Biostratigraphy of the Gurpi Formation in Chenareh section based on calcareous nannofossils

Autor: Zeinab Bayat, Anoshiravan Lotfali Kani, Jahanbakhsh Daneshian
Jazyk: English<br />Persian
Rok vydání: 2024
Předmět:
Zdroj: Journal of Stratigraphy and Sedimentology Researches, Vol 40, Iss 2, Pp 59-86 (2024)
Druh dokumentu: article
ISSN: 2008-7888
2423-8007
DOI: 10.22108/jssr.2024.141481.1289
Popis: Abstract The Gurpi Formation was measured and sampled in a stratigraphic section located on the northern limb of the Chenareh anticline to the north of Andimeshk. The thickness of the Gurpi Formation in this section is 288.5 meters and its lower boundary with the Ilam Formation is abrupt while its upper boundary with purple shales of the Pabdeh Formation is gradual and continuous. Paleontological studies led to the identification of 67 species belonging to 45 genera from 20 families of calcareous nannofossils, which made it possible to recognize CC18 to CC26 biozones from Sissingh standard classification scheme and NP1 to NP5 biozones from Martini standard classification scheme equivalent to biozones CNP1 to CNP7 from Agnini et al.. Based on the identified biozones, the duration of deposition of the Gurpi Formation was determined to be early Campanian–Selandian. In this section, the Emam Hassan Limestone Member was recognized with a thickness of 112.8 meters having been deposited during the early Maastrichtian. Both the lower and the upper boundaries of the Emam Hassan Limestone Member within the Gurpi Formation are continuous and gradual. The position of the Campanian–Maastrichtian, Maastrichtian–Danian (Cretaceous–Paleogene) and Danian–Selandian transitions in the Gurpi Formation were also determined in the studied section. Keywords: Gurpi Formation, Chenareh anticline, Calcareous nannofossils, Cretaceous–Paleogene transition Introduction The Gurpi Formation was deposited in a deep sedimentary basin during the Late Cretaceous–Paleocene in the geological zone of Zagros. The type section of the Gurpi Formation in Mount Gurpi located in Khuzestan province, consists of 320 meters of marls and bluish-gray shales and subtly includes thin layers of clayey limestones (Stöcklin and Setudehnia 1970). The Gurpi Formation has two formal members, Emam Hassan Limestone (light gray and weathered white) and Seymareh Limestone (brown in appearance), and an informal member called Mansuri limestone (Motiei 1993). The Type section of the Emam Hassan Limestone Member in Mount Emam Hassan, is located in Lorestan province and includes 114 meters of clayey limestones with interlayers of marls. The Emam Hassan Member is spread only in Lorestan and part of Khuzestan (Stöcklin and Setudehnia 1970). Calcareous nannofossils are one of the most powerful tools used in the biostratigraphy of the Mesozoic (especially Cretaceous) and Cenozoic rocks, and they have always been emphasized in the Geologic Time Scale (GTS) along with foraminifera and ammonites. In many cases, in sediments with soft lithology such as the Gurpi Formation, using calcareous nannofossils, it leads to achieving a very clear biostratigraphic classification. The age of the Gurpi Formation in Lorestan and part of Khuzestan is from Campanian to Paleocene and in Fars and another part of Khuzestan from Santonian to Maastrichtian. The Emam Hassan Limestone Member was deposited during the Maastrichtian period (Stöcklin and Setudehnia 1970). In the Lorestan subzone, recent paleontological studies in the Gurpi Formation often confirm the Campanian–Paleocene age range. But some studies also mentioned Santonian (Hadavi and Rasaizadi 2009; Solgi 2015; Maghfouri Moghaddam 2015; Taherzadeh 2018; Mahmoodian 2019; Mohammadian 2019). The purpose of this study is to investigate the lithostratigraphy and biostratigraphy (based on calcareous nannofossils) and finally to determine the age of the Gurpi Formation in the Chenareh section. Material & Methods Chenareh geological section is located in the Chenareh anticline, within theLorestan subzone of the Zagros Basin (Fig. 1). The geographic location of this section is between two provinces of Lorestan and Khuzestan. Due to the lack of a complete succession outcrop, this section was taken as a composite section: The coordinates of the base and the top of the first part of the Chenareh section