How to cite: Espinasa-Pereña, R. (2026). Influence of Nevado de Toluca’s volcanic activity on the karst development of the Cacahuamilpa area, Guerrero, Mexico. Revista Mexicana de Ciencias Geológicas, 42(3), 122–139. DOI: https://dx.doi.org/10.22201/igc.20072902e.2026.2.1911

Revista Mexicana de Ciencias Geológicas, v. 43, num. 2, August 2026, 122–139

DOI: https://dx.doi.org/10.22201/igc.20072902e.2026.2.1911

Influence of Nevado de Toluca’s volcanic activity on the karst development of the Cacahuamilpa area, Guerrero, Mexico

Ramón Espinasa-Pereña

Sociedad Mexicana de Exploraciones Subterráneas A.C. Ingenieros 29, Colonia Escandón, C.P.11800, Mexico City, Mexico.

Corrresponding author (R. Espinasa): respinasa@yahoo.com.mx

0000-0002-3619-3954

ABSTRACT

Cacahuamilpa Cave, along with several adjacent inactive systems and two lower active river caves, originated in response to Pleistocene volcanic processes associated with Nevado de Toluca, located approximately 50 km to the north. A catastrophic sector collapse of the volcanic edifice generated a large debris avalanche that evolved into the Pilcaya debris flow, which was channeled southward along the paleovalleys of the Chontacoatlán and San Jerónimo rivers. Prior to this event, these rivers converged to form the ancestral Amacuzac river, which incised the Cacahuamilpa limestone range via the Michapa canyon, establishing the regional base level for karst development.

Subsequent remobilization of the volcanic deposits produced the Mogote debris flows, which overtopped the canyon and infilled the upper reaches of the Amacuzac valley, creating the Michapa plain and advancing approximately 20 km further downstream. The resultant depositional surface, convex in a direction transverse to the regional drainage, forced the two rivers to diverge toward the deposit’s periphery. The eastern drainage evolved into the San Jerónimo River, which continued to breach the limestone through the original canyon before skirting the western margin of the Mogote deposits across the Michapa plain. Conversely, the western drainage became the Chontacoatlán River, which carved a new course through the limestone massif via a preexisting low saddle.

The increased hydraulic gradient across the limestone range promoted significant river infiltration into the karst aquifer. The combination of substantial discharge and a dense fracture network within the limestone facilitated the formation of extensive vadose and water-table–controlled conduits. A later rejuvenation of the Amacuzac river’s gradient resulted in the abandonment of the upper-level caves and the development of younger, currently active river caves at lower elevations. Continued volcanic activity at Nevado de Toluca triggered additional laharic episodes, the deposits of which were emplaced within these younger caves, where they remain preserved as inset terrace remnants. Similar volcanic-derived sediments have also been documented within Cacahuamilpa Cave.

Keywords: Karst geomorphology; Volcanic debris flows; Speleogenesis; Nevado de Toluca; Pleistocene volcanism; Mexico

 

RESUMEN

La gruta de Cacahuamilpa, varias cuevas inactivas cercanas y dos cuevas fluviales activas a menor altitud deben su origen a la actividad volcánica pleistocénica del Nevado de Toluca, situado aproximadamente 50 km al norte. Un colapso sectorial catastrófico del edificio volcánico desencadenó una gran avalancha que evolucionó hacia el flujo de escombros Pilcaya, desplazándose hacia el sur a lo largo de los paleovalles de los ríos Chontacoatlán y San Jerónimo. Antes de este evento, dichos ríos convergían para formar el río Amacuzac, el cual cruzaba la sierra calcárea de Cacahuamilpa a través del cañón de Michapa, estableciendo así el nivel de base regional.

La removilización subsecuente de los depósitos volcánicos generó los flujos de escombros Mogote, que atravesaron el cañón y rellenaron los tramos superiores del valle del Amacuzac, formando la llanura de Michapa y extendiéndose unos 20 km adicionales aguas abajo. La superficie de depósito resultante, convexa en dirección perpendicular al drenaje principal, desvió a ambos ríos hacia los márgenes del depósito. El drenaje oriental se convirtió en el río San Jerónimo, el cual continuó cruzando la sierra calcárea por el cañón original antes de bordear el margen occidental del depósito Mogote sobre la llanura de Michapa. El drenaje occidental, a su vez, dio origen al río Chontacoatlán, que excavó una nueva ruta a través de la sierra calcárea aprovechando un collado preexistente.

