Middle Devonian reef facies and development in the Oued Cherrat Zone and adjacent regions (Moroccan Meseta)

Abstract

Middle Devonian reefs were widespread on the various synsedimentary tectonic blocks of the Moroccan Meseta, which formed the southern external Variscides. So far, poorly studied biostromes and small to medium-sized bioherms have grown in the southern subtropics, separated by the ca. 3000-km-wide Prototethys from the well-known reefs of the southern Laurussian shelf. Autochthonous, displaced, and reworked Givetian reefal limestones of the Oued Cherrat Zone south of Rabat and from adjacent regions were studied by outcrop facies logging and microfacies analysis. The new data are used to reconstruct their paleoecology, developments in time and space, and contribute to the understanding of reef facies differences along a paleolatitudinal gradient. Eighteen microfacies (sub) types are recognized, described, and illustrated. They represent thick inner-reef platforms (Ain Khira South, Jennabia), different parts of a synsedimentarily tectonized bioherm (Ain-Al-Aliliga), a small, open neritic biostrome/low bioherm (Ain-as-Seffah), the episodically retro-and prograding outer margin of a thick biostrome to bioherm sequence (Sidi Mohamed Smaine), and reworked reef remnants preserved in proximal to distal olistolith units (Ain Dakhla, Sidi Ahmed Lhemdoun, Biar Setla Conglomerate). Meseta reefs differed from those of stable northern Gondwana (western Anti-Atlas to southern Algeria) but show many similarities with the more distant Rhenish organic buildups. This suggests a very low Givetian paleoclimatic gradient in the southern hemisphere. The lack of facies types dominated by calcimicrobes/calcareous algae is distinctive for the study area.