Depositional Framework of the Sandy Mid-Fan Complexes of the Proterozoic Chaibasa Formation, E.Singhbhum, Bihar

The Proterozoic Chaibasa Formation belonging to the Singhbhum Group was deposited in small overlapping submarine fans along the eastern cratonic margin of an unstable foreland trough during the early phase of the Singhbhum Orogeny. The following features suggest deposition within the mid-fan complex: (I) a relatively high proportion of coarse, massive psammitic layers with small proportion of pelitic layers and classic graded sandstones; (2) the presence of shallow sand-filled channels; (3) the frequent associations of olistostromes; (4) association of thick coarse sandstone with thin layers of fine sandstone separated by thick shale units; (5) the presence of both positive and negative cycles in the stratigraphic record.

Sandstone bodies characteristic of the unchanneled fan lobes, represent deposition associated with basinward progradation of the lobe complex. Accumulation of a thick siltstone on top of the coarse sandstone facies during the active progradational phase is related to a channel cutoff in the upfan region. In contrast, the sandstone bodies present in the channel show an overall graded nature, lack thick siltstone horizon, contain erosional structures, and a higher percentage of positive cycles. The development of the submarine fan was related to external and internal factors, that is, the flow velocity, the rate of sediment supply coupled with channel migration, frequency of major storms or earthquakes to initiate submarine gravity flows. This development of alternating graded sandstone units and siltstone blankets may be a more useful criterion (than sedimentary structures) for the recognition of ancient mid-fan lobe sequences.