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SIGEP

Geological and Paleontological Sites of Brazil - 019

TOCA DA BOA VISTA, Bahia state
the longest known cave in the Southern Hemisphere

Date:04/10/1999 

Augusto S. Auler 
aauler@hotmail.com
A.Auler@bristol.ac.uk
Rua Piauí, 1195 apt. 1101 - Belo Horizonte – MG
30150-321 - Brasil

Peter L. Smart
P.L.Smart@bristol.ac.uk 
School of Geographical Sciences
University of Bristol - Bristol, BS8 1SS -England

© Auler,A.S.; Smart,P.L. 1999. Toca da Boa Vista, Bahia state - the longest known cave in the Southern Hemisphere. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 4/10/1999 on Internet at the address http://www.unb.br/ig/sigep/sitio019/sitio019english.htm [Actually http://sigep.cprm.gov.br/sitio019/sitio019english.htm

(The above bibliographic reference of author copy rights is required for any use of this article in any media, being forbidden the use for any commercial purpose)

 ABSTRACT

    Toca da Boa Vista, the longest known cave in the Southern Hemisphere with 84 km of mapped passages, is one of the most important speleological and palaeontological sites in Brazil. Together with the neighbouring caves of Toca da Barriguda, Toca do Calor de Cima, Toca do Pitú and Toca do Morrinho, they represent a geological site of global interest. Morphological, hydrochemical and isotopic evidence suggest that cave genesis was due to oxidation of sulphide within the Una Group dolomite bedrock. Toca da Boa Vista is the longest cave in the world known to have been generated by such a process. Radiometric dating of secondary carbonates and fossil bones have allowed the reconstruction of Quaternary palaeoclimate events in the area and suggest periods of increased precipitation at the last glacial maximum. Among the many fossil remains found in these caves, there are remarkably complete and well preserved skeletons of many extinct species, allowing a significant advance on the knowledge about these fossil groups. The remote and rural location of the caves reduces risk from anthropogenic activity, but some formal protection should be granted to the site in recognition to its exceptional technical and scientific value.

INTRODUCTION

     Toca da Boa Vista is at present (1999) the longest known cave in the Southern Hemisphere and 16th longest in the world, with 84 km of mapped passages. Together with the neighbouring Toca da Barriguda, presently the second longest cave in Brazil at 19.5 km and other related caves, Toca do Calor de Cima, Toca do Pitu and Toca do Morrinho, it comprises a site of major scientific importance. With the exception of Toca do Morrinho, these caves probably represent a unified cave system now bisected by collapses and later sedimentation. The hypogene sulphuric acid speleogenetic processes responsible for the development of these caves are relatively uncommon, and result in a distinctive morphology. Secondary carbonate precipitates (speleothems) within the caves have been dated and provide an important record of late Quaternary climatic change which suggest that more humid conditions were previously present in this now semi-arid area. This is complemented by a rich and remarkably well preserved fossil fauna.

 LOCATION

    The Toca da Boa Vista is located in northern Bahia State, within the limits of the municipality of Campo Formoso (Fig. 1). Toca da Boa Vista (main entrance 10º09'45"S, 40º51'35"W) lies 11 km east from the small village of Laje dos Negros, the entrance being adjacent to the road that connects Laje dos Negros and Abreus. Toca da Barriguda (10º08'26"S, 40º51'08"W) and Toca do Calor de Cima (10º08'26"S, 40º50'55"W) are 2 km further to the east along the same road. Toca do Pitu (10º07'44"S, 40º50'18"W) is located in the same area, closer to the village of Barriguda. Toca do Morrinho (10º12'32"S, 40º55'05"W) lies a few kilometres south of Laje dos Negros, near the village of Casa Nova. The cave entrances are not easily located amidst the featureless local landscape, and help from locals is highly recommended. Visits to the site should be conducted with the help of experienced cavers, due to the complexity, dry atmosphere and lack of water in the caves.

Figure 1. Location of the site.

    The Toca da Boa Vista is in the semi-arid region of northeastern Brazil. Average annual precipitation for the area is around 490 mm, with a deficit (precipitation minus evapotranspiration) of over 1400 mm (Martins, 1986). Vegetation comprises a dry shrub drought resistant arbustive forest known as caatinga. Soil is thin and in places covered with silica cobbles which are residues from dissolution of the cherty carbonates. The area is within the catchment of the ephemeral Salitre River, which enters the São Francisco River near Juazeiro. The sole perennial river, the Pacuí River, originates in springs a few kilometres to the east of the caves, which form the discharge point of the carbonate aquifer that contains the caves.

 DESCRIPTION OF THE SITE

     The entrances of Toca da Boa Vista and the other caves have been long known to the local inhabitants. Saltpetre, for manufacture of gunpowder, was extracted in some of the caves until late in the twentieth century, and the water at the bottom of Toca do Pitú was regularly collected for drinking and washing purposes. J. Telesphoro de Araújo, a local government officer at Campo Formoso, was particularly interested in ascertaining the potential of local caves and, in the absence of local caving groups, summoned the help of cavers from southeastern Brazil. The first exploratory party to enter Toca da Boa Vista comprised cavers from SEE (Sociedade Excursionista e Espeleológica) who descended one of the vertical entrances of the cave but apparently did not recognise the potential as there was no follow-up visit to the site.
   
In early 1987 cavers from GBPE (Grupo Bambuí de Pesquisas Espeleológicas) visited the area, and guided by Telesphoro, entered the horizontal entrance of Toca da Boa Vista now known as the "classical entrance". The party quickly realised the potential of the cave, and dedicated the remaining days of the expedition to mapping the complex maze of passages. A connection between the classical entrance and the two vertical entrances (Bode and Sapo Pits) was achieved, and the newly explored cave was left with 2.8 km of surveyed passages at the end of the first expedition.
   
Since 1987 GBPE have regularly returned to the area and after 15 major expeditions and a number of other shorter trips over 110 km of passages have been mapped in the caves. These expeditions have been largely self financed, but support from the municipality of Campo Formoso, Rede Globo (Brazilian TV network) and the United States National Science Foundation has been received. Research has been supported by the Brazilian Research Council (CNPq) and NSF through grants for MSc and Ph.D. thesis research. Nowadays, in addition to the exploratory and topographical work carried out by GBPE members, research groups from Universidade Federal de Minas Gerais (Geology and Biology Institutes) and Universidade de São Paulo (Geosciences Institute) regularly work in the area.

     Geology

    The caves have developed in dolomites of the late Proterozoic Salitre Formation, Una Group. Recent geological mapping by the Brazilian Geological Survey (CPRM, 1998) places the carbonates that outcrop above the cave into the Gabriel Unit, comprising calcosiltites with flat bedding sometimes dolomitised with local levels of dolomitic calcarenite. Cave passages are mostly developed in two facies of dolomite, a pure dolomite that usually hosts the larger chambers, and a very cherty dolomite that contains most of the cave passages. The Salitre carbonates in the area occupy a relatively restricted basin, limited in the west and north by Mid Proterozoic Espinhaço Supergroup quartzites, and in the south by shales that probably belong to the Archaean Rio Salitre Belt. These are exposed in valley bottoms and are overlain by the Plio/Pleistocene Caatinga Limestone, a freshwater lacustrine limestone which overlies unconformably the Una Group carbonates to the south of the caves. The schematic profile in Fig. 2 illustrates the geological relationship in the area.

Figure 2. Schematic geological profile of the Toca da Boa Vista area.

    Morphology of the caves

    Toca da Boa Vista and Toca da Barriguda represent sections of a very extensive system that has been bisected by surface denudation. Although still separate caves, it is possible that they will be linked during future explorations, as the gap between the closest passages is now at 700 m. It is believed that a minimum of 100 km of unexplored cave passages still remain to be mapped in the area. A line survey of the two major caves is presented in Fig. 3. 

Figure 3. Plan of Toca da Boa Vista and Toca da Barriguda showing survey line. Mapping by Grupo Bambuí de Pesquisas Espeleológicas.

    They are characterised by a bidimensional maze system with very irregular wall and ceiling contours, and abrupt and ungraded passage junctions. Following the classification of Palmer (1991), the pattern is predominantly ramiform, with some spongework and network sectors. Passage size ranges from a maximum of 15 m high and 50 m wide to very small passages that preclude exploration. The cave passages appear to follow certain bedding horizons, and undulate in accord with minor folds. Passages reach a little over 50 m below the surface in the deeper sectors where the water table is intersected. Passages are terminated by either breakdown or sedimentation. Vadose dissolutional features are absent. The passages appear to be a single phase system, i.e., they formed in a single continuous event, and they bear no genetic relation to the present day surface. The present entrances correspond to fortuitous collapses that occurred when surface denudation lowered the surface to intersect the pre-existing cave passages (Fig. 4). Further morphological information is provided in Auler (1999).

