Organic Petrology and Geochemistry of Lignite Layer From Siwalik Sediments Near Dehradun, India

Lignites have been documented from ancient foreland settings related to collisional orogens worldwide. These lignite samples are excellent indicators of paleoenvironments. In this study, we report a thin lignite body from the Middle Siwalik sandstones near Dehradun, India. The low huminite reflectance values (<0.25%) in the studied samples categorize them as low‐rank lignite. The biomarker signatures (carbon preference index [CPI] >2.0, Pr/ n C17 ∼ 1) of the studied samples indicate immature, terrestrially derived organic matter (OM) deposited under anaerobic waterlogged conditions with notable bacterial activity during peat accumulation. The predominance of huminite‐group macerals (>90 vol%) and rare inertinite‐group macerals (<1 vol%) further reveal anoxic conditions during OM accumulation. Sulfate‐rich water formed (framboidal) pyrite, suggesting early diagenetic mineralization took place in a reducing environment. A high degree of gelification of plant tissues under reducing conditions is further supported by maceral indices. The Pr/Ph range from 0.857 to 1 further suggests an anoxic depositional environment. The deduced information is novel in Middle Siwalik stratigraphy around the Dehradun area and differs significantly from the braided stream‐alluvial fan paleoenvironment deduced from its clastic rocks.