Geological and Paleontological Sites of Brazil - 093
Narendra K. Srivastava
Antônio José Dourado
© Srivastava,N.K.; Rocha,A.J.D. 1999. Fazenda Cristal - Mesoproterozoic stromatolites, Bahia state, Brazil. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 29/12/1999 on Internet at the address http://www.unb.br/ig/sigep/sitio093/sitio093english.htm [Actually http://sigep.cprm.gov.br/sitio093/sitio093english.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: The Cristal Farm, situated in the municipality of Morro do Chapéu, Eastern part of Chapada Diamantina (State of Bahia), contains excellent outcrops of bioherms and biostromes of columnar, stratified, domal and oncolitic stromatolites. These structures occur in shallow marine tempestite carbonate deposits and are at the base of the Caboclo Formation ( Chapada Diamantina Group) of Mesoproterozoic age. The spherical and domal carbonate bioherms of about six meters in diameter are composed of non- branching or ramified columnar stromatolites of upto ten centimeters in height and three centimeters in diameter. The carbonate biostromes composed of stratified stromatolites (planar biolithites), oncolites and domal stromatolites, show thin layers of evaporite- pseudomorphs, milimetric intercalations of red pelites, desiccation cracks and evidences of pressure and dissolution features ( stylolites ). Above this stratigraphic interval occurs a thick bed of lenticular siltites deposited in subtidal regime through the action of tempestites , in which are found large collapse structures represented by the "Hole of Possidonio "( SW of the town of Morro do Chapéu ) and the "Hole of Buracão "( at the Cristal Farm ), which are tourist attractions of the region.
Fazenda Cristal contains one of the most expressive outcrops of biohermas and biostromas composed of columnar, stratiform , domal and oncolitic stromatolites , constituting na excellent example of calcareous stromatolites associated with tempetite sedimentary structures. The nature of these outcrops, allied to the presence of caves and collapse structures, in an area of easy access and low demographic density, together with their importance for mesoproterozoic geology and paleontology of Brazil, justify that the suitable measures may be undertaken for their preservation.
The domain selected for the characterization of Fazenda Cristal embraces an area of approximately 3,25 square kilometers delimited by the meridians 41( 19 ' 57 " and 41( 18 ' 07 " longitude west and the parallels 11( 18 ' 05 " and 11( 19 ' 47 " latitude south, located in the municipal district of Morro do Chapéu, at the Eastern part of Chapada Diamantina ( Diamantina Plateau ) , in the heartland of the state of Bahia (Figure 1) .
From Salvador, the access to the area can be made by asphalted highways, initially by BR-324 until Feira de Santana (108 km); then, by BR-052 until Morro do Chapéu (290km). Later on by the highway BA-046 until the village of Catuaba. The Fazenda Cristal lies four kilometers to north of this village and is reached by a dirt road (Figure 1 ).
Figure 1 - Localization map of the area of Cristal Farm site.
Moving around the geological and paleontological site doesn't present any difficulty, inspite of the area to be poorly populated . The principal outcrops are located in a scarp, with approximately 2km of extension, enclosed with barbed wire.
In the area of interest for this description, the dirt road which gives access to the ranch runs parallel with the scarp, which is situated just some dozens of meters in distance. The entrances to the site are situated at the headquarters of the homonymous farm of the property of Mr. José Carlos Ferraz.
The first geological mapping of the area, at ther scale of 1 : 250 000, was executed by Pedreira et al. (1974), through the Projeto Bahia, executed by a joint venture of DNPM/CPRM. This investigation registered the ocurrence of Caboclo Formation comoposed of metassiltites, metarenites and metargilites. In 1988, the area was mapped, at the scale of 1:50.000, by Maia & Pena Filho , during the Course in Sedimentary Terranes, offered by the Integrated Center of Geological Studies (CIEG), Morro do Chapéu, as a part of the training program for the technical teams of CPRM. This work presented the first description of the carbonate rocks of the area. In 1989, Srivastava realised a study on the stromatolite bioherms and biostroms of the ranch, and described four main groups: columnar stromatolites , nodular stromatolites , columnar/ stratiform stromatolites and ramified columnar stromatolites. The carbonate sequences and the stromatolites of the Caboclo Formation were examined by Rocha et al. (1990). Later, Souza and Souza Jr. (1992), made a quantitative approach to the taxonomic studies of the stromatolites through the studies of laminar profiles. Rocha (1997) presented the results of the geological mapping of the Morro do Chapéu region, at the scale 1:100.000. In 1998, the same author presented the stratigraphy of the Chapada Diamantina and Una Groups at the scale of 1:250.000. In both cases, the studies described the litofácies associations correspondent to the stratigraphic interval.
