Silurian acanthomorphitae acritarch from the Shiala Formation, Tethys Garhwal Himalaya, India

The Tethyan-Tibetan sedimentary belt extends from Nanga Parbat in the west to Namcha Barwa in the east and divisible into five sub basins, which are resting over the Central Crystalline basement in the south. These continuous sedimentary sequences vary in thickness from 5000 to 16000 m and range in age from Precambrian to Eocene (?). In north they are terminated by the Indus-Tsangpo Suture Zone. The five subdivisions are developed in the Kashmir, Zanskar, Spiti-Kinnur and Garhwal-Kumaon Basin. The present study is confined to the Lower Palaeozoic sequences of Garhwal-Kumaon basin, which is best known for biostratigraphy and sedimentological studies (Shah and Sinha 1974, Sinha 1989, Sinha et al. 1998, Sinha et al. 2005, Sinha and Mishra 2006, Bagati et al. 1991, among others) in the TethyanTibetan belt. The Ordovician-Silurian strata known as Shiala Formation in the Garhwal-Kumaon basin vary from 400-500 m thickness and are not easily accessible for field work. The Shiala Formation conformably overlies the Garbyang Formation, which is characterised by green shales at the bottom and arenaceous component increases gradually reflecting coarsening upward sequence. The Shiala Formation has been assigned a mid to late Ordovician age based on index forms of brachiopod taxa (Sinha