Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India

Recent work on Himalayan tectonics indicates that prior to the Cenozoic collision of India and Asia, an enigmatic Cambrian-Ordovician event may have strongly influenced the regional geology of the Himalaya. Stratigraphic and sedimentological analyses of well-preserved Cambrian deposits are critical for understanding the nature of this early tectonic event and its influence on the later tectonic evolution of the Himalaya. The Parahio Formation, defined herein, of the Parahio Valley, Spiti region, in the Tethyan Himalaya of India, is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya. This formation consists, of > 1350 m of dominantly siliciclastic deltaic deposits. The formation ranges from upper-most Lower Cambrian (Lungwangmiaoan Stage) to middle Middle Cambrian (Hsuchuangian Stage), representing a time span or similar to 5-10 m.y. It contains numerous medium-scale shoaling cycles that range from storm-influenced offshore deposits to thick trough cross-bedded fluvial facies. Many thin carbonate beds with abundant trilobite fossils directly overlie the fluvial facies and represent transgressive systems tract deposits. The cycles are interpreted to have resulted from delta-lobe switching, based on a lack of systematic stratigraphic changes in cycle or facies thicknesses. This paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep-sea flysch to shallow-marine ticialites. In addition, our paleoenvironmental analysis and paleocurrent data suggest that the uppermost Lower to Middle Cambrian deposits of the Lesser and Tethyan Himalaya are parts of the same ancient northward-prograding, fluvial-deltaic depositional system or the palco-Tethys margin of India. An angular unconformity with overlying Ordovician conglomeratic rocks has considerable local relief, with meter-scale scours and a valley fill > 100 m thick. The scours have northeast-southwest orientations, which parallel both the paleocurrents in the underlying Parahio Formation as well as published paleocurrent readings from the coarse red beds of the overlying Ordovician strata. The Cambrian-Ordovician unconformity is of regional extent, and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in Spiti is similar to 15 m.y. Our sedimentological analysis and associated paleocurrent data from the Parahio Formation, along with additional data from units both above and below the unconformity, indicate that published models portraying foreland basin development at this time with southward-directed thrusting are problematic. An alternate possibility that uplift took place south of the Tethyan Himalaya - is also problematic, because no published stratigraphic or structural evidence exists for such an uplift to the south for either the Greater or the L esser Himalaya lithotectonic zones.