Ordovician strata of the Indian subcontinent

Abstract

Abstract Ordovician rocks of the Indian Tethyan Himalaya contain a conspicuous angular unconformity between mostly marine Cambrian and overlying terrestrial Ordovician strata, which is a record of the Kurgiakh Orogeny. This tectonic event is traceable across the Tethyan Himalaya from Pakistan to Bhutan. The Pin Formation in the Spiti Valley provides a high-resolution account of the marine depositional history, palaeontology and isotope geochemistry of Late Ordovician events. The middle (Takche) member is late Katian and the upper Mikkim Member is lower Silurian (Llandovery), based on an ozarkodinid conodont fauna. The Pin Formation records the Boda event, the last warming interval prior to Hirnantian glaciation. The δ 13 C carb chemostratigraphic data allow precise global correlation, and recognition of the Paroveja positive excursion, the last major excursion of the Katian. The Mikkim Member records a ‘lower HICE’ (Hirnantian isotopic carbon excursion) of the Katian–Hirnantian boundary interval. Palaeontological data indicate that there are no known fossils diagnostic of any Ordovician ages older than the Katian Stage in India. Evidence of Ordovician sedimentary rocks in the Lesser Himalaya is intriguing, but presently equivocal. The widespread absence of pre-Katian strata on the Indian subcontinent is due to erosion associated with the Kurghiak orogeny and delayed onlap onto topographically high areas.