ABSTRACT

The Aroe Jari Cave, located in the Chapada dos Guimarães Plateau, Northern Mato Grosso State, is a rare and good example of cave in siliciclastic sedimentary rocks. It occurs in the Ordovicio-Silurian Alto Garças and Vila Maria formations (Rio Ivaí Group) of the Paraná Basin, at its Northwestern Border. Other caves than the Aroe Jari are also registred nearby (Lagoa Azul and Kiogo Brado). Their genesis is attributed to the resistence of the Vila Maria Formation diamictites to surface erosion, which sustain the local relief and characterize the ceiling of the cave; and to the erosion by piping of the lower Vila Maria and upper Alto Garças formations sandstones and conglomerates, which characterizes the walls and floor of the caves. Piping is consequence of a high friability of the rocks due to regional fracturing (N70^oE), and to lithostatic unload of these lowermost lithostratigraphic units of the Paraná Basin.

RESUMO

A caverna Aroe Jari, localizada na Chapada dos Guimarães, ao Norte do Estado de Mato Grosso, é um exemplo bom e raro de caverna formada em rochas sedimentares siliciclásticas. A caverna ocorre nas formações Alto Garças e Vila Maria (Grupo Rio Ivaí), de idade ordovício-siluriana, na bacia do Paraná, em sua borda Noroeste. Outras cavernas além da Aroe Jari, ocorrem nas proximidades (Lagoa Azul e Kiogo Bardo). A gênese dessas cavernas é atribuída à resistência dos diamictitos da Formação Vila Maria à erosão superficial, os quais sustentam o relevo local e caracterizam o teto da caverna; e à erosão por piping dos arenitos e conglomerados das formações Vila Maria inferior e ALto Garças superior, os quais caracterizam as paredes e piso das cavernas. O mecanismo de piping é conseqüência da alta friabilidade das rochas dessas formações devido a um fraturamento regional (N70^oE), e decompactação por carga dessas unidades mais basais da bacia do Paraná.

INTRODUCTION

The Aroe Jari Cave site belongs to the physiographic domain of the Chapada dos Guimarães Plateau, and to the geological domain of the Paraná Sedimentary Basin. This region possesses geological significance for containing expressive outcroppings of the Alto Garças and Vila Maria formations. The authors propose to these formations, respectively, a lectostratotype and parastratotype in the region. Also, the region possess a geomorphological significance due to the fact of the presence of caves in sandstones, a process not usually observed in the surface modelling; a paleontological significance becouse trace-fossil abundance; and speleological significance as the cave occur in siliciclastic sedimentary rocks, an uncommon fact.

Near this cave, others occur such as the Lagoa Azul, with clear waters ("Blue Lagoon"), and the Kiogo Brado, belonging to a system of caves, of which the Aroe Jari is the most renowned. Those other caves are also treated in the site context.

The Aroe Jari and Kiogo Brado caves are registered by the SBE (Brazilian Speleological Society), respectively under the numbers SBE MT-038 and SBE MT-037.

LOCATION

The Aroe Jari cave is located in the Chapada dos Guimarães County (Figure 1), Northern Mato Grosso State, among the Aricá-Açu, Caveira and Monjolinho creek headwaters (Lat. 15°36'26 " S, Long. 55°29'30 " WGr).

To reach the cave, from the Chapada dos Guimarães City, it takes the road MT-251 (not paved), in a course of 28 km, as far as the detour to the road MT-301; in this road it takes 4 km to reach the Nossa Senhora Medianeira Farm entrance; starting from this entrance, it takes about 14 km to the South, crossing the Caveira and Monjolinho creek headwaters, successively. The last three kilometers are done on foot by trails. The Aroe Jari cave possesses two accesses, the first one is reached by following the Monjolinho Creek upstream, and, the second, by trail Southwesternward of the first entrance. The other caves are also reached Southwesternward by trails (successively Lagoa Azul and Kiogo Brado caves).

HISTORICAL

In terms of human occupation, the Aroe Jari cave (Gruta das Almas or French's Cave) may have been visited several times by indians, since prehistoric times. However, the only physical recorded of that occupation are the recent cemeteries of Bororo and Caiapó tribes, that inhabited the Chapada dos Guimarães region at the same time of the first European settlers arrival. Several popular reports make us believe that the Aroe Jari and other caves were known by the end of the last century by the "tropeiros" and local inhabitants. Nevertheless, the early scientific reports can be attributed to the speleologist Ramis Bucair, which could have visited it in the beginning of the 70s, according to the report of Maria Lúcia Pardi on the archaeological sites of the Chapada dos Guimarães. During the mid 80s, Tércio Soares Barreto reports the cave mapping and its detailed topography accomplished by the SBE. (Mattos 1999)

