Integrated geophysical approach for detection and size-geometry characterization of a multiscale karst system in carbonate units, semiarid Brazil

4.1. Surface analysis of karst features using satellite elevation data (mega- to giga-scale porosity)

From a geomorphological point of view, the southernmost edge of the Irecê basin represents a moderately flat carbonate plain surrounded by quartzite scarps on the E, W, and S sides (Figures 2, 3, 4a, and 5). The surface drainage runs mainly from north to south and from west to east within the basin (yellow lines and colored arrows in Figure 5). Major streams are sinking underground in the central sector of the Iraquara karst system, where karst landforms and cave entrances are concentrated (Figures 4a and 5). The discontinuity of the surface drainage in the area where karst landforms and caves are concentrated (Figure 5a and b) indicates that groundwater (yellow arrows in Figure 5a) flows eastward through a cave system to a large river on the eastern border of the carbonate plain (Figure 3a). On the eastern boundary of the carbonate basin, a wide E-W-oriented canyon cuts across the eastern cliff, allowing surface water to escape from the valley (red arrows in Figure 5a).

Figure 4

(a) Residual DEM of the southern edge of the Irecê basin, showing the Iraquara karst system (white rectangle), cave entrances (white dots) and the location of gravity profiles (BB′ and CC′) (Figure 8) and ERT profiles (Figures 9 and 10); and (b) the morphostructural map. (c) Rose diagrams display the strike of relief lineaments within the quartzite basement (black lines) and the Irecê carbonate basin (blue lines) and LD and TR cave systems. The Bambuí Speleological Group (https://bambuiespeleo.wordpress.com/) mapped the cave locations and internal geometries.

Figure 5

(a) Residual DEM of the Iraquara karst system, showing major karst landforms and superficial drainage and main fluvial (red arrows) and underground (yellow arrows) water flow. (b) ASA of the DEM, highlighting relief features. The location of the area is provided in Figure 4.

A morphostructural analysis depicts a high concentration of NNW-SSE-trending lineaments in both the basement and the basin areas (Figure 4b). In the basement, these lineaments define the lateral limits of the basin. In contrast, a secondary set of WNW-ESE-trending lineaments occurs in the western part of the basin. N-S-trending lineaments are concentrated along the main streams within the basin, whereas relief features are widely distributed in different directions over areas outside river valleys.

The Bambuí Speleological Group (Grupo Bambuí de Pesquisas Espeleológicas, https://bambuiespeleo.wordpress.com/) mapped 101 cave entrances in the Iraquara area. This group also mapped the internal geometry of the caves. Their maps of the two major cave systems (Lapa Doce – LD – and Torrinha – TR) reveal 3D mazes of orthogonally oriented conduits (Figure 4c). These cave systems extend over areas of 4.8 km² (LD) and 1.17 km² (TR), with conduits up to 2.3 km long. The preferred conduit directions are NNW-SSE and E-W, which coincide with the spatial distribution of the karst landforms. Dolines and karst valleys expose some cave entrances. The caves connect other dolines, forming a complex karst network. A series of dolines coalesce, forming uvalas and sometimes karst valleys, where a few main streams excavated canyons in the karstic zones. In addition, fluvial-related karst landforms are largely absent in the southern sector of the Irecê basin.

In the study area, dolines have semicircular to oval shapes, preferably oriented along the NNW-SSE and E-W directions, forming sets of dolines along two right-angle axes (Figures 5 and 6). The streams and karst landforms show a similar orthogonal pattern, indicating that the same regional fault and joint systems control the karst system and the surface drainage. A total of 118 dolines were found within the study area of 476.5 km², resulting in an overall density of 0.24 dolines per km² (Figure 6). Suppose that the area with no dolines is excluded. In that case, the remaining area of 131.5 km² has a density of 0.9 dolines per km², reaching a density of 1.4 dolines per km² in the area with the highest concentration of dolines. Considering only the areas occupied by dolines, the region with a higher density of dolines (10–77 dolines/km²) is located in the central valley of the basin, bounded by the main streams on both the western and eastern edges (Figure 6a). Two zones of high doline density stand out at the eastern margin of the mainstream along the eastern border of the Irecê basin (Figure 6b). They are not fully covered by the gravity survey and, therefore, are not the subject of the present study.

