The Mindinho’s Cave – Portugal – English
Objectives and introduction
At 2010 in continuation of the works developed on Minde’s Poldje we began working on another cave located near by Minde’s Podlje. This cave can be considered a “classical cave” of Portuguese speleology, although, as far as we know, it had been without large scale work for quite several years. The cave has already been explored to a considerable length in the past. The main objective of the works was to produce and publish a quality cave survey and also to take a fresh look at the cave in order to evaluate if the discovery of unexplored zones was still achievable.
The cave is located at the Northern border of Mindinho’s depression (quite close to the town of Mira de Aire) and is known as Mindinho’s Cave.
The existence of Mindinho’s cave is known for several decades, probably even from the 1940ths, when the exploration of the nearby Moinhos Velhos-Pena-Contenda system began. Mindinho’s cave may even be connected with the cave system above mentioned.
According to C. Thomas, 1985, an exploration involving sump pumping was conducted during 1985 by Olímpio and Maria João Martins. The speleometric data collected during the exploration were published by C. Thomas, 1985. The data consisted on a simple cave plan and profile. According to C. Thomas, 1985 during the exploration about 600m of galleries were discovered and a maximum depth of 70m was reached.
In the 1950ths the University of London expedition to Serra de Aire conducted some work in the cave.
The Sociedade Portuguesa de Espeleologia also conducted exploration in the cave. According to Crispim, 2007 due to work conducted on the 1970ths and 1980ths a maximum cave length of about 1Km and 100m depth were reached.
The cave entrance is approximately at 275m high; close by to a small cliff. Mindinho´s cave, at the extension explored at the 2010 campaign, is composed by a entry room, where one access by a 10m vertical, following the room, one descends a few ramps (about 20m long) that end on a gallery.
The gallery has a variable diameter, generally never less than 1m, reaching 5m at some points. The gallery develops for both NW and E. To NW after pumping 2 sumps the exploration stopped on a third sump. This length of the gallery is about 40m long. To E one reached a larger extension, around 170m long. In this sector one had to pump a small sump, in order to access the reaming of the gallery, the gallery still had 2 other sumps, that were only partially filled followed by a 8m vertical, the gallery continues until reaching a third sump. This last sump, has a large volume, and was closed about 7m long. A cave dive allowed the sump to be crossed reaching another dry sector of the gallery. The cave diver estimated the dry sector to be about 40m long ended on another sump.
The surveyed zone of the cave has a parametric development of 286m, all the explored cave has been surveyed with the exception of the gallery zone located beyond the E branch third sump (the zone were the access was only possible by cave dive).
Fig.4. Gallery access ramp. Photo: Marta Borges
Some notes about geology and geomorphology
The cave develops at the South border of São Mamede Plateau, Portugal. This plateau is one of the three geomorphologic units that compose the largest and more important Portuguese limestone massif, the Maciço Calcário Estremenho (MCE), as defined by Fernandes Martins (1949). The cave entrance opens at the Northern border of Mindinho’s depression. This depression lies right at the border of one of MCE’s largest depressions: Mindes’s Poldje. The bottom of Mindinho’s depression is about 275m high, while Minde’s Poldje bottom is about 200m high.
According to Thomas 1985, Mindinho’s depression is a suspended Poldje. . By other side Crispim, 1995, mentions that at beginning of the karst evolution, on the sector where Minde’s Poldje is located there should have been a serial of depressions, perhaps all poldje like, along NW-SE fractures, placed on steps from São Mamede Plateau to Covão do Feto. At West of these depressions Minde’s Poldje had a more intense evolution. The Mindinho’s depression can be one of this karst depressions, with an evolution close by to Minde´s Poldje, but preserved and kept at a higher altitude due to tectonic and or a different hidrogeological evolution form Minde’s Poldje.
The Mindinho’s depression is limited at North and South by two faults with a roughly NW-SE orientation, just like Minde’s Poldje. These two faults are part of the so called Alvados Minde Fault. Regarding the geological structure, and based on a first analysis of Sheet 27-A Vila Nova de Ourém of Portugal Geological Map at 1/50000 scale, the cave develops on a flank of regional fold. The flank can be regarded as being locally a monoclinal structure with a NW-SE/60S to WNW-ESE/50S attitude.
The cave develops on the rock formation of Calcários Mícriticos da Serra de Aire, considered by Crispim, 1995 as being one of best rock formations for Karst development on the MCE. Such formation controls the cave development, so far only galleries going NW and SE (and developing on the above mentioned formation) were found. No gallery developing South of the fault, that limits, at North, Mindinho’s depression, on less karsificable formations were found.
