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Abstract

Carbonate platforms affected by salt tectonics form important hydrocarbon reservoirs. In an effort to gain new insights of the impact of diapirism on carbonate systems we have undertaken an integrated structural and sedimentological study of Jurassic carbonate platforms of the Moroccan High-Atlas basin. In this natural laboratory, the scale of outcrop exposure is similar in area to a large offshore seismic data set, and field observations provide high details on the geometries and facies distributions around diapiric structures.

The Atlas intracontinental basin initiated during the Triassic, contemporaneously with Atlantic rifting. The Triassic synrift sequence includes thick shales and evaporite deposits accumulated in multiple tectonic sub-basins. A thick (>5000m) Jurassic sequence was deposited during an overall post-rift stage in a west-southwest/east-northeast shallow-marine basin open towards the NeoTethys. Since the Sinemurian, sedimentation was mainly carbonates. However, geodynamic events linked with the evolution of the Atlantic margin produced several phases of clastic influx leading to the development of mixed systems (Toarcian and Bathonian).

During the Early Pliensbachian, an extensional tectonic event triggered synsedimentary diapiric movements which locally lasted until the Cretaceous. These movements were responsible for the development of narrow diapiric ridges of large extent (>100km), controlled by normal west-southwest/east-northeast relay faults. These ridges were separating several kilometers-wide elongated mini-basins, which subsidence was induced by salt/shale withdrawal.

Regionally, diapiric movements have been discontinuous in time and space, leading to significant thickness variations within the different stratigraphic units. However, diapirism has not had any major influence on the nature and distribution of sedimentary systems at the basin scale. The impact of diapirs remains essentially localized in the immediate vicinity of these structures (km-scale), where they affected both stratigraphic geometries and facies distribution. This impact appears to be very different in oolitic and mixed ramp systems in which subtle differentiation of depositional profiles controlled progressive facies variations, or in bioconstructed carbonate systems in which diapiric movements had a major role on the location and morphology of platform margins and associated “micro-rim-basins.” In return, the geometry of the diapirs has been clearly influenced by the lithology of surrounding rocks.

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