Cambrian Depositional History of the Zanskar Valley Region of the Indian Himalaya: Tectonic Implications

A well-preserved Cambrian section in the Zanskar Valley of northern India has previously been interpreted to record the transition from a passive to an active tectonic margin related to Cambrian–Ordovician orogenesis. This interpretation has been used to support the tectonostratigraphic interpretation of other successions across the Tethyan Himalaya. Our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these Cambrian deposits: no definitive record of impending Cambrian–Ordovician orogenesis is recorded in these late Middle Cambrian rocks. A critical transition from an , 125-m-thick, stromatolite-bearing carbonate deposit, the Karsha Formation, into shale and sandstone of the Kurgiakh Formation, was interpreted to represent tectonically induced drowning of a carbonate platform. Siliciclastic strata of the Kurgiakh Formation were thought to record deep-water flysch deposition in a tectonically active foreland basin next to an arc-trench system. This interpretation was based on sandstone beds with classic Bouma sequences. We show that these event beds in the Kurgiakh instead contain hummocky cross-stratification, quasi-planar lamination, and combined-flow ripple stratification, all of which reflect deposition in shallow-marine, storm-influenced environments. Thus, although the Karsha carbonate platform may have been drowned, it did not culminate in deep-sea flysch deposition, and this in turn eliminates a major line of evidence linking Kurgiakh deposition to the onset of Cambrian–Ordovician orogenesis. Other aspects of Cambrian–Ordovician deposits of northern India also shed doubt on the proposed link between Kurgiakh sedimentation and the Cambrian–Ordovician orogenic event. First, our improved biostratigraphic database suggests that the transition from the Karsha carbonate to the Kurgiakh Formation may have predated the main phase of Cambrian–Ordovician orogenesis, as recorded by overlying Ordovician molasse, by as much as 20–30 My. Second, published data from the Ordovician molasse indicate northward paleocurrents, which are parallel to those recorded by siliciclastic deposits of the Parahio Formation below the Karsha, and thus are at odds with standard models of foreland basin development for the Cambrian–Ordovician event. Our sedimentological analysis of depositional cycles of the Parahio Formation indicates that these strata record storminfluenced environments from offshore marine to shoreface to fluvial settings. This is at odds with previous paleoenvironmental interpretations that ranged from deep-sea flysch to intertidal deposits. Paleocurrent data for marine and fluvial facies of the Parahio Formation in both Zanskar and the Spiti Valley to the south indicate northeast sediment transport. This supports the view that the Parahio and overlying carbonate of the Karsha Formation record the ancient northern passive margin of India during the Cambrian and that these strata may be distal equivalents of the younger Cambrian deposits of the Lesser Himalaya.