The cave structural control is of complex interpretation. This is due to the alteration of the original morphology, due to breakdown or by the deposal of concretions that disguise a lot of the cave original appearance. Structural control by the layers strike seems to be predominant on the studied zone of the cave. The layers attitude, measured inside the cave, varied from N50-60W/20S to E-W/20S. This values are roughly according to the strike values reported on Sheet 27-A Vila Nova de Ourém of Portugal Geological Map at 1/50000 scale. At one of the galleries an even a small fold was found (perhaps a drag fold caused by the nearby fault) .The cave section between the second and third sump of the cave NW branch is however controlled by of two families of discontinuities with an attitude of E-W/70S and N-S/5OW.
The NW-SE fault that limits, at North, Mindinho’s depression should have played a role on the cave development. This fault could be responsible by some of the cave structural control and at the very least acted as a NW-SE oriented structure of preferential water flow due to the contact of permeable rock formations, at North of the fault, with less permeable rock formations, at South of the fault.
- The East branch vertical (Photo: Marta Borges)
Water circulation regime and Karst zones
The cave sediments (mostly sinter) and breakdown have disguised most of the cave original morphology, hiding evidences of the way water flowed in the cave. According to Bogli, 1980 one of the better evidences are the erosion cusps. The cave presents a lot of erosion cusps, although most of these are symmetric, and as such don’t reveal the way water flowed. The non symmetric cups are located mostly on whirl zones, indicating opposite directions of flow on close by cusps. Only in the NW termination of the cave were found some asymmetric cusps on places where there wasn’t likely to have whirls. These cusps point the water flowing from NW to E, from the “clay sump” to the East zone of the cave.
Another indicator of the way water flow is sediments. Sinter development also indicates flow from NW to E, from the higher to the lower zone of the cave. One must mention that the sinter as been formed under a vadose regime, and so reflects water flow only on the vadose zone. This evidence agrees with the model of infiltrating water infiltrating and crossing the cave on their way to the Phreatic Zone. The sand and clay deposits found on the cave sumps also indicate a water flow from NW to E.
According to Crispim, 2007, a tracer test was conducted proving the connection between sinkholes located on Mindinho’s depression and Olho de Mira (a spring located on Minde’s Poldje). C. Thomas, 1985 had already mentioned the possibility of the connection between Mindinho and Olho de Mira, because Mindinho’s cave East branch develops toward Olho de Mira. Olho de Mira has, according to Crispim, 2007, an extension of about 80m and a depth of 100m, before reaching a water level. The above mentioned author states a speed between 55 a 70 m/h to water flow and conducts cross sections between 7,3 and 9,2m in diameter. The author refers this diameters are larger than the ones actually known in both caves. Only a small part of the tracer injected on Mindinho’s sinkhole was recovered on Olho de Mira. This means that there is more than one path between the sinkhole and the spring. The tracer test proves that, at least in high precipitation periods, the water flow in Mindinho’s cave from NW to E.
The cave lies surely on the Vadose Zone. Considering that the cave develops much higher than the present phreatic level, the cave probably lies on the Inactive Vadose Zone (according to Bogli, 1980 classification. The cave presents temporary water flow, most likely on vadose regime, with infiltrating water coming from sinkholes at the surface, and going to the deeper parts of the cave. The sump’s water should most likely coming from the surface. The water should circulate, at the so fare cave studied zone, from NW to E, in direction to Minde’s Poldje and São Mamede´s Plateau. The cave was first developed in the Phreatic Zone, passing in to the Vadose Zone due to the lowering of phreatic level and or tectonic movements.
According to Crispim, 1995 a radiometric dating was performed on a concretion collected in a fossil cave at the East slope of Mindinho´s depression. The author doesn’t specify if the concretion was collected in Mindinho’s cave. If not in Mindinho’s cave it is was probably collected in close by cave of similar surrounding. According to the same author the dating returned a quite old age (<1.25My), but too much imprecise to allow important conclusions, only allowing to state that the retreat of groundwater circulation from Minde’s Poldje and adjacent depressions is of Quaternary age.
Ancient groundwater circulation regimes
One can question if the actual water circulation regime was always the one occurring in the cave. As mentioned above the cave had to be created on the Phreatic Zone. The cave entrance position, at the East border of Mindinho’s depression, is similar to the close Minde’s Poldje springs (all located on the poldje’s East border) and also the existence of cave galleries of phreatic origin, make us to consider the hypothesis that the cave entrance as acted, in the past, as a spring. With the lowering of the phreatic level the former spring was converted in to a sinkhole.
