Foto 1 - Escarpa do Tombador vista da serra das Palmeiras, visando sul.
Photo 1 - Tombador escarpment seen from Palmeiras range, looking south

Foto 2 - Morro do Cruzeiro visto da serra do Tombador, visando norte.
Photo 2 - Cruzeiro hill seen from Tombador range, looking north.

ABSTRACT

The Tombador range that is part of the Chapada Diamantina eastern escarpment, is formed by clastic sedimentary rocks of the Tombador Formation that is older than one billion years. These rocks are composed by conglomerates and sandstones of eolian, fluvial and deltaic facies, with predominance of the first one. The Tombador Formation was described in the first decade of the present century is deposited on a basement composed by TTG (tonalite-trondjhemite-granodiorite) orthogneisses, acid meta-volcanic rocks, granodiorites and porphyroclastic monzonites. In the Tombador range itself, this unconformity (non-conformity) between this basement and the sedimentary rocks of the Tombador Formation may be observed. They consist of sandstones with bimodal granulometry and large scale cross bedding. In the middle part of the formation, the cross bedding co-sets that have levels with rain pits are bounded by surfaces with thin beds of horizontally bedded sandstones. These surfaces are interpreted as result of discontinuous elevations of the water table. The lithologies and structures of the Tombador Formation characterize it as a perfectly preserved paleo-desert. Owing to an elevation of its level, the sea transgressed on the Tombador Formation, depositing upon it argillites and siltstones of the Caboclo Formation in a tidal flat environment. The bimodal sandstones of the Tombador Formation are exploited as slabs for floor revestment.

Key words: Precambrian, Chapada Diamantina, Tombador range, desertic environment

INTRODUCTION

The Tombador range is an escarpment longer than 75km, where crops out the Mesoproterozoic Tombador Formation, in the center of Bahia State. The escarpment, that trends NNE-SSW is part of the eastern edge of the region named Chapada Diamantina (photo 1). The Tombador range, in its north-south development, takes several local names as Tombador do Araújo, Palmeiras range and Gameleira range. Its main sector is between the BR-324 and BA-052 roads, that cross the Tombador and Gameleira ranges. Towards north and south, its altitudes decrease, and to the north it is covered by Neoproterozoic sedimentary rocks. The stratigraphic, sedimentologic, tectonic and geomorphologic features of the Tombador range have been object of research since the beginning of this century. In this range may be advantageously studied sedimentation processes older than one billion of years, as well as the tectonic and geomorphologic evolution of the rocks along the geological time.

LOCATION

The Tombador range diagonally crosses the 40⁰ 45' meridian and is bounded north and south by the parallels 11⁰ 00' and 12⁰ 00' (figure 1). This situation makes the Tombador escarpment to be almost paralel to the Jacobina range, from which is separated by a wide valley.

Figura 1 - Mapa de localização da serra do Tombador. O encarte mostra a situação da região no Brasil.
Figure 1 - Location of the Tombador range. Inset shows the situation of the region in Brazil.

HISTORICAL DATA

The travel notes of Mr J. A . Allen, ornitologist of the Thayer Expedition, refer to his traverse from the hamlet of Chique-Chique on the São Francisco river, to the town of Bahia (Salvador) approximately in the year 1868. In these notes, he comments about his crossing of the Jacobina Plateau, the highest region where is presently located the hamlet of Laje do Batata (figure 1). From this plateau, one descends eastwards "through a narrow and precipitous canyon".

In the first decade of this century, the region was studied by J.C. Branner, a british geologist of the Geological and Mineralogical Survey of Brazil, who traversed along the several roads that cross the range. He described the basement that separate it from the Jacobina range as composed by " granites, gneisses, schists and old eruptives...The hills made of these rocks are mostly low and rounded ". The rocks of the range that he named Tombador Series, were described as " sandstones and quartzites and the series is unbroken by intercalated shales...about a hundred meters thick... the rocks are rather soft and much joited, while in color are pinkish, yellowish, reddish and grayish...Along the face of the bluffs as seen from the old trail a couple of kilometers north of the present trail, the false beds are often three or four meters high...Going westward or northwestward down the slope of the range, one passes from the Tombador sandstones to a series whitish flinty beds that the writer has called the Jacuipe flints... They are well developed in that part of the range locally known as the Serra da Gameleira...Next above the Jacuipe flints and resting conformably upon them is a series of shales that were given the field name of Caboclo shales..." . Branner 's descriptions are perfect: taking into account the latter studies and the evolution of the knowledge, that determined the basement composition as TTG orthogneisses, acid metavolcanic rocks, granodiorites and porphyroclastic monzonites (Sampaio et al., 1995), and the false stratification as large scale cross bedding, there is nothing to add. Branner's paper is illustrated with his drawings, one of them reproduced in figure 2.

