ABSTRACT

Domo de Araguainha (Araguainha Dome) is the largest known astrobleme in South America. It is a 40-km wide eroded complex crater, formed near to the Permian-Triassic boundary (245 Ma.), when a celestial body struck what was probably a shallow sea. The rocks affected by the impact include the Ordovician crystalline basement (granite) exposed at the center of the 6,5-km wide central uplifted core of the astrobleme and sedimentary units from the Paleozoic Paraná Basin (Furnas, Ponta Grossa, Aquidauana and Estrada Nova formations), disposed in rings around the central uplift. Impact metamorphic features include shatter cones in sandstones from the Furnas Formation, several types of impact breccias, planar features in quartz, mica and feldspar from granites and from breccias, severe deformation and hematite impact bombs.

INTRODUCTION

    Collision of celestial bodies is currently considered a common geologic phenomenon throughout the solar system and recognized as a dominant process in the early geological history of some planetary bodies, including the Earth (Grieve, 1987). The most recent impact cratering record of the Earth (Grieve et al., 1995) lists about 150 impact craters (or astroblemes) and crater fields known on Earth, to which about 3 to 5 new ones are discovered each year. This author emphasizes that this is just a small and biased sample of a probably much larger number of craters. The majority of the known structures are less than 200 Ma, usually located in cratonic areas such as North America and northern Europe-western Russia, areas that have been specifically searched for impact structures.
    In planetary bodies that lack or have very thin atmospheres and on those that have retained portions of their earliest crusts, such as the Moon, impact craters dominate the surface’s morphology, being responsible for the major topographic and stratigraphic features. Unlike these planetary bodies, the Earth’s surface shows comparatively few impact craters, due to highly active geologic environment of the Earth, which has obliterated, masked or modified the terrestrial cratering record through geologic time.
    In South America, there are only eleven known impact craters. Eight of them are located in Brazil: Domo de Araguainha, Serra da Cangalha, Riachão, Vargeão, São Miguel do Tapuio, Colônia, Cerro Jarau and Piratininga (Crósta, 1987; Hachiro et al., 1996). Two other craters are located in Argentina (Campo del Cielo and Rio Cuarto) and one in Chile (Monturaqui). Of all the impact craters in South America, Domo de Araguainha (Araguainha Dome) is the largest, possibly the oldest and the best known.
    Araguainha Dome covers an area of approximately 1,300 km², with a diameter of 40 km. It was formed over sedimentary rocks of the Phanerozoic Paraná Basin, an intracratonic basin that covers the southern portion of Brazil, stretching to Argentina, Paraguay and Uruguay. The first reference to the existence of a dome-type structure at Araguainha was made by Northfleet et al. (1969). These authors interpreted the structure as the result of a Cretaceous syenite intrusion that uplifted and deformed the Phanerozoic sediments in the shape of a dome.
    Due to this proposition, and considering the economic potential of the alkaline magmatism of the Cretaceous Iporá Formation elsewhere in Goiás State, Silveira Fº & Ribeiro (1971) carried out a reconnaissance geological survey. These authors described Araguainha Dome as a crypto-volcanic structure, with the central portion being formed by a granitic block, belonging to the crystalline basement of the Paraná Basin. According to these authors, the central portion of the dome was surrounded by volcanic rocks (lavas, breccia and tuffs) of trachytic composition and by tectonically deformed Paleozoic sediments
    Dietz & French (1973) and Dietz et al. (1973) proposed a new hypothesis for the formation of Araguainha, relating it to the impact of a large cosmic body against the Earth's surface. They recognized features due to shock metamorphism in samples from the center of Araguainha Dome, such as shatter cones and planar features (shock lamellae) in quartz, with orientations corresponding to {1013}, {1012}, {1122} and {1121} planes, together with the impact breccia around the core of the structure.
