Jurassic rifting to post-rift subsidence analysis in the Central High Atlas and its relation to salt diapirism

The subsidence evolution of the Tethyan Moroccan Atlas Basin, presently inverted as the Central High Atlas, is characterized by an Early Jurassic rifting episode, synchronous with salt diapirism of the Triassic evaporite-bearing rocks. Two contrasting regions of the rift basin - with and without salt diapirism - are examined to assess the effect of salt tectonics in the evolution of subsidence patterns and stratigraphy. The Djebel Bou Dahar platform to basin system, located in the southern margin of the Atlas Basin, shows a Lower Jurassic record of normal faulting and lacks any evidence of salt diapirism. In contrast, the Tazoult ridge and adjacent Amezra¿ı basin, located in the centre of the Atlas Basin, reveals spectacular Early Jurassic diapirism. In addition, we analyse alternative Central High Atlas post-Middle Jurassic geohistories based on new thermal and burial models (GENEX 4.0.3 software), constrained by new vitrinite reflectance data from the Amezra¿ı basin. The comparison of the new subsidence curves from the studied areas with published subsidence curves from the Moroccan Atlas, the Saharan Atlas (Algeria) and Tunisian Atlas show that fast subsidence peaks were diachronous along the strike, being younger towards the east from Early-Middle Jurassic to Late Cretaceous. This analysis also evidences a close relationship between these high subsidence rate episodes and salt diapirism

Synsedimentary record of diapiric activity related to the Jbel Azourki ridge, Central High Atlas, Morocco : Impact on depositional geometries and facies distribution of the Lower Jurassic carbonate and mixed systems

