Guelb er Richat and other craters of magmatic doming origin

Structures of probably magmatic doming origin in North-Africa

Guelb er Richat (Mauritania)
21° 04' N # 11° 22' W

Richat enigma: Doming and hydrothermal karstification above an alkaline complex ?

"The stratigraphy of the Guelb er Richat dome includes limestone, dolomite, sandstone, mudstone and chert. Field and petrologic studies indicate that the breccia is composed of angular and rounded heterolithic chert fragments in a silica cement. Silica dissolution and replacement were observed in the breccia whereas dolomitization and sulphidation were observed in the limestones. Early ductile deformation in the chert indicates an early development of silicification. The chert fragments display a bimodality in the boundary fractal dimension (Euclidian distance mapping) that implies two phases of breccia formation. Field observations suggest that the breccia was form by a process of syngenetic karst-dissolution collapse. The most likely interpretation is that the Richat chert was formed during a shelf high-stand and persisted in a sub-emergent setting from the Proterozoic up to the Cretaceous. Thus, the Richat chert appears to be an indication of a long term high-standing structural position."

"A model is proposed in which the intrusion of an underlying magmatic body resulted in the bulging of the overlying crust and production of fluids, thus creating a favorable setting for the dissolution of sedimentary rocks. Formation of voids has led to the collapse and brecciation of overlying units. Thus, Guillaume Matton and Michel Jebrak of the University of Quebec in Montreal, and James K.W. Lee of Queen’s University in Kingston, Ontario suggest that this unique structure had a terrestrial origin, ultimately forming from the effects of an intrusion originating from Earth's mantle."

Guelb er Richat: Latitude 21° 04' N; Longitude 11° 22' W Internal structure of Guelb er Richat	The 38 km wide Richat structure on the Gres de Chinguetti Plateau in the Adrar region of central Mauritania is an eye-catching feature when seen from space. Although reminiscent of multiringed impact structures, field and laboratory evidence suggests that the Richat structure is a dome of endogenic origin. The strata dip outward, so that weathering and erosion of the alternating resistant and non resistant 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 centre 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 30°NE 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 in the past have interpreted Richat as an impact structure because of the nearly circular pattern and the reported presence of coesite (?). All geomorphologists now consider Richat to be an endogenic structural dome of purely terrestrial origin: Dips of strata are relatively gentle; strata are even flat-lying at the centre 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 pseudotachylite-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. An unrealistic amount of erosion would be needed if Rich at 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. Panorama



	Semsiyat dome, 50 km west-southwest of Richat and centred 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.
	Christian Laroubine, Societe Astronomique de France, has told me: "I sampled breccias on the central zone of Richat. I do not think that we can say that this zone is the central peak of an impact crater, but there is a doubt about a tectonic origin. The components of my samples do not find their origin with a very great depth as could be to it a magma or nearby products. This breccia (left) is astonishing by its structure and its composition. Today, nobody can affirm without controversy if it is of tectonic or impact origin." I believe, that these breccia can have emerged also in consequence of endogenic processes. In the doubt is the context important. The Richat - structure has unquestionably a endogenic origin. The finds of this breccia and Coesit (?) are the proof for it, that endogenic processes and an impact - event can produce similar forms. R. F. Fudali: Department of Mineral Sciences, Smithsonian Institution, Washington - Coesite from the Richat Dome, Mauritania: A Misidentification The "shattered sandstone" from Richat reported to contain coesite is a tectonic breccia and probably represents a shear zone developed during the structural doming. An optical and x-ray examination of concentrates from this breccia demonstrated that the supposed x-ray reflections of coesite are actually due to barite, introduced into the permeable crushed zone by groundwater.

Tin Bider (Algeria)
27° 36' N # 05° 07' E

"Tin Bider is a multi ring structure approximately 6 km in diameter. The target rocks are alternating sequences of upper Cretaceous clay and limestone. Massive continental sandstone of lower Cretaceous is exposed in the centre having been uplifted approximately 500 m from normal depth. Shatter cones (no valid) were initially reported, but were not officially documented. Subsequent investigation yielded conically striated ventifacts created by wind erosion. PDF’s (PF's ?) have been identified in some mineral grains, which indicate an impact origin for the structure, as does the stratigraphic evidence. There is a distinct lack of allochthonous material, which suggests that the structure has been eroded to a depth below that of the original crater floor. One feature that tends to distinguish Tin Bider from other terrestrial impact structures is the extreme prominence of ductile deformation, where brittle deformation is typical for other impacts."

