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1. Introduction
The Moroccan Triassic basins are characterized by two sedimentary episodes (detrital and evaporitic). The deposition of the detrital episode started during (1) the Early Triassic in the Argana Valley, [e.g. Hofmann et al. 2000; Tourani et al. 2010]; (2) the Middle Triassic in the Central High Atlas [e.g. El Arabi et al. 2006] and in the Oujda mountains [e.g. Courel et al. 2003; Crasquin-Soleau et al. 1997; Oujidi and Elmi 2000], and during (3) the Upper Triassic in the Middle Atlas basins [e.g. Baudelot et al. 1990; Lachkar et al. 2000] and in the Moroccan Atlantic margin (Essaouira basin, e.g. [Hafid 2000; Slimane and El Mostaïne 1997]; Doukkala basin, e.g. [Hminna et al. 2013]; Khémisset basin, e.g [Et-Touhami 1994; Taugourdeau-Lanz 1978]; Mohammedia–Benslimane–ElGara–Berrechid basin (MBEB), [Afenzar 2018]).
The Moroccan salt series (evaporitic episode) are formed during the Upper Triassic as in the other Triassic continental basins, whether in the North of Gondwana (Algeria, e.g. [Aït Salem et al. 1998; Bourquin et al. 2010; Courel et al. 2003]; and Tunisia, e.g. [Soto et al. 2017], [Soussi and Ben Ismaïl 2000], [Soussi et al. 2001]) or in Europe (Portugal, [Alves et al. 2006; Ramos et al. 2017; Soto et al. 2017]; Spain, e.g. [Barrón et al. 2006; Bourrouilh et al. 1995; Ferrer et al. 2012; González de Aguilar 2015; Ortí 2004; Reolid et al. 2014; Roca et al. 2011]; France, e.g. [Bourquin and Guillocheau 1993, 1996]; Germany, e.g. [Aigner and Bachmann 1992; Kozur and Bachmann 2008]) or in Canada [Leleu et al. 2016; Miall and Balkwilla 2019; Olsen 1997; Wade et al. 1995; Welsink and Tankard 2012].
In Morocco, existing data for the Triassic salt successions are scarce [e.g. Peretsman and Holser 1988; Salvan 1974]. In the Atlasic domain, the saliferous series is more reduced and is represented in outcrop by gypsum levels (Central High Atlas, e.g. [Biron 1982; Benaouiss et al. 1996; Courel et al. 2003; El Arabi 2007; Baudon et al. 2012; Vergés et al. 2017]; Argana Valley, e.g. [Hofmann et al. 2000]; Middle Atlas, e.g. [Lorenz 1976; Laville et al. 1995; Ouarhache et al. 2012]). Towards the Mesetien domain and the Moroccan Atlantic margin domain, these successions become much more important. In all Triassic basins belonging to these two domains, the saliferous successions are similar. They are subdivided into two large parts recognized in borehole: (1) the lower part attributed to the Upper Triassic and (2) the upper part dated from the Lower Liassic, separated by the Central Atlantic Magmatic Province, i.e. CAMP basalt (Essaouira Basin, [Echarfaoui et al. 2002b; Hafid 2000]; Doukkala Basin, [Echarfaoui et al. 2002a; Salvan 1984]; Khémisset Basin, [Et-Touhami 1994, 1996, 1998; Salvan 1982]; MBEB Basin, [Afenzar 2018; Lyazidi 2004; Salvan 1984]).
The MBEB Basin consists of a detrital and evaporitic sedimentary series of about 1500 m [Afenzar 2018; BRPM 1973]. This sedimentary series is subdivided into two main formations: a sandy-conglomeratic formation at the base (Formation A) and argillaceous-saliferous formation in the middle and top of the series (Formation B) [Afenzar 2018; Afenzar and Essamoud 2017] recovered by Triassic-Liassic basalts [Peretsman 1985].
The aim of this study, from a detailed sedimentological analysis is (1) to reconstruct palaeoenvironment and (2) for the first time, to propose sequence stratigraphy analyses to constitute a basis of a stratigraphic correlation with other Triassic Atlantic basins. Moreover, the correlations in terms of sequence stratigraphy allow to constrain the spatial and temporal evolution of salt series and could guide the exploration and thus have an economic impact.
2. Geological setting
At the early Mesozoic, the Pangea continent was affected by an initial break-up associated with the early stages of the opening of the Central Atlantic Domain. During this phase, the Moroccan and North American Margins were subjected to an extensive tectonic regime that led to the opening of a set of rift basins [e.g. Courel et al. 2003; Hafid 2000; Leleu et al. 2016; Le Roy and Piqué 2001; Medina 1995; Olsen 1997; Piqué and Laville 1995; Piqué et al. 1998]. The Moroccan basins are placed geographically in the central segment of Central Atlantic Domain and are laterally equivalent to the Nova Scotian basins [Hafid 2000; Leleu et al. 2016]. The MBEB basin is a part of a Moroccan margin like the Khémisset, Doukkala, Essaouira, Souss and Tarfaya basins (Figure 1). Among these basins, the closest to MBEB Basin are Khémisset Basin in the north (separated by the Paleozoic basement of central Morocco) and Doukkala Basin in the south (separated by the Paleozoic of Casablanca block).
Paleogeographical location of Moroccan Triassic basins in the Late Norian (synthetic map based on [Courel et al. 2003] and [Leleu et al. 2016]).
