Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger

Accepted Manuscript Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger Trey Lobpries, Thomas Lapen PII:

S1464-343X(18)30303-0

DOI:

10.1016/j.jafrearsci.2018.09.020

Reference:

AES 3332

To appear in:

Journal of African Earth Sciences

Received Date: 30 May 2018 Revised Date:

26 September 2018

Accepted Date: 26 September 2018

Please cite this article as: Lobpries, T., Lapen, T., Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger, Journal of African Earth Sciences (2018), doi: https:// doi.org/10.1016/j.jafrearsci.2018.09.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Title: Remote sensing evidence for a possible 10 kilometer in diameter impact structure in north-central Niger Authors: Trey Lobpriesa,b*, Thomas Lapena,c a

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Department of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research 1, 3507 Cullen Blvd, Rm. 312, Houston, TX 77204, USA,

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e-mail: [email protected]

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e-mail: [email protected]

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*Corresponding Author: Trey Lobpries, Department of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research 1, 3507 Cullen Blvd, Rm. 312, Houston, TX 77204, USA, email address: [email protected]

Declarations of interest: none

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All figures should be printed in color.

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This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Abstract Remote sensing investigations of West Africa reveal a roughly 10 km in diameter, circular feature in north-central Niger. The circular feature is located within Early

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Devonian fluvio-marine sedimentary strata about 100 km to the north of a suite of

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igneous ring dikes located in the Aïr Massif. Whether the circular feature is related to

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the ring dikes, structural doming, or an impact crater can be evaluated from earlier

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geologic mapping (Black, 1967) and image analyses. Previous mapping indicates the

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circular feature is defined by outcrops of Devonian Idekel sandstone and the Late

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Proterozoic, weakly-metamorphosed molasse of the Proche-Ténéré Formation. Absent

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in published lithologic descriptions of these sedimentary and metasedimentary rocks are

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diapiric evaporite deposits that could produce the observed circular morphologies.

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Lithologies that define the circular feature are confirmed by classification techniques

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using the ENVI software platform and band ratios/math from Landsat 8 and Sentinel 2A

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data. Spectral profile comparisons between the igneous intrusive rocks exposed in the

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Aïr Massif, outcrops of sedimentary rock that are proximal to the circular feature, and

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desert sand confirm that the sedimentary rocks mapped by Black (1967) define the

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circular feature. The shallow subsurface structure, revealed through Spaceborne

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Imaging Radar-C data, and 3-D modeling using Sentinel 1A data, is also not consistent

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with nearby ring dike structures. These data suggest that the circular feature did not

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likely form as a magmatic intrusion close to the time of sedimentary rock deposition. We

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propose that the circular feature formed from either a meteorite impact after deposition

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of the host sedimentary units or by some other, but less likely, geologic process.

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Confirmation of an impact explanation for the circular feature would require sample

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analysis and inspection for shock-metamorphosed materials.

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Keywords: Remote sensing, impact crater

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1. Introduction

The Earth has been subject to meteorite impacts since its formation (Marvin,

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1990; Grieve, 1991; Grieve and Shoemaker, 1994), however, much of this record is lost

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to geologic processes such as weathering, erosion, and burial. In oceanic basins, the

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consumption and production of oceanic crust over geologic time limits the preservation

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of impact craters to those no older than about 200 Ma (Muller et al., 2008). Presently,

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oceanic crust has no confirmed impact sites, and only 4 unconfirmed impact sites

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(Abbott et al., 2005; Levin et al., 2006; Glass and Koeberl, 2006). Thus, all confirmed

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impacts are found within continental crust and over 85 percent of these occurred during

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the Phanerozoic (Earth Impact Database, 2011).

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Identification of possible impact sites can be made from geomorphic and spectral

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analyses of satellite imagery. Remote sensing has been well-established as a viable

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means for the identification of various land cover types (Sultan and Arvidson, 1986;

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Abrams et al., 1987; Rothery, 1987; Sultan et al., 1987; Gad and Kusky, 2006; Kavak et

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al., 2009; Bishop et al., 2011). In addition, remote sensing has been used to detect

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possible impact structures for future ground truth confirmation, particularly in arid

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environments, including the Arkenu (Paillou et al., 2004), Jebel Hadid (Schmieder et al.,

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2009), and Ibn Batutah (Ghoneim, 2009) structures in Libya, the Gilf Kebir region in

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Southwest Egypt (Paillou et al., 2003), the Faya Basin and Mousso structure in northern

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Chad (Buchner and Schmieder, 2007; Schmieder & Buchner, 2007, 2010), and the

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Uneged Uul structure in Mongolia (Schmieder et al., 2013). Subsequent ground truth

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investigations on the Arkenu structures (Cigolini et al., 2012) and Gilf Kebir (Orti et al.,

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2008) indicated intrusive magmatic origins for these features. Thus, similarities

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between the morphologies of impact-generated structures and those related to

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terrestrial geologic processes, such as ring dike igneous intrusions, igneous or

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structural domes, and salt diapirs require ultimate confirmation of impact structures by

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documentation of shock-related metamorphism (Reimold, 2007; Pinter and Ishman,

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2008; French and Koeberl, 2010).

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Shatter cones are usually easily identifiable in hand specimens (French, 1998; French and Koeberl, 2010). Tektites and microtektites, forms of impact melt glass, are

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common in distal impact ejecta. They are variable in size from the sub-mm to cm scale,

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but are less stable than crystalline minerals on a geologic timescale (Barnes, 1964;

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Taylor, 1973; King, 1977; Melosh and Vickery, 1991; Dressler and Reimold, 2001;

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Glass and Simonson, 2012). Impact metamorphism acting on minerals such as quartz,

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plagioclase, zircon, and monazite produces planar deformation features (PDFs), and at

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higher grade of metamorphism form high pressure polymorphs such as coesite and

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stishovite from quartz (SiO2), or reidite from zircon (ZrSiO4) (Chao, 1967; Cavosie et al.,

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2010; Schmieder et al., 2011). However PDFs and shock-produced high-pressure

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polymorphs require additional petrologic-microstructural analyses for proper

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identification. Cavosie et al. (2010) showed that shocked zircon grains could survive

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and be recycled in modern sands upwards of 2 billion years after an impact event.

