Biostratigraphy and geology of the neogene Siwalik group of the Surai Khola and Rato Khola areas in Nepal

An almost complete sequence of Neogene molasse sediments of the Siwalik Group in the Surai Khola (‘khola’ means small river in Nepali) area in Nepal, which was divided into five lithological units, includes the upper part of the Lower Siwaliks, the Middle and the Upper Siwaliks. It discloses a tectonically uncomplicated southern belt (i.e. the Surai Khola sequence) and a tectonically much disturbed northern belt. The Upper Siwaliks at Surai and Rato Khola yield a rich vertebrate fauna, contained within thick bodies of multistoried sandstones. The data from the pollen and plant fossils indicate vegetational changes from tropical evergreen forests in the Lower and Middle Siwaliks to dry, deciduous forests and grasslands by the beginning of the Upper Siwaliks. Biostratigraphically the Upper Siwalik sediments can be compared with the Hexaprotodon sivalensis and the Elephas planifrons Interval Zones of Pakistan and with the Tatrot and Pinjor formations of Northwest India. Palaeomagnetic dating of the Surai Khola sequence indicate a time scale from 13 to ca. 1 million years.