L’évolution des systèmes sédimentaires jurassiques (Pliensbachien-Bajocien) dans le secteur de Zaouiat-Ahançal (Haut Atlas, Maroc) est localement influencée par des mouvements diapiriques associés à la ride du Jbel Azourki. Cette structure tectonique complexe suit un tracé en baïonnette d’orientation globale OSO-ENE sur près de 60 kilomètres, ponctué par six affleurements de matériel diapirique triasique. L’objectif de cette thèse est de caractériser l’impact du diapirisme sur la géométrie des dépôts et sur la distribution des faciès au sein de trois systèmes de dépôt successifs : (1) un système carbonaté de plate-forme peu profonde (Formations d’Aganane, de Jbel Choucht et d’Assemsouk) (2) des systèmes de rampe mixte silicoclastique et carbonatée (Formations de Tamadout, d’Amezraï, de Tafraout et d’Aguerd-n-Tazoult), (3) et un système carbonaté oolitique (Formation de Bin-El-Ouidane). Une cartographie détaillée des unités stratigraphiques et des unités de faciès, ainsi qu’une série de quatorze coupes géologiques de la ride diapirique du Jbel Azourki ont ainsi été réalisées. L’étude des interactions entre sédimentation et diapirisme révèle un enregistrement continu de la déformation diapirique sur l’ensemble de la série sédimentaire, et a permis d’établir une chronologie de l’activité diapirique dans le secteur de Zaouiat-Ahançal. L’unité des calcaires inférieurs (1) enregistre une déformation polyphasée marquée par le développement localisé, au sein de la plate-forme, de bassins d’extension kilométrique caractérisés par une sédimentation hémipélagique et gravitaire (rim basins). Ces bassins circonscrits aux diapirs sont limités par des bordures de plate-forme bioconstruites à Lithiotis, de type aggradant ou érosif. À partir du Pliensbachien terminal, les déformations syndiapiriques se manifestent dans les unités mixtes (2), d’une part à l’échelle plurikilométrique avec l’accumulation de plusieurs milliers de mètres de dépôts, contrôlée par la variation latérale du taux de subsidence entre et au sein des compartiments nord et sud de la ride, et d’autre part à l’échelle hectométrique de la bordure du diapir avec des géométries caractéristiques et des variations de faciès (micro plates-formes à oolites et coraux). La ride diapirique perce en surface pendant le dépôt des unités mixtes, puis est recouverte par l’unité transgressive peu déformée des calcaires supérieurs (3) à l’Aalénien terminal. Les paramètres de contrôle de la géométrie des dépôts et de la distribution des faciès autour de la ride diapirique du Jbel Azourki correspondent aux variations locales de subsidence liées aux mouvements de la couche de sel en profondeur, au taux de sédimentation et à leur rapport relatif. Ils s’inscrivent dans un contexte tectonique régional, et climatique global, qui définit l’accommodation générale et le type de remplissage sédimentaire du bassin atlasique. Le type de sédimentation, carbonatée ou mixte, joue un rôle prépondérant dans ces manifestations tectono-sédimentaires.The evolution of the Jurassic sedimentary systems (Pliensbachian-Bajocian) in Zaouiat-Ahançal area (High Atlas, Morocco) is locally controlled by diapiric movements related to the Jbel Azourki ridge. This nearly-60-kilometer-long complex tectonic structure follows an overall WSW-ENE “bayonet-shape” outline, punctuated by six Triassic diapiric outcrops. The aim of this thesis is to characterize the role of diapirism on depositional geometries and facies distribution in three successive sedimentary systems: (1) a shallow-carbonate platform system, (2) mixed siliciclastic- and carbonate-ramp systems and (3) an oolitic-carbonate system. Therefore, a detailed geological map with the stratigraphic and facies units has been realized, as well as a set of fourteen geological sections across the Jbel Azourki diapiric ridge. The analysis of the interactions between sedimentation and diapirism has revealed a continuous recording of diapiric deformation by the entire sedimentary succession, allowing the establishment of a chronology of diapiric activity in the Zaouiat-Ahançal area. The lower-carbonate unit (1) records a polyphase deformation with the development of localized kilometer-scale basins within the platform, characterized by hemipelagic and gravity-flow deposits (rim basins). These basins are confined around the diapirs and bounded by Lithiotis-bioconstructed platform margins, which can be aggradational or erosional. From the late Pliensbachian, the mixed units (2) were affected firstly by syn-diapiric deformations at a pluri-kilometer scale, with the accumulation of several thousand-meter-thick deposits, controlled by lateral variations of the subsidence rate in and between the northern and southern flanks of the ridge, and secondly by syn-diapiric deformations at a hectometer- “diapir-edge” scale, with characteristic geometries and facies variations (oolite- and coral-rich micro platforms). The diapiric ridge reached the surface during the deposition of the mixed units and was finally capped by slightly deformed transgressive upper-carbonate unit (3) in the late Aalenian. The controlling factors on depositional geometries and facies distribution around the Jbel Azourki diapiric ridge are the local variations of the subsidence rate related to salt-movement, the sedimentation rate, and their relative ratio. They are part of the regional tectonic and global climate settings which defined the overall accommodation rate and the sedimentary filling of the atlasic basin. The type of sedimentation, carbonate- or mixed-dominated, played a major role in these tectonic-sedimentary responses

Synsedimentary record of diapiric activity related to the Jbel Azourki ridge, Central High Atlas, Morocco : Impact on depositional geometries and facies distribution of the Lower Jurassic carbonate and mixed systems

