Contenda cave – english version
This page describes briefly the works conducted on Contenda cave during six years of campaigns (2007-2014), and presents some of the results.
Contenda cave develops on the South border of the São Mamede plateau, Portugal. The São Mamede plateau is a geomorphologic unit, part of Portugal’s largest and more important karst massif, the Maciço Calcário Estremenho, (Fig. 1) defined by Fernandes Martins (1949). The cave entrance is located on the Northern border of Minde’s Polje (Fig. 2); along witch several springs are located. Contenda is one of the above mentioned surgences, possibly an episodic spring, according to the Bögli, 1980, classification. Contenda cave is at least partially filled with water during most of the year, making the fieldwork, on most of the cave, only possible for a period of 3-4 months an year.
This cave is close by the Moinhos Velhos cave who is the second longest and second deepest cave of Portugal. The Moinhos Velhos and Contenda caves connection has been proved by dye tracing in 1986 (Almeida et all, 1995). The dye tracing conducted on Moinhos Velhos revealed an average velocity of 16 m/h for a travel distance of 800 meters up to Contenda cave and that the flow direction found in this sector is north to south. Contenda cave is, according to the Ramsar report, an overflow spring of the Moinhos Velhos System.
Till now no caver has clammed to have done the physical connection between Contenda and Moinhos Velhos. The cave system composed by Moinhos Velhos, Contenda and another cave (Poio) who has been physically connected to Moinhos Velhos cave is known as Moinhos Velhos – Pena – Contenda cave system, for a few years.
Cave exploration history
Before continuing we must refer the historical works conducted on this cave. According to C.Thomas, 1985, the cave as been explored, since the 1950´s, by SPE – Sociedade Portuguesa de Espeleologia. There is a partial cave map of the Moinhos Velhos- Contenda- Pena System, made by SPE that can be found exposed at the entrance of the tourist cave of Mira de Aire (the commercial name of Moinhos Velhos cave).. This map although of public domain has not been published. The only published cave map is partial, and resulted of a topographical survey conducted by C.R.G. (Cave Research Group of Great Britain). The survey was conducted on 1957. The cave map was published on 1959 on a paper called: “The caves of the Serra de Aire massif, Central Portugal”. This cave map was republished in the book by C.Thomas, 1985, Grottes et Algares du Portugal. We must also refer that about 1/3 of Contenda cave is, since 1979, part of the natural park of: “Serra de Aire e Candeeiros”.
During six years (2007 to 2013) a group of speleological associations as joined efforts in order to conduct several works in Contenda cave. The works conducted were: topographical survey, geological survey, biological sampling, several desobstructions, and twoo cave dives.
The works have been conducted by members of the following speleological associations, here announced 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 Actividade Especiais da Liga Portuguesa de Protecção da Natureza, GEM- Grupo de Espeleologia e Montanhismo NEC- Núcleo de Espeleologia de Condeixa, NEUA- Núcleo de Espeleologia da Associação Académica da Universidade de Aveiro, ECTV- Espeleoclube de Torres Vedras, SAGA- Sociedade dos Amigos das Grutas e Algares
Topographical survey results
Almost the entire known cave has been surveyed. So far the cave length is 1418m and the cave depth is 90m (+17m (cave highest point), -73m (cave deepest point)). The depth measurements are given in relation to the cave entrance.
The cave map is presented on Fig. 4.
The cave presents a curvilinear branchwork pattern, according to Palmer, 2003 classification. The cave has two entrances, one of those entrances is a small shaft that is nowadays obstructed, and the other entrance is the actual spring. The access to the cave is only possible by this last entrance. The cave is composed of a main gallery who develops roughly first to Southwest and then to North. This gallery encounters at about 270m from the entrance a major affluent gallery who develops to South. The affluent gallery presents a small canyon close to a bifurcation where it divides in to two galleries, both of those end on sumps. The affluent gallery is around 130m long. The main gallery, at about 550m from the cave entrance, enters the pits zone.
Two sets of pits can be found. One of the sets, is about 60m deep has the last pit base cover by clay and ends on a sinkhole. The other set of pits, also about 60m deep, allow access to two inferior galleries. One of these galleries is called the “River gallery” the name is due to the underground river found there. This gallery has a known extension of about 100m. The other gallery is called “SPE 66 gallery” and develops about 230m to North and is 7m higher than the river gallery. The terminal zone of this gallery ends on a pit filled with water. The underground river, the sumps on the affluent gallery and the water level on the pit at the end of SPE66 gallery should all correspond to the regional groundwater level.
The cave develops, according to the geological map, sheet 27-A-Vila Nova de Ourém at the scale 1:50.000, on limestone of Bathonian age (Dogger), as one can see on Fig 4. The limestone belongs to the Formation of Calcários Mícritos da Serra de Aire, composed mostly of micrite limestone. Regarding the structural geology, and based upon an expedite analysis of the above mentioned geological map; the cave is developed on a flank of a regional fold. The flank can be considered locally has a monocline with an attitude roughly NW-SE/20S.
