ORIGINAL PAPER Characterization of nodular structure in shale with a proposed growth model in the Sitakund anticline, Bengal basin, Bangladesh Md. Yousuf Gazi 1 & Badrul Imam 1 & S.M. Mainul Kabir 1 & Subrota Kumar Saha 1 Received: 12 May 2018 /Accepted: 15 November 2018 # Saudi Society for Geosciences 2018 Abstract Nodular shales commonly occur in silty shales near the axial (proximity to core) regions of anticlinal structures (Sitakund anticline, Sitapahar anticline, Patharia structure, and Mirinja anticline) of eastern fold belt of Bangladesh. Stratigraphically, they are pro- nounced in the Surma group of Neogene succession. In many outcrops, these are found in the incompetent beds bounded by the competent ones. An initial investigation has been followed by comprehensive petrographical and geochemical studies including thin section microscopy, laser particle size analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray fluores- cence (XRF) to characterize the nodular shale of Sitakund structure. From laser diffraction analysis, it is evident that nodular shales are silty in nature containing approximately 60% silt (mainly quartz). XRD pattern shows that nodular shale contains quartz, feldspar, illite, kaolinite, chlorite, and expandable mixed layer clay mineral including minor amount of calcite and siderite. Geochemical analysis shows that there is no significant variation from host rocks in context of major and trace element concentra- tion. Microcracks within the quartz grains and squeezed clay minerals were observed in some nodular shale samples correlating the influence of regional tectonics and local structural dynamics in the axial regime of the anticline. Projection of 15 nodular shale long axes in outcrop shows their orientation in NNW-SSE that is parallel to the fold axis. The study suggests a new name of conven- tionally called nodular shales based on their origin, composition, and occurrence. The proposed name is BClay Cabbage^. Diagenesis involving the conversion of montmorillonite to illite clay through a mixed layer illite/montmorillonite along with the regional and local tectonics is responsible for the formation of nodular shale. A new model naming as BTectono-Diagenetic (TD) Model^ is proposed in this study concerning the origin of nodular shale. Fluid pressure during diagenesis along with axial deformation due to folding produces hemispherical bodies by brittle and plastic flow that initiates the core of nodular shale at greater depth. Spiral geometry of the body is attained through several deformation stages with the evolution of the folded pattern. Keywords Nodular structure . Shale . Growth model . Sitakund anticline . Bengal basin Introduction Interest in mudrock lithounit has increased overwhelming- ly since the understanding that mudrocks hold enormous wealth of information about the sedimentary history of the earth. Numerous studies have focused on the characteri- zation of mudrocks in different parts of the world (Harbor 2011; Arthur and Sageman 1994; Dawson 2000; Loucks and Ruppel 2007; Olusola et al. 2014; Schultz and Grundl 2004). They focused mainly on the facies characteriza- tion, stratigraphic architecture, mineralogy, and geochem- istry of the mudrocks. Interests in mudrocks have also increased due to its role as unconventional oil and gas plays, i.e., shale gas and oil shale (Zhang et al. 2017). In the neighboring India, several works are also reported on the mudrocks. Gogoi and Sarmah (2013) studied the geochemical characteristics of shale of Disang group of Eocene age. Varma et al. (2015) outlined the petrographic characterization of shales from Raniganj Coal Basin, and Farrimond et al. ( 2015 ) studied detail geochemical * Md. Yousuf Gazi yousuf.geo@du.ac.bd Badrul Imam badrul@du.ac.bd S.M. Mainul Kabir smkabir@du.ac.bd Subrota Kumar Saha sks@du.ac.bd 1 Department of Geology, University of Dhaka, Dhaka, Bangladesh Arabian Journal of Geosciences (2018) 11:739 https://doi.org/10.1007/s12517-018-4080-7