R E S E A R C H A R T I C L E
Role of Fracturing in the Organization of the Karst Features of Azrou Plateau (Middle Atlas, Morroco) Studied by Remote Sensing Imagery
Faiza Menjour1•Toufik Remmal1•Mustapha Hakdaoui2•Fouad El Kamel1• Kawtar Lakroud1•Fouad Amraoui1•Iz-Eddine El Amrani El Hassani3• Benjamin Van wyk de vries4• Pierre Boivin4
Received: 13 July 2015 / Accepted: 14 December 2016 ÓIndian Society of Remote Sensing 2017
Abstract Karst formation geometry can be controlled by fractures and faults, and by other lithologies. Here we show the organization of kastic collapse features related to structures and to extensive basaltic lava flows in the Middle Atlas of Morocco. A lineament map of major faults and fractures has been created for the Middle Atlas using Landsat 7 ETM?satellite images. This shows a dominant NE–SW regional direction and less prominent NNW–SSE and ENE–WSE directions. All these directions coincide with the alignments of karstic depressions that have formed in the Liassic limestones. The basaltic flows covering these formations on the Middle Atlas limestone plateau, have allowed the generation of cryptokarst, geometrically organized a long these major lineament directions. Karst landforms probabaly existed before the eruption of the lavas, but there were partly invaded by intrusions and volcanism. The extensive basaltic flows allowed for increased infiltration, and subsurface water flow, increasing the rate of kast formation after eruptions. Some basins show evidence of increased subsidence after lava emplacement (Aguelmam Sidi Ali Lake) and some maar- like craters also have subsided after eruption, by karts
formation. We lay out the structural and lithological con- trols on Karstic formation in an intraplate volcanic field based on limestones and evaporites.
Keywords Middle AtlasLandsat 7 ETM?Fracture Cryptokarst
Introduction
The Azrou plateau is part of the tabular Middle Atlas Causse (Causse=upland limestone plateau), and it is mostly underlain by Liassic dolomitic rocks and lime- stones. The Triassic clay-evaporitic appears particularly on the edge at the northwest end of the plateau (Martin1981;
Fedan 1989). There area is extensively covered by Neo- gene volcanoes arranged along east–west or north–south lineaments (Harmand and Moukadiri 1986). These volca- noes are themselves surrounded by extensive lava fields that cover a large part of the plateau (Fig.1).
The Middle Atlas Causse is considered to be the water tower and hydrological regulator of Morocco because of its heavy rain and snow precipitation, estimated to be about 730 mm per year (Bentayeb and Leclerc 1977). The karst process has been developed through a combination of cli- matic, lithological and structural factors as well as regional or local circumstances. The Karst has in turn contributed to the configuration of the Azrou plateau volcanism, and both have interacted to produce crypto-karstic landforms (Mar- tin1981).
When looking at collapse alignments in volcanic areas it is tempting to explain cavity alignments by the presence of underground networks, such as lava tubes, especially when the strings of holes that are nearly coalescing follow clo- sely the flow direction of the basaltic lavas. However many
& Faiza Menjour
f.menjour@yahoo.com
1 Laboratoire de Ge´osciences Applique´es a` l’Inge´nierie de l’Ame´nagement, Faculty of Sciences, Hassan II University, Casablanca, Morocco
2 Faculty of Sciences Ben M’Sik, Hassan II University, Mohammedia, Morocco
3 Institut Scientifique, Mohammed V University, Rabat, Morocco
4 Laboratoire Magma et Volcans, Blaise Pascal University, Clermont Ferrand, France
DOI 10.1007/s12524-016-0646-6
of the alignments of holes on the plateau cannot be linked in this way with the collapse of lava tunnels as they are oblique or even perpendicular to the direction of these flows. In addition, many of the collapse features are very large (several hundred meters wide) and are too large to be formed from tube collapse. Thus it seems that both vol- canic collapses and karst collapse may be present, and possibly combined volcano-karstic features.