Comment: Crater is similar to Guelb er Richat.

Ouarkziz (Algeria)
29° 00' N # 07° 33' W

The crater is 3.0 km in diameter. "This structure is situated in sedimentary rocks in the rocky desert of northwest Algeria. It displays a well-defined ring that is partly open to the south. The impact origin is suggested from the occurrence of planar deformation features (PFs?) within rocks returned >30 years ago from the only geologic expedition ever to visit the structure."
"The crater ... has a central uplift structure. The external rim has a height of 100m from top to bottom, and is made up of strongly dipping sedimentary rocks...."
"Quarkziz has limited evidence of an impact origin, in that no impact glass or meteorite fragments have been found. However, petrographic studies have identified planar deformation features in quartzites from the central uplift structure, and from the outer edges of the crater...."

Comment: No clear proofs for an impact.

Talemzane (Algeria)
33° 19' N # 04° 02' E

"The Talemzane crater (locally named: Cratère de Maadna) is located Algeria, 120km southeast of Laghouat. The crater is 1.75 km in diameter and 70m deep. The crater has a raised rim that is made up of outward dipping limestones. The crest of the rim is made up of large, disoriented limestone blocks, ranging in size from 1m to 10m.
The target rocks in the area are limestones, and they compose the crater walls. In many places the limestone has been brecciated, and the blocks are upturned almost vertically. Several highly fractured zones are observed in these crater walls, as are brecciated rocks. There is an abundance of breccia veins in the crater walls. These veins are usually parallel or perpendicular to the strata. The veins differ in distribution and thickness, with some veins as thick as 1m. These veins are filled with a breccia with sub-angular limestone clasts as wide as 10cm, and a calcite cement.

In the lower parts of the crater walls, the limestones are intensely fractured, and grade into a monomict breccia. This breccia can also be found outside of the crater, extending about 500 m past the rim. At the foot of the crater wall is a polymict breccia made up of limestone clasts from different layers that show different stages of weathering. The cement in this polymict breccia is also calcite, and forms up to 50% of the breccia. The centre of the crater is partially filled with alluvial sands and clays.
There is petrographic evidence for Talemzane's impact origin. Thin sections cut from several different rocks have revealed planar elements in quartz, sometimes accompanied by fluid inclusions. Although no other evidence of shock metamorphism has been found, planar elements in quartz are a fairly conclusive sign of an impact event.
The age of Talemzane is hard to estimate. While the crater is somewhat eroded, the presence of an ejecta blanket suggests that the age is not too old. However, the crater postdates Pliocene folding of the local rocks."

Principal investigators: Philippe Lambert, Center for Meteorite Studies, Arizona State University, Tempe, AZ, 85281; John F. McHone Jr., Department of Geology, University of Illinois, Urbana, IL 61801; Robert S. Dietz, Department of Geology, ASU, Tempe, AZ 85281; Messaoud Houfani, SONAREM, El Harrach, Algiers, Algeria

STRUCTURAL AND GEOPHYSICAL CHARACTER OF THE MAADNA METEORITIC CRATER
D. Beihai, H.A. Baker and M. Hamoudi, Institut des Sciences de la Terre, Universite des Sciences et de la Technologie Houari Boumediene, Algeria.

"The so-called Talemzane crater as is usually reported in the literature is in fact located near the Maadna zone southeast of the M'zab region in the Algerian Sahara desert. The geographical coordinates of the crater are approximately 33° 19 North and 4°02 East. Maadna crater is a roughly circular feature. It has an average diameter of 1750 m. In cross-section, the crater has the form of a shallow bowl with an average depth of 75 m from the rim crest which culminates at 652 m to the floor where the centre is at 575 m above the mean-sea level.
Geologically, the Maadna crater overlies the Saharan Platform where the plateau exhibits low-deformed to flat-lying strata. Within the crater, the various cretaceous-tertiary units show impact character. The autochthonous target rocks are hardly fractured, some strata are nearly vertical or inverted and contain breccia, shatter-cones (valid ?) and pseudo-tachylites associated with very high temperature fused materials.
The subsurface structure under the filling plio-quaternary Sediments of the Maadna crater is poorly known from previous works so combined geological-geophysical extensive investigations are performed inside and outside the impact zone in order to better understand the structure of the crater and to get a 3D-image of the subsurface interior. The results thus obtained are consistent with the impact cratering hypothesis."