In the area of Fazenda Cristal are exposed the sedimentary rocks of Caboclo Formation of Chapada Diamantina Group, which is na integral part of the Espinhaso Supergroup of Mesoproterozoic age. The Caboclo Formation, which in this area, according to Rocha ( 1998 ), is about 400 meters thick and is composed of the following lithofacies associations :
a) Algal laminites- columnar stromatolites - supra- through subtidal - Top
b) Lamites- sandstones - deep marine
c) Algal laminites - Oolite calcarenites - supra- through intertidal
d) Conglomeratic sandstones - fluvial braided
e) Lamitos - Sandastones - deep marine
g) Algal laminites - Oolite Calcarenites - supra- through intetidal
h) Conglomeratiuc sandstones - fluvial braided
i) Lamite - Sandstsones - deep marine
j) Amalagamated lenticular siltites - subtidal (Photos 1 and 2)
k) Algal laminites - Calcarenites- Columnar stromatolites- (supra- through subtidal) -Base
Photo 1-General view of a part of the outcrop in Cristal Farm showing the association of algal laminite - calcarenite - columnar stromatolite lithofacies ( inter through subtidal ),
Photo 2-Details of the previous photo evidencing a sharp contact between two association of lithofacies.
The stromatolites outcrops which motivated the suggestion for the creation of the geological and paleontological site of the Cristal Ranch, belong to Algal laminite - Calcarenite - Columnar stromatolites lithofacies asscociation and represents the base of the Caboclo Formation and is equivalent to stratigraphic interval denominated by Braanner ( 1910) as the Jacuipe Flints, in the region of the scarps of Tombador Mountain.. At the Cristal ranch this litofácies association is composed of columnar stromatolitess , intraclastic calcarenites , algal laminites , calcilutites and silexites.
(1) Stratiform Stromatolites : this type of microbial bio-sedimentary structures form biostromes that possess intercalations of columnar stromatolites , domal stromatolites, onclites and bioclastic calcarenites with crossed beddings (Photo 3), besides siltites and red argillites (Photo 4 and Figure 2). The stratiform stromatolites are composed of irregular or crenulated , micritic , dark (rich in organic matter) and clear, sparitic ( constituted of trapped ou precipittaded carbonate material ). It is common to observe the presence of the pseudomorphs of evaporite minerals and sílica nodules, with a thickness of some centimeters and several meters in extension accompanying the bedding planes (Photo 5). The aerial exposure of the stratiform stromatolites provoked the rupture of thin laminae thus forming layers bioclastic breccias. The thickness of stratiform stromatolites beds is rarely more than 1,5meters. Considering the external features and internal textures Srivastava (1989) recognized the Forms Stratifera Korolyuk and Irregularia Korolyuk. However, no microfossils were were found associated wirh these stromatolites.
Photo 3 -Beds of intraclastic - oncolitic calcarenite with cross- stratifications and ripple marks, besides a concentration of clasts at the base of the bed due to action of tempestites.
Photo 4 - General view of an another part of the coutcrop with a thickness of 50 meters at the Cristal Farm ( Figure 2 )
Photo 5 - Irregular layers and nodules of quartz parallel to the stratification of algal laminmites resulted by the substitution of previous laayers of evaporites.
Figure 2 - Lithological profile of algal laminite - calcarenite - columnar stromatolite facies.
(2) Columnar Stromatolites : These types of bio-sedimentary structures are found at two levels of varying thickness ( 0,7 to 1,5 meters thick) on the western part of the site, while on the eastern part, they form bioherms ( Photo 6 ) and domal and tabulate biostromes interdigitated with amalagated lenticular siltite lithofacies deposited under the influence of storm processes.
