Biostratigraphy of the Upper Devonian Khoshyeilagh Formation in NE Iran based on conodonts and other faunas

Abstract

The Khoshyeilagh Formation located in the northeast of Iran yielded seven conodont species and sub-species and 19 macro- and microfossil taxa that allow recognition of two conodont biozones and one biozone based on calcareous microfossils. The latest Frasnian age (the Upper rhenana to linguiformis zones) is attributed to the topmost strata with Icriodus alternatus. Its replacement with I. cornutus indicates the Famennian (Lower triangularis to Lower crepida zones) corresponding to the Umbellina Zone. The fossil assemblages identified in the Khoshyeilagh Formation represent a shallow marine environment with a tropical climate at the time of deposition. The fossil species from the Khoshyeilagh Formation and the sedimentary basins in Algeria, Libya, Morocco, Poland, and Russia reveal a close connection between the Iranian plateau and the northern parts of Gondwana in the Late Devonian. The biofacies and lithofacies analyses show a sea-level decline at the end of Frasnian, followed by a considerable sea-level drop, as in other regions of the world. After a short time, in the Famennian, the deepening occurred in some parts of the area and the open marine facies (bioclast spicule wackestone-packstone) were deposited. This study is the first attempt to determine the Frasnian-Famennian boundary based on conodont assemblages and other fossil species such as umbellulids, tentaculites, and ostracods. The distribution of these species is interpreted in sedimentological, stratigraphic, sequence stratigraphy, and the global eustatic context. Keywords: biofacies, conodont, Frasnian-Famennian boundary, Late Devonian, NE Iran.