    Crósta et al. (1981) and Crósta (1982) reported on the results of geological mapping, geomorphologic characteristics and petrographic analysis of samples from the granitic basement, the impact breccias and from the Furnas Formation, all bearing evidence of shock metamorphism. Their results reinforced the origin of the structure by impact. Among the evidences recognized by these authors were shatter cones in sandstones of the Furnas Formation at two localities, shock lamellae in quartz, mica and feldspar, changes from crystalline to amorphous state in minerals without melting, with the vitreous phase retaining the morphologic and textural characteristics of the crystalline phase, and kink bands in minerals. Crósta (1982) also presented the result of the first isotopic age dating for the shocked granitic basement exposed at the central uplift of Araguainha, using the K-Ar technique. This author obtained an age of 283.6± 17.2 Ma for K-feldspar, pointing out that this age should be interpreted as a mixture of the age of formation of the granite (Pre-Devonian), and not the actual age of the impact, since the shock stage of the granite sample analyzed wasn’t advanced enough for a total degassing of pre-existing Ar. The mafic portion of the same granite sample gave an age of 362.6± 13.2 Ma, also an anomalous young age for the granitic basement of the Paraná Basin.
    Theilen-Willige (1981) pointed out that geomorphologic, petrographic and geophysical evidence found in the Araguainha Dome supported the origin of the structure by meteoritic impact. Among the recognized features there were the circular shape of the structure, the impact-related morphology, including a central uplift, ring walls and a basin rim of escarpments, evidences of shock metamorphism, the presence of suevite and impact breccias, shatter cones and negative magnetic anomalies at the center of the structure.
    Deutsch et al. (1992), based on isotopic dating of two samples from Araguainha, estimated an upper limit for the age of the impact as 243± 19 Ma, based on a Rb-Sr model age for alteration products of cordierite from a sample of the impact melt (fine-grained crystalline matrix with quartz, albite, minor K-feldspar, biotite, titano-magnetite, ilmenite, hematite and barite). For a sample of a partially shocked granite from the central uplift, these authors obtained a Rb-Sr isochron with an age of 449± 9 Ma. Their results placed the impact event near the Permian-Triassic boundary.
    Engelhardt et al. (1992) presented a detailed petrographic and geochemical study of the interior part of the 6.5 to 8 km-wide central uplift of the Araguainha Dome, focusing on the shocked granite and the breccias. They presented a geologic map of the inner part of the central uplift, showing the distribution of three types of breccias: impact breccia with melt matrix, polymict impact breccia and monomict impact breccia from sandstones. They also found 10-100 cm thick and tens of meters long red dykes cutting the granite, formed bycataclastic mixtures of unshocked, shocked and shock-melted granitic materials, which were intruded into the granite under conditions of extensional stress, probably during the uplift of the granitic bottom of the transient crater during the modification phase of the impact. These authors also determined the age of the impact event by ⁴⁰Ar/³⁹Ar dating of impact melt samples from the lower unit of polymict breccias, which showed ages of 247± 5.5 and 245.8± 5.5 Ma for two different grain size fractions of the same sample. These ages confirm that the impact event occurred close to the Permian-Triassic boundary.
    Hammerschmidt & Engelhardt (1995) presented the results of a second ⁴⁰Ar/³⁹Ar dating of impact and also of the granitic basement. Two size fractions of the impact melt rocks provided plateau ages of 245.5± 3.5 Ma and 243.3± 3.0 Ma, respectively, confirming that the Araguainha impact event occurred close to the Permian-Triassic boundary. Total degassing ages of biotites of the granite from the central uplift produced values ranging from 326 to 481 Ma, with this variation being explained by the authors as due to Ar losses. They interpreted the oldest age obtained for the biotite as the evidence that the ascending granite passed the 300^o C isotherm earlier than 481 Ma. (Pre-Devonian).
    Masero et al. (1994) and Fischer & Masero (1994) conducted a magnetotelluric survey across Araguainha astrobleme, in order to determine the depth of the crystalline basement under the structure and also to estimate the uplift of its central region. They found that the basement forms a well defined symmetric ring at a depth of 1,000 meters between the radii of 9 and 20 km from the center, dropping further away outside 20 km. Inside the ring and toward the center, the top of the granite rises, outcropping at 1.5 to 2 km from the center. Masero et al. (1997), through 2-D and 3-D modelling of magnetotelluric data, found a disk-shaped body within the granite at depths of 3 to 7 km, characterized by resistivity values below the bulk value of the upper crust. These authors relate the anomaly to impact-induced faulting and brecciation of the crust.
    Hippert & Lana (1998) studied impact bombs that occur in association with the polymict breccias in the central uplift of Araguainha Dome. The bombs contain shocked quartz particles captured during cooling and show enrichment in platinum-group elements (Rh-4x, Pd-5x, Pt-30x) relative to the target rocks.

