Diapiric ridges and minibasins in the Central High Atlas: impact on geometries and facies distribution (Lower-Middle Jurassic, Morocco)

¹ Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France
² Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Spain
³ Departament de Geologia, Universitat Autònoma de Barcelona
⁴ Group of Dynamics of the Lithosphere, Geosciences Barcelona, Geo3Bcn-CSIC, Barcelona, Spain
⁵ TPD RDI, Equinor, Bergen, Norway

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 January 2025
Accepted: 29 September 2025

Abstract

The Central High Atlas of Morocco is the result of the Alpine tectonic inversion of an intracontinental Mesozoic basin, the infill of which started with thick Upper Triassic clay and evaporite deposition. This slightly deformed mountain range is characterized by narrow ENE-WSW ridges that expose a Triassic core. These ridges bound unique synclines filled by thick Jurassic successions showing well-preserved halokinetic depositional sequences, interpreted as salt-related minibasins. This paper aims to describe and analyze the evolution of diapiric movements and their interactions with Lower and Middle Jurassic carbonate dominated sedimentation in the Imilchil area, which corresponds to the axial part of the Atlas basin. Four diapiric ridges have been identified in this region: the Tasraft, Tassent, Ikkou, and Amagmag, which expose Triassic shales and are intruded by Triassic and Jurassic magmatic rocks. These ridges delineate three minibasins: Ikassene, Lakes (Lake Plateau), and Tilmi (Ikkou), each 5 to 10 km wide, preserving over 6,000 m of Upper Liassic and Dogger deposits. The Atlas basin is filled with five third-order transgression-regression cycles organized as one regressive megasequence. This long regressive trend is expressed by the succession of three depositional systems: i) a distal to proximal carbonate ramp system from the Toarcian to Late Bajocian; ii) a shallow mixed carbonate-siliciclastic system from the Late Bajocian to Early Bathonian; and iii) a fluvial system from the Bathonian to Early Callovian. Throughout this period, the development of coeval diapiric ridges significantly influenced these sedimentary systems, though the intensity of the diapiric activity varied over time. During the Late Bajocian, ridge growth slowed considerably amid the peak progradation of the carbonate systems. It then increased significantly from the latest Bajocian onward, due to a drastic rise in siliciclastic flux and sedimentation rate in the Atlas basin. The effects of diapirism on sedimentation are observed regionally (several kilometers), as minibasins evolve as depocenters, each exhibiting distinct subsidence rates and sedimentary thicknesses. Despite these local variations, the regional scale paleogeography and the sequential organization of the deposits remained largely unaffected. On a smaller scale (hectometers to kilometers), diapiric structures are always characterized by halokinetic depositional patterns. Internal angular unconformities only occur in very shallow-marine to continental deposits during periods of maximum ridge growth and subaerial exposure (notably in the Bathonian). At this scale, diapiric movements lead to local facies variations within shallow-marine deposits, fostering the development of bioconstructed or grainy carbonate sediment bodies along the ridges. However, in outer ramp environments, the submarine topographies created by ridge growth were generally insufficient to induce similar facies variations.