The columnar stromatolites are cylindricals to subcylindricals, ramified or not, bifurcated, could be coalescentes at the top, varying in size, upto 10 centimeters in height and 3 centimeters in diameter. The external and internal features suggest their affinity with the Forms Collumnacollenia, Planoconophyton and Pseudokussiella (Srivastava, 1989).
Photo 6 -Bioherm of columnar stromatolites ( intertidal ).
(3) Domal Stromatolites: Intercalated with columnar and stratiform stromatolites occur diverse forms of cumulative , nodular, domal and columnar-stratiform stromatolites of small dimensions, generally with irregular internal and crenulated laminations (Photo 7). Some of these occurrences were tentatively classified as Planicollenia, Cryptozoon and "Collenia"
Photo 7 -Domal stromatolites ( Figure 2 ).
(4) Oncolites: Spherical, oval and asymmetric concretions of various sizes ( 0,7 - 60 mm ) , with milimetric laminations composed of alternate, micritic dark ( rich in organic matter) and sparitic, incolour (poor in organic matter) laminae, with poor selection are found within thin layers (6cm) of bioclastic ( intraclasts of microbial mats) calcarenites ( Figure 2 ). The microfacies association of these oncólitos suggests a shallow marine environment of relatively high energy, probably in tide channels. On the basis of their morphology, these oncolites can be classified as Type C of Radwanski and Szulczewski (1966) or as SS- type stromatolites of Logan et al. (1964). They are also similar to the Palaeozoic forms Osagia of Maslov (1960) due to the presence of very distinct internal laminations. The biostratigraphic importance of these oncolites is still being discussed, since only few investigations have so far been realizsed on Precambriano oncolites, especially in Russia (Zhuravleva, 1979) and these studies are not sufficient to make comparisons, or classify them and furnish conclusions concerning their biostratigraphic status in the Proterozoic of Eastern part of Chapada Diamantina.
Born and Horta (1995) described four cavities at site of Fazenda Cristal: the caves Cristal I, Cristal II and Pé de Manga ( Foot of Mango ), besides the doline Buracão. The most important occurrence is the Cristal I (Photo 4), which is practically devoid of speleothems , but has a characteristic reticulated labyrinthic pattern (Figure 3). Due to its complexity this cave is still not totally explored, but its physical and genetic behavior suggest a great potential concerning the development of intercommunicating galleries . The cave has four entrances and the visitation is practically restricted to local people. Being a dry cave it is dependent on the bats whose function is of energy importers, since their excrements and cadavers form the basis of alimentary chain of the animals that inhabit the interior of the cave. Deforestations and intense visitation are the principal cause that shun away the bats, which in its turn provokes desequilibrium and even extinction of cave life. The large thickness of sediments at the area could be potentially rich in fossils, which imparts a greater importance to the cave.
According to these author the geomorphological feature denominated as Buracão constitutes a cylindrical doline (of collapse !) with an ellipsoidal contour and a variable diameter between 70 and 120m and depth between 25 and 50m (Photo 8).The contour is always conformed by steep walls, where are trees and bushes . The doline is covered by a savanna vegetation, developed within rubbles of rock- boulders, and fringed by savanna and pasture areas.
Photo 8 -Buracao Dolina. A geomorphological feature developed in the silitites due to the dissolution of asasociated carbonates laayers.
Figure 3 - Map of the Cristal I cave.
The geological-paleontological site of Fazenda Cristal contains a series of outcrops composed of calcareous stromatolites of several morphologies which represent the basal part of the Caboclo Formation (Chapada Diamantina Group ) of Mesoproterozoic age. The stromatolites are associated with the siliciclastic tempestites , with diverse facies associations and sedimentary structures and offer excellent outcrops with well preserved bioherms and biostromes still poorly studied, although they are understood to be important for the proterozoic stratigraphy. Therefore it is necessary that this site may be preserved, not only for scientific research, but also as a natural and tourist patrimony.
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