Figure 6

Doline density map of the Iraquara karst system derived from the digital elevation model, showing dolines (red polygons) and superficial drainage (black lines) (a) and high-gradient gravity zones (gray areas – Figure 7b) and cave entrances (blue dots) (b). The location of the area is provided in Figure 4.

4.2. Gravity method (mega- to giga-scale porosity)

The residual gravity anomaly map of the Iraquara karst system (Figure 7a) was derived from 594 new gravity measurement stations (Figure 2) and separated from the regional trend using a matched filter. In the study area, the residual gravity anomalies allow one to investigate the internal basin architecture from hundreds of meters deep (giga-scale) to karstified zones at the near surface (mega-scale). The gravity anomalies vary from −3.6 to 3.9 mGal, which is a range of 7.5 mGal. Gravity lows occur in the central sector of the karst system, where karst landforms and cave entrances are concentrated, especially in the karst valley formed at the southernmost end of the mainstream, oriented N-S (Figures 4 to 6). The gravity minima are separated by positive anomalies that trend approximately NW-SE and E-W, reflecting the structural control in karstification, which is equally revealed by morphological lineaments (Figure 4). Peripheral gravity lows seem to be related to areas with more intense fluvial drainage and sedimentation and karst features with incipient surface expressions.

Figure 7

Residual gravity anomaly (a) and ASA (b) maps of the Iraquara karst system over the DEM (grey scale). Yellow lines – superficial drainage, white lines – dolines. Location of gravity profiles BB′ and CC′ shown in Figure 8.

The presence of gravity lows and highs causes strong gravity gradients, setting up high-amplitude zones (>0.15 mGal/m) in the analytical signal (ASA) map (red zones in Figure 7b). These sectors represent a high susceptibility zone of karst features with a surface area of ∼30 km². Considering the superficial and deep cavities in the host rock as the main source of gravity anomalies in this geological context, the high amplitude zones should indicate the areas where the karst features are concentrated. In fact, the main high-amplitude zone (Figure 7b) occurs in the central sector of the karst system, where a high doline density occupies an area ca. 7.5 km long and approximately 6.4 km wide (Figure 6). Furthermore, 62% of the cave entrances are located within the zone with a high gravity gradient. This close relationship between the spatial distribution of dolines, cave entrances, and gravity gradient suggests a promising potential for using the gravity method for mapping karst systems at a semiregional (mega- to giga-) scale, which is discussed in Section 5.

The E-W- and N-S-oriented gravity profiles (BB′ and CC′ in Figures 4 and 7, respectively) depict asymmetric intercalation of positive and negative residual anomalies, which are slightly displaced in the central portion of the Iraquara karst system (Figure 8). The main karstic zone, defined by ASA values >0.15 mGal/m, occupies areas that are 6.2 and 4.6 km long in profiles BB′ and CC′, respectively. Gravity maxima reach ∼3.0 and 0.56 mGal, and gravity minima reach −3.2 and −2.2 mGal in BB′ and CC′ profiles, respectively. Variations of ∼6.2 mGal in the residual gravity field reflect the mass deficit due to sinkholes, caves, and density contrasts within the basin and in the underlaid basement. To support this assumption, we calculated 2.5D three-layer gravity models, including the basement, composed of quartzites (density of 2.67 g/cm³) and the Irecê basin, filled by diamictites (2.58 g/cm³) and limestones (2.71 g/cm³). The densities of quartzites and diamictites were extracted from the classic literature [46]. In contrast, the average density of the limestones was based on rock sample measurements carried out by Borges et al. [47] in the Irecê basin. In addition, we included zones with high-density carbonate rocks (HDC of 2.77 g/cm³) in the gravity models to fit the high-amplitude positive anomalies satisfactorily. The presence of dolomites in the study area, with densities ranging from 2.28 to 2.90 g/cm³, according to Telford et al. [46], can justify carbonates with such high densities in the models. We assigned a 0.0 g/cm³ to the karst landforms and caves since most rivers and caves remain dry over the years. DEM and ERT data and the locations of the cave entrances constrained the location and internal geometry of the karst features. The insignificant gravity effect of the thin sediment layers that fill some caves was neglected in the gravity modeling.

Figure 8

2.5D gravity models along profiles BB′ (a) and CC′ (b) of the southernmost edge of the Irecê basin. Numbers in parenthesis are densities in g/cm³. RMS error is annotated in each profile. ASA – analytic signal amplitude. HDC – high-density carbonate rock. Profile locations in Figures 4 and 7.

