Geochemical assessment of the Upper Bhuban shales from the Surma Group, Mizoram, northeast India

Abstract This study presents the first bulk geochemical analysis of shale samples from the Upper Bhuban Formation near Tuirial in Saitual District, Mizoram, northeast India. The Hydrogen Index (HI) range from 33.33 to 95.65 mg HC/g TOC indicates that the studied shale contains type III-IV kerogen and is derived mainly from ligno-cellulosic organic matter mixed with oxidized plant debris. Low total organic carbon (0.20–0.33 wt%), poor hydrocarbon generation potential (0.09–0.30 mg HC/g rock), and minimal volatile matter (0.10–0.99 wt%) indicate poor preservation of pyrolyzable organic matter. High ash yield (90.60–98.48 wt%) and strong infrared signatures of minerals suggest heavy clastic sediment influx, which likely diluted organic matter and promoted oxidizing conditions favorable for microbial degradation. The SiO₂ content (56.9–60.37 wt%), Al₂O₃/TiO₂ ratio (15.13–24.00), and CaO–K₂O–Na₂O relationships, indicate an andesitic to granodioritic source. Major oxide compositions indicate derivation from an Oceanic Island Arc (OIA) tectonic setting linked to the early collisional phase between the Neo-Tethyan oceanic slab and the Eurasian plate. Weathering indices such as the chemical index of alteration (CIA: 68.40–79.46) and chemical index of weathering (CIW: 69.61–83.05), along with A–CN–K plots, reflect moderate chemical weathering under cold to semi-arid or sub-humid climatic conditions. The index of compositional variation (ICV: 0.84–1.11) and A-CN-K diagram further suggest smectite dominance over kaolinite, consistent with moderate weathering intensity. Overall, these weathering proxies collectively point to progressive aridification, cooling, and moderate rainfall intensity in northeast India during the Late Miocene.