Structural and facies architecture of a diapir-related carbonate minibasin (lower and middle Jurassic, High Atlas, Morocco)

• Published in: Marine and petroleum geology 2017-03, Vol.81, p.334-360
• Main Authors: Teixell, Antonio, Barnolas, Antonio, Rosales, Idoia, Arboleya, María-Luisa
• Format: Article
• Language: English
• Subjects: Carbonate facies; High Atlas; Jurassic; Minibasin; Morocco; Salt tectonics
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2017.01.003

We report the structural geometry and facies architecture of a small diapir-related carbonate-dominated basin from the Jurassic rift of the Moroccan High Atlas. The Azag minibasin is a lozenge-shaped depocenter completely enclosed by tectonic boundaries that we interpret as welds after former salt anticlines or salt walls. The exposed ca. 3000 m-thick infill of the Azag minibasin is asymmetric; layers are tilted to the W defining a rollover geometry. Areally-restricted sedimentary discontinuities and wedges of growth strata near the basin margins indicate sedimentation contemporaneous with diapiric rise of a Triassic ductile layer. Facies evolution through the basin reflects local accommodation by salt withdrawal and regional events in the High Atlas rift. The early basin infill in the Sinemurian and Pliensbachian shows thickness variations indicative of low-amplitude halokinetic movements, with reduced exposed thicknesses compared to surrounding areas. The exposed Toarcian and Aalenian deposits are also reduced in thickness compared to areas outside the basin. Subsidence increased dramatically in the Bajocian-early Bathonian (?), the main phase of downbuilding, when over 2600 m of carbonates and shales accumulated at a rate > 0.5 mm/a in the depocentral area of the minibasin governed by W-directed salt expulsion. The stratigraphic units distinguished often show maximum thicknesses and deeper facies in the depocentral area, and rapidly change to shallower facies at the basin margins. The Bajocian carbonate facies assemblage of the minibasin include: reservoir facies as microbialite-coral reefs in the basin margins (formed during periods of strong diapir inflation and bathymetric relief), basin-expansive oolite bars (formed during episodes of subdued relief), and organic-rich, dark lime mudstones and shales that show source-rock characteristics. The Azag basin is a good analog for the exploration of salt-related carbonate plays in rifts and continental margins where source-rock and reservoir can form in a same minibasin. •The Azag minibasin is a small lozenge-shaped basin bounded by welds deriving from former salt walls.•Growth strata and unconformities attest to sedimentation contemporaneous to halokinesis.•Lithofacies of Jurassic carbonates filling the minibasin are defined and grouped in units.•Facies evolution reflects local accommodation by salt withdrawal and regional events in the Atlas rift.•The Azag minibasin is a good analog for salt-related carbo