This study aims to map the faults and fracture systems that could be susceptible to underground network development in Liassic rocks, to study their impact on the distribution of cryp- tokarstic cavities of Azrou plateau, and finally to explore the relationship between the volcanism and cryptokarst formation.
Morphostructural Aspects
The Middle Atlas is a NE–SW elongated intracontinental belts, organized into two structural units that are separated by the North Middle Atlas Fault ‘‘NMAF’’ (Martin1981;
Hollard1985; Charriere1992) (Figs. 1,2):
1. The Middle Atlasic Causse with elevation ranging from 1400 to 2000 m, consists of Jurassic and Cretaceous limestone plateau, which in a large part are covered by Quaternary alkali basalt lavas stretch- ing over 250 km2, among which rise Neogene mono- genetic volcanoes;
2. The Folded Middle Atlas, which offers a mountain landscape region, with greater elevation (900–2800 m), bounded by two NE–SW orientated faults, the NMAF and the South Middle Atlasic Fault (SMAF), which becomes Ait Oufella Fault (AAF) to the southwest. This latter fault describes complex duplex structures with southeast vergence and over thrusts the Middle Atlas onto the adjacent Missouri and Upper Moulouya basins. Inside the chain, several anticlinal ridges associated with major faults bounding large depocentres or sub-basins are individualized (Guigou-Timahdite, Bou Anguer, Ain Nokra, Bekrite, Boulmane) (Charriere 1984; Fedan 1989; El Arabi et al.2001).
Fig. 1 Geological map of the Middle Atlas (modified from Martin1981)
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The North Middle Atlasic Fault area between the Causse and the Folded Middle Atlas is sinuous and has a sinistral strike-slip motion (Martin1981; Gomez et al.1996). It is affected by one or more long faults that relay transverse faults oriented 120°N and 80°N (Fig.1).
The uplift of the Atlas system is probably due to a thermal dome (Zeyen et al.2005) and began in the upper- middle Miocene based on geophysical and tectonic-sedi- mentary considerations (Chellaı¨ and Perriaux1996; Gomez et al.2000; Babault et al.2008). This period is associated with intense magmatic activity highlighted by under
saturated-alkaline to sub-alkaline volcanism characterized by extensive lavas and small monogenetic volcanoes (Moukadiri 1999; El Azzouzi et al.2010). The main the volcanism initiated in the Eocene (El Azzouzi et al.1999), and has continued to the Quaternary similar to the timing in the Western European Rift (Merle and Michon 2001;
Cloetingh et al. 2005). A compilation of the average ages for sites suggests a migration of magmatic activity in the Middle Atlas to the North East (Guercif Basin) and south west (Siroua) (Missenard et al. 2006; Fullea Urchulutegui et al.2006).
Fig. 2 Colored altitude digital elevation model of the Middle Atlas, showing the topographic framework of the study area. Heights are in meters above sea level
Types of Volcano-Karstic Cavities
Holes, depressions and cavities that affect the basalt flows of the plateau, especially those close to Azrou, come in two main forms (Martin1981): the fluvio-karst corridors at the plateau edges and the cryptokarstic cavities (Fig.3).
The Fluvio-Karst Corridors
This is a system of small sinuous dry valleys developed at the contact between with basalt and limestone strata (Fig.4a). The basalt flows fill valleys, and produce steep slopes where eroded. The limestone hill slopes is gentler Fig. 3 The volcanic edifices (cones and maars) and cryptokarstic cavities of the Middle Alas plateau between Azrou and Timehdite
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and covered by colluvial deposits and coalescing blocky scree fans. The clearest examples are around Jbel Irfoud, Bou-Ikhte`ne and in the northeastern side of the Ain Kahla dome (Figs.3,4b). Sinkholes are formed at the contact as runoff water encounters the basalts or just behind the lava edge by infiltration directly into the basalt flow.
