Ordovician–Silurian acritarch biostratigraphy of the Tethyan Garhwal Himalaya, India

Abstract

The Ordovician–Silurian succession of the Shiala and Yong Limestone formations, Tethyan Garhwal Himalaya (Uttar Pradesh), reveals a rich assemblage of acritarchs. The assemblage includes sphaeromorphs, acanthomorphs, netromorphs, polygonomorphs, diacromorphs and herkomorphs. Seventy species belonging to thirty-seven acritarch genera have been identified. The new forms are described. Four distinct local acritarch assemblage zones are proposed. They are: (IV) Leiofusa algerensis– Multiplicisphaeridium osgoodence; (III) Tetradinium minutum– Dactylofusa sp. cf. D. oblancae; (II) Domasia trispinosa– Deunffia monospinosa; (I) Baltisphaeridium longispinosum var. longispinosum– Multiplicisphaeridium ornatum. The palynological assemblages identified in the Garhwal Himalaya correlate with Ordovician–Silurian assemblages of the United Kingdom, Belgium, Russia, North Africa, Saudi Arabia and China, etc. The presence of biostratigraphic index forms suggest a Late Ordovician to Middle Silurian age for the Shiala Formation and a Middle to Late Silurian age for the Yong Limestone. Thus, the Ordovician–Silurian boundary lies within the Shiala Formation and negates an earlier view that this boundary lies at the base of the Yong Limestone Formation. In the systematic part, five new species of acritarchs, Neoveryhachium distinctum Sinha, Prasad and Srivastava, sp. nov., Tetradinium minutum Sinha, Prasad and Srivastava, sp. nov., Triangulina densus Sinha, Prasad and Srivastava, sp. nov., Geron indicus Sinha, Prasad and Srivastava, sp. nov., and Leiofusa shialensis Sinha, Prasad and Srivastava, sp. nov., are described and illustrated.