The cave entrance is close by to the Northern border of Minde´s Polje. This polje, according to Almeida et all (1995), is about 4 km by 2 km and its bottom has an average altitude of 200 m. Several temporary springs, all of them associated with important caves (Almeida et all, 1995 after Crispim, 1987), are located on its NW and NE limit. Periodically the sinkholes located near the town of Minde are insufficient to drain the polje and it becomes transformed into a lake. All of the cavities, whose entrance is located near the northern edge of Minde´s polje are located near a fault (with a NW-SE direction) which borders the North of the polje. Apparently this fault produces a barrier effect by allowing the lateral contact between Dogger limestones, with a large karstification potential, with Malm limestones of lower permeability. This fault probably acts as a barrier to water flow in the direction perpendicular to the fault but also as a line of preferential flow of water parallel to the fault. Crispim, 1995 recognises the importance of this fault by putting in to contact rocks of very different permeability and also states that the emergency zones are located along the contacts between the rocks of different permeability.
The cave has several kinds of structural controls on different areas. On a first approach the main gallery seems to be controlled mostly by the bedding planes, in some areas by the layers strike and in other areas by the layers dip. The bedding planes measurements, conducted inside the cave reported values of N40W to N50W/20S for the layers strike and dip. This value is in agreement with the value above mentioned for the layers attitude of the area. Several fractures cross the main gallery and also have some influence in the gallery development. The area where the main and the affluent gallery meet, a fracture with an attitude of N50W /vertical controls the development of both galleries. Fractures with an attitude roughly N30E/vertical also control other parts of the main gallery.
The affluent gallery development is controlled mostly by the layers dip and in some areas by several fractures. Those fractures have the following attitude N40W to N50W/vertical, N10W/vertical, and E-W/vertical.
The SPE 66 gallery is structurally controlled mostly by a family of fractures of attitude roughly N10W/80E, although some small portions of the gallery are controlled by fractures of another family of attitude N60E to N80E/Vert.The areas controlled by this last family of fractures connect the areas controlled by the fractures of the N-10W/80E family.
Water circulation patterns
The entire surveyed cave galleries can be considered has being on the high-water zone of the vadose zone as defined by Bögli, 1980. Regarding the thickness of the high-water vadose zone on Minde polje’s the Ramsar report refers a 100m water level fluctuation on that area. Also according to Crispim, 1987, in the close by Moinhos Velhos cave, the zone of temporary oscillation of the phreatic level has maximum 80m thickness. As referred on the introduction, Contenda cave is considered a Moinhos Velhos outflow spring. According to this interpretation one can consider the entire cave depth has representative of the local vadose high-water zone thickness, and in deed the cave depth (counting from the cave entrance) is around 70m, this value is quite close to the one mentioned on Moinhos Velhos cave.
The cave galleries are of phreatic origin. The only exception is the canyon found on part of the affluent gallery who seems to have been formed under a vadose regime, that succeeded, in that area, the original phreatic regime.
Based on the scallops and sediments found in the cave, the water should flow from the deepest part of the cave (SPE 66 gallery, river gallery and affluent gallery) to the main gallery where it flows until reaching the cave entrance.
Regarding the canyon on the affluent gallery, the scallops, on the upper part of a canyon transversal section, indicate flow of water coming from the sumps to the main gallery, although the scallops on the lower part of the same section indicate flow of water on the opposite way, from the main gallery to the sumps. The interpretation of this morphology can be summarized on the following steps:
a) Formation of the affluent gallery (on a phreatic regime) by water flowing from unknown galleries, located beyond the sumps, to the main gallery. The scallops on the upper part of the gallery section (pointing in to main gallery) were formed mostly during this period.
b) After a certain point of the cave development, the cave started acting not only as spring, but also a receiver of water coming from the surface. The water is provided by infiltration from locals of diffuse or concentrated surface recharge areas. The water reaches the cave along some small diameter galleries that end on the affluent gallery. The water coming from the lowering of the polje temporary lake, when the cave acts as a sinkhole also takes part on this process. The incoming water reaches the affluent gallery, circulating along it until reaching the sumps. This incoming water circulates on a vadose regime, and has much probably excavated the canyon and produced the scallops on the lower zone of the gallery section (pointing to the sumps).
c) Nowadays the water circulates from the sumps into the main gallery only when the cave fills with water rising from the phreatic zone, on season of high groundwater level. This circulation may occur, even now, on a phreatic regime. The remaining time, which is probably most of the hydrologic year, the affluent gallery receives water circulating on a vadose regime and deepening the canyon.
Considering the cave morphology, the water circulation above described, and the fact that Contenda cave seems to be a episodic spring, one may consider that groundwater circulation is probably limited, for most of the year, to the “river gallery” (where there is usually groundwater circulation even in the summer), perhaps to the SPE 66 gallery, and to an unknown gallery(s) to where the affluent gallery should connect.
Cave dives at SPE 66 gallery
As mentioned above Contenda cave is quite close to the Moinhos Velhos cave. The terminal part of the SPE 66 gallery ends on a pit filled with water. This pit is about 100m (on a straight line) from Moinhos Velhos Cave, according to the topographical surveys of Moinhos Velhos (Crispim, 1987) and Contenda. The dives were performed in this pit, which is at roughly about 800m from the cave entrance. The journey to reach the pit takes around 3-4 hours of a hard progression including the crossing of some lakes. As far as we know no dive had ever been performed in this cave. The objective of the dives was first to access if the pit had a passable connection to a gallery and if so to explore the gallery.
