**4.1 Lithology and deposition process**

The Lechmine n'Aït El Haj maar is located in the volcanic plateau near Azrou, in the eastern part of the province (**Figure 2**). The maar deposits are mainly formed of pyroclastic breccia consisting of accidental lithics extracted from the substrate

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**Figure 3.**

*evolution of the eruptive dynamics of the maar.*

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

associated with juvenile fragments, and volcanic ash. Several lithological sections were established around the rim of the LNH crater, using a graphical semiology that shows the cadence of the eruptive sequences and their variability according to the nature of the activity phase (**Figure 3**). Two criteria allowed understanding the environmental condition at the time of the eruption: The outcrop of the Liasic

The LNH maar is set on the limestone of the Middle Lias. It occupies less surface

On the northern flank, between the limestone and the first phreatomagmatic unit appears a 1,5 m thick level of reddish deposits (**Figure 4a**). This facies appears also in small outcrops cleared by streams (**Figure 4b**). It is formed by a mixture of fine tuffs and clay interspersed with centimetric levels of breccia with juvenile and limestone fragments (4 mm to 4 cm) (**Figure 4d**). This aspect indicates an

*Volcanic map of the maar of Lechmine n'Aït el Haj in the Causse of the Middle Atlas with logs describing the* 

comparing to the dolomite of the Lower Lias in the Causse of Middle Atlas. The distribution of limestone follows often the structural undulations in the region [6]. With their white color, their stratification is clearly visible. Typical karstic forms

limestone, and the lacustrine deposits in the northern flank of the maar.

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

(lapiez) are distinguished at the surface.

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*

associated with juvenile fragments, and volcanic ash. Several lithological sections were established around the rim of the LNH crater, using a graphical semiology that shows the cadence of the eruptive sequences and their variability according to the nature of the activity phase (**Figure 3**). Two criteria allowed understanding the environmental condition at the time of the eruption: The outcrop of the Liasic limestone, and the lacustrine deposits in the northern flank of the maar.

The LNH maar is set on the limestone of the Middle Lias. It occupies less surface comparing to the dolomite of the Lower Lias in the Causse of Middle Atlas. The distribution of limestone follows often the structural undulations in the region [6]. With their white color, their stratification is clearly visible. Typical karstic forms (lapiez) are distinguished at the surface.

On the northern flank, between the limestone and the first phreatomagmatic unit appears a 1,5 m thick level of reddish deposits (**Figure 4a**). This facies appears also in small outcrops cleared by streams (**Figure 4b**). It is formed by a mixture of fine tuffs and clay interspersed with centimetric levels of breccia with juvenile and limestone fragments (4 mm to 4 cm) (**Figure 4d**). This aspect indicates an

### **Figure 3.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

depression of the Guigou Valley and to the West by the Wadis of the Beht and Oum Rbia. Most of the volcanoes are strombolian (70%) disseminated in the entire volcanic area. on the other hand, maars are focused in the eastern part of the volcanic province [5] (**Figure 2**). The karstic-carbonate nature of the region combined with the fluctuations in rainfall contributes to the formation of the water table in the epikarstic level [34, 35]. The variation of the karstic water at this level has an impact on the volcanic dynamic and the changing of the eruptive style. In The MAVP, 8% of volcanic vents have witnessed a shifting between wet and dry dynamics during

*Geographical location of the study area. Most of the volcanoes and lava flow units are settled between Azrou* 

The Lechmine n'Aït El Haj maar is located in the volcanic plateau near Azrou, in the eastern part of the province (**Figure 2**). The maar deposits are mainly formed of pyroclastic breccia consisting of accidental lithics extracted from the substrate

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**Figure 2.**

the same monogenic eruption [5].

**4.1 Lithology and deposition process**

**4. Case of study: the Lechmine n'Aït el Haj maar**

*and Timahdite where the cryptokarstic cavities are dominating.*

*Volcanic map of the maar of Lechmine n'Aït el Haj in the Causse of the Middle Atlas with logs describing the evolution of the eruptive dynamics of the maar.*

### **Figure 4.**

*The lacustrine deposits at the base of the first phreatomagmatic unit (U1) of the maar of Lechmine n'Aït el Haj.*

explosive dynamism that occurred while the sediment was still rich in water. Towards the summit appears fine gray ash overlain by gray and reddish vacuolar and scoriaceous beds revealing the eruptions that preceded the hydromagmatic explosion (**Figure 4c**). A palynological analysis was carried out on these lacustrine deposits at the GEOBIO laboratory of the Scientific Institute of Rabat. It showed that these deposits are extremely poor in palynomorphs. The gray deposits are rich in bisaccate called "Pinus pollens" that have existed from the Paleozoic to the present day [36, 37].