    

Figure 4. Sapo Pit, one of the vertical entrances of Toca da Boa Vista.

    Nearly all passages in the caves contain sediment deposits. These range from secondary carbonates (speleothems) to clastic deposits such as breakdown or silt or clay deposits, and include extensive fossil bat guano and fossil bone accumulations. These deposits have yielded important information concerning the geomorphology and Quaternary evolution of the area.

    Scientific value

    Toca da Boa Vista and associated caves are now among the best studied caves in Brazil, but much remains to be done. Three theses have been based on the area, and an extensive dating program has been performed on speleothem, bone, clastic and biogenic sediments. Additionally, several rock and speleothem samples have been analysed by X-ray diffraction, sulphur isotopes and quantitative chemical composition. Many distinct fossil species have been collected in the site. A synthesis of the geomorphological and palaeoclimatological research in the area is provided in Auler (1999), and the palaeontology is described in Cartelle (1995). The site offers much potential both for the detailed study of speleogenesis by hypogenic processes, and for unraveling the long term geomorphic evolution of passive margins. Such studies, and the palaeoclimate inferences which may be derived from them, require a level of detailed scientific study that had not yet been possible in this area. Toca da Boa Vista finds few parallels amongst other caves in the world, and its exceptional scientific importance places it among the most significant geological monuments in Brazil. 

    GEOMORPHOLOGY

    Toca da Boa Vista shares many features common to hypogenic caves, an unusual group of caves which are generated by acids released from the subsurface rather than those released by biogenic activity at surface. According to Palmer (1991), hypogenic caves comprise less than 10% of global caves, all others being formed by carbonic acid derived from the atmosphere and soil incorporated in meteoric water. However, almost all of the hypogenic caves described in the literature correspond to caves generated by deep-seated processes, by sulphuric or carbonic acids derived either by migration from hydrocarbon reservoirs or generated by volcanic processes. Toca da Boa Vista clearly does not belong to this type of hypogenic caves, because the local geology is not favourable to the existence of hydrocarbons, and volcanic processes are absent. Morphological, hydrochemical and isotopic evidence (reviewed in Auler, 1999) suggest that Toca da Boa Vista was largely generated by sulphuric acid produced within the carbonate by pyrite oxidation. Such type of cave has been recognised in other areas such as the United States (Morehouse, 1968), but mostly comprise small caves. The relatively slow rate of pyrite oxidation (Palmer, 1990), and the low concentration and dispersed nature of pyrite in carbonates (Ball and Jones, 1990, Palmer, 1991) are usually given as reasons for the minor importance of pyrite oxidation in the formation of caves. However, the existence of large masses of sulphide within Una Group sequences (Misi and Kyle, 1994), although not directly observed near the caves, could promote significant carbonate dissolution, as could the extensive timescales available for karst development (since the Palaeozoic). If such a model of shallow hypogenic development is correct, Toca da Boa Vista represents a rare geomorphological feature and by far the longest cave system generated by sulphide oxidation known in the world.
   
Palaeomagnetism of cave sediments has shown that most of the passages of Toca da Boa Vista have been dry for a minimum of 778 ka. Apatite fission track studies of the São Francisco Craton indicate slow rates of denudation, on the range of 30 ± 10 m/Ma (Harman et al., 1998). Such rates, if applied to the vertical range of the now drained cave, suggest that their upper passages would have been drained for a minimum of 1.5 Ma, and possibly much longer. It appears reasonable to suppose that Toca da Boa Vista and some of its clastic sediments are Tertiary in age. Full details on the geochronology of Toca da Boa Vista and associated geomorphic interpretation are presented in Auler (1999).

    QUATERNARY PALAEOCLIMATE

    Toca da Boa Vista contains abundant speleothem deposition (Fig. 5). These deposits, usually composed of calcite, were generated by water that infiltrated from the surface. At present, due to the semi-arid climate, no percolation water reaches the cave, and thus there is no speleothem deposition. Uranium-series dating of growth phases of speleothems in the area can provide information on past periods of increased humidity. Such a study is underway at present and will provide a record of late Quaternary past intervals of increased humidity in the area.