The gravity models resulting from 2.5D modeling show RMS errors close to 0.15%, revealing an up-to-600-m-thick basin with an E-W-oriented synform geometry, thinning to the W, E, and S borders and filled with diamictite and limestone layers (Figure 8a and b). To the north, the basin infill shows a constant thickness of ∼450 m toward the central sector of the Irecê basin. Profile BB′ shows a striking gravity gradient of 6.0 mGal in the main karstic zone, which was modeled as an abrupt thickness variation in the carbonate layer (Figure 8a). This internal basin architecture suggests an ∼5.0 km-wide graben-like structure in the central part of the basin. The abrupt borders of this structure are coincident with the two major N-S-oriented shear zones that are deep seated in the Precambrian basement, whose subvertical boundaries could represent the faulted borders of the Irecê syncline. Otherwise, the top of the carbonate layer is smoother along profile CC′ (Figure 8b) parallel to the main axis of the Irecê syncline.

In the uppermost part of the basin, the gravity-modeled cave system extends for more than 4.5 and 3.2 km in profiles BB′ (E-W – Figure 8a) and CC′ (N-S – Figure 8b), respectively. These caves have developed in limestones and high-density carbonate rocks (dolomites). No caves have been mapped outside the main karstic zone in gravity profiles. Most likely, the low coverage of gravity stations and the limited spatial resolution of the method make it difficult to identify smaller and more isolated caves along the profiles.

4.3. ERT (mega-scale porosity)

Three resistivity models resulting from 2D numerical inversions are shown in Figures 9 and 10. The geoelectric models generally display a relatively low resistive superficial layer (∼70 to 550 Ωm), corresponding to reddish soil and alterite, which is the residual carbonate weathering product. The thickness of the uppermost geoelectric layer varies from a few meters to >25 m, presenting abrupt lateral changes, probably associated with less weathered rock zones. These zones, here called underground karst towers, are relatively more resistive subvertical blocks that are partially controlled by the local fracturing system and properly imaged on GPR profiles (see the next section). The high-density carbonate rocks, modeled in the gravity profiles (HDC in Figure 8), could represent the regional expression of these more resistive subvertical blocks. Furthermore, a high-resistivity layer (650–50,000 Ωm) extends downward from the surface and the base of the upper low-resistivity layer to the final ERT investigation depth (∼65 m). We interpret this resistivity layer as the less weathered and fresh carbonate bedrock, and the very high-resistive zones (>50,000 Ωm) might be associated with caves that interconnect the karst landforms at depth.

Figure 9

Satellite image (a), residual gravity anomaly (b), and inversion model of electrical resistivity tomography (c) of the profile ERT01. RMS error is 8.3% after three iterations. Unit electrode spacing is 10 m. Profile location in Figure 4.

Figure 10

Satellite image (a), residual gravity anomaly (b) and inversion model of electrical resistivity tomography (c) and (d) of the profiles ERT02 and ERT03. RMS errors are 4.9 and 7.7%, respectively, after three iterations. Unit electrode spacing is 10 m. Profile locations are presented in Figure 4.

The ∼E-W-trending profile ERT01 crosses semicircular dolines roughly oriented NNW-SSE (Figure 9). A high-resistivity zone (>150,000 Ωm) is located near 330 and 430 m in this profile and between 20 and 45 m deep, revealing a presumable underground passage between these dolines and the large karst valley to the north (yellow star in Figure 4a). The residual gravity anomaly depicts a slight low between distances of 250 and 300 m along the high-resistivity zone (Figure 9a and b), indicating a subtle mass deficit related to the cave. This local gravity low is embedded within a 500-m-wide positive gravity anomaly, suggesting that the cave and/or underground passage formed within denser carbonate rocks, which in turn concealed the local gravity effect of the caves. More expressive gravity lows occur at both profile ends, where dolines with steeper cliffs occur. Likewise, profiles ERT02 and ERT03, located 2.5 km to the east (yellow star in Figure 4a), display similar geophysical patterns (Figure 10). A large doline occupies the SW sector of the area, connected to a smaller doline by an underground passage. A third oval-shaped doline occurs 100 m to the north (Figure 10a). The underground passage can be observed from the surface, with its top at a depth of ∼14 m. Profile ERT03 crosses the area between the two dolines. It shows a high-resistivity zone (>26,000 Ωm) in the first 110 m of the profile and at a depth of 13.5 m (Figure 10d). High-resistivity anomalies (>25,000 Ωm) also occur along profile ERT02 between distances of 0–250 m and 380–490 m (Figure 10c), revealing a buried karst network that spreads northward from the dolines with no evident surface expression (Figure 10a). Residual gravity lows support the presence of caves (Figure 10b), which deepen slightly to the east (Figure 10c).