L’évolution des systèmes sédimentaires jurassiques (Pliensbachien-Bajocien) dans le secteur de Zaouiat-Ahançal (Haut Atlas, Maroc) est localement influencée par des mouvements diapiriques associés à la ride du Jbel Azourki. Cette structure tectonique complexe suit un tracé en baïonnette d’orientation globale OSO-ENE sur près de 60 kilomètres, ponctué par six affleurements de matériel diapirique triasique. L’objectif de cette thèse est de caractériser l’impact du diapirisme sur la géométrie des dépôts et sur la distribution des faciès au sein de trois systèmes de dépôt successifs : (1) un système carbonaté de plate-forme peu profonde (Formations d’Aganane, de Jbel Choucht et d’Assemsouk) (2) des systèmes de rampe mixte silicoclastique et carbonatée (Formations de Tamadout, d’Amezraï, de Tafraout et d’Aguerd-n-Tazoult), (3) et un système carbonaté oolitique (Formation de Bin-El-Ouidane). Une cartographie détaillée des unités stratigraphiques et des unités de faciès, ainsi qu’une série de quatorze coupes géologiques de la ride diapirique du Jbel Azourki ont ainsi été réalisées. L’étude des interactions entre sédimentation et diapirisme révèle un enregistrement continu de la déformation diapirique sur l’ensemble de la série sédimentaire, et a permis d’établir une chronologie de l’activité diapirique dans le secteur de Zaouiat-Ahançal. L’unité des calcaires inférieurs (1) enregistre une déformation polyphasée marquée par le développement localisé, au sein de la plate-forme, de bassins d’extension kilométrique caractérisés par une sédimentation hémipélagique et gravitaire (rim basins). Ces bassins circonscrits aux diapirs sont limités par des bordures de plate-forme bioconstruites à Lithiotis, de type aggradant ou érosif. À partir du Pliensbachien terminal, les déformations syndiapiriques se manifestent dans les unités mixtes (2), d’une part à l’échelle plurikilométrique avec l’accumulation de plusieurs milliers de mètres de dépôts, contrôlée par la variation latérale du taux de subsidence entre et au sein des compartiments nord et sud de la ride, et d’autre part à l’échelle hectométrique de la bordure du diapir avec des géométries caractéristiques et des variations de faciès (micro plates-formes à oolites et coraux). La ride diapirique perce en surface pendant le dépôt des unités mixtes, puis est recouverte par l’unité transgressive peu déformée des calcaires supérieurs (3) à l’Aalénien terminal. Les paramètres de contrôle de la géométrie des dépôts et de la distribution des faciès autour de la ride diapirique du Jbel Azourki correspondent aux variations locales de subsidence liées aux mouvements de la couche de sel en profondeur, au taux de sédimentation et à leur rapport relatif. Ils s’inscrivent dans un contexte tectonique régional, et climatique global, qui définit l’accommodation générale et le type de remplissage sédimentaire du bassin atlasique. Le type de sédimentation, carbonatée ou mixte, joue un rôle prépondérant dans ces manifestations tectono-sédimentaires.The evolution of the Jurassic sedimentary systems (Pliensbachian-Bajocian) in Zaouiat-Ahançal area (High Atlas, Morocco) is locally controlled by diapiric movements related to the Jbel Azourki ridge. This nearly-60-kilometer-long complex tectonic structure follows an overall WSW-ENE “bayonet-shape” outline, punctuated by six Triassic diapiric outcrops. The aim of this thesis is to characterize the role of diapirism on depositional geometries and facies distribution in three successive sedimentary systems: (1) a shallow-carbonate platform system, (2) mixed siliciclastic- and carbonate-ramp systems and (3) an oolitic-carbonate system. Therefore, a detailed geological map with the stratigraphic and facies units has been realized, as well as a set of fourteen geological sections across the Jbel Azourki diapiric ridge. The analysis of the interactions between sedimentation and diapirism has revealed a continuous recording of diapiric deformation by the entire sedimentary succession, allowing the establishment of a chronology of diapiric activity in the Zaouiat-Ahançal area. The lower-carbonate unit (1) records a polyphase deformation with the development of localized kilometer-scale basins within the platform, characterized by hemipelagic and gravity-flow deposits (rim basins). These basins are confined around the diapirs and bounded by Lithiotis-bioconstructed platform margins, which can be aggradational or erosional. From the late Pliensbachian, the mixed units (2) were affected firstly by syn-diapiric deformations at a pluri-kilometer scale, with the accumulation of several thousand-meter-thick deposits, controlled by lateral variations of the subsidence rate in and between the northern and southern flanks of the ridge, and secondly by syn-diapiric deformations at a hectometer- “diapir-edge” scale, with characteristic geometries and facies variations (oolite- and coral-rich micro platforms). The diapiric ridge reached the surface during the deposition of the mixed units and was finally capped by slightly deformed transgressive upper-carbonate unit (3) in the late Aalenian. The controlling factors on depositional geometries and facies distribution around the Jbel Azourki diapiric ridge are the local variations of the subsidence rate related to salt-movement, the sedimentation rate, and their relative ratio. They are part of the regional tectonic and global climate settings which defined the overall accommodation rate and the sedimentary filling of the atlasic basin. The type of sedimentation, carbonate- or mixed-dominated, played a major role in these tectonic-sedimentary responses