The MBEB basin is located in the North-West of the Moroccan coastal Meseta, about twenty kilometers northeast of Casablanca (Figure 2). The structure of this basin has been interpreted several times. The interpretations of El Wartiti et al. [1992] were based on the boundary with the Hercynian basement, which are unconformity contacts materialized by border faults controlling the individualization of the basin and conditioning its filling.
Usually, this basin is presented as an immense shallow depression which seems to have originated from N–S to NE–SW half-graben structure [Afenzar 2018; El Wartiti et al. 1992]. At the end of the Paleozoic and the early Mesozoic, this half-graben is developed and filled by an important detrital and evaporitic syn-rift sedimentary series; with a magmatic activity belonging to the CAMP [Manspeizer and Cousminer 1988; Peretsman 1985].
Geological map of the MBEB basin with the sections and boreholes analyzed as well as correlation transects in genetic stratigraphy (A: Chaâbat Al Hamra section, B: Chaâbat Lhmira, C: Assikriat section, D: Tlet Ziaida, E: Sidi Amour, F: Sidi Bouchaibe, G: Barrage Oued El Maleh, H: Sidi Bou Amar, I: Ouled Jhaich J: ElGara, TI, TII, TIII: correlation transects).
Lithostratigraphic subdivision of the detrital and evaporite infra-basaltic series of the Mohammedia–Benslimane–ElGara–Berrechid basin (A: [BRPM 1973]; B: [Lyazidi 2004] C: [Afenzar 2018]).
This basin was subjected to a NW–SE extension with a slight deformation component [El Wartiti et al. 1992]. The structure is controlled by a deep detachment, which is probably an ancient Hercynian weakness zone, and which plunges slightly towards the NNW. This is related to the opening of the proto-Atlantic domain [El Wartiti et al. 1992; Medina 1994].
3. Lithostratigraphic framework
According to the old nomenclature, the lithostratigraphic series of the MBEB basin was subdivided into five main zones attributed to Permo-Triassic deposits [BRPM 1973]: (1) Zone argileuse inférieure; (2) Zone salifère inférieure; (3) Zone basaltique; (4) Zone salifère supérieure; (5) Zone argileuse supérieure (Figure 3A).
According to the latest works, the sedimentary series in the MBEB basin is subdivided into two major series: Lower argillaceous-salt series attributed to Triassic and Upper argillaceous-salt series attributed to Liassic [Hssaida et al. 2012; Lyazidi 2004]. The two parts are separated by a basaltic complex (Figure 3B).
This basalt was dated Late Triassic-Early Liassic (200 Ma) and belonged to the CAMP by several works based on radiometric dating data [Peretsman 1985]. Subsequently the infra-basaltic series can be attributed to the Upper Triassic [Afenzar 2018; El Wartiti et al. 1992; Lyazidi 2004; Salvan 1984].
In this study, we have subdivided this Upper Triassic part into two main formations: the sandy-conglomerate Formation (A) at the base of the series and the argillaceous-saliferous Formation (B), subdivided into two members, (1) Mudstone-siltstone Member and (2) Argillaceous-saliferous Member (Figure 3C).
4. Method
This sedimentological study consists in a detailed analysis of the field sections (outcropping in the northeastern part of the basin) and drill cores of four boreholes (PB43 (712 m), PB44 (650 m), POM1 (1070 m) and POM2 (1350 m), Figure 2) for a better view of the sedimentary bodies and their spatial arrangements. This allows to characterize different facies and their associations as well as the architectural elements [Allen 1983; Miall 1985, 1996] which allow us to reconstruct the depositional environments and their evolutions over time and space.
In fact, the vertical and horizontal arrangement of these facies give birth to the characterized architectural elements according to several criteria determined by Allen [1983] and Miall [1985, 1996], which are: the nature of the upper and lower bounding surfaces, the external geometry, the scale, and the internal structures. The last step is the determination of paleoenvironment which is based on the nature and types of lithofacies already characterized and also on types of architectural elements.
The genetic stratigraphy applied in this study consists of a high-resolution correlation of all the field sections as well as the boreholes in the basin, which makes it possible to individualize isochronous markers separated by a few tens to hundreds of thousands years [e.g. Bourquin and Guillocheau 1993, 1996].
After the characterization of sedimentary facies, deduction of depositional processes, identification of facies association and interpretation of depositional environments, and setting up a sedimentological model, the genetic unit are characterized and then stacking pattern showing the evolution over time of the depositional environment are established.
The second step consists in correlation of the genetic sequences according to three transects and based on reference levels which can serve as isochronous markers. For this objective four levels have been identified: (1) unconformity between the Hercynian basement and the first facies deposited in the basin; (2) contact between the siltstone-mudstone member and the basal part of the argillaceous-saliferous member; (3) contact between the top part of the argillaceous-saliferous member and the basal part of the member containing pure halitic facies; (4) the lower part of the basalt formation considered as the limit between the Upper Triassic and Lower Liassic [Peretsman 1985]. A cartography of the genetic sequences was carried out below in order to obtain 2D and 3D geometries for a better interpretation of the evolution of these genetic units as well as the lateral passages of the facies.
5. Facies analysis
5.1. Identification of lithofacies
Fourteen facies were identified, described and interpreted in terms of depositional processes based on lithology, grain sizes and sedimentary structures [Miall 1978, 1996] (Figures 4, 5 and 6, Table 1).
Illustration of the different facies identified in the basin. Description and interpretation in Table 1.
