Biostratigraphy of Paleocene- Eocene deposits based on foraminifera in south of Birjand, Iran

The studied section is located in the eastern margin of Lut block, 38 kilometers south of Birjand (Iran) in the east of Ali Haji village. It is situated within the geological sheet map of Birjand (Ohanian and Tatevosian, 1978).The present study aimed to identify foraminifera assemblage in the measured section and present a biostratigraphic scheme. In addition, we evaluated the correlation of the studied section with other previously studied sections in the vicinity of Birjand.

This study was conducted on 10 rock samples and 29 soft shale and marl samples. The thin sections were photographed. After washing and separating the foraminifera from the sediments, imaging was performed using Scanning Electron Microscope (SEM).

The section starts with a basal conglomerate, which is overly on the ophiolites (peridotite) unconformably. This conglomerate is composed of peridotite fragments, which are gradually converted into sandstone and calcareous sandstone. The succession continues with thick, cream-colored foraminifera limestone. The upper portion of the lower sequence and the middle segment mostly contain light-to-dark gray marl with thin layers of shale (colored gray-to-olive). The upper portion could be divided into two parts; the lower part contains most of the gray shale and sandstone with some trace fossils, and the upper parts mostly contain a grayish-green shale, which is formed between the layers of marl and gradually disappears, causing a reddish-brown shale to appear on top of the sequence. In addition, the biostratigraphic studies in Ali Haji section led to the identification of twenty genera and thirty species of benthic, as well as five genera and nine species of planktonic foraminifera. As a result, five biozones were introduced, including ; 1- Alveolina assemblage zone 2- Morozovella aragonensis interval zone 3- Turborotalia cf. possagnoensis taxon range zone4- Subbotina bolivariana taxon range zone 5- Uvigerina jacksonensis assemblage zone. Based on the introduced zones, the age of the studied section is suggested a late Paleocene to late Eocene. Furthermore, the comparison of the Ali Haji section in the south of Birjand showed correlations with the Ching Dar and Grong sections, the Dahan Rud section, and the Friznook section in the north of Birjand. The thickness of the detrital units at the base of the section during the Paleocene was observed to be more significant in the north of Birjand compared to the south, which could be attributed to the further subsidence of the basin due to the activity of synsedimentary faults, earlier transgression in the north, or higher altitudes in the south (paleo-high) in Paleocene.The limestone units in the southern part will be transformed into marl and marly limestone units. In the northern part, the thickness of limestone was considered significant in Ching Dar and Dahan Rud sections.In the Dahan Rud section, sandstone and conglomerate intercalations were thicker compared to the other sections. Although limestone and marl units were identified in the Friznook section, the detrital facies was thoroughly spread possibly due to flysch deposits with fossil trace diversity.

According to the results, the Ali Haji section in the south of Birjand with a thickness of 560 meters had conglomerates, limestone, nummulitic limestone, marly limestone, marl, shale, and sandy limestone. Due to the distribution and identification of foraminifera in the Ali Haji section, a late Paleocene to the late Eocene age is speculated. Given the disconformity boundary of ophiolites and Paleocene deposits, it could be concluded that the ophiolite placement occurred before the Late Paleocene.