The Central High Atlas in Morocco: A Snapshot of a Jurassic Diapiric Rifted Basin

AAPG 2013 ANNUAL CONVENTION AND EXHIBITION, May 19-22, 2013, Pittsburgh, Pennsylvania, USAThe Central High Atlas in Morocco exposes an inverted Triassic-Jurassic rift basin with major ENE-WSW northern and southern compressive boundaries. Internally, the Moroccan High Atlas is constituted by long and continuous NE-SW trending anticlines, which are oblique to the present limits of the compressive range. Interpretation of these anticlines is still under debate with different solutions: a) formed by magmatic intrusions in a transtensional setting; b) by Cenozoic compression and c) by Jurassic diapirism. Nonetheless, most of these anticlines (also named ridges) show Triassic shales, evaporites and basalts in their cores, which are commonly intruded by Jurassic magmatic rocks. Typically, these anticlines show strongly dipping and subparallel flanks with different Jurassic stratigraphic sequences on each limb. Thinning, onlaps, truncations and/or sedimentary facies changes decreasing in paleowater depth towards the axis of the anticlines are also characteristic features. We interpret these structural features as linked to elongated diapiric walls and coeval sedimentation as halokinetic. Some of these diapiric structures are welded without Triassic materials between the limbs. The interpretation of geological maps, confirmed by remote sensing mapping and fieldwork, indicates a secondary NW-SE trending system of diapir walls and welds. These are interfering with the major NE-SW system resulting in a polygonal pattern of thick Jurassic minibasins, which is especially evident in the Imilchil region and south of Demnate. We corroborate with our observations that the Central High Atlas was a transtensional diapiric province characterized by an intricate pattern of salt walls and minibasins, which formed mainly during Jurassic times. The diapiric system controlled the position, extent and growth of the early and middle Liassic carbonate platforms (Jbel Choucht and Aganane formations) and the fast deposition of the thick mixed platform upper Liassic Zaouiat Group. While some halokinetic structures like the Tazoult ridge were practically inactive during the earliest Middle Jurassic, salt migration continued though Middle Jurassic times developing 2-3 km-thick sedimentary minibasins in the Imilchil region (Lake Plateau and Ikkou minibasins). Unambiguous examples of Jurassic diapiric growth have been reported in the Atlas in Algeria and Tunisia. These results can be applied to buried large salt-related provinces in both rift basins and continental margins

The Central High Atlas in Morocco: a Snapshot of a Jurassic Diapiric Rifted Basin

24e Réunion des Sciences de la Terre à Pau du 27 au 31 Octobre 2014 au Palais Beaumont.The Central High Atlas in Morocco exposes an inverted Triassic- Jurassic rift basin with major ENE-WSW northern and southern compressive boundaries. Internally, the Moroccan High Atlas is constituted by long and continuous NE-SW trending ridges (anticlines), which are oblique to the present limits of the compressive range. Most of these ridges show Triassic argillites, evaporites and basalts in their cores, which are commonly intruded by Jurassic gabbros. Typically, these ridges show strongly dipping and subparallel flanks and in most cases showing different Jurassic stratigraphic sequences on each limb. Bed thinning, onlaps, truncations and/or sedimentary facies changes decreasing in paleowater depth towards the axis of the ridges are also characteristic features. We interpret these structural ridges as linked to elongated diapiric walls and coeval sedimentation as halokinetic. Some of these diapiric walls are welded diapirs. The interpretation of geological maps, confirmed by remote sensing mapping and fieldwork, indicates a secondary NW-SE trending system of diapir walls and welds. These are interfering with the major NE-SW system resulting in a polygonal pattern of thick Jurassic minibasins. While some diapiric structures like the Tazoult ridge were partially inactive during the earliest Middle Jurassic, migration of mobile material continued though Middle-Upper Jurassic times developing 2-3 km-thick sedimentary minibasins in the Imilchil region (Lake Plateau and Ikkou minibasins). This diapiric region in High Atlas may represent a well-exposed example of Jurassic diapiric growth reported in the Atlas in Algeria and Tunisia but buried at depth

The Central High atlas in Morocco: A snapshot of a Jurassic diapiric rifted basin

