Geological Setting

The geological context of the Colombian territory is dominated by the convergence of the Nazca and Caribbean tectonic plates of oceanic affinity and the continental South American plate; that convergence is the main source of stress that propagates towards the continent, giving rise to diverse structural styles and complex fault systems. In Colombia, the Andes Cordillera is bifurcated into three mountain ranges known as the Occidental, Central and Oriental cordilleras, separated by the inter-Andean valleys of the Cauca and Magdalena rivers. The study area is in the Middle Magdalena Valley Basin (MMVB), towards the northeast boundary with the Eastern Cordillera, near the Bucaramanga fault, northeast of the structure known as the Nuevo Mundo Synclinal (Figure 1).

Figure 1 Relief map and geological map of the municipality of Rionegro (adapted and modified after ^{Ward et al., 1973}). 

The MMVB is an intra cordilleran basin tilted towards the east, with homoclinal tendency, disturbed by some folds and faults (^{Pérez and Valencia, 1977}). The stratigraphic sequence that commonly describes the VMM basin is made up of the Los Santos, Rosa Blanca, Paja, Tablazo, Simití, El Salto, La Luna and Umir Cretaceous Formations. The “Doña Lucrecia” cave was developed in the Rosa Blanca Formation. According to ^{Morales (1958)}, it is a fossiliferous unit (predominantly mollusks in diverse fragmentation states), very cemented, hence its appreciable hardness, with minor intercalations of marls, calcareous shales, and a tendency to decrease the content and size of macro-fossils towards the ceiling. The Rosa Blanca formation. According to ^{Etayo-Serna and Guzmán-Ospitia (2020)} it is composed of 5 members: 1. Lagunetas member, of Berrasian age (composed of dolomitic limestones and fossiliferous micrites), 2. Carrizal member, of Lower Valanginian age (composed of wackstone alternated with mudstone), 3. Zo member, of Lower Hauterivian age (composed of packstone and wackstone), 4. El Sapo member, of Lower Hauterivian age (composed of wackstone alternating with terrigenocalcareous mudstones) and 5. Zapatoca member of Lower Hauterivian age (composed of wackstone and packstone alternating with muddy sandstones). The thickness reported in the type locality is 450 m, which decreases significantly in the E and W edges of the northern sector of the MMVB (^{Clavijo, 1996}). The area of interest is in an active tectonically area, with the NW-trending Bucaramanga-Santa Marta fault system as the main structure influencing the zone, along with the Cachira River Fault with a dextral strike slip kinematics. It is located northwest of this karstic area, from it, the Sardina Fault and other inferred structures are detached. The Sardina Fault is located to the east of the area, shows a vertical inverse movement and is closely parallel to the eastern limit of a rhyolitic intrusion affecting the Bocas Formation (^{Ward et al., 1973}).

Karst geomorphology

Exokarst: Figure 2A illustrates a typical exokarst morphology associated to the “Doña Lucrecia ‘’ cave, with karren as the main superficial karstic forms, which reveals the occurrence of surface dissolution processes. An example of lapiaz is illustrated in Figure 2B, it can be controlled by lithological and structural, topographic, pedological, meteorological or biological factors (e.g., ^{Soriano, 1985}; ^{Gilli, 2011}). The forms of karren observed in the study area correspond to rinnenkarren, trittkarren and rillenkarren, which are formed mainly by continuous flow of water. The rinnenkarren are channels of a few centimeters of width and depth, but nevertheless can develop several meters in length, Tributary channels were observed; additionally, there are two forms in their channels, and seen in transverse section there are channels in “V” form and in “U” form, denominated Type A and Type B respectively (Figure 2C). The trittkarren are stepped forms with a vertical surface of 1 to 2 cm and horizontal one, which can later evolve into rinnenkarren (Figure 2D). Finally, the rillenkarren consist of small grooves with a few centimeters (cm) wide and deep (1-2 cm) that can reach tens of centimeters in length (Figure 2E).

Figure 2 Superficial karstic forms associated with the immediate surroundings of the “Doña Lucrecia” cave. 