5.2. Architectural elements
In this study and according to architectural elements of [Miall 1996, 2016] we used the abbreviation AE (Architectural Element) for the coding of these elements. This analysis made it possible to characterize six architectural elements (noted AE1 to AE6, Figures 5a, 5b, 5c and 5d) and two facies associations (AFP and AFE).
Architectural element AE1
This constitutes the basis of the series, it was determined in the Chaâbat El Hmira area on a vertical extent from 4 to 5 m. AE1 is formed mainly by facies association Fc1 and Fc2 corresponding to facies Gms and Gm of Miall [1978, 1996]. It is at the base of the series (in contact with the Paleozoic basement) and associated with the architectural elements AE2 and AE3 (Figure 5a). It is formed by gravity flow deposits, mainly pebbles and gravels poorly sorted, formed in the proximal areas of alluvial fans. According to all the criteria, this element corresponds to the element SG (Sediment Gravity Flow) of Miall [1985, 1996, 2016].
Architectural element AE2
AE2 (Figures 5a and 5b) is formed by lithofacies Fc2, Fc4, Fc5, Fc6 and Fc9. Regarding the internal structure, it is formed by poor matrix and imbricated pebbles facies (Fc2) showing channels lag and sieve deposits. In other cases, this element is formed by sandstone lithofacies characterized by horizontal planar bedding: upper flow regime (Fc6), and by some sandstone lithofacies showing the planar crossbeds (Fc5). Moreover, architectural element AE2 has fourth order basal boundary surfaces (4th: minor erosion) and over (5th: surface bounding generally flat to slightly concave-upward) [Miall 1988]. These limits are sometimes erosive and slightly planar and in other cases erosive and concave to the top. Inside the element, and between the lithofacies, small boundaries can be identified. From these characteristics, we can say that AE2 resembles to the architectural element CH (Channels) of Miall [1985, 1996, 2016].
Architectural element AE3
It is formed mainly by assemblages of coarse lithofacies containing imbricated pebbles and gravels, showing horizontal stratification, sometimes planar crossbedding: Fc2 and Fc5 (Figure 5a). Sometimes minor lithofacies were identified between these major facies (Fc4 and Fc6). Most facies of this element are organized as tabular bodies of five to six meters thick. Also, it is formed from a 4% to 6% of fine to medium sandstones. AE3 corresponds to the architectural element GB (Gravel bars and bedforms). It is usually coarse deposits formed at gravels bars, these coarse deposits are sometimes intercalated by thin levels sandstones formed at low flows (speed decrease) [Massari 1983; Miall 1985, 1996, 2016].
Architectural element AE4
This architectural element (Figures 5b and 5c) is formed by medium to fine lithofacies assemblages: facies Fc5, Fc6 and Fc7 corresponding respectively to facies Sp, Sh and Sl of Miall [1978, 1996, 2016], but it is dominated by Fc7. The Fc6 and Fc7 facies are sometimes separated by a very fine facies: Fc8. Architectural element AE4 passes laterally into element OF. It is characterized by internal boundaries of second to third order, while its outer boundaries are fourth order. For this, AE4 has many similarities with the architectural element SB (Sand Bedform) of Miall [1985, 1996, 2016]. This architectural element characterizes in our case, crevasse channels and/or crevasse splays deposits.
Description and interpretation of sedimentary facies of Triassic series of MBEB based on Miall] (1978] and 1996]), Bridge [2003] and Opluštil et al. [2005]
| Facies code | Lithology and sedimentary structures | Depositional process | Interpretation |
|---|---|---|---|
| Conglomeratic facies | |||
| Fc1 | Massive conglomerate (1 m to 4 m thick) with angular, disorganized and poorly sorted gravels. The matrix is formed by the fine sandstone. Without sedimentary structures (A, B Figure 4). | Gravity flows, Mass flow deposits | Equivalent to Gms of Miall [1996]. The absence of sedimentary structures and the existence of a mixture of fine and coarse materials suggest that this conglomerate is deposited by gravity flows: debris flow at proximal alluvial fans environment [Miall 1978, 1985, 1996; Opluštil et al. 2005]. |
| Fc2 | Clast-supported stratified conglomerates with centimeter to decimeter-sized pebble-gravels, showing angular to sub-angular shapes and horizontal bedding imbrication (B & C, Figure 4). | Aggradational deposit | Equivalent to Gm of Miall [1996]. This facies (0.4 m to 2.5 m thick) has been deposited at median to distal alluvial fans or longitudinal bars in the braided rivers system. It can also be interpreted as sieve and/or lag deposits [Miall 1978, 1985, 1996]. |
| Fc3 | Planar crossbeds conglomerate (0.5 m to 3 m thick). The matrix is formed mainly by clastic materials (clast-supported) which are very fine sandstones (D, Figure 4). | Progradational deposit | Equivalent to Gp of Miall [1996]: Deposit of longitudinal bar in a braided rivers system with shallow channels [Miall 1978, 1985, 1996]. This facies present a similarity with conglomerate of the lower part of Bigoudine formation (T6) in the Argana valley [Hofmann et al. 2000]. |
| Sandstone facies | |||
