Richat Structure

Known by its Arabic name, قلب الريشات, romanized as Qalb ar-Rīšāt (Hassaniyya: [galb er. riːʃaːt]), the Richat Structure is a well-known circular geological formation located in the Adrar Plateau in the Sahara. It is situated in Mauritania’s Adrar Region, close to Ouadane. Feathers is the meaning of rīšāt in Hassaniya Arabic. It is also called tagense locally, which refers to the circular aperture of the leather pouch used to gather water from nearby wells.

With a circumference of 40 kilometers (25 miles), it is an eroded geological dome that displays sedimentary rock in strata that resemble concentric rings. Inside, exposed igneous rock has hydrothermally altered gabbros and rhyolites, as well as a central megabreccia. Moreover, the edifice is home to remarkable collections of Acheulean archeological relics. The International Union of Geological Sciences (IUGS) named it one of the top 100 geological heritage sites with the greatest scientific importance.

Description

With a circumference of 40 kilometers (25 miles), the Richat Structure is a slightly elliptical dome that has been severely eroded. The Late Proterozoic sedimentary rock found in the dome’s center and the Ordovician sandstone that surrounds it are the two ages of the exposed sedimentary rock.

The sedimentary strata constituting this formation tilt outward at 10–20°. High-relief circular cuestas have been produced by the differential erosion of resistant layers of quartzite. A siliceous breccia that spans at least 30 kilometers (19 mi) in diameter is located in its middle.

Numerous intrusive and extrusive igneous rocks are seen inside the Richat Structure. They consist of kimberlites, gabbros, carbonatites, and rhyolitic volcanic rocks. The rhyolitic rocks are thought to be the eroded remnants of two maars.

They are composed of lava flows and hydrothermally altered tuffaceous rocks that are a component of two separate eruptive centers. Aeromagnetic, gravimetric, and field mapping data indicate that the gabbroic rocks form two concentric ring dikes.

The inner ring dike is located 3 kilometers (1.9 km) from the Richat Structure’s center and is around 30 meters (98 feet) wide. The outer ring dike is about 8 kilometers (5.0 mi) from the structure’s center and is roughly 70 meters (230 feet) wide.

Thirty-two carbonatite dikes and sills have been identified within the structure. The dikes are normally between one and four meters (3.3 and 13.1 feet) broad and 300 meters (980 feet) long. They are made up of large, generally vesicle-free carbonatites.

According to dating, the carbonatite rocks cooled between 94 and 104 million years ago. Several sills and a kimberlitic plug have been discovered in the northern section of the structure.

The age of the kimberlite plug has been estimated to be 99 million years. According to interpretation, these intrusive igneous rocks point to the existence of a large alkaline igneous intrusion that was formed by elevating the surrounding rock and presently underlies the structure.

The Richat Structure includes spectacular hydrothermal phenomena. These consist of core megabreccia formed by hydrothermal dissolution and collapse, as well as the widespread hydrothermal alteration of gabbros and rhyolites.

The thickness of the siliceous megabreccia varies from a few meters on the borders to at least 40 meters (130 feet) in the middle. The breccia is intensely silicified and is composed of quartz-rich sandstone, diagenetic cherty nodules, stromatolitic limestone, and shards of white to dark gray cherty material. Using the 40Ar/39Ar technique, the hydrothermal alteration that produced this breccia has been dated to 98.2 ± 2.6 million years ago.

Interpretation

The building was initially referred to as the Richât Crater or Richât buttonhole (boutonnière du Richât) in the 1930s and 1940s. According to Richard-Molard (1948), it is the outcome of a laccolithic elevation.

A geological mission to Mauritania led by Théodore Monod in 1952 identified four “crateriform or circular irregularities” (accidents cratériformes ou circulaires) in the region, Er Richât, Aouelloul (south of Chinguetti), Temimichat-Ghallaman and Tenoumer. As with the other three, it was first thought to be an impact structure, but a more thorough examination in the 1950s and 1960s revealed that it could have been generated by terrestrial processes instead.

