Middle and Late Devonian sea-level changes and synsedimentary tectonics in the eastern Anti-Atlas (Morocco)

Abstract

Few areas in the world offer such favorable preconditions for the reconstruction of Paleozoic sea-level changes and synsedimentary tectonics as the eastern Anti-Atlas of Morocco. This review article is focused on these significant depositional parameters during the Middle and Late Devonian. All the global eustatic fluctuations can be well recognized in coeval deposits of this area. The Middle Devonian Choteč, Kačák, Taghanic and Frasnes events are predominantly expressed by black shales reflecting distinct sea-level rises. The highstand during the Kellwasser interval, however, lasted much longer than in the type localities in central Europe, i. e. from the late Frasnian (Lower rhenana Zone) into the early Famennian (Lower rhomboidea Zone). The upper part of this interval is diachronic as is shown by different lithologies which substitute the typical dark fetid limestones and shales. Subsequent widespread highstands occurred in the middle Famennian (Lower to Middle marginifera Zone), late Famennian (Lower to Middle expansa Zone) and during the latest Famennian. The “Strunian” regression following the latter represents the most drastic reconfiguration which levelled almost the entire pre-existing paleotopography. The effects of synsedimentary tectonics during the Middle and Late Devonian are manifested as faults, angular unconformities, neptunian dykes/sills, and folds. On the basis of striking paleogeograhic discrepancies, the eastern part of the Great Anti-Atlas Fault is now sketched farther north, before it continues, dislocated by a Variscan transform fault, into the Ziz Valley. All recognized normal faults were not reactivated by the Variscan orogeny. Neptunian dykes are the most common result of weak dilatation and originate always from obvious erosional unconformities. For the first time isoclinal and recumbent folds have been recognized, which have affected Givetian to upper Famennian strata and were levelled by upper Famennian grainstones or “Strunian” shales or sandstones. They testify a Late Devonian precursor phase of the Late Carboniferous Variscan orogeny. The coincidence of the major transgressions during the Late Devonian with the frequency of synsedimentary movements depends on previous breaks in sedimentation and consequently on early lithification which is to be expected only on pelagic platforms.