Figure 5. Speleothems (columns) at Toca da Boa Vista.

    Standing water is only present at a few restricted places in Toca da Boa Vista at present, but there is extensive geomorphic evidence of higher Quaternary water levels. Because in carbonate aquifers ground water is frequently saturated with respect to CaCO3, degassing in standing water bodies can cause calcite to precipitate at or below the water level. Such subaqueous speleothems, including cave rafts, shelfstones and coralloids, are widespread in some sections of the caves (Fig. 6). Uranium series dating of these deposits have provided a chronology of such episodes of higher water table. 

Figure 6. Subaqueous speleothems (cones and rafts) at the Discos Voadores Passage.

    Samples from one area of Toca da Boa Vista where subaqueous deposits are present in kilometres of passages yielded ages between 17.3 - 20.1 ka, indicating that the water table in the area was 13 ± 1 m higher at the peak of the last glaciation than at present. In a second area, which could not be tied to the present water table, a higher water table was present at around 145 ka. We believe that higher water table within the cave correlates with more humid phases in the Quaternary. This suggestion is supported by presence of extensive travertine deposits which include the remains of large forest trees in the nearby Salitre Valley. Uranium series dating indicates that the most recent phase of travertine deposition spanned a time period from 21 to 9 ka, coincident with the most recent water table stand in Toca da Boa Vista (Auler and Smart, in preparation). Such results represent a significant departure from previously held views of a drier late glacial maximum, derived from the wetter areas to the southeast, and is not depicted in recent General Circulation Model simulations.

    PALAEONTOLOGY

    Toca da Boa Vista and neighbouring caves have yielded a rich and significant fossil faunal assemblage (Table 1), reviewed by Cartelle (1995) (Fig. 7). Among the most significant fossil finds at Toca da Boa Vista is the first complete skeleton of the primate Protopithecus brasiliensis (Hartwig and Cartelle, 1996), and a nearly complete skeleton of a new genus of primate, Caipora bambuiorum (Cartelle and Hartwig, 1996). These remarkably complete and well preserved monkey skeletons have demonstrated that diversity in neotropical primates was greater in the recent past (Hartwig and Cartelle, 1996) and their body mass (>20 kg) shows that they underwent body size expansion at the late Pleistocene as did other mammals (Cartelle and Hartwig, 1996). Other major fossil finds at these caves include a foetus of the extinct terrestrial ground sloth Nothrotherium maquinense, and nearly complete skeletons of the extinct canid Protocyon troglodytes and the bear Arctotherium brasiliense (Cartelle, 1995). These well preserved fossil skeletons represent the best remains ever found of these specimens, and are allowing a much improved understanding of the systematics of these extinct species.

Ordem/Order

Espécie/Species

Chiroptera

Mormoops megalophylla

 

Pteronotus parnelli

 

Chrotopterus auritus

 

Loncophylla mordax

 

Desmodus rotundus

 

Desmodus draculae*

 

Eptesicus brasiliensis

 

Tadarida brasiliensis

Edentata

Nothrotherium maquinense*

 

Scelidodon cuvieri*

 

Myrmecophaga tridactyla

 

Euphractus sexcistus

Rodentia

Coendou preensilis

Artiodactyla

Tayassu tajacu

 

Lama guanicoe

 

Mazama gouazoubira

Carnivora

Cerdocyon thous

 

Protocyon troglodytes*

 

Arctotherium brasiliense*

 

Protocyon cancrivorus

 

Conepatus semistriatus

 

Felis pardalis

 

Felis tigrina

 

Felis yagouaroundi

 

Felis concolor

 

Smilodon populator*

Primates

Protopithecus brasiliensis*

 

Caipora bambuiorum*

Table 1. List of mammals discovered at the site. From Cartelle and Hartwig (1996). * denotes extinct species.

 

Figure 7. Fossil bones at Toca do Calor de Cima.

    Extensive deposits of fossil bat bones and guano are also found at several places in the caves. Both the large bodied fauna, which includes some living species, and the bats suggest a much wetter and more forested landscape than that at present. Czaplewski and Cartelle (1998) have obtained an AMS radiocarbon date on bat remains of 24 ka (calibrated age), which agrees with a calibrated conventional radiocarbon age obtained by Auler (1999) on guano. Results show good agreement with dates from speleothem and travertine. Auler (1999) has also obtained U series data on calcite enclosing some of the mammalian deposits which show a considerable spread over the last glacial and penultimate glacial periods. Toca da Boa Vista and associated caves represent one of the richest Pleistocene palaeontological sites in Brazil. With the present suite of around 15 radiometric ages these caves are arguably the best chronologically constrained palaeontological site in Brazil.

