THE RECORD OF CAMBRIAN AND ORDOVICIAN OROGENESIS IN NORTHERN INDIA AND BHUTAN

An early Paleozoic tectonic event, the Kurgiakh orogeny, has long been known from the western Tethyan Himalaya, and is recorded most conspicuously by an angular unconformity between Cambrian and Ordovician sedimentary strata, an overlying coarse conglomerate, and widespread syn- to post-tectonic granitic plutons. Although the lateral extent of this event is poorly known, two regions in the central and eastern Himalaya, the Kumaon of India and several klippe of Bhutan, contain poorly described conglomerate units that may be correlative. In the Kumaon, we show that the Ralam Formation contains a basal polymictic conglomerate unit with detrital zircon grains as young as (512 +/-5 Ma), is overlain by sandstone containing the arthropod walking trace Diplichnites gouldi. The late early Cambrian trilobite Redlichia sp. from the Martoli Group shortly below the Ralam Formation, indicate that conglomerate is post early Cambrian and, based on stratigraphic analysis, also likely records the Kurgiakh orogeny. In western Bhutan, the Lower Paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix. These units have also been assigned to a variety of ages, but rocks that apparently overlie these strata contain the Furongian (late Cambrian) zonal trilobite Kaolishania granulata. Here we show that detrital zircon ages of a clast (youngest grains = 781+/-8 Ma) are distinctly older than those of the matrix, which contains grains as young as ~448 +/-5 Ma, which is a maximum depositional age of the unit. Our data suggests that the Bhutan conglomerate units, and possible equivalents in Tibet, also correlate to those of the central to western Himalaya, significantly increasing their known along-strike extent by over 1000 km, and increasing the total extent to about 1200 km. This suggests that the Kurgiakh orogeny spanned much of the Himalayan orogen along the northern Gondwanan margin.