Biostratigraphy of Shishtu 1 Formation in Howz-e-Dorah section based on the conodonts (eastern south of Tabas)

The study area is located in the zone of Central Iran based on the structural divisions of Iran. Late Devonian deposits in central Iran are widespread. The Shishtu Formation is a two-stroke stratigraphic unit that is subdivided into two sub-formations called Shishtu 1 and Shishtu 2. Below the Shishtu 1 Formation is the Late Devonian (Frasnian-Famenian) and the Shishtu 2 Formation is early Carboniferous (Tournisian-Visean). The boundary between them is marked by a horizon of black charcoal shales called the Mouse horizon. The Paleozoic and Lower Triassic sediments in the Lut Zone are essentially similar to the Shotori Mountain Formations.

Howz-e-Dorah section is located 57 km from Tabas town and 5 km from Deyhok village. After conducting library studies and selecting the appropriate shear in terms of thickness and separation of layers, the tectonic status of the area and the intended purpose by measuring layer thickness, fossil record collection, photography and sampling, extraction was conducted to extract the conodont elements.

The Shishtu Formation 1 is exposed at a thickness of 152 m in the Howz-e-Dorah section. The lower boundary of this sequence with the Bahram Formation is in disconformity with Middle Devonian and its upper boundary is in conformity with the Shishtu 2 Formation. Lithology of the Shishtu Formation 1 alternates from medium to thick bedded brown sandstones, green shales and thin bedded dolomitic white limestones, thin to thick bedded gray limestones, and highly diverse masses. It is high in conodont microfossils and macrofossils such as brachiopods, bryzoas, corals and crinoids. Notable points in this section are the presence of coral limestones, especially brachiopods, which are of late Devonian age in central Iran. The uppermost and last rock unit of the studied section is a 28 m outcrop of cephalopod horizon, outcropped as intermediate red limestone, indicating a short period of no sedimentation. Based on conodonts element studies, the sequence of late Shishtu Formation 1 to Late Devonian (Early Frasnian - Late Feminine) introduces 4 genus, 13 species, 4 subspecies with 4 conodonts biozonesThe anterior part of Shishtu 2 Formation ages back to Late Tournisian - Late Visean. 6 genus, 13 species, 4 subspecies, 13 species and 2 subspecies were identified in Howz-e-Dorah section. According to the studied conodonts elements, the sedimentary environment of the studied section is shallow and close to shore, and the conodonts species in the Shishtu 2 Formation are formed in oxygen and continental slope environments.

The study of Shishtu Formation 1 and the basal part of Shishtu Formation 2 resulted in the identification of a high diversity of conodonts in the Howz-e-Dorah section. In addition to the high diversity of conodonts, the diversity of invertebrates, especially brachiopods, has been very high. Depending on the biofacies, depth and depositional environment of the Shishtu Formation 1, due to biofacies, the environment was offshore. Also, the presence of conodonts species is characteristic of tidal environments in Shishtu 1 Formation. The base part of Shishtu 2 is formed in low oxygen environments which are common in lower continental slopes.