     BIOLOGY

    The present day cave fauna of the site is markedly impoverished due to the dry nature of the caves, and lack of input of biogenic material. However, at least two troglobitic species (species morphologically adapted to living in caves) occur in the area. The moth Coletinia brasiliensis, described from Toca do Morrinho (Mendes and Ferreira, in press) is the only species of the genus Coletinia known outside Europe (Rodrigo Ferreira, pers. comm.). The amphipoda Spelaeogammarus bahiensis occurs at Toca do Pitu. Ecological studies on bat guano fauna at Toca do Morrinho (Ferreira, 1998) have provided important data on the diversity and distribution of cave dwelling spiders (Ferreira and Martins, 1998).

    Scenic value

    The caves of Toca da Boa Vista area are not pleasant sites for recreational visits. The high internal temperature (27-29 ºC) makes cave exploration an uncomfortable activity and obstacles along the cave passages render many of its more scenic sections unsuitable for non-speleologists. Nevertheless, despite lacking attributes such as flowing rivers and clean washed passages common in many commercial caves, these caves contain many sites that are among the most spectacular in Brazilian caves.
   
The Discos Voadores passage is an extensive series of chambers and passages that contains excellent examples of subaqueous speleothems such as cave rafts, cave cones, and shelfstones. These are among the most extensive in Brazilian caves. Gypsum and bassanite speleothems, including small chandeliers are present in a few sites. More conventional speleothems, such as stalactites, stalagmites, flowstone and rimstone dams abound in many sections of the caves. In the far north of the cave, a dry riverbed filled with milky white calcite layers, runs over reddish clay, creating a stunning visual contrast. Septaria deposits (desiccation cracks filled with calcite) in the middle section of the cave are among the best examples of such type of speleothem in the world (Hill and Forti, 1997). Even where speleothem decoration is absent, the morphology of many of the passages is quite attractive. Round vaulted chambers, and beautiful chert intercalations in the cave walls contribute overall to a very interesting cave morphology (Fig. 8).

Figure 8. Afonso Pena Passage at Toca da Boa Vista.

    Technical and sporting value

    Toca da Boa Vista has provided a training ground for hundreds of cavers from Brazil and overseas. The very complex maze pattern has called for specialised surveying techniques specially adapted for dealing with hundreds of interconnecting galleries (Auler et al., 1991). Toca da Boa Vista was the first cave where cave surveying software was applied in Brazil (1990), and today the representation of such a complex system still requires considerable labour. The entire cave survey is now being digitised in order to improve handling of the data.
   
The very dry and hot nature of the cave is in marked contrast with most other long caves in the world. Cave exploration is limited by water availability, and long trips are becoming increasingly physically demanding. As the cave continues to expand away from the known entrances, new techniques for exploration and mapping are constantly being sought. Toca da Boa Vista has contributed in improving technical skills in Brazilian speleology, as many of Brazilian active cavers have been involved with work on exploratory expeditions

PRESERVATION OF THE SITE

    Toca da Boa Vista and the related caves remain essentially in a natural state. The only environmental impacts observed underground are occasional speleothem breakage and removal by local inhabitants in the areas close to the entrances. A wall and gate at the main entrance built by the local authorities in the early years of exploration detract from the surrounding natural features, but do not impact air flow or alter in any other way the dynamics of the cave. Additional, and mostly unavoidable, environmental impact in the site are related to the exploration and mapping activities. Such minor alterations consist of trails across sediments and plastic survey station markers, essential to the mapping process. In summary, the site remains in very good state of preservation.

    Recommendations for environmental protection

    The remote location of the site appears to be its main ally against anthropic impact. Very few visitors, except for cavers and occasional locals, enter the caves. There is no impact from water pollution or soil erosion and quarrying is not of concern as the site is located away from any major urban centre. Nevertheless, some formal protective status for the site is recommended, in recognition of its scientific value, but no major management or environmental programs appear necessary.

