DEPOSITIONAL HISTORY OF THE MIDDLE CAMBRIAN KUNZUM LA FORMATION, INDIAN HIMALAYAS

Over 1,300 m of dominantly siliclastic deltaic deposits of the Middle Cambrian Kunzum La Formation were analysed from sections in the Spiti Valley region of the northern Indian Himalayas. This formation ranges from the lowermost Middle (Maochuangian Stage) Cambrian to middle Middle Cambrian (Hsuchuangian Stage). These Tethyan deposits record deposition in a variety of inner shelf to fluvial environments, which contradicts previous interpretations of this unit as a flysch succession. These are in cases, arranged in large-scale shoaling cycles that range from storm-influenced shelf deposits to thick trough cross-bedded fluvial facies. These 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 rise may have been triggered by large-scale avulsion of river channels. Reduction in sedimentary input associated with avulsion, and continued regional subsidence due in part to sediment loading, led to further transgression and associated deposition of dark, organic-rich mud. Paleocurrents for the Kunzum La Formation are generally towards the north-northeast for both marine and fluvial facies. suggesting that the hinterland was towards the south. High accumulation rates, on the order of 23 cm/1000 yrs (before decompaction), support the interpretation of these rocks as deltaic in origin. An unconformity with Ordovician conglomeratic rocks at the top of the Kunzum La Formation has considerable local relief, with valley-fills over 100 m thick. Ordovician deposits are not in obvious angular unconformity with the underlying Cambrian strata, thus suggesting that tilting was regional in scale and that any intense structural deformation was somewhat distal to the field area.