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Unfortunately, due to some localized geopolitical complexities, not every region where a

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suspected terrestrial impact site is identified can be easily or safely sampled and

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studied for shock-related metamorphism. This work seeks to characterize a circular

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feature with attributes consistent with an impact structure that, when geopolitics allow, a

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ground-truth investigation can be conducted.

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Satellite images accessed through Google Earth document a circular feature with a diameter of approximately 10 km observed at 21°21 '14.56"N Latitude and 9°

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8'32.24"E Longitude in northern Niger (Fig. 1). The Republic of Niger covers an area of

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over 1,186,000 km2, 80% of which is composed of Sahara Desert. Forty meteorites of

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various types have been found in Niger to date, yet despite the favorability of the

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modern Saharan climate in preserving geologic features, there are currently no

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confirmed meteoritic impact sites (Earth Impact Database, 2018; Meteoritical Bulletin

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Database, 2018). Based on cratering rate estimates of Grieve, 1984, undiscovered

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impact sites should exist and be preserved in the Sahara (Paillou et al., 2003), though

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Hergarten and Kenkmann (2015) calculate that the only remaining undiscovered craters

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on Earth are less than 6 km. Rossi (2002) presented two possible impact structures of

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0.5 and 1 km diameter using ASTER imagery in southern Niger at approximately 16.5°N

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and 8°E, but no ground truth information has been c ollected due to difficulties of access.

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This study uses remote sensing data and compilations of geologic data to determine the

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lithologies and morphology of the circular feature and to use these data to distinguish

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likely mechanisms for its formation.

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1.1 Geologic Background Precambrian aged units form much of the basement rock throughout Niger but are covered by extensive Quaternary sand deposits and other Cenozoic weathering

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products (Schlüter, 2008). In north-central Niger, Precambrian basement is exposed

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latitudinally with onlapping Paleozoic and Mesozoic sedimentary strata to the east and

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west, much of which is covered by Quaternary deposits (Persits et al., 2002). The

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circular feature occurs in an Early Devonian sedimentary sequence in the Ténéré basin

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of Tamesna, wedged between the Hoggar Mountains to the north in Algeria and the Aïr

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Massif to the south (Figs. 2a, 2b). Modern eolian deposits cover much of the region

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with vast dune fields to the east of the circular feature. Based on geologic maps by

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Black (1967), the majority of the rocks that compose the floor and visible rim of the

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circular feature belong to Idekel Sandstone of the Anou Izileg Formation. The Idekel is

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an Early Devonian 80m thick, coarse, arkosic, fluvial deposit with occurrences of

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silicified wood that formed approximately 410-419 Ma (Black, 1967; Zanguina et al.,

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1998, Mergl and Massa, 2004; Schlüter, 2008). The Anou Izeleg Formation forms the

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base of the Devonian series of Tamesna that reaches an overall thickness of up to

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155m; a 20m thick conglomerate locally occurs at the base, and shallow marine

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brachiopod fauna occur in the finer sands higher up in the sequence (Faber and

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Bertrand, 1983). It is transgressive towards the south and occurs unconformably over

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both Silurian graptolite shales and older Proterozoic rocks. At the location of the

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circular feature, the Idekel sandstone directly overlies Upper Proterozoic molasse of the

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Proche Ténéré Formation. The Proche Ténéré outcrops as elevated ridges and peaks

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on the floor of the circular feature and to the northwest, west, and south of the site (Fig.

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2b). The Proche Ténéré Formation is made up of weakly metamorphosed sediments

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with basal polygenic conglomerates composed of clasts ranging from decimeter to

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meter scale with a schistose matrix believed to be of glacial origin (Raulais, 1959, Black

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et al., 1967, Fabre and Bertrand, 1983). The unit fines upward into arkose and hosts

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some localized bi-modal igneous intrusions. Remnants of slightly older, weakly

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metamorphosed arkoses, black conglomerates, and quartzites of the Coin Formation lie

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unconformably below the Proche Ténéré and outcrop to the northwest along with a suite

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of granodiorite intrusions. All of these sedimentary and metasedimentary units lie on

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middle to lower Proterozoic basement rocks composed of granitoids and the granulite-

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facies Tafourfouzete metamorphic rocks. These are associated with the oldest rocks in

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the Aïr region, the gneissic Azanguerene formation, which outcrops further west of the

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circular feature (Fabre and Bertrand, 1983). Faulting is common in the region, and the

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majority of faults in the region display either a dominant NW-SE or NE-SW strike, but

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their type is undifferentiated (Black 1967). To the south of the circular feature in central

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Niger, the exposed Precambrian basement of the Aïr Massif is aged ~580-730 Ma and

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comprises the southeastern extension of the Hoggar Mountains (Moreau et al., 1994).

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This is a large peralkaline granitic range covering over 60,000 km2 which represents an

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extension of the Tuareg Shield of the Pan-African belt (Cahen et al., 1984). Features

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similar in morphology to the circular feature occur within these mountains and are

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described as anorogenic ring plutons (Fig. 2b).