The Cryptokarstic Cavities
These correspond to collapse cavities puncturing the basalt lava flow surfaces, which are locally masked by an infill of red clay and silt. The most spectacular are cylindrical sinkholes or perfect funnels exceeding 40 m depth of 200 m in diameter. These have steep, clearly-defined rims with large basalt fallen blocks. Others are shallow flat- bottomed forms 5–10 m in diameter and more than 2 m depth (Figs.3,5).
All these structures are the result of a sudden or gradual subsidence of the basalt cover caused by removal of the underlying karst galleries. The arguments in favor of this
cryptokarstic origin are setting, the presence of alignments and evidence of collapse of the limestone substrate (Ter- mier1936; Martin1981).
However, it must be emphasized that some aligned depressions may to have formed during lava flow devel- opment, and can only be explained by lava tunnels with locally collapsed rooves (IFRI-Ouskra, Chedifate) (Figs.3, 6).
The volcanic activity at the Atlas plateau contributes to the accentuation of the karst processes. Indeed, the lava flows by their permeability favor the retention and the circulation of water, which increase the dissolution of limestone and/or evaporite bedrock (Martin 1981). This phenomenon has been observed in basaltic effusions that fill the depression of Aguelmam Sidi Ali (ASA) and par- ticipate in riding the lake at the bottom of which dispose karst sinkholes.
The eruptive succession of Lachemine N’Ait Lhaj maar conceals the collapse structures affecting the tuff ring deposits and the lava cascades (Mountaj et al. 2014) Fig. 4 aSketch of different types of basalt contacts with limestone
reliefs (modified from Martin 1974). b The cryptokarst chains elongated in the structural direction NE–SW and/or oriented in the
direction of basalt flows locally around J. Irfoud in plateau of Azrou sketched on image of Google-Earth
suggesting that maars are often deepened by karst pro- cesses post eruptive. There are many sinkholes that share the same general trends as the volcanoes, indicating a common structural link, and a possible use by magma of the karst system for pathways both to the surface and laterally.
Remote Sensing Applied to Mapping of Fault and Fracture Lineaments in the Middle Atlas Causse
The geological database used includes geomorphological maps of the Middle Atlas at 1:100,000 scale (Martin1981) and the geological map of Azrou 1:50,000 scale (Ministry of Energy and Mines2005). In addition to these data, we
used Landsat 7 ETM?(Enhanced Thematic Mapper Plus) scenes p200r037 and p201r037, respectively acquired in May 2, 2000 and April 12, 2001 with a resolution of 30 m for all bands except the panchromatic band that have 15 m.
The digital image processing for the identification and extraction of lineaments is essential in mapping and structural geology over a large area, such as here (Siegal and Abrams 1976; Condid and Chavez 1979). The appli- cation of filtering processes (spatial and directional fre- quency) allows the highlighting of lithological and structural discontinuities. Recent studies have used these methods in different parts of Morocco for the purpose of risk assessment (Chotin et al. 1995; Ait Brahim et al.
2000), hydrogeological (Boutaleb et al.2009), or structural (Emran et al.1988; Mahmoud1996; Himyari et al.2002) Fig. 5 Cryptokarst cavities at the foot of the of Tit-Ougmar maar
(Southwest J. Hebri). Figure2illustrated by the ‘‘Types of Volcano- Karstic Cavities’’ Section shows a Cryptokarst in bucket with a flat
bottom (5–10 m in diameter and 6 m depth) locally covered by a red clay loam surface formation
Fig. 6 Sketch lava tub of Ifri- Ouska (modified from Martin 1974)
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and lithological mapping, particularly in the Middle Atlas (De Waele et al.2009).
The image processing methods used on previously georeferenced and orthorectified images include Principal Component Analysis (PCA), the color composition, spatial and frequency filtering techniques. The organization chart (Fig.7) summarizes the different processing used with Image processing and other GIS tools.
