Biostratigraphy, microfacies, sedimentary environments of the Gurpi and Amiran formations in the Gerdbisheh section, High Zagros

Abstract

Abstract Biostratigraphy, microfacies and depositional environments of the Upper Cretaceous deposits (Gurpi and Amiran formations) were carried out in the Gerdbisheh section. The Gurpi Formation predominantly consists of shales interbedded with some argillaceous limestone layers. This formation disconformably overlies the Sarvak Formation and gradually underlies the Amiran Formation. The Amiran Formation is represented by grey to greenish shales, siltstones and sandstone intercalations which underlay the informal siliciclastic-carbonate unit 1. Planktonic foraminiferal assemblages have led to the recognition of six zones that reveal the late Coniacian?–early Maastrichtian age for the Gurpi Formation. Radiolaria and scattered planktonic foraminifera are the two main fossil groups present in the Amiran Formation. Therefore, based on the precence of the index planktonic foraminfera and the stratigraphic position the early-middle? Maastrichtian age is proposed for the Amiran Formation. We determined two microfacies in the interval of the Gurpi Formation and three petrofacies in the Amiran Formation. According to the obtained data the Gurpi Formation was deposited in the basinal setting, whereas the Amiran Formation is deposited as submarine fans on the slope part of a carbonate shelf under the control of turbidity currents. Keywords: Gurpi Formation, Amiran Formation, High Zagros, Foreland Basin, Late Cretaceous Introduction During the Cretaceous, the Neo-Tethys Oceanic crust started subducting under the Iranian Plate which was followed by foreland basin development (Alavi 1994, 2004; Piryaei et al. 2011). The late Coniacian through the late Maastrichtian (in some areas Paleocene) interval is represented by the Gurpi Formation which constitutes the main part of the newly created foreland basin (Ziegler 2001; Alavi, 2004; Piryaei et al. 2011; Orang et al. 2018). In the studied area the Cretaceous successions including Kazhdumi, Sarvak, Gurpi, Amiran and Tarbur formations are outcropped. The Gurpi Formation (Coniacian–Thanetian), due to spectacular expostions in most parts of the Zagros Basin, is easily accessible, yields high faunal richness and diversity (especially planktonic foraminifera and nannofossils), spanning the K/Pg (Cretaceous–Paleogene) transition and the potential of source and cap rocks, is considerd as one of the main lithostratigraphic units of the Upper Cretaceous successions in Iran (Wynd 1965; Motiei 2003; Beiranvand et al. 2014a, b; Razmjooei et al. 2018; Ezampanah et al. 2022). During the Maastrichtian in the more internal parts of the Zagros fold-and-thrust belt the siliciclastic successions of the Amiran Formation are deposited (Alavi 2004). At the type section, the Amiran Formation with 871 m-thick is composed of cherty conglomerates, sandstones, siltstones, greenish to grey shales and limestones which all show a shallowing upward trend (Motiei 2003; Casciello et al. 2009; Homke et al. 2009). Among the Upper Cretaceous strata in the studied area, the shallow marine carbonates of the Tarbur Formation are studied more in detail and received lots of attention (Moosavi and Asgari Pirbaloti 2008; Khazaei et al. 2010; Asgari Pirbaloti et al. 2012; Vaziri-Moghaddam et al. 2013; Ghanbarloo et al. 2021; Ghanbarloo and Safari 2023). Some researchers also focused on the biostratigraphy of the Gurpi Formation in the Borujen area (Tabaei et al. 2005; Vaziri-Moghaddam et al. 2007, 2013; Shahriari 2009; Senemari and Forougi 2019). In the current research biostratigraphy, facies and depositional environment of the two formations (Gurpi and Amiran) were performed in the Gerdbisheh section (south of Borujen city). Material & Methods The Gerdbisheh section was logged and sampled in the south of Borujen city (coordinates: 31°33′19.99′′ N, 51°12′38.17′′ E). For this study, we investigated 210 thin sections through the entire Gurpi and Amiran (159 m) formations, plus a few meters of the underlying Sarvak and the overlying informal siliciclastic-carbonate unit 1. The identification, biostratigraphic ranges and the zonal framework used for planktonic foraminifera in this study follow Robaszynski and Caron (1979a, b), Premoli Silva and Verga (2004), Haynes et al. (2015), Coccioni and Premoli Silva (2015) and Huber et al. (2022). The petrographic characterization of the carbonates facies and depositional environments is based on Dunham (1962) and Flügel (2010). Discussion of Results & Conclusions The Gurpi Formation (692 m-thick) is predominantly composed of shales interbedded with some argillaceous limestone layers. Based on the field observations and laboratory-obtained data the lower boundary of the Gurpi Formation with the Sarvak Formation is represented by a prominent hiatus, whose stratigraphic gap encompasses the late Cenomanian? to middle Coniacian. Such an unconformity is also reported in the other parts of the Zagros Basin (James and Wynd 1965; Ghasemi-Nejad et al. 2006; Vaziri-Moghaddam et al. 2007; Taheri and Soradeghi 2011; Piryaei et al. 2011; Vincent et al. 2015; Ezampanah et al. 2022) and in the Arabian platform (van Buchem et al. 2002, 2011; Aqrawi et al. 2010). Within the late Coniacian through the middle Maastrichtian interval of the studied section 45 species belonging to 19 genera of planktonic foraminifera, and three genera of benthic foraminifera were recognized. The first and last occurrences of the index planktonic foraminiferal species have led to the recognition of seven Upper Cretaceous zones (Dicarinella asymetrica Total Range Zone to Gansserina gansseri-Contusotruncana contusa? integrated zone) were introduced in the Gurpi and Amiran formations. These biozones reveal the latest Coniacian to early Maastrichtian age for the Gurpi Formation. It should be mentioned in the Maastrichtian interval of the studied interval a change in the depositional environment occurred and deep marine deposits of the Gurpi Formation turn into turbidity and siliciclastic successions of the Amiran Formation. For this reason, in the Amiran Formation index planktonic foraminifera are rare and the early–middle Maastrichtian age is mostly proposed based on the stratigraphic position. Petrographic studies of the studied interval led to the recognition of two microfacies in the Gurpi Formation and three petrofacies in the Amiran Formation. According to introduced microfacies and identified biota, the Gurpi Formation is deposited in the deep marine setting. Lateral age, facies and thickness variations of the studied formations in the studied area point to the fundamental role of basement faults, which substantially controlled sedimentation. The Amiran Formation sandstones (rich in radiolarians) and its other clastic successions were derived from the erosion and reworking of the obducted radiolarite and ophiolite sequences during the Late Cretaceous (Berberian and King 1981; Robertson 1987; Broud 1987; Ziegler 2001; Motiei 2003; Casciello et al. 2009; Piryaei et al. 2010) deposited under turbidity currents on the slope to the toe of slope environments. In contrast to the Amiran Formation, no evidence of gravity flow deposits is observed in the Gurpi Formation.

Keywords

• gurpi formation
• amiran formation
• high zagros
• foreland basin
• late cretaceous