Table 10.1

Facies summaries and interpretations, Palaeo-Mesoproterozoic lithostratigraphic units, Singhbhum craton

Lithostratigraphic unitLithofaciesInferred depositional environment
Chandil FormationThe Chandil Formation includes quartzites, mica schists, carbonaceous slate/phyllite, weakly metamorphosed felsic volcanic and volcaniclastic rocks (including vitric and lithic tuffs), and amphibolites. The sandstones are medium- to coarse-grained and compositionally and texturally immature (Chatterjee et al. 2013). Some fine-grained sandstone contains rounded quartz grain (Fig. 10.7) and bimodal textures. The uppermost sandstone units are relatively well sorted and texturally mature and have mud couplets.The compositional and textural immaturity of the medium- to coarse-grained sandstones constituting the lower part of the Chandil succession in combination with their poor sediment sorting, lenticular geometry and unimodal cross-strata orientation are indicative of their terrestrial (fluvial) origin (Mazumder 2005). Some relatively finer-grained sandstones, having sheet-like geometry and occurring on top of the coarse-grained sandstone facies, are relatively better sorted, and the presence of rounded quartz grains (Fig. 10.7) and bimodal texture indicate that they were subsequently reworked by aeolian processes. The topmost sandstones are probably shallow marine and/or lacustrine (Chatterjee et al. 2013).
Dalma FormationThe Dalma Formation is represented by a thick sequence of mafic–ultramafic volcanic rocks with lenses of basic agglomerates (Gupta et al. 1980; Bose 1994; Mazumder & van Loon 2012). Pillow structures are common within the basalts along lower stratigraphic levels (Mazumder et al. 2012a, b). The agglomerates are coarse-grained, characterized by angular basaltic fragments embedded in a lava matrix and are interlayered with the basalts. In places, the volcaniclastic rocks exhibit planar cross-stratification (Mazumder 2005).The inferred palaeogeography and tectonic setting of the Dalma Formation ranges from continental rift (Dunn & Dey 1942; De 1964) to island-arc (Naha & Ghosh 1960) and even back-arc (Bose & Chakraborti 1981; Bose 1994, 2000). This discrepancy stems from the geochemical bias of the previous research, which neglected the importance of the associated volcanogenic low-grade meta-sediments to constrain the palaeogeography. Mazumder & van Loon (2012) have interpreted the precursors of the metamorphosed Dalma basaltic agglomerates as pebbly mudstones (diamictite) formed by mass flow processes in a terrestrial setting.
Dhalbhum FormationThe Dhalbhum Formation is made up of sandstones and shales. The fine-grained sandstones are relatively better sorted and have low-amplitude ripples and wrinkle structures (Fig. 10.5a–c). Some sandstones are medium- to coarse-grained, poorly sorted (Fig. 10.5d) and are feldspar-rich. Penecontemporaneous deformation structures (Fig. 10.5e) and dune to upper-stage plane bed transition are common (Mazumder 2005). The shales are generally massive.Compositional immaturity, coarser grain size, poor sorting, fining-upward cycles, coupled with a unimodal palaeocurrent pattern, suggest that the Dhalbhum sandstones are of fluvial origin (Mazumder 2005; Mazumder et al. 2012a, b). Frequent overturning of the cross-strata possibly took place as a consequence of variable discharge, suggesting thereby a braided character for the fluvial depositional system (Mazumder 2005, Fig. 10.5e). The fine-grained, relatively well-sorted sandstones with very low-amplitude ripples (ripple index 20) occurring on top of coarse- to medium-grained sandstones are of aeolian origin (Mazumder 2005; Mazumder et al. 2012a, b).
Chaibasa FormationThe Chaibasa Formation is characterized by the interbanding of sandstones, a heterolithic (very fine sandstone/siltstone–mudstone) and shale facies in different scales. The sandstone beds are pervasively cross-stratified with characteristic double mud drapes (Fig. 10.4a). The heterolithic facies contain profuse wave-generated structures including hummocky cross-stratification and numerous slumps and slides (Fig. 10.4b, c). The shale facies may contain generally thin but sometimes very thick fine sandstone beds with a variety of penecontemporaneous deformation structures (Fig. 10.4d). In significant contrast to the lower Chaibasa shale facies, the upper Chaibasa shale facies bears superimposed ripples and desiccation cracks (Bhattacharya 1991).The Chaibasa sandstones were formed in a subtidal setting. The heterolithic facies formed in a shelf setting between the fair-weather and storm wave base. The lower Chaibasa shale facies formed in a shelf setting below the storm wave base (Bose et al. 1997; Mazumder 2004, 2005; Mallik et al. 2012; Mazumder et al. 2012a, b). The upper Chaibasa shale facies formed in an intertidal setting (Bhattacharya 1991; Mazumder 2005).
Dhanjori FormationThe Dhanjori Formation is made up of two members: phyllites, quartzites and thin conglomerate comprise the lower member, whereas volcanic and volcaniclastic rocks along with some quartzites and phyllites are important components of the upper member (Fig. 10.2). The sandstone bodies are either massive or cross-bedded, and appear to be broadly lenticular, occurring in units up to 30 m wide. The Dhanjori sandstone is medium- to coarse-grained, locally granule rich and poorly sorted, with matrix content generally 10–12% but occasionally >15%. Grains, where they retain their primary boundaries, appear subangular to subrounded.Poor sediment sorting, compositional immaturity, lenticular geometry and broadly unimodal palaeocurrent patterns indicate the Dhanjori sandstone is a fluvial deposit. The conglomerate–sandstone assemblage at the base of the lower member has been interpreted as the distal fringe of an alluvial fan deposit. The Upper Member does not include any sheet flood and sieve deposits and is constituted solely by channel and mass flow deposits (Mazumder & Sarkar 2004; Mazumder 2005).