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Moreau et al. (1994) distinguished three different types of plutons within the

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Massif: the Taghouaji type, made up of plutonic alkaline and peralkaline syenite and

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granite, with minor metaluminous granite; the Goundaï type, characterized by rhyolitic

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tuff and ignimbrite with minor quartz-syenite ring plutons; and the Ofoud type,

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composed mainly of troctolite, leucogabbro, and anorthosite, with alkaline to peralkaline

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syenitic and granitic intrusions. Complexes range in size from 0.4 to 34 km in diameter

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and have circular to elliptical shapes. Of the 28 recognized, most are lenticular in cross-

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section with only one true ring dike present, Meugueur-Meugueur, with a diameter of

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~65 km. The majority of these intrusions have topographic relief of 200 to 1000 m

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relative to their surroundings. Early dating discrepancies for pluton emplacement

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between Bowden et al. (1976) and Karche and Vachette (1978) were resolved by

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Moreau et al. (1994) giving an average Rb-Sr whole rock isochron date of 407 ± 8 Ma.

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However, these analyses were only done for 3 of the 28 complexes. The oldest of the

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remaining 25 igneous complexes is 435 ± 8 Ma based on Rb-Sr (Karche and Vachette,

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1978). Extrusive Cenozoic volcanic rocks have minor occurrences in the southern

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portions of the Aïr Massif as cinder cones and small (1-3 km in diameter) lava flows

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(Persits et al., 2002).

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2. Methods

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2.1 Data

Remote sensing analysis of the circular feature and surrounding regions,

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including within the Aïr Massif, was completed utilizing various satellite data. C-band

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radar Sentinel 1A images were acquired on 29 May 2016 (Frame 521, path 22; spatial

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resolution 20 m). Additional C-band data from the Shuttle Radar Topography Mission

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(SRTM) was acquired on 11 February 2000 (spatial resolution 1 arc second). Landsat 8

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OLI (Operational Land Imager) Level 1T radiometric and terrain corrected satellite

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images were acquired on 16 April 2016 (circular feature: scene path 189, row 045; Aïr

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Massif: scene path 189, row 047, spatial resolution 30 m). The Landsat 8 OLI Quality

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Assessment band in each image was screened for and found to have minimal

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irregularities. Sentinel 2A MSI (Multispectral Instruments) Level 1-C processed data

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were accessed from both the USGS (United States Geological Survey) Earth Explorer

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Website and the European Space Agency’s (ESA) Copernicus Open Access Hub

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website. Scenes were checked for passing quality inspections and had average cloud

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cover of <2.3%. The Sentinel 2A scenes required a range of dates to maintain data

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quality, but were chosen to be as close together temporally as possible while still

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passing inspection standards (circular feature, Cambro-Ordovician Sedimentary Rock

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East of circular feature, Northern Aïr Massif, Southern Aïr Massif: February 3, 2016;

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Devonian Sedimentary Rock West of circular feature: March 7, 2016; circular feature

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Scene: February 20, 2016; spatial resolution 10, 20, and 60 m based on band). All

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Sentinel 1A, Landsat 8 OLI, and SRTM data were obtained through the USGS Earth

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Explorer website. Additional CNES/SPOT (French Space Agency, Centre National

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d’Etudes Spatiales) and Landsat 7 ETM+ (Enhanced Thematic Mapper Plus) imagery

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utilized by the program Google Earth (Google, Inc.) was used to display various

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geologic features. Multispectral processing and classifications were completed using

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ENVI 5.1 and 5.3 (Excelis Visual Information Solutions). Histogram cutoffs, map

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displays, and 3-D rendering were completed with ArcGIS 10.3 and 10.4 (Esri, Inc.).

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Band ratios (dividing one spectral band by another to produce a false-grayscale output

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band), band math (addition or subtraction of input spectral bands to produce a false

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output), and false color composites were used to obtain comparison results for the

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various sites. Commonly used Landsat 7 ETM+ bands for band ratio analyses (Sabins,

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1999; Kusky and Ramadan, 2002; Gad and Kusky, 2006; Schmieder et al., 2013) were

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initially converted to comparable Landsat 8 OLI bands and finally to Sentinel 2A MSI

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bands (Werff and Meer, 2016) (Fig. 3). Sentinel 2A data were recognized by ENVI as 3

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separate datasets based on 10, 20, and 60 m resolutions. Classification required

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loading each band individually into ArcGIS, and exporting the 20 and 60 m resolution

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datasets to a resized 10 m resolution so all bands had the same number of pixels.

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Exporting was used instead of re-sampling to preserve the data values of the original

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pixels. The bands could then be merged with the Composite Bands tool for

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classification as a single dataset in ENVI.

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2.2 Analysis

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2.2.1 Structure and Morphology

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Several different modes of analysis were used to better characterize the

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morphology of the circular feature. Since the circular feature is partially obscured by

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eolian deposits, radar data from Sentinel 1A and the SRTM were used to resolve its

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subsurface structure. Longer wavelength radar bands have the greatest penetration,

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especially in dry soils such as desert sand, with the L-band being most commonly used

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to penetrate 1-2 meters and P-Bands up to 6 meters (McCauley et al., 1982; Schaber et

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al., 1986). Unfortunately, no L- or P-band data are currently available for the region, so

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data from Sentinel 1A, which carries C-band Synthetic Aperture Radar, was used. C-

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band still has the potential to penetrate up to a depth of ~0.5 meters of sand in these

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arid regions (Schaber et al., 1986; 1997).

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2.2.2 Classification Techniques Classification techniques were used to characterize the various mapped (e.g. Black, 1967) rock types that make up the region around the Aïr Mountains and circular

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feature in order to confirm the lithologies that define the circular feature. These

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classifications were initially completed with Landsat 8 datasets but yielded ambiguous

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results. Results from the higher resolution Sentinel 2A (S2A) data were used in this

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study because they produced better defined spectral distinctions between lithologies.

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Ten pixels were chosen for each endmember composition, based on previously mapped

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exposures, and were plotted on a spectral chart (Fig. 5). Pixels were selected to

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represent a range of reflectance values.

