The Furongian break-up (rift/drift) unconformity in the central Anti-Atlas, Morocco

Abstract

This paper presents a stratigraphic, sedimentological and palaeontological study of the Cambrian–Ordovician transition in the central Anti-Atlas in order to characterize the rift-drift transition of the Cambrian Atlas Rift. The event stratigraphy and sedimentology of the Cambrian–Ordovician transition in the central Anti-Atlas, and the endemic character of the latest shelly faunas from the Cambrian Atlas Rift have been reviewed, the latter by sampling the Bailiella Formation and the base of the Jbel Lmgaysmat Formation. In the vicinity of Foum Zguid, central Anti-Atlas, several unconformities (joined laterally into one single angular discordance or paraconformity) can be recognized across the Cambrian–Ordovician transition: (i) a ravinement surface capped by lenses of polymictic conglomerates marking the top of the Azlag Formation and reflecting reworking of the substrate under sharp transgressive conditions; (ii) a skeletal calcareous siltstone bed at the base of the Jbel Lmgaysmat Formation preserving surficial silicification (silcretization related to broad uplift, denudation and weathering of an exposed landscape dominated by silica-rich rocks), postdated by fissuring associated with hydrothermal mineralization of Fe (Ba-Mn-Pb) ore deposits, linked to the opening of the so-called El Graara graben; (iii) generalized uplift and denudation, followed by sedimentation of alluvial-to-fluvial and proximal coastal-plain deposits (lower member of the Lower Fezouata Formation); and (iv) a final drowning and shaly blanketing of the inherited palaeorelief by the upper member of the same formation, which marks the beginning of passive-margin conditions. The Bailiella Formation has yielded the trilobite species Planolimbus hessikouanus, Maladioidella? destombesi n.sp. and Bailiella sepulcra n.sp., and the Jbel Lmgaysmat Formation Bailiaspis? glabrata, which point to biogeographic connections with the neighbouring Avalonian and west-European margins of Northwest Gondwana and Baltica.