Abstract
This study provides a combined analysis on the palynology, fossil charcoal and biomarkers of the subsurface coal deposits from a borehole RMB #2 drilled at the Dhulia Coal Block, Rajmahal Basin, India, in attempts to establish the chronology of sedimentation and to propose palaeobotanical as well as geochemical evidence for the occurrence of wildfires in these sediments. The palynological investigation suggests a Scheuringipollenites barakarensis palynoassemblage from the lower Barakar Formation, dated as Artinskian (Early Permian) in age. This assemblage reveals the dominance of Glossopteridales and sub-dominance of taxa belonging to Cordaitales and Coniferales. Fossil charcoal in sediments is usually recognized as a direct indicator for the occurrence of palaeo-wildfires. More data involving the anatomical features of fossil charcoal analyzed by Field Emission Scanning Electronic Microscope broaden our knowledge on Early Permian wildfires from the peninsula of India. The studied macroscopic charcoal fragments exhibit anatomical details such as homogenized cell walls, uniseriate simple and biseriate alternate pitting on tracheid walls and rays of varying heights pointing to a gymnospermous wood affinitity. The excellent preservation of charcoal fragments, shown by their large sizes and almost unabraded edges, suggests a parautochthonous origin. The embedded biomarker study performed for charcoal sediments and its characterization demonstrate the presence of n-alkanes, isoprenoids, terpenoids and aromatic compounds. A bimodal distribution pattern of n-alkanes with a Cmax at n-C25 is identified. Diterpenoids and pentacyclic terpenoids are identified, indicating the input of an early conifer vegetation and bacterial activity, respectively. The identified polyaromatic hydrocarbon (PAH) compounds, together with the charcoal fragments, clearly suggest that repeated wildfire events occurred during the deposition of these Artinskian sediments in the Rajmahal Basin.
Highlights
1 Introduction The Rajmahal Basin contains an excellent succession of the upper Gondwana sediments in India, which is represented by the Dubrajpur Formation (Lower Triassic to Lower Cretaceous) and the Rajmahal Formation (Lower Cretaceous)
Banerjee and D'Rozario (1988), Banerjee and D'Rozario 1990) reported three assemblage zones for the lower Permian sediments of the Chuperbhita Coalfield: (1) the Plicatipollenites-Parasaccites Zone comprising a dominance of Ginkgocyladophytus and a sub-dominance of monosaccate pollens followed by Callumispora, Brevitriletes and an acritarch (Quadrisporites), indicating an Early Permian age (Karharbari Formation); (2) the nonstriate bisaccate Scheuringipollenites Zone exhibiting a dominance of non-striate bisaccate Scheuringipollenites along with Laevigatosporites, Brevitriletes, Lophotriletes and Marsupipollenites, belonging to the late Early Permian (Barakar Formation); and, (3) the striate bisaccate Striatopodocarpites–Striatites Zone showing a dominance of striate bisaccate pollens, belonging to the late Early Permian
The main objectives of this study are: (1) to date the succession of the borehole RMB#2 palynologically, and to highlight the biostratigraphic status of the Barakar Formation of Rajmahal Basin and correlate its biostratigraphy with contemporary palynoassemblages recorded from other Indian Gondwana basins; (2) to analyze the anatomy of macroscopic fossil charcoal embedded in two sediment samples at depths of 88 m and 93 m of the core; and, (3) to provide a biomarker analysis of saturated and aromatic fractions on the two samples and to identify significant fire-related compounds as well as various other biomarkers in these samples
Summary
The Rajmahal Basin contains an excellent succession of the upper Gondwana sediments in India, which is represented by the Dubrajpur Formation (Lower Triassic to Lower Cretaceous) and the Rajmahal Formation (Lower Cretaceous). The lower Gondwana sediments, represented by the Talchir and Barakar formations, can lithologically be attributed to an Early Permian age (Raja Rao 1987). Later on, Srivastava and Maheshwari (1974) performed a palynological study for an outcrop in the Brahmani Coalfield and identified an assemblage comprising 27 spore-pollen genera characterized by the dominance of Faunipollenites, Gondwanapollenites, Densipollenites and Sulcatisporites (=Scheuringipollenites), indicating the Late Permian (Barren Measures Formation). Tripathi (2001) conducted a palynological analysis for the sedimentary succession of borehole RCH-51
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