Early Triassic conodonts of the Sorkh-Shale Formation in Rabat-Shor section of (West of Tabas, East of Central Iran); investigation of their alteration index concerning the hydrocarbon production capacity

Sorkh Shale Formation in Rabat-e-Shur stratigraphic section, west of the Tabas Depression, has been biostratigraphically investigated in this research. The Sorkh Shale Formation, with 48 meters thickness in this section, consists of shale, marl, and marly limestones, which are underlain by the Permian Jamal Formation and are overlain by the Triassic Shotori Formation dolomites. Nine conodont species belonging to four genera, Ellisonia, Hadrodontina, Parachirognathus, and Pachycladina were identified for the first time from this formation. The mentioned conodont assemblage is divided into three biozones: Hadrodontina aequabilis, Hadrodontina anceps, and Pachycladina obliqua. These taxa are in good agreement with the previously reported euryhaline assemblages from the near-shore and shallow water Western Tethyan realms in the south and east of Europe in the Early Triassic (Late Griesbachian-Middle Smithian). These conodont biozones are used for a better understanding of the distribution and affinities of the Early Triassic conodonts in West Asia, their paleobiogeographical rank, their significant role in the Early Triassic chronostratigraphy and their importance in bio-correlation of different section worldwide. Examination of the conodont alteration index of the obtained taxa from the Sorkh Shale Formation demonstrates CAI=5 during the late Griesbachian, CAI=4 in the Smithian, and CAI=6-8 in the Dienerian substages which is placed in the barren Zone for hydrocarbon production.