On a geological point of view, despite of the great amount of general knowledge, the Paraná Basin is poorly known in its Northwestern Border, particularly in the Chapada dos Guimarães region. In that region, according to the common geological literature, outcrops the Furnas, Ponta Grossa, Botucatu and Bauru formations, and a "Cobertura Detrito-Laterítica" (detritic lateritic regolith). However, Borghi & Moreira (1996), during field works related to project "Stratigraphic Revision of the Basal Sequence of the Paraná Basin" of the Federal University of Rio de Janeiro, accomplished in the Chapada dos Guimarães, Campo Verde and Santo Antônio do Leverger counties, Eastern part of the Chapada dos Guimarães region, revealed the occurence of an unknown stratigrapphic interval represented by the Alto Garças and Vila Maria formations, which belong to the Rio Ivaí Group, resting in nonconformity on the Cuiabá Group (basement) and in unconformity below the Furnas and Ponta Grossa formations, which belong to the Paraná Group.

That lower Palaeozoic stratigraphic interval, which the caves here described carve, was erroneously attributed to the Furnas Formation since the Oliveira & Mühlmann's (1965, 1967) mapping, but was mapped as Rio Ivaí Group in the scale of 1:500.000 by Borghi & Moreira (1998a) (Figure 1). Such fact is due to the lithologic similarity among those lithostratigraphic units, where even the Vila Maria Formation's diamictites can be described as sandstones with granules and pebbles, and mistaken as sandstones of the Furnas Formation, from which it may not be clearly differentiated.

GEOLOGY OF THE REGION

In the caves' site, Borghi & Moreira (1998a) mapped the Rio Ivaí Group. This group was proposed by Assine et al. (1994) to identify the succession of sandstones of the Alto Garças Formation, diamictites of the Iapó Formation, and shales and sandstones of the Vila Maria Formation, which rests in nonconformity on the basin's basement (Precambrian), and stratigraphically below the Furnas Formation (Devonian, base of the Paraná Group), in unconformity (Figura 2). The Rio Ivaí Group spans from 460 until 420 million of years, involving the end of the Ordovician Period and beginning of the Silurian Period (Early Paleozoic).

ALTO GARÇAS FORMATION

The Alto Garças Formation, formalized by Assine et al. (1994), was used to designate the main sandstone interval which rests stratigraphically on the Paraná Basin's basement, and below the Iapó Formation, in unconformity, in the wells 2-AG-1-MT (Alto Garças) and 2-RI-1-PR (Rio Ivaí), both of PETROBRÁS.

In the caves' region, the Alto Garças Formation form an abrupt scarp (first scarp), above the "Baixada Cuiabana" (Cuiabana lowland), with NW-SE direction, which altimmetry is around 600 m, extending from the Formosa River headwaters, in the district of Chapada dos Guimarães County, as far as the Aricá-Mirim Creek headwaters, in the Campo Verde County (Figure 1). Its best exposures are in the scarps next to the caves (Figura 1, point 2), in the Chapada dos Guimarães County and in the Torrinhas locality (Figura 1, point 5), in the Campo Verde County, where it reaches about 40 m thick. Further, the Alto Garças Formation occur in the Ranchão Hill (Figure 1, point 4), a monadnock at the Southwest of the region of the caves, in the Santo Antônio do Leverger County.

In this region, the Alto Garças Formation rests in nonconformity on the basement, represented by the phyllites of the Cuiabá Group or in unconformity on the "Campo Verde sandstone" (sensu Borghi & Moreira 1998b); and unconformity below the basal conglomerate of the Vila Maria Formation (sensu Faria 1982), associated to a sequence limit of type 1, according to Borghi & Moreira (1997). In faciologic terms, the Alto Garças Formation is represented, in its base, by massive conglomerate facies, and, in its top, by massive sandstone, plan-parallel laminated sandstone in tabular bed, and sandstone with Skolithos linearis (icnofácies Skolithos), all interpreted as the record of a sandy shallow marine system (Moreira & Borghi, 1999a)

In paleontological terms, Borghi et al. (1997) identified the icnofóssil Skolithos linearis Haldeman, 1840, as typical of the Alto Garças Formation in the Chapada dos Guimarães region. This trace-fossil occurs in a dense assemblage usually designated "piperock ", which exemplifies a monoichnospecific ichnofabric, common of the icnofácies Skolithos in Palaeozoic strata (Moreira et al. 1997).