El aumento resultante en el gradiente hidráulico a través del macizo calcáreo favoreció la infiltración del agua de los ríos en el sistema kárstico. La combinación de caudales abundantes y una alta densidad de fracturas en la caliza facilitó el desarrollo de extensos sistemas de cuevas en condiciones freático-vadosas. Un descenso posterior del nivel de base del río Amacuzac condujo al abandono de las cuevas del nivel superior y a la formación de las cuevas fluviales activas actuales. La actividad continua del Nevado de Toluca ha generado lahares más recientes que se han depositado dentro de estas cuevas fluviales, dejando restos de terrazas en sus interiores. Depósitos laháricos similares también se han identificado en la cueva de Cacahuamilpa.

Palabras clave: Geomorfología kárstica; flujos de escombros volcánicos; espeleogénesis; Nevado de Toluca; volcanismo del Pleistoceno; México

 

 

INTRODUCTION AND PREVIOUS STUDIES

Gruta de Cacahuamilpa is arguably the most renowned show cave in Mexico and has been protected as a National Park since 1936 (Gómez-Aguado de Alba & Palacio Prieto, 2016a, 2016b). Situated in the northern portion of Guerrero State, near the borders with Morelos and the State of Mexico (Figure 1), the cave comprises a single, monumental passage ranging from 20 to 110 m in width and 20 to 70 m in height. The tunnel is profusely adorned with large stalagmites, flowstone accumulations, and other speleothems, which partition the conduit into a succession of expansive chambers. The explored section of the cave extends for nearly 2 km before terminating at a major collapse and speleothem obstruction.

 

 

Additional caves are present within the same limestone range, including Carlos Pacheco, Agua Brava, Pedro Asensio, Pilares, and the subterranean sections of the San Jerónimo and Chontacoatlán rivers. The latter two river caves each exceed 5 km in length (Figure 2c) and traverse the range approximately 80 to 100 m below the principal passage of Gruta de Cacahuamilpa.

A further karst system, Gruta de la Estrella, emerges within a nearby hill in the Ixtapan Valley (Figures 2a and 2b). This cave exhibits a tiered morphology with an inactive upper level and an active lower conduit. Owing to its well-preserved stratigraphic and morphological features, Gruta de la Estrella has been used by prior researchers as a genetic model for the larger cave systems in the region and was therefore included in this study.

 

 

As one of Mexico’s most well-known caves, Gruta de Cacahuamilpa has generated an extensive body of literature, much of which consists of travelogues and descriptive accounts of visits to the show cave (see Gómez-Aguado de Alba & Palacio Prieto, 2016a, 2016b, and references therein). In contrast, rigorous geological and speleogenetic analyses remain comparatively limited. Since the earliest reports, numerous authors have recognized the presence of volcanic agglomerate plains flanking the limestone range to the east and west. Others have noted the occurrence of rounded volcanic boulders within the main passage of the cave—an early indication that it may once have been traversed by a major river (e.g., Bárcena, 1874; Bolívar Pieltain, 1940; Müllerried, 1944; Bonet, 1971). These interpretations generally support the hypothesis that Gruta de Cacahuamilpa represents an abandoned, inactive segment of the ancestral San Jerónimo River’s underground course.

A markedly different view was advanced by Bretz (1955), who argued that Gruta de Cacahuamilpa, along with Carlos Pacheco and the upper levels of Gruta de la Estrella, originated as phreatic conduits that predate both the incision of the limestone massif and the emplacement of the volcanic agglomerates. Bretz asserted that these caves bear no geomorphic evidence of former fluvial occupation and instead reflect bathyphreatic conditions associated with a deep, confined aquifer. In his interpretation, only the lower-level caves—such as Chontacoatlán, San Jerónimo, and the active lower level of Estrella—were later modified by epigenetic capture of post-volcanic surface streams, developing predominantly through vadose processes while locally reoccupying pre-existing phreatic

voids.

Enjalbert (1964) offered an alternative hypothesis, suggesting that Gruta de Cacahuamilpa was formed prior to the deposition of the volcanic agglomerates. According to his model, a single river—resulting from the confluence of the San Jerónimo and Chontacoatlán rivers within the Ixtapan Valley—originally flowed through the cave. The emplacement of the volcanic agglomerates subsequently blocked and infilled the passage, preserving only its terminal section. In this view, the current subterranean courses of both rivers developed only after the agglomerate deposits were established.

In contrast, Bretz’s (1955) visit to Cacahuamilpa—accompanied by the Spanish émigré geologist and speleologist Federico Bonet—led to significantly different conclusions. Over the ensuing 15 years, Bonet conducted intermittent fieldwork aimed at demonstrating the vadose origin of the cave systems. His findings, published in the Boletín del Instituto de Geología (Bonet, 1971), were informed by detailed surveys of regional caves carried out by members of the Grupo Espeleológico Mexicano (GEM). Bonet ultimately argued for a fully vadose origin, asserting that all caves postdate the valley-filling volcanic deposits, and that the inactive and active cave levels represent successive stages of base-level adjustment.