Principal Component Analysis
The Principal Component Analysis (PCA) is an operation made on multiple channels in order to improve image quality, remove duplication of information and compile data (Emran et al.1988; Ranjbar et al.2004).
In order to map most of the fractures affecting the basaltic Azrou plateau, we conducted a Principal Compo- nent Analysis by multi-sensor fusion of multi-spectral images of Landsat ETM?that has a spatial resolution of 30 m with a panchromatic image from the sensor that has a spatial resolution of 15 m.
Color Compositions
The combination of the channels through a RGB system (Red, Green, and Blue) gives a large number of color compositions (Fig.8). We have limited ourselves in this case the visible and infrared bands (VIR), the most rec- ommended in Earth Science (Scanvic1983; Emran et al.
1988; Nicolas et al. 2005). Only a few of these combina- tions were maintained, including the color composition of channels 7, 4 and 2, which provided a clearer and more
mixed picture; the channel 7 (SWIR) is interesting for the recognition of rocky environments, especially carbonates.
Spatial Filtering Techniques
The result of Principal Component Analysis (PCA) allowed us to select the first components with a good compression of the spectral information.
These images were used as entries for the application of more relevant techniques such as filtering (Bonn and Rochon 1992) for the identification of lineaments (litho- logical or structural discontinuities).
Filtering consist of applying in a neighborhood of the current pixel, a convolution of the image with a mask that represents the filter. This treatment allows us to detect discontinuity signals of the grayscale the current pixel relative to its neighbors within a particular direction (Touzi et al. 1988; Nezry et al. 1991). We use in this case as a treatment, directional filters of Sobel, Prewitt and Yesou.
The masks representing these filters (Fig.9)were designed to highlight specific features or hide specific characteristics of an image based on their frequency related to the texture (Himyari et al. 2002; Jourda et al.2006; Ta et al. 2008).
Applying filters to images from the treatment of PCA may increase by 20% on the whole listed lineaments (Himyari et al.2002).
Mapping and Analysis of the Fracturing
A map of new lineaments is drawn using the processed images. This map shows a high density of lineaments highlighted by the contrast of light and dark areas of variable extent from a few hundred meters to several kilometers length. The validation of this lineament map was done using structural data derived from the geological and geo-morphological maps (Martin 1981) and comple- mented by the field surveys.
Overall, the fracture map (Fig.10) highlights three structural directions NE-SW, NW–SE and EW. The NE–
SW orientation is the most dominant in number (65%) and extent (75%), followed by the transverse direction NW–SE (28, 18%) and to a lesser extent EW (8, 7%). Some of the main faults and lineaments are described below.
Tizi N’Tretten Fault
This fault intersects the whole plateau (Fig.1); it is sub- divided into two segments: an eastern one with a N40E trend and a western segment at N80E. This fault was limited to small amplitude vertical movements in the upper Miocene (Charriere 1990). Around Michlifene, this fault becomes puts limestone of the Middle Lias in overlapping contact with dolomite of the Lower Lias (Figs.8,11).
Fig. 7 Organization chart of satellite image processing and GDEM methodology
Fig. 8 Image in color composition RGB with bands 7, 4 and 2 of Landsat 7 ETM?
