جستجوی مقالات مرتبط با کلیدواژه « نانواستراتیگرافی » در نشریات گروه « زمین شناسی »

Calcareous nannofossils are one of the most important groups used for oceanic sections biostratigraphy. This research has studied calcareous nannofossils of the deposits attributed to Oligo- Miocene in Lut Block (eastern Iran). Previous nannofossils studies on Paleogene and Neogene deposits of east Iran were done by Hadavi & Zohravi (2010); Jalili et al. (2017). The present research is another investigation of calcareous nannofossils in the Avich section. The studied section with 550 meters thickness is located in the southwest of Qayen city and the south of the Avich village. The lithology of the deposits consists of sandstones, argillitic limestone, and green marl with interbedded sandstone. This study aims to determine the calcareous nannofossil assemblages and discuss the possibility of applying the standard zonation and age determination.

In this study, 20 samples from the upper part of the deposits attributed to Eocene, 100 samples from the lower part of the deposits attributed to Oligo-Miocene, and 100 samples from the upper part of these deposits from the Avich section have been studied. Samples were prepared as a smear slide method (Bown & Young, 1998). The examination of nannofloras was performed at 2500x magnification. All images were taken in either cross-polarized light (XPL) or (PPL). All calcareous nannofossil specimens were identified by using the taxonomic schemes of Perch- Nielsen (1985) and Bown & Young (1998).

Calcareous nannoplanktons are widespread in the present oceans, from coastal areas to open ocean settings. They are suitable for biostratigraphical studies since they are abundant, planktonic, rapidly evolving, and largely cosmopolitan. Regarding calcareous nannofossils, 45 species belonging to 18 genera have been recognized at the studied interval in the Avich section. In the present samples, structures of the central area in many species such as Helicosphaera and Chiasmolithus; indicate the calcareous nannofossils in this section have medium to relatively low preservation. The most important identified species are Chiasmolithus oamaruensis, Isthmolithus recurvus, Discoaster druggii, Triquetrorhabdulus carinatus, Discoaster kugleri, Catinaster coalitus, and Discoaster hamatus. Some species belonging to Discoaster, Catinaster, Triquetrorhabdulus and Isthmolithus are existed from the lower part or upper part of the studied section. Other taxa such as Hughesiu, Coccolithus, Tetralithoides, Sphenolithus, and Helicosphaera are present in most samples.In the present study, Martini's standard calcareous nannofossil zonation (1971) was applied. In this zonation, the NP abbreviation represents Paleogene nannoplanktons (Nannoplankton Paleogene), and NN abbreviation means Neogene nannoplanktons (Nannoplankton Neogene). In this study, according to the first and last occurrences of marker species of nannofossils, two calcareous nannofossil biozones NP18 (Chiasmolithus oamaruensis Zone) and NP19 (Isthmolithus recurves Zone) in the upper part of Eocene deposits, NN2 (Discoaster druggii Zone) and NN3 (Sphenolithus belemnos Zone) biozones in the lower part of Miocene deposits, and NN7 (Discoaster kugleri Zone) and NN9 (Discoaster hamatus Zone) in the upper part of Miocene deposits were recognized.Calcareous nannoplankton zones, especially the presence of NP19 at the uppermost part of Eocene deposits and the presence of NN2 at the lowermost portion of Miocene deposits, indicate that the investigated boundary has the age of Priabonian – Aquitanian in this area. Other determined biozones represent the age of the Aquitanian-Tortonian for the studied deposits attributed to Oligo-Miocene in the Avich section. In this section, the absence of NP20-NN1 biozones may be due to the facies changes and the lack of calcareous nannofossils in sandstone rocks of the succession or the tectonic activities between the Eocene and Miocene deposits.

The nannofossil assemblages of the Oligo-Miocene deposits of the Avich section are relatively good in diversity with medium to relatively low preservation. Index calcareous nannofossils allowed us to infer the biozonation and record bioevents in these deposits. In this section, the presence of NN2 at the lower part of Miocene deposits and the presence of NN9 at the upper part of the studied deposits show that the investigated deposits are Aquitanian - Tortonian in age.