Subsequent cartographic work by Coons (1976) resulted in the first detailed maps of the subterranean river systems. Since that publication, no major studies have directly addressed the origin of these caves or the broader karst evolution of the area. However, paleoseismological investigations by Garduño-Monroy et al. (2011) and Mejean et al. (2015) examined seismically disturbed stalagmites within Gruta de Cacahuamilpa, providing insights into the region’s neotectonic activity.

This study examines the role of the Pilcaya–Mogote debris flows in reorganizing regional drainage and initiating karst development in the Cacahuamilpa limestone range. It further analyzes the volcaniclastic deposits preserved within active and inactive cave levels to determine their origin, depositional processes, and significance for reconstructing the timing and evolution of speleogenesis in the

region.

 

METHODOLOGY

Surface and subsurface geological and geomorphological investigations were conducted intermittently beginning in 1989. Existing geological maps of the area (Fries, 1960, De Cserna and Fries 1981, Bonet, 1971) were modified through the interpretation of aerial photographs at an approximate scale of 1:50000, acquired in 1990 from Compañía Mexicana de Aerofoto. These were later supplemented and refined using high-resolution imagery from Google Earth and CalTopo.

With the exception of the San Jerónimo and Chontacoatlán river caves, all caves discussed in this study were surveyed or resurveyed by the author and members of the Sociedad Mexicana de Exploraciones Subterráneas (SMES) between 1989 and 2022. Surveys were performed using traditional nylon survey tape and, in later years, a laser rangefinder for distance measurements, alongside Suunto clinometer and compass for inclination and azimuth readings. At each survey station, measurements were collected for station-to-station distance, inclination, and azimuth, as well as transverse dimensions (wall, ceiling, and floor distances) perpendicular to the traverse line. Survey stations were securely marked on cave walls, ceilings, or floors to minimize spatial uncertainty.

This method established a precise polygonal traverse along the principal cave passages. Using COMPASS and CaveXO software, three-dimensional cave models were constructed and georeferenced to the regional topography, enabling spatial comparison with surface geomorphic features. Concurrently, structural characteristics and other geological features within the caves were documented and analyzed.

In February 2010, small stratigraphic trenches were excavated on the floors of both Cacahuamilpa and Carlos Pacheco caves to document their sedimentary sequences. Rounded pumice grains recovered from sand deposits in the Carlos Pacheco trench were collected and submitted for geochemical analysis to the Laboratory of Ultrapure Chemistry (LABQUP) and Mass Spectrometry (ICP-MS) at the Institute of Geology, National Autonomous University of Mexico (UNAM). Analyses were conducted using an Agilent 7500ce inductively coupled plasma mass spectrometer (ICP-MS). Sample preparation involved acid digestion following modified procedures based on Eggins et al. (1997).

To ensure analytical reliability, four certified reference materials (BHVO-1, RGM-1, GSR-2, SDO-1) were analyzed in parallel, with accepted reference values from Govindaraju (1994). In addition, the syenite standard IGLa-1 was analyzed as an independent control for assessing analytical accuracy, precision, and reproducibility (Lozano & Bernal, 2005).

During through-trips of the Chontacoatlán River cave in April 2017 and the San Jerónimo River cave in March 2018, volcaniclastic deposits were described in situ and subsequently plotted onto the existing cave maps of Coons (1976).

 

REGIONAL GEOLOGY

The regional stratigraphy of the Cacahuamilpa area was initially described by Fries (1960) and subsequently refined by De Cserna and Fries (1981). In addition to the geological map published by Bonet (1971), broader regional surveys including the Cacahuamilpa region were later provided by Morán-Zenteno et al. (1998, 2007), though these maps lack detailed resolution due to their small scale. The maps presented here were based on aerial photo interpretation and field work (Figures 2a and 2b).

The carbonate rocks that constitute the Cacahuamilpa mountain range belong to the Morelos Formation, as originally defined by Fries (1960). Based on fossil assemblages, this unit has been assigned an Albian–Cenomanian age. The formation is composed of very pure limestones and dolomites, typically thick- to massively bedded, frequently recrystallized, and with a cumulative thickness exceeding 1000 m (De Cserna and Fries, 1981).

Overlying the Morelos Formation in conformable contact is the Mexcala Formation, also described by Fries (1960). This unit consists of a flysch-like succession of thinly bedded calcareous shales, sandstones, and argillaceous limestones, deposited during the Turonian to Coniacian stages of the Late Cretaceous. Both formations were significantly deformed during the Laramide orogeny at the close of the Cretaceous.