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Data products from Sentinel 2A provide higher resolution at 10 and 20 m for nonatmospheric correction bands. The 60 m resolution bands 1, 9 and 10 were not used

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due to their atmospheric use (meaning low atmospheric transmission for ground based

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sensing) and low resolution. Without the 60 m bands, the areal resolution of Sentinel

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2A is between 100 and 400 m2 compared to 900 m2 for Landsat 8, an increase in

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resolution of 2.25 to 9 times per pixel. This makes Sentinel 2A a promising candidate

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for geologic remote sensing as shown by initial comparisons between data products of

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the two satellites (Mandanici and Bitelli, 2016; Werff and Meer, 2016).

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Even with the improved 10-20 m resolutions of Sentinel 2A data, the possibility

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was not ignored that the units that comprise the circular feature could be a mixture (e.g.

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plutonic and sedimentary rock) resulting in multiple materials occurring within a single

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pixel. In order to address this issue, a matched filtering technique was used, which

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visualizes the response of a known end-member by checking for spectral alignment

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within each pixel. This minimizes the appearance of unknown end-member spectra or

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noise by checking for the presence of the known spectra within a pixel and maximizing

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the signal to noise ratio. This technique was used because it does not require

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knowledge of every possible end-member within a scene and provides a way to rapidly

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detect for specific materials. A spectral library was built from pixels that occurred within

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regions of interest that comprised end-members of potential rock and land cover types

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around the circular feature: desert sand, rocks of the Aïr basement, Aïr intrusive rock

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from the ring plutons, Aïr Cenozoic extrusive volcanic rocks, and Devonian sedimentary

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rock. These were confirmed using the geologic maps and visual inspection of Sentinel

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2A satellite images of the region. Resulting images were histogram-stretched so that

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positive correlations with the respective end-member are displayed as brighter pixels

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within the scene. A blue to red color ramp was then applied to each scene making

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variations easier to see, where red indicates a positive correlation to an end-member

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and blue a negative correlation.

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Additional distinctions in composition were found between the sites using a

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variety of band ratio and band math methods. Band ratios require dividing one spectral

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band by another to produce a false output band; band math is the addition or

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subtraction of input spectral bands to produce a false output; and false color composites

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display the false output bands as RGB colors to visualize compositional differences

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between the various sites. Band math/ratios used in the present study were converted

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from older Landsat 7 studies seen in Table 1.

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Lastly, the Supervised Spectral Angle Mapper (SAM) Classification was compiled for the Sentinel 2A data. This technique requires user input to select spectral

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endmembers. The software then calculates the angle between spectra and groups

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them from their angular similarity rather than overall reflectance values. This helps to

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eliminate reflectance based bias from illumination and albedo effects, i.e. if an outcrop is

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in shadow versus full sun. Different materials, rock types and soil cover, were selected

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based on geologic maps of the region (desert sand, Cambro-Ordovician sedimentary

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rock, Devonian sedimentary rock, ring complex rocks, and basement rocks and

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Cenozoic extrusive volcanic rocks). The SAM classification technique was applied to

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the scene containing the circular feature as well as two stitched scenes that comprised

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the majority of the Aïr Massif. To improve the confidence of the classification, each

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endmember was averaged from 500-1000 pixels and a higher discrimination of 1/20th

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radian or ~2.86° was used between angles for a spec tral match.

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2.3 Results

Radar images of the circular feature show the morphology of the ring structure

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and the central portions (Fig. 4a). The central structure is more apparent as a

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continuous body of rock as opposed to random individual outcrops. An annular

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depression is still seen, covered by desert sand that is thicker than the penetration

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depth of the sensors, followed by the outer annular ring. Figure 4B shows a C-band

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image from the SRTM color sliced to reveal an ancient drainage channel cutting across

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the eastern half of the circular feature. This erosional modification explains the

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discontinuous nature of the annular ridge as well as the possible asymmetry of the

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central topographic high. As mapped by Black (1967), the central topographic high contains outcrops of

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the Proterozoic Proche Ténéré Formation at higher elevations than surrounding, and

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stratigraphically higher Idekel Sandstone. While there is no direct constraint on the

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thickness of the units at the site, the Idekel Sandstone has been reported to be as thick

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as ~155 m while the Proche Tenere Formation is approximately 3000 m thick (Fabre

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and Betrand, 1983). Additionally, in regards to central uplifts, it is common for oblique

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impacts to produce structures with bilateral symmetry such as the Jebel Waqf as

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Suwwan impact structure in Jordan, the Matt Wilson, Spider and Gosses Bluff impact

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structures in Australia, Upheaval Dome in Utah, and Sierra Madera in Texas (Wilshire

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and Howard, 1968; Abels, 2005; Scherler et al., 2006; Kenkmann and Poelchau, 2009;

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Kenkmann et al., 2017). The central uplift structures of the circular feature are difficult

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to ascertain due to erosion and cover, but bilateral symmetry is not obvious in the

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central topographic high. Faults are not visible in the satellite images or the processed

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Sentinel 2A data, but were mapped by Black (1967). On Google Earth, tabular layers of

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Devonian Idekel Sandstone can be distinguished in the outcrops of the annular ridge,

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but are absent in the central topographic high. The Proche Ténéré Formation is

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exposed centrally and is prone to gentle folding throughout the region, but complex

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folds have not been shown to occur (Fabre and Betrand, 1983). Black (1967) mapped

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12 faults within the circular feature that occur in the Idekel Sandstone having NE-SW

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orientations (Fig. 2B). Three faults occur within the Proche Ténéré and run NW-SE,

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similar to orientations seen in the region, and one arcuate fault exists along the

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southern outcrop of the Idekel Sandstone that represents the southern exposed rim of

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the circular feature.

