Petrographic, palynofacies, geochemical, and spectroscopic insights into depositional environments and palaeowildfires in the Barakar Formation (Artinskian), Godavari Valley Coalfield, India

The Early Permian (Artinskian) Barakar Formation in the Godavari Valley Coalfield (GVC), southern India, preserves a detailed record of terrestrial ecosystem responses to changing atmospheric and climatic conditions. This study integrates petrographic, palynofacies, geochemical, and micro-Raman spectroscopic data from coal and shale samples of the Gautam Khani Open Cast (GKOC) mine to reconstruct depositional settings and assess the nature and intensity of palaeowildfires. The studied samples are dominated by the inertinite group of macerals (avg. 45.5 vol.%), followed by the vitrinite group (avg. 32.8 vol.%) with lower liptinite (avg. 15.7 vol.%). The palynofacies composition is characterized by the co-domination of opaque phytoclasts (avg. 38.8%) and non-opaque biostructured (NOB) phytoclasts (avg. 37.0%), palynomorphs (avg. 5.2%). The vitrinite content and NOB phytoclasts reflect a significant input from higher plants. Moderate carbon preference (CPI) and odd-even preference (OEP) indices, along with n-alkane distribution (n-C₁₅ to n-C₂₉), suggest mixed terrestrial and aquatic sources. The presence of the detrovitrinite sub-group and non-opaque non-biostructured (NONB) components indicates herbaceous vegetation contribution and/or fluctuating levels of bacterial degradation. Moreover, the maceral-derived indices suggest that peat precursors were deposited in dry-wet forest swamp settings with ombrotrophic to mesotrophic hydrological conditions. The dominance of inertinite group, high pristane to phytane (Pr/Ph) ratio and of opaque phytoclast point to oxic conditions (with intermittent shifts). Likewise, various palynofacies assemblages also indicate that the deposition of these sediments generally occurred under low-energy aerobic conditions (with occasional shifts to suboxic conditions) in proximal settings.Micro-Raman spectroscopy of whole-rock samples reveals first-order D (disorder) and G (graphite) bands, with peak positions at S ~ 1070-1260 cm^-1, D2 ~ 1240-1250 cm^-1, D1 ~ 1350-1375 cm^-1, D3 ~ 1440-1480 cm^-1, D4 ~ 1500-1570 cm^-1, and G ~ 1590-1600 cm^-1. The ID1/IG ratios range from 0.84 to 0.86, while the AD1/AG ratios vary between 0.54 and 3.03. G-D values span 149.81-218.81 cm^-1. The measured vitrinite reflectance (VR_o%) ranges from 0.50% to 0.59% (avg. 0.54%), classifying the samples as medium-rank bituminous D coals. In addition, the studied samples exhibit a dominance of fluoranthene (Fla) and pyrene (Py) among the unsubstituted polyaromatic hydrocarbons (PAHs), indicating organic matter combustion. The values of methyl phenanthrene to phenanthrene (Mph/Ph) and Fla/(Fla + Py) ratios further support the pyrogenic origin of these compounds. The moderately high presence of semifusinite and fusinite macerals (micro-charcoals) in the examined samples with preserved cellular structure suggests the occurrence of wildfire event(s) during their deposition. Consequently, the fusinite reflectance values vary between 1.15% and 4.91%, with an average of 2.07%, signifying a combustion temperature of 320-762 ℃. Furthermore, these micro-charcoals were largely formed under the combined influence of ground (57%) and surface (31%) fires. The inertinite content in the studied samples suggests that a relatively high atmospheric oxygen (pO₂; 27%) concentration could have ignited the palaeowildfire.