4.4. GPR (macro to mega-scale porosity)

Figures 11 and 12 show the typical 200 MHz antenna GPR response of the shallowest part of the Iraquara karst system, dominated by weathered carbonate rocks (alterite) and soil, associated with strong signal attenuation. Our interpretation is based on the study by Conti et al. [48], who measured dielectric permittivities and electrical conductivities in unweathered and weathered carbonate rock samples. These authors pointed out that the decrease in the GPR signal amplitude occurs in alterites of the Irecê basin due to an overall reduction in EM impedance contrast and increased GPR attenuation as a result of weathering (ghost-rock karstification) and soil formation. Subhorizontal reflections are attenuated, but their lateral continuity is preserved (Figure 11a and b). Vertical high-amplitude zones are interspersed along the GPR profiles with widths from 0.5 to 2.0 m and 2.0 to 12 m apart, depending on the intensity of the karstification affecting each area. They are associated with non- or less-altered carbonate bedrock, constituting underground karst towers due to differential resistance to weathering. In regions with advanced weathering, the removal of the alterite forms karst pinnacles and towers a few tens of meters high (Figure 12a and b). The larger high-amplitude zones are up to 10 m wide and are normally divided into two columns by a karstification halo around fractures. They form narrow, weathered corridors observed at distances of 12 and 47 m in profile GPR01 (Figure 11) and at distances of 14 and 20 m in profile GPR02 (Figure 12c). This suggests that the abrupt and vertical edges of the high-amplitude zones are caused by the lateral expansion of weathering from vertical fractures and horizontal bedding (Figure 12d).

Figure 11

Typical GPR signal (200 MHz antennas) of the karstified carbonate rocks (alterite) showing vertical high-amplitude zones related to tower-like fresh unweathered (or unaltered) carbonate and stratified residual soil at the profile top (a). GPR closeup on high-amplitude zones: processed GPR signal (b), texture attribute (c), and interpreted section (d).

Figure 12

GPR profile along the doline cliff above the LD cave entrance (a) and (b) 500 m to the north of the profile ERT01 (location in Figure 4). GPR profile (200 MHz antennas) with vertically segmented high-amplitude zones, whose tops are deepening to NW-wards (c). Schematic diagram showing preferential pathways of descending fluid (red arrows) that causes overall ghost-karstification and leads to the formation of tower-like karst features comprising fresh rock blocks (d).

Profile GPR02 was measured 500 m north of resistivity profile ERT01 (location in Figure 4) and at the NE border of the doline that marks the entrance of the Lapa Doce (LD) cave (Figure 12a and b), which is the longest known cave of the Iraquara karst system [39]. The top of the cave is ∼15 m deep and, therefore, is not imaged by the GPR data, which penetrates only 7.5 m below the surface. Several high-amplitude zones occur along the entire GPR profile, 2–10 m apart from each other (Figure 12c). The aerial photograph displays steep walls of doline, comprising orthogonal faulted blocks with subtle horizontal bedding planes (Figure 12b). The exposed carbonate rocks exhibit gray or beige colorations, denoting different degrees of ghost-rock karstification. The high-amplitude GPR zones are vertically segmented, whose tops are deepening to the NW and form a set of at least seven arcuated levels. These geometries would result from intense weathering along the preferential underground water pathways, namely, the subvertical fracture network and the bedding planes, in this case, subtle arcuated NW-wards features (Figure 12d).

Some cavities were identified in the GPR profiles based on attenuated to transparent EM signals (yellow areas in Figure 11d). Partially or completely emptied cavities and voids are supposedly present throughout the karstified region; however, they are more easily identified within the high-amplitude zones because the GPR patterns show greater contrast (Figure 11b and c). In the GPR profiles, the mapped main cavities range from 0.5 to 2.5 m in width and from 0.2 m to 1.5 in height (Figure 11d). The strong attenuation of the GPR signal in the alterites prevents the identification of smaller cavities or other karst features.

