ABSTRACT

The Furnas Structural Escarpment zone is a rare geomorphological Site in Brazil. It constitutes a group of relict landforms that keeps important stratigraphic and palaeoenvironmental records. It is an imposing topographical projection about 5 km wide, which extends along about 260 km between the states of São Paulo and Paraná. The Escarpment has altitudes among 1,110-1,200 m and amplitudes between 100 and 200 m, what has permitted exposing the whole Furnas Formation sequence formed by Eodevonian sandstones. On the base of the Escarpment is the erosional discordance limiting the Furnas Formation and the underlain Proterozoic-Eopaleozoic geological units. The development of the Escarpment zone is related to a series of geodymamic processes started with the Gondwanaland drift (Jurassic) and followed by differential erosion processes led under arid to semi-arid and hot climate conditions during the Late Cretaceous to the Tertiary time. The Site still presents important landforms including ruinform relief, residual hills (monadnocks), pediments, pinnacles, canyons, falls and rapids. Besides, there are small caves, grottos and shelters formed into sandstones, which keep archaeological vestiges.

INTRODUCTION

Structural escarpments are steep slopes terminating high lands abruptly, which result of lingering and severe action of differential erosion working on rocks with special geological structures (horizontal or steep bedding, folds, fractures and even very ancient faults), for hundreds of millions years (Bloom, 1991). In Brazil those escarpments occur associated to Palaeozoic sedimentary basins and Proterozoic low-grade metamorphic rocks. In general, they expose different geological formations and litologies and they do not show uniform topographical expressions. Besides, the majority had been affected by ruptile tectonic events, being strongly controlled by faults. The Furnas Structural Escarpment had only been implanted on the Furnas Formation sandstones. It presents continuous and uniform topographical expression and Mesozoic and Cenozoic tectonic activities just weakly deformed it.

The Furnas Structural Escarpment zone is located along the eastern edge of the Paraná Sedimentary Basin, in the states of São Paulo and Paraná. It is a rare Brazilian Geological Site of geomorphological and stratigraphic natures, still keeping important palaeoenvironmental informations, speleologic features and archaeological vestiges. It constitutes a well preserved relict landform, whose genesis started in the Late Mesozoic and led ahead due lingering and continuous differential erosional processes mainly active under arid and semi-arid climates during the Tertiary and Early Quaternary. Those geodynamic processes acting on hard sandstones had been responsible for the development of important landforms including ruinform relief, canyons, residual hills (monadnocks), pinnacles and pediments. The stratigraphic significance of the Escarpment is that it has exposed the whole Furnas sequence and the erosional discordance between this and the underlain Proterozoic and Eopaleozoic geological units. Small caves and grottos represent speleologic features, which had been shelters for the prehistoric populations. Nowadays, they still keep important archaeological vestiges as lithic and ceramic artefacts, rupestrian paintings and human bones remains.

SITE LOCATION

The Furnas Structural Escarpment zone extends along the southern portion of the State of São Paulo and the oriental side of the State of Paraná, approximately between 24°00’S to 25°30’S and 49º00’W to 50°20’W (Figure 1).

Figure 1.Localisation map of the Furnas Structural Escarpment and vertical section showing local geomorphological aspects.

The accesses to the Site may be made either by the State of São Paulo, through the cities of Itapeva, Itararé and Bom Sucesso de Itararé, or by the State of Paraná, throughout by Sengés, Eduardo Xavier da Silveira, Jaguariaíva, Ventania, Piraí do Sul, Castro, Ponta Grossa, Abapã, Itaiacoca and Campo Largo.

The best views of the Furnas Structural Escarpment zone are along main and secondary roads and trails that are well known by the local inhabitants. The most important places are located at the following main roads (Figure 2): Itapeva-Bom Sucesso de Itararé (Serra da Fazenda Pouso Alto), Itararé-Bom Sucesso de Itararé (Serra da Ventania), Jaguariaíva-Piraí do Sul (Serra das Furnas), Ventania-Piraí do Sul (Serra das Furnas), Castro-Ponta Grossa (Serra de São Joaquim) Abapã-Itaiacoca (Serra de Itaiacoca), and Ponta Grossa-Campo Largo (Serra da Prata).