| Fc4 | Massive coarse sandstone (1 m to 3 m thick) without any sedimentary structures. Sometimes it presents isolated large fragments (A & C, Figure 4). | Rapid deposits Gravity flow deposits | Equivalent to Sm of Miall [1996]. The presence of isolated large fragments is probably related to their falling along the slope [Sohn et al. 1997], associated with the deposition mechanism itself or a movement in a high load flow [Postma and Cruickshank 1988]. According to [Einsele 1992; Miall 1985, 1996], these facies occur in alluvial fans environment. |
| Fc5 | Fine to coarse sandstone (0.4 m to 2 m thick), showing the planar crossbeds (E, Figure 4). | Progradational deposit | Equivalent to Sp of Miall [1996]: Linguoid or transverse bars deposits (lower flow regime) [Bridge 2003; Miall 1985, 1996; Todd 1996]. It is similar to some sandstone beds of the lower part of Oukaïmeden sandstone [Benaouiss et al. 1996]. |
| Fc6 | Fine to coarse sandstone (0.2 m to 1.5 m) characterized by horizontal laminations with parting or streaming lineation (F, Figure 4). | Plane-bed flow | Equivalent to Sh of Miall [1996]. This structure is generated by small longitudinal vortices affecting the entire turbulent boundary layer. It results from upper flow regime deposits [Miall 1985, 1996]. |
| Fc7 | Fine sandstone (0.5 m to 2 m thick) characterized by low angle (<10°) planar cross beds. | Scour fills | Equivalent to Sl of Miall [1996]. It is a crevasse channel and/or a crevasse splay deposit often formed in the floodplain at anastomosed fluvial system. |
| Fine facies | |||
| Fc8 | Massive to horizontal laminated siltstone. They have a reddish appearance with grey to greenish levels of mottling (E & F, Figure 4). | Overbank deposit | Equivalent to Fl of Miall [1996]. Vertical accretion deposit showing a laminar flow of very low energy. This facies is interpreted as a flood plain, overbank or playa deposits [Miall 1985, 1996]. |
| Fc9 | Massive reddish mudstone (0.1 m to 6 m thick) showing mottling spots (C & H, Figure 4). | Overbank deposit | Equivalent to Fm of Miall [1996]. These mudstones can be deposited in (1) an alluvial plain of a braided system, (2) floodplain and playas, and sometimes in (3) distal alluvial fans [Mader 1985]. These mudstones are interpreted as lacustrine or overbank deposit [Miall 1996]. This facies and Fc8 sometimes have cyclicity similar to that described by Hofmann et al. [2000] in the Argana valley (T4, T5 and T7). |
| Evaporite facies | |||
| Fc10 | Gypsum beds facies (0.1 m to 0.5 m). It is presented in the form of centimeter banks alternating with the siltstones and the mudstones facies (H, Figure 4). | Evaporation in a hot and humid environment | These facies are formed in relatively hot and humid environments by the precipitation of sulfated ions in supersaturated solutions subjected to intense evaporation [Warren 2006, 2010]. |
| Fc11 | Fibrous gypsum (0.1 m to 0.2 m thick) | Diagenetic facies | Diagenetic origin [Afenzar and Essamoud 2017; Afenzar 2018; Et-Touhami 1994, 1996]. |
| Fc12 | Milky clean halite deposited as decametric to metric beds rarely associated with very fine anhydrite laminae (I, Figure 4). | Evaporation in a hot and humid environment | The rhythmicity of this facies with the mudstone facies is probably due to the interventions of the slightly turbid continental waters which propagate on the surface of the brine [Afenzar 2018; Et-Touhami 1994, 1996; Sonnenfeld and Hodec 1985]. The alternation of this halite with detrital and sulphate levels indicates that it is probably deposited in saline mudflats or evaporite flats. |
| Fc13 | Phenoblastic halite with limpid crystals (J, Figure 4). (0.5 m to 0.5 m thick) | Diagenetic facies | Filling of dissolution cavities. Diagenetic origin [Afenzar 2018; Et-Touhami 1994, 1996]. |
| Fc14 | Millimeter to centimeter veins of fibrous halite. | Diagenetic facies | Filling of pre-existing fractures in the mudstone levels. It is presented as thin fibers elongated perpendicularly to the walls of mudstones. This halite is formed probably during diagenesis in the mudstone [Dumas 1988; Et-Touhami 1994, 1996; Hovorka 1983]. |
Architectural element AE5
It is an assemblage of fine to very fine lithofacies. This element is presented as sandstone sheets with horizontal laminations from 40 to 80 cm thick: facies Fc6, it is intercalated with thin purplish siltstones and mudstones laminated: facies Fc8 and Fc9. This architectural element resembles to architectural element LS (sand laminated sheets) of Miall [1985] (Figure 5c). Lithofacies forming this architectural element have been interpreted as the product of flash floods [[Miall 1985; Rust 1978; Sneh 1983; Tunbridge 1981, 1984]]. The architectural characteristics of this element are well described by Tunbridge [1981] and Sneh [1983]. The sand sheets are deposited on flat surfaces slightly eroded, laterally, they can spread over hundreds of meters.
Architectural elements characterized in the basin (a: in Chaâbat Lhmira outcrop, b: in the Chaabat Al Hamra outcrop, c: in the top of Chaâbat Lhmira outcrop, d: in Assikriat outcrop).
Synthetic sedimentary log of the basin with minor and major cycles of base-level variations. a and b: conglomerate; c: sandstone; d: siltstone; e: mudstone; f: pure halite; g: argillaceous halite; h: gypsum; i: basalt.
