ABSTRACT

Pai Inácio Hill, together with the Camelo and Morrão hills, consist of erosional outliers located in the limbs of Pai Inácio Anticline, in the central region of Bahia State. In this fold, whose axis trends north-south, is exposed the contact between the Paraguaçu and Chapada Diamantina groups: the former consists of fine grained sandstones, siltstones and argillites and the latter, represented by the Tombador Formation, is formed by sandstones and occasionally diamond bearing conglomerates. The contact between the two groups is a surface that sepparates two depositional system tracts: a lower one, of transitional environment, and upper one, of continental environment.

The landscape of the hills, the geological struture, and the vegetal species that grow in the region, form a unique system for multidisciplinary study. The region is located within the limits of the Chapada Diamantina National Park and the Marimbus-Iraquara Environmental Protection Area, and is classified as "rigorous protection area".

INTRODUCTION

The Pai Inácio Hill (photo 1), considered by many as the milestone of the Chapada Diamantina, constitutes na erosive outlier 140 meter high and with an altitude of 1.120m, preserved along the western limb of Pai Inácio Anticline (photo 2). It is located in Palmeiras municipality that is part of the Central Chapada Diamantina region that encompasses, wholly or in part, the Lençóis, Andaraí, Mucugê, Seabra, Itaetê, Ibicoara and Iraquara municipalities.

Photo 1 – Eastern escarpment of Pai Inácio Hill, seen from BR-242 road. Photo by Fileno P. dos Santos.

The Chapada Diamantina is the extension in the State of Bahia, of the Espinhaço Range System. With average altitudes around 1.000 meters and peaks 1.700 meters high, this platean soars like a wall that separates the São Francisco river valley from the eastern terrains that extend until the coast. The climate in the region is semi-moist tropical with the rainy months between November and February and the dry ones betwen May and September. The temperature in the region is mild, with yearly average of 20^oC. The vegetation in the summit of the hill consists of orchids, cacti, lichens and mosses, among other species. Some of them dwell in the smooth surfaces of the rocks.

Geologically the area is in the domain of the terrigenous meta-sediments of the Chapada Diamantina Syneclisis and is characterized by gentle folds with north-south trending axes. The predominting lithologies are fine grained sandstones, siltstones and argillites of the Paraguaçu Group, overlain by conglomerates, sandstones and pelites of the Chapada Diamantina Group, represented in the Pai Inácio Hill, by Tombador Formation. The conglomerates of this formation are considered as the secondary repositories of the diamond mineralizations of the region. The washing of diamonds in the region, activity that survives until the present times, began late in the XVIII century, reaching its peak by middle XIX century. Then, the hamlet of S. Isabel do Paraguaçu (presently the town Mucugê), had more than 25.000 inhabitants.

From the summit of the hill, one looking south sees the majestic sight of the Pai Inácio Anticline with its north-south trending axis and bounded west by the Bacia range and east by Chapadinha range. About 10km south, outstants the stony "castle"of the Morrão, whereas 3km to north one sees the Camelo or Calumbi Hill, back ground of the opening scenery of a TV opera sometime ago.

The argillites, siltstones and fine grained sandstones of the Paraguaçu Group have lesser resistance to the runoff than the conglomerates and sandstones of the Tombador Formation. On the other hand, the former rocks fold more easily than the latter; these in general are fractured. So, with the formation of the anticline the Tombador Formation fractured in several places, easing the penetration of the rain water, that eroded the softer rocks of the Paraguaçu Group. These processes gave birth to the present landscape, schematicaly shown in figure 3 and in the accompanying photos.

LOCATION

Pai Inácio Hill is located in the central region of Bahia State, where the meridian 41^o28’30’’ crosses the parallel 12^o27’30’’ and is 28km far from the town of Lençóis (figure 1). It marks the northern limit of the Chapada Diamantina National Park, with na area of 1520 sq. km., established on Sep. 17, 1985, by the Federal Decree n^o 91.655.

Figure 1 – Location map of the Pai Inacio Camelo and Morrão Hills. Inset shows the situation of the region in Brazil.