The most beautiful and important canyons, waterfalls and rapids which occur along the Escarpment zone are associated to the following main rivers (Figure 2): Taquari-Guaçu River (SP), Pirituba River (SP), Itararé River (SP/PR), Jaguaricatu River (PR), Jaguariaíva River (PR), Iapó River (PR) and Pitangui River (PR).

Figure 2. Regional geological context of the Furnas Structural Escarpment.

HISTORICAL

Derby (1878) was the first author who referred to the existence of the Escarpment like "an escarpment exposing massive horizontal beds of white, thick and soft grés, of Devonian age, which is situated in Campos Gerais area, westward from Curitiba".

The name Furnas was used for the first time by Oliveira (1912), when he designated the Devonian sandstones exposed at the scarps of the Serra das Furnas, at Itapeva (SP), and Serrinha, at the western side of Campo Largo (PR). However, only almost after three decades, Petri (1948) did the formal stratigraphic denomination applied for the Furnas Formation (Paraná Group), which was later reviewed by Bigarella et al. (1966).

Maack (1947) denominated of "Devonian Scarp" the relief associated to the Furnas rocks, whose extends along the edges of the Paraná Basin. In spite of the term "Devonian Scarp" is frequently found in the geological and geomorphological literatures, mainly in the State of Paraná, we are considering that "Devonian" is an inadequate designation to the Escarpment, because it is related to the age of the Furnas deposition, instead of the age of the Escarpment, which is younger.

Maack (1968) elaborated a map of the "Devonian Scarp" in which he included special landforms like mountain ridges and residual hills associated. He also attributed the scarp origin to erosional processes.

Ab'Saber & Bigarella (1961) assumed the both surfaces overlaying the top and the base of the scarp in Paraná might be correlated to Purunã Surface (1,190 m high) and Predevonian Surface (950 m high), respectively. For the State of São Paulo, Almeida (1964) associated them to the Japi Surface (Late Tertiary) and Itapeva Surface (Predevonian age), respectively.

Amongst the most important regional geomorphological studies concerning the escarpment zone, some authors can be mentioned: Ab'Saber (1964), IBGE (1968), IPT (1981a), FIBGE (1990a) and Ross (1997).

Countless regional and local geological studies concerning the Furnas Formation have been accomplished in Paraná and São Paulo. Amongst the ones involving stratigraphic, sedimentological, palaeoenvironmental and tectonic aspects, they are the most important: Petri (1948), Bigarella et al. (1966), Bigarella & Salamuni (1967), Lange & Petri (1967), Popp & Barcellos-Popp (1986), Zalán et al. (1987), Pereira & Bergamaschi (1989), Rodrigues et al. (1989), Bergamaschi (1992), Assine et al. (1994) and Milani (1997).

SITE DESCRIPTION

As commented previously, the Furnas Structural Escarpment zone constitutes a special Brazilian landscape and it still keeps important informations of palaeoenvironmental and stratigraphic nature and speleologic-archaeological records. The whole those characteristics are described bellow.

Regional landscape

The 260 km long Furnas Structural Escarpment zone extends from the southern State of São Paulo through the oriental side of State of Paraná about 100 km far from Curitiba city. The Escarpment zone is situated between two regional geomorphological compartments (Figures 1 and 2) denominated Patamares of the Paraná Basin and Paranapiacaba Mountain Ridge (FIBGE, 1990b). The escarpment line is very irregular, presenting a festoon standard with several deep amphitheatres cut by rivers forming canyons. Either ancient faults or joints and basic dikes, whose NS and EW orientations are transverse to the regional directions of the Escarpment (NE-SW and NW-SE), in general, control these canyons. This configuration has created an Escarpment influence zone of about 5 km wide.

