Department of Geology, Shimane University, Matsue, Japan
Department of Geology and Mineralogy, Hokkaido University, Sapporo, Japan
The Siwalik group (Churia group) in and around western part of the Chitwan Dun are divided into the lower, middle and upper formations, roughly correlable to the Lower, Middle and Upper Siwaliks of India and Pakistan, respectively. The lower formation (more than 500 m) consists of alternating beds of fine sandstone and siltstone. The middle formation (200 m thick) is characterized by thick-bedded, medium to coarse sandstones. The upper formation (more than 500 m thick) is composed predominantly of gravel beds. The Siwaliks comprise a coarsening upwards sequence as a whole, reflecting the rising Himalayas during Neogene Period. Many molluscan fossils (Unionidae and fresh-water gastropods) are found in the middle formation. This is the first discovery of well-preserved mollusks from the Siwaliks in Nepal. Mud-cracks and rain-drops are also found in them. These are important for interpretation of the Siwalik sedimentation.
Concerning the paleomagnetic polarity stratigrapny it is possible that the normal polarity zone in the upper formation corresponds to an event in the Matsuyama Reversed Epoch or a part of the Gaus Normal Epoch, and that a long normal polarity zone found in the lower formation corresponds to an older Epoch than Epoch 5, although no fossil and geochronologic evidences have been obtained yet.