5.1. Karst detection and spatial distribution

Due to the subhumid to semiarid climate, the rivers are ephemeral and carry water only after intense rainfall. They flow SE-wards from the headwaters in the quartzite hills to the south and from higher carbonate plains to the north and sink into the carbonate rocks in the Iraquara karst zone (Figures 3–6). They are the major water sources for epigenetic karstification, and the preferential pathways were provided by opened fractures, empty voids, and cave systems resulting from previous karstogenesis. The topographic and doline density maps (Figures 3–6) display a significant concentration of karst landforms (dolines, uvalas, and karst valleys) in a regional, u-shaped valley surrounded by quartzite hills (Figures 3a and 4a). The highest density of karst landforms is located between the main streams (Figures 5a and 6a), which sink underground, feeding a groundwater flow through the cave system and providing strong hydrodynamic conditions for karstification with total removal and cave collapse (zones 1 and 2 in Figure 13). Ghost karstification predominates in the areas further away from the streams (zone 3 in Figure 13), conditioned only by the scarce rainfall regime typical of the semiarid climate.

Figure 13

Schematic diagram of the Iraquara karst system, showing fresh unweathered carbonate (a), weathered/ghost-rock karstified (b), hypogenic cave network (c), dolines (d), uvalas (e), and karst valley (f). The karst system is divided into three domains: (1) underground karstification by total removal (cave network); (2) superficial karstification by total removal (karst landforms); and (3) ghost-rock karstification (alterites and residual soil). Adapted from the thermodynamic concept of karstification described by Dubois et al. [7].

The karst landforms are preferentially oriented in the NNW-SSE or WNW-ESE directions, coinciding with the two main trends of the morphostructural lineaments (Figure 4b). Furthermore, the cave entrances are concentrated in the zones with the highest density of dolines (Figures 4a and 6b). In addition, the internal geometries of the main cave systems present orthogonal galleries oriented in the NW-SE and E-W directions (Figure 4c). This spatial distribution of karst features both at the surface and at depth indicates the influence of structural-tectonic conditions on the development of the Iraquara karst system at a regional scale (mega- to giga-scales).

At the outcrop scale (mega-scale), short-wavelength gravity lows indicate mass deficits caused by the removal of karstified material at depth (caves) and on the surface (karst landforms) (Figure 7a). In contrast, gravity highs are related to the dolines and uvalas edges, constituted by denser carbonate rocks (dolomites). Zones with high-amplitude analytical signal (ASA) indicate high concentrations of karst features (Figure 7b) and, consequently, high gradients of gravity anomalies. In these areas, the densities of dolines vary from 10 to 77 dolines/km², especially in the central valley of the basin, bounded by the main streams on both the western and eastern edges of the karst system (Figure 6b). Zones of high doline density also occur outside the central valley. However, the sparse or lack of gravity coverage does not allow proper investigation of the presence of dolines based on gravity patterns in these areas. Similarly, more than 60% of cave entrances are also located in high-gravity gradients (Figure 6b), confirming that the generation of cave systems and karst landforms must have taken advantage of the same structural conditions. It is worth mentioning that the fact that almost 40% of the cave entrances are distributed outside the high-gravity gradient zones may be due to the low-gravity coverage and the regional character of the cave mapping. Certainly, further studies are necessary to establish precise relations between gravity patterns and karst occurrences. For example, a second high-gradient gravity zone is located on the western edge of the study area, where few dolines are observed. The presence of a cave system is not expected for this region. Therefore, density variations within the sedimentary pile and/or of the underlying basement would be likely causes of the local gravity anomalies.

The 2.5D gravity modeling indicates that the main karstic zone occurs over a graben-like structure in the central portion of the synclinal basin, where the sedimentary pile reaches up to 600 m thick (Figure 8). The subvertical boundaries of the graben-like structure in the central part of profile BB′ (Figure 8a) could indicate the NNW-SSE-striking faults related to regional dextral transtensional tectonics responsible for the onset of the Irecê syncline [37]. This structural control in the distinct stages of the Iraquara karst system evolution is corroborated by morphostructural lineaments (Figure 4b). The main NNW-SSE- and, secondarily, E-W-trending structures accommodated tectonic events that deformed the Archean/Proterozoic basement, forming the internal basin architecture. This structural framework imprinted on the sedimentary pile was reactivated by compressional events, which led to hydrothermal fluid uprising and intense hypogenic karstification [22,38]. Our results indicate that the same structural setting controlled the bedrock fracturing and surface drainage, which determined the hydrodynamic conditions and pathways for both ghost-rock karstification and karstification by total removal that was overlaid on the hypogenic cave system.