The exuberant and continuous topographical expression of the Furnas Escarpment zone does it easily identifiable in products on several scales, either on satellite and radar images or on aerial photographs, as well on topographic maps.

The top altitudes of the Furnas Escarpment are quite regular and they are always among 1,100 and 1,200 m. The amplitude also varies a little along, between 100 and 200 m.

The configuration of the Site in plan reminds a parabolic arch with a pronounced N50W axis (Figures 1 and 2). It results from a basement structural high of Proterozoic-Eopaleozoic age, which was reactivated and uplifted during the Mesozoic-Cenozoic times. That structure is well known as Ponta Grossa Arch (Sanford & Lange, 1960; Zalán et al., 1987a).

Local geomorphological aspects

The Furnas Escarpment zone includes four sectors as are known: face, base, reverse and front (Figures 1, 3, 4 and 5).

The escarpment face corresponds to vertical steep rocky slopes that reaches about 120 m high. Along the canyons it is possible to find nice waterfalls falling down the escarpment face (Figure 6).

The base of the escarpment presents lower steep slopes dipping 25° to 40°, which are represented by pediments and talus (debris) deposits. Pediments were developed under arid to semi-arid and hot climates during the Tertiary, while talus deposits have been formed under more humid conditions of subtropical climate since the Early Quaternary.

The reverse of the escarpment is represented by a very smooth surface dipping inland, which has deeply been cut by rivers forming canyons, countless springs, waterfalls (Figure 6) and rapids.

In front of the escarpment and shared from it there are several residual hills (monadnocks) and pinnacles. They are constituted by less fractured silicified sandstones that have been shared from the escarpment face, but they have been resisting to the whole erosional process. Actually, residual hills and pinnacles are resultant forms arising from ruinform relief, which always occurs along the escarpment face and produce odd forms (Figure 5). Severe and lingering differential erosion acting on stratified sandstones controlled by joints systems have been the most important to the development of those kind of landforms.

Figura 3.Reverse, face and base of the Furnas Structural Escarpment.

 

Figure 4. Front of the Furnas Structural Escarpment showing mainly residual hills (monadnocks) and pinnacles.

Figure 5. Ruinform relief and odd forms associated to the Furnas Structural Escarpment.

 

Figure 6. Palmito Mole Fall, Pirituba River canyon (SP).

Origin, evolution and age

The origin and evolution of the Furnas Structural Escarpment zone is associated to a series of geodynamic processes which affected the whole south-southeastern area of Brazil and were initiated with the break-up of the Gondwanaland (180 to 170 Ma.). The followed processes led by continental basic magmatism (140 to 130 Ma.), marginal uplift and continental drift with marginal coastal basins development and evolution (120 Ma. up to Recent). Simultaneously, severe and lingering erosional processes occurred under climate alternations between arid to semi-arid and hot conditions and humid and hot conditions, which led since the Late Cretaceous and during the Tertiary and Early Quaternary. Those processes had played a fundamental hole for dissection and retrogradation of the Escarpment zone. The Escarpment evolution during the Late Quaternary is mainly associated to the talus deposits development and excavation of the valleys, under subtropical climate.

The authors have accomplished some studies at Bom Sucesso de Itararé (SP), concerning a small sedimentary basin sunk into Proterozoic metasedimentary rocks (Itaiacoca Group), which is far from the Escarpment zone about 7 km. They have discovered an alluvial fan sequence associated to a braided fluvial system that was deposited under semi-arid climate, probably during the Miocene, and which its main sedimentary source was the Furnas Formation (Souza, 1990). That basin was originated under weak Tertiary ruptile tectonic events. It may be correlated to the sedimentary basins of the Continental Rift from southeastern Brazil that was described in Riccomini (1989). All of those facts are important evidences to prove that the Furnas Structural Escarpment has already constituted an imposing geomorphological feature, at least since the Middle Tertiary.