Endokarst (“Doña Lucrecia” cave): The “Doña Lucrecia” cave forms part of the karstic ecosystems of Santander not yet reported and it is located approximately 19 km NW of the Rionegro municipality, MMVB, that is being hosted in limestones and mudstones of the Cretaceous Rosablanca Formation. The morphological features of the cave are depicted in the geomorphological map of Figure 3. The cave system can be divided into several sectors and is distinguished by the size of its gallery, up to 5 m (2-3 m on average) wide and 10 (3-4 m on average) high, and variety of speleothems (stalagmites, stalactites, columns, eccentrics, etc). It has small corridors connecting eight halls, and the last of them preserve speleothems. It has good ventilation due to the presence of two pits open to the surface. However, there are no facilities to permit access to the cave. The cave is located at about 1000 m.a.s.l. and it does not present notable variations in altitude, which do not exceed 10 m. Each sector is named based on distinctive features, usually derived from outstanding speleothems or some significant event.

Figure 3 A. “Doña Lucrecia” cave map. B. Rose diagram showing a principally NW-SE trend in the directions of the “Doña Lucrecia” cave´s passages. 

The route of this cave begins with an entrance of approximate 1 m high and 1.5 m wide that follows a course towards the W. The 1st room (main hall) with SE-NW orientation is about 22 m long, 2-4 m high and 2.5 m wide (on average), where a large number of stalactites and stalagmites some of them in process of fossilization, teeth of saw, castings and some fallen blocks occur. The 2nd room (radial shape) with EW orientation and connected by a SN gallery, is 6 m high (on average) and 3 m radius, with a lot of castings, curtains, and guano. Crossing this gallery, it is found the 3rd room with SN orientation, which is about 30 m long and 8 m high (on average), where fallen blocks, curtains, moon milk and a chasm occur. Other gallery with SE-NW orientation and 6 m long, 2 m wide and 4 m high that ends with a gap of 1.5 m in diameter to enter the 4th room with SE-NW orientation, which is 16 m long, 4 m wide and 3 m high (on average), where a lot of castings, blocks and stalagmites occur. Then, it is found the access to a gallery with SE-NW orientation, which is 10 m long, 3 m wide, and 2 m high, the 5th room has a EW orientation, which is 15 m long, 5 m high and 6.5 m wide (on average), in which flood deposits, many fallen blocks and few speleothems are observed. Afterwards, the 6th room is found, which has SE-NW orientation and is 12 m long, 7 m wide and 2 m high (on average), with columns, guano, fallen blocks and a depression to enter a gallery of about 6 m that will lead to the next room. The 7th room with SE-NW orientation is about 20 m long, 5 m wide (on average) and 3 m high (on average), occurring castings, moon milk, fallen blocks, helicitites and antodites. At the end of this room, there is a deviation towards the NE that is about 5 m long and is composed of detrital deposits. Leaving this room, a gallery about 10 m long, 2 m wide and 2 m high is found, which is characterized by the presence of numerous fallen trees and the development of some small speleothems. The 8th room found has E-W orientation and is 6.5 m long and 3 m wide, with a large amount of flood deposits. Finally, going up, a small gallery of 4 m long and 1.5 m wide, the last room (9th room) has a radial shape and is 8 m high and 2.5 m radius, where flood deposits such as columns, stalactites, stalagmites, and some fallen blocks occur. The main features of the “Doña Lucrecia” cave are depicted in Table 1

Speleothems

Zenithal forms: The most common and familiar of all speleothems are the stalactites (Figure 4A). Frequently, stalactites finish their growth process due to lack of “hydric nutrition” because of the path of percolation moving towards other easier areas of the fissure, developing stalactite curtains (Figure 4B), which can be flags (Figure 4C) forming a box work as a secondary form of flow. All these forms have been called ortho geotropes, because of their growth in the sense of gravity (^{Llopis, 1970}). As a consequence of its emigration along the fissure, stalactite curtains can be also considered as parietal forms (Figures 4D). A cham (Figure 4E) of 1.3 meter in diameter communicating the cave with the outside, which was originated by a karstic erosive process in the calcareous rock or collapse of the ceiling of a cavity through which the water percolates to lower levels. It is usually the degeneration of a sinkhole.

There are also plagio-geotropic speleothems, that is, oriented transversally in relation to gravity. These forms have been called eccentrics (Figures 4F-4G), such as antidotes and helictites, for not having a vertical axis like the other stalactites. In the ceiling there are cupolas (Figure 4H) that are concentric “hollows” of yellowish-brown color with diameters that do not exceed 30 centimeters and where bat groups settle, which can be the result of the action of urine of bats (^{Dandurand et al., 2019}). However, there are many other possible origins, such as condensation, water erosion with the cave filled with sediment, hypogenesis, etc.

