This study concerns the geological interpretation of aeromagnetic data
of Bou Chber region located in Central Morocco. Processed magnetic maps
obtained through different standard methods known in aeromagntism were
compared to field geological structures and ancient geological maps. We
propose then a geological scheme of the Bou Chber non-mapped area based
on the newly interpreted magnetic anomalies of this regionEn este estudio se presenta una interpretación geológica de datos aeromagnéticos
en la región de Bou Chber (Marruecos Central). Se han usado
mapas aeromagnéticos procesados con métodos convencionales en aeromagnetismo,
que se comparan con estructuras geológicas de campo y con
mapas geológicos antiguos. Se propone un nuevo esquema geológico de la
zona de Bou Chber según la interpretación de anomalías magnéticas aquí
descrit

Bouya, Nawal

El Ouardi, Hmidou

Arias Montano: Institutional Repository of the University of Huelva

ABSTRACTThis study concerns the geological interpretation of aeromagnetic dataof Bou Chber region located in Central Morocco. Processed magnetic mapsobtained through different standard methods known in aeromagntism werecompared to field geological structures and ancient geological maps. Wepropose then a geological scheme of the Bou Chber non-mapped area basedon the newly interpreted magnetic anomalies of this region.Key-words:Aeromagnetism, field geology, magnetic mapping, Central Mo-rocco.RESUMENEn este estudio se presenta una interpretación geológica de datos ae-romagnéticos en la región de Bou Chber (Marruecos Central). Se han usadomapas aeromagnéticos procesados con métodos convencionales en aero-magnetismo, que se comparan con estructuras geológicas de campo y conmapas geológicos antiguos. Se propone un nuevo esquema geológico de lazona de Bou Chber según la interpretación de anomalías magnéticas aquídescrita.Palabras clave: Aeromagnetismo, geología del campo, cartografía mag-nética, Marruecos Central.Geogaceta, 56 (2014), 39-42. Fecha de recepción: 28 de enero de 2014ISSN (versión impresa): 0213-683X Fecha de revisión: 29 de abril de 2014ISSN (Internet): 2173-6545 Fecha de aceptación: 30 de mayo de 214Copyright© 2014 Sociedad Geológica de España / www.geogaceta.comIntroductionAeromagnetic data have been used re-cently in Morocco for geological mappingafter its extensive use in the miningprospecting (Bouya et al., 2013a, b; Bouya,2014) and throughout the world mainly inregional geological studies (Galdeano,1980; Blakely, 1995; Grauch et al., 2001;Aspler et al., 2003; Naba et al., 2004;Arya-manesh, 2009; Groune et al., 2009; Ran-drianasolo, 2009).Taking into account mag-netic susceptibility, it allows highlighting dif-ferent petrographic types at different mapscales (Gleizes, 1992; Bouchez, 1997).However, it is always difficult to assign amagnetic anomaly to a specific geologicalfacies (Henkel, 1991; Clark, 1997; Naba etal., 2004; Naba, 2007; Randrianasolo,2009). The data used in this study havebeen processed with the Oasis Montaj soft-ware.The data were acquired during a pro-gram of measurements organized by Mo-roccan Ministry of Energy and Mines. Inorder to use these techniques, we have dig-itized data and built a database; so that wecan compare numerical anomalies and ge-ological maps of the region.The aim of this work is twofold, first toprocess airborne magnetic data of the BouChber area, and then to define the geolog-ical significance of the detected magneticanomalies.Geological setting, data andmethodsThe study area corresponds to the BouChber region located at the Eastern part ofthe Central Moroccan Massif (Fig. 1). It is asegment of the famous Fourhal-Telt synclinetrending NE-SW alike the whole MoroccanMeseta structures. Stratigraphic series cor-respond to alternate clays and sandstonesattributed to the Visean-Namurian stage,which are overlain by Devonian-Visean al-lochthonous limestones. These series areNE-SW trend folded as the rest of CentralMoroccan Meseta.Aeromagnetic data used in this workwere acquired from airborne geophysicalsurvey in the Central Moroccan Massif, dur-ing April – June, 2000.The acquisition was performed using aEurocopter type AS35OB3, equipped