The studied section is located in east Iran, Lut block (a part of flysch basin east of Iran) and west of Afzalabad (north Birjand city). The mentioned flyschoids are more than 4200 m thick. The studied section consists of two units including 200 m shale and sandstone of Paleocene age (abbreviated as Pef in the geological map) and 300 m of shale and sandstone, marl, sandstone and limestone of Eocene (as Ef in the geological map).

290 samples were taken from this sequence. The upper and lower parts of the mentioned sequence were sampled over a smaller distance. In order to avoid weathering, samples were often taken from a depth of 50cm. Most of the samples were from shale and marl. Most samples were prepared using smear slide method and others by gravity method. Nannofossils were studied using Polarizing Light Microscope Olympus (BH2 model) in PPL and XPL lights, and most of them were photographed.

In order to determination calcareous nannofossils of this sequence, (Romein, 1979; Perch-Nielsen, 1985; Bown & Young, 1985) reports were used. According to these studies, 21 genera and 56 species of calcareous nannofossils have been identified in the mentioned sequence. Martini, 1971 biozonation is amongst common biozonations for Paleogene and Neogene. Recently other complementary studies were conducted by Agnini et al., 2014. They have adapted their biozonation to that made by Martini, 1971 using some minor changes. Biozonations introduced in this sequence include:Discoaster binodosus Zone (NP11)
This biozone has been defined based on Martini, 1971 biozonation from the last appearance of Tribrachiatus contortus to the first appearance of Discoaster lodoensis. The age of this biozone is early Eocene (Ypresian). It is worth mentioning that Agnini et al., 2014 have defined their CNE3 biozone, which is started a bit before NP11, at lower edge by the first appearance of Tribrachiatus orthostylus and its upper edge exactly matches that of NP11. In our studied sequence the first appearance of Tribrachiatus orthostylus occurs at the base of the sequence and the first appearance of Discoaster lodoensis was seen 230 m from the sequence base which is the upper boundary of biozone NP11 of Martini, 1971. The age of this biozone is early Eocene (Ypresian).
Tribrachiathus Orthostylus ZONE (NP12)
According to Martini, 1971 biozonation, this biozone continues from the first appearance of Discoaster lodoensis to the last appearance of Tribrachiatus orthostylus. The age of this biozone is early Eocene (Periabonian). In the studied sequence the first appearance of Discoaster lodoensis was seen as the base of NP12 biozone and the upper boundary of this biozone was determined based on the last appearance of Tribrachiatus orthostylus.
Isthmolithus recurvus ZONE (NP19)
Based on NP19 biozone (Martinin, 1971), the lower boundary of this biozone in this sequence was also determined by the first appearance of Istmolithus recurvus and the upper boundary of this biozone was determined by the first appearance of Sphnolithus pseudoradians. The age of this biozone is Late Eocene (Priabonian). The thickness of this biozone is 100 m in Afzalabad sequence.

In Afzalabad flyschoid (north Birjand), 21 genera and 56 species of calcareous nannofossils were identified. Based on the identified species, NP11 and NP12 biozons (Martini, 1971) were identified for lower part of this sequence which nearly matches CNE3 and CNE4 biozons from Agnini et al., 2014 biozonation. In the upper part of the studied sequence NP19 biozone (Martini, 1971) was determined. Agnini et al., 2014 biozonation measures were not seen in this section. According to the identified biozons, the age of studied sequence, Ypresian (early Eocene) to Periabonian (Late Eocene) is suggested for the studied thickness. Considering the presence of NP11 biozone in the upper part of Pef and the lower part of Ef, the pass of mentioned units is continuous and gradual.
Keywords: Nannostratigraphy; Flyschoid; Afzalabad; Birjand.
References
Agnini, C., Fornaciari, E., Raffi, I., Catanzariti, R., Palike, H., Backman, J., & Rio, D., 2014. Biozonation and biochronology of Paleogene calcareous nannofossils from low and middle latitudes. Newsletters on Stratigraphy, 47 (2): 131-181.
Martini, E., 1971. Standard Tertiary and Quaternary Calcareous Nannoplankton Zonation. In: Farinacci, A., (ed.), Proceeding of 2nd International Conference of Planktonic Microfossils, Roma, 2: 739-785.
Perch-Nielsen, K., 1985. Cenozoic calcareous nannofossils. In: Bolli, H.M., Saunders, J.H., & Perch-Nielsen, K., (eds.), Plankton Stratigraphy, Cambridge University Press, 427-554.
Romein, A.J.T., 1979. Lineagesin early Paleogene Calcareous Nannoplankton Utchret Microplaentol. Utrecht micropaleontological bulletins, 22: 1-230.