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The results of the classification techniques display positive identification of the end-members and rock types within the study areas that are supported by previous

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geologic mapping by Black (1967). The spectral data between Landsat 8 and Sentinel

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2A (Fig. 5) indicates that desert sand has the highest reflectance values throughout the

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electromagnetic spectrum. With Sentinel 2A, what is notable is that the circular feature,

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sedimentary rock outcrops, and the Aïr weathering spectra all appear sub-parallel

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except for SWIR bands 6 and 7. Similarly, all the pluton types plot parallel to each other

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with slight differences in the SWIR bands. Ring complexes and weathering products

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from the Aïr Massif are distinct from the sedimentary and circular feature spectra. In

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both plots, Cenozoic extrusive volcanic rocks maintain a distinction from all other

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endmembers and display the lowest apparent reflectance.

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Matched Filtering results for the outcrops of the circular feature show at least some degree of correlation with all end-members except the desert sand (Fig. 6). The

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lowest overall positive correlation is with the intrusive igneous rock types except for one

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outcrop in the granitic end-member (Fig. 6E, F). A moderate correlation is seen with the

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extrusive volcanic rock that is not supported by the quantified spectral plots (Fig. 6D)

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and a high correlation is seen with both sedimentary rock end-members. The desert

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sand surrounding the outcrops of the circular feature shows a high correlation with the

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desert sand end-member as expected, but also displays a negative correlation with the

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outcrops of the circular feature, indicating that the classification was effective (Fig. 6C).

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Both the Cambro-Ordovician and Devonian sedimentary end-members displayed highly

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positive correlations with the outcrops of the circular feature, however, the Devonian

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end-member also shows a correlation with the weathering products immediately

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surrounding the outcrops (Fig. 6A, B). The band math and band ratio results that displayed the greatest contrast in

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rocks types can be seen in Figure 7. The true color images (Fig. 7A-B) display exposed

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rocks of the circular feature which appear appreciably darker than those of the pluton.

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This same difference in shades is apparent in the 753 RGB composite images (Fig. 7C-

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D). Weathering products around each feature display variable colors which are an

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indicator of compositional differences (Yazdi et al., 2013; Bishta et al., 2015). Structural

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differences can also be distinguished as well with this composite (Kusky and Ramadan,

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2002). Differences in outer ring thicknesses and continuity are apparent, and the

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internal outcrops of the circular feature show irregular geometries compared to the

331

distinct fracture patterns within the ring complex. Bright pixels in short wave infrared

332

(SWIR) band ratio 6/7 images (Fig. 7E-F) highlight hydroxyl-bearing alteration, silicate

333

alteration, laterite, and argillic rocks (Chavez and Kwarteng, 1989; Kusky and

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Ramadan, 2002; Meer et al., 2012; Werff and Meer, 2016). The rock types appear to

335

be complete spectral opposites with the circular feature displaying a bright white tint

336

whereas the ring pluton shows deep shades of grey and black. The hydroxyl content is

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unique to the circular feature outcrops and is not seen in the desert sands around the

338

site. The 6/7 ratio image was combined with a false color composite showing bands,

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6/4, 4/2, and 6/7 in RGB respectively (Fig. 7G-H). This not only highlights the argillic

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content and alteration (bands 6/7), it also shows high overall iron oxide content in the

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4/2 ratio in hues of green and ferrous iron oxides with 6/4 in red (Chavez and Kwarteng,

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1989; Sabins, 1999; Meer et al., 2012, Werff and Meer, 2016). Band ratios and

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composite images reveal distinctions between the composition of the circular feature

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and the Aïr Massif rocks, yet these analyses only look at a limited portion of the spectral

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data provided.

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With the Sentinel 2A SAM classification (Fig. 8), the circular feature shows only sedimentary end-members with no plutonic signature. Some pixels remain unclassified,

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particularly as fluviatile deposition within the Aïr Massif, but this is expected with the

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strict ~2.86° angle of discrimination. All other e nd-members appear to be properly

350

classified. In Figures 8 B and D, proper classification occurs for the desert sand in tan

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as well as the ring plutons and basement rocks in tones of blue. Fluvial channels within

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the Aïr Massif classify as Cambro-Ordovician sedimentary rock, while the circular

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feature outcrops classify as a mix of Cambro-Ordovician and Devonian sedimentary

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rock. Cenozoic extrusive volcanic rocks were properly classified in the southern Aïr

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Massif, and do not occur further north nor in the circular feature scene.

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3. Discussion

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3.1 Structure and Morphology

Remote sensing and rock classification techniques provide important constraints

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on the possible origin of circular geologic features that are difficult to access in the field.

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This includes an understanding of the structure and morphology of such features

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compared to those having a confirmed origin. Ring intrusion complexes are commonly

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lenticular, but a raised outer perimeter does occasionally occur. Unlike the circular

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feature, minimal deformation is seen within the plutonic ring complex. If the circular

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feature were a true ring dike such as Meugueur-Meugueur, it is unclear if related

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outcrops would have formed within it. The presence of a drainage channel (Fig. 4B)

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could explain the asymmetric nature of the rocks within the interior, if these outcrops

368

constitute a possible impact-produced central uplift. When comparing the radar-derived

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structure as well as the true color composite of the circular feature (Figs. 4A, 8A) to the

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Imaghlane ring complex (Fig. 7B), the differences in the way the structures weather

371

becomes apparent. The weathering of the crystalline rocks of the Imaghlane ring

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complex appears to be controlled primarily by the network of joints and fractures that

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occur across its surface. This is in contrast to the more irregular, hummocky weathering

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that occurs at the circular feature.

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The circular feature appears similar to two confirmed impact sites in Chad (Fig. 9). These include the Aorounga impact structure with its nearby circular features as

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well as the Gweni Fada impact structure. Both of these sites have an estimated age of

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< 345 Ma and occur in sandstones believed to be of Upper Devonian age or younger

379

(Koeberl et al., 2005; Earth Impact Database, 2011; Reimold and Koeberl, 2014). The

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Aorounga and Gweni Fada structures are 16 km and 21-23km in diameter respectively

381

(Koeberl et al., 2005). The morphology of these structures bears similarities to the

382

circular feature. Similar to both Chad craters, the circular feature shows an annular

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ridge and central topographic high separated by an annular depression, and like the

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Gweni Fada structure, SRTM data of the circular feature displays modification of the

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circular structure by drainage channels.