The interconnection between karst landforms and cave systems is also well established at the mega-scale, as observed in some doline cliffs and by ERT and gravity data (Figures 9–12). High-resistivity zones (>50,000 Ωm) are located between 20 and 45 m deep, revealing underground passages that connect karst landforms at depth by cave conduits (Figures 9 and 10). Some underground passages can be observed from the surface (Profile ERT03 – Figures 10a and d), while most caves show no surface evidence (profiles ERT01 – Figure 9 and ERT02 – Figure 10a and c). Residual gravity lows associated with high-resistivity zones support the presence of caves along the ERT profiles. Exceptionally, caves and underground passages can grow in denser carbonate rocks. In these cases, the expected gravity response will be a local minimum inserted in a longer wavelength positive anomaly, as occurs in profile ERT01 (Figure 9). In fact, interferences from other causative sources of gravity anomalies can make it difficult to detect karst features, which represents an important limitation of the simple use of the gravity method.

The GPR data provide high-resolution imaging of the uppermost portion of the Iraquara karst system, where ghost-karstification prevails. This geophysical method did not investigate the interconnections between caves and dolines, as caves occur at depths greater than 10 m. In macro- to mega-scale characterization of ghost karstification using GPR, it is possible to individualize shadow zones (low-amplitude reflections), interpreted as alterites and residual soil, from high-amplitude zones related to slightly weathered to unweathered carbonate rocks (Figures 11 and 12). According to Conti et al. [48], the loss of GPR energy is due to a decrease in the dielectric permittivity and an increase in the electric conductivity of the karstified bedrock, reducing the EM impedance and intensifying the attenuation of the GPR signal. The high-amplitude zones are subvertical and separated by a few to dozens of meters. They are segmented at depth, forming arcuated levels of high-amplitude reflections (Figure 12c). This peculiar GPR pattern can be associated with non- or less-altered carbonate bedrock comprising underground karst towers due to differential resistance to weathering. Karst towers are exposed on the doline cliffs with more prominent relief and gray coloration (Figure 12b).

Pueyo-Anchuela et al. [3] proposed a three-step integrated geophysical routine for detecting doline areas for applied and engineering geology. The whole study area is examined in the first step to determine anomalous subsoil behaviors. We analyzed gravity data associated with a high-resolution DEM to identify karst landforms and their spatial distribution in the regional approach. Our processing and modeling of gravity data are coincident with residual gravity lows interpreted as buried caves and dolines by Martínez-Moreno et al. [5] and Argentieri et al. [15]. Both studies show 2D/2.5D models constructed from gravity anomalies, revealing caves at depths similar to our models in Figure 8. The second Pueyo-Anchuela et al. [3] survey step concerns imaging subsoil karst structures using ERT and GPR. Our ERT profiles display high-resistivity zones consistent with the ERT response of air-filled caves (Figures 9 and 10). Resistivities increase (500 to > 10,000 Ωm) if the karst features are filled with air [11,12,49]. In the Iraquara area, the cave roofs are deeper than 15 m, which is beyond the investigation depth of our GPR survey using 200 and 400 MHz antennas. Even GPR data acquired with 80 MHz antennas, which allow greater penetration depth, display no clear response of the cave roofs in the study area. For this reason, we used only GPR data in the third stage of the survey routine, characterizing more karstified and weathered levels of the near-surface karst system from shadow zones in the GPR profiles (Figures 11 and 12), as described by Fernandes et al. [16] and Conti et al. [48]. Certainly, lower-frequency antennas (20–40 MHz) should be able to image the top of the cave system under the local geoelectrical setting.

5.2. Karst characterization

In terms of karstification processes, the Iraquara karst system is divided into three domains (Figure 13): (1) underground karstification by total removal (cave network); (2) superficial karstification by total removal (karst landforms); and (3) ghost-rock karstification (alterites and residual soil). The superficial ghost-rock karstification evolved at the southern border of the Irecê basin along the N-S main axis of the central valley surrounded by quartzite hills that channeled surface drainage from N and W to SE throughout the Iraquara karst system (Figure 4). This topographic setting associated with an orthogonal fracture network (NNW-SSE and E-W directions) created higher hydrodynamic potential across the central valley, where sets of dolines, uvalas, and karst valleys developed (Figures 4, 5, and 13). In the areas surrounding the fracture corridors, low hydrodynamic energy leads to ghost-rock features filled with residual alterite, decreasing the karstification intensity toward the basin edges. The weathering is represented by a maze of unweathered strips controlled by the bedding and the vertical fracture sets (Figures 11 and 12).

