Lithological and Geochemical Characteristics of Cenozoic Sedimentary Rocks of the Surma Basin, Bangladesh: Implications for Depositional Environment and Provenance

ABSTRACT

Surma Basin in northeastern Bangladesh contains a thick Cenozoic sedimentary rock succession, offering a valuable record of the tectono‐sedimentary evolution from the Eocene to Mio‐Pliocene. This study combines lithology, organic matter (OM) geochemistry and elemental composition of sedimentary rocks to assess sediment and OM sources, depositional environments and tectonic influences. Geochemical proxies, including TOC (0.02%–2.68%), C/N (0.15–26.34), C/S (0.36–11.36) and δ ¹³ C (−26.50‰ to −23.42‰), indicate variable depositional settings and OM sourced from both marine to terrestrial origins. MFW and A–CN–K diagrams, along with weathering indices (CIA: 62.2%–89.7%, ICV: 0.32–1.16, K ₂ O/Na ₂ O: 1.36–7.50), reveal moderate to intense chemical weathering and sedimentary rocks form primarily from felsic to intermediate rock in humid climatic conditions. Tectonic discrimination diagrams show a transition from passive to active continental margin settings, linked to the Himalayan uplift and evolving monsoonal regimes. Lithological and geochemical records indicate that Sylhet Limestone Formation was deposited in a stable shallow shelf, the Kopili Shale Formation in an outer continental shelf to deltaic environment and OM originating from nearby landmasses. Subsequent basin subsidence adjacent to the uplifted Himalayas resulted in rapid deltaic to fluvial sedimentation of the Barail (Renji) Sandstone Formation. Global sea‐level changes along with continued Himalayan uplift, intensified monsoonal activity, influenced the deposition of the Bhuban and Boka Bil formations, Tipam Sandstone Formation and Girujan Clay Formation in outer shelf deltaic to fluvial–lacustrine systems. Tectonics, monsoonal activity and sea‐level fluctuations collectively affected basin evolution, sedimentation and OM preservation over time.