HISTORICAL DATA

The name Pai Inácio Hill was given after a legend envolving a slave named Inácio, who secretively flirted the daughter of his master, who owned large diamond washings. Once discovered the affair, the master put several accomplices to chase the slave. During the flight, Inácio found shelter in the hill, but was discovered and then, he jumped from the summit, using his umbrella as a parachute. It is told that, after the jump he was scen running in a nearby valley, but was never found again.

The geology of Pai Inácio Hill is closely related to the geology of the Chapada Diamantina, that has been studied since the XIX century. The discovery of diamonds in 1844 in Mucugê region (Cathtarine, 1986), lead to intesive studies about the region.

The oldest work, is the report of the General Inspector of the Diamond-bearing Terrains, Benedito Marques da Silva Acauã, presented to the Imperial Government on April 15, 1847 (Acauã, 1847). In this report, he described the geology, geomorphology and mineral resources of Santa Isabel do Paraguaçu (presently Mucugê), Chique-Chique (presently Igatu), Andaraí, Lençóis and Palmeiras (figure 1), but without giving names to the formations. Later, in 1906, Derby named the lower sequence of Chapada Diamantina sediments of Paraguaçu Group, and the upper one, Lavras Group. In 1959, W. Kegel described in the section Seabra-Lençóis-Amparo (hamlet located 40km west of Itaberaba; figure 1) that includes Pai Inácio Hill, the whole clastic sequence cropping out in this sector of the Chapada Diamantina. He described the Upper, Middle and Lower Lavras units; the former two respectively correspond to the Lavras and Paraguaçu groups. About 150km northeast from the Pai Ináçio Hill, in the Tombador range (Pedreira & Rocha, 1999), J. C. Branner in 1910 described the Tombador and Caboclo formations, suggesting that the diamond bearing conglomerates that cropped out above them in the neighborhood of Morro do Chapéu and Ventura were correlated to Derby Lavras Group.

In 1969, J. .F. Mascarenhas adopted the stratigraphy proposed by Brito Neves (1967) that added above the Caboclo Formation, the Morro do Chapéu Formation, describing above the latter, the Lençóis Formation. Later, Pedreira & Mascarenhas (1974), demonstrated that the Tombador Formation that crops out in the Tombador range was the same that crops out the Sincorá range, so that the correlation made by Branner (1910) between the diamond bearing formations of Morro do Chapéu and Ventura, and the diamond washings studied by Derby (1905), was not valid; between the two, there is a pelitic sequence (Branner’s Caboclo Formation or Mascarenhas' Lençóis Formation). In 1990, the CPRM – Mineral Resources Exploration Company mapped the central-eastern sector of the Chapada Diamantina emphasizing the more modern concepts of depositional systems. In the sector of Pai Inácio Hill, the lower depositional sequence corresponds to the Paraguaçu Group and comprises sandy and clayey-sandy lithofacies deposited in a continental-transitional environment. The upper sequence (Tombador Formation) comprises fluvio-deltaic and eolian lithofacies, mostly sandy. So, in this region is exposed the boundary between two depositional system tracts (Pedreira, 1994; figure 2).

Caption: 1 – Microconglomerate; 2 – Sandstone; 3 – Argillite; 4 to 7 – Cross bedding; 4 – large scale, trough; 5 – medium to small scale, trough; 6 – tabular; 7 – herring bone; 8 – Horizontal bedding; 9 – Sigmoids / sandwaves; 10 – Fluidization.

Figure 2 – Graphic sedimentary log of Pai Inacio Hill (thickness, 150m). Modified from Guimarães & Pedreira, 1990.

DESCRIPTION OF THE SITE

Taking into account that the Pai Inácio Hill is a punctual feature, this paper describes some other surrounding sites that are geologicaly and geomorphologicaly correlated with it.