are respectively 32° 54ʹ 03ʹʹ N and 48° 06ʹ 47ʹʹ E and 32° 54ʹ 32.45ʹʹ N and 48° 05ʹ 57ʹʹ E (from base of the section to 279 meters). The coordinates of the base and the top of the second part of the section are respectively 32° 53ʹ 86.22ʹʹ N and 48°08ʹ 52.17ʹʹ E and 32° 53ʹ 89.22ʹʹ N and 48° 08ʹ 08.18ʹʹ E (top final 21 meters). In total 194 samples were collected from 300 meters of thickness in this section, the first three samples (0– 1 meter level of the section) belong to the Ilam Formation and the last eight samples (in the last 10.5 meters) belong to the Pabdeh Formation (Purple Shale Member). A total of 187 samples have been prepared for the study of nannofossils by the gravity-settling method of Bown and Young (1998). The prepared slides were studied by a Nikon Optiphot II Pol optical microscope equipped with a Nikon D-3300 digital camera at 1000x magnification. In order to identify calcareous nannofossils, a combination of references such as Perch-Nielsen (1985a and b), Young and Bown (1997), Young et al. (1997), Burnett (1998) and the Nannotax website were used. Biostratigraphic classification for the Late Cretaceous is based on the standard scheme of Sissingh (1977) revised by Perch-Nielsen (1985a) (CC) and for the Paleocene is based on the two schemes of Martini (1971) revised by Perch-Nielsen (1985b) (NP) and Agnini et al. (2014) (CNP). Discussion of Results & Conclusions The Gurpi Formation in the Chenareh section (288.5 meters thick) can be divided into seven lithological units and is predominantly composed of shales (including units 2, 3, 4, 6 and 7) interbedded with some argillaceous limestone layers and subordinate marls (units 1, 2) (Fig. 2). Unit 5 or the Emam Hassan Limestone Member is 112.8 meters thick. According to the 1:100,000 geological map of Balarud (Sahabi and Macleod, 1969), the Gurpi Formation is covered by the Amiran Formation on the northern limb of the Chenareh anticline, and the Pabdeh Formation on its southern limb. Despite the presence of the Chenareh section on the northern limb, the top of the Gurpi Formation is definitely overlain by the Purple Shale Member of the Pabdeh Formation (Figs. 1 and 3). The time of deposition of the Gurpi Formation was determined early Campanian up to the earliest part of the Selandian (the earliest part of NP5 biozone). The Emam Hassan Limestone Member was determined to have the earliest Maastrichtian age range (CC23b to CC25a). The last part of the Ilam Formation (last 1 m) was assigned to the middle of the early Campanian (biozone CC18a) and the beginning part of the Pabdeh Formation was assigned to the Selandian (NP5). According to the field observations, the lower boundary of the Gurpi Formation with Ilam Formation is sharp while the upper boundary with the Purple Shales of the Pabdeh Formation is continuous and gradual. Based on the laboratory studies, the transition of the Ilam to Gurpi formations is ambiguous, but the boundary of Gurpi to Pabdeh formations is chronologically continuous. The lower and upper boundaries of the Emam Hassan Limestone Member within the Gurpi Formation based on both field observations and laboratory-obtained data are continuous and gradual. Three stratigraphic transitions of Campanian–Maastrichtian, Maastrichtian–Danian (Mesozoic–Cenozoic) and Danian–Selandian were identified in the Chenareh section. The Campanian–Maastrichtian transition (Maastrichtian base) is placed within the immediate limestone-shale sequence of the base of the Emam Hassan Member and between the two events of the last appearance of A. parcus and the last appearance of Q. trifidum (closer to the last appearance of A. parcus). The Maastrichtian–Danian transition (Danian base) was identified by the first abundant appearance of the genus Thoracosphaera at the boundary between the shale-limestone sequence (unit 6) and the dark grey shales (unit 7). Thus, the first sample of the dark shales of unit 7 is attributed to the Danian age (NP1). The Danian–Selandian transitions (base of Selandian) was determined based on the first appearance of Fasciculithus ulii and F. billii, F. janii, F. pileatus within the dark grey shales of unit 7 (the last sample in unit 7) and immediately before the beginning of the base of the Purple Shales (Fig. 6).
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