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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 , it is the second largest known impact crater in South America, and possibly the oldest one.
The crater has most recently been dated to 254.7 ± 2.5 million years ago, when the region was probably a shallow sea. The margins of error of this date overlap the time of the Permian–Triassic extinction event, 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 wide and deep, which then widened to 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 breccias; then another ring of ridges and mountains, in diameter and up to high, consisting of folded and steeply tilted Devonian sandstones. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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