DEPOSITIONAL HISTORY OF MIDDLE CAMBRIAN OF INDIAN HIGH HIMALAYAS: TECTONIC AND PALEOGEOGRAPHIC IMPLICATIONS

The Middle Cambrian Kunzum La Formation consists of over 1,300 m of dominantly siliclastic deltaic deposits in the Spiti Valley region of the Indian High (Tethyan) Himalayas. The formation ranges from the lowermost Middle (Maochuangian Stage) Cambrian to middle Middle Cambrian (Hsuchuangian Stage). It is dominated by large-scale shoaling cycles that range from storm-influenced shelf deposits to thick trough cross-bedded fluvial facies. The fluvial deposits are in cases overlain by thin carbonate beds with abundant trilobite fossils. These beds, which are scattered throughout the formation, represent the initial transgressive deposits of these cycles and their bases are marine flooding surfaces. Relative sea level rises may have been triggered by large-scale avulsion of river channels, and falls by reduction in sedimentary input associated with avulsion along with continued regional subsidence due in part to sediment loading. Paleocurrents for the Kunzum La Formation are generally towards the north-northeast for both marine and fluvial facies, which suggests that the hinterland was towards the south. High Himalayan strata are generally reconstructed as distal equivalents of Lesser Himalayan passive margin deposits. Detrital zircon age data from nearly age equivalent strata from each area reveal similar age populations, but show differences in population abundances and degree of maturity of zircon grains. Palinspastic reconstruction of these regions is difficult because of uncertainty of Cenozoic fault displacements. Thick deltaic deposits of the Kunzum La Formation are difficult to reconcile as distal equivalents of the Lesser Himalayas without considerable spatial changes in source terranes and shoreline progradation. The Cental and High Himalayan rocks could alternatively represent a separate basin that was juxtaposed with India during the early Paleozoic. There is, however, no structural evidence in the form of folds or microstructures to support Neoproterozoic to lowermost Paleozoic orogenesis.