Parietal forms: These forms are developed in the walls of the cave by different physical and chemical processes. The wash or casting is a laminar flow over a certain surface which facilitates the loss of carbon dioxide (Figures 4G-4L). The forms can sometimes reach thicknesses of tens of meters, reaching to fill large galleries. Moon milk mainly occurs over the surfaces of some stalactites and flowstones. According to ^{Self and Hill (2003)}, it is a microcrystalline to nanocrystalline coagulation of disordered acicular individuals, porous and plastic in nature, and containing 40-80% water by weight. Blisters are rounded deposits filled with calcite or gas and it is formed in walls from solutions under capillary pressure (Figure 4I).

Paved forms: The most known and attractive for scientists are the stalagmites (Figures 4M-4Ñ), because of its shape (negative ortho geotrope) and position of deposits, are the materials that most resemble the clastic sediments, so that the stalagmite crust may have the same chronological value.

Mineralogy and petrography of speleothems

Stalactites: The stalactites in this cavity are conical or cylindrical growths from cave ceilings and they are made up of concentric layers of crystals elongated perpendicular to an axis (Figure 5) running from the cave ceiling to floor. Therefore, the petrographic description is made from the center towards the flanks, where it is observed different levels that vary in their mineral composition and thickness (Figure 5A), with layers oscillating between: 1) level of large calcite crystals with characteristic rhombohedral equant texture; 2) level of aragonite with crystals of different sizes developing needle habit (Figure 5B); 3) level of elongated calcite crystals (Figure 5B); 4) level of a boundary between siderite and magnesite with presence a sectored dissolution (Figure 5A). The last two levels are interleaved, maintaining equal thicknesses and similar characteristics.

Figure 4 Speleothems of the “Doña Lucrecia” Cave. A. Stalactites. B. Curtains following a fissure from the ceiling to the wall. C. Flags. D. Parietal curtains. E. Cham. F. and G. Eccentrics. H. Cupola generated by bats. I. Blisters and stalactites variety “turnip” in the upper part. J. Eccentric grouping in the wall of the cavity. K. Eccentric grouping in parietal forms. L. Incipient appearance of moon milk in the wall of the cave. M. Long stalagmites. N. Stalagmite “variety fried eggs”. Ñ. Stalagmites in formation. 

Figure 5 A. Polished surface photograph of the stalactite in longitudinal (left) and cross-sectional (right) views. B. Stereomicroscopy views of the polished surface of the stalactite; note the layers of the stalactite with its limits and changes in the mineralogy. Arg, aragonite; Cal, calcite; Sid, siderite; Mgs, magnesite. 

As mentioned before, this type of speleothem is made up of concentric layers of crystals elongated perpendicular to an axis. Nevertheless, near axis the direction of crystals grown is subparallel like aragonite level, with the level of elongate calcite is developing columnar aggregates and presence of fluid inclusions whilst in the boundary of siderite and magnesite level presents dissolution with sectored filling as shown in the Figure 6. This is an elongated stalactite, this means that the saturation of the Ca+ anion was high on the water when the minerals formed; the change of the crystals composition is the consequence of the different precipitation events, some of them rich in elements as Fe and Mg.

Figure 6 Cross section of stalactite, showing interleaved calcite, siderite+magnesite (dissolution structure in these limits) and aragonite (growth direction subparallel to the symmetry axis). Equant calcite in the core presents a symmetry axis perpendicular to the cave ceiling. 

Elongated calcite crystals, with a texture that varies from braided to columnar (Figure 7) and siderite-magnesite level develop elliptical aggregates (Figures 7A-7B). On account of the occurrence of carbonates like siderite and magnesite, calcite (Figures 7A-7B), some dissolution structures are fulling alumina-silicate and oxides fluids containing Si, Al, Fe, K, Mg, affected by different diagenetic processes such as recrystallization and dissolution (Figures 7A, 7C). The aragonite recrystallized into rhombohedral crystal of calcite and towards crystal edges demonstrates the recrystallization by the aragonite relic within calcite crystal (Figure 7D). The differences in the morphology of carbonate crystals are the result of changes in the growth mechanisms under specific atmospheric conditions in the past. According to ^{Frisia et al. (2000)}, columnar and fibrous crystals grow when speleothems are continuously wet, and from fluids at close equilibrium conditions (low supersaturation), through a screw dislocation mechanism, whereas the highly defective microcrystalline carbonates form at the same supersaturation range but under variable discharge and the presence of growth inhibitors. However, temperature and drip rate monitoring is necessary to explore what controls the temperature of speleothem forming drip water.

