CARBONIFEROUS CONODONTS INDICATE REVISED AGE OF “FAMENNIAN” CRINOID HORIZON IN SOUTHEASTERN IRAN

The Devonian/Carboniferous stratigraphic section southeast of Hutk, southeastern Iran is easily accessible and one of several good reference sections within the region. It is well exposed, fossiliferous, and has been the subject of previous studies, including part of the 1998 IGCP 421 field conference. In the Hutk section the Bahram Formation was reported to be Late Devonian and Early Carboniferous, including over 100m of both Frasnian and Famennian strata. A similar stratigraphic section, slightly less accessible, occurs northeast of Shams Abad, 35 kilometers southwest of Hutk. A rich crinoid horizon, one to two meters thick, was found in what had been considered the Famennian, approximately 15m above the Frasnian/Famennian contact. The fauna, numerically and systematically dominated by glossocrinids, contains additional cladids, camerates, and flexibles. The crinoid-bearing beds initially were considered to be Famennian because the crinoid fauna compares more closely with Famennian than with coeval or nearly coeval Tournaisian crinoid faunas from Germany, England, and North America, and because of the previously assigned stratigraphic age. Conodonts retrieved from matrix of in situ blocks containing the crinoid specimens came from two localities. Conodonts from the Shams Abad area (lower horizon) included Polygnathus bischoffi, P. bartensis, and P. communis communis. The combination of P. bischoffi and P. bartensis indicates an Early Carboniferous age within the Lower crenulata Biozone to isosticha-Upper crenulata Biozone. Conodonts from southeast of Hutk (upper horizon) yielded Protognathus praedelicatus, Gnathodus typicus, and Polygnathus communis communis, which suggest that the horizon can be no older than isosticha-Upper crenulata Biozone (because G. typicus first occurs within this zone) and no younger than anchoralis-latus Biozone (because all three taxa are known to occur in this biozone). Consequently, the crinoidal interval may span parts of two Early Carboniferous conodont biozones. This new information also indicates that the Famennian, if present, is much thinner in the Hutk region than previously recognized.