Conodont biostratigraphy of the Frasnian-Famennian boundary in the Esfahan and Tabas areas, Central Iran

Abstract

The Frasnian-Famennian (F-F) boundary in Central Iran has been investigated on the basis of conodont faunas (34 species and subspecies) from four sections: Chahriseh near Esfahan, and Kal-e-Sardar, Howz-e-Dorah and Ghale-Kalaghu near Tabas. The F-F boundary in the Chahriseh section is located in a one-metre interval between beds EX1 and F-F9 whereas in the Kal-e-Sardar section it is at the base of bed Cly1. The F-F boundary cannot be recognized in the Howz-e-Dorah and Ghale-Kalaghu sections because of unconformable relationships and erosion of the uppermost late Frasnian beds indicated incidentally by reworked boulders (indicating contemporaneous tectonic activity) and by the abrupt appearance of contrasting environments, including tempestites at the base of the lower Famennian. The best conodont and palaeoenvironmental data were obtained from the Kal-e-Sardar section where a deeper marine environment prevailed during the late Frasnian, becoming shallower in the early Famennian; the Chahriseh section displays small fluctuations in sea level during the early Famennian. The conodont faunas display the inception of the Polygnathus communis group in the late Frasnian (rhenana-linguiformis zones) and the appearance of Icriodus alternatus mawsonae in the Late rhenana Zone. A new age-range is suggested for Polygnathus aequalis Klapper and Lane, from the transitans to the linguiformis zones. Three conodont biozones are represented in the late Frasnian to early Famennian of the Chahriseh section, two late Frasnian and one early Famennian in the Kal-e-Sardar section and two biozones in the late Frasnian and early Famennian of both the Howz-e-Dorah and Ghale-Kalaghu sections. Two new species are described: Polygnathus tabasianus (Early to Late crepida zones) and Polygnathus vachiki (Late rhenana-linguiformis zones).