VILA MARIA FORMATION

The Vila Maria Formation was formalized by Faria (1982) to designate a succession of diamictite, shale and sandstones that outcrop at the homonymous city, in the Goiás State, Northeastern Border of the basin (cf. Moreira & Borghi 1999a). In respect to the age, according to Gray et al. (1985) and Grahn (1992), the Vila Maria Formation was dated as Llandovery (early Silurian) based on palynomorphs.

Assine & Soares (1989) and Assine et al. (1994) redefined the Vila Maria Formation proposed by Faria (1982) in its stratigraphic amplitude, excluding the diamictites from its base. However, that amendment proposal to the diagnosis of the Vila Maria Formation was not followed in this work.

In the caves' region (Figure 1), the Vila Maria Formation extends throughout the plateau between the first scarp (Alto Garças Formation) and a second one (Furnas Formation). The Vila Maria Formation is about 20 m thick, as can be observed in the scarps near the caves (Figura 1, point 2) and in the area of the Serrinha Farm (Figura 1, point 3). In the Ranchão Hill, the Vila Maria Formation presents a maximum thickness of about 70 m, standing for the most complete succession of it (Figura 1, point 4).

The Vila Maria Formation, according to Moreira & Borghi (1999a), is faciologically complex in the region, being constituted, from the base to the top, as a succession of: (i) cross-bedded conglomerate of a fluvial system; (ii) conglomerate and sandstone interbedded in tabular layers and sandstones with Arthrophycus alleghaniensis (icnofacies Cruziana) of a rudaceous shallow marine system; (iii) shales with Chondrites isp. and Teichichnus isp. (icnofacies Cruziana), climbing-rippled sandstones and diamictites of a glaciomarine system; and (iv), cross-bedded or wavy cross-laminated sandstones in tabular layers, and flaser, wavy and linsen bedded shales and sandstones, sandstones with Arthrophycus alleghaniensis and Palaeophycus isp. (icnofacies Cruziana), and sandstones with Skolithos linearis (icnofácies Skolithos) of a sandy shallow marine system.

It rests unconformably on the Alto Garças Formation (sequences limit of type 1) and below the conglomerate of the Furnas Formation in an apparent angular unconformity (cf. Borghi & Moreira 1996a, 1998a).

In paleontological terms, Moreira et al. (1999) identified the following ichnogenus: Arenicolites Salter, 1857, Arthrophycus Hall, 1852, Aulichnites Fenton & Fenton, 1937, Chondrites Von Sternberg, 1833, Diplocraterion Torell, 1870, Lockeia James, 1879, Palaeophycus Hall, 1847, Skolithos Haldeman, 1840 and Teichichnus Seilacher, 1955; Machado et al. (1997) identified the of ctenodontidae bivalve genus Tancrediopsis Beushausen, 1895 in Serrinha Farm locality and in the Ranchão Hill; and Mussa et al. (1997) described megafossils of primitive vascular plants in the Serrinha Farm locality, related to the species Cooksonia hemisphaerica Edwards, 1979 and Cooksonia caledonica Edwards, 1970.

DESCRIPTION OF SITE

The several caves are very close, occuring in a N70ºE lineament, constituting a system of caves, among which the Aroe Jari cave (Figure 3) is the first found from NE; it follows the Lagoa Azul (Figura 4) and Kiogo Brado (Figura 5) caves, which don't have a physical connection. All of have their larger extension oriented in the same direction of its lineament. In particular, the Lagoa Azul Cave shows prominent fractures in N80^oW direction, which can constitute the conjugated pair with the fractures that create the caves, however, without conditioning the formation of any known cave.

In lithological terms, the diamictites of the Vila Maria Formation, that occur in the top of the stratigraphic succession of the caves, sustains the local relief and make the ceiling of the caves; while the conglomerate and sandstones of the Vila Maria Formation and the sandstones of the Alto Garças Formation, that make the walls and floor of the caves, occur in succession below the diamictites, and are extremely friable. This friability induced the caves' generation by a supposed piping process.

In the diamictites the sand fraction, considered as matrix, prevails; it ranges from very fine to coarse, and is generally poorly sorted, some times bimodal, quartzose in composition; frequent milky quartz pebbles, angular to subrounded, and rare euhedric pebbles of hyaline quartz and phyllite and quartzite pebbles and boulders. Despite of the absence of fines (silt and clay) in the diamictite's matrix, its poor sorting reduces their permo-porous characteristics in comparison to the other rocks of the region, what gives them an "impermeable" nature, becoming less susceptible to the erosion. In paleoenvironmental terms, these diamictites are the evidence of the Ordovicio–Silurian glaciation registered in the Gondwana (Caputo & Crowell 1985).

