Conodont biostratigraphy of Upper Silurian formations from the Rabat-Tiflet area (northwestern Meseta, Morocco)

Abstract

Abstract Silurian sections from the Rabat-Tiflet area (northwestern Moroccan Meseta) have been systematically sampled for conodonts. The conodont faunas are characterized by two index species of the European Silurian zonation (Ozarkodina sagitta sagitta and Ozarkodina remsceidensis eosteinhornensis) and two other index species of the north American zonation (Kockelella stauros and Kockelella variabilis). Recognition of the sagitta/stauros through lower variabilis zones in the lower part of the Silurian limestone and shale succession from the Rabat-Tiflet area, proves for the first time the presence of Wenlock below Ludlow-Pridoli, in the Silurian sequences of this area. Most of the Silurian from the northwestern Moroccan Meseta is represented by black graptolitic shales with intercalations of limestone (beds or lenses) in its upper part. The Silurian biostratigraphy of Morocco is generally based on graptolites [Willefert in Destombes et al., 1985]. In the present contribution the age of some Silurian sections (fig. 3) from the Rabat-Tiflet area is re-assessed by using conodonts. The stratigraphically important taxa have been described in Benfrika [1999]. Section Oued Grou I (G I) Situated near the barrage Mohamed Ben Abdellah, this section shows 30 m of alternating black shales and limestones. The lower part of this unit (I) provided : Ozarkodina sagitta sagitta, Kockelella absidata, Kockelella stauros, Ozarkodina bohemica, Ozarkodina excavata, Dapsilodus obliquicostatus, Panderodus unicostatus, Pseudooneotodus bicornis. The first appearance of Oz. sagitta sagitta and the K. stauros indicates the lower boundary of sagitta Zone established by Walliser [1964] in the Carnic Alps and also the lower boundary of stauros Zone proposed by Barrick et Klapper [1976] for North America (fig. 2). These species are of Wenlock age. Section Oued Grou II (G II): This section is located at approximately a hundred meters north of the preceding section. It is subdivided into 2 units : Unit IIA : 12 m of greyish to black limestones interbedded with green shales ; Unit IIB : 65 m of shales interbedded with some limestones. The conodonts identified in unit A are : Ozarkodina bohemica, Kockelella absidata and Kockelella stauros. This fauna is of Wenlock age. The first occurrence of Kockelella variabilis in sample indicates the lower variablis zone (lower part of Ludlow). Section Al Khaloua Iron Mines (AK) The limestones interbedded with shales of the lower part of the Al Khaloua section yield Polygnathoides emarginatus and Kockelella variabilis. This association is of Ludlow age. The conodonts from the upper part of this section (Caudicriodus sp. and Belodella devonica) indicate a Lower Devonian age. Section Oued Tiflet syncline (T) Two units are recognized : Unit A : 18 m of alternating black shales and greyish black limestones providing : Kockelella absidata, Ozarkodina bohemica, Decoriconus fragilis, Dapsilodus obliquicostatus, Ozarkodina excavata. This fauna is of Upper Wenlock to Lower Ludlow age ; Unit B : 34 m of black shales with some limestone intercalations, is probably of Ludlow-Pridoli age. Conclusion The Silurian conodont succession recognized in the Rabat-Tiflet area (northwestern Moroccan Meseta) allows to attribute the lower part of alternating limestones and shales to the Wenlock and the upper part to the Ludlow-Pridoli. The presence of the Wenlock in the Rabat-Tiflet area and also in the Oued Cherrat (under investigation) is demonstrated by conodonts. This suggests that the Silurian transgression has reached this area of the Moroccan Meseta during the Wenlock and not the Ludlow, as generally admitted. Furthermore, the conodont faunas allow to recognize affinities with those from North America and Central Europe. Two index species of the conodont zonation established by Walliser [1964] for the Carnic Alps have been recognized in Morocco, two other index species of the conodont zonation proposed by Barrick and Klapper [1976] for North America have been also identified in Morocco.