Figura 2 - Serra do Tombador, vista da Fazenda Santa Cruz (esboço de J.C. Branner). À direita, está o morro do Cruzeiro, onde a Formação Tombador aflora separada da escarpa principal (foto 2).
Figure 2 - Tombador range seen from Santa Cruz ranch (sketch by J.C. Branner). At the right hand side, is the Cruzeiro hill, where the Tombador Formation crops out separated from the main escarpment (photo 2).

The Projeto Bahia (Pedreira et al., 1975), restablished the original names of Tombador and Caboclo formations. In the Tombador range, where the "Jacuípe flints" do not crop out, the contact between the Tombador and Caboclo formations was described as transitional.

In 1987 the National Department of Mineral Production (DNPM) jointly with the Mineral Resources Exploration Company (CPRM) opened in the town of Morro do Chapéu (figure 1), the Integrated Center of Geologic Studies (CIEG), in order to train technicians of the two agencies, execute and give support to studies regarding the sedimentary rocks that crop out in the region. One of these studies was the survey along the roads that cross the escarpment of the range (Gameleira and Palmeiras ranges, Angelim creek, Gogó da Gata and Tombador range), to make the faciologic analysis of the Tombador Formation. The panel-diagram of the figure 3 displays and correlates these sections. In the panel is outstanding the predominance of eolian facies in the Tombador Formation

Figura 3 - Correlação entre as litofácies da Formação Tombador analisadas nos perfis das serras da Gameleira e das Palmeiras, riacho Angelim, Gogó da Gata e serra do Tombador.
Figure 3 - Correlation among the profiles of Tombador Formation analysed in the profiles of Gameleira and Palmeiras ranges, Angelim creek, Gogó da Gata and Tombador range.

Finally, recent observations show that under the view point of sequence stratigraphy, Tombador Formation is a lowstanding systems tract, and its contact with the Caboclo Formation is a transgressive surface; the latter represents a highstanding systems tract.

Since the town of Jacobina, looking west, one can see far away the Tombador range. Following the BR-324 road towards Lage do Batata, about 18km west of Jacobina it is possible to observe the unconformity between the basement and the Tombador Formation. In the photo 3, this unconformity is below the vertically fractured beds that continue to the right side of the tree. The lighter colored hill is formed by weathered rocks of the basement.

Foto 3 - Discordância (não-conformidade) entre os arenitos da Formação Tombador e o embasamento cristalino. Rodovia BR-324, cerca de 18km a oeste de Jacobina.
Photo 3 - Unconformity (non-conformity) between the sandstones of the Tombador Formation and the crystaline basement. BR-324 road, about 18km west of Jacobina.

This unconformity can be observed in detail about two kilometers farther west (photo 4). In this place it is marked by a thin bed of fluvial conglomerates, overlain by whitish eolian sandstones. The conglomerate has green quatzite pebbles whose source area is the Jacobina range. Also it may be observed that the unconformity is not a planar surface: the sandstones of the Tombador Formation infill basement depressions.

Foto 4 - Detalhe da discordância entre a Formação Tombador e o embasamento cristalino (acima do automóvel), 20km oeste da cidade de Jacobina.
Photo 4 - Detail of the unconformity between the Tombador Formation and the crystaline basement (above the car), 20km west of the town of Jacobina.

