Biostratigraphy and conodont biofacies of Upper Devonian-Lower Carboniferous (Mississippian) deposits at Tuye-Darvar section, Southwestern Damghan, Eastern Alborz

Close to the DCB, Hangenberg Event have occurred, due to rising global temperatures, evidence of sea level rise, eutrophic conditions, anoxic conditions, rising carbon burial, formation of the Hangenberg Black Shale, and depletion of shallow and especially deep marine fauna in deposits (De Vleeschouwer et al, 2013). The Jeirud Formation in its type section (consists of sandstone, brachiopod-bearing dark shales, and phosphate horizons) diconformably covers the Mila Formation, and is continously underlain by the Mobarak Formation (Bozorgnia, 1973).

The Tuye-Darvar section location The Tuye-Darvar section is located Eastern Alborz Zone, adjacent to Darvar village (southwest of Damghan city) with base of the section: 36° 01' 27.31" N, 53° 53' 17.33" E; top of the section:36° 01' 19.32" N, 53° 53' 33.03" E.

Method of study:

After detailed field studies, 44 rock samples were collected to study the conodont fauna based on standard methods (Jeppson and Anehus, 1995). After sediment dissolution, washing and hand picking under binocular microscope, stub preparation and gluing conodonts on aluminum base, determining the color alternation index of conodonts with the help of standard tables, preparing SEM photos, naming and identifying conodonts and their biozones was performed based on global refrences (Sandberg et al, 1978; Ziegler et al, 1990; Hartenfels, 2011; Corradini et al, 2016; Spalletta et al, 2017).

Lithostratigraphy Jeirud Formation: The thickness of the Jeirud Formation in this section is 166 m, and includes the lower siliciclastic part with a thickness of 84 m and consists of 3 Lithostratigraphy units (included sandstone, conglomerate and shale) and the upper carbonate-siliciclastic part with a thickness of 82 m and consists of 3 Lithostratigraphy units (included sandstone, dolomites, microconglomerate, shale and limestone).Mobarak Formation: The Mobarak Formation disconformably covers the Jeirud Formation, with thickness 81.5 m thickness and underlies the vermiculite limestones of the Elika Formation (Triassic) through a fault surface. Based on the lithological characteristics, three stratigraphic units can be distinguished in this interval (including dark limestone and shale).

Conodont biostratigraphy:

Study on the conodonts in the mentioned section led to the discrimination 32 species within 10 genera: Bispathodus, Branmehla, Clydagnathus, Gnathodus, Polygnathus, Pseudopolygnathus, Palmatolepis, Protognathodus, Scaphygnathus, Siphonodella According to the age range of conodont species, 12 conodont zonation were identified, of which 9 biozones belong to the Late Devonian (Late Famennian) and 3 biozones belong to the Early Carboniferous (Mississippian).The Late Devonian biozones included: 1. Pseudopolygnathus granulosus Zone, 2. Polygnathus styriacus Zone, 3. Palmatolepis gracilis manca Zone, 4. Palmatolepis gracilis expansa Zone, 5. Bispathodus aculeatus aculeatus Zone, 6. Bispathodus costatusZone, 7. Bispathodus ultimus Zone, 8. Siphonodella praesulcata Zone to ckI Zone, 9. Protognathodus kockeli Zone And the Late Carboniferous included: 10. Siphonodella sulcata Zone – L. Siphonodella crenulata Zone, 11. Siphonodella isosticha-U. Siphonodella crenulata Zone – Gnathodus typicus Zone, 12. Scalignathus anchoralis-Doliognathus latus Zone Sea level changes and the paleoenvironment of the Tuye-Darvar section With combination of the 2 biofacies models to the Late Devonian deposits according to Sandberg and Dreseen (1984) studies and to the Early Carboniferous deposits according to Sandberg and Gutschick (1984) studies, conodont biofacies in the studied section was studied in the Late Devonian and the Early Carboniferous, that according to that, dominant biofacies in Late Devonian included (Polygnathid, Pseudopolygnathid, Bispathodid and Bispathodid-polygnathid) and dominant biofacies in the Early Carboniferous included (Bispathodid-polygnathid, Polygnathid, Pseudopolygnathid, Gnathodid and Gnathodid- pseudopolygnathid).

The lithostratigraphy study of the Tuye-Darvar section in Eastern Alborz Zone due to identified 9 lithostratigraphy unit belong to Jeirud Formation with the Late Devonian period and the Mobarak Formation with the Early Carboniferous period. 32 number of species within 10 genera were recognize, that base on this, 12 biozones in the Late Devonian and Early Carboniferous were separated. With study of biofacieces, in the Late Devonian and the Early Carboniferous, that according to that, dominant biofacies in Late Devonian included (Polygnathid, Pseudopolygnathid, Bispathodid and Bispathodid-polygnathid) and dominant biofacies in the Early Carboniferous included (Bispathodid-polygnathid, Polygnathid, Pseudopolygnathid, Gnathodid and Gnathodid- pseudopolygnathid).