Characterization of nodular structure in shale with a proposed growth model in the Sitakund anticline, Bengal basin, Bangladesh

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 pronounced 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 fluorescence (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 concentration. 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 conventionally called nodular shales based on their origin, composition, and occurrence. The proposed name is “Clay 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 “Tectono-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.