Pai Inácio Hill

Leaving the town of Lençois towards Seabra following the BR-242 road, just after the bridge on the Mucugêzinho river, one sees in the right side of the road, in front of a motel and gas station, the imposing Pai Inácio Hill (photo 1). Presently, the access to the hill was eased due to the construction of a road until close to its summit, that leads to a comunications tower (Telemar). The road is kept in excelent condition, allowing the ascent and parking of buses. From the tower to the summit of the hill, the climb is by foot along a well marked trail, taking about 20 minutes. From the summit, one has a breathtaking view of the Sincorá range (photo 2). In the base of the hill winds up the BR-242 road (Salvador – Brasília), and to the south there is na excellent observation of the Pai Inácio Anticline, eroded along its axis (Cercado Valley) and the Morrão. Northwards is the Camelo or Calumbi Hill.

Photo 2 – Sincorá Range seen form the summit of Pai Inácio Hill. Photo by João Dalton de Souza.

The lower half of Pai Inácio Hill is formed by delta front sediments of the upper Paraguaçu Group, comprising argillites and siltstones intercalated to tabular cross bedded fine/medium grained sandstones. The upper half is built by fluvial-deltaic and eolian sediments of the Tombador Formation, comprising poorly selected, rather feldspatic and pebbly sandstones and well selected bimodal sandstones.

Funch (1997), reports about the vegetation of the hill: "Sitting down in the hill summit pay attention to the surrounding vegetation. Taking advantage of all cracks and sinks on the stones, orchids, cacti, lichens, mosses and other plants, form a garden on the unhospitable surface of the rock. One may think about the climatic conditions in the summit of the hill, as a kind of cold desert. It rains much, but as there is no soil, the water runs off very fast, and the plants are exposed to the dry wind and hot sun. To depend themselves from these unfavorable conditions, the plants are adapted to retain and store moisture. The leaves in general are small, hard and thick, and covered by hairs and waxes, to lessen the water loss. The Pai Inácio Hill area is small, with fragile and easily degradation prone ecosystems, so that it needs a high degree of understanding for its presevation."

Pai Inácio Anticline

The Pai Inácio Anticline (photo 3) is a long erosional window up to 25km wide, that exposes in its inner part sedimentary rocks of the Paraguaçu Group, overlain by the sandstones and conglomerates of the Tombador Formation (figure 3). It is a south plunging asymetric structure, that in Paty valley and Capão regions turns southeastwards plunging and disappearing under the rocks of the Chapada Diamantina Group.

Photo 3 – Pai Inacio Anticline. Photo 100king south.

The axial surface of the anticline dips steeply to east. The dips in the eastern limb are gentler (5^o-15^o) than those of the western one, that may reach 30^o.

Figure 3 – Location of Pai Inácio and Camelo hills in relation to the folds of Chapada Diamantina. Modified from Kegel, 1959.
Caption: 1 – Una Group; 2 – Caboclo Formation; 3 – Tombador Formation; 4 – Paraguaçu Group.

Morrão

Morrão is the most beatiful witness of the regional geologic history (photo 4). Its geology is similar to that of Pai Inácio Hill. Posted in the center of Pai Inácio Anticline, is about 10km south of Pai Inácio Hill and is about 210m high (altitude 1.418m).

Photo 4 - Morrão Hill (see location in figure 1).

Morro do Camelo

Located about 2km north of Pai Inácio Hill, the Camelo Hill is about 170m high with an altitude of 1.090. Its geology and vegetation show little difference from the Pai Inácio Hill. Observed from the summit of Pai Inácio Hill, some people associate the shape of the hill to a woman laying down and looking oposite from the observer. From another looking point, following along the BR-242 road towards the town of Seabra, it takes the shape of a camel, origin of its name (photo 5). From any direction however, the Camel Hill is always impressive for its grandeur and beauty.

Photo 5 – Camelo Hill (see location in figure 1).