The Balsas Group unconformably overlies all preexisting lithologies in the region (Fries, 1960). Within the study area, this unit is predominantly represented by conglomerates composed of limestone and sandstone clasts—derived from the underlying Morelos and Mexcala formations—set within a clay-rich matrix enriched in red iron oxides and cemented by calcite. Based on its stratigraphic position, the Balsas Group is interpreted to be of Eocene age (De Cserna and Fries, 1981), and marks the onset of post-orogenic continental deposition in the region. Locally, these deposits primarily correspond to talus accumulations at the base of a prominent escarpment formed by a major normal fault that delineates the western margin of the limestone range.

Immediately south of the mapped area, resting atop the Balsas Group with slight angular discordance, is the Tilzapotla Rhyolite. This unit comprises a substantial sequence of ignimbrites and rhyolitic pyroclastic density current deposits. The stratigraphy and structural relationships of the Tilzapotla Rhyolite and its contact with the Balsas Group have been comprehensively investigated by Morán-Zenteno et al. (1998), Alaniz-Álvarez et al. (2002), and Morán-Zenteno et al. (2007).

North of the study area lies the prominent Nevado de Toluca volcano, which underwent a complex and well-documented eruptive history during the Late Pleistocene (Bloomfield & Valastro, 1974, 1977;

Cantagrel et al., 1981; García-Palomo et al., 2000, 2002; Arce et al., 2003, 2005; Capra et al., 2006, 2008). Of particular relevance to this study is a voluminous volcaniclastic sequence that was emplaced southward along two graben-bound valleys, extending as far as the Cacahuamilpa region. These deposits were originally mapped by De Cserna and Fries (1981) as components of the Chontacoatlán Formation. Subsequent work by Capra and Macías (2000) distinguished the uppermost units as the Pilcaya and El Mogote volcanic debris flows deposits (Figure 3). According to Capra & Macías (2000), intense hydrothermal alteration weakened the southern flank of Nevado de Toluca, promoting gravitational instability. This culminated in a sector collapse, generating a large volcanic debris avalanche. As the debris avalanche descended, it incorporated water and rapidly transformed into a cohesive debris flow—the Pilcaya deposit. High water content, combined with the abundance of fine altered material, allowed the flow to behave as a plastic mass capable of long runout. The flow followed the tectonically controlled Coatepec Graben. After emplacement of the Pilcaya deposit, intense rainfall and surface runoff remobilized its unconsolidated upper portions. This process generated two additional deposits known as the Mogote debris flows, which followed the ancestral Amacuzac drainage system, overfilled the limestone canyon and extended onto the Michapa plain, reaching near present-day Amacuzac.

 

 

 

PHYSIOGRAPHY AND KARSTIC GEOMORPHOLOGY

Cacahuamilpa Cave, along with several associated systems, is developed within a north–south-oriented limestone massif situated immediately south of the Trans-Mexican Volcanic Belt. The Nevado de Toluca stratovolcano lies 56 km to the northwest (Figure 2a). The southwestern, southern, and southeastern flanks of the volcano drain into two principal rivers, the Chontacoatlán and the San Jerónimo, which occupy tectonically controlled grabens. Upstream of the limestone range, both rivers are entrenched within deep canyons that dissect a conspicuously planar surface formed by extensive volcaniclastic sequences of debris-flow deposits (Figure 3). This geomorphic surface and the mountain ranges that bound it to the east and west was termed the Ixtapan Valley by Bonet (1971) after the principal settlement located within its bounds, to refer to the paleo-valley that was infilled by the volcaniclastic deposits.

Upon reaching the southeastern margin of the valley, where it contacts the limestone uplands, both rivers are captured by large, well-developed swallow holes (sumideros) (Figure 2c). Their former surface courses are preserved as dry valley remnants incised into the limestone (Figure 4; De Cserna & Fries, 1981). Both rivers reemerge approximately 100 m apart at the upper end of the Dos Bocas (“two mouths”) canyon, situated immediately below Cacahuamilpa Cave.

 

 

The limestone range is characterized by a series of relatively uniform, rounded hills, punctuated by four prominent collapse dolines, which are described in detail below (Figure 4). At the eastern margin of the massif, where the limestone contacts the Mexcala flysch, lies the flat-floored Hoya de Corralejo, a marginal polje that hosts the resurgence cave Agua Brava. The resurgence waters transit the polje before sinking again into the Resolladero of the Chontacoatlán River (Figure 5). Small, isolated outcrops of the Mogote cohesive debris flow southeast of the polje confirm that its formation postdates the emplacement of this volcaniclastic unit (Figures 2b and 4).

