Geological and Paleontological Sites of Brazil - 061
FAZENDA ARRECIFE ( BAHIA )
© Srivastava,N.K.; Rocha,A.J.D. 1999. Fazenda Arrecife (Bahia) - Neoproterozoic stromatolites. In: Schobbenhaus,C.; Campos,D.A.; Queiroz,E.T.; Winge,M.; Berbert-Born,M. (Edit.) Sítios Geológicos e Paleontológicos do Brasil. Published 3/11/1999 on Internet at the address http://www.unb.br/ig/sigep/sitio061/sitio061english.htm [Actually http://sigep.cprm.gov.br/sitio061/sitio061english.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)
The Arrecife Ranch, occupying an area of about 7.5 square kilometers, in the municipality of New Várzea, in the eastern part of of Diamantina Plateau (State of Bahia), contains excellent exposures of bioherms composed of columnar stromatolites pertaining to the Supergroups Kussiellida and Conophytonidas (Kussiella, Columnacollenia and Colonella) of the Neoproterozoic age (Salitre Formation of Una Group). These bioherms are associated with carbonate storm deposits. The bioherms of three to five meters in diameter are normally subspherical to oval shaped and contain non - branching or ramified, columnar stromatolites of up to eighty centimeters in height and fifteen centimenters in diameter. They are generally non phosphatic or poorly phosphatic and predominate in the area. Locally, smaller ( up to six centimenters in height and two centimenters in diameter), phosphatic stromatolites belonging to the Supergroups Gymnosolenida and Kussiellida (Gymnosolen, Jurussania and Boxonia) also occur dispersed in bioherms of larger stromatolites. This selective phosphatization of stromatolites is probably due to microbial control. Nevertheless, these smaller columnar stromatolites do not form biological constructions and are rare in the locality. Pseudomorphs of evaporite minerals are frequent in certain bioherms and within the columnar stromatolites indicating specific paleoenvironmental settings and diagenetic processes. The outcrops are exposed in the large, low lying, poorly habitated area, relatively far away from urban concentrations. However, the locality is frequently used for agriculture and cattle breeding. These activities, along with the mineral exploration may destroy or even obliterate this impressive geological and paleontological site in the future.
A Fazenda Arrecife, situada no município de Várzea Nova, na Chapada Diamantina Oriental (Bahia), abriga belas exposições de biohermas, compostas por estromatólitos colunares dos supergrupos Kussiellida e Conophytonida (Kussiella, Columnacollenia e Colonella), que ocorrem na formação Salitre, do grupo Una, de idade Neoproterozóica, associadas a sedimentos carbonáticos, com evidências da ação de tempestades. As bioermas subesféricas a dômicas, com diâmetros de até cinco metros, são compostas de estromatólitos colunares, bifurcados ou com colunas isoladas, de até 80cm de altura e de 15cm de diâmetro. Localmente esses estromatólitos colunares, não fosfáticos, são encontrados associados aos estromatólitos colunares fosfáticos, de até 6cm de altura e 2cm de diâmetro (Gymnosolen, Jurusania e Boxonia), indicando provável controle microbiano na formação de dois diferentes grupos de estromatólitos: a) os maiores, não-fosfáticos (supergrupo Kussiellida e Conophytonida); e b) os menores, fosfáticos (supergrupos Gymnosolenida e Kussiellida). Freqüentemente são observados pseudomorfos de minerais evaporíticos dispersos, tanto em bioermas como também em estromatólitos colunares, sugerindo claramente controles paleoambiental e diagenético. As ocorrências dessas estruturas bio-sedimentares, em planta e em corte, estão espalhadas numa área plana, desértica, pobre em fauna e flora (caatinga), relativamente longe dasconcentrações urbanas, embora freqüentemente utilizada para a pecuária. De modo geral, o sítio da fazenda Arrecife encontra-se relativamente abandonado, o que aparentemente caracteriza um baixo grau de risco para a conservação dos afloramentos. Entretanto, as atividades relacionadas à agropecuária, aliadas à possível explotação de minérios (fosfato, calcário, etc.), podem, no futuro, representar grandes riscos para a preservação desse impressionante sítio geológico-paleontológico.