8th Workshop of the ILP-Task Force Sedimentary BasinThe Central High Atlas in Morocco exposes an inverted Triassic-Jurassic rift basin with major ENE-WSW northern and southern compressive boundaries. Internally, the Moroccan High Atlas is constituted by long and continuous NE-SW trending anticlines, which are oblique to the present limits of the compressive range. Interpretation of these anticlines is still under debate with different solutions: a) formed by magmatic intrusions in a transtensional setting; b) by Cenozoic compression and c) by Jurassic diapirism. Nonetheless, most of these anticlines (also named ridges) show Triassic shales, evaporites and basalts in their cores, which are commonly intruded by Jurassic magmatic rocks. Typically, these anticlines show strongly dipping and subparallel flanks with different Jurassic stratigraphic sequences on each limb. Thinning, onlaps, truncations and/or sedimentary facies changes decreasing in paleowater depth towards the axis of the anticlines are also characteristic features. We interpret these structural features as linked to elongated diapiric walls and coeval sedimentation as halokinetic. Some of these diapiric structures are welded without Triassic materials between the limbs. The interpretation of geological maps, confirmed by remote sensing mapping and fieldwork, indicates a secondary NW-SE trending system of diapir walls and welds. These are interfering with the major NE-SW system resulting in a polygonal pattern of thick Jurassic minibasins, which is especially evident in the Imilchil region and south of Demnate. We corroborate with our observations that the Central High Atlas was a transtensional diapiric province characterized by an intricate pattern of salt walls and minibasins, which formed mainly during Jurassic times. The diapiric system controlled the position, extent and growth of the early and middle Liassic carbonate platforms (Jbel Choucht and Aganane formations) and the fast deposition of the thick mixed platform upper Liassic Zaouiat Group. While some halokinetic structures like the Tazoult ridge were practically inactive during the earliest Middle Jurassic, salt migration continued though Middle Jurassic times developing 2-3 km-thick sedimentary minibasins in the Imilchil region (Lake Plateau and Ikkou minibasins). Unambiguous examples of Jurassic diapiric growth have been reported in the Atlas in Algeria and Tunisia. These results can be applied to buried large salt-related provinces in both rift basins and continental margins

Characterization of syn-diapiric Jurassic sedimentation in the Imilchil Area, Central High-Atlas, Morocco

19th International Sedimentological Congress, 18-22 August, Geneva, Switzerland, International Association of SedimentologistThe Jurassic Atlas intracontinental basin in the Central High-Atlas is characterized by a succession of ENEWSW narrow anticlinal ridges and wide synclines, developed by the diapirism of Triassic sediments and later magmatic intrusion along major synsedimentary faults. The core of the diapiric ridges is made of Triassic red shales, magmatic rocks and some evaporites. These diapiric structures are particularly well preserved in the Imilchil region, an ideal area to study the interaction between sedimentation and diapirism. The objective of this study is to place local diapiric movements and associated deformation and facies changes within the context of regional subsidence and facies patterns. The effects of diapir movements on sedimentation are demonstrated by intra-formational geometries (thickness variations, unconformities), facies variations close to the diapirs (local influence on the depositional profiles), and/or by specific events (erosional surfaces, breccia, condensed levels). Toarcian to Bathonian halokinetic sequences define two main phases of diapiric activity. The first major phase of diapiric movements occurred during the Toarcian/Aalenian, and is characterized by the presence of progressive unconformities within relatively deep marls and carbonate deposits. In the regional context, this diapiric activity is concurrent with a period of normal faulting. In contrast, reduced diapirism during the Bajocian is recorded by the progradation of oolitic and bio-constructed carbonate platform systems. The second phase of major diapiric activity occurred from the end of the Bajocian to the Bathonian, in relation with a new phase of deformation in the Atlas Basin, associated with magmatic intrusions and tectonic movements. The latter are thought to be responsible for an increase of clastic influx. Carbonate platforms were subsequently replaced by relatively shallow mixed carbonate-siliciclastic and ultimately continental systems, themselves affected by significant synsedimentary diapiric activity and associated deformation. At this stage, the diapirs tended to emerge, and disconnect the mini-basins. The high subsidence rate in these mini-basins allowed the accumulation of 2000 to 3000 meters of sediments during the Bathonian