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1 1 1 2 1 1 1 1 1 2 3 2 1 1 1 2 3 4 3 2 1 0 0 0 0 0 0 0 -1 -2 -3 -4 -3 -2 -1 -1 -1 -2 -3 -2 -1 -1 -1 -1 -1 -2 -1 -1 -1
SOBEL filtre : N-S
-1 -1 -1 0 1 1 1 -1 -1 -2 0 2 1 1 -1 -2 -3 0 3 2 1 -2 -3 -4 0 4 3 2 -1 -2 -3 0 3 2 1 -1 -1 -2 0 2 1 1 -1 -1 -1 0 1 1 1
SOBEL filtre : E-W
+1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 -1 -2 -3 -7 1 1 1 -1 -2 -3 -3 1 1 1 -1 -2 -2 -2 1 1 1 -1 -1 -1 -1 1 1 1
PREWITT filtre 0 1 1 1 1 1 2
-1 0 2 2 2 3 1 -1 -2 0 3 4 2 1 -1 -2 -3 0 3 2 1 -1 -2 -4 -3 0 2 1 -1 -3 -2 -2 -2 0 1 -2 -1 -1 -1 -1 -1 0
SOBEL filtre : NE-SW
2 1 1 1 1 1 0 1 3 2 2 2 0 -1 1 2 4 3 0 -2 -1 1 2 3 0 -3 -2 -1 1 2 0 -3 -4 -2 -1 1 0 -2 -2 -2 -3 1 0 -1 -1 -1 -1 -1 -2
SOBEL filtre : NW-SE
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
YESOU filtre Fig. 9 Sobel, Yesou and Prewitt filters (matrix 797) serving to extract the lithological and structural discontinuities
Fig. 10 Detailed map of the lineaments of the Middle Atlas established from the analysis of Landsat 7 ETM?and GDEM images, illustrated on the first principal component (Landsat, 15 m)
Lbouatene—Jebel Irfoud Fault
Jebel Irfoud is a monoclinal ridge dipping 30°to the south (Figs.8,12), bounded on the north by an N70 sub-vertical fault which extends eastward to the anticlinal dome of
Lbouatene, formed essentially by liasic limestone pave- ments. This section is affected by small scale N20–30 oriented fractures cause by sinistral strike-slip faulting (Harmand and Moukadiri1986).
Fig. 11 Localization of Tizi n’Tretten fault in the North of the Michlife`ne Maar sketched on image of Google-Earth
Fig. 12 The Jbel Irfoud fault with relays in the limestone dome of Lbouate`ne sketched on image of Google-Earth
Fig. 13 Aguelmane Sidi Ali fault sketched on image of Google-Earth
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Aguelmam Sidi Ali Fault
This forms part of the North Middle Atlas Fault that separates the tabular and folded Middle Atlas. It is sinuous and has a NE–SW to ENE–WSW sinistral strike-slip fault movement (Martin 1981; Gomez et al. 1996). It is associated with several fault splays with 120°N and 80°N directions (Figs.8, 13). This fault had first already operated in extension to form the Aguelmam lake depression, probably during the upper- middle Pliocene (Hinaje2004). At that time, the regional tectonic regime was extensional and oriented NNW–SSE, producing N60°–N90°normal faults that formed the ENE–
WSW to NE–SW oriented basin that are now partly filled with fluvio-lacustrine sediments and lavas.
In the Neogene, the volcanic activity concentrated in a small pull-apart basin to the north of the principal fault.
The pull-apart was limited to the north by a major N50 fault and to the east and west by N160 normal faults.
The arrangement of volcanic vents as chaplet oriented NNW–SSE (Fig.14) is only an apparent alignment. The emplacement of these volcanoes is rather in favor of NE–
SW faults or into the relay areas of these faults, as the case in ASA and other structures parallels in the northern of plateau notably the North Middle Atlasic Fault and the Tizi N’Tretten Fault (Harmand and Moukadiri1986).
The principal’s directions enumerated above reflect the evolution of the Middle Atlas chain since its initialization in early Triassic, in favor of a rift system developed in the path of NE–SW faults inherited from the Hercynian history (Hafid 1999; Frizon De Lamotte and Zizi2008). The car- bonate platform is installed at the Lower Lias consecu- tively to thermal subsidence and has dislocated since the Fig. 14 Distribution Map of
mio-Quaternary volcanism with the major structural alignments of the Middle Atlas
Middle Lias into a mosaic of tilted blocks by the activation of longitudinal and transverse faults (El Arabi et al.2001;
El Hammichi et al.2007).