with avideo recording system (PAL camera). Thesesurveys were flown with a fixed-wing aircraftalong NW-SE traverse lines spaced 500 mapart and 60 m altitude clearance, with NE-SW tie lines spaced 5 km. The magnetome-ter used was a Scintrex type cesium sensorof Geometrics, brought to a nominal altitudeof 30 m with a sensitivity of 0.01 nT.The values of the residual magnetic fieldobtained were interpolated to a 125 m cell-Geological interpretation of aeromagnetic data from Bou Chberarea, Central Moroccan MassifInterpretación geológica de los datos aeromagnéticos de la region de Bou Chber, Macizo Central de MarruecosNawal Bouya and Hmidou El OuardiCartoTec Research Group, Moulay Ismail University, Faculty of Sciences, Department of Geology, PB. 11201 Zitoune, Meknes, Morocco.nawal-geol@hotmail.com, hmidouelouardi@yahoo.fr39GEOGACETA, 56, 2014size grid. The method was to digitize all theintersection points between flight lines andisovalue curves available onto map (9950points were digitized) and later the pro-cessing was done by Oasis Montaj program.Once the database is compiled in Ar-cGis, it is exported onto Oasis Montaj to un-dergo standard procedures used in suchstudies.ResultsIt is known that magnetic anomaliescould be induced either by crystalline rocksmetamorphic basement or can be gener-ated by linear tectonic contacts (Grauch etal., 2006).The first obtained processed mapis the residual magnetic anomaly map thatwe use to apply different conventional fil-ters used in aeromagnetism. The reduction-to-pole (RTP) technique applied to the resid-ual magnetic field permits to obtain the RTPmap (Fig. 2A), in order to eliminate distor-tions caused by the tilt of the Earth’s mag-netic field vector. The RTP map will be usedas the base map for applying diverse math-ematical filters.So the RTP map or pseudogravity mapallows finding different geological contoursas well as major geological structures thatcharacterize this region.The obtained maps permit distinguish-ing areas of strong anomalies and others ofrelatively low response. On these maps, thepresence of four areas that show strongmagnetic anomalies (1 to 4, Fig. 2C) is de-picted. The north end of the map coincideswith the edge of the plateau of Agourai. Astrong anomaly E-W oriented in the east-central part of the map is related to theOued Ifrane. And finally, two strong anom-alies in the southern part of the map arealso represented (Fig. 2C). Moreover, theapplication of the Euler deconvolution tothe magnetic data in the region of BouChber, with a structural index of 0 and 3,allow us to obtain the maps presented inFig. 2D. So that, many geological contactsare observed, two semicircular contacts oc-curring in the southern part of the map, oth-ers NE-SW oriented in the NW area and fi-nally N-S linear contacts observed in theeastern part of the map. Euler map of struc-tural index (SI=3) confirms the aforemen-tioned structures and highlights a semicir-cular structure in the southern part of themap that could be caused by a deeper mag-netic source.Geological interpretation andconclusionsThe studied mapped magnetic anom-alies of different scales can be caused eitherby magnetic sources of different depths orinduced by the magnetic bodies of differentdimensions (Fig. 4). The first strong anom-aly, substantially elongated E-W, detectedin the northern part of the map at theboundary with the Agourai plateau, obvi-ously corresponds to the Triassic outcropsbordering this Mesozoic plateau (Figs. 3 and4). The second strong circular anomaly de-tected, a few km south, corresponds to Car-boniferous basalts, which have been studiedby Driouch et al. (2010). These are basicrocks interbedded within Visean-Namurianflysch of the Fourhal - Telt Synclinorium.Thestrong magnetic signature of these outcropscorresponds to their mineralogical compo-sition and geochemical characteristics ofbasic and ultrabasic rocks (Thomas et al.,2002). The third strong magnetic anomalyoccurring at the central part of the map,with a remarkably elongated E-W trend, co-incides with a Quaternary basalt flow, whichemphasizes the bedding of the Oued Ifrane,close to Amghasse town (Fig. 3). These dif-ferent Carboniferous, Triassic and Quater-nary basaltic bodies have been mapped andinterpreted in the geological sheet(1/100.000) of Aguelmous (Figs. 3 and 4).The Middle Atlas and the Eastern part ofCentral Moroccan massif is characterized bythe presence of basaltic Plio-Quaternary vol-canism which is a magnetic marker of thisGEOGACETA, 56, 2014 N. Bouya and H. El Ouardi40 Geofísica / GeophysicsFig. 