There are four levels of these sandstones, separated by sub-horizontal surfaces. The lower one ends against the road, whereas the upper one is in the right hand corner of the photo 5. The beds that separate these levels are horizontally bedded and are interpreted as bounding surfaces, formed by the elevation of the water table (Medeiros et al., 1971). The lowest level would be a dune field; the rise of the water table fixed the dunes, so that the wind removed the loose sand above it. Upon this flat surface a new dune field formed and a new rise of the water table fixed it. How many times this process occurred is impossible to say: in the Tombador range are preserved only these four levels; others, if present, were eroded.

Foto 5 - Superfícies de truncamento, formadas pela elevação do nível do lençol de água subterrânea, que separam conjuntos de estratificação cruzada de grande porte.
Photo 5 - Bounding surfaces formed by the elevation of the water table, separating large scale cross bedding cosets.

In the western end of the upper level, large scale cross beds can be observed besides the road. In the photo 6, the upper beds have shallow dips to the left; the lower ones, where the hammer is, have steeper dips to the right side.

Foto 6 - Estratificação cruzada de grande porte em arenitos eólicos da Formação Tombador. O círculo assinala um martelo como escala.
Photo 6 - Large scale cross bedding in eolian sandstones of the Tombador Formation. Hammer within the circle as scale.

In the lower part of the outcrop shown in the photo 6, there are some levels with a great number of tiny circular holes (photo 7). These holes are interpreted as rain pits and, according to McKee (1979), are diagnostic features of dune deposits.

Foto 7 - Pingos de chuva preservados na parte inferior do afloramento mostrado na foto 6.
Photo 7 - Rain pits preserved in the lower part of the outcrop shown in photo 6.

In the top of Tombador range, the bedding of the sandstones is horizontal, owing to the marine invasion that deposited the Caboclo Formation. The tidal currents reworked the unconsolidated sands , changing the cross beds into horizontal ones. The Caboclo Formation consists of an alternance of argillites and siltstones (photo 8). In these rocks were found contractional structures, that allow to interpret them as tidal flat deposits, corresponding to an elevation of the level of the sea that transgressed upon the Tombador Formation.

Foto 8 - Argilitos e siltitos interacamados da Formação Caboclo no povoado de Meio, topo da serra do Tombador.
Photo 8 - Interbedded argillites and siltstones of the Caboclo Formation in the hamlet Meio, top of Tombador range.

The Tombador range is not just a succession of outcrops along a road. It represents a perfectly preserved paleo-desert older than a billion years, where the processes that lead to its formation can be examined: the filling of the basement depressions, the wind direction and the variations of its speed, the elevations of the water table, the occasional rains, and the final marine invasion.

PROTECTIVE ACTIONS

The bimodal granulometry of the Tombador Formation sandstones, eases the mining of slabs used as dimension stones. This activity is exerted intensively in the Tombador range as shows photo 9, without intervention of the governamental authorities.

Foto 9 - Extração de lajes de arenito na Formação Tombador, para produção do "Arenito Jacobina".
Photo 9 - Mining of sandstone slabs in Tombador Formation for the production of the "Jacobina Sandstone".

REFERENCES

Branner, J.C.,1910. The Tombador Escarpment in the State of Bahia, Brazil. American Journal of Science, 30: 335-343.

Mc Kee, E.D. 1979. Ancient Sandstones Considered to be Eolian. In: Mc Kee, E.D. (ed.) A Study of Global Sand Seas. Washington, D.C., U.S. Government Printing Office, Geological Survey Professional Paper 1052, p. 187-238.

Medeiros, R.A .; Schaller, H.; Friedman, G.M.,. 1971. Fácies Sedimentares; Análise e Critérios para o Reconhecimento de Ambientes Deposicionais. Rio de Janeiro, Petrobrás, CENPES, Divisão de Documentação Técnica e Patentes, 123 p.

Pedreira, A .J.; Arcanjo, J.B.; Pedrosa, C.J.; Oliveira, J.E.; Silva, B.C.E. 1975. Projeto Bahia; Relatório Final; Geologia da Chapada Diamantina. Salvador, DNPM/CPRM, 2v. (Relatório inédito).

Sampaio, A .R.; Santos, R.A .; Rocha, A .J.D. (orgs.), 1995. Jacobina; Folha SC.24-Y-C; Escala 1:250.000. Brasília, CPRM. (Mapa Geológico).