STRATIGRAPHIC CONSTRAINTS UPON THE NATURE AND TIMING OF THE HIMALAYAN CAMBRIAN–ORDOVICIAN EVENT

Cambrian strata of the west-central Himalaya provide a critical record of the early history of the orogen. In the Tethyan Himalaya of India, Middle Cambrian carbonate platform deposits of the Karsha Formation are overlain by the Kurgiakh Formation, a shale and sandstone succession previously interpreted to record deep-water flysch deposits. The carbonate-to-siliclastic transition is hypothesized to represent a tectonic transformation from a passive margin to an active deep marine, foreland basin setting that was adjacent to a newly developed arc-trench system. This interpretation is problematic for three reasons. First, the interpretation of sandstone beds of the Kurgiakh as turbidites with classic Bouma sequences is incorrect since these strata contain evidence of shallow-marine, storm-influenced deposition within shoaling deltaic cycles. Secondly, paleocurrent data in overlying Ordovician conglomeratic molasse(?) range from eastward with a subordinate northwest mode in the Zanskar Valley, to north and northeast in the Spiti Valley. Such transport directions are inconsistent with standard models of foreland basin development. Finally, our improved biostratigraphic database suggests that the carbonate-to-siliciclastic transition may have significantly predated the main phase of Cambrian–Ordovician orogenesis. Earliest dated Ordovician deposits in the west-central Himalaya are no older than Middle Ordovician and the Kurgiakh is latest Middle Cambrian. Thus, tectonic uplift and erosion could have occurred as much as 20-30 million years after deposition of the Kurgiakh Formation, and therefore Kurgiakh deposition may have significantly predated the Cambrian–Ordovician orogenic event. These data are inconsistent with recent models for the Himalayan C-O event that includes significant thrust-induced subsidence and southward sediment transport.