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3.2 Classification Techniques

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When looking at the quantified spectra between the two satellite platforms (Fig. 5), with the Sentinel 2A data, the slopes between rocks of similar composition display

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similar trends with only minor perturbations of the parallelity between spectra seen in

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one or two bands. Parallel spectra represent similar compositions but can have

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different degrees of reflectivity, so the degree to which two spectra are askew between

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bands indicates the degree of variation in composition between the two for that portion

394

of the electromagnetic spectrum. Spectra from pixel counts for the higher resolution

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Sentinel 2A data sets reveal a much better correlation between the desert sand,

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sedimentary rocks, and the circular feature and indicate similar composition between

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them. Variations in reflectance between relatively parallel spectra indicate analogous

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compositions with variable grain sizes (Okin and Painter, 2004). The Devonian

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sedimentary rock endmember taken from outcrops ~250 km to the west (near 21.17°N,

400

6.77°E), presumably from the same strata as the hos t rock of the circular feature, shows

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the greatest correlation with it. However, a higher reflectance in SWIR bands 11 and 12

402

is evident and may be explained for a few reasons. Devonian sedimentary rock is the

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only endmember spectrum whose data is from a different date and orbital pass due to

404

its westerly location and excess cloud cover for datasets with better temporal resolution.

405

Differences in multiple variables such as atmospheric and orbital conditions may result

406

in differences between the datasets. The SWIR difference, where Devonian

407

sedimentary rock shows a relative increase in reflectance in Bands 11 and 12 when

408

compared to the circular feature, is possibly due to a slight variation in composition

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between the circular feature and the pixels of the Devonian sedimentary rock ROI, or it

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may indicate a difference in alteration or different amounts of influence from desert sand

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between the two. This would also explain the higher standard deviations seen between

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these pixels. Small perturbations in spectral angles between igneous rock and igneous

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weathering spectra may represent relatively minor differences in compositions. The

414

dissimilarity between materials weathered from the Aïr Massif and the circular feature in

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particular shows that the circular feature is not a weathered ring complex. Another

416

important note is how the Aïr weathering products plot in Figure 5B relative to the

417

sedimentary spectra and the igneous spectra. Even though the weathering products

418

are now sedimentary in nature and composed of grains, they are defined by a plot that

419

distinctly shows their genetic relation to their source rocks. This indicates that the plots

420

are more compositionally based as opposed to reflecting the physical characteristics of

421

sedimentary products such as grain size or sorting. Cenozoic extrusive volcanic rocks

422

show almost no apparent compositional relation to any other end-members in both

423

Landsat 8 and Sentinel 2A. It is clear that relative to Sentinel 2A, the Landsat 8 spectra

424

are not nearly as definitive at showing the major distinctions between rock types,

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presumably due to the differences in resolution between the two satellite platforms.

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It is interesting to note that in the results of Matched Filtering the igneous rocks

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show a moderate correlation with the sand cover surrounding the outcrops of the

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circular feature, particularly with pixels of obvious sand that did not correlate well with

429

the desert sand end-member. The sedimentary rocks also show moderate correlation to

430

the sand since one would expect they would share similar components. When applying

431

the same technique with Landsat 8 data, the distinction between sedimentary rock and

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desert sand was considerably less discernable, lending credence to the ability of

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Sentinel 2A for positive identification of land cover types. It is unusual that the Cenozoic

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extrusive volcanic rocks showed some correlation with the circular feature outcrops, but

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the intrusive igneous rocks also displayed some degree of correlation possibly owing to

436

some similarities in mineral content. This same correlation was not seen in the

437

quantified spectral plots or with the SAM classification. Regardless, the sedimentary

438

end-members showed the strongest correlation with the circular feature, in agreement

439

with the findings of the quantified spectral plots and with the SAM classification. The

440

Devonian sedimentary end-members showed the best correlation, and interestingly

441

showed a high correlation with the pixels within the central outcrop and coming off of the

442

annular ring complex outcrops. This is likely to indicate a correlation with the least

443

altered weathering products from the outcrops of the circular feature, as evidenced by

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the downwind correlation indicating transport direction of eroded material. If this

445

interpretation is accurate it would also indicate that the central topographic high was

446

more prominent before reaching its current state of low relief.

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The Sentinel 2A band math/ratio composites showed no appreciable differences, aside from increased resolution, over the original Landsat 8. This highlights how

449

previous studies were able to highlight differences in composition primarily by this

450

method. The shortcoming is that band ratio/math analyses are limited by the number of

451

bands that can be utilized, which limits the overall usage of the electromagnetic

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spectrum relative to classification techniques that “see” a greater proportion of the

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spectrum during a single analysis.

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Following the SAM classification, the fluvial channels within the Aïr Massif, as

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well as parts of the circular feature, classified as Cambro-Ordovician sedimentary rock.

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This is somewhat unusual when looking at the differences in spectral plots for similar

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end-members with Sentinel 2A data. Aïr weathering products showed a distinctly

458

different profile from both sedimentary plots. Older Cambro-Ordovician sedimentary

459

rocks do onlap onto the Aïr Massif and may have overlain the exposed basement rocks

460

at some time in the past only to be eroded and deposited into these drainages. Air

461

weathering products may be classified as Cambro-Ordovician sedimentary rocks,

462

unclassified, or a combination of the two. However, the current geologic maps do not

463

indicate these details at this scale. Also, the spectral plots used very specific pixels to

464

obtain accurate spectra of pure endmembers at the pixel scale, while the SAM

465

classification used larger regions of interest containing 500-1000 pixels to obtain more

466

complete average spectra. The large pixel base has an increased chance of

467

contamination from other materials but also provides less bias when discriminating

468

between pixels containing mixed media during classification of a scene as a whole. The

469

circular feature outcrops classify as a mix of Cambro-Ordovician and Devonian

470

sedimentary rock on the north and south face, respectively of almost every outcrop.

