Revised Lithostratigraphy and Age of the Gondwana Sequence in the Eastern Nepal Himalaya

ABSTRACT The Gondwana sequence in eastern Nepal is exposed as isolated patches in the frontal part of the Lesser Himalaya at the hanging wall of the Main Boundary Thrust (MBT). The stratigraphic classification and age of the Gondwana sequence in this area have long been ambiguous and debated. The present study in the Barahakshetra-Ranitar and Takure areas of eastern Nepal aims to clarify the lithostratigraphy and ages of the Gondwana sequence. For this purpose, geological mapping at a 1:25000 scale, route mapping at larger scales, direct measurement of stratigraphic sections, petrographic study, sedimentological study, study of plant fossil prints, and palaeoflow analyses were carried out along four stratigraphic sections, namely: Rakche Khola, Saptakoshi River, Kokaha Khola and Takure. The present study, along with previously published U-Pb zircon ages, reveals that the Gondwana sequence in the area can be divided into five formations, namely the Kokaha Diamictite, Tamrang Formation (with Baraha volcanics), and Kali Khola Formation, belonging to the Lower Gondwana and Sehera Khola Formation and Sapt Koshi Formation, belonging to the Upper Gondwana. Baraha volcanics, comprising tuff, volcanoclastic sandstone, and agglomerate, are the reworked volcanic materials probably from older volcanoes away from the depositional basin. The Tamrang Formation has been confirmed as part of the Lower Gondwana based on the occurrence of Permian cold-water bivalve Eurydesma. The Sapt Koshi Formation, previously thought as the oldest unit of the Gondwana, is comparable with the Late Cretaceous-Palaeocene Amile Formation in the Tansen area. Upper Gondwana plant fossil Ptilophyllum has been reported for the first time from Sehera Khola in the Takure area.