Stalagmites: These speleothems grow upwards from the cave floor with a cylindrical to conical growth pattern (^{Fairchild et al., 2007}) (Figure 8A), and the description was carried out from top to bottom where the lower layers are the oldest as established by the principle of superposition. Figure 8B illustrates stereomicroscopy views of the polished surface of the analyzed stalagmite. Calcite is the main mineral that makes up this speleothem, throughout the cross section exists variations: towards the flanks, elongated crystals of calcite and in the core, calcite is minor size, these changes are due to decreases of the dripping erosive force; Type E surface indicates periods of higher drip and Type L surface indicates periods of lower drip, these surfaces indicate variations in local precipitation (^{Railsback et al., 2013}).

Figure 7 Photomicrographs of the stalactite: A. two more recent levels; B. last three levels showing the dissolution and filling of oxides; C. last three levels without presence of dissolution; D. SEM image showing the dissolution and its respective filling. Arg, aragonite; Cal, calcite; Sid, siderite. 

Figure 8 A. Polished surface photograph of the stalagmite in longitudinal (left) and cross-sectional (right) views; B. Stereomicroscopy views of the polished surface of the stalagmite; note the layer-bounding surfaces in stalagmite. 

Figure 9 also shows the typical microcrystalline fabric, indicating variations in the crystal size and texture of the calcite, and the corresponding EDS spectra. Towards the flanks, crystals elongate have a braided texture indicating high efficiency of CaCO₃ precipitation (^{Fairchild et al., 2007}), whilst in the core occur the microcrystalline texture denoting decreasing on CaCO₃ precipitation. This microcrystalline variety reflects rhombohedral exfoliation (Figure 9) where some fluid inclusions are observed. The presence in the core of some very small crystals of zircon indicates that the stalagmite is not only generated chemically but also argues for a detrital terrigenous origin, which creates floods somewhere in the cave

Figure 9 Above, cross section of the stalagmite, showing a rhombohedral exfoliation (R.E.) reflected in the crystal jointing patterns on the microcrystalline fabric. Below, BSE image of the microcrystalline texture and EDS spectrum of the calcite. 

Flowstones: Flowstones are laminated deposits which form on cave floors or walls, from thin sheets of flowing water film that is slightly supersaturated with respect to calcium carbonate (Figure 10) (^{Frisia and Borsato, 2010}). An advantage for a paleoenvironmental study is that flowstones can be cored with relatively little damage to the cave environment, and they can grow over tens of thousands of years (^{Fairchild et al., 2007}). Flowstones in this cavity appear in different interleaved types of carbonates like calcite (CaCO₃) and siderite+magnesite (Mg, FeCO₃) (Figure 10).

The greater presence of siderite+magnesite (Mg, FeCO₃) (Figure 11) indicates a large contribution of Mg-Fe during its growth and could be related to the layers found in the stalactite, a different event or record of various events with annual climate cycles. These layers share a tendency to display undulations in surface morphology and the lamina structure is dominantly parallel and continuous, but in detail there are many subtypes like this case (Figure 11) not has an axis of symmetry due to the variation on thickness and quantity of layers, reflecting local slopes, water supply and other factors (^{Hill and Forti, 1997}).

Figure 10 A. Polished surface photograph of the flowstone in longitudinal view. B. Stereomicroscopy views of the polished surface of the flowstone; note the changes in the continuity and quantity of the layers on flowstone, in the type of precipitate carbonate, crystal size and fabric. Cal, calcite; Mgs, magnesite; Sid, siderite. 

Figure 11 Cross section of the flowstone, variations on the thickness and parallelism of the layer boundaries (L.B.) due to the asymmetry with the axis perpendicular ceiling. 

