Brachiopods and rugose corals in an upper Serpukhovian (Mississippian) biostrome: preliminary results from the Djebel Arhlal (Béchar Basin, Algeria)

Abstract

The Djebel Arhlal is the southernmost outcrop of upper Serpukhovian strata in the Béchar Basin. Here the Djenien member of the Djenien Formation is three-folded and it contains a 10 m-thick coral biostrome in its middle unit formed by Siphonodendron, Diphyphyllum and Lithostrotion. Distances between in situ colonies are in the order of several decimetres and the space between them is filled with bioclastic limestone containing coral rubble. Few brachiopods and solitary corals are found as dwellers within the biostrome, but these groups are much more common in the strata below and above the coral biostrome. This is especially true for the brachiopods, which reach diversities of more than a dozen species in specific horizons. They mainly belong to the orders Productida, Spiriferida and Athyridida. Agitated open-marine platform interior or platform margin settings are the general facies encountered in the Djenien member at the Djebel Arhlal. The field data, including brachiopod coquinas and many fragmented brachiopods and corals, are confirmed by microfacies analysis. However, the coral biostrome records a quieter setting at the interface of middle and outer ramp settings, as seen in the co-existence of in situ coral growth, input of reworked material, deposition of carbonate mud, and sparitic textures. This autoparabiostrome at Djebel Arhlal is compared to other rather thick and of exceptional horizontal extension upper Serpukhovian biostromes (few kilometres).