The tephrostratigraphic sequence of deposits shows color and dip variation corresponding to four eruptive phases (U1–4). The first two units are describing a phreatomagmatic activity with heterogeneous lithology; U1 is highlighted by alternating beds of variable thickness rich in limestone blocks, while U2 shows relatively thin regular bedding compared to the first deposit, with a white basal level formed almost of blocks and fragments of accidental lithics overlain by another level of red

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*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

scoriaceous to finely pulverized fragments. The third unit (U3) results from strombolian activity. Its eruptive products are composed of massive breccia tuffs rich in volcanic bombs and mantle-derived xenoliths. The activity of the maar end with a final phreatomagmatic activity highlighted by the deposition of pyroclastic breccia

The first phreatomagmatic deposits show a heterogeneous lithology characterized by the association of juveniles formed by basalts and accidental lithics from the substratum of limestone. The pyroclastic projections fall around the maar in crossed strata centered on an axis that describes a semi-annular geometry. Depending on the abundance, size, and sequential organization of the constituent elements, the phreatomagmatic unit is subdivided into two units U1 and U2 [13]. **Unit 1:** it corresponds to the initial explosive phase, declined in two sequences of deposits with a weak external dip of 15° to 20° (**Figure 5a**). These two sequences indicate a variation in fragmentation intensity during the activity of the maar. The first phase (U1a) of vent opening is highlighted by an alternation of beds of variable thickness between 50 cm and 1.5 m, composed of breccia and fine tuffs, rich in accidental lithics (1 to 10 cm in diameter) (**Figure 5b**). In the second stage (U1b) describes thin regular bedding compared to the first deposit with a clear decrease in the accidental lithic and an increase of juveniles (**Figure 5c**). The beds at the base correspond to lapilli-tufts with calcareous interstitial ramifications, resulting from

**Unit 2:** It shows a succession of heterogeneous cross-bedded layers of lapilli tuffs (**Figure 6a**). It is distinguished by a white color at the bottom (U2a) because of the abundance of blocks and fragments of accidental lithics (**Figure 6d**). This level indicates the resumption of hydromagmatic activity since enclaves of breccias from the first explosion are packed into the deposit (**Figure 6b**). it is overlain by (U2b), a thick column (40 to 50 m) of parallel, locally crossbedded, strata, formed essentially of scoriaceous to finely red pulverized material interspersed with fine beds of accidental lithics (**Figure 6c**). In this unit a range of sedimentation features characterizing maar projections are observed, notably mud crack at the summit of

The maar is surrounded on the northern and southern sides by strombolian deposits (U3) (**Figure 7a**) which announces the transition to an activity where water participation has substantially decreased. The first deposits on the eastern and southern flanks are essentially rust-colored, centimetric scoria, well-classified, more or less vesicular (U3a) (**Figure 7b**). They correspond to pyroclastic fallout from a plume. The color variations indicate a beginning of alteration suggesting that the pyroclastics were wet at the time of their deposition. The beds become heteromeric with centimetric scoria with dense and angular bombs. Towards the top, the fallout has a relatively chaotic character (U3b). The hot fallout can weld together locally and form conical accumulations of scoria (spatter cones), piled up lava, and blocks or bombs of lava (**Figure 7c**). The summit of this unit is composed of a mixture of scoria and bombs (U3c). The whole is globally homogeneous with a reddish color due to hot oxidation. It is an eruption in a regular regime but with some slightly more powerful explosions; levels richer in bombs (**Figure 7d**). On the western flank, the U3c unit is poorly deposited. The lava flow that cap U3b is formed by agglutination

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

*4.1.1 The phreatomagmatic phase*

the circulation of water.

*4.1.2 The strombolian phase*

the phreatomagmatic deposit (**Figure 6e**).

located in the north and south of the crater [13].