Architectural element AE6
Architectural element AE6 (Figures 5b and 5c) is considered as a major element in the basin, its thickness can reach forty meters. It is presented as large sheets (2 to 50 m thick), formed by purplish or red brick siltstones and mudstones, these siltstones and mudstones sometimes show a massive aspect with crude planar laminations indicating a quiet depositional setting. In other cases, they show horizontal laminations with very thin intercalations (5–15 cm) of fine sandstones. By these characteristics we can say that AE6 corresponds to the architectural element OF (Overbank fines) of Miall [1985, 1996, 2016].
According to Miall [1985, 1996], in most cases this element has a sheet geometrical form, reflecting its origin by vertical aggradation. In the vicinity of the active channels, these sheets are separated by crevasse splays. This architectural element may fill abandoned channels, provided it has concave-up basal contact and ribbon to lenticular geometry of the channel itself [Ethridge et al. 1981; Miall 1985].
Facies association of Playa (AFP)
It is a lithofacies combination of 5 to 6 m thick. It is formed by siltstone and mudstone lithofacies assemblage (Fc8, Fc9) and by fine sandstones sometimes showing horizontal flat beddings (lithofacies Sh). The facies association AFP presents a cyclicity of the sandstone, siltstone and mudstone facies; which shows that it is deposited in Playa Lake. The presence of sandstone deposits also shows that these playa lakes are shallow [Liu and Wang 2001].
Evaporite facies association (AFE)
It is an association of mudstone (Fm) and evaporite facies (Fc10: beds gypsum, Fc11: fibrous gypsum, Fc12: milky halite, Fc13: phenoblastic halite, Fc14: fibrous halite). The thickness of the facies varies between 1 and 1.5 m for the siltstone, between 10 and 20 cm for the gypsum and between 10 cm and 2 m for the halite. These evaporite facies are often of primary and in other cases diagenetic origin. They are formed by the evaporation of saline waters in mudflats and lagoons in a hot and humid climate in relation to a “pellicular” sea that covered the basin in the Upper Triassic.
6. Paleoenvironment reconstruction
6.1. Proximal alluvial fans system
This fluvial model is characterized by massive conglomerate with angular, disorganized and poorly sorted gravels, and by massive coarse sandstone (two to three meters thick without any sedimentary structures). These lithofacies are associated in two architectural elements: AE1 (Sediment gravity flow SG) and AE2 (Channels CH). AE1 units are interbedded with channelized beds of AE2. By these architectural element characteristics (facies assemblage, geometry, bounding surfaces…), we can deduce that this depositional environment is similar to the model No 1 of Miall [1985, 1996, 2016]. It is proximal alluvial fan with sediment gravity flows of gravelly rivers [Afenzar 2018; Afenzar and Essamoud 2017; Miall 2016]. The frequency of debris flows depends strongly on source rock weathering characteristics, so that adjacent fans, the headwaters of which flow across contrasting bedrock units, may show quite different lithofacies assemblages [Hooke 1967; Miall 1996, 2016].
6.2. Shallow channels of a braided rivers system
The sediments characterizing this environment style are coarse to medium. It is a clast-supported stratified conglomerate with centimeter to decimeter-sized pebble-gravels, showing sub angular shapes and horizontal bedding imbrication (Fc2), and planar crossbeds conglomerate (Fc3). The medium to fine deposits are presented by massive coarse sandstone without any sedimentary structures (Fc4) and fine to coarse sandstone characterized by horizontal laminations with parting or streaming lineation (Fc6). The facies association of this style forms the architectural element AE2 (CH: channel) and AE3 (GB: gravel bars). In this environment, the architectural element AE3 is the most abundant. During the fluctuations stage, bar complexes become emergent, and are crossed by minor channels in which thin deposits of AE2 may form [Miall 2016]. These fluvial style characteristics resemble to those of model No 2 of Miall [1985, 1996, 2016].It is a proximal braided rivers system characterized by shallow channels and gravel bars.
6.3. Floodplains in anastomosed rivers system
This depositional environment is characterized essentially by fine to very fine deposits with a large thickness. In that case, these facies are organized into two architectural elements: AE4 (SB: Sand Bedforms) deposited in the crevasse splays, and architectural element AE6 (OF: Overbank Fine) formed in flood plains and abandoned channels. The element AE4 has a limited lateral extent and passes laterally into AE6. This type of fluvial style is much less studied and interpreted, unlike other types of depositional environment [Farrell 1987; Kraus and Bown 1988; Miall 1985, 1996, 2016]. However, it is linked to an anastomosed environment characterized by low-energy floods with crevasse channels and crevasse splays.
6.4. Coastal plains, playa lakes, mudflats and lagoons system
This is a set of more distal deposit environments. They are characterized by deposits recorded in the middle and top part of the argillaceous-saliferous formation (B) and which is finally capped by Triassic-Liassic basalts. This system is characterized by massive to horizontal laminated siltstone, massive reddish mudstone showing mottling spots, gypsum beds facies alternating with the siltstones and the mudstones facies, fibrous gypsum, milky clean halite deposited as decametric to metric beds rarely associated with very fine anhydrite laminae and phenoblastic halite with limpid crystals.
This system probably corresponds to the large coastal plain downstream characterized by the development of playa lakes, mudflats and lagoons where the evaporite facies are formed by the evaporation of marine waters [Peretsman 1985; Peretsman and Holser 1988] under a hot and humid climate and in relation to a pellicular sea that covered the domain during the Upper Triassic time.