The aim of this research was to interpret environmental conditions of the Pabdeh Formation in Ilam section، North West Zagros basin، based on calcareous nannoplankton. According to identified biozones which is equivalent to NP10-NP25 biozones of Martini (1971)، the age of Pabdeh Formation in studied section is Early Eocene – Late Oligocene. Abundance of Discoaster and Sphenolithus in the lower part of section indicates an oligotrophic condition in sedimentary basin which is related to global warming. Toward the top of section، the Cyclicargolithus floridanus، Sphenolithus moriformis، Coccolithus pelagicus، Reticulofenestrid nannofossils are dominant. On the other hand، the presences of genera Reticulofenestra، Cyclicargolithus، Dictyococcites that are taxa of temperate and cold water، indicate mesotrophic – eutrophic conditions in the upper part of section. Increasing of Discoasters shows warming trend during the Late Oligocene time.

This study presents the nannostratigraphy and nannofossil events in the lower boundary of Kalat Formation in the East and West of the Kopeh- Dagh basin. The Kalat Formation comprises of coarse grained detritus limestone with subordinate sandstone intercalations. In the current study, six sections have been chosen in the East and West of the basin which are as follow: Dobaradar section, Kalat section, Chahchaheh section, Sheikh section, Qaleh Zoo section and Jozak section. In the Dobaradar section 22 species have been determined, in the Kalat section 25 species, in the Chahchaheh section 32 species, in the Sheikh section 28 species, in the Qaleh Zoo section 20 species, and in the Jozak section 18 species have been determined. The boistratigraphy based on calcareous nannofossils allows the identification of nannofossil standard zones CC25- CC26 in the East, and CC21 and CC26 in the West of the Kopeh- Dagh Basin in all of the sections, indicating that the investigated deposits are Late Maastrichtian – late Late Maastrichtian in age in all sections in the East, and early Late Campanian – late Late Maastrichtian in all sections in the West of the Kopeh- Dagh Basin. The nannofossils response to the Maastrichtian climate evolution is investigated in the lower boundary of Kalat Formation. Warm water indicators (Uniplanarius sissinghii, Micula murus and Micula prinsii) suggest warm surface water conditions in the studied thickness. In the lower boundary of the Kalat Formation, based on Lithraphidites spp. and Watznaueria barnesae, lower fertility conditions with low productivity at the end of the Maastrichtian were suggested.

In this study، Gurpi Formation at Mook section has been sampled and studied. Taxonomical studies of calcareous nannofossils lead to identification of 23 genera and 44 species at the Mook section. According to the first occurrence of index species and fossil assemblages، seven calcareous nannofossil biozones (CC21- CC26 of Sissingh، 1997) and biozone NP1 of Martini (1971) equivalent to biozone CP1a of Okada and Bukry (1980) at the Mook section have been recognized. Based on these biozones، age of the Gurpi Formation in SE of Shiraz، is Late Campanian to Early Danian. On the bases of paleoecological interpretation، Gurpi Formation at the Mook section was deposited in low latitude and deep marine environment. But، the depth of the basin decreases toward the top of the Gurpi Formation at this section.