### *Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*

scoriaceous to finely pulverized fragments. The third unit (U3) results from strombolian activity. Its eruptive products are composed of massive breccia tuffs rich in volcanic bombs and mantle-derived xenoliths. The activity of the maar end with a final phreatomagmatic activity highlighted by the deposition of pyroclastic breccia located in the north and south of the crater [13].

### *4.1.1 The phreatomagmatic phase*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

explosive dynamism that occurred while the sediment was still rich in water. Towards the summit appears fine gray ash overlain by gray and reddish vacuolar and scoriaceous beds revealing the eruptions that preceded the hydromagmatic explosion (**Figure 4c**). A palynological analysis was carried out on these lacustrine deposits at the GEOBIO laboratory of the Scientific Institute of Rabat. It showed that these deposits are extremely poor in palynomorphs. The gray deposits are rich in bisaccate called "Pinus pollens" that have existed from the Paleozoic to the

*The lacustrine deposits at the base of the first phreatomagmatic unit (U1) of the maar of Lechmine n'Aït el Haj.*

The tephrostratigraphic sequence of deposits shows color and dip variation corresponding to four eruptive phases (U1–4). The first two units are describing a phreatomagmatic activity with heterogeneous lithology; U1 is highlighted by alternating beds of variable thickness rich in limestone blocks, while U2 shows relatively thin regular bedding compared to the first deposit, with a white basal level formed almost of blocks and fragments of accidental lithics overlain by another level of red

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present day [36, 37].

**Figure 4.**

The first phreatomagmatic deposits show a heterogeneous lithology characterized by the association of juveniles formed by basalts and accidental lithics from the substratum of limestone. The pyroclastic projections fall around the maar in crossed strata centered on an axis that describes a semi-annular geometry. Depending on the abundance, size, and sequential organization of the constituent elements, the phreatomagmatic unit is subdivided into two units U1 and U2 [13].

**Unit 1:** it corresponds to the initial explosive phase, declined in two sequences of deposits with a weak external dip of 15° to 20° (**Figure 5a**). These two sequences indicate a variation in fragmentation intensity during the activity of the maar. The first phase (U1a) of vent opening is highlighted by an alternation of beds of variable thickness between 50 cm and 1.5 m, composed of breccia and fine tuffs, rich in accidental lithics (1 to 10 cm in diameter) (**Figure 5b**). In the second stage (U1b) describes thin regular bedding compared to the first deposit with a clear decrease in the accidental lithic and an increase of juveniles (**Figure 5c**). The beds at the base correspond to lapilli-tufts with calcareous interstitial ramifications, resulting from the circulation of water.

**Unit 2:** It shows a succession of heterogeneous cross-bedded layers of lapilli tuffs (**Figure 6a**). It is distinguished by a white color at the bottom (U2a) because of the abundance of blocks and fragments of accidental lithics (**Figure 6d**). This level indicates the resumption of hydromagmatic activity since enclaves of breccias from the first explosion are packed into the deposit (**Figure 6b**). it is overlain by (U2b), a thick column (40 to 50 m) of parallel, locally crossbedded, strata, formed essentially of scoriaceous to finely red pulverized material interspersed with fine beds of accidental lithics (**Figure 6c**). In this unit a range of sedimentation features characterizing maar projections are observed, notably mud crack at the summit of the phreatomagmatic deposit (**Figure 6e**).

### *4.1.2 The strombolian phase*

The maar is surrounded on the northern and southern sides by strombolian deposits (U3) (**Figure 7a**) which announces the transition to an activity where water participation has substantially decreased. The first deposits on the eastern and southern flanks are essentially rust-colored, centimetric scoria, well-classified, more or less vesicular (U3a) (**Figure 7b**). They correspond to pyroclastic fallout from a plume. The color variations indicate a beginning of alteration suggesting that the pyroclastics were wet at the time of their deposition. The beds become heteromeric with centimetric scoria with dense and angular bombs. Towards the top, the fallout has a relatively chaotic character (U3b). The hot fallout can weld together locally and form conical accumulations of scoria (spatter cones), piled up lava, and blocks or bombs of lava (**Figure 7c**). The summit of this unit is composed of a mixture of scoria and bombs (U3c). The whole is globally homogeneous with a reddish color due to hot oxidation. It is an eruption in a regular regime but with some slightly more powerful explosions; levels richer in bombs (**Figure 7d**). On the western flank, the U3c unit is poorly deposited. The lava flow that cap U3b is formed by agglutination