The conglomerates of the Vila Maria Formation are monomitic, fine to medium, with subangular to subrounded megaclasts, quartzose (milky and hyaline quartz), whose matrix is constituted by a quartz-arenites, fine to coarse, poorly sorted, generally angular to subarrounded. In paleoenvironmental terms, they were deposited in the upper soreface, reworked by storm waves.

The Vila Maria and Alto Garças formations sandstones are quartzose and cemented by quartz, and shows intense fracturing and secondary porosity resultant from the decompacting of the strata by erosion. This situation increases the permo-porous characteristics of the sandstones what makes easier its erosion by piping. In paleoenvironmental terms, those sandstones were deposited in the shoreface reworked by waves with variable energy, from storms to fair-weather.

In tectonic terms, it is notable the regional presence of glaciogenic deformations observed in the diamictites beds or, as in the Aroe Jari cave, in the contact observed between the diamictites with conglomerates and sandstones, below, inside the lower Vila Maria Formation. In this interval the main structures found are sheath folds, boundins, grooves, detachement faults and internal shearing surfaces (Moreira & Borghi 1998). Such structures were interpreted as gravitational slump, in compressional zone. The vergence of the folds and the sense of the sedimentary structures stretching show the slump movement toward SSE.

In stratigraphic terms, the caves obey an interval that covers the basal part of the Vila Maria Formation (conglomerate, sandstones and diamictites) and the top of the Alto Garças Formation (Figure 6). That stratigraphic covering is smaller in the Northeast entrance of the Aroe Jari Cave and it increases toward SW, successively, in the Southwest entrance of the Aroe Jari Cave, in the Lagoa Azul Cave and in the Kiogo Brado Cave, as they approach the scarps of the basin's border (Figure 7). This fact seems to be related, in geomorphologic terms, to the difference of base level of the local drainage (basin of drainage of the river Aricá-Açu). The unconformity among the Alto Garças and Vila Maria formations can be well observed in the ceiling of the Lagoa Azul and Kiogo Brado caves.

In paleontological terms, the trace-fossils Skolithos can be easily identified in the Alto Garças Formation sandstones, particularly in the Lagoa Azul and Kiogo Brado caves; whereas Arthrophycus trace-fossil can be seen in the sandstones interbedded with conglomerates of the Vila Maria Formation, particularly at the ceiling of the Lagoa Azul Cave. It is notable the abundance in which Skolithos occur, described as "piperock" (Moreira et al. 1997).

Finally, Borghi & Moreira (1996b) and Moreira & Borghi (1999b) suggested the site of the Aroe Jari Cave (Figure 1, point 2) as a reference section (lectostratotype) of the Alto Garças Formation, due to its easy access, excellent exposure and mapeability. The Alto Garças Formation presents very few outcrops described in the literature, yet debatable. Its best exposures are in the Lagoa Azul and Kiogo Brado caves.

In respect to the Vila Maria Formation, its lower interval is quite complete and includes, distinctly of the type-area, conglomerates and sandstones in its lowermost part, what allowed Moreira & Borghi (1999b) to propose a complementary stratotype in the area (parastratotype). Its best exposures are in the Aroe Jari Cave.

MEASURES OF PROTECTION

In the mid 80s, the Aroe Jari Cave site and surroundings areas didn't show appreciable effects of anthropic erosion. By the beggining of 90s, the tourists' influx, many of which using motorcycles and jeeps, brought serious local impact as result of the constant crossing of the trails that lead to the caves (cf. Mattos 1999). This fact was associated to the popularization of the caves in the media and to the valorization of the ecoturism, without an appropriate environmental monitoring. The environmental impact today is exposed as deep "voçorocas" that cut the landscape besides the roads.

In 1989, the Aroe Jari Cave was included inside the "Plano de Reordenamento de Ocupação de Chapada", created by FEMA (State Foundation for Environment) with funds of the National Program for Environment (Mattos 1999). The region was considered an area of environmental preservation by the Decree Nr. 99556/90, implemented by the Regulation Nr. 09/97/IBAMA/MT. The first protectionist action was observed already in 1997 by the authors, when a hut of the IBAMA/MT was installed in the site, and the access for the public, controlled. It is worthy to mention, that at that time, the presence of a non-governmental organization named IPECA (Curupira Arara Research Institute), which pawns in the defense of the site.

The visitors influx control and the descontinous usage of alternative trails to access the caves may be a profilatic rule to avoid deep erosion ("voçorocas").

AKNOWLEDGEMENT

Acknowledgements are due to the FAPERJ (Proc E-26/170.293/99), to the 12^th District of DNPM, to MME/MT and to the Sub-Reitoria de Ensino para Graduados e Pesquisa of UFRJ, which along almost one decade supported the studies in the region.

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