Two dives were performed, the first one in 2008 and the second in 2009. Both were performed by Rui Pinheiro, with the technical support of other divers. The 2008 dive revealed the pit accessed a submerged gallery, but the technical limitations didn’t allow much more exploring.
The 2009 dive reached unexplored galleries. The pit gives access to a lower gallery of North-South orientation. The gallery continues to the North and then to East for about 12m until reaching a dry zone, the cave continues, but the exploration of this new zone wasn’t possible. To the South, of the pit, the gallery (larger than the North side) continues to SE, always submerged until it ends a few tenths of meters ahead. Although it ends laterally this gallery gives access to three pits with an estimated depth of 9m or more, these pits continue deeper in to the phreatic zone. With this dive the Contenda cave is now approximately at 90m (on a straight line) from the Moinhos Velhos – Pena cave.
Cave dives at River gallery 2011
At 2011 two cave dives were performed at East and West sumps of the River gallery.
A first dive was conducted in the West sump, after a 100m swin trough a partially inundated gallery, of phreatic origin, the dive revealed a gallery with a section 2,5mx3m, completely underwater for about 10m, after witch it has air again, but only on the upper 0,5m, for about 25m long. At this point the gallery dives and becomes again completely underwater, the visibility improves to about 2m, much due to the passage of a clayey to a sandy floor. At about 60m of the beginning of the dive and a total depth above 7m, the visibility becomes controlled only by the reaching limit of the flashlight. After a 85m extension dive and 9m deep, the diver reached a gallery section of 1,5mx1,5m, at this point the air reserve was close to the 2/3 thirds and the diver returned.
The West sump was also dived. After this sump the gallery develops with a smaller section and clayey floor for the first 10m, after it the sand becomes sandy, The gallery has a uniform section without large changes of direction. After 45m the gallery enlarges into a small room in which the directions changes 90º. At 50m dive and below 8m deep the dive returned, because he was reaching the air reserve safety limit, but the gallery continues.
The cave dives didn’t reached no secondary galleries
Cave dives at River gallery 2013
After a year interval the group of cavers that since 2007 works in the cave returned. This year the institutional suport was provided by the following associations (by alphabetic order): ARCM, GEM, NEL and NEUA with the support of a large number of caver from several associations. The works consisted on more dives at the river gallery and surveying one lateral gallery also at River gallery, who had been discovered at 2011.
The dives were performed by António Mendes (NEUA). The first dive objective was to place ariadne line at West branch of River gallery, but the poor visibility in the water made it impossible. The second dive was performes in the East branch of the same gallery, the visibility was much better, because the water current was East to West. The dive allowed the installation of 65m of ariadne line, 15m of it on unexplored zone. The other 50m had already been dived, by the same diver, at 2011.
The survey of the lateral gallery revealed it is about 20m long and is quite inclined. It ends on two chimneys (one that might be penetrable) located under the bottom of the cave pit zone.
One can find in here a small film recorded at early January 2010. In this film is visible Contenda cave entrance acting as a spring. This is the only cave entrance and during summer the water level is about 70m lower. The film author is André Reis (CEAE), we also thank Paulo Campos (ARCM) for uploading the video on the web.
Be aware when caving on Contenda Cave
Contenda cave exploration requires some precautions. The upper gallery, for most of the year, is only water free until about 200m of the cave entrance. In this place one can find a sump, that opens by itself late spring. Further inside the upper gallery about 20m from the place where the upper gallery meets the affluent galley there is another sump. This sump is particularly dangerous because in case of, even a slight rainfall, it rapidly fills with water (a few hours top) even when it is initially completely empty. This sump when closed stops any access to the remaining of the cave or to the exit.
As such in case of any rainfall forecast this cave shouldn’t be visited.
This work was only possible due to thesupport and hard fieldwork of the following cavers: José Silva, Ildegardo Granjo, Luís Sobral, Margarida Jalles, Mário Matos, Miguel Lopes, Paulo Almeida, Paulo Alexandre, Paulo Rodrigues, Ana Calambra, António Mendes, Pedro Robalo, João Paulo Janela, Alvaro Jalles, Rui Andrade, Paulo Rodrigues, Rui Francisco (Lóia) Rui Pinheiro, Pedro Pinto, André Reis, Beatriz Silva, Patrícia Silva, Celso Santos, Paulo Campos, Diana Campos, Luís Costa (Costinha), Vitor Amendoeira, Marta Borges, Mário Pereira, Vitor Toucinho, Ana Barros, Ulisses Lopes, Fortunato Videira, João Pinheiro, Costa Pereira, Luís Meira, Marco Costa, Flávio Lucas, Marco Dias, Pedro Sabino Koch, Sandra Lopes, Fátima Carvalho, Denise Fialho, Bruno Pais e Sofia Reboleira.
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