LOCATION

    The center of the Araguainha Dome is located at the coordinates 16º47´S and 52º59´W, in between the villages of Araguainha and Ponte Branca, in Mato Grosso State (Figure 1). The Araguaia River cuts across the middle of the structure, which is located partly in Mato Grosso State and partly in Goiás State.
    Araguainha Dome can be easily reached by car, from Goiânia or from Cuiabá, by the BR-364 interstate. The village of Ponte Branca, located on the NE border of the structure, is 100 km from the interstate, accessible by an unpaved road which goes to Barra do Garças (MT). Another unpaved road (MT-306) links Ponte Branca to Araguainha, cutting across the central uplift which lies in between these two villages, closer to Araguainha (Figure 4). Some of the best outcrops of the geological units of the central uplift (shocked sandstones and granite, breccias and shatter cones) can be found along road MT-306, or not far from it.

Figura 1 – Localização do Domo de Araguainha

Morphology

    Araguainha Dome forms a remarkable ring-shaped structure, consisting of an uplifted core (the central uplift), annular ridges, ring depressions and valleys, arcs of isolated hills and terraced walls. Figure 2 shows a Landsat MSS band 7 image, depicting the annular aspect and general overview of the astrobleme.

Figure 2. Araguainha Dome (States of Mato Grosso and Goiás) in a colour composition (channels 4, 5 e 3 - infrared and false colour) of Landsat satelite (Thematic Mapper) . The image covers an area of 40 x 40 km.

A small elliptical basin forms the core of the central uplift, corresponding to the area of the granitic basement. An inner ring of hills, formed mostly by shocked granite and overlying breccias (Figure 5), surrounds the basin, drained by the Seco Creek. This ring is in turn surrounded by another ring of ridges and mountains, 6.5-8 kilometers in diameter, formed by folded and steeply inclined Devonian sandstones of the Furnas Formation (Figure 3). In the northern part of the central uplift, blocks of the Furnas sandstone, locally shock-metamorphosed into quartzites, can reach up to 150 meters in height.

Figure 3. External view of the central uplift showing its morphology. Folded and steeply inclined sandstones of the Devonian Furnas Formation form the hills, reaching up to 150 meters in height.

    The central elevation is surrounded by a ring depression, with a slightly undulated floor and a few isolated hills, corresponding to the areas of exposure of the Devonian Ponta Grossa and the Carboniferous Aquidauana formations. According to the description given by Theillen-Willige (1981), the most conspicuous aspect of the Araguainha structure is the existence of concentric multiple rings of ridges, hills, depression belts and valleys. The annular ridges have been dissected by drainage systems. Crósta (1982) showed that these morphologic features are controlled by annular and radial fault systems, typical of impact craters. This ring depression is cut by the Araguaia River.
    The external border of Araguainha Dome is formed by rim crests, representing the remnants of semi-circular grabens formed by annular collapse faults dipping towards the center of the structure. These grabens contain highly deformed Permo-Carboniferous sediments of the Passa Dois Group/Estrada Nova Formation. Generally speaking, Araguainha Dome is a deeply eroded complex impact crater, or an astrobleme. The depth of the excavation was estimated by Engelhardt et al. (1992) as 2,400 meters, which is one-tenth of the diameter of the transient crater (24 kilometers), formed just after the impact. This represents 0.6 of the diameter of the final crater (40 km).

