Calpionelles et microfaunes pélagiques du Jurassique supérieur – Crétacé inférieur dans les Formations Shal et Kolur (Montagnes du Talesh, chaîne de l’Elbourz, Nord-Ouest Iran). Répartition stratigraphique, espèces nouvelles, révision systématique et comparaisons régionales

Abstract

Two sections corresponding respectively to the Shal (Callovian – Lower Berriasian) and Kolur Formations (Berriasian – Valanginian in the sampled interval) have been studied in the Talesh Mountains (North-West Iran), an area whose sedimentary successions formed on the northern margin of the Eastern Tethys during the Mesozoic. This study allowed the identification of the different calpionellid zones and sub-zones defined in basin successions of the Western Tethys. In the upper part of Shal Formation the Chitinoidella boneti (base of the Upper Tithonian) and B1 sub-zones (base of the Calpionella alpina zone, lowermost Berriasian) have been recognized. The rich and diverse calpionellid faunas allowed us to recognize in the Kolur Formation B3 (top of the C. alpina zone), C1, C2 (Calpionella elliptica zone), D1, D2, D3 (Calpionellopsis zone) subzones and E (Calpionellites zone) and F (Tintinnopsella zone). The E zone has been subdivided in two sub-zones: E1 and E2, which is characterized by the disappearance of the genus Calpionellopsis. Thirty calpionellid species have been described. Fifteen of them are new: Calpionellites medius nov. sp., Calpionellites minor nov. sp., Remaniella longocadischiana nov. sp., Remaniella minicadischiana nov. sp., Tintinnopsella curva nov. sp., Tintinnopsella duranddelgai nov. sp., Tintinnopsella elmii nov. sp., Tintinnopsella largovalata nov. sp., Tintinnopsella lehegarati nov. sp., Tintinnopsella longovalata nov. sp., Tintinnopsella maxilonga nov. sp., Tintinnopsella minicarpathica nov. sp., Tintinnopsella pseudoblonga nov. sp., Tintinnopsella pseudocarpathica nov. sp. and Tintinnopsella pseudosimplex nov. sp. Amongst the other pelagic microfossil groups, Globochaete and calcispheres (“microproblematics”), often abundant in Western Tethyan Upper Jurassic – Lower Cretaceous pelagic successions, are absent in the Shal Formation and extremely rare in the Kolur Formation despite its lithofacies usually suited for preservation of such microfossils. It is worth noting that in Kolur section the peak of Cadosina fusca in the D1 sub-zone and the occurrence of Stomiosphaera wanneri in the C zone have been observed similarly to successions of Moroccan external Rif. As in most basins of the Western Tethys, Saccocoma remains are frequent in the upper part of the Shal Formation and disappear after the appearance of Chitinoidella in the uppermost Tithonian. Radiolarians with spherical tests are represented by two morphologies:one of common size, called morphology A, occurs at different levels of both Shal and Kolur Formations, while the larger one, called morphology B, occurs in the Upper Tithonian of the Shal Formation and becomes frequent in the Berriasian – Lower Valanginian beds of the Kolur Formation. The first morphology is relatively frequent in coeval successions of Western Tethyan basins, while the second is rarely reported in the literature. Holothurian sclerites are often abundant in the Lower Berriasian – Lower Valanginian interval, whereas they are rare (or scarcely reported in the literature) in coeval successions of the southern margin of the Western Tethys. Chitinoidella, calpionellids, Saccocoma, protoglobigerinid forams, radiolarian of the morphology A are fossil groups that commonly occur in Tethyan basins. On the contrary, Globochaete and calcispheres were seemingly adapted to local ecological conditions, which were probably different from those of the study area. However, preservation bias for these particular microfossils cannot be excluded in the Kolur section, which is locally affected by faulting.