Ordovician stratigraphy and benthic community replacements in the eastern Anti-Atlas, Morocco

The Anti-Atlas contains a thick, volcanic-free Ordovician succession that originally deposited in a passive-margin basin. Three main sedimentary packages are bounded by major unconformities: (i) the Tremadocian–Floian Lower Fezouata and Upper Fezouata formations, which unconformably overlie a palaeorelief of Cambrian rifting volcanosedimentary complexes, and are subsequently topped by a Dapingian para-conformable gap; (ii) the Darriwilian–Katian Tachilla Formation and First Bani and Ktaoua groups, the latter unconformably overlain by a Hirnantian glaciogenic succession; and (iii) the Second Bani Group, which subsequently infilled the former glaciogenic palaeorelief. Due to the scarcity of carbonate interbeds for etching analyses, leading to rare references of conodonts, the global Ordovician chart is interpolated on the basis of microphytoplancton (acritarchs and chitinozoans), regional graptolites and brachiopods. The Ordovician counter-clockwise rotation of Gondwana led its Moroccan margin from mid-to high-latitude positions, leading to the onset of a siliciclastic, wave-and storm-dominated platform. Flooding surfaces are marked by shelly silty carbonate interbeds that reflect the episodic development of echinoderm–bryozoan meadows during Katian times; in areas protected from siliciclastic input, they reached massive and bedded bioaccumulations (Khabt-el-Hajar Formation). The Hirnantian glaciation controlled the incision of numerous tunnel channels, infilled with both alluvial to fluvial sediments and glaciomarine diamictites. The Hirnantian palaeorelief was definitively sealed during Silurian times.
The authors thanks the useful and detailed revisions made by J. Colmenar, T. Servais and one 661 anonymous referee. This work is dedicated to the memory of Jacques Destombes who left us 662 during the redaction of this synthesis.This paper is a contribution to Spanish project CGL2017-663 19 87631-P from Spanish Ministry of Science, Innovation and Universities and IGCP 653 (IUGS-664 UNESCO) project ‘The Onset of the Great Ordovician Biodiversification Event’.