Structuring of the chain is acquired in the Cenozoic (ante-Late Miocene). The Cretaceous and Tertiary forma- tions (Paleocene–Eocene) are deformed as synclinal basins parallel to the longitudinal wrinkles NE–SW. Folding is accompanied by a sinistral shear movement in view of the obliquity of the Middle Atlas chain relative to the short- ening direction NS (Fedan1989; Charriere1990).
Discussion
The digital lineament extraction method offers a better analysis of satellite imagery by locating with more accu- racy the position and direction of faults and fractures than from previously made maps based on fieldwork (e.g. Refs).
The interpretation of the filtered images revealed previ- ously unreported lineaments and increased the density of structures on known ones. Lineaments we detected with the following orientations, in order of occurrence: N40°–N60°, N150°–N170°and N100°–N120°.
The NNW-SSE revealed by this study is poorly repre- sented across the plateau. The fractures are rare and often interspersed even if we can admit the screen effect caused by the outpouring of basaltic nappe.
The surface distribution of the karst system fits with the distribution of the various lineaments (Fig.15). The model of spatial density of karst holes compared with the network of fractures shows this relationship (Fig.16).
The dissolution phenomena are the result of different interactions between humid climate (1000 mm of Mediterranean rainfall on the Azrou plateau), altitude (&2000 m), lithology and tectonics. The meteoric waters drain off the Liasic uplands and basalt flows, and infiltrate Fig. 15 Concordance map of
cryptokarsts and nivokarsts chains with the major structural alignments of the Middle Atlas
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through weakness areas, such as faults, fractures, and joints. The underground galleries are eroded-dissolved within the Liasic substratum. Possibly enhanced in areas of greater rainfall where more water can drive more through flow.
The cryptokarsts developed in the Quaternary basaltic flows revealed particularly the relationship to faulting, as illustrated by the cryptokarst collapses detected around Lachemine N’Ait Lhaj volcano that are arranged into two alignments in the direction of the N60 and N160 faulting (Figs.8,17). The emplacement of lava above underground galleries causes a collapse under the final load which results in the various forms of depressions seen.
Conclusions
We need a point by point conclusion here.
1. The Middle Atlas of Morocco is formed of mainly liassic limestones and dolomites, largely covered by extensive lava flows erupted from monogenetic volcanoes.
2. Karst has formed before and after volcanism, in response to the high rainfall. The Karst features follow the tectonic structures which present an essential factor in process of dissolution, and develop in relation to the particular limestone-lava geometry. Major sink holes develop at the lava edges.
Fig. 16 Spatial density model of the karst cavities in the Middle Alas plateau
3. Lava flows allow for greater infiltration and water retention, further driving dissolution and promoting cryptokarst formation.
4. Lineament analysis from remote sensing has identified the fault network of the middle Atlas, the new lineaments which was hitherto not mapped or updated in the maps of Morocco geological service, and the lineaments show a strong correlation with the karstic structures. Where lineaments are denser, there are more collapse features.
5. Some small collapse features may be lava tube collapse, and some craters may have enlarged by karstic collapse.
6. Possibly, dykes also have followed the structural trends, affecting subsurface water flow, and generating kastic collapse along the dyke strike.
Acknowledgement This work is part of the framework of the research project entitled: Multidisciplinary research on the Geoma- terials and Volcanic Geosites of Morocco: need for their valorisation Fig. 17 Crossing of two alignments of cryptokarst cavities around the Lechmine-n-Ait-el-Lhaj crater sketched on image of Google-Earth
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and exploitation in the prospects for a sustainable development, supported and financed by Hassan II Academy of Sciences and Techniques and staged in partnership with a consortium of institu- tions composed of the Faculty of Sciences of Hassan II University—
Casablanca, the Scientific Institute in Rabat and the Faculty of Sci- ences and Techniques of Hassan II University—Mohammadia.
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