In the central valley, ghost-rock karstification overlapped the ancient hypogenic cave system, where sufficient hydrodynamic conditions gathered to cause mechanical erosion of the alterite and collapse of cave roofs. Dolines coalesced, forming uvalas and karst valleys, whereas main streams connected the karst landforms (Figure 5). The cliffs around the doline are formed by rectangular pinnacles and karst towers of less weathered rocks (Figure 12b), whose GPR signal is represented by vertical high-amplitude zones (Figures 11 and 12c). As revealed by the ERT profiles, one of the doline faces commonly marks a cave entrance connecting other sinkholes in an intricate underground karst network (Figures 9 and 10). The area that concentrates the cave system interconnected with karst landforms extends over >30 km², as revealed by the zones with high doline and cave entrance concentrations and high-gradient gravity values (Figures 6b and 7b, respectively).

5.3. Karst evolution

Carbonate rocks can form low-porosity and impermeable sedimentary rocks, whose weathering product is thin, clayey soil in semiarid settings [7,39,50]. However, fracturing and other discontinuities, such as bedding and structural features, create preferential pathways for acidified fluid percolation within the bedrock, triggering chemical dissolution and mechanical erosion [7]. Discontinuities and hydrodynamic energy are key parameters that dictate the type and intensity of karstification, which can cause only in situ weathering of the carbonate with the removal of soluble materials (ghost-rock karstification) or promote the total removal of minerals and create open voids (karstification by total removal) [8,9]. The voids may develop into karst networks and eventually expand to large cave systems. Both types of karstification can occur synchronously in the same region or during karstification episodes that are distinct in time, as seems to be the case in the Iraquara karst system [38,39].

On the basis of GPR data and field observations, we proposed a five-stage model for carbonate rock formation and karst evolution in the Iraquara region (Figure 14). The Neoproterozoic carbonate sequence of the Irecê basin was deposited in a shallow epicontinental marine system and subjected to diagenetic processes, such as recrystallization, dolomitization, and stratification [34, and references therein] (Figure 14a). Continental-scale tectonic events caused intense fracturing and gentle folding in the carbonate layers, which served as preferential pathways to ascending hydrothermal fluids under confined conditions [38,40] (Figure 14b). Upwelling acidic groundwater from deep hydrothermal sources dissolved carbonate minerals in a confined environment of high chemical and hydrodynamic energies (Figure 14c). Karstification by total removal of dissolved material resulted in enlarged fractures and cavities (Figure 14d), culminating in the development of the longest cave system in South America [51]. Klimchouk et al. [38] identified at least three speleo-stratigraphic stories in the caves of the northern sector of the Irecê basin, whose mechanical and fracture stratigraphy controlled the rising fluid recharge, lateral distribution and outflow components [52]. This process spread out to the whole carbonate platform of the Neoproterozoic basin, including the Iraquara cave system.

Figure 14

Schematic cartoon showing the evolution of the Iraquara karst system: Depositional and diagenetic stage (a); tectonic events caused fractures and gentle folding of the carbonate layers (b); upwelling hydrothermal fluids from deep sources under confined conditions along preferential pathways (red arrows) (c); hypogenic karstification by total removal of the dissolved bedrock (d); and superficial ghost-karstification nucleated along the fractures and beddings and formation of residual soil (e).

On the basis of the topographic and geophysical data and field observations, we verified that a posterior epigenic ghost-rock karstification used the same fracture and bedding networks to control meteoric water infiltration and dissolve the carbonate rocks. This process formed from alterites and residual soil under low-hydrodynamic conditions (Figure 14e), to karst landforms, where fracture corridors are present, channeling and potentiating hydrodynamic energy. In the macro- to mega-scales, structural control is evidenced by vertical high-amplitude GPR zones related to unweathered carbonate rocks. In contrast, low-amplitude GPR signals (shadow zones) characterize alterites and residual soil (Figures 11 and 12). At the mega- to giga-scales, the spatial distribution of karst landforms and cave entrances are concentrated in the central valley, as unveiled by topographic and magnetic lineaments [22] and high-gradient gravity zones. A synthesis of the different types of karst features found in the Iraquara karst system is shown in Figure 13.