Following field and lab investigations in the 1960s, no noteworthy evidence of shock metamorphism or any other deformation suggestive of a collision with an alien object at high velocity was discovered. In 1969, an additional study revealed that barite had been mistakenly labeled as coesite, despite the fact that coesite is indicative of shock metamorphism and was first reported to be found in rock samples from the structure. The 1990s saw the completion of the structure’s dating process. According to Matton et al.’s (2005, 2008) analysis of the structure’s genesis, it was not an impact structure.

The structure is the result of ring faults that led to gabbroic ring dikes over a sizable intrusive body of magma and the uplifting and later erosion of a dome through intense hydrothermal activity through the fractured substructure, according to further analysis of the deep structure beneath the surface, including with aeromagnetic and gravimetric mapping. Due to the consequent alternating hard and soft rock layers’ differential erosion over time, this can create cuestas. The Cretaceous epoch is represented by the underlying alkaline igneous complex that has been revealed by erosion.

IUGS Geological Heritage site

The Richat Structure was inscribed in October 2022 by the International Union of Geological Sciences (IUGS) as “a spectacular example of a magmatic concentric alkaline complex” in its global collection of 100 geological heritage monuments.

An IUGS Geological Heritage Site is described by the organization as “a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.”

Archaeology

There are remarkable collections of Acheulean antiquities at the Richat Structure. These Acheulean archaeological sites are situated beside wadis that sit inside this structure’s outermost circular depression. In the same regions, pre-Acheulean stone tools have also been discovered.

The raw quartzite outcrops that are connected to these sites served as the source of raw materials for the creation of these artifacts. The northwest of the outer ring contains the most significant Acheulean sites and the outcrops that are linked with them. Wadi Akerdil runs east from these sites, while Wadi Bamouere runs west.

Neolithic spear points and other artifacts have also been discovered, sparsely and widely distributed. But since Théodore Monod originally unearthed these sites in 1974, mapping of the artifacts in the vicinity of the structure has revealed that they are largely absent from its deepest indentations.

Neither identifiable man-made structures nor identifiable midden deposits have been identified and reported from the building thus far. This is taken to mean that the region of the Richat Structure was solely utilized for transient hunting and the production of stone tools. The accumulation and mixing of surface artifacts caused by deflation across several glacial-interglacial cycles is what gives the area its seeming riches.

There are artifacts in silty sand, clayey sand, muddy gravel, late Pleistocene to early Holocene gravelly mud, redeposited, deflated, or both types of mud. These sediments are frequently bound together with calcrete to form concretionary bulk or beds. Generally, ridges are composed of extensively worn bedrock, which is indicative of truncated Cenozoic paleosols that originated in tropical settings.

The Pleistocene to Middle Holocene sediments are found along wadis as thin accumulations that range in thickness from less than a meter to several meters in the interior annular depressions to three to four meters (9.8 to 13.1 feet) at the outermost annular depression of the structure. Slope scree, debris flow, and fluviatile or even torrential flow deposits are all mixed together to form the gravelly deposits. Playa lake and eolian deposits make up the finer-grained, sandy sediments.

Freshwater fossils in the latter are in excellent condition. The majority of these sediments accumulated between 15,000 and 8,000 BP during the African humid era, according to multiple concordant radiocarbon dates. The bedrock underneath these deposits is severely worn and degraded.

Site of Atlantis: fringe hypothesis

There have been speculative suggestions that the Richat building is the location of Atlantis, which Plato referenced in his works. The main evidence for this assertion is the structure’s concentric design, which on the surface corresponds with Plato’s depiction of the city. The majority of classicists think that Plato invented the mythical city of Atlantis as a rhetorical device rather than as a genuine place. The assertion was criticized by skeptic Steven Novella, who said that the building contradicted Plato’s account of Atlantis and that there was no proof of a city ever being at the site.

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