Neritic–pelagic correlation in the Lower and basal Middle Devonian of the Dra Valley (Southern Anti-Atlas, Moroccan Pre-Sahara)

Abstract

Marine Lower Devonian successions are widely exposed in the Dra Valley (Southern Anti-Atlas, Moroccan Pre-Sahara). Resulting from new studies, especially on brachiopods, conodonts, and dacryoconarid tentaculitids, the chronostratigraphic assignments of the Lower Devonian formations are revised. Thanks to lateral and vertical facies variations, it is possible to correlate pelagic and neritic successions and corresponding biostratigraphies. Pelagic conodont, dacryoconarid and goniatite faunas allow correlations and dating in the sense of the Bohemian and global chronostratigraphies, whereas units of the traditional Rhenish subdivision can be identified by means of neritic brachiopods.

Morphological trends and new species of Cyphaspis (Trilobita, Otarioninae) in the Devonian of Morocco, Turkey, Germany and Belgium

“…Haas regarded the occurrence of the ammonoid Gyroceratites gracilis (Bronn, 1837) in the Gebze Formation as additional evidence in support of an Eifelian age. Jansen et al (2007) underlined the Emsian age of G. gracilis while referring to Bultynck & Hollard (1980) for its occurrence in the Khebchia Formation (early upper Emsian) in Morocco. This last unit is also the origin of C. lerougei, a species which is very similar to C. goerlichi.…”

Zlíchovian faunas with early ammonoids from Morocco and their use for the correlation of the eastern Anti-Atlas and the western Dra Valley

“…A "Rich" is typically composed of a lower limestone unit (crinoidal, marly to nodular limestones) overlain by alternating beds of claystones, siltstones and fine sandstones and, at the top, coarse, cross-bedded, massive sandstones with a neritic fauna (Hollard 1981a, Jansen 2001, Becker et al 2004d, Jansen et al 2007. A certain lateral variation characterizes these Rich sequences and allows distinguishing between an eastern and a western succession.…”

“…We focus here on the upper part of Rich 2 (upper part of the Merzâ-Akhsaï Formation) and on the lower part of Rich 3 (lower part of Mdâour-El-Kbîr Formation) in the eastern Dra. The lower part corresponds to the TST of the Rich 3-cycle and the base is usually interpreted as corresponding to the basal Zlíchov Event sensu Chlupáč & Kukal (1988); see Jansen et al (2007), Ouinaimi & Lazreq (2008) and Lubeseder et al (2009). The overall increasing thickness of sedimentary rocks towards the SW indicates a predominant provenance of siliciclastic supply and the presence of erosive land masses towards the SW and W (Becker et al 2004d, Lubeseder et al 2009).…”

“…Mdâour-El-Kbîr Formation was studied recently, about 25 km SW of Foum Zguid (Jansen 2001;Jansen et al 2004Jansen et al , 2007, but there, the lower part of the Mdâour-el-Kbîr Formation with the basal limestone is not exposed (Jansen et al 2004). It is underlain by the Merzâ-Akhsaï Formation, which is also present in the SW Dra Valley.…”

“…Recently, the Lower Devonian lithostratigraphy and biostratigraphy of the Dra Valley area were reviewed by Jansen (2001), Becker et al (2004d) and Jansen et al (2007). A correlation between the two regions has proved to be difficult because of the more neritic facies of the Dra Valley and the rather pelagic facies of the Tafilalt and partially also the Maïder.…”