Biostratigraphy of Late Devonian-Early Carboniferous deposits in the Zoo section (northeast of Iran) based on conodont fauna

Devonian-Carboniferous boundary is an exciting issue for researchers all over the world. In this boundary, significant events have occurred on marine organisms (specially conodonts elements) and terrestrial organisms, consisting of weather and sea-level changes, and relative coincidence with the Hangenberg bio-event (Mc. Ghee, 1998; Sandberg et al., 2002; Caplan & Bustin, 1999; Kaiser et al., 2006, 2011, Bahrami et al., 2011, 2019; Konigshof et al., 2021; Sattari et al., 2021). According to ICS, the Devonian-Carboniferous boundary, as defined by the first Appearance Datum (FAD) of the conodont species Siphonodella sulcata within the Siphonodella praesulcata- Siphonodella sulcata lineage and the GSSP is located in the La Serre Trench E’ section in France (Paproth et al. 1991). Khoshyeilagh Formation in Zoo section consists of shallow marine deposits attributed to the Late Devonian (Frasnian-Famennin) (Najjarzadeh, 1997). This study focuses on identifying and reviewing the Devonian-Carboniferous boundary in a part of the Eastern Alborz area and study the biostratigraphy of deposits near this boundary based on conodont fauna.

58 carbonate and 11 non-carbonate samples (3-4 kg/each) were collected and processed for conodonts. The samples were processed with the conventional acetic acid technique, but formic acid 10% with 10 hours was also used for some silty samples and compacted limestones. Then picked remaining deposits with the handpicking method. The following steps include pasting the conodont specimens on stab, photographing them under an SEM microscope, and naming conodonts elements based on available references (e.g., Branson & Mehl, 1934; …). Accordingly, the biozonation of the studied sequence using the global standard zonings of Ziegler & Sandberg (1990), Sommervile (2008), Poty et al. (2006), Spalletta et al. (2017).

Khoushyeilagh and Mobarak formations Late Devonian-Carboniferous conodonts fauna of Zoo section, (NE Jajarm), led to identifying 10 conodont genera, 23 species, and 8 subspecies. The conodont faunas which be obtained in this research indicate the presence of these biozones: Bispathodus ultimus Zone (Latest Famennian), From the base of Siphonodella duplicata to within Siphonodella crenulata interval (Tournaisian), and Pseudopolygnathos multistriatus Zone (Visean). According to this study, the Fm/T boundary is located at the end of Khoushyeilagh formation, 6m above the diabase layer. The conodonts, as mentioned above, are indicators for Polygnathid biofacies. These sequences are similar to other Famennian-Tournaisian deposits in the east Alborz, both in the litho-facies and bio-facies peculiarities. The conodont Color Alteration Index in the Carboniferous conodonts element increased compared to the Late Devonian elements.