INTRODUCTION:The Arrecife Ranch or Reef Ranch , occupies an area of about 5,0 km2 in the municipal district of New Várzea, Eastern Diamantina Plateau , in the central part of the state of Bahia, delimited by the meridians of 41° 01 ' 26 '' and 41° 02 ' 40 '' of longitude west and parallels of 11° 05 ' 50 '' and 11° 06 ' 50 '' latitude south (Figure 01) and contains one of the most impressive exposures of columnar stromatolite bioherms . The stromatolites and the associated sedimentary structures are well preserved underlining their importance for the study of the Neoproterzoic geology of Brazil. The nature and beauty of the geological and paleontological features are exposed in a poorly populated and easy accessible areas and justify their preservation.
The stromatolites are biosedimentary structures formed throughmicrobial activities (cyanobacterias, algae, mushrooms) in aquatic environments. They are considered as being the oldest evidence of macroscopic life on the Earth (~ 3.5 Ga Warrawoona Group, Australia), and are found in all the continents and in sedimentary rocks of all geological ages, mainly in Precambrian rocks, where they have reached the maximum of diversity and development. Now a days, they are observed, in growth form, in several saline alkaline lakes and hypersaline lagoons (Shark Bay, Australia; Lagoa Salgada, Brazil; Salt Lake, USA; Bahamas and Bermuda).
The importance of the stromatolites of Fazenda Recife, due to their excellent preservation, lies in their utilization in several geological studies, such as (1) identification of depositional environments (salinity, paleocurrents, depth; (2) dating of host carbonate sequence through stromatolite association and its eventual biostratigraphic correlation with other areas; (3) reconstruction of paleogeographic conditions of the area; (4) determination of paleoastronomical rhythms and interpretation of tidal regimes ; (5) computation of sedimentation rates, and (6) search for microfossil communities responsible for stromatolite constructions.
Figura - 01 - Mapa de localização da
Fazenda Arrecife ( Bahia ), Brasil.
Figure - 01 - Location map of Fazenda Arrecife ( Bahia ), Brazil.
LOCATION: Fazenda Arrecife is situated in the heartland of the state of Bahia, at the Eastern part of Chapada Diamantina ( Diamantina Plateau ), to the north of the city of Morro do Chapéu ( Figure 01), and is reached by highway BR - 426 (Morro do Chapéu - Várzea Nova) through a dirt road towards Tábua, whose entrance is located about 14 km beyond New Várzea. The distance between Fazenda Arrecife and the crossing point with BR - 426 is about 18.2 km. From the town of Jacobina, Fazenda Arrecife can be reached through highway BR - 324 going up to Serra do Tombador. The entrance to Fazenda Arrecife is about 14 km beyond the town of Lajes do Batata.. From Salvador, the access to the area can be made by asphalted highways, initially through BR - 324 (Salvador - Feira de Santana - 108 km), then BR - 052 (Feira de Santana Morro do Chapéu - 290 km) or through BR - 324 (Feira de Santana - Jacobina Lajes do Batata - 252 km), then BR - 426 (Lajes do Batata to the entrance for Fazenda Arrecife).
Moving around the ranch does not present great difficulties, since the area is poorly populated and the Arrecife Ranch is located on both sides of the dirt road and is surrounded by barbed wire. The entrances to the ranch are located at the central part of the ranch where three old houses still exist, although they are rarely inhabited.
HISTORICAL NOTES: The first reference, in the literature, concerning the stromatolites of the ranch, was made by Cassedanne (1964), in the which he described the bioherms of Collenia with data on the chemical composition of the host rocks while investigating the lead and zinck mineralizations in the area.. On the basis of stromatolites he suggested a Late Precambrian ou Early Cambrian age for these carbonate occurrences. Later, Srivastava (1990) surveyed the area for Proterozoic stromatolites in conjunction with a detailed study of stromatolites from Mesoproterozoic Caboclo and the Neoproterozoic Salitre Formations from the eastern part of Chapada Diamantina (Bahia ) with the intention of investigating taxonomy, biostratigraphy, paleoenvironments and geochemistry of the stromatolites and carbonate host rocks. However, no systematic work on the stromatolite bioherms of Fazenda Arecife and their importance to the geology of the region has been executed until now.