Therefore, the Furnas Structural Escarpment constitutes a relict landform formed mainly during the Late Cretaceous to the Tertiary.

Stratigraphic, palaeoenvironmental and palaeotectonic characteristics of the Furnas Formation

Furnas Formation presents a flat-lying sandy sedimentary sequence of Eodevonian age (410 to 390 Ma.), whose the palaeogeographic reconstruction is possible thanks to the presence of the Escarpment zone.

The Furnas Formation is constituted by whitish sandstones of coarse to medium grain size, badly to moderately sorted, presenting lenses of medium and fine sandstones, gravels and conglomerates, and besides pelitic beds with white clay balls and small lenses. Quartz, feldspars and muscovite are the predominant minerals in those rocks. Sedimentary structures are abundant and they range of small to large size, occurring horizontal-parallel lamination, angular to tangential and trough to festoon cross bedding, current and wave ripple marks and cut-and-fill structures. Sandstones present groups of concordant and discordant joints, nets of orthogonal joints being predominant.

Controversies still exist about the depositional site environment of the Furnas Formation. Some authors suggested marine conditions (Almeida, 1954; Sanford & Lange, 1960; Bigarella et al., 1966; Bigarella & Salamuni, 1967; Lange & Petri, 1967), while others attributed it only to a fluvial environments (Ludwig & Ramos, 1965; Schneider et al., 1974; Zalán et al., 1987b; Pereira & Bergamaschi, 1989). On the other hand, the authors have studied a Furnas sequence outcropping at Itararé-Bom Sucesso de Itararé road, and they have concluded that those sediments had been deposited in a coastal fluvial system.

The Furnas Formation is discordantly overlying upon five different geologic units (Figure 2): metasedimentary rocks of the Açungui and Itaiacoca Groups (Proterozoic), granitoid rocks of the Cunhaporanga and Três Córregos Complexes of Brasiliano age (Late Precambrian), diamictitic rocks of the Iapó Formation (Ordovician to Silurian), volcanic-sedimentary association rocks of the Castro Group (Ordovician), and molassic rocks of the Camarinha Formation (Eopaleozoic).

Recent regional studies (Milani, 1997) suggest that the discordant surface (erosional discordance) over which the Furnas Formation is lain, it was originated after a great marine regressive episode happened in the Late Ordovician to the Silurian. So, the Furnas sandy sequence was deposited in a wide golf, which was opened to southwestward and connected with the Panthalassa Ocean in the southern margin of the Gondwanaland.

Speleologic features and archaeological vestiges

The grottos and small caves formed into Furnas sandstones are presented along the whole Escarpment zone. The Barreira Grotto is one of the most beautiful and interesting examples of sandstone cave, situated at Itararé close to the limit between the states of São Paulo and Paraná.

The most important archaeological and historical vestiges have already been found in the Site are shelters located into caves and grottos and around the base of the escarpment, where have been discovered lithic and ceramic artefacts, rupestrian paintings and human bones remains (Araripe, 1887, apud Araújo, 1995; Aytai, 1970; Blasi, 1972; Araújo, 1995).

PROTECTION MEASURES

In the State of Paraná, the Furnas Structural Escarpment zone comprises an Environmental Protection Area (APA) denominated "Devonian Scarp APA". It encloses an area of about 395,000 ha, including several parks, among them Vila Velha (Pontes F^o. et al., 1993) and Guartelá (Melo et al., in this book).

In the State of São Paulo, the Furnas Structural Escarpment does not constitute an area of environmental preservation or conservation yet. However, there are six units of environmental conservation in its neighbourhood: Itapeva Ecological Station, Itaberá Ecological Station, Alto Ribeira-PETAR State Park, Fazenda Intervales State Park, Xitué Ecological Station and Capão Bonito National Forest (SMA, 1998).

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