 

 

Within the Ixtapan valley, another notable cave system—Gruta de la Estrella—was formed by a tributary of the Chontacoatlán river (Figures 2a and 2b). The system comprises inactive upper passages and an active lower level that transects a limestone hill entirely surrounded by volcaniclastic deposits. Gruta de la Estrella has been widely used as a conceptual model for the speleogenesis of the nearby cave systems (Bretz, 1955; Bonet, 1971).

Downstream of the limestone range, the combined Chontacoatlán and San Jerónimo rivers form the Amacuzac river, which follows a deeply incised valley carved in the Mesozoic rocks, capped by volcaniclastic sequences.

 

MORPHOLOGY OF THE CAVES

Cacahuamilpa

This renowned cave comprises a single, exceptionally large, slightly sinuous passage, measuring between 20 and 110 m in width and 20 to 70 m in height, with a total surveyed length of 1907 m (Figure S1, of the Supplementary Material). The entrance is partially obstructed by breakdown material derived from the weathering of the overlying cliff. The cave interior is richly adorned with monumental stalagmites—some reaching up to 40 m in height—and extensive flowstone deposits (sheet-like speleothems formed when thin films of mineral-rich water flow over cave walls or floors and precipitate layers of calcite as carbon dioxide degasses (Hill & Forti, 1997; Palmer, 2007)). The floor is predominantly flat, gently sloping toward the entrance, and mantled by speleothems and clay. The passage terminates at a zone of collapse and speleothem deposits. Throughout its course, the passage trends approximately parallel to, and at an elevation of about 80 m above, the final segment of the San Jerónimo River cave.

Volcaniclastic deposits are preserved in two distinct areas of the cave, consisting primarily of subrounded andesitic clasts up to 1 m in diameter, lacking imbrication and cemented within a matrix of andesitic pebbles and sand, with occasionally discrete sand lenses exhibiting crossbedding (Figure 6).

 

 

Along much of the first half of the passage, a prominent “bathtub ring” or horizontal waterline is visible on the cave walls and covering stalactite formations, marking a late-stage episode during which the cave was partially flooded nearly to the elevation of the entrance breakdown deposits (Figure S1).

 

Carlos Pacheco

Slightly smaller in diameter than Cacahuamilpa and located at approximately the same elevation, this cave consists of a single passage with a surveyed length of 487 m, ranging from 12 to 25 m in width and 10 to 27 m in height. The entrance occurs at a ceiling collapse located at a right-angle bend in the tunnel (Figure S2, of the Supplementary Material). The upstream segment of the passage, extending to the southwest, runs parallel to the terminal section of the Chontacoatlán river cave. It is nearly obstructed midway by a massive speleothem accumulation and contains several large stalagmites. Nevertheless, much of the floor is covered by a relatively flat, clay-rich fill. The upstream passage terminates abruptly where the ceiling and floor descend, marking what is interpreted as the downstream side of a sediment-choked sump. The downstream passage, trending southeast, averages approximately 10 m in width and 20 m in height, ending against a flowstone wall.

In the large chamber of the southwest passage, just before the sump, a trench was excavated into the floor, exposing a sequence of fluvial sands and clay layers, including a distinctive bed of cross-bedded sand containing rounded pumice pebbles (Figure 7).

 

 

San Jerónimo river cave

The entrance to this cave is located at the contact between the Balsas Conglomerate and the underlying Morelos Limestone, at the base of a gorge incised into the floor of the dry Michapa canyon, which marks the former surface course of the San Jerónimo–Amacuzac rivers (Figures 2c and 4). The cave consists of a large tunnel, generally 30–50 m wide and 40–60 m high, with a floor composed of fluvial deposits. These include large, imbricated andesitic boulders up to one meter in diameter, sand and gravel deposits, and, in certain sections, exposed and polished limestone bedrock.

Seepage from the cave walls has produced extensive flowstone formations along portions of the tunnel. These deposits are continually undercut by the active river, resulting in episodic collapses. The largest flowstone structure, known as Fuente Monumental, is fed by a stream emerging from a small lateral passage. Formerly around 60 m high and over 300 m long, its central portion collapsed during the early 1980s. The total surveyed length of the main passage is approximately 5.6 km (Figure S3, Supplementary Material).

Almost every bend in the passage, as well as many walls and ledges, contains volcaniclastic deposits several meters thick, in some cases partially covered by flowstone. These deposits are composed of large subrounded to subangular clasts, some up to one meter in diameter, lacking imbrication and set within a matrix of gravel and sand of similar lithology. Other layers are formed by angular andesitic clasts a few centimeters in diameter in a matrix of silt size particles of similar composition (Figures 8 and 9).