Primary columnar calcite has been observed in different phases of the stalactite (Figures 6, 7A-7B) and towards the flanks of the stalagmite. The stratigraphic position varies depending on the drip conditions and the type of speleothem. For the case of the stalactite, the columnar calcite appears at different levels identifying two types of fabrics that are columnar and braided (Figure 7). These fabrics are interspersed at the outermost-old levels, the columnar fabric is characterized by elongated crystals parallel to subparallel in the direction of the growth of approximately 300 to 450 μm in length and between 50 to 60 μm wide. Braided calcite appears in the stalactite in the last level of this, which differs from the previous fabric because the parallelism is slightly lost in the edges of the crystals and its size decreases varying between 100-160 μm in length and 20-30 μm wide, this fabric is also identified in the stalagmite towards the flanks (Figure 8). Primary acicular aragonite shows a fibrous texture (Figure 5) in the direction of sub parallel growth to the cave ceiling (Figure 6). It appears in one of the most recent levels of the stalactite (Figure 6) with a thickness of approximately 3000 μm, limiting with the rhombohedral crystal calcite core, the aragonite crystals are approximately 130 μm long. Sparite cement is composed of equants crystals of sizes approximately of 40 μm, they are developed in the boundaries of the calcite and aragonite levels in the stalactite (Figure 5B); whereas micrite is very smaller size than the previous one with a microcrystalline texture, which predominates towards the center and all the vertical axis of this one in the stalagmite (Figure 9B). Isolated rhombohedral crystals can be identified in the central level of the stalactite, which represent the most recent crystal growth with an equant texture and with the twinning characteristic of the carbonates, being a single crystal of an approximate size 2000 μm (Figure 7C). Siderite represents the oldest stratigraphic levels and an approximate thickness of 340 μm in the stalactite (Figures 5-6) and in the flowstone appears on different layers (Figures 10-11), developing concentric and elliptical aggregates mostly, with an approximate diameter of 30 μm (Figures 8A-8B). At the level where this carbonate variety occurs, there are dissolution structures (D.S.) towards the most recent limit (Figures 5-6), where in some dissolved places there is a filling with aluminosilicate and ferrous fluids (Figures 7A-7D). Magnesite occurs along with siderite in the stalactite and flowstone with similar properties (Figures 5-10), due to enrichment of rate Mg/Fe (Figure 11) and a decrease in siderite precipitation. Zircon is observed in some layers of the stalagmite with very small sizes that were noticed and verified through the microscopy techniques.

Most carbonate petrologists consider Mn²⁺ and Fe²⁺ to be the only trace elements responsible for cathodoluminescence in carbonates. Figure 12 illustrates an example of the cathodoluminescence microscopy and micro-Raman spectroscopy of the flowstone. Checking the oscillatory zones of calcite (generally brown colored). In contrast, the samples of stalactite and stalagmite did not show cathodoluminescence, which can reveal a secondary calcite origin. From the premise of finding fluid inclusions in the crystals of the mineral present in the wall sample and due to their fibrous nature, it is observed that the fluid inclusions do not show a sufficient development for the measurement of the gasses contained in them. However, measurements were made on several crystals contained in the sample; different possible matches of the resulting micro-Raman spectrum with the comparison of the database are observed with different colors. With more rigorous fluid inclusions for a pre-feasibility study, it is possible to identify measurable fluid inclusions, in order to generate a doubly polished section and to make a more complete microthermometric investigation along with micro-Raman.

Speleothem microstratigraphy

^{Martín-Chivelet et al. (2017)} introduced the Speleothem Architectural Analysis (SAA) to establish the growth conditions of speleothems (stalagmites), to genetically interpret stalagmites on a multi-temporal scale. The detailed stratigraphic analysis of speleothems constitutes a fundamental base in the reconstruction of the paleoclimatic conditions of the Quaternary history (e.g., ^{Gázquez and Calaforra, 2016}; ^{Martín-Chivelet et al., 2017}; ^{Jones et al., 2018}).

Figure 12 A. Cathodoluminescence microscopy and B. micro-Raman spectroscopy of the flowstone. 