7. Genetic stratigraphy and correlation
7.1. Identification of genetic sequences
In the continental domain, as in our case, the identification of genetic sequences is complicated. A genetic sequence is usually represented by a period of erosion, by pass or stacked of fluvial deposits linked to the base level fall (i.e. progradation) and a period of aggradation accompanied the base level rise (i.e. retrogradation). The periods of base level fall correspond to a weak preservation of the facies whereas the period of aggradation corresponds to a significant sedimentary preservation [Bourquin et al. 1998, 2009; Hamon and Merzeraud 2005; Homewood et al. 1992; Merzeraud 1992].
Genetic sequence UG1
It is a genetic sequence in which an alluvial fan system evolves to a braided system with an alluvial plain. The period of base level fall (progradation) is characterized by a weak preservation of the thin conglomerate facies without sedimentary structure with coarse, angular and poorly sorted elements limited by erosion surfaces indicating the base level fall. The base level rise period (retrogradation) is characterized by the important preservation of alluvial fan facies and the development of conglomeratic facies (showing horizontal stratifications with imbricated and well-sorted pebbles-gravels) and sandstone facies showing horizontal planar bedding. These facies are organized into architectural elements: AE1, AE2 and AE3 formed in channels and bars in a braided river system.
Genetic sequence UG2
The base level fall period is characterized by the deposition of coarse massive sandstone facies without sedimentary structures. The base level rise is marked by well-organized facies with horizontal stratifications and other conglomerate facies showing planar cross-beds organized into architectural elements AE2 and AE3 deposited at the channels and bars in a braided fluvial system.
Genetic sequence UG3
This genetic sequence formed during the increase of the base level (retrogradation), is characterized by the development of distal fine facies forming the architectural element AE6 and deposited in the floodplain. The basal boundary of this sequence is marked sometimes by the passage of coarse facies formed at the bars in a braided system to fine facies of the floodplain.
Genetic sequence UG4
This sequence is characterized by its formation during the rise of the base level marked by the deposition of sandstone, siltstone and mudstone facies. These facies are associated in two architectural elements (AE2: CH and AE6: OF) formed in anastomosed channels with an immense floodplain. This period of base-level rise is also characterized by a decrease of grain-size, passing from channel sandstone facies to the siltstone-mudstone facies of the floodplain.
Genetic sequence UG5
UG5 is characterized by a period of base level rise associated with the development of sandstone and mudstone facies organized into architectural elements AE4, AE5 and AE6 deposited in sand bars and crevasse splays at a floodplain related to an anastomosing fluvial system. The facies association is characterized by a gradual decrease of grain-size.
Genetic sequence UG6
This genetic sequence is formed during the base level rise period. It is represented by sandstone and siltstone-mudstone facies deposited in floodplains and probably in coastal plains characterized by architectural element AE6 and facies associations of playa (AFP).
Genetic sequence UG7
It is characterized by a period of base level rise, and formed by sandstone, siltstone, mudstone and evaporite facies. These facies form the association of Playa (AFP) and/or facies-association of evaporite (AFE) formed at playa lakes and at lagoons with an immense coastal plain.
Genetic sequences UG8 to UG14
The UG8 to UG14 genetic sequences are characterized by alternating siltstone, mudstone and evaporites facies (organized into architectural elements AE6, AFP and AFE) formed in a coastal plain associated with mud flats and lagoons. These genetic units have been identified mainly in boreholes at depths ranging from 500 m up to 1000 m. The maximum flooding surfaces of these sequences are between the mudstone facies and the evaporites facies.
7.2. Genetic sequences correlations
The genetic units are correlated from one section to the other (Figure 7) and from borehole to another (Figure 8). This correlation is based on the determination of reference levels. For this objective four limits have been identified:
- the first one is the unconformity between the Hercynian basement and the first facies deposited in the basin,
- the second reference level is the contact between the detrital facies and the first evaporites facies (gypsum) deposited in the basin and corresponds to the contact between the Mudstone-Siltstone Member and the basal part of the Argillaceous-Saliferous Member (Figure 3C),
- the third reference level concerns the contact between the upper part of the Argillaceous-saliferous Member and the basal part of pure halitic facies, located at the top of this Member,
- the fourth one is the beginning of the Basalt considered as the limit between the Upper Triassic and Lower Liassic [Peretsman 1985].
For more precision, this correlation was carried out according to three transects: the first one (TI) N–S on Oued El Maleh river. The second transect (TII) N–S on the Oued Nfifikh and the last transect (TIII) N–S which links between the boreholes PB43, PB44, POM1 and POM2 (Figure 2). The stacking pattern of the genetic sequence allows the constitution of four minor cycles of base level variation (Figure 6).
Correlation of genetic sequences based on the study of field sections on the Oued Nfifikh-ElGara axis (Transect II).
The first cycle (C1) is characterized by conglomerate and sandstone deposits unconformably deposited on the Hercynian basement. The maximum flooding surface (MFS1) is marked by the passage of thin conglomerate facies without sedimentary structure with coarse, angular and poorly sorted elements limited by erosion surfaces to conglomerate facies, deposited in proximal alluvial fans and conglomerate and sandstone facies formed in a braided fluvial system with a progressive rise of base level (UG1 and UG2).
The second cycle (C2) (Figures 9, 10 and 11) has a remarkable base level increase. It is characterized by a vertical aggradation of the sandstone, siltstone and mudstone facies formed in an alluvial plain in braided system and/or in an anastomosing river system.
