THE MIDCARBONIFEROUS TO LOWERMOST PERMIAN SUCCESSION OF SPITI (PO GROUP AND GANMACHIDAM FORMATION - TETHYS HIMALAYA, NORTHERN INDIA) - GONDWANA GLACIATION AND RIFTING OF NEO-TETHYS

The upper Lower Carboniferous to lowermost Permian terrigenous success ion of the Spiti Valley can be subdivided into five formations (Thabo Formation, Fenestella Shale, Kabjima Quartzarenite, Chichong Formation and Ganmachidam Diamictite), here described in derail and subdivided into members or lithozones. The Po Group, overlying the platform carbo nates and gypsum deposits of the Lipak Formation, records an increase of subarkosic to quartzarenitic terrigenous detritus derived from upli ft and erosion of continental blocks in the south during the initial s tage of Neo-Tethyan rifting. Increasing mineralogic stability through the Thabo Formation broadly coincides with a climate change from tropi cal arid to temperate humid conditions during the Visean-Serpukhovian. The Fenestella Shale, containing rich brachiopod associations of Bash kirian age, documents a stage of widespread subsidence and transgressi on, followed by the regressive Kabjima Quartzarenite, which records re cycling of quartzose sedimentary sequences in the south. The Chichong Formation marks another transgressive event, associated with a signifi cant increase of granitoid detritus, partly from nearby Lesser to High Himalayan source areas. Chaetetid, cephalopod and brachiopod faunas h int at a Moscovian age for the ''Chaetetid beds'', at the top of which varve-like lamination and scattered pebbles suggest glacially-influen ced deposition. The overlying glacio-fluvial (?) cobble conglomerates (''Pebbly beds'') reflect onset of rapid tectonic uplift. Abundant det ritus from sedimentary rocks characterizes the overlying Ganmachidam D iamictite, deposited in glacio-marine environments; cold-water marine faunas of Asselian age occur in its middle part. Erosion of progressiv ely older pre-rift sedimentary successions is ascribed to basin invers ion, associated with unroofing of anatectic granitoids of the Lesser a nd High Himalayas. Basaltic to rhyolitic volcanic detritus documents a lkalic magmatism at the climax of continental rifting.