471

This may be due to differences in illumination at the time the datasets were acquired,

472

which is supported by the circular feature reflectance plotting between the two

473

sedimentary rock endmembers in Figure 5B.

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3.3 Possible Circular Feature Origins The circular feature did not show any relation to plutonic rocks in the Sentinel 2A

477

classification. One reason for this may be that the circular feature is in fact a ring pluton

478

that has different spectral characteristics from the Aïr Massif complex due to

479

incorporation of the host sedimentary rock during emplacement. Past geologic mapping

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expeditions indicated the circular feature outcrops are sedimentary rock, and the

481

present study supports this hypothesis. A wide variety of igneous rocks types occur

482

within and around Aïr, so it would seem more likely to be an igneous analog if

483

classification of the circular feature displayed a greater variety of associated rocks or an

484

increased relation to the weathered portions of the Massif in the spectral plots.

485

Microgabbro and dolerite intrusions occur minimally within the Idekel Sandstone, but

486

these appear as very small (
487

the circular feature represents an extension of the igneous complexes, then the Early

488

Devonian Idekel Sandstone would have been intruded shortly after deposition. Black

489

(1967) didn’t report any intrusive rocks in or around the circular feature. Along the same

490

lines, a ring pluton is unlikely as they occur exclusively in Proterozoic basement, not in

491

overlying formations, with the northern most known occurring about 94 km to the south

492

at Adrar Bous.

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An extrusive volcanic origin is also unlikely. The quantified spectral plots of Cenozoic volcanic rocks showed no genetic relation to sedimentary or other igneous

495

rocks around the Aïr region. Extrusive volcanic rocks within the Aïr Massif were either

496

unclassified or showed up as plutonic rock in the SAM classification. Additionally, none

497

of the extrusive formations in the region approach the size of the circular feature, they

498

are known to only occur in the southern Aïr Massif, and all of them display a

499

considerable drape of lava flows and ash that is not present around the circular feature.

500

The morphology, classifications, and age of the circular feature supports the possibility

501

of it being an eroded complex impact structure similar to Aorounga and Gweni Fada

502

craters, but other types of sedimentary formations may explain its appearance. The

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circular feature is also similar in appearance to the mud volcano ruin northwest of the

504

Chandragup mud volcanoes in Pakistan, but no such structures are known to exist in

505

the Saharan region (Delisle, 2004; Schmieder et al., 2013). The geology of the region

506

does not indicate the presence of any mud-related structures, save for Silurian

507

graptolites shales and Proterozoic schists, unlikely candidates for producing mud

508

volcanoes. If a domal pluton or salt diapir exists below the surface at the circular

509

feature, then this could explain uplift and subsequent erosion of sedimentary strata in a

510

concentric shape. However, based on Black’s (1967) published map, the Idekel

511

Sandstone itself is fluvial in origin with Proterozoic molassic metamorphic rocks below.

512

So, it seems unlikely that evaporites would be present in these units.

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Impact structures on Earth larger than ~2-4 km in diameter have a complex

514

morphology resulting from the rebound and structural uplift of the target rock at the

515

crater center during an impact event (Grieve, 1983; French, 1998; and Mohr-Westheide,

516

2011). Central uplifts in complex impact structures display distinct structural properties:

517

(1) Complex impact structures are typically characterized by structural uplift (by roughly

518

one tenth of the crater diameter) at their center (Grieve and Therriault, 2004), and (2)

519

the central uplifts of impact structures are typically characterized by a complex system

520

of faults and folds that are commonly exposed and visible in arid regions, unless the

521

structure is buried by sand and other younger sediments (Wilshire and Howard, 1968;

522

Kenkmann et al., 2017). Since the central topographic high contains stratigraphically

523

lower units of the Proterozoic Proche Ténéré Formation surrounded by the younger

524

Idekel Sandstone, this may provide evidence consistent with a central uplift and

525

exhumation and exposure of older units. Although bilateral symmetry is not obvious in

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the central topographic high, symmetry may have existed before modification via

527

drainage channels. According to Black (1967), the orientation of faulting is irregular

528

relative to the general orientation of the rest of Niger’s fault system. Within the circular

529

feature, mapped faults run NE-SW, at 90° to the NW- SW orientations seen elsewhere in

530

the region. While a few faults outside the circular feature in Figure 2B do show the NE-

531

SW orientation, there is a much higher concentration occurring within the circular

532

feature particularly in the Idekel Sandstone. Additionally, the annular faulting previously

533

mapped around the southern concentric ridge is similar to that of an impact structure.

534

The Proche Ténéré Formation is prone to gentle folding throughout the region but since

535

no images show high enough resolution to compare the outcrops of the central

536

topographic high to others nearby, there is no way to confirm from the available image

537

data if there is a difference in the degree of deformation within the circular feature

538

versus exposed outcrops of the unit elsewhere in the region. While the metamorphic

539

rocks in the area are gently folded, none of them display a circular pattern similar to that

540

of the circular feature. Folding of the underlying Proche Ténéré formation occurred prior

541

to the deposition of Devonian units, so there would be no obvious reason during

542

deposition for the Idekel Sandstone to produce a resistant annular ridge around the

543

circular feature.