Comment: No clear proofs for an impact. It however appears to be a typical form of an impact crater.

Amguid (Algeria)
26° 05' N # 04° 23' E

"Amguid is located in southwestern Algeria. Amguid formed in lower Devonian sandstones, and has a diameter of 450m. These sandstones dip outward within the crater walls, and become progressively steeper in the upper parts of the wall. On the northwest side of the rim, the sandstone beds have been overturned, and these beds dip gently in the northeast part of the rim. The top of the rim seems to be covered by blocks of sandstones that are several meters in diameter, and form a nearly continuous debris blanket 100m from the rim. The centre of the crater is flat, and is filled by compacted eolian silts. The crater walls do not have visible breccia veins or shatter cones (valid ?). Despite this, planar deformation features (PFs ?) were discovered in some quartz grains. Intense deformation was also visible in some of the grains, as well as fracturing. The fact that planar deformation features (PFs ?) are observed so rarely at Amguid are probably a result of the crater’s formation in sandstones, and of lower pressures that occur in the formation of small craters." Comment: No clear proofs for an impact. It however appears to be a typical form of an impact crater.
At the crater's outside

Eheri (Algeria)

22° 53' N # 04° 27' E

Two up to now still unknown structures in western direction of Tamanrasset.
They have an important similarity to the Clayton Craters in the Gilf Kebir (Egypt). The Clayton Craters contain plugs with basaltic columns.

Comment: Nobody has examined this up to now.

Ilebjane (Mali)
18° 28' N # 01° 06' E

This structure does not appear in any of the current databases. It is located in the Sahara Desert near Kidal, in the northeastern area of Mali. It is approximately 18 km in diameter.

Comment: It appears me this is a broken-in-dome.

Ouro Ndia (Mali)
15° 11' N # 04° 33' W

"The crater has a diameter of about 3 km and it is occupied by a small lake. It is located inside Niger inland delta, on Quaternary deposits. Based on the overall morphology and the geological and geomorphological setting a recent age of the formation could be hypothesized. This crater shows a lobate-like pattern around the rim. If the impact hypothesis is correct this could be interpreted as fluized ejecta. This observation could be consist with the geological setting."

Source: http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1309.pdf

Comment: Nobody has examined this up to now.

Aorounga (Chad)
19° 06'N # 19° 15'E


	The circular depression of Aorounga structure, which is included on some geological maps, has a diameter of 12.6 km and is situated in Northern Chad, about 110 km southeast of the Emi Koussi volcano in the Tibesti Massif. French researchers have collected a few samples from the structure, which show evidence of shock metamorphism. The host rock of the crater is fine-grained, well-sorted, carbonate-bearing sandstone of probably Upper Devonian age. However, the structure was described in an earlier photogeological study by ROLAND (1976), who concluded that it is a diapir. The structure has an outer and an inner ring wall, which both rise about 100 m above the mean level of the surrounding plain. The two rings are separated from each other by a depression of uniform width. A central uplift is located near the centre of the central depression. The sandstones forming the outer ring dip steeply towards the depression.
AOROUNGA AND GWENI FADA IMPACT STRUCTURES, CHAD, CENTRAL AFRICA: PETROLOGY AND GEOCHEMISTRY OF TARGET ROCKS Christian Koeberl, Wolf Uwe Reimold, Pierre M. Vincent, and Dion Brandt Our 15 samples from the Aorounga structure are derived from the top or outer slope of the inner ring structure. We conclude from our petrographic studies that all these samples represent normal quartzite or sandstone without any unusual properties. Lithologically, a number of rock types have been sampled, including pure orthoquartzite, quartzites and sandstones of variable grain size and grain shape distributions, hematite-cemented sandstone, as well as arenites of variable degrees of recrystallization. These samples are shock metmorphosed to variable degrees. This ranges from unshocked rocks to evidence of low (short microfractures are present in quartz), moderate (single sets to PDFs in only a few quartz grains per thin section), to high degree of shock metamorphism. In the latter case, nearly every quartz grain shows at least one, but frequently up to five sets of different crystallographic orientations. Two to three sets of PDFs are most abundant. In a few barely deformed samples. no shock-characteristic deformation effects were found, but indications of compression, such as short fractures emanating from a contact point between two quartz crystals, were observed. The results of the orientation measurements show clearly that the shock-characteristic orientations are present. No obvious siderophile element anomalies have been observed in any of the samples. Only one sample from Aorounga (a highly shocked rock, possibly a breccia) shows an Ir concentration barely above the detection limit at about 0.25 ppb, with somewhat elevated Ni values. No clear evidence for the presence of an extraterrestrial component was found in any of the samples. Source: http://www.lpi.usra.edu/meetings/LPSC98/pdf/1103.pdf Comment: Finds of PDF (PF?) are no sufficient proofs for an impact event.