PROTECTIVE ACTIONS

According to written communication (1999) from Roy Funch, Director of the Environmental Department of the town of Lençóis, the Pai Inácio Hill is in the "buffer area" of the Chapada Diamantina National Park, so that it receives a certain legal protection from the IBAMA (Brazilian Institute of the Environment). This institute deals with actions that could have negative influence on the park itself. The Pai Inácio Hill is also within the Marimbus/Iraquara Environmental Protection Area (State Decree N^o 2.216 of June 14, 1993), being classified as "Rigorous Protection Zone", the most restrict of all. Also there is preservation by the IPHAN (National Institute of the Historic and Artistic Heritage), that poses severe restrictions on any enterprise in the area.

BIBLIOGRAPHIC REFERENCES

Acauã, B. M. S.; 1987. Relatório dirigido ao governo imperial em 15 de abril de 1847, pelo inspector geral dos terrenos diamantinos na província da Bahia. Inst. Hist. Geogr. Ethnogr. Brasil Rev. Trim, t.9: 227-260.

Bomfim, L. F. C.; Pedreira, A . J.(Orgs) 1990. Lençóis; folha SD.24-V-A-V.Estado da Bahia. Texto explicativo. Brasília: DNPM, 130p. (Programa Levantamentos Geológicos Básicos do Brasil-PLGB).

Bomfim, L. F. C.; Krug, L.; Nolasco, M. C. 1996. Excursão E5: Roteiro Geoturístico da Chapada Diamantina.. In: CONGRESSO BRASILEIRO DE GEOLOGIA, 39 Roteiro de Excursão. Salvador: SBG - Núcleo Bahia - Sergipe.

Branner, J. C. 1910 The Tombador Escarpment in the State of Bahia, Brazil. Am. Jour. Sci. 30 : 335-343.

Brito Neves, B. B. de, 1967. Geologia das Folhas de Upamirim e Morro do Chapéu.Recife, CONESP, 53p. (Relatório 17).

Catharino, J. M. 1986. Garimpo-Garimpeiro-Garimpagem. Rio de Janeiro, Philobiblion, Museu Eugênio Teixeira Leal. 270p.

Derby, O. A. 1906. The Serra do Espinhaço, Brazil. The Journal of Geology,14 : 314-401.

Funch, R. 1997. Um guia para o visitante a Chapada Diamantina: o Circuito do Diamante. Salvador: Secretaria da Cultura e Turismo, 209p.(Coleção Apoio, 15).

Guimarães, J. T.; Pedreira, A . J. ( Orgs ). 1990. Utinga; folha SD.24-V-A-II . Estado da Bahia. Texto Explicativo. Brasília: DNPM, 141 p. (Programa Levantamentos Geológicos Básicos do Brasil – PLGB).

Kegel, W. 1959. Estudos Geológicos na Zona Central da Bahia. Rio de Janeiro, DNPM-DGM, 40p. (DNPM-DGM Bol. 198).

Lima, C. C. U. de; Nolasco, M. C. 1997 Lençóis, Um Aporte entre a Geologia e o Homem. Feira de Santana: UEFS. 152 p.

Mascarenhas, J. F. 1969. Contribuição à Estratigrafia da Chapada Diamantina na Região entre Itaetê e Seabra- Bahia. In: CONGRESSO BRASILEIRO DE GEOLOGIA 23. Resumo das Comunicações. Salvador, SBG, 1969. (Boletim Especial, n. 1). p. 59-60.

Pedreira, A . J. & Mascarenhas, J. F. 1974. A Seqüência Estratigráfica da Chapada Diamantina NE, Bahia, Brasil. In: CONGRESSO BRASILEIRO DE GEOLOGIA 28. Anais. Porto Alegre, SBG, v.4.p.19-26.

Pedreira, A. J. 1994. O Supergrupo Espinhaço na Chapada Diamantina Centro - oriental, Bahia : Sedimentologia, Estratigrafia e Tectônica. São Paulo, USP – Instituto de Geociências. 126p. (Tese de doutoramento inédita).

Pedreira, A. J.; Rocha, A. J. D. 1999. Serra do Tombador; Sítios Geológicos e Paleontológicos do Brasil. http://www.unb.br/ig/sigep/sitio031/sitio031.htm (30 Ago. 99)