In order to this hypothesis to be true the cave entrance, which is currently at about 275m, must have been at the same high than the phreatic level. Nowadays at Minde’s Poldje the phreatic level, reaches at most, only a few metres above the springs. We will consider the Minde’s Poldje springs to have 200m of altitude. We believe there are two hypotheses who explain how the ancient phreatic level reached terrain nowadays 75m higher, than the current phreatic level:
a) the phreatic level was then quite higher than the present one
b) tectonic movements raised, after the cave formation, Mindinho’s depression above Minde’s Poldje.
In any of the above hypotheses the result was the passage of the cave from the phreatic to the vadose zone.
There are in literature several references to regional water levels (either interior lakes, either sea level) at higher altitudes than nowadays. Some of these levels have reached terrains nowadays 300m high. The above mentioned references are:
- Fernandes Martins, 1949, refers the existence of an ancient permanent lake, during Quaternary, in Minde’s Poldje (remember that nowadays Minde´s Poldje is 200m high).
- Suzanne Daveau, 1973 considers as of sea origin sand and boulders found on the West flank of Serra dos Candeeiros (Serra dos Candeeiros is the most Western elevation of the MCE and at around 10 Km West of Mindinho). The sand formation is about 300m high. This is quite close to the heights of Mindinho’s depression base and the entrance of Mindinho’s cave. Zybszewski, 1963 also refers the existence of a sea level on Serra dos Candeeiros represented by a deposit of sand and rolled boulders. G.Manuppela et al, 2000 once again mentions this deposit, adding that it extends on a rectilinear outcrop dislocated to the 300m high. The same author states that the deposit can be interpreted as the base of the fossil sea cliff that limited the Littoral Platform (the platform extended from the present sea shore until Serra dos Candeeiros). Still according the above mentioned author, the last phase of the work on the Littoral Platform is, by Ferreira et al 1998, Calabrian ((1.8 Ma.—781.000 years ago) or Placencian by Carvalho, 1953, or even Tiglian by Cabral 1993. G.Manuppela et al, 2000 once again mentions the same sand deposits saying that the deposits are adjacent to Candeeiros Fault, and quotes Cabral, 1995, according to who no unequivocal evidences of recent activity of this fault were found.
- Also Crispim, 1995 mentions the existence, on a region South of Minde’s Poldje, of a fine white sand deposit, similar to the ones described by Suzanne Daveau, 1973. This deposit was found inside a cave on Cabeço Morto, about 300m high.
Beside the above mentioned higher sea level stands, there are also evidences of an ancient sea level at 200m high and at lower heights. These sea level stands certainly had influence no groundwater network of the MCE.
Fernandes Martins, 1949, refers the existence of a platform caused by sea abrasion. Part of this platform is still preserved nowadays by two levels, sculpted on the landscape, the levels of Fragusta and Pombal, 180 to 190m high. These two levels, once just one, are now separated due to regressive erosion. According to Zybszewski, 1963 a Pliocenic sediment consisting of sandy clay with rolled boulders, deposited between 175 and 215m high, along the towns of Aljubarrota and Cruz da Légua is a evidence that sea has reached and maintained a level about 200m high. According to the above mentioned author this level is a testimony of the final part of a sedimentation cycle connected with a relatively high sea level stand represented by a deposit of sand and boulders on the slopes of Serra dos Candeeiros.
If one considers the model in which there is no tectonic movement, the effects of a 300m sea level stand, following, according to Zybszewski, 1963, by a 200m high level stand must have been felt on the groundwater network. The effect would be caused by the establishment of a 300m base level (who is regionally the sea level), latter on lowering to 200m, and then to 120m (height of at least part of the littoral platform according to G.Manuppela et al, 2000). As the sea level lowered from the 300m to the lower altitudes, the caves, formed in the Phreatic Zone, passed into the Vadose Zone.
Cave exploration works
The exploration works, including cave dive, and sump pumping, were conducted from August to October 2010, with the objectives of exploring and surveying the cave. The works were conducted at 21, 22 and 28 of August, 4, 5, 11, 12, 25, 26, 29 of September, 1, 2, 9, 14, and 17 of October.