    Potential for ecotourism

    The site is located away from any existing tourist route. The precarious state of the unpaved roads and the lack of any tourist infrastructure in the nearby villages do not favour the development of an active tourist trade. The cave is itself not conducive to a pleasant leisure visit. Thus, Toca da Boa Vista does not appear to hold any potential for ecotourism. Nearby caves, such as the more spacious Gruta do Convento, or the pleasant Pontes do Sumidouro, should readily absorb any local demand for cave related tourism.

ACKNOWLEDGEMENTS

    Much of the scientific data presented in this chapter has been supported by CNPq grants to various researchers. Grupo Bambuí de Pesquisas Espeleológicas, the driving force behind Toca da Boa Vista exploration and survey, was instrumental in revealing the importance of the site. CPRM at Salvador kindly allowed the use of unpublished geological data. We acknowledge reviews by Ezio Rubbioli and Lilia Horta. Rodrigo Lopes Ferreira provided additional information and Adriana Paiano produced the figures.

 REFERENCES

 

Auler,A.S. 1999. Karst Evolution and Palaeoclimate of Eastern Brazil. PhD Thesis, University of Bristol.

Auler,A.S.; Rubbioli,E.L.; Masotti,F.S. 1991. Evolução metodológica no mapeamento da Toca da Boa Vista, Campo Formoso/BA. Espeleotema, 16:25-39.

Auler,A.S.; Smart,P.L. (em preparação). Late Quaternary palaeoclimate in Northeastern Brazil from U-series dating in travertine and water table calcite.

Ball,C.K.; Jones,J.C. 1990. Speleogenesis in the limestone outcrop north of the South Wales Coalfield: The role of microorganisms in the oxidation of sulphides and hydrocarbons. Cave Science, 17: 3-8.

Cartelle,C. 1995. A fauna local de mamíferos pleistocênicos da Toca da Boa Vista (Campo Formoso, BA). Tese para professor titular, Universidade Federal de Minas Gerais.

Cartelle,C.; Hartwig,W.C. 1996. A new extinct primate among the Pleistocene megafauna of Bahia, Brazil. Proceedings National Academy of Sciences USA 93: 6405-6409.

CPRM. 1998. Mapeamento geológico da região de Laje dos Negros. Mapa inédito.

Czaplewksi,N.J.; Cartelle,C. 1998. Pleistocene bats from cave deposits in Bahia, Brazil. Journal of Mammalogy (1998), 79:784-803.

Ferreira,R.L. 1998. Ecologia de comunidades cavernícolas associadas a depósitos de guano de morcegos. Dissertação de mestrado, Universidade Federal de Minas Gerais.

Ferreira,R.L.; Martins,R.P. 1998. Diversity and distribution of spiders associated with bat guano piles in Morrinho Cave (Bahia State, Brazil). Diversity and Distributions, 4: 235-241.

Harman,R.; Gallagher,K.; Brown,R.; Raza,A.; Bizzi,L. 1998. Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous. Journal of Geophysical Research  (1998),103: 27091-27105.

Hartwig,W.C.; Cartelle,C. 1996. A complete skeleton of the giant South American primate Protopithecus. Nature (1996), 381: 307-311.

Hill,C.; Forti,P. 1997. Cave Minerals of the World. Second Edition, National Speleological Society, Huntsville.

Martins,M.R. (ed.). 1986. Avaliação dos Recursos Hídricos das Bacias Hidrográficas do Estado da Bahia - Bacia do Rio Salitre. Centro de Estatística e Informações, Salvador.

Mendes,L.F.; Ferreira,R.L. (no prelo) . On a new cave-dwelling Nicoletiidae (Zygentoma: Insecta) from Brazil. Garcia de Orta.

Misi,A.; Kyle,J.R. 1994. Upper Proterozoic carbonate stratigraphy, diagenesis, and stromatolitic phosphorite formation, Irecê Basin, Bahia, Brazil. Journal of Sedimentary Research(1994), A64: 299-310.

Morehouse,D.A. 1968. Cave development via the sulfuric acid reaction. National Speleological Society Bulletin (1968), 30: 1-10.

Palmer,A.N. 1990. Groundwater processes in karst terranes. In Groundwater Geomorphology (Higgins, C.G.; Coates,D.R. eds.). Geological Society of America Special Paper, 252: 177-209.

Palmer,A.N. 1991. Origin and morphology of limestone caves. Geological Society of America Bulletin (1991), 103: 1-21.