Geology

    The geological map of Figure 4 shows the distribution of the main geological units that occur in the area of the Araguainha Dome. The geology of the central uplift is presented in more detail in Figure 5. The description presented next is mostly based on the work by Crósta et al. (1981) and Engelhardt et al. (1992), and will concentrate on the shock metamorphic features. These features occur at the central uplift, involving lithologies from the granitic basement and the Furnas formation, besides the breccias formed by the impact.

GRANITIC BASEMENT

    The outcrops in the inner portion of the uplift are composed of alkali-feldspar granite. Although some of the original characteristics of the rock may still be seen, all the outcrops exhibit impact-induced features at some stage. The texture of the granite varies from hypidiomorphic to porphyritic, with predominance of porphyritic granites with K-feldspar phenocrysts up to 5 cm in size. The main constituents are quartz, K-feldspar, albite, biotite and muscovite, with the following accessories: zircon, tourmaline and calcite.

Figure 4 - Geologic   Map of Domo de Araguainha - Brazil 

Figure 5. Geologic map of the inner portion of the central uplift (after Engelhardt et al., 1992)

These rocks, together with the breccias, exhibit the majority of impact-induced deformation at Araguainha. The most common deformation is the development of planar deformation structures ("shock lamellae") in quartz (Figure 6a). Up to four different sets of planar features were observed by Engelhardt et al. (1992) in a single quartz grain from the granite. Crósta (1992) also observed shock lamellae in feldspar and muscovite (Figure 6b). The presence of {1012} planar feature in quartz indicates that the granite now exposed in the central uplift of Araguainha was shocked at peak pressures in between 20 and 25 GPa.

Figure 6. Planar deformation features (shock lamellae) in quartz (a) and muscovite (b), from the shocked granite. Two sets of deformation features can be seen in each mineral.

    Engelhardt et al. (1992) described the occurrence of 10-100 cm thick dikes of red color and breccia veins of gray color cutting the granite. The dykes are composed of angular mineral fragments showing a fluidal texture. Quartz fragments within the dykes show planar deformation features or are re-crystallized to a mosaic of interlocking crystals. Indications of incipient melting are seen on quartz and feldspar grains and glass is quite common in these dikes. The breccia veins reach up to few centimeters in thickness, composed of angular clasts of granites and granite minerals. Both, the dikes and veins are interpreted by the authors as mixtures of unshocked, shocked and shocked-melted granitic materials, which were intruded into the granite under conditions of extensional stress. Several deformation features were described in the granite by Crósta et al (1981) and Engelhardt et al. (1992), including plastic and brittle deformations of mineral grains (intragranular deformation) and movement of individual grains against each other (intergranular deformation). Features such as undulatory extinction in K-feldspar, kink bands in micas and plagioclase, disintegration of quartz grains along stress fractures, selective transformation of minerals (mainly feldspar) into an amorphous phase (glass) without melting, general oxidation of iron-bearing minerals (biotites), among others.

IMPACT  BRECCIAS

    Impact breccias occur in the central uplift of Araguainha, around and covering the granitic basement (Figure 4). At least three different types of breccias were described by Crósta (1982) and later mapped and characterized by Engelhardt et al. (1992): impact breccia with melt matrix (IBM), polymict breccia and monomict breccia from sandstones.
    IBM typically overlies the granite, appearing macroscopically as light to dark gray coloured rock with lighter inclusions of feldspar and quartz grains of irregular to rectangular shape, exhibiting fluidal textures (Figure 7a). Chemical analysis showed that IBM and granite are identical in main components and in trace elements.
    Polymict breccias mostly surround the granite in the north and northwest. These are well exposed in cuts along the road from Araguainha to Ponte Branca, forming as an unsorted and unstratified mass of constituents of various shapes, ranging from irregular masses and angular fragments (Figure 7b). These fragments are several meters to centimeters in size, together with bodies embedded in a fine-grained groundmass. The fluidal texture indicates turbulent movements and flow and the distorted/twisted shape of some components show that they were plastically deformed.
    Monomict impact breccias from sandstones were found in the hills bordering the central basin in the south and southeast. Their area of occurrence is poor in outcrops and covered by cerrado-type vegetation and the few samples described by Engelhardt et al. (1992) consist of thermally altered and fused quartz sandstones with angular grains.

