Biostratigraphy, biofacies, and palaeoecology of the Upper Devonian deposits (Khoshyeilagh Formation) based on conodonts in the northeast of Iran (around Bojnord and Jajarm)

Abstract

AbstractIn this study, the paleoecology, biostratigraphy, and biofacies of the Khoshyeilagh Formation hve been evaluated based on the found conodont species. The Khoshyeilagh Formation includes terrigenous-carbonate sequences, which contain conodonts of shallow environments (Icriodus Polygnathu). The identification of seven conodont species and sub-species led to the identification of 2 conodont biozones which are Upper rhenana-linguiformis, Lower triangularis-Lower crepida, respectively, and also biofacies named as Icriodus-Polygnathus. The identified biozones prove Late Devonian (Frasnian–Fammenian) age for the studied parts of the Khoshyeilagh Formation. The determined biofacies and the recognized conodonts in the studied sections represent a shallow marine environment and warm tropical climate during the deposition time. Considering the obtained fossil assemblages and comparing with the other sedimentary basins, the studied sections belong to the northern parts of Gondwana land. The analysis of biofacies and lithofacies show a decrease of depth at the end of Frasnian which is followed by an intense sea level drop similar to global examples. After a period of time and during Fammenian, the depositional environment became deeper and open marine sub-environments (bioclast spicule wackestone-packstone) were deposited.Keywords: Paleoecology, Biostratigraphy, Late Devonian, Conodont, NE Iran, Bojnord, Jajarm IntroductionIn 1954, limited stratigraphic studies were carried out in the studied areas while comprehensive evaluations were performed by Bozorgnia et al. (1979). Asilian-Mahabadi (1995) published the first report of Devonian deposits of the area. Bozorgnia (1979) divided the Khoshyeilagh Formation into six units in the type section based on fossil assemblages. Brice et al. (1974) studied the brachiopods of the area and introduced the Late Devonian age for these successions. Ghavidel-Syooki and Owens (2007) studied the palynomorphs of the Khoshyeilagh Formation in Kuh-e Uzon section and estimated the Late Devonian age and determined warm tropical climate and shallow environment during the formation time. Wendt et al. (2002) determined Frasnian–Fammenian boundaries in the Khoshyeilagh Formation based on the recognized conodonts and the other fossil species. Ahmadzadeh Heravi (1975) studied the brachiopods and conodonts of the Khoshyeilagh Formation and reported the Early to Late Devonian ages for these successions. Hamdi and Janvier (1981) studied the Khoshyeilagh Formation and measured the thickness of this formation 1510 m and attributed the Early to Late Devonian age to this Formation. Weddige (1984a, b) studied the conodonts of the Khoshyeilagh Formation and proposed the Eifelian to Givetian ages for the lower part of the Formation. Ashouri (2006) reported six new species of conodonts in the type section of the Khoshyeilagh Formation and proposed Eifelian to Tournaisian age for this unit. The main objective of this study is the evaluation of the conodont species of the Khoshyeilagh Formation, which have not been studied in detail earlier. Material & MethodsAfter preliminary field evaluations, three stratigraphic sections including Gardaneh-e Pelmis, Robat-e Ghareh Bil, and Kuh-e Uzon sections were selected in order to study the Khoshyeilagh Formation. The samplings were carried out systematically and according to determined intervals. The samples were taken regarding facies changes and close to the formation boundaries sampling was done in short intervals. A total of 59 samples from limestone units (i.e., 17 samples from Pelmis section, 19 samples from Robat-e Qarehbil section, and 23 samples from Kuhe Ozon section) were collected for conodont fauna studies. To obtain isolated samples, about 4 kg of each sample was leached and placed inside 85% formic acid for a week. After dissolution, the obtained sediments were passed through 60, 150, and 200 mesh sieves and leached. The conodont elements and the other microfossils (the fish remains, ostracoda, brachiopoda, and gastropods) were identified, separated, and studied under an optical microscope. Eventually, the required photos were taken using a scanning electron microscope (SEM; Leo 1450VP) at the laboratory of the National Iranian Oil Company (NIOC). Discussion of Results & ConclusionIn its simplest sense, the word “Paleoecology” means studing the interaction between fossilized species and living environments. The determination of palaeoecological parameters of extinct conodonts is a complicated process, especially that the biological relationship between these species and their contemporary living species has not been well understood.An ideal ecological identification depends on comparing conodonts with other components. In this regard, the lack of zoological relationship has made this work more complicated. The carried out studies on conodonts show different zonations. During Late Devonian, the endemic forms of conodonts were decreased while several forms such as cosmopolitan fossils show an increase (Sandberg and Dreesen, 1984). Regarding the negligible occurrence of the Late Devonian conodonts and considerable distribution of some species (cosmopolitan fossils), the studied conodonts show similarities with the Gondwana and Alpine fossil assemblages.The presence and lack of conodonts in different sedimentary facies are related to energy level, temperature, salinity, and the nutrient availability. In other words, the best environment for conodonts is bedded limestones containing brachiopods, corals, and fossil-bearing deposits in well-oxygenated waters (Sandberg and Ziegler 1979).Conodonts are frequently found in glauconitic deposits, whereas they are rarely found in stromatolites (Sandberg and Ziegler 1979). Recent studies represent an adaptation between conodonts and benthic to pelagic environments. Previous studies performed in the study area have focused on conodonts, brachiopods, and their palynomorphs in the type section, while the conodonts of the three mentioned sections have not yet been studied.By studying 59 conodont samples, seven species and subspecies of Icriodus and Polygnathus have been recognized: Icriodus alternatus alternatus, Icriodus expansus, Icriodus symmetricus, Icriodus brevis, Icriodus cornutus, Icriodus sp., Polygnathus praepolitus, Polygnathus xylus xylus, Polygnathus sp.The evaluation of conodonts in Khoshyeilagh Formation with Late Frasnian–Early Famennian age revealed two conodont biozones including Upper rhenana-linguiformis of the Late Frasnian and Lower triangularis-Lower crepida of Early Famennian. Based on the recognized conodont species and Icriodus-Polygnathus biozones, the shallow and shelf margin reef environments have been estimated for these parts of the Khoshyeilagh Formation. Thin section studies show grainstone texture related to a bioclastic bar setting. The comparison of biofacies of Late Devonian in the study area represents different conodont species related to the depth of the environment such that shallow marine species are found frequently whereas pelagic species are absent. Considering the obtained conodont species from the study areas and comparedwith the Gondwana sedimentary basins in Australia, Iranian plateau was most likely connected to the Gondwana land during Devonian. The sedimentary deposits of Devonian in Iranian plateau show different lithofacies formed in different tectonic settings. It is worth noting that the changes in the thickness of sediments and their situation in the stratigraphic columns have been changed due to the function of main faults and the formation of horst-graben structures.

Keywords

• paleoecology
• biostratigraphy
• late devonian
• conodont
• ne iran
• bojnord
• jajarm