REGIONAL GEOLOGY: The region where the Fazenda Arrecife is located, has an aulacogen type structural, evolutive model, in which the Xique Xique Complex, Espinhaço and San Francisco Supergroups, of the Paleoproterozóoc, Mesoproterozoic and Neoproterzoic ages respectively are widely exposed. The area, where the Fazenda Arrecife is located, is composed of Neoproterzoic sediments of the San Francisco Supergroup, including the Bebedouro Formation (metasiltites and metaargilites with scattered pebbles of varied composition - metatilites) with structures like dropstones, suggesting a glacial origin. Overlying this sequence are carbonates of shallow marine environments of the Salitre Formation, extremely rich in bioherms and biostromes of stromatoliteos of diverse nature and sizes, whose best and largest exposures are in the Irecê Basin. Bomfim et al. (1985), Pedreira et al. (1987), Monteiro et al. (1987) and Souza et al. (1993) have divided the carbonate sequence of the Salitre Formation into the following lithofaciologic units : Nova America, Jussara and Irecê. The columnar stromatolites are preferentially abundant in the Nova America Unit . In the delimited area of the Fazenda Arrecife site only the biogenic carbonates of the Nova America Unit are exposed.
STROMATOLITES: At Fazenda Arrecife, two " main types " of calcareous stromatolites belonging to the two " taxonomic " groups are present:
(1) the first type, the larger ones, form domal to subspherical, isolated bioherms (litoherms) , which are close to one another (the distance between the bioherms varies from one to two meters) and are up to five meters in diameter (Figures 02, 03, 04). They are associated with the tempestites composed of interclast calcirudites (Figure 05) produced by storm-waves (hummocky cross stratification). The columnar stromatolites are cylindrical to subcylindrical, sometimes but not always ramified and bifurcated,although sometimes coalescing at the top (Figure 06). The nature of the ramification is usually passive or parallel, alpha-type (where the ramification width is constant), but beta-type ramification (quick increase of width of the main column before ramification) also ocurrs. In plan-view ( outline of the columns, perpendicular to the main axis of growth) they are usually lobate to ellipsoidal (Figure 07). The average height of the columns is eighty centimeters, with a maximum height of one meter and the width of the columns is fifteen centimeters. On the basis of height / width ratio these stromatolites are considered fine in their attitudes (nature of column - axis relative to the substratum) they are erect. At the borders of the bioherms , in contact with the storm sediments in the " inter- bioherms zones " the stromatolite columns are usually tilted or even recumbent, pointing in same direction of paleocurrentes , usually North South. On the basis of the diameter variation along the main axis of the columns, the stromatolites of this group are considered uniform but sometimes have a smaller diameter at the top. The flat internal laminations are usually slightly to moderately convex, but sometimes slightly rectangular. The synoptic relief of the lamination is usually low, the degree of inheritance of nature of the lamination is high, and laterally laminations continue bending down without forming any external wall or entering in intercolumnar sediments. The lamination rhythmicity is well developed, due to parallel accretion and significant alternation of color, thickness and composition of the laminations. Based on external and internal characteristics of columnar structures these stromatolites are tentatively assigned to the Supergroups Kussielida and Conophytonida reaspectively (according to the classification of Raaben, 1969) representing Kussiella, Columnacollenia and Colonella or as SH stromatolites (vertically stacked hemispheroids) according to the classification of Logan et al. ( 1964).
(2) the second type of columnar stromatolites present in the ranch is smaller ( 6 cm tall and with a diameter of 2 cm) and does not form bioherms or biostromes, but is found sparsely dispersed in bioherms of above-mentioned larger stromatolites. The individual stromatolites are cylindrical, straight, actively ramified (beta, parallel-type), bifurcated, sometimes, coalesced and usually uniform variable in their diameters. The internal laminations are flat and slightly convex. The synoptic relief of the laminations is low and the degree of lamination -inheritance is high and has lateral continuity, bending down, forming a flat external wall. The lamination rhytmicity is well developed. The external and internal characteristics of these stromatolites suggest their affinities with Supergroups Gymnosolenida and Kussielllida representing Gymnosolen, Jurussania and Boxonia respectively or SH- type stromatolites of the classification of Logan et al. ( 1964).