Figure 7. Macroscopic aspects of impact breccias. (a) Impact breccia with melt matrix; (b) Polymict breccia with angular fragments of granite, sandstone and other components.

SHATTER CONES

    Shatter cones, as well as planar features (shock lamellae) in quartz, are among the most well established evidences of meteoritic impact events, as they were only recognized in association with impact craters worldwide. The occurrence of shatter cones at Araguainha Dome was first mentioned by Dietz et al. (1973) and further detailed by Crósta et al. (1981) and Theilen-Willige (1981).
    Shatter cones were found in sandstones of the Devonian Furnas Formation that surround the granitic core and the breccias, in the western and northwestern portions of the central uplift. They form aggregates of conic structures (Figure 8), each cone reaching up to 10 cm in size. The gray sandstones show shock-induced quartz and mica recrystallization and cataclasis. Shatter cones were also found by Engelhardt et al. (1992) in sandstone fragments within the impact breccias.

Figure 8. Shatter cones in sandstone of the Devonian Furnas Formation

    Hematite aggregates were described in association with impact breccias at Araguainha by Crósta et al. (1981). Optical and scanning electron microscope (SEM) analysis of these aggregates by Hippert and Lana (1998) showed that these aggregates are impact bombs. They show an asymmetrical external geometry, with convex and concave sides, which also corresponds to their internal structure, comprising porous hematite in the convex side and massive hematite in the concave side (Figure 9a). Both, porous and massive domains are composed of an aggregate of individual units with a conical geometry and showing a feather-like morphology when viewed across longitudinal sections (Figure 9b). According to the authors, these microstructures reflect a well-ventilated cooling process during their ballistic trajectory. Quartz fragments with planar deformation features occur in the porous domains, representing ejected solid particles captured by the bombs during cooling. Hippert and Lana (1998) also conducted a neutron activation analysis of the hematite bombs for platinum-group elements. They found enrichments such as Rh (4x); Ru, Ir and Pd (5x); and Pt (30x) relative to the normal level of target sediments, relating these enrichments to the presence of a projectile-derived component in the constitution of the hematite bombs.

Figure 9. Hematite bombs. (a) Transition between massive (ma) and porous (po) hematite associated with the front and the rear of the bombs, respectively. Feather-like structures (F) appears in the massive domain; (b) Transversal section, showing a detail of the feather-like structures. The arrow indicates the atmospheric ablation during the ballistic trajectory of the bomb; (c) morphology of the rear portion of the bomb (from Hippert and Lana, 1998)

    Araguainha Dome is a well-preserved example of a complex astrobleme, with plenty of classical macroscopic and microscopic evidences of shock metamorphism. As such, it is unique in Brazil and South America and extremely valuable as a permanent study site for impact events. Due to the age of the impact event, close to the Permian-Triassic boundary, a period in which one of the most important mass extinction events in the history of the Earth occurred, this site may be of key importance for the study of the Earth’s cratering record in relation to mass extinction events.
    As with most sites of geological interest, the population that lives nearby is not aware of the origin and history of the Araguainha Dome and the reason why it is of unique scientific and cultural interest. The first step towards conservation of this site is therefore to let the local population be aware of its value and how to better preserve it. This could be achieved through an education program focusing on school children and also adults living in the villages of Araguainha and Ponte Branca.
    The great majority of outcrops that show evidence of shock metamorphism are located along the road MT-306, or very close to it. These outcrops need to be preserved, since most of them show medium to severe weathering effects, making the rocks quite fragile. This is the case particularly of the shatter cones in Furnas sandstones and impact breccias. Maintenance works have already damaged several well-exposed examples of the impact breccias in road cuts, first seen by this author in 1979.
    A program directed towards the preservation of the outcrops in the region of the central uplift of Araguainha Dome, combined with awareness programs for the local population, are therefore strongly recommended as protection measures of this valuable site.