The third cycle (C3) (Figures 9, 10 and 11) was identified on the borehole (PB44). It is a retrogradational/aggradational cycle and is characterized by the appearance of the first evaporite facies. Its base is represented by gypsiferous mudstone and alternations of mudstone and bed gypsum. Then, there is a transition to alternations of halite ( + ∕− closed lagoon) and mudstone formed in playa and mudflats.
The fourth and last cycle (C4) (Figure 11) is characterized by the rise of the base level, with the formation of pure halite facies, which shows a marine intervention stopped abruptly by basaltic effusions at the end of the Upper Triassic.
Correlation of genetic units based on the study of the central and northern boreholes of the basin (Transect III: N–S).
First transect (TI) N–S of correlation and cartography of genetic sequence and minor cycles of base-level variation.
7.3. Basin-wide genetic sequences cartography
The identification of different orders of genetic sequence stacking and their correlation allow the cartography of their extent. The result is a paleogeographic reconstruction for each genetic sequence. This stratigraphic correlation makes it possible to study the distribution of facies in space within each sequence, and their evolution over time (Figures 9, 10 and 11).
Second transect (TII) N–S of correlation and mapping of genetic sequence and minor cycles of base-level variation.
Third transect (TIII) N–S of correlation and mapping of genetic sequence and minor cycles of base-level variation.
At the base of MFS1, conglomerates have larger thicknesses in the center than at the ends for transect (TI) (Figure 9). For the transect (TII) (Figure 10), these conglomerates have significant thicknesses along the entire transect. At the north, some conglomeratic facies of the retrogradational part of the genetic sequence UG1 pass laterally to sandstone facies. The genetic sequence (UG1) of this cycle (C1) is formed in proximal deposition environments: alluvial fans, proximal braided system with remarkable variations of palaeoslope.
Between MFS1 and MFS2, an increase of base level leads to a change in the depositional environment to an alluvial plain characterized by sandstone and siltstone deposits. This rise of the base level is shown by well-developed braided channel facies and by a gradual decrease of grain size and then by the appearance of the well-sorted facies, with the disappearance of the angular elements and finally by the passage of the massive facies without sedimentary structure (Gms and Sm) at Planar cross-beds facies (Gp and Sp) then at Horizontal lamination facies (Gm and Sh) [[Bourquin et al. 1998, 2009; Hamon and Merzeraud 2005; Homewood et al. 1992; Merzeraud 1992; Poli 1997]]. Lateral transition of facies is observed at the north of the transect (TI) (Figure 9) and the entire transect (TII) (Figure 10).
Between MFS2 and MFS4, we have noticed an appearance of evaporites facies that are concentrated in the middle of the transect (TI, Figure 9) and pass laterally to siltstone and mudstone at the same genetic sequence. For the transect (TIII, Figure 11), this interval (MFS2–MFS4) is characterized by a genetic sequence with purely saline facies. This passage of evaporites facies to mudstone facies is probably due either to the migration of deposition environment during the same time interval, or to the dissolution and erosion of these facies exposed in outcrop. The last cycle (C4) (Figures 10, 11) is characterized by a deposition of pure halite facies recovered by basalts, which shows the marine incursion before these basaltic effusions at the end of Triassic and early Jurassic.
These genetic sequences and progradational/ retrogradational cycles show an evolution of the proximal fluvial environments (alluvial fans passing laterally to braided system) at the base of the TI and TII (respectively Figures 9 and 10, orange and beige color) to a distal environments (anastomosed rivers and floodplains) in the middle of TI and TII and at the base of TIII (respectively Figures 9, 10 and 11, yellow and green color below MFS2), and then at a transition environment (coastal plain with lagoons and mudflats) at the top of the TI and TII and throughout the transect TIII (respectively Figures 9, 10 and 11, green and pink color).
8. Discussion
Paleoenvironmental evolution
Sedimentary analysis has shown that the studied basin is characterized by siliciclastic sedimentation at the beginning and then evaporitic at the end of the syn-rift sedimentary filling (Figure 6) [Afenzar 2018]. First-cycle debris eroded from a mountainous source area are deposited at the edges of the basin in coarse alluvial fans, or transported to a coastal plain [Miall 2016].
Based on the identified facies, and the characterized architectural elements and the alluvial styles, the depositional environments are evolved over time from:
- proximal alluvial fans system characterized by accumulation of gravity-flow sediments to a proximal braided system characterized by conglomeratic and sandstone bars;
- subsequently, the depositional environment has changed to an anastomosed system with a vast floodplain characterized by crevasse splays that pass laterally to overbank deposits (middle of the sedimentary series); these anastomosed systems occur in areas of active vertical aggradation, such as coastal systems during a time of rapidly rising base level;
- finally, these environments eventually evolve to a coastal plain where playa lakes, mudflats and lagoons have developed. In this phase, the syn-rift sedimentary series recorded a marine incursion at the Upper Triassic with saliferous sedimentation Afenzar [2018]. It is deduced from the presence of a thick saliferous series with a large lateral extension whose isotope ratios of sulfur and bromine contents indicate their marine origin [Peretsman 1985; Peretsman and Holser 1988]. These marine waters are probably of Tethyan origin and are also related to the opening of the Proto-Atlantic [Et-Touhami 1994]. This marine incursion remains thin compared to the southwest European Triassic basins. In these cases, the well-developed marine domain is indicated by the presence of carbonates rich in marine species (e.g. Alpujarride carbonates in Betic Cordillera: [Martin-Rojas et al. 2009]). Leleu et al. [2016] specified that these marine ingressions from the Tethys domain are inferred from dolomite and marine fauna in Portugal and from thick salt deposits in Morocco and offshore Canada during the late Rhaetian.
