Stratigraphy, facies analysis and evolution of paleo-drainage of the Late Mesozoic continental succession in AdrarAglagal syncline (Central High Atlas, Morocco)

Adrar Aglagal syncline is located at the southern front of Central High Atlas (Morocco), about 150 km southeast of Marrakech. This area is characterized by a Lower Jurassic-Upper Cretaceous succession, resting unconformably... [ view full abstract ]

Adrar Aglagal syncline is located at the southern front of Central High Atlas (Morocco), about 150 km southeast of Marrakech. This area is characterized by a Lower Jurassic-Upper Cretaceous succession, resting unconformably over Triassic clastics and volcanic deposits. This succession includes thick continental, mostly fluvial, deposits that have not been analyzed in previous studies in terms of facies architecture and paleo-drainage orientation. Syncline’s limbs are made up of marine-transitional Lower Jurassic limestone, dolomite and gypsum; the core is characterized by a continental clastic succession, being the local expression of a Late Mesozoic continental complex, regionally known as Couches Rouges. Such complex attests to a Middle Jurassic-Lower Cretaceous regressive phase, debated for the understanding of the tectono-sedimentary evolution of the Central High Atlas. This study aims at illustrating a detailed stratigraphy and facies analysis of this continental succession, in order to understand the tectono-sedimentary context in which it was deposited. The succession consists of mudstones, sandstones and conglomerates referred to fluvial-alluvial paleoenvironments, characterized by braided to low-sinuosity rivers and alluvial fans; facies analysis and recognition of different rank erosional, angular and progressive unconformities, allowed to subdivide the succession into five unconformity-bounded stratigraphic units that record the infill of successive paleovalleys. Each unit shows a cyclical facies trend, testifying base level fluctuations. The lithological composition of the conglomerate facies shows a gradual decrease of clasts from the Liassic carbonates and dolomites in favor of Triassic and Paleozoic volcanic and meta-sedimentary rocks; this trend records the progressive denudation of gradually deeper crustal levels, indicating syndepositional uplift of a structural high. Finally, paleocurrents analysis testifies significant changes in the paleo-drainage of the fluvial systems: units 1 to 3 show mean directions oriented N-NE (subaxial to the chain), whilst units 4 to 5 show mean directions oriented to the South (transversal to the chain). The reconstructed tectono-stratigraphic setting, characterized by angular and progressive unconformities and syn-sedimentary reverse faults, joined to the facies architecture and the evolution of paleo-drainage, suggests a predominant active tectonic control during deposition.