Gweni Fada (Chad)
17° 25' N # 21° 45' E

The second impact crater in Chad, the 14 km diameter Gweni Fada structure, was first noted on Landsat images and aerial photography, and later visited on the ground by a French team, who reported in 1996 that preliminary petrographic studies showed the presence of shock metamorphic effects in quartz grains from sandstones. The structure is asymmetric and appears deeply eroded. An external depression, with a diameter of 12 km, forms a crescent around two thirds of the inner disturbed zone. The external limit of the depression is pronounced and marked by steeply dipping sandstones. The depression is surrounded on the north side by an elevated outer ring of outward dipping sandstones. On the south side the external depression is absent, but tilted or folded sandstones are present. The inner zone, with a diameter of about 10 km, consists of a rugged terrain with hills of several hundred meters elevation. Sandstones show no uniform dip direction. The structure is exposed within upper Devonian sandstones; the Precambrian basement was not found to be exposed in the central uplift.

AOROUNGA AND GWENI FADA IMPACT STRUCTURES, CHAD, CENTRAL AFRICA: PETROLOGY AND GEOCHEMISTRY OF TARGET ROCKS
Christian Koeberl, Wolf Uwe Reimold, Pierre M. Vincent, and Dion Brandt

Unfortunately only a limited number of 6 samples from Gweni Fada was available for our study. These samples also represent a suite of arenitic lithologies, including quartzite, hematite-stained sandstone, a pebbly sandstone, and a quartz conglomerate. They are derived from various locations of the central disturbed area, as well as from the outer margin of the ring depression. Some samples are apparently unshocked, but display minor evidence of compressional deformation. Others contain few to abundant grains with single or multiple sets of PDFs, with two sets of PDFs being most common. In addition, local brecciation in the form of micro-veins of cataclastic material or off more pervasive brecciation was noted in individual specimens. It appears as if the presence of cataclastic zones is limited to samples that have very few or no PDFs (i.e., shock pressure <15 GPa).
No obvious siderophile element anomalies have been observed in any of the samples. Only one sample from Aorounga (a highly shocked rock, possibly a breccia) shows an Ir concentration barely above the detection limit at about 0.25 ppb, with somewhat elevated Ni values. No clear evidence for the presence of an extraterrestrial component was found in any of the samples.

Source: http://www.lpi.usra.edu/meetings/LPSC98/pdf/1103.pdf

Comment: Finds of PDF (PF?) are no sufficient proofs for an impact event.

