返回Google圖書搜尋

Terrestrial Impact Structures

其他書名	Their Detection and Verification with Two New Examples from Brazil
出版	University of Illinois at Urbana-Champaign, 1986
URL	http://books.google.com.hk/books?id=Z2ooAQAAMAAJ&hl=&source=gbs_api

註釋Two circular features detected on satellite images of Brazil's Maranhao sedimentary basin are verified as probable meteoroid impact structures. Impact recognition guidelines and criteria are presented in an extended synthesis section. Serra da Cangalha (12 km dia., 08°05'S; 046°52'W) is a multi-ring structure. A central bowl of steeply dipping Devonian shale 400 m above regional elevation is surrounded by a 3 km dia. mountain ring of contorted Mississippian sandstone 350 m in relief and uplifted as much as 500 m. An outer ring, about 4 km wide, of outward-dipping Pennsylvanian sandstone, shale, and argillite is eroded to a ring-trough 100 m below regional flat-lying Permian chert and silty sandstone which define an abrupt circular perimeter. Oriented optical planar elements in quartz grains and shatter cones confirm an impact origin. Riachao Ring (4 km dia., 07°43'S; 046°39'W) is a dome of convoluted Permian sandstone within a densely forested ring-trough of Permian chert, shale, and sandstone and an outer rim of weakly consolidated Triassic(?) sandstone 30 m above surrounding undisturbed Permian elastics. Quaquaversal slabs of impact ejecta breccia are embedded in rim sandstones. Some quartz grains from breccia clasts contain oriented optical planar elements diagnostic of meteoroid impact. Both sites contain shock metamorphism effects diagnostic of impact histories. Furthermore, non-impact origins are ruled out by their solitary occurrences, lack of associated volcanics, absence of thick diapir-forming evaporites, and innermost rock units which are contorted and thickened by centripetal movements.