The global Annulata Events: review and new data from the Rheris Basin (northern Tafilalt) of SE Morocco

Abstract

AbstractA review of the literature shows that the Famennian global Annulata Event(s) can be recognized as a transgressive, often hypoxic and eutrophic, interval that interrupts an overall regressive eustatic trend in more than 40 regions of North America, Europe, North Africa, Asia and Australia. According to differences in palaeogeography, sedimentology and biota, these occurrences are assigned to 10 event settings. The first detailed data on facies, ammonoid and conodont faunas are presented for the Rheris Basin of the eastern Anti-Atlas (southern Morocco) and compared with previously studied sections of the adjacent Tafilalt Platform, Tafilalt Basin and Maider Basin. The rather argillaceous succession at El Gara resembles the Tafilalt Basin (Hassi Nebech section) in its lack of black shales/limestones and similar ammonoid and conodont assemblages. However, the Sulcoclymenia sulcata Zone (Upper Devonian III-C2) below the Annulata Events contains ammonoid taxa that are unique for all of the Anti-Atlas and North Africa: Protornoceras ornatum Dybczynski, 1913, Genuclymenia aff. angelini (Wedekind, 1908), Protactoclymenia aff. implana (Czarnocki, 1989) and ?Pleuroclymenia sp. juv. The first regional record of the marker conodont Pseudopolygnathus granulosus Ziegler, 1962 also distinguishes the pre-event assemblage. As in many other regions, there is a major decline in ammonoids well before the Lower Annulata Event, which suggests an episode of extreme oligotrophy. Both Annulata Events at El Gara are whitish-weathered marly shales with only small specimens of Platyclymenia and Prionoceras (sensu lato), which are also typical for the annulata Zone (UD IV-A) of other Tafilalt sections, but benthonic organisms are nearly absent. This suggests local low-oxygen conditions, but only a moderate production of organic carbon, insufficient for black shale formation, unlike many German sections or in the Maider Basin (section Mrakib). The latter region represents a deeper shelf basin that had much higher productivity and a unique ‘Gundolficeras–Erfoudites–Protactoclymenia–Stenoclymenia–Guerichia biofacies’ of the Lower Annulata Shale. The upper part of the annulata Zone at El Gara is characterized by Platyclymenia (Platyclymenia) levata n. sp. Other new taxa of the same zone in the Anti-Atlas are Posttornoceras ascendens n. sp. and Stenoclymenia rectangula n. sp. Whilst the ammonoid faunal overturn between UD III-C and UD IV-A was severe, the strong reduction in conodont diversity with the two Annulata Events was mostly (apart from two taxa) a palaeoecologically triggered, only episodic, feature. The comparison of the various Anti-Atlas Annulata Event beds and assemblages enables the distinction of event biofacies types, which reflect local differences of bathymetry, trophic conditions and seafloor ventilation.