The chemical analyses of these two groups of columnar stromatolites demonstrated that those of Supergroup Gymnosolenida (the smaller ones) are richer in phosphate ( > 30, 5% P ) as compared to those of larger ones of the Supergroup Kussiellida, which are devoid of phosphate or poorly phosphatic (< 3,5% P2 O5). This could mean that the microbian community responsible for the formation of phosphatic stromatolites was selective towards phosphate. The carbonate microfacies and sedimentary structures associated with the biosedimentary constructions suggest a patch-reef environment in a shallow subtidal regime with oscillations in depths controlled by frequent storms.
DESCRIPTION OF THE SITE : The geological paleontological site of Fazenda Arrecife (Recife) occupies an area of approximately 7.5 square kilometers, inside a vast, privately- owned, homonymous area, and is enclosed by barbed- wires and divided by a dirt-road which links the village of Tabuá to highway BR 426. The stromatolite outcrops are located on both sides of the dirt-road, near the main office of the farm, which consisting of three poorly conserved and inhabited houses. After a short walk, one can reach the outcrops through wooden gates and cattle trails among the savanna vegetation. The bioherms are found frequently covered by vegetable debris (leaves and dry branches) and gravel. The outcrop, located in a natural water tank for cattle on the left side, when coming from BR -426, offers excellent examples of bioherms associated with storm sediments and composed of ramified columnar stromatolites . This outcrop is protected by a stone fence made up of stromatolite blocks. This protective wall is being gradually destroyed by the natural action of wind and rain. Frequently, remains of vertebrate fossils of Cenozoic age are found in this locality. On the right hand side, when coming from BR - 426, the access to outcrops is made through badly preserved wooden gate. On this side are found, in plan- view, impressive displays of oval shaped bioherms ( Figures 08, 09).
Figura - 02 - Bioherma de
estromatólitos colunares em rochas carbonáticas
Figure - 02 - Bioherm of columnar stromatolites in carbonate rocks.
Figura - 03 - Estromatólitos
colunares ramificados ( vista em corte )
Figure - 03 - Columnar ramified stromatolites ( view of vertical sections ).
Figura - 04 - Vista geral de bioherma de
estromatólitos, lateralmente em contato com rochas depositadas sob ação de tempestades.
Figure - 04 - General view of bioherm, laterally in contact with carbonate rocks deposited under action of storms.
Figura - 05 - Detalhe dos tempestitos,
mostrando truncamentos das laminações ( vista em corte )
Figure - 05 - Details of tempestites, showing the truncated laminatioms ( vertical section ).
Figura - 06 - Estromatólitos colunares com
ramificações passivas e laminações internas ( vista em corte ).
Figure - 06 - Columnar stromatolites with passive ramifications and internal laminations ( vertical section).
Figura - 07 - Estromatólitos colunares
mostrando ramificações ( vista em planta ).
Figure - 07 - Columnar stromatolites showing ramifications ( plan- view )
Figura - 08 - Bioherma dômica de
estromatólitos ( vista em planta ).
Figure - 08 - Dome- shaped bioherm of stromatolites ( plan - view ).
Figura - 09 - Biohermas de estromatólitos
alongados tipo manchas, com canais indicativos de paleocorrentes.
Figure - 09 - Stromatolite bioherm, patch-reef type, with canals indicative of the direction of paleocurrents.
PROTECTIVE MEASURES: The geological - paleontological site contains important and well preserved outcrops of biosedimentary constructions of Neoproterozoic age, without parallel in their beauty and preservation, but little studied. Therefore, it is essential to preserve this ranch , not only for scientific researche, but also as a natural and tourist attraction, being part of the Brazilian heritage
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