The Bashkirian “Fenestella Shales” and the Moscovian “Chaetetid Shales” of the Tethys Himalaya (South Tibet, Nepal and India)

Abstract

The “Fenestella shales” are a mid-Carboniferous marker unit which has long been described from classic localities of the NW Himalaya (Kashmir, Spiti). Correlative shaly units have recently been traced in central Nepal and as far as South Tibet, where they yielded varied brachiopod assemblages indicative of Bashkirian age. A second distinct interval of black shales, characterized by the abundance of chaetetids and directly underlying the widespread Gondwanan diamictites, has been dated as Moscovian in Spiti and represents the youngest fossiliferous horizon hitherto identified in the Upper Carboniferous of the Tethys Himalaya. The “Chaetetid shales” are recognized also in Manang, whereas in South Tibet the stratigraphic framework still needs improved definition. These major fossiliferous black shale units, marking repeated transgressive events in the middle part of the Himalayan rift sequence, have not only major stratigraphic significance but also represent a fundamental landmark in palaeogeographic and palaeoclimatic reconstructions of Northern Gondwana. With the onset of continental rifting, arid tropical climates at the close of the Tournaisian were replaced by temperate humid conditions in the Visean-Serpukhovian, when diamictites were deposited in South Tibet. After this first cooling stage, the “Fenestella shales” mark a widespread transgression at the very beginning of the Late Carboniferous, coupled with reduced tectonic activity and temperate to temperate-warm climates. After renewed tectonic activity during a second cooling episode, marked by local deposition of diamictites in central Nepal, the “Chaetetid shales” represent another major transgression in the Moscovian, shortly preceding the final and most intense cooling event marked by deposition of glacio-marine diamictites in the whole Tethys Himalaya from Kashmir to South Tibet during the latest Carboniferous/earliest Permian. Two fossiliferous horizons containing very similar brachiopod faunas of early Late Carboniferous age have recently been found also in North Karakorum, at lower southern latitudes, where climatic conditions always remained temperate and there is no trace of Upper Palaeozoic glacial deposits or ice-rafted debris.