 

 

Chontacoatlán River Cave and Cueva de los Pilares

The Chontacoatlán river is diverted underground near the stratigraphic contact between the El Mogote debris-flow deposit and the underlying limestone. Prior to this capture point, the river has incised a canyon approximately 700 m in length into the limestone bedrock; its dry, suspended continuation represents the abandoned surface course of the river (Figures 2c and 4).

An upper-level inactive passage, known as Cueva de los Pilares, lies at the cave entrance, approximately 20 m above the active river level. This passage, which receives minor inflow from a small tributary containing substantial flowstone deposits, begins as a broad gallery measuring approximately 20 m in width and height and contains abundant stalagmites. After extending for roughly 200 m, the passage constricts into a low, sinuous tunnel only 1–2 m in height and width. The floor of this constricted section is entirely composed of well-cemented volcaniclastic debris-flow deposits, over which the limestone ceiling remains intact. This tunnel continues for nearly 500 m, terminating in a large chamber—Sala Urquijo—measuring approximately 20 m in both height and width. The chamber is almost completely filled with the aforementioned debris-flow deposit, which has subsequently been re-excavated by the tributary stream. The exposed floor surface corresponds to the upper surface of this volcaniclastic deposit. The inlet stream emerges from a small, still-unexplored conduit formed between the debris fill and the overlying limestone ceiling (Figure S4, Supplementary Material).

The main passage of the Chontacoatlán River cave is slightly smaller in scale than that of the San Jerónimo system, generally measuring 20–40 m in width and height. The passage floor is composed of coarse fluvial deposits, including large imbricated andesitic boulders and sand. Flowstone accumulations are common throughout the cave, with the most substantial deposits typically issuing from small, impenetrable or unmapped tributary passages. As in the San Jerónimo system, collapsed remnants of once-massive flowstone aprons are frequently observed.

Volcaniclastic deposits are conspicuously present along nearly every bend, wall, and ledge, forming accumulations several meters thick and occasionally overlain by a flowstone caprock. These deposits consist of subrounded to subangular clasts, some reaching up to one meter in diameter, set within a matrix of smaller pebbles and sand of comparable composition. Other layers are formed by angular andesitic clasts a few centimeters in diameter in a matrix of silt size particles of similar composition.

Approximately midway through the cave, the ceiling height increases abruptly at a collapse entrance known as El Resolladero, situated at the base of the Hoya de Corralejo polje. Water emerging from Cueva de Agua Brava crosses the flat floor of the polje and sinks into El Resolladero, generating an impressive flowstone cascade. Unstratified volcaniclastic debris-flow deposits are adhered to the southwestern wall of this chamber to an elevation approximately 70 m above the present river level and 40–50 m above the ceilings of both the upstream and downstream passages (Figure 10). This exceptional deposit likely overtopped the cave and spilled into the Corralejo polje.

 

 

The surveyed length of the main tunnel is approximately 5.6 km. Including the Pilares upper-level passage system, the total mapped extent of the cave reaches 6475 m (Figure S5, Supplementary Material).

 

Hoyanco Grande, Hoyanco Chico, and Cueva de Pedro Asensio

Two prominent collapse dolines are situated above the subterranean courses of the regional rivers and are locally referred to as Hoyanco Grande and Hoyanco Chico. These features are located on the eastern flank of Cerro La Corona, almost directly above Cueva Carlos Pacheco. Hoyanco Grande measures approximately 460 × 320 m, while Hoyanco Chico spans 370 × 240 m. Both depressions exhibit vertical to subvertical walls and may be classified as tiankengs according to the criteria outlined by Waltham (2006) (Figures 2c and 4). A third, unnamed and significantly degraded collapse doline is present near the summit of Cerro La Corona, although none of these features currently provides direct access to cave passages.

A fourth collapse structure, Cueva de Pedro Asensio, measuring 140 × 115 m, is located on the northern slope of Cerro Temasól, above the upstream sector of the Chontacoatlán river cave. The northern margin of this doline forms a gently inclined slope, whereas the southern margin is characterized by a vertical to overhanging wall. At its base, the collapse allows entry to a short segment of phreatic passages situated approximately 70 m below the surface. These passages occupy an elevation comparable to that of the Corralejo polje floor, the distal reaches of the Los Pilares branch of the Chontacoatlán river cave, and the majority of conduits within Cueva de Agua Brava (Figure S6, Supplementary Material). All accessible passages terminate in clay-rich sediment fill, indicating cessation of active karstification at this level.

