Glauconite within Albian-Cenomanian Aitamir Formation, Kopet-Dagh Basin, northeastern Iran: origin and implications of c
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ORIGINAL PAPER
Glauconite within Albian-Cenomanian Aitamir Formation, Kopet-Dagh Basin, northeastern Iran: origin and implications of cretaceous seawater Mahdi Jafarzadeh 1 & Tathagata Roy Choudhury 2 & Azizollah Taheri 1 & Santanu Banerjee 2
&
Arman Jafarian 3
Received: 20 April 2020 / Accepted: 2 September 2020 # Saudi Society for Geosciences 2020
Abstract An integrated petrographical, geochemical, and mineralogical investigation reveals the wide variation in the mineral chemical composition of glauconites within the Albian-Cenomanian Aitamir Formation. Bounded between a black shale (Sanganeh Formation) below and chalky limestone (Abderaz Formation) above the Aitamir Formation represents an inner shelf deposit. Glauconite may occur either as pellets or as altered zones within feldspar and mica. Poor sorting and deeply penetrative crack indicate the authigenic nature of glauconite pellets. The contents of Fe2O3, MgO, Al2O3, and SiO2 of glauconites vary widely while the K2O exhibits a limited spread indicating the evolved nature of the glauconite. The glauconite matures at a fixed and high value of K2O with variable Fe2O3 (total) supporting the replacement origin. The X-ray diffraction study reveals a negligible interstratification within glauconites. The micro-textural study attests to the highly evolved nature of the glauconite. The shallow marine glauconite within the Albian-Cenomanian Aitamir Formation formed under sub-oxic, shallow depositional setting during the middle Cretaceous global warming phase within the Tethyan domain. Keywords Glauconite . Black shale . Mineral chemistry . Aitamir Formation . Kopet-Dagh Basin . Tethys domain . Palaeo-Tethys
Introduction The presence of glauconite within a marine sedimentary succession is usually considered an indicator of stratigraphic condensation (Odin and Matter 1981; Amorosi 1995, 1997, 2012; Banerjee et al. 2012a, b, 2016b; Bansal et al. 2017; LópezQuirós et al. 2019). Particularly the authigenic glauconite, containing more than 6% K2O, occurs in condensed deposits (Amorosi 2012). However, paleoclimatic and paleooceanographic implications of authigenic glauconite are less Responsible Editor: Beatriz Badenas * Santanu Banerjee [email protected] 1
Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran
2
Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
3
State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences Beijing, Beijing 100083, China
explored. The Cretaceous Epoch is significant for warm and greenhouse climatic conditions, the formation of extensive epeiric seas, and faunal diversity. However, little is known about the authigenic mineral formation on the seafloor in response to greenhouse climatic conditions. Recent studies indicate that glauconite is abundant in the marine Cretaceous deposits across the Tethyan belt, which is possibly related to climatic and oceanographic conditions (Garrison et al. 1987; Pasquini et al. 2004; Westermann e
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