops overlying limestone bedrock, which contains the aquifer. This limestone has two important characteristics: (i) lithologic properties that change with depth and (ii) numerous dissolution cavities ranging from small pores to caves, some of them filled with soil (soil pockets). Due to soil shallowness, differences in rock properties are relevant because they restrict or favor root growth; in particular, caves, crevices, and soil pockets allow preferential movement of soil, water, and roots. These features may be important for water and nutrient availability, especially for tree species, but they have not been investigated within an ecological context. We studied the capability of ground‐penetrating radar (GPR) for identifying various limestone layers—the laja (0.3–2.5 m), the sascab (2.5–5 m), and the coquina (5–9 m)—and dissolution cavities. Research was conducted in a limestone quarry so that radar records could be compared with karst features revealed as blasting and rock extraction advanced. Radar records obtained using a 200‐MHz antenna were generally of good interpretative quality; observation depths ranged from 3 to 5 m. In areas that lacked a soil mantle, the interface between the laja and the sascab was clearly identified by GPR at about 2 m. However, the sascab–coquina, and coquina–aquifer boundaries could not be identified. Areas of deep soils (>1 m), microrelief (mounds and plains), and large soil pockets within the limestone matrix were also identified. Main sources of GPR signal attenuation were suspected to be the higher clay and water contents of soil material contained in underlying cavities. As a noninvasive tool, GPR can help to determine properties of the limestone and its dissolution features that are critical to vadose zone–forest interactions.

Malachite, an indicator of major Pliocene Cu remobilization in a karstic environment (Katanga, Democratic Republic of Congo)

Malachite [Cu 3(OH) 2(CO 3)] is a common mineral in the Central African Copperbelt, where it constitutes a high-grade (~ 57% Cu) copper ore and is exploited industrially and by artisanal miners. The genesis of this supergene ore in Katanga has yet received little attention. Here we focus on the genetic processes giving rise to this regionally important mineral. Malachite is secondary to primary copper sulfides, which indicates that it ultimately formed by oxidation of the latter. In fine-grained siliciclastic host-rocks, it is often partly impregnative, commonly with variations in malachite content parallel to the bedding; this type may form either by absolute enrichment or by in situ replacement of sulfides. Within voids of various types, including dissolution cavities in carbonate host-rocks, it precipitates in distinctive forms, such as thick laminated crusts and speleothemes (up to several tens of centimeters in length). REE geochemistry suggests that malachite formed in groundwater environments, through in situ recombination of carbonate ions originating from dissolving host-rock carbonates and downward percolating Cu 2+ (leached from copper sulfides). Thermodynamic modeling confirms this genetic environment, and shows that chrysocolla [(Cu,Al) 2H 2Si 2O 5(OH) 4·nH 2O] coexists with malachite when Si content increases in the groundwater fluids. The importance of karstic features in Katanga is also re-evaluated. Karstic features appear to be rather frequent and to constitute ideal site/traps for the formation of secondary base metal ores.