### **Figure 5.**

*The northern flank of the Lechmine n'Aït el Haj crater; (a) the contact between the limestone base and the first phreatomagmatic phase (U1); (b) the first sequence (U1a) showing an opening facies with unsorted accidental lithics (level a) and beds rich in scoriaceous basalt (level b); (c) the second sequence (U1b) with regular and less thick bedding, where the proportion of accidental lithics decrease.*

of packages of low-viscosity lava flowing towards the center of the crater (**Figure 8**), through a path oriented ENE-WSW probably linked to a sectorial collapse.

### *4.1.3 Terminal phreatomagmatic deposits*

The activity of LNH ended with a phreatomagmatic phase, highlighted by the deposition of pyroclastic lapilli tuff layers thicker on the northern flank of the maar (10 to 15 m) than on the southern flank (2 m to 5 m). This unit (U4) is formed essentially of scoria and rare lava blocks mixed with accidental lithics. The whole forms a rhythmic sequence that is particularly well represented on the southern flank (**Figure 9**).

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**n'Aït el Haj**

**Figure 6.**

strombolian deposits (**Figure 11**).

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

**4.2 Volcanic dynamics and structural context of the formation of Lechmine** 

*Volcanic lithofacies accompanying the two sequences U2a and U2b of the second phreatomagmatic phase (U2); (a) the succession of the first and the second sequence of U2 (U1a and U2b) with varying thicknesses and compositions; (b) enclave of the first phreatomagmatic phase (U1) packed in the fallout of the second phreatomagmatic phase (U2a); (c) the second sequence (U2b) with reddish color richer in juvenile pyroclasts; (d) variation of composition and color between U2a and U2b. (e) Mud cracks structure at the summit of U2b.*

The LNH maar is set on the path of the Lbouatène tectonic corridor (ALB) (**Figure 2**), between the Fault of Tizi n'Traten (TTF) and North Middle Atlas Fault (NMAF). This area is particularly distinguished with cryptokarst cavities. These are aligned according to the orientations of the major faults in the Causse of the Middle Atlas (**Figures 2, 10**). It has been shown [8] that the formation of the cryptokarsts in the Quaternary basaltic flow is controlled by the fracturing. Around the maar, the cryptokarstic cavities are arranged in two alignments N60 and N160 conforming to those recognized in the Causse of Middle Atlas [8, 38] highlighting the karstic context of the formation of the maar. In order to analyze the instability at the scale of LNH, fracturing measurements were made in both phreatomagmatic and

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*

### **Figure 6.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

of packages of low-viscosity lava flowing towards the center of the crater (**Figure 8**),

*The northern flank of the Lechmine n'Aït el Haj crater; (a) the contact between the limestone base and the first phreatomagmatic phase (U1); (b) the first sequence (U1a) showing an opening facies with unsorted accidental lithics (level a) and beds rich in scoriaceous basalt (level b); (c) the second sequence (U1b) with regular and* 

The activity of LNH ended with a phreatomagmatic phase, highlighted by the deposition of pyroclastic lapilli tuff layers thicker on the northern flank of the maar (10 to 15 m) than on the southern flank (2 m to 5 m). This unit (U4) is formed essentially of scoria and rare lava blocks mixed with accidental lithics. The whole forms a rhythmic sequence that is particularly well represented on the southern flank (**Figure 9**).

through a path oriented ENE-WSW probably linked to a sectorial collapse.

*4.1.3 Terminal phreatomagmatic deposits*

*less thick bedding, where the proportion of accidental lithics decrease.*

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**Figure 5.**

*Volcanic lithofacies accompanying the two sequences U2a and U2b of the second phreatomagmatic phase (U2); (a) the succession of the first and the second sequence of U2 (U1a and U2b) with varying thicknesses and compositions; (b) enclave of the first phreatomagmatic phase (U1) packed in the fallout of the second phreatomagmatic phase (U2a); (c) the second sequence (U2b) with reddish color richer in juvenile pyroclasts; (d) variation of composition and color between U2a and U2b. (e) Mud cracks structure at the summit of U2b.*