 

Cueva de Agua Brava

Situated within the flat-floored Corralejo polje, Cueva de Agua Brava serves as a seasonal resurgence during the rainy months. The outflowing stream traverses the length of the polje before cascading into El Resolladero, the large collapse skylight that opens into the Chontacoatlán river cave. The entrance passage of Agua Brava is relatively low-ceilinged and heavily coated with mud, suggesting intermittent submergence during high-flow conditions. After several hundred meters, the conduit abruptly increases in size, attaining widths of more than 20 m and heights exceeding 10 m, and terminates in a deep freshwater lake. Beyond this point, a network of smaller passages connects to a broad but low-ceilinged chamber, known as Sala Federico Bonet, which marks the current limit of exploration (Figure S7, Supplementary Material).

 

Gruta de la Estrella

Located within the Ixtapan Valley (Coatepec structural depression), Gruta de la Estrella is developed in a limestone hill entirely encircled by the Pilcaya–Mogote debris-flow deposits (Figure S8, Supplementary Material). The cave captures a surface stream that has incised predominantly into these volcaniclastic deposits, draining a catchment area of approximately 8 km². The entrance opens at the lithologic contact between the volcanic agglomerate and the underlying limestone bedrock. Above the entrance, the remnant of an abandoned dry valley marks the former course of the stream prior to its subterranean capture. The cave has been partially modified and is currently accessible as a show cave.

The active entrance passage measures ~10 m in width, with the stream entrenched into the passage floor and descending through a series of short cascades into a canyon-shaped gallery 10–20 m wide and up to 20 m high. This segment can be followed for 364 m before the stream resurges at the base of the hill, approximately 100 m above the Río Salado, a tributary of the Chontacoatlán river.

Approximately 60 m from the upper entrance, just before the stream descends into the lower active conduit, the tourist path ascends a constructed ladder through a vertical chimney on the left, providing access to an inactive upper level. Upon entry, this upper passage extends briefly back toward the entrance before becoming obstructed by speleothem deposits and breakdown blocks, features consistent with an older, abandoned phreatic or shallow vadose conduit. Directly above the chimney, a conglomerate of stream-rounded cobbles cemented by flowstone is exposed (Figure 11), providing compelling evidence of former fluvial activity at this higher elevation.

 

 

Following a descending trend, the upper inactive passage forms a broad, ancient meander loop that reconnects with the active stream passage within a few meters of the chimney. At this junction, additional cemented cobbles are visible adjacent to the modern tourist pathway (Figure 11). Two short side passages diverge from the meander loop but terminate in speleothem-choked chambers. A separate, small-diameter tunnel branches from the active stream passage and narrows into impassable fissures (Figure S8, Supplementary Material).

 

SPELEOGENESIS OF THE CACAHUAMILPA CAVE SYSTEM

Capra and Macías (2000) attribute the catastrophic flank collapse of the southern sector of Nevado de Toluca volcano during the Pleistocene to extensive hydrothermal alteration of the edifice. The resulting sector failure mobilized as a large debris avalanche which, owing to syn-event water saturation and the mechanical weakness of the altered lithologies, rapidly transformed into a cohesive debris flow (the Pilcaya deposit; Table 1). This lahar was funneled along a narrow tectonic depression—the Coatepec Graben (García-Palomo et al., 2000)—which coincides with the paleo-valley of the Chontacoatlán river. The debris flow traveled approximately 40 km before emerging onto a low-gradient alluvial plain, ultimately reaching distances of up to 55 km from the volcanic edifice.

 

 

A key geomorphic transition occurs east of Pilcaya and south of Tonatico, where the paleo-Chontacoatlán valley intersects the Villa Guerrero graben (García-Palomo et al., 2000), which is the paleo-valley of the San Jerónimo river. At this confluence of the two paleo-rivers, a marked change in valley width and gradient—from ~5° to <1°—induced a rapid deceleration of the debris flow. This decrease in transport efficiency facilitated substantial flow thickening and deposition. The Pilcaya deposit increases in thickness from roughly 15 m upstream to over 40 m in this area and contains isolated megaclasts up to 15 m in diameter (Capra and Macías, 2000). The surface morphology of this zone is characterized by a hummocky topography typical of cohesive debris-flow terminal deposits.

Following the emplacement of the Pilcaya debris flow, intense post-event rainfall and associated surface runoff partially reworked the uppermost portions of the deposit, mobilizing two secondary cohesive debris flows. These events, known as the Mogote debris flows (Capra and Macías, 2000), traveled an additional ~25 km along the ancestral course of the paleo-Amacuzac river—formed by the confluence of the paleo-Chontacoatlán and paleo-San Jerónimo drainages. They crossed the Cacahuamilpa limestone range and spread onto the modern Michapa alluvial plain, eventually reaching the area of the present-day town of Amacuzac (Figure 2a).

