Occurrence of authigenic beidellite in the Eocene transitional sandy sediments of the Chu-Saryssu basin (South-Central Kazakhstan) Valentin Robin a,b, ⁎, Benoit Hebert a,b , Daniel Beaufort a , Paul Sardini a , Emmanuel Tertre a , Olivier Regnault b , Michael Descostes b a Université de Poitiers/CNRS, UMR 7285 IC2MP, équipe HydrASA, Bat. B8 rue Albert TURPAIN, TSA 51106, 86073 Poitiers CEDEX 9, France b AREVA, Business Group Mines, R&D Department, Tour AREVA, 1 place Jean Millier, 92084 Paris La Défense, France abstract article info Article history: Received 6 November 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online 16 March 2015 Keywords: Authigenic smectite Beidellite Eogenesis Illitization Sandstone diagenesis This paper describes the petrographic properties and the clay mineralogy of Eocene sandy sediments of the Chu-Saryssu basin (South-Central Kazakhstan), in which dioctahedral smectite formed during shallow burial diagenesis (eogenesis). Evidence from petrography and clay mineralogy supports the successive occurrence of kaolinite and dioctahedral smectite during the eogenetic processes, which may have resulted from a change from wet and humid to semi-arid or arid climatic conditions. An original result of this study is the predominantly beidellitic nature of the authigenic smectite, which was determined via X-ray diffraction (XRD, using the Hofmann–Klemen test) and Fourier transform infrared spectroscopy (FTIR) investigations and chemical microanalysis. The crystal-chemical investigations indicate a rather homogeneous chemical composition of the smectite at the regional scale, and the following unit formula is proposed: (Si 4+ 3.65 Al 3+ 0.35 )(Al 3+ 1.65 Fe 3+ 0.21 Mg 2+ 0.14 )O 10 (OH) 2 Na + 0.10 Mg 2+ 0.11 Ca 2+ 0.04 K + 0.07 . This type of smectite has been interpreted as representative of mixed layers of montmorillonite and beidellite. The fact that the smectite that formed primarily in the unconsolidated sandy sediments is close to beidellite, rather than montmorillonite, may have a major impact on the rate of further illitization in the Chu-Saryssu basin. Indeed, the crystal chemistry of beidellite is more favorable to illitization, and the presence of this mineral provides new insights concerning the different rates of illitization between sandstones and shale formations observed worldwide. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The crystal chemistry of clay minerals formed during the early stage of diagenesis (eogenesis) and their further transformation processes are topics of primary concern for scientists interested in basin evolution and reservoir modeling (Primmer et al., 1997; Ketzer et al., 2003; Worden and Morad, 2003). This interest is particularly strong for dioctahedral smectites, which are considered to be the precursor material necessary for the crystallization of illite through a series of illite–smectite mixed- layer minerals in siliciclastic formations (Hower and Mowatt, 1966; Mc Kinley et al., 2003; Ferrage et al., 2011). However, most of the studies on sandstones have been focused on illite–smectite mixed layers or il- lite, and the actual presence of a pure authigenic smectite precursor has seldom been described (Carrigy and Mellon, 1964; Wilson and Pittman, 1977; Smosna, 1988). Currently, most of the data regarding dioctahedral smectites in sedimentary basins concern the formation of mudstones, shales and bentonites. In these clay-rich sedimentary formations, the smectite precursor to illite is generally considered to be montmorillonite, i.e., a dioctahedral smectite with a 2:1 layer charge essentially located in the octahedral sheet (Mc Kinley et al., 2003; Worden and Morad, 2003). However, there is a dramatic lack of information concerning the nature of the dioctahedral smectites formed in sandy sediments during eogenesis (Mc Kinley et al., 2003). Typically, the characterization of smectites in sandstones is not detailed (Wilson and Pittman, 1977; Smosna, 1988; WoldeGabriel et al., 2014) or has been restricted to their di- or trioctahedral nature (Velde and Iijima, 1988; Freed and Peacor, 1989; Mc Carty and Thompson, 1991; Pollastro, 1993). Without appropriate crystal chemical data, the dioctahedral smectites of sandy sediments are often considered to be montmorillonite (Carrigy and Mellon, 1964), similar to the smectites of mudstones and shales (Niu and Ishida, 2000). However, the actual chemical composition of smectites developed in sediments can display a wide range of composi- tions, ranging from montmorillonite to beidellite (Nieto et al., 1996; Drief and Nieto, 2000). The shortage of data concerning the diagenetic process in very shallow sandy sediments can be explained by the lack of interest in exploring for petroleum or gas in such sediments and the difficulty in collecting core samples that have avoided mechanical disaggregation or mud injections during the core drilling operation. Sedimentary Geology 321 (2015) 39–48 ⁎ Corresponding author at: Université de Poitiers/CNRS, UMR 7285 IC2MP, équipe HydrASA, Bat. B8 rue Albert Turpain, TSA 51106, 86073 Poitiers CEDEX 9, France. E-mail address: valentin.robin@univ-poitiers.fr (V. Robin). http://dx.doi.org/10.1016/j.sedgeo.2015.03.004 0037-0738/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo