- (1968). Взрывные кратеры на Земле и планетах. , М.: Мир
- Вальтер А.А., Гуров Е.П. (1979). Установленная и предполагаемая
распространённость взрывных метеоритных кратеров на Земле и их
сохранность на Украинском щите.. Метеоритные структуры на поверхности
планет, М.: Наука, с. 126-148
- Зоткин И.Т., Дабижа А.И. (1982). Эволюция метеоритного кратера
как процесс случайных перемещений.. Метеоритика, Issue 40, с. 82-90
- Алексеев А.С. и др. (1991). Оценки частоты падения небесных тел
на Землю, исследование возможности заблаговременного их обнаружения и
изменения траекторий. Отчёт по НИР, АН СССР ВЦ, Новосибирск , 128 с.
- Хазанович-Вульф К.К. (2007). Диатремовые шлейфы астроблем или
"болидная модель" образования кимберлитовых трубок. Из-во "Геомастер",
Петрозаводск, 272с.
- Jarmo Moilanen (2003). List of probable and possible impact structures of the World.
- D. Rajmon (2009).
This is a very prominent multiring structure in aerial photographs
and from satellites. Suggested as impact structure, but lacking any
shock metamophic evidences it has been discredited by most scientist as
impact structure.
Impact explanation was put forward when coesite was found from
samples. Later coesite identification was noted to be incorrect.
Misidentification was caused by barite (French 2010).
(Jarmo Moilanen).
Реферат:
По территории Мавритании и Марокко, протянулась на 1000 км
полоса-цепочка метеоритных кратеров, самый крупный из которых Ришат (38
или 50 км в диаметре) находится на юго-западном ее конце.
Предположительное направление полета метеорного роя - с
север-северо-востока на юг-юго-запад. Эта зона представляется очень
перспективной на поиски "диатремовых шлейфов" и может быть рекомендована
для постановки аэрогеофизических исследований.
Список кратеров Маррокко-Мавританской зоны, перспективной на обнаружение диатрем:
| Кратер | Страна | D, км | Возр.,млн.л | Координаты
| | Тиндуф | Марокко | 3 - 4 | ? | 27 40' | -8 06'
| | Темимшат | Мавритания | 0,5 | 35? 2-6? | 24 15' | -9 39'
| | Тенумер | ,, | 1,8 | 108? 2-5? | 22 55' | -10 24'
| | Ришат | ,, | 38-40 | <300 | 21 09' | -11 24'
| | Семсият | ,, | 5,0 | 5? | 21 01' | -11 50'
| | Аоэлул | ,, | 0,25 - 0,37 | 3,1 - 0,46 | 20 15' | -12 41'
| | Агейр | ,, | ? | ? | 19 25' | -11 30'
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Марокко-Мавританская цепочка астроблем как перспективная зона на
обнаружение полей диатрем (поле штриховки). В числителях - диаметр в км,
в знаменателях - возраст млн. лет.
(Хазанович-Вульф К.К., 2007).
Guelb er Richat Structure:
The bull's-eye target pattern of the Richat structure on the barren
Gres de Chinguetti Plateau in the Adrar region of central Mauritania is
an eye-catching feature when seen from space, as is evident in the color
Plate photograph taken looking south from the 17th Shuttle mission.
Sand has drifted northward and encroaches on part of the southern half
of the 38-km wide structure. The Makteir desert abuts the plateau to the
north Although reminiscent of multiringed impact structures on the
Moon, Mercury, and Mars, field and laboratory evidence suggests that the
Richat structure is a dome of endogenic origin.
Richat, slightly elliptical and with its long axis oriented
northeast, lies in a depression in which drainage is annular and radial
and topographic relief is about 100 m. The strata dip outward, so that
weathering and erosion of the alternating resistant and nonresistant
Late Precambrian and Early Paleozoic rocks have produced a series of
inward-facing cuestas. Quartzite forms the resistant circular ridges,
and less resistant rocks underlie the intervening annular depressions,
some of which contain seasonal lakes. The center of the structure, the
Guelb er Richat exposes flat-lying limestone and some meta-arkose
surrounded by a massive ridge of chert and chert breccia. The overlying
part of the stratigraphic section, well exposed outward in the dome,
consists of Upper Precambrian/Lower Paleozoic shelf facies rocks.
