Editing Richat Structure (section)

==Description==
[[File:Richat structure cropped.png|left|thumb|Cross section of the structure. Magenta is the volcanic intrusion, while purple, green and grey represent sedimentary layers]]
The Richat Structure is a deeply [[Erosion|eroded]], slightly [[elliptical dome|elliptical]] [[Dome (geology)|dome]] with a diameter of {{convert|40|km|mi}}. The sedimentary rock exposed in this dome ranges in age from Late [[Proterozoic]] within the center of the dome to [[Ordovician]] [[sandstone]] around its edges. The [[sedimentary rock]]s composing this structure [[Strike and dip|dip]] outward at 10–20°. Differential erosion of resistant layers of [[quartzite]] has created high-relief circular [[cuesta]]s. Its center consists of a [[Silicon dioxide|siliceous]] [[breccia]] covering an area that is at least {{convert|30|km|mi}} in diameter.<ref name="Matton2008a">{{Cite web |last=Matton |first=G. |year=2008 |title=The Cretaceous Richat Complex (Mauritania); a peri-Atlantic alkaline |url=http://bibvir.uqac.ca/theses/030084214/030084214.pdf |publisher=[[Université du Québec à Chicoutimi]] |place=Chicoutimi, Quebec, Canada |access-date=7 September 2011 |archive-date=2 April 2012 |archive-url=https://web.archive.org/web/20120402073745/http://bibvir.uqac.ca/theses/030084214/030084214.pdf |url-status=dead }}</ref><ref name="MattonOthers2005a">{{Cite journal |last1=Matton |first1=Guillaume |last2=Jébrak |first2=Michel |last3=Lee |first3=James K.W. |year=2005 |title=Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex |url=http://www.archipel.uqam.ca/272/1/Matton_etal05.pdf |journal=Geology |volume=33 |issue=8 |pages=665–68 |doi=10.1130/G21542AR.1 |ref={{sfnref|Matton|2005}}}}</ref><ref name="Wooley2001a">{{Cite book |last=Woolley |first=Alan Robert |url=https://www.worldcat.org/oclc/16448519 |title=Alkaline Rocks and Carbonatites of the World |year=1987 |isbn=0-292-70389-9 |publisher=University of Texas Press|location=Austin |oclc=16448519}}</ref>[[Image:ASTER Richat.jpg|thumb|250px|Satellite picture of the Richat Structure (false color)]]Exposed within the interior of the Richat Structure is a variety of [[Intrusive rock|intrusive]] and [[Extrusive rock|extrusive]] [[igneous rock]]s. They include [[Rhyolite|rhyolitic]] volcanic rocks, [[gabbro]]s, [[carbonatite]]s and [[kimberlite]]s. The rhyolitic rocks consist of [[lava]] flows and [[Hydrothermal alteration|hydrothermally]] altered [[tuff]]aceous rocks that are part of two distinct eruptive centers, which are interpreted to be the eroded remains of two [[maar]]s. According to field mapping, [[Aeromagnetic survey|aeromagnetic]], and [[Gravimetry|gravimetric]] data, the gabbroic rocks form two concentric [[ring dike]]s. The inner ring dike is about {{convert|30|m|ft}} in width, {{convert|3|km|mi}} from the center of the Richat Structure. The outer ring dike is about {{convert|70|m|ft}} in width, {{convert|8|km|mi}} from the center of the structure.<ref name=":0" /> Thirty-two carbonatite dikes and [[Sill (geology)|sills]] have been mapped within the structure. The dikes are generally about {{convert|300|m|ft}} long and typically {{convert|1 to 4|m|ft}} wide. They consist of massive carbonatites that are mostly devoid of [[Vesicular texture|vesicles]]. The carbonatite rocks have been dated as having cooled between 94&nbsp;and 104&nbsp;million years ago. A kimberlitic plug and several sills have been found within the northern part of the structure. The kimberlite plug has been dated to around 99&nbsp;million years old. These intrusive igneous rocks are interpreted as indicating the presence of a large alkaline igneous intrusion that currently underlies the structure and was created by uplifting the overlying rock.<ref name="Matton2008a" /><ref name="MattonOthers2005a" /><ref name="NettoOthers1992a">{{Cite journal |last1=Netto |first1=A.M. |last2=Fabre |first2=J. |last3=Poupeau |first3=G. |last4=Champemmois |first4=M. |year=1992 |title=Datations par traces de fissions de la structure circulaire des Richats |journal=Comptes Rendus de l'Académie des Sciences de Paris |volume=314 |pages=1179–1186}}</ref><ref name="MattonOthers2014a">{{Cite journal |last1=Matton |first1=Guillaume |last2=Jébrak |first2=Michel |year=2014 |title=The 'eye of Africa' (Richat dome, Mauritania): An isolated Cretaceous alkaline–hydrothermal complex |url=https://www.academia.edu/9220436 |journal=Journal of African Earth Sciences |volume=97 |pages=109–124 |bibcode=2014JAfES..97..109M |doi=10.1016/j.jafrearsci.2014.04.006}}</ref>

[[File:GuelbRichatTopo.png|thumb|Topographic map of Guelb el Richat. Elevation in meters. 10 m contour interval with major contour line every 50 m|left]]

Spectacular hydrothermal features are a part of the Richat Structure. They include the extensive [[hydrothermal alteration]] of rhyolites and gabbros and a central [[Breccia|megabreccia]] created by hydrothermal dissolution and collapse. The siliceous megabreccia is at least {{convert|40|m|ft}} thick in its center to only a few meters thick along its edges. The breccia consists of fragments of white to dark gray [[chert]]y material, [[quartz]]-rich sandstone, diagenetic cherty nodules, and [[Stromatolite|stromatolitic]] [[limestone]] and is intensively silicified. The hydrothermal alteration, which created this breccia, has been dated to have occurred about 98.2&nbsp;± 2.6&nbsp;million years ago using the [[Argon–argon dating|<sup>40</sup>Ar/<sup>39</sup>Ar]] method.<ref name="Matton2008a" /><ref name="MattonOthers2005a" /><ref name="MattonOthers2014a" />