The stalagmite of interest in this study shows growth axis based on the general shape and geometry, despite not having isotopic evidence to establish the age and duration of growth of the speleothem (stalagmite) or to determine if the changes observed in its texture are associated with hiatus. Figure 13 shows that, according to the micro stratigraphic stacking patterns (fourth order elements) proposed by ^{Muñoz-García et al. (2016)}, the analyzed stalagmite may be progradational, aggradational, and retracted. Macroscopic analysis of the longitudinal polished section of the examined stalagmite (Figure 13) reveals discrete growth layers, with the growth axis showing slight changes in direction, in some cases, the axial laminae flatten to define “stalagmite top” morphologies like those described by ^{Rowe et al. (2020)}, who considered that there is no evidence that growth stopped at any of these morphological surfaces. The core of the speleothem, which is constituted by pale creamy carbonate layers, is flanked by a prominent band of brown carbonate layers. However, we do not have clear evidence on the occurrence of corrosion surfaces, which can be interpreted as a hiatus. The areal limits of the upper surface are marked by a line that defines a slope break and the change into the flanks of the stalagmite. Therefore, the diameter of the apex represents the distance between the two opposite breakpoints observables in the stalagmite polished section. Stalagmites are not perfectly cylindrical, this approach should be adopted with caution, assuming the inherent limitations of 2D microstratigraphy (^{Martín-Chivelet et al., 2017}). These diameters, horizontally measured, constitute the micro stratigraphic stacking patterns log (^{Muñoz-García et al., 2016}). Both the fabrics and the micro stratigraphic stacking pattern of growth layers, stalagmite, from the “Doña Lucrecia” cave provide valuable information about the drip behavior and can be used as a paleoenvironmental indicator, which is very useful when not petrographical sections are available.

Speleothems constitute geological archives of great importance as they can provide valuable information on chronology, stratigraphy, growth rate, geographical distribution, or paleoclimatology (e.g., ^{McDermott, 2004}; ^{Wong and Breecker, 2015}). According to the microstratigraphic stacking patterns observed in the analyzed stalagmite, we concluded that growth rate could have varied significantly over time. However, a lack of knowledge of the relationships between crystal texture and growth rates does not allow a better interpretation. There were no clear hiatus events present in the stalagmite studied.

Figure 13 Fourth order architectural elements: Method for differentiating successive stacking patterns sets within a longitudinal polished section of a stalagmite, as proposed by ^{Muñoz-García et al. (2016)}. To obtain quantitative logs, the distance between the “breakpoints” of each growth layer can be measured along the stalagmite. Variations in this distance through the growth axis allows us to define trends ; thus, retracted, progradational or aggradational sets. 

This speleothem grew in successive layers perpendicular to its longitudinal axis and its growth rate was undoubtedly controlled by the drip rate and the concentration of CaCO3 in the water. However, several parameters, such as water supply rates, chemical kinetics and flow conditions on the Surface of the speleothem probably controlled the growth of speleothems, which may reveal palaeoclimatic signals inscribed into the morphology and stratigraphy of the analyzed stalagmite. Besides, its relationship with climatic oscillations makes a very useful archive to carry out palaeoenvironmental reconstructions. The presence of minerals of detrital origin such as zircon in the stalagmite may be associated with ancient flooding of the cave due to periods of intense rains in which the infiltrated water could cause the underground river to flood the cavity where the stalagmite was forming. However, it must be supported by isotopic studies to contribute to the knowledge of the absolute ages of such speleothem and the paleotemperatures of formation of growth rings or bands.

Karst hydrogeology

The karst hydrogeology characteristics of the “Doña Lucrecia” cave reveal that it is an active one because it has a constant flow of water inside, the speleothems mostly dripping. Hydrodynamically, and under these criteria, the cavity can be considered hydro geologically active. A general knowledge of this karstic cavity is summarized below. Branching networks of conduits has been developed along fractures and bedding throughout the limestones of the Lower Cretaceous Rosablanca Formation, developing secondary porosity and creating suitable pathways for the circulation of fluids in the downstream direction, which constitutes one of the most important karst characteristics of interest in this study. Limestones have a specific hydrogeological behavior because of several factors that control karstification; although the tectonic activity is the one of the major factors that played a very important role in the development of the karst in this region.

