FOSSIL AND CHRONOSTRATIGRAPHIC CONSTRAINTS ON THE QUARTZITE MEMBER, BLACK MOUNTAIN REGION, BHUTAN AND ITS GEOLOGICAL SIGNIFICANCE

A fossil assemblage containing trilobites and rhynconelliform brachiopods from the Quartzite Member of the Pele La Group in the Black Mountain region of Bhutan is indicative of an age of about 494Ma, within Stage 9 of the Cambrian System, Furongian Epoch. This fossil assemblage contains three trilobite genera, Kaolishania, Taipaikia, and Lingyuanaspis, and these are the youngest Cambrian trilobites known in any part of the Himalaya. Although Kaolishania is common throughout equatorial peri-Gondwana at this time, it is only known to co-occur with Taipaikia and Lingyuanaspis at one horizon in one section on the north China block. Although the presence of these young Cambrian rocks in this sector of the Tethyan Himalaya suggests regional variations in Cambrian strata preserved along the margin. Detrital zircon spectra from the Quartzite Member mimic spectra of similar age from Tethyan and Lesser Himalayan sections in other parts of northern India. This provides support for a continuous Himalayan margin at this time. The presence of zircons as young as 500Ma implies rapid exhumation and erosion of source of the zircon grains followed shortly by the deposition of the Quartzite Member. These young zircons may reflect the Cambro-Ordovician orogenic event evident in the western Himalaya.