The catastrophic emplacement of the combined Pilcaya–Mogote deposits led to the complete infilling of the former Ixtapan valley. The resulting depositional surface was likely convex in a direction transverse to the original drainage axis, forcing both river systems to bypass the central axis of the deposits. The eastern drainage was redirected and evolved into the modern San Jerónimo river, which maintained its original incised course through the limestone range before being deflected along the western margin of the deposit near Michapa. Conversely, the western drainage was captured by a previously undeveloped low saddle in the limestone, forming the new course of the Chontacoatlán river. The traces of both paleochannels are still discernible in the modern landscape (Figure 4).

The diversion of the Chontacoatlán river onto a lower elevation along the eastern flank of the limestone range increased the hydraulic gradient and triggered a regional lowering of the water table. This hydraulic reorganization induced a significant shift in subsurface flow, drawing groundwater from both the Chontacoatlán and San Jerónimo drainages toward the new confluence point and accelerating infiltration into the underlying carbonate strata (Figure 12).

 

 

Throughout the Cacahuamilpa system, cave morphology is dominated by relatively straight, single-conduit passages, with minor anastomotic development confined to Gruta de la Estrella. This pattern is characteristic of conduit systems fed by sinking allogenic streams originating in non-karstic terrain (Palmer, 2007). The combination of substantial discharge and the presence of a well-developed joint and fracture network in the limestone facilitated the formation of extensive vadose/water-table–level conduits, consistent with Stage IV in the speleogenetic model proposed by Ford and Williams (1989), which represents mature cave development under mixed vadose-water table conditions.

Subsequent downcutting by the Amacuzac river further rejuvenated local base levels, leading to the abandonment of upper-level conduits and the entrenchment of the lower, presently active river caves (Figure 13).

 

 

The comparable elevations of the main conduit in Cueva de los Pilares, the lower phreatic passages of Cueva de Pedro Asensio, and the surveyed galleries of Agua Brava and Carlos Pacheco suggest that these caves may represent inactive or partially reoccupied remnants of a former speleogenetic level of the Chontacoatlán River’s subterranean course. By analogy, Cacahuamilpa Cave appears to record an abandoned level of the San Jerónimo system, reflecting a broader pattern of multi-level karst development governed by shifts in regional base level and fluvial incision.

Gruta de la Estrella, which transects a limestone ridge in the center of the Ixtapan valley, documents a comparable sequence of geomorphic evolution. As the stream incised its valley, it was deflected against a resistant limestone spur, creating a saddle that remains topographically distinguishable. Paleo-meanders preserved in the upper-level passages, combined with fluvial deposits incorporating large, rounded volcanic boulders, demonstrate the allogenic and fluvial origins of these now-abandoned conduits.

Volcanic activity at Nevado de Toluca persisted synchronously with the development of the karstic system. Volcaniclastic deposits have been identified at multiple sites within the principal conduit of Cacahuamilpa Cave. These sediments could only have been emplaced via transport by a high-energy debris flow or lahar while the cave was still hydrologically active.

In the Chontacoatlán river cave, the upper-level passage known as Pilares is almost entirely occluded by fine-grained volcaniclastic sediments, whose upper surface lies approximately 40 m above the modern stream level. The morphology and sedimentology of the Pilares passage suggest that it represents a former active level of the main river cave that was later overwhelmed and infilled by a single, large lahar event.

At El Resolladero, also within the Chontacoatlán system, debris-flow deposits reach elevations up to 70 m above the present stream floor, indicating complete infill prior to significant re-excavation by fluvial processes. In the San Jerónimo River Cave, the outcrop shown in Figure 8 is exposed within daylight below the lower entrance and is observable from the tourist trail. Along both caves, volcaniclastic deposits are preserved as benches and ledges on walls, bends, and projections, marking former sediment surfaces. Their characteristics indicate emplacement by mudflows or debris flows (lahars).

In Carlos Pacheco Cave, excavation of a trench in the floor of the primary passage revealed fine-grained fluvial sediments containing rounded pumice clasts. Geochemical analyses indicate these pumices exhibit compositional affinities with eruptive products from Nevado de Toluca (Table 2, Figure 14), providing the first direct evidence that this cave also formed within a fluvial environment and further corroborating the volcanic provenance of the sedimentary infill.

 

 

 

Editors:

Natalia Pardo Villaveces

 

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