The modal data for the Barail and Surma Groups clearly indicate a
recycled orogen provenance, and LmLvLs relations are consistent with
derivation from a suture belt (Fig. 2). Within such orogens, sediment
sources are dominantly sedimentary and metamorphic rocks exposed
to erosion by orogenic uplift of fold belts and thrust sheets (Dickinson
and Suczek, 1979; Dickinson, 1985). This is reflected by the
dominance of sedimentary and metamorphic lithics in the samples
analyzed here. The Barail Group sandstones contain little feldspar and
have LsNLm, and thus were derived from a mixed sedimentary and
metamorphic source. Ls and feldspar contents are slightly enriched in
the middle and upper parts of the Surma Group, whereas Lm is more
abundant than Ls in its lower part. This suggests a greater
metamorphic component in the source in the lower part, and
derivation of lithic fragments from extensive sedimentary and lowgrade metamorphic terranes in the middle and upper parts. Slight
decrease in quartz and increase in total lithic fragment and feldspar
contents upward from the Barail to the upper Surma Group suggests
uplift and erosion of the Eastern Himalaya during the Miocene, with
increased exposure of crystalline rocks