Viséan transgression and reworking at Boudouda (NW Benahmed, western Moroccan Meseta)

Abstract

At Boudouda NW of Benahmed, upper Frasnian goniatite shales are in unconformable contact with massive, partly dolomitized conglomerate boulders of the new Oued Ayada Formation. They represent debris flow deposits of localized channels and contain reworked supposed Emsian reefal organisms and Frasnian to lower/?middle Famennian conodonts. Rare foraminifers prove upper Visean (Upper Asbian) sedimentation. Laterally, a ca. 35 m thick sequence of bioclastic limestones with crinoids, mostly fragmented brachiopods or gastropod-rich forms a second member. Thin-sections show a dominance of variably sorted wacke-, pack- and grainstones with abundant coated grains (initial ooids) and subangular fine quartz sand. Biota include calcareous green algae, foraminifers, and various Algospongia (Aoujgaliidae and Palaeobereselliidae). Throughout the succession, there are reworked Frasnian to middle Tournaisian deeper-water conodonts, which shows that a probably small-sized pelagic platform existed once in the region, which had been uplifted during the Eovariscan 2 tectonic phase. A single, juvenile, originally pyritic goniatite gives limited evidence for an upper Tournaisian/lower Visean goniatite shale, which has no equivalents elsewhere in the Meseta. Based on a rich record of foraminifers (more than 70 taxa) and stratigraphically meaningful other calcareous microfossils, polyphase erosion and re-deposition by storms occurred on a shallow-water, photic zone carbonate ramp in the upper Visean. The Oued Ayada Formation correlates with the transgressive limestones of the basal Melilla Formation in the southern Mdakra Basin and the Bled Mekrach Formation in the NE Rehamna.