Investigating the role of rock weathering and clay mineralogy in landslide occurrences within the exposed Tertiary Formations in Rangamati area

Abstract Background Landslides are a major and recurring hazard in the folded Tertiary sedimentary rock units of Southeastern Bangladesh. The mechanical strength and stability of these rocks are strongly influenced by their mineralogical composition and geochemical alteration, which can affect their susceptibility to weathering and collapse. However, the role of these factors in triggering landslides remains poorly understood. Objective This research addresses this gap by linking rock weathering intensity and clay mineral composition to landslide mechanisms in the outcropped Tertiary Formations in the Rangamati region. Methods Detailed geological field work has been conducted on four major exposed Tertiary Formations: Dupi Tila Formation, Tipam Sandstone Formation, Boka Bil Formation, and Bhuban Formation. A comprehensive analysis of 30 landslides is carried out to evaluate the geological influence, with 22 samples (both fresh and weathered) examined using X-ray Diffraction (XRD), X-ray Fluorescence (XRF), and Scanning Electron Microscope (SEM) techniques to determine clay content and the extent of weathering. Shale percentages are estimated from outcrops as well as from wireline logs using empirical equations. Results The Bhuban Formation exhibits the highest shale content, followed by the Boka bil, Dupi Tila, and Tipam Sandstone formations. Illite is identified as the dominant clay type and they are more prevalent in the older formations. Smectite is also present in varying proportions contributing significantly to landslide occurrences through its expansive properties. The Upper Marine Shale in Bokabil Formation contains the most smectite, followed by the Dupi Tila and Bhuban formations. The degree of weathering is evaluated through field observations and oxide analysis, with average Chemical Index of Alteration (CIA) values exceeding 75, indicating intense weathering in all formations, as clearly reflected in the outcrops. Four primary types of landslides: flow, fall, slide, and complex—are identified across all formations in varying proportions. Flow is predominant in formations dominated by single rock type, such as the Bhuban and Tipam Sandstone formations, while slides are more common in formations with alternating layers of sandstone and shale, such as the Boka bil and Dupi Tila formations. Conclusion The findings highlight that higher shale volume, clay content, and CIA values significantly elevate the landslide susceptibility of geological formations, especially when these factors are compounded by primary triggers like intense rainfall and human-induced slope modifications.