NEW STRATIGRAPHIC AND GEOCHRONOLOGIC DATA FOR THE TETHYAN OF BHUTAN

The Quartzite Formation of the Pele La Group in the Black Mountains region of Central Bhutan contains a fauna of trilobites and rhynconelliform brachiopods that have been assigned ages that range from Middle Cambrian to Early Ordovician. These strata are roughly time-equivalent with Tethyan Himalayan strata in northern India, Nepal and Tibet. Our sedimentological analysis, the first on these strata, indicate that this part of the Tethyan was a storm-influenced prograding shelf and shoreline. Thick upward-coarsening successions are capped by amalgamated hummocky cross-stratified sandstone and minor white limestone beds. The limestone beds and a few coquinas within the shoreface sandstone contain a trilobite fauna that include the genus Kaolishania, which are tentatively assigned to the middle Late Cambrian. The fauna have similarities with others found in equatorial peri-Gondwana continental fragments, central Iran, and northern China. The Bhutanese succession lacks a prominent Cambrian–Ordovician unconformity and conglomerate, which is present across northwestern Himalaya. This fact and a similarity in both lithofacies and faunas with peri-Gondwanan tectonic blocks raises the possibility that this succession was not a contiguous part of the northern Indian margin at this time, as has been assumed. Age spectra of detrital zircons indicate that earlier estimates that place these strata in the Middle Cambrian are incorrect and that a latest Cambrian to Early Ordovician age is more likely. Our data for the underlying deposits also refutes earlier assertions that these are Lower Cambrian.