The pumping operations allowed us to cross three sumps. A forth sump was dived because its large size and position made the pumping operation quite difficult. Two of the pumped sumps are located at the NW branch of the cave, being known as the “clay sumps” due to the abundant clay that covers the sumps bottom. Before pumping operations began, an attempted was made to dive the first clay sump. However the dive was unsuccessful due to a narrow passage and to total lack of visibility. After successful pumping of both “clay sumps”, one reached a 25m long gallery that ended on another sump. The pumping of this last sump was began, but the rain who felled at mead October forced us to abandon the pumping.
At the East branch of the cave the exploration began with the pumping of a small sump allowing access to more 75m of gallery. The gallery after an 8m vertical fall ended on a second small sump, almost completely filled of water, however still crossable. After this small sump a third sump was found. This one has a quite large volume (several tents of cubic meters) and diving into the sump water we found an old dive guide line. After pumping the second small sump it was easier to carry all the necessary diving gear for the large sump. The diver followed the already existing guide line.
The sump is 7 meters long and about 0,5metres high. The reduced height of the passage forced the diver to use a “sidemount” configuration. After crossing the sump, the diver, found an air filled gallery about 40metres long, of phreatic origin, with some “gours and walls presenting abundant erosion cusps”. The gallery continues onto a descending steep slope until reaching another sump. The diver, who was alone, decided by safety reasons, no to continue the exploration and return to the origin point.
The entire cave from the entrance to the terminal sumps of both branches was surveyed; a preliminary geological survey was also conducted. During field work some climbing and desobstructions were performed. Some of those were successful giving access to small branches of cave without any considerable possibility of further discoveries.
Cave development potential
As mentioned above Mindinho’s cave is, according to Bogli 1980 definition, on the Vadose Zone. The thickness of the Active Vadose Zone in Minde’s Poldje is, according to the Ramsar report, of 100m, this is the value reported as the variation of phreatic level. According to Crispim, 1987, the Moinhos Velhos- Pena- Contenda system, located less than 1 Km from Mindinho’s cave, with springs in the base of Minde’s Poldje (200m high), has a phreatic level temporary oscillation zone of about 80m meters. The dye tracing performed by Crispim, 2007 proofs the connection between Mindinho’s depression to the spring of Olho de Mira and to Minde’s Poldje (where is known than most of the springs are part of the Pena- Contenda- Moinhos-Velhos cave system). Based on this one can assume a possible connection between Mindinho’s cave and the above mentioned cave system or at least an association between Hydrological zones of Mindinho’s depression and Minde’s Poldje.
Using Bogli, 1980, karst zonation and assuming the above mentioned connection, Mindinho’s depression would have about 75-100m of Inactive Vadose Zone, followed by about 80-100m of Active Vadose Zone and them by the Phreatic Zone. A cave on Mindinho’s depression, just like the explored cave, would have a potential depth of 150-200m of at least partially air filled cave zone, before reaching the permanent water filled cave zone.
Caving associations who took part on field work
The exploration, pumping and surveying were conducted by speleologists of the following associations, here listed by alphabetic order: : AES – Associação de espeleólogos de Sintra, AESDA – Associação de Estudos Subterrâneos e Defesa do Ambiente, ARCM-Alto Relevo Clube de Montanhismo, CEAE- LPN – Centro de Estudos e Actividades Especiais da Liga de Protecção da Natureza, ECTV – Espeleoclube de Torres Vedras, GEM – Grupo de Espeleologia e Montanhismo , NEALC – Núcleo de Espeleologia de Alcobaça, GPS – Grupo Protecção de Sicó, NEUA – Núcleo de Espeleologia da Associação Académica da Universidade de Aveiro, SAGA- Sociedade de Amigos das Grutas e Algares.
This work was only possible due to the support and hard work from all the following cavers: :José Silva, Carlos Lemos, Carlos Gomes, João Neves (Jr.), Mário Matos, António Mendes, Orlando Elias, André Gaspar, Pedro Robalo, Alvaro Jalles, Rui Andrade, Paulo Rodrigues, Rui Pinheiro, Paulo Campos, Raquel, Vitor Amendoeira , Marta Borges, Vitor Toucinho, Ana Barros, Ulisses Lopes, Fortunato Videira, Gonçalo, Costa Pereira, Luís Meira, Marco Costa, Bárbara Monteiro, Sérgio Medeiros, Pedro Alves, Sérgio Barbosa e Daniela.
From these these cavers, who made this work possible, we must outline four names: Orlando Elias e André Gaspar always present on the activities, either on weekends either during the working week, allowing us to push forward the Works, Rui Pinheiro the cave diver and also Pedro Robalo who draw the cave map.
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