Mineralogical and deformational signatures of the Neogene Surma Group sandstones in the Bengal Basin from high-resolution petrography and provenance

The Neogene Surma Group sandstones of the Sylhet Trough, Bengal Basin, archive the interplay of rapid flexural subsidence, high-flux sedimentation, and active fold-and-thrust deformation along the Indo-Burman margin. We analyzed core samples from the Fenchuganj-2 and Kailashtila-2 wells using polarized-light microscopy, Gazzi–Dickinson point counting, and detailed microstructural inspection to constrain mineralogy, provenance, diagenesis, and deformation. The sandstones are quartz-rich sub-lithic to lithic arenites with moderate to high textural maturity. Diagenetic evolution proceeds from early syntaxial quartz overgrowths and chlorite pore-linings, through intermediate feldspar alteration (kaolinization, albitization, sericitization) and pressure-solution seams, to late blocky to poikilotopic calcite cement. These diagenetic changes progressively alter pore structure and mechanical behavior with burial depth. Notably, early cements stiffen the framework and reduce primary porosity, promoting brittle deformation at shallow levels, whereas pressure solution, phyllosilicate alignment, and feldspar alteration facilitate more distributed ductile strain at greater depths. Microstructural observations reveal a systematic depth progression, with grain-boundary pressure solution, intragranular fractures, and undeformed plagioclase twins at ~ 3.1–3.6 km, mica kinking, sutured contacts, and quartz subgrains at ~ 3.7–4.3 km, and transgranular fractures, undulose extinction, and penetrative foliation at ~ 4.7 km depths. These features record a brittle-to-ductile transition under ongoing thrust loading and flexural subsidence in the Sylhet Trough. This diagenesis–deformation coupling explains the observed depth trends and bears directly on reservoir quality, with dissolution-derived secondary porosity locally offsetting cement occlusion where fracture networks are well developed. Ternary provenance plots place all samples in the recycled-orogen field, implicating the uplifted Shillong Plateau and Barail-Cachar Ranges as principal sediment sources. The results refine sediment-routing models, highlight diagenetic controls on reservoir quality, and emphasize the necessity of integrating petrographic and structural datasets to decipher provenance and deformation in complex foreland basins.