Definitions

crypto volcanic

Araguainha crater

The Araguainha Crater or Araguainha Dome is an impact crater on the border of Mato Grosso and Goiás states, Brazil, between the villages of Araguainha and Ponte Branca. With a diameter of 40 km, it is the largest known impact crater in South America, and possibly the oldest one.

The crater was formed 244.40 ± 3.25 million years ago in the Triassic era, when the region was probably a shallow sea. The date is actually quite close to the Permian/Triassic boundary, one of the largest mass extinction events in Earth's history. The impact punched through Paleozoic sedimentary units belonging to the Paraná Basin formations, and exposed the underlying Ordovician granite basement rocks. It is estimated that the crater was initially 24 km wide and 2.4 km deep, which then widened to 40 km as its walls subsided inwards.

Description

Araguainha is a complex crater with annular and radial faults, exposed to the surface and eroded, crossed by the Araguaia River. The crater has an uplifted central core, shaped like an elliptical basin, consisting of exposed basement granite. Surrounding this core is a ring of shocked granite and overlying brecias; then another ring of ridges and mountains, 6.5 km in diameter and up to 150 m high, consisting of folded and steeply tilted Devonian sandtones. This central region is surrounded by an annular depression floored by rocks from Devonian and Carboniferous sandstone formations. The outer rim of the crater consists of remnants of semi-circular grabens in highly deformed Permo-Carboniferous sediments. Evidences of impact origin include shatter cones, impact breccias, and shocked quartz.

History and studies

The earliest report on the Araguainha structure was published in 1969 by Northfleet et al., who interpreted it as an uplift of the Phanerozoic sediments caused by a Cretaceous syenite intrusion. A reconnaissance geological survey by Silveira Filho and Ribeiro (1971) noted the occurrence of lavas, breccias and tuffs around the central core, and deduced that Araguainha was as a crypto-volcanic structure. In 1973 R.S. Dietz and B.M. French reported the occurrence of impact breccias and shocked quartz, and recognized the structure as an impact crater. A detailed study of the crater by A.P. Crósta in 1981--1982 reported further petrological and mineralogical evidence of the impact. Further geomorphologic evidence was published by Theilen-Willige in 1981. The impact was first dated (at 243 ± 19 million years ago, with Rb-Sr method) by Deutsch et al. in 1992. In 1992 Engelhardt et al. (1992) published a detailed study of the uplifted core and revised date of about 246 million years ago, later revised to about 244 million years ago. A magnetic survey was condurcted in 1994 by Fischer and Masero.

Access and conservation

The Araguainha Dome can be reached by car from Goiânia or from Cuiabá. The unpaved state road MT-306, between Ponte Branca and Araguainha, cuts across the central uplift. As of 1999, the local population was not aware of the Dome's nature and scientific importance.

References

External links

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