5.1. Major elements
Barail Group sandstones have SiO2 contents ranging from 73 to
78 wt.%, and Al2O3from 11 to 13 wt.%. SiO2contents in the Surma (66–
85 wt.%) and overlying Tipam (79–84 wt.%) Group sandstones range to
higher values, and are higher still in Dupitila Group sandstones (82–
85 wt.%). Average SiO2 abundances in the Sylhet mudrocks are
characteristically less (b∼70%) than those in the sandstones (Table 2).
In both sandstones and mudrocks, SiO2 contents in the Tipam and
Dupitila Groups are greater at a given Al2O3than in the Barail and Surma
Groups; this is also the case for the Jaintia mudrocks (Fig. 3a)