The quaquaversal dip of the beds increases inward 20 to 25';
locally, dips are as great as 35'. Some minor radial and tangential
faults occur in the outer parts of the structure. The north part of the
structure is cut by a fault system oriented N30'E that appears to be
unrelated to the formation of the dome. Reconstruction of the eroded
beds of the structure suggests that they were uplifted 3 to 4 km to form
the dome.
Reconnaissance gravity data do not support the presence at a
reasonable depth of an igneous or diapiric intrusive mass nor do they
suggest the presence of low-density breccia layers characteristic of
impact structures. However, deep erosion could have removed the
brecciated lens. Thus, the gravity data neither confirm nor preclude an
impact origin for the structure.
Dolerite sills and dikes crop out at several localities within the
structure. Associated analcime-rich rock has been interpreted by some to
be the result of hydrothermal alteration of rhyolite sills and dikes.
No evidence of volcanic activity exists at Richat.
Some geomorphologists have interpreted Richat as an impact structure
because of the nearly circular "bulls-eye" pattern, the reported
presence of coesite, the uniqueness of the structure in the region, its
interior drainage, and the presence of nearby impact
structures-Aouelloul , some 150 km to the southeast, and Tenoumer
to the north. Most geomorphologists now consider Richat to be an
endogenic structural dome of purely terrestrial origin. An impact origin
is discounted by Monod and Pomerol (1973) for these reasons:
Dips of strata are relatively gentle; strata are even flat-lying at
the center of the structure. Nowhere are beds severely disrupted and
contorted.
Although breccia is abundant in the structure, the breccia does not have the characteristics of that produced by impact.
Injection breccia and pseudotachylyte-like material, shatter cones,
and other shock-metamorphic effects have not been identified.
The coesite reported from the rocks of the structure is almost certainly barite that was misidentified (Fudali, 1969).
An unrealistic amount of erosion would be needed if Richat were an
impact structure in order to remove shock-metamorphic effects. Studies
elsewhere have indicated that severe effects extend downward below the
base of a typical impact crater to about one-fifth its diameter; less
severe shock-metamorphic effects extend even deeper. At Richat, then,
detectable shock-metamorphic effects might be expected to extend to a
depth of perhaps 10 km. The fact that none is observed in the rocks of
the structure argues for at least 10 km of erosion if, in fact, Richat
is an impact structure. Geologic history of the region does not allow
the possibility of that degree of erosion nor does the structure itself
show evidence of secondary uplift or deformation, or both.
Reconstruction of the beds suggests a maximum uparching of 3 to 4 km,
which is a reasonable upper limit to the amount of erosion that has
occurred at Richat.
The proximity of Aouelloul and Richat is coincidental; they are of
significantly different ages. Aouelloul, a small bowl-shaped crater, is
probably of Quaternary age, whereas Richat is an ancient structure that
has undergone significant planation.
Semsiyat dome , 50 km
west-southwest of Richat and centered at latitude 210'N and longitude
1105'W, has a diameter of 5 km. Although the style of deformation is
similar to that of Richat, Semsiyat is barely detectable on the ground.
The structure lies on the Chinguetti Plateau and has only a few meters
of topographic relief. Strata dip so slightly that field measurement is
difficult. Exposures are poor, and the rocks are extremely weathered. No
evidence of shock metamorphism has been discovered at Semsiyat; like
Richat, it is believed to be a dome of endogenic origin (Dietz et al.,
1967). (GCW: J. R. Underwood).
Geomorphology from Space
: 1986; NASA publication edited by Nicholas M. Short, Sr. and Robert W. Blair, Jr.
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