Serra da Cangalha

Serra da Cangalha is a substantial impact crater situated in the state of Tocantins, Brazil. With an outer perimeter measuring between 12 and 13 kilometers in diameter, it holds the distinction of being the second-largest known impact crater within Brazil. The geological event that created Serra da Cangalha is estimated to have occurred approximately 220 million years ago, during the Triassic period. The name "Serra da Cangalha" itself translates from Portuguese to "Pack-Saddle Mountains," a descriptor likely inspired by the crater's topographic features.

Description and Features

The crater's outer boundary is defined by a circular inward scarp, approximately 12.5 kilometers in diameter. This scarp is set within the largely undisturbed Cretaceous and upper Silurian sediments of the Parnaíba basin. Drainage valleys have breached the scarp on the west, north, and south sides. Internally, the structure exhibits a series of concentric circular valleys and a central basin, all existing at roughly the same elevation and separated by distinct ring walls. Advanced imagery, such as from the Shuttle Radar Topography Mission, reveals further concentric features: a faint outer ring around 11 kilometers in diameter, a second ring of gentle hills about 5.5 kilometers in diameter, and an inner ring of steeper hills, approximately 3 kilometers in diameter and rising up to 420 meters high. This inner ring surrounds a central basin measuring about 2.2 kilometers in diameter, which is open to the northwest.

Evidence of Impact

The origin of Serra da Cangalha as an impact structure is strongly supported by several key geological indicators. The site contains impact breccias, which are rocks composed of angular fragments cemented together by a matrix of finer material formed during an impact. Additionally, quartzite shatter cones have been identified, a distinctive conical fracture pattern found only in rocks subjected to extreme shock pressures from impacts or explosions. The presence of shocked quartz, a form of quartz containing microscopic planar deformation features indicative of high-pressure shock, further corroborates its impact origin. Studies suggest the bolide struck the surface at a low oblique angle, estimated between 25 and 30 degrees, on dry land. Radial faults are also observed within the crater, with some extending up to 16 kilometers from the center. Disturbed and steeply inclined sediments from the Carboniferous and Devonian periods are found within the crater. Geophysical surveys, including magnetic surveys, indicate that the deformation caused by the impact extends to a depth of approximately 2 kilometers beneath the surface.

Historical Studies

The scientific identification of Serra da Cangalha as an impact crater was first formally published in 1973 by researchers Robert Dietz and B.M. French. Subsequent studies have built upon this foundational work. Shatter cones were specifically reported by Beatty in 1980, adding further weight to the impact hypothesis. In 1986, McHone's thesis documented the presence of impact breccias, impact melting, and shocked quartz, providing detailed petrographic evidence. More recently, a comprehensive magnetic survey of the structure was published by A.A. Adepelumi and colleagues in 2005, contributing to a deeper understanding of the subsurface deformation and extent of the impact event.