Faya (Chad)

18° 11' N # 19° 34' E

Comment: Probable a subvolcanic structure

The Faya basin (N Chad, Africa) - A possible impact structure?
by Martin Schmieder & Elmar Buchner
Institute of Geology, University of Stuttgart, Germany, November 2006

The Faya basin, an almost circular structure, has been discovered by remote sensing using Landsat and Shuttle Radar Topographie Mission (SRTM) data. It is centred on 18°11' N and 19°34' E, about 55 km ENE of the city of Faya (Largeau), and has a diameter of ~2 km. The Faya basin is a singular geological feature that formed in sandstones of most likely Late Devonian age. As so far observed and interpreted in satellite imagery and digital elevation models, the Faya basin exhibits various structural features and the typical morphology of small, complex impact craters: an elevated rim surrounded by concentric faults, an annular depression, and a central topographic high. It is difficult to explain the formation of the Faya basin by common geological processes. No volcanic features are known in this area. Eroded magmatic intrusions and volcanic calderas should usually be different in size and shape, and should show an inhomogenic rock distribution on satellite images. Further, no salt diapirs, sinkhole structures, or glacial erosion features have been reported in this region. Therefore, we suppose that the Faya basin might have an impact origin, but final confirmation will require studies of rock samples from the structure.

Mousso (Chad)

17° 58' N # 19° 53' E

Comment: Spider-like outflows at the crater ?

Mousso structure: A deeply eroded, medium-sized, complex impact crater in northern Chad?
by Elmar Buchner & Martin Schmieder
Institute of Geology, University of Stuttgart, Germany, June 2007

The Mousso structure, a complex, ~3.8 km circular structure centred on I7°58' N and 19°53' E and located near the Mousso oasis, northern Chad, displays structural features typical for complex impact structures: a circular rim with concentric faults, an annular basin, and a central peak. Remote sensing investigations based on Landsat-7 Enhanced Thematic Mapper Plus (ETM+) and Shuttle Radar Topographie Mission (SRTM) data reveal a specific morphological-structural resemblance of the complex central peak of the Mousso structure to the central uplift of the Spider impact structure. This is consistent with the layered sedimentary rocks associated with all of these structures. No endogenic geological processes such as magmatism, diapirism, karst dissolution, and glacial or fluvial erosion can conclusively explain the formation of the Mousso structure within a large area of flat-lying early Paleozoic sandstones. Thus, this paper proposes that the Mousso structure might represent a deeply eroded, medium-sized, complex impact structure. As field investigations are currently impossible, the search for shock-metamorphic effects in rocks of the Mousso structure remains outstanding.

Uri (Chad)
21° 17' N # 19° 20' E

Comment: No impact crater.

EVIDENCE OF TWO NEW POSSIBLE IMPACT STRUCTURES IN CHAD: SEPARATE IMPACTS, BINARY ASTEROIDS OR AOROUNGA IMPACT CHAIN?
Emilio Gonzalez and Sergio Alonso, Astroseti.org, Barcelona, Spain

The Uri structure presents a 5 km diameter and a multi-ring morphology. The structure is harder to appreciate because it is in a mountainous area, and because it is also more eroded than the northern one. Despite the previous description, there are two continued rings -separated by a depressed zone- and even a third one extending the affected area to 8 km. There is presence of some volcanic buildings developed from linear fractures in the rings, suggesting that the fractures and magma emissions are more recent than the structure formation. The depressions between rings indicate a compressed genesis rather than accumulation of pyroclastic elements and lava coming from fractures.

Source: http://www.geo.su.se/file.php?id=4928

Oun (Chad)
21° 44' N # 19° 20' E

EVIDENCE OF TWO NEW POSSIBLE IMPACT STRUCTURES IN CHAD: SEPARATE IMPACTS, BINARY ASTEROIDS OR AOROUNGA IMPACT CHAIN?
Emilio Gonzalez and Sergio Alonso, Astroseti.org, Barcelona, Spain

The Oun structure is 8 km minimum in diameter with three rings separated by depressions (as seen when using elevation techniques with NASA World Wind Landsat images). The external ring has discontinuities apparently caused by the presence of wind deposits (sand). In NE direction the ring loses circularity due to the action of a fracture. Inner circles show more continuity and present radial dips in the central zone. The most inner ring delimits a more depressed zone, crossed by a fluvial morphology. Images show landslides in the inner ring walls, and some volcanic buildings developed from fractures that affected -presumably post impact- the most external ring.

Source: http://www.geo.su.se/file.php?id=4928

Comment: No impact crater, an erosion-structure.

last update: 04.09.2010
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