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4. Conclusions

Remote sensing investigations of West Africa reveal a roughly 10 km in diameter,

547

circular feature in north-central Niger. The circular feature is located within Early

548

Devonian fluvio-marine sedimentary strata about 100 km to the north of a suite of

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igneous ring dikes with similar morphologies located in the Aïr Massif. C-band radar

550

data reveals a central morphologic high and thinner outer ring for the circular feature not

551

seen within the ring complexes. Sentinel 2A data allowed for distinct classification and

552

indicates that the circular feature is sedimentary, not igneous. Other sedimentary and

553

structural explanations for the circular feature are not supported by the local geology,

554

and extrusive volcanism is ruled out by classification and geologic maps. We propose

555

that the circular feature may have formed from a meteorite impact after deposition of the

556

Idekel Sandstone. Confirmation of an impact explanation for the circular feature would

557

require sample analysis and inspection for shock-metamorphosed materials.

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Acknowledgements: Support was provided by SSERVI Cooperative Agreement

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#NNA144AB07A subcontract 02235-06 to TJL. This study benefitted from discussions

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with and help from Henry Chafetz, Shuhab Khan, Melissa Lobpries, Diana Krupnik, and

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Unal Okyay. We appreciate the very helpful manuscript reviews by Martin Schmieder

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and Christian Koeberl.

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Figure Captions

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Figure 1. Google Earth Image of the circular feature, a roughly 10 km in diameter

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circular feature at 21°21'14.56"N Latitude and 9° 8 '32.24"E Longitude. An outer rim is

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visible with some scattered outcrops seen within.

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Figure 2A. Geologic Map of Niger and the surrounding region’s surface geology.

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Precambrian rocks are basement units with younger sedimentary strata of the indicated

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age overlying it. Intrusive and extrusive rocks are noted separately with a hashed

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appearance. The circular feature is noted to the north in red. The ring plutons of the Aïr

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Massif occur as Paleozoic intrusive rocks in the Precambrian units due south of the

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circular feature, noted in blue. Cenozoic extrusive volcanic rocks are seen as mottled

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yellow, just south of the ring plutons.

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Figure 2b: A geologic map of the area surrounding the circular feature. Basement

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outcrops to the west, with later Proterozoic units resting unconformably above.

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Devonian sandstone drapes unconformably on the Proterozoic rocks. Dune fields

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encroach from the east, and faults have a general NW-SE orientation. Note the

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irregular outcrop and fault patterns displayed at the circular feature.

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Figure 3: Landsat 7 ETM+, Landsat 8 OLI, and Sentinel 2A MSI bands are compared.

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Bands commonly used for remote sensing for the identification of land cover types with

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Sentinel 2A and Landsat 8 require conversion from Landsat 7 bands based on the

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newer band numbers.

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Table 1: Landsat 7 ETM+, Landsat 8 OLI, and Sentinel 2A MSI bands are compared.

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Band ratios commonly used for remote sensing from studies prior to the launch of

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Landsat 8 require conversion from Landsat 7 bands based on the newer band numbers.

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Previous studies that utilized these ratios are listed along with their respective band

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math/ratio.

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Figure 4: Radar images show shallow subsurface features that would otherwise be

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obscured by sand cover. (A) Sentinel 1A composite polar image reveals additional

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details of the outcrops within the rim. (B) SRTM image enhanced with 200 raster color

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slices from the grayscale image. An ancient drainage channel is denoted by the red

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arrows, which explains eroded sections of the outer ring and inner outcrops

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Figure 5: Comparison between quantified data of (A) Landsat 8 OLI versus (B) Sentinel

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2A on bands used for classification. Spectra represent 10 averaged pixels for each

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endmember with error bars representing standard deviation of pixel reflectance.

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Figure 6: Results for Matched Filtering of Sentinel 2A data for the circular feature.

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Endmembers for each image are (A) Devonian sedimentary rock, (B) Cambro-

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Ordovician sedimentary rock, (C) desert sand, (D) Cenozoic extrusive volcanic rock, (E)

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Aïr plutonic rock, and (F) Aïr granitic basement. Red pixels indicate a positive

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correlation with the respective endmember, while blue indicates no correlation.

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Figure 7: Color Composite and band ratio images of Sentinel 2A data for the circular

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feature and Imaghlane ring complex. A, C, E, and G display the circular feature, while

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B, D, F and H show the Imaghlane ring complex. A and B are true color RGB

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composites of bands 432. C and D are 753 RGB composite images. E and F are 6/7

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band ratio images, and G and H are the 6/4, 4/2, and 6/7 RGB color composites.

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Figure 8: Sentinel 2A Supervised Spectral Angle Mapper Classification results for the

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circular feature (top) and the central Aïr Massif (bottom). A and C are true color images

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for bands 4, 3, and 2 while B and D display the results for the classification. The colors

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for the endmembers used for classification are tan = desert sand, dark red = Cambro-

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Ordovician sedimentary rock, bright red = Devonian sedimentary rock, light blue =

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basement rocks, dark blue = ring complex rocks, purple = Cenozoic volcanic rocks, and

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black = unclassified.

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Figure 9: This Google Earth image shows the relation of the circular feature to the

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Gweni Fada and Aorounga Craters in Chad. The Aïr Massif and its associated plutons

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can be seen to the South-Southwest of the circular feature. The morphology of these

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two confirmed impact structures displays similarities to that of the circular feature.

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Table 1

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7, 4, 2 RGB 5/7

7, 5, 3 RGB

12, 8, 3 RGB

6/4, 4/2, 6/7 RGB

11/4, 4/2, 11/12 RGB Gad and Kusky, 2006

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Band Math and Band Ratios converted from previoues studies Landsat-7 ETM+ Bands Landsat-8 OLI Bands Sentinel 2a MSI Bands Previous Landsat-7 Studies

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Highlights A 10 km in diameter circular feature is newly documented in north-central Niger.

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Remote sensing indicates it is unrelated to igneous intrusions or diapiric doming.

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The circular feature is potentially an impact structure.

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