## **4.2 Volcanic dynamics and structural context of the formation of Lechmine n'Aït el Haj**

The LNH maar is set on the path of the Lbouatène tectonic corridor (ALB) (**Figure 2**), between the Fault of Tizi n'Traten (TTF) and North Middle Atlas Fault (NMAF). This area is particularly distinguished with cryptokarst cavities. These are aligned according to the orientations of the major faults in the Causse of the Middle Atlas (**Figures 2, 10**). It has been shown [8] that the formation of the cryptokarsts in the Quaternary basaltic flow is controlled by the fracturing. Around the maar, the cryptokarstic cavities are arranged in two alignments N60 and N160 conforming to those recognized in the Causse of Middle Atlas [8, 38] highlighting the karstic context of the formation of the maar. In order to analyze the instability at the scale of LNH, fracturing measurements were made in both phreatomagmatic and strombolian deposits (**Figure 11**).

### **Figure 7.**

*Strombolian pyroclastic fallout on the eastern flank of LNH; (a) succession of phreatomagmatic deposits overlain by the strombolian unit (U4); (b) air fallout from the first plume of the strombolian phase; (c) formation of spatter cone near the emission zone; (d) fallout of scoria with bombs far from the volcanic eruptive center.*

In the phreatomagmatic deposits (U1, U2) general subsidence is marked by conjugate fault-systems (**Figure 12d**) found also in the limestone basement (**Figure 11**). In the uppermost part of U2 (**Figure 12e**), there is a shift from an extension by normal fault perpendicular to the NE–SW structural direction, to a strike-slip system by permutation of the stress axes σ<sup>1</sup> - σ<sup>2</sup> (**Figure 13**).

The distribution of strombolian deposits have an elliptical shape in map view with a 900 m long axis (550 m short axis) oriented N60E which corresponds to the regional structural direction. The NE and SW extremities of the major axis are distinguished by markers that reflect a general northward collapse movement. This distension controls the injection of basalt and its massive westward flow;

• On the western flank, a shear has been observed at the southern limit, cutting the blocks of the massive basalt flow with a right lateral movement of 30° dip towards the foci as shown by the striation on the fault plane (**Figure 12a**). On this flank,

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**Figure 8.**

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj…*

of an extension oscillating swinging between WNW-ESE and NNW–SSE.

**4.3 Characterization of the structure of the LNH maar by applied geophysics**

The treatment of the magnetic anomalies of the Middle Atlas allowed highlighting the existence of anomalies of short and long wavelengths. These last are linked to magnetized sources, notably the plio-quaternary basalts. Other anomalies

*Modality of expression of the eruptive activity in the maar of Lechmine n'aït el Haj in the Causse of Middle Atlas; (a) flow of basalt emitted during a collapse of the western sector of the crater (1) overlain by strombolian (2) and then phreatomagmatic fallout (3); (b) basalt prisms (1) overlain by Strombolian fallout* 

*of U3b (2); (c) corded lava flow resulting from the agglutination of lava.*

• On the eastern flank, the faults are sometimes decametric where the scarp is outlined by the bleached zone of alteration (**Figure 12b**). Subparallel cracks are associated with conjugate fault system related to this sectoral collapse

basalt flows are piled up or corded. These flows form lobes that slide towards the center of the crater or that follow the gaps between the blocks of lave of the initial massive flow. The overlying scoriaceous layers attesting the strombolian explosion are affected by conjugate faults systems (**Figure 12c**) which could be an indicator

*DOI: http://dx.doi.org/10.5772/intechopen.94756*

NW-SE (**Figure 12d, e, f**).

*Study of Monogenic Volcanism in a Karstic System: Case of the Maar of Lechmine n'Aït el Haj… DOI: http://dx.doi.org/10.5772/intechopen.94756*

basalt flows are piled up or corded. These flows form lobes that slide towards the center of the crater or that follow the gaps between the blocks of lave of the initial massive flow. The overlying scoriaceous layers attesting the strombolian explosion are affected by conjugate faults systems (**Figure 12c**) which could be an indicator of an extension oscillating swinging between WNW-ESE and NNW–SSE.

• On the eastern flank, the faults are sometimes decametric where the scarp is outlined by the bleached zone of alteration (**Figure 12b**). Subparallel cracks are associated with conjugate fault system related to this sectoral collapse NW-SE (**Figure 12d, e, f**).