1.- Geological map of central Moroccan Massif and location of the study area (square). 1) post-paleozoic; 2) Permian; 3) Namurian-westphalian; 4) LowerNamurian; 5) Visean; 6) Devonian-Dinantian; 7) Silurian; 8) Tardi-hercynian granite; 9) Lower Palaeozoic (Cambrian- Silurian). Colour version on the web.Fig. 1.- Mapa geológico del macizo central de Marruecos y ubicación del área de estudio (recuadrado). 1) Post-paleozoico; 2) Pérmico; 3) Namuriense-Westfaliense; 4) Namuriense inferior; 5) Viseense; 6) Devónico-Dinantiense; 7) Silúrico; 8) granito Tardi-Hercinico; 9) Paleozoico inferior (Cámbrico-Silúrico).Versión en color de la figura en la web.Geofísica / Geophysics 41Geological interpretation of aeromagnetic data from Bou Chber area, Central Moroccan Massif GEOGACETA, 56, 2014Fig. 2.- Processed magnetic mapsof Bou Chber area (colour versionon the web). A) RTP map; B) Verti-cal gradient map; C) Horizontalgradient map; D) Euler deconvolu-tion (SI=3) map.Fig. 2.- Mapas magnéticos proce-sados del área Bou Chber (figuraen color en la web). A) Mapa RTP;B) Mapa de gradiente vertical; C)Mapa de gradiente horizontal; D.Mapa de deconvolución de Euler(SI=3).Fig. 3.- Geological map of the Bou Chber area (extracted from the geologi-cal map of Morocco 1/1000000). Colour version on the web.Fig. 3.- Mapa geológico de la région de Bou Chber (extraído del mapageológico de Marruecos 1/1000000). Figura en color en la web.Fig. 4.- Geological scheme of Bou Chber area and interpretation of the mainmagnetic anomalies. Colour version on the web.Fig. 4.- Esquema geológico de la región de Bou Chber y de las principalesanomalías magnéticas. Figura en color en la web.magmatic province (El Azzab and El Wartiti,1998; El Azzab et al., 2001). Finally, thefourth strong anomaly was highlighted inthe southern part of the map. This anomalymay be caused by a deep magnetic source.From the intensity of the magnetic signal,we can assume that this deep source maycorrespond to buried basic magmatic rocksanalogous to basic flows exposed in thenorthern region of Bou Chber.The estimateddepth of this magnetic source from Eulerdeconvolution and the analysis of the mag-netic signal is about 2100 m deep (Figs. 4and 5). Geological field data permit to con-firm the presence of many basaltic dykesand sills in this area hosted in the Car-boniferous flysch.AcknowledgementsThis work is supported by the PHCMaghreb project MAG/11/01 (027/STU/11).We want to thank the Moroccan-FrenchVolubilis program. We thank also twoanonymous reviewers for their relevant re-marks and suggestions.ReferencesAryamanesh, M. (2009). World Applied ScienceJournal 6(10), 1429-1432.Aspler, L.B., Pilkington, M. and Miles,W.F. (2003).Interpretations of Precambrian basementbased on recent aeromagnetic data, Mcken-zie Valley, Northwest Territories. Geologicalsurvey of Canada, Current research-C2, 11 p.Blakely, R.J. (1995). Potential Theory in Gravityand Magnetic Applications. Cambridge Uni-versity Press, Cambridge, 441 p.Bouchez, J.L. (1997). In: Granite: from segrega-tion of melt to emplacement fabrics, (J.L.Bouchez, D.H.W. Hutton and W.E. Stephens,Eds.), Kluwer Academic Publishers, Dordrecht,95-112.Bouya, N. (2014). 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El Ouardi42 Geofísica / GeophysicsFig. 5.- Depth estimation of magnetic source from the magnetic signal (see location in Fig. 2A).Fig. 5.- Estimación de profundidad del cuerpo magnético (localización en Fig. 2A).

Interpretación geológica de los datos aeromagnéticos de la region de Bou Chber, Macizo Central de Marruecos

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Interpretación geológica de los datos aeromagnéticos de la region de Bou Chber, Macizo Central de Marruecos

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