There is no doubt that the fractures played a major role in the initiation and enhancement of the karstification process, yet it depends on the density and occurrence of fractures. Several flow zones were probably developed along such plans of weakness and the enlarged openings in the limestones generated a groundwater flow, which produced further dissolution of these rocks, although without evidence of the formation of sinks and springs. This can be explained if we consider that horizontal cave systems tend to form at the intersection of vertical fractures and horizontal seams between depositional layers. The formation of the “Doña Lucrecia” cave is likely a complex process

that involves multiple factors. One key factor is the flow of water through the surrounding rock. As water percolates through the rock above the cave, it likely carries dissolved minerals and other materials with it. When the water reaches the cave, it may deposit these materials in layers, creating the distinctive formations that we see nowadays. However, the process is not necessarily straightforward, depending on the location of the cave and the geology of the surrounding area, the flow of water may be intermittent or irregular. This could result in different parts of the cave forming at different times and under different conditions. For instance, some parts of the cave may have formed when the water table was higher, while others may have formed during drier periods when the water table was lower. In addition, it is worth noting that the formation of caves is a slow process that can take thousands or even millions of years. As a result, the cave may have formed over a long period of time, with different layers forming at different stages of the process.

Overall, while we can make some knowledgeable guesses about the process that led to the formation of the “Doña Lucrecia” cave, it’s important to remember that there is still much we do not know. Further research and exploration may be necessary to fully understand how this unique geological feature came to be. This cave shows evidence of hydric activity with horizontal and vertical water flows, such as underground ravines or drip of speleothems, which changed significantly over geologic time, as part of the complex hydrological and hydrochemical behavior of water percolating through the karstic system. The recharge area is represented by the entrance and very small chasms of cavities that present mainly vertical and intermittent flow of water, which depends mainly on atmospheric conditions and water only filters when there is rain. As shown in Figure 3, the location of the stagnant water bodies in the two wells.

The lack of underground flows such as streams or waterfalls suggests that the cave is not actively being fed by a consistent source of flowing water. Instead, the presence of small bodies of stagnant water in the wells implies that the cave may be affected by changes in the piezometric level, which could be caused by fluctuations in the water table elevation. This interpretation is supported by the fact that the Wells are located at the lowest points in the cave system; therefore, it may represent the current water table level. These observations are important in understanding the hydrology and geomorphology of the cave, as well as the potential implications for the cave ecosystem and management strategies. It would be beneficial to conduct further studies to monitor the water levels in the wells and to explore any potential connections between the piezometric level and the cave’s geological features. Another explanation to this can be related to rainwater that was deposited on a waterproof layer. The water has definitively left the cavity, which begins to undergo fossilization due to lithogenesis. Figure 14 shows a sketch of the “Doña Lucrecia” cave.

Figure 14 Karst ecosystem associated with the “Doña Lucrecia” cave. 

Speleothems as an indicator of neotectonic and paleoseismic activity

The analysis of the orientation of the passages of the caves represents an original and useful approach for the study of neotectonics (^{Littva et al., 2015}). Detailed observations of recent seismic activity are required to determine whether broken or deformed speleothems can be used as effective proxies for paleoseismic activity (^{Zhao et al., 2020}).

The structural analysis in the “Doña Lucrecia” cave consisted of comparing the trends of the orientations of the passages with the youngest regional stress tensor, defined by works carried out in the Middle Magdalena Valley basin, relatively close to the area of study. The passages of this cavity present two main patterns of orientations ranging from N-S to NW-SE (Figure 3B), which coincide with the direction of the minor faults that come off the Cachira River Fault, generally the orientation of cave passages is perpendicular to the orientation of the tensor of greater extension, the direction of extensional component of the stress field has been assumed to be perpendicular to the orientation of cave passages (^{Littva et al., 2015}). Local structures are part of a complex orogeny of the Andean region (Oligocene) (e.g., ^{Lozano and Zamora, 2014}).

Bearing in mind that in the regional stress tensor, the greater compression (σ1) has approximately NW-SE tendency and the lower compression (σ3) NE- SW, it is deduced that the preferred directions of the passages are perpendicular to σ3 and parallel to σ1, which suggests that the “Doña Lucrecia” cave was formed under the action of this regime of stress. The cave has different periods of the karstic cycle with the predominance of maturity, the last room are in senility period and have embrionary conducts in NE sense (^{Llopis, 1970}). In “Doña Lucrecia” cave, fallen blocks, including collapsed stalactites that had fallen from the ceiling, fractures in speleothems (stalactites, stalagmites, and flowstones), are well preserved due to the lack of tourists, and on the cave ceilings, new broken stalactites mixed with old broken stalactites, and the presence of a corridor showing displacement, represent evidence of active tectonics in the area in recent years.