Petrographical-geochemical characteristics and floral-faunal compositions of the Valia lignite deposits from Cambay Basin (Gujarat), western India

Abstract

Lignite-bearing sequence of the Valia mine, associated with the Cambay Shale Formation (early Eocene) in Cambay Basin of western Indian State Gujarat, has been studied. An integrated approach (including organic petrography, palynofacies, palynology, entomology, geochemistry) has been adopted to assess the origin and characters of lignite deposits and their potential for the hydrocarbon generation, besides to understand the evolution and interaction of different biotic (floral-faunal) groups of the geological time. The studied samples (lignites/shales) are characterized by the domination of huminite group (av. 67 vol.%)/phytoclasts (av. 58%) along with the abundance of short-chain n-alkanes (bacterial activity). The frequencies of detrohuminite sub-group, non-biostructured elements, macrophyte-derived amorphous organic matter (AOM, mainly), and high values of proxy aqueous are indicating the elevated bacterial degradation of the OM and/or also suggest the significant contribution of the herbaceous vegetation/aquatic macrophytes/algal input to the palaeo-peat. The relative abundance of hopanes and low values of the steroids/hopanoids ratios further indicate a high bacterial activity. On the other hand, the contents of telohuminite, liptinite group, and cuticle elements together with the high carbon preference index, low proxy wax, and terrigenous/aquatic ratios confirm both terrestrial and macrophyte contributions during the peat accumulation. Fascinatingly, we have found Dipterocarpus-derived amber (fossil resin) within the lignite seam (lower) with remarkably well-preserved arthropod inclusions consisting of arachnids and insects. Generally, insects have an association with flora and provide important ecosystem functions. In the studied amber samples, the insects to arachnids (spiders and pseudoscorpions) ratio is 3:1. Their quantitative analysis indicates the dominance of the Chironomidae (non-biting midges 32%) followed by Araneae, Myrmicinae, Cecidomyiidae, and Mycetophilidae. A rare entity such as pseudoscorpions has also been found during the investigation