Control factors
Detrital sedimentation in the MBEB basin can be interpreted as resulting from the filling of a ditch in the form of half-graben [El Wartiti and Fadli 1985; El Wartiti et al. 1992], the replay of the Hercynian faults which caused the collapse of this ditch and the activation of erosion. According to Salvan [1984], distension and subsidence were localized in Meseta points (ex. MBEB basin), which led to the accumulation of salts deposits.
The MBEB basin is therefore an open continental zone, which has favored the registration of alluvial fans, high-energy fluvial systems, and then gradually filling up, we have moved to lakes, and to evaporites. It is a typical succession associated with geodynamic context of rift type.
In this basin, the silico-clastic sedimentation is the result of continental alteration, and therefore a good part of the budget of erosion will arrive in the basin. After the period of erosion (the ante-Triassic reliefs), the basin is an area of sedimentation. It is an extensive basin whose deposits are initially aggradational and then retrogradational.
The MBEB basin also recorded a budget of parameters (mainly tectonic, climate) that created accommodation and a parameter that fills this space: the sediment supply. The sediments are mostly red. This reflects the oxidizing conditions in which the sedimentation was carried out [[Biron 1982; Van Houten 1973]]. The evaporites indicate a relatively hot and humid climate that has favored their precipitation.
The palaeoenvironmental evolution was also controlled by the evolution of palaeoslope. The decrease of the latter is found in most Triassic rift basins belonging to the central Atlantic domain. Leleu and Hartley [2010] suggest that in the Fundy and Minas basins, the palaeoenvironmental transition is diachronous, such that it cannot be related to climatic controls of global or megaregional extent and the fining-upward profile can be explained by a decrease in source area relief by erosion within a hydrologically closed basin.
Paleogeography
The evaporitic deposits could correspond to a Late-Triassic transgression from the Tethys to the north and an epicontinental marine domain to the west [Beauchamp 1988; Beauchamp et al. 1995; Oujidi et al. 2000]. A “pellicular” sea covered most of the Moroccan basins with intermediate facies, which was therefore a flat surface affected by localized subsidence (The MBEB basin is part of it) where the evaporitic thick layers accumulated. The marine character of the evaporites [Peretsman and Holser 1988] denotes a first transgressive episode [Salvan 1984] generalized throughout the Atlasic-Mesetien domain [Oujidi et al. 2000]. The detrital basins on either side of the future Atlantic Ocean are considered primarily purely continental, with the Newark Basin as a model [Smoot 1991].
The idea of a secondary communication between the Tethyan marine domain and the proto-Atlantic via the basins of Khémisset, Roumani and MBEB, operating discontinuously is quite conceivable [Et-Touhami 1994].
Relationship with Atlantic rifting
At the beginning of the Mesozoic, the northwestern part of the African continent was affected by an initial fracturing associated with the early stages of the opening of the central Atlantic (Atlantic rift).
Several authors consider that the opening of several Moroccan Triassic sedimentary basins (at least those in the southern edge of the Tethys) was initiated during this Triassic rifting. This opening was controlled by the reactivation of the preexisting weakness zones in the Paleozoic basement and inherited from the Hercynian orogeny [Courel et al. 2003; Hafid 2000; Laville et al. 2004; Leleu et al. 2016; Le Roy and Piqué 2001; Medina 1994; Olsen 1997; Piqué and Laville 1995; Piqué et al. 1998].
The MBEB basin is part of the western province of Triassic deposits in Morocco, which correspond to all the basins of the Moroccan Atlantic margin (Doukkala, Argana, Essaouira…) in direct relation with the Atlantic rift [Beauchamp et al. 1985; Salvan 1984; Van Houten 1977]. The Triassic deposits are considered as syn-rift.
9. Conclusion
This sedimentological analysis carried out for the first time in this basin allowed the reconstruction of palaeoenvironments and thus the syn-rift sedimentary filling history during the Upper Triassic. During rifting, the MBEB Basin passed through three major phases of sedimentary filling. The first phase is purely continental. During this period, the first deposits arrived in the basin are of alluvial fan origin. Subsequently, the decrease in palaeoslope and the rise of the basal level resulted in paleoenvironmental changes (proximal fluvial system to a distal depositional environment). During the third phase, the syn-rift sedimentary series recorded a marine incursion at the Upper Triassic with saliferous sedimentation.
The correlation and cartography of the genetic sequences as well as the progradational/ retrogradational cycles made it possible to obtain 2D/3D geometries of the basin according to the three correlation transects. This indicates a growth in the thickness of these sequences vertically (growth towards the top of the sandy-conglomeratic formation) and laterally (increase of the thickness of the genetic sequences while passing of borders to the center of the basin). These correlations also show lateral passages of the mudstone facies (NE-SE border) to the evaporite facies (basin center). The economic importance of these 2D/3D geometries lies in their orientation of the exploitation of pure halite facies whose thickness and quality increase towards the center of the basin.
From a paleogeographic point of view, the Mohammedia–Benslimane–ElGara–Berrechid Basin is part of the western Moroccan Triassic province, which corresponds to all the basins of the Moroccan Atlantic margin (Doukkala, Argana, Essaouira, Tarfaya) in relation to the rifting of Central Atlantic Domain. In this context, this study consists of a basic approach for all future studies concerning the stratigraphic correlations between this basin and the Triassic basins of the African Atlantic margin and Northeastern American margin.