A prominent shear zone runs E-W across Singhbhum. Structural analysis indicates three phases of folding, all with E-W axial planes dipping northerly or subvertically to the north of the shear zone. South of this shear zone there are two phases of folding

Abstract

The diagenetic and metamorphic evolutions of the Cambro-Ordovician silicictastic sediments of the Luna area, which formed part of the Lower Palaeozoic stable shelf, and of the Sil area, which was situated in the adjacent geosyncline, were determined and compared. The overburden in the Luna area reached 2500-3500 m, based on stratigraphic evidence. This is in agreement with the mineral association occurring in the sediments (authigenic illite with minor amounts of detrital kaolinite and authigenic muscovite and chlorite), indicating a depth of burial of 2000-3000 m. The sediments were subject to diagenetic changes, which occurred shortly after deposition before lithification (syndiagenetic phase), in the lithified rocks (anadiagenetic phase) and after uplifting and erosion of the area (epidiagenetic phase). Based on stratigraphic evidence, the overburden of the sediments studied in the Sil area ranged from 5000 to 14000 m. The mineral associations observed in the sediments (illite-muscovite-chlorite in the upper part and muscovite-chlorite-biotite in the lower part of the succession) are not in agreement with the estimated depth of burial. The sediments of the Sil area underwent diagenetic alterations similar to those of the sediments in the Luna area, but the effects are obscured by metamorphic recrystallization, which occurred during the Hercynian orogeny. During this period the area was affected by regional metamorphism.