Crósta, A. P. 1982. Mapeamento geológico do Domo de Araguainha utilizando técnicas de sensoriamento remoto. Instituto Nacional de Pesquisas Espaciais (INPE), São José dos Campos. Dissertação de Mestrado 90 p.

Crósta, A. P. 1987. Impact structures in Brazil. In: J. Pohl (ed.) Research in Terrestrial Impact Structures. Wiesbaden, Vieweg & Sohn, 30-38.

Crósta, A. P., Gaspar; J. C.; Candia, M. A. F 1981. Feições de metamorfismo de impacto no Domo de Araguainha. Revista Brasileira de Geociências, 11: 139-146.

Deutsch, A.; Buhl, D.; Langenhorst, F. 1992. On the significance of crater ages: new ages for Dellen (Sweden) and Araguainha (Brazil). Tectonophysics, 216:205-218.

Dietz, R. S.; French, B. M. 1973. Two probable astroblemes in Brazil. Nature, 244:561-562.

Dietz, R. S.; French, B. M.; Oliveira, M. A. M. 1973. Araguainha Dome (Goiás) and Serra da Cangalha (Mato Grosso): probable astroblemes? In: SBG, Congresso Brasileiro de Geologia, 27, Resumo das Comunicações, 1:102.

Engelhardt, W. v.; Matthäi, S. K.; Walzebuck, J. 1992. Araguainha impact crater, Brazil. 1. The interior part of the uplift. Meteoritics, 27:442-457.

Fischer, G.; Masero, W. 1994. Rotational properties of the magnetotelluric impendance tensor – the example of the Araguainha impact crater, Brazil. Geophysical Journal International, 119:548-560.

Grieve, R. A. F. 1987 Terrestrial impact structures. Episodes, 10:86.

Grieve, R. A. F.; Rupert, J.; Smith, J.; Therriault, A. 1995. The record of terrestrial impact cratering. GSA Today, 5:189-196.

Hachiro, J.; Coutinho, J. M. V.; Coimbra, A. M.; Fernandes, L. A. 1996. Fossil tracks of a falling star in the Cretaceous of the Paraná Basin (Brazil). Anais da Academia Brasileira de Ciências, Resumo das Comunicações, 68:618-619.

Hammerschimidt, K.; Engelhardt, W. 1995. ⁴⁰Ar/³⁹Ar dating of the Araguainha impact structure, Mato Grosso, Brazil. Meteoritics, 30:227-233.

Hippert, J.; Lana, C. 1998. Aerial crystallization of hematite in impact bombs from the Araguainha astrobleme, Mato Grosso, central Brazil. Meteoritics & Planetary Science, 33:1303-1309.

Masero, W.; Schenegg, P. A.; Fontes, S. L. 1994. A magnetotelluric investigation of the Araguainha impact structure in Mato Grosso-Goiás, central Brazil. Geophysical Journal International, 116:366-376.

Masero, W.; Fischer, G.; Schnegg, P. A. 1997. Electrical condutivity and crustal deformation from magnetotelluric results in the region of the Araguainha impact, Brazil. Physics of the Earth and Planetary Interiors. 101:271-289.

Northfleet, A. A.; Medeiros, R.A.; Muhlmann, H. 1969. Reavaliação dos dados geológicos da Bacia do Paraná. Boletim Técnico da Petrobrás, 12:291-346.

Silveira Filho, N. C.; Ribeiro, C. L. 1971. Informações geológicas preliminares sobre a estrutura vulcânica de Araguainha, Mato Grosso. DNPM/Distrito Centro-Leste (relatório interno).

Theilen-Willige, B. 1981. The Araguainha impact structure, Central Brazil. Revista Brasileira de Geociências, 11:91-97.