STRATIGRAPHY AND SEDIMENTOLOGY OF MIDDLE TO UPPER CAMBRIAN STRATA, ZANSKAR REGION, INDIAN HIMALAYAS

The Karsha and Kurgiakh formations in the Zanskar Valley consist of approximately 1280 m of dominantly siliciclastic marine and fluvial rocks described from five measured sections. The Mauling Member of the lower Karsha Formation consists of shale, sandstone tempestites, and thin carbonate beds that are arranged in decameter-scale shoaling cycles. These are deltaic deposits that are lateral equivalents of the Kunzum La Formation of the Spiti Valley to the east. Trilobite fauna help to correlate the top of the Mauling Member to the top of the Kunzum La Formation. The presence of more abundant trough cross-bedded fluvial sandstone and amalgamated HCS sandstone in the Kunzum La Formation indicates that the Mauling Member in Zanskar is generally a more distal succession relative to the Spiti Valley strata. The Karsha is overlain by the Kurgiakh Formation, a dominantly siliciclastic unit that ranges upward from shale to a thick sandstone-dominated unit and back to a shale-dominated unit at the Cambrian–Ordovician unconformity. This transition has been interpreted to represent a shift from passive margin deposition to flysch deposition in an active foreland basin based on the purported presence of turbidites within the Kurgiakh Formation. However, we document amalgamated sandstone intervals and shoaling cycles up to several meters thick that include abundant hummocky cross-stratification and other storm-generated structures. These indicate deposition in a shelf setting above storm wave base. These interpretations remove the underlying basis for the interpretation of these strata representing a major tectonic event in this region.