Devonian conodont and ammonoid succession of the eastern Tafilalt (Ouidane Chebbi section), Anti-Atlas, Morocco

Abstract

Devonian conodonts and ammonoids occurring in association in the Ouidane Chebbi section of the eastern Tafilalt (Anti-Atlas, Morocco) are described and analysed in terms of stratigraphy. The excellently exposed sequence spans the entire Devonian; it includes open-marine carbonates and shales deposited outside the tropical realm. Except for its Middle Devonian part, the Ouidane Chebbi section is not condensed. Although the sequence was only sampled in a preliminary fashion, most of it lithological units have been dated with some precision. The study is an attempt to enhance the precision in the correlation between the Devonian conodont and ammonoid sequences. Graphic correlation method was used for estimating the precise position of the zonal and stage boundaries for the Middle and Upper Devonian intervals. The stratigraphy of the Lower Devonian at Ouidane Chebbi is still poorly defined. The Eifelian/Givetian boundary is well constrained, especially by both conodonts and ammonoids. It occurs within the uppermost part of the Kacak Event-Level, a characteristic shale horizon that begins in the ensensis Zone and ranges into the hemiansatus Zone. Biostratigraphic indications from conodonts show that the top of the Pharciceras limestone unit is located within the norrisi Zone and the Givetian/Frasnian boundary should be placed within the characteristic black styliolinites level of the Frasne Event. The onset of the Kellwasser facies falls within the Zone 12 (winchelli Zone) of the Frasnian and it extends into the rhomboidea Zone. The combined evidence from ammonoid and trilobite data suggests that the marine deposition persisted at Ouidane Chebbi at least up to the time of the Upper praesulcata Zone.