Biostratigraphy and biogeography of the Tethyan Cambrian sequences of the Zanskar Ladakh Himalaya and of associated regions

Abstract

The Cambrian sections studied so far exhibit 4 km thick and apparently conformable successions of Neoproterozoic and Cambrian strata in Zanskar Spiti basins. The Cambrian sediments of this area are generally fossiliferous comprising trilobites, trace fossils, hylothids, cystoids, archaeocythid and brachiopod, etc. In parts of Zanskar – Spiti and Kashmir region rich trilobite fauna is collected from the Cambrian successions. The trilobites constitute the most significant group of fossils, which are useful not only for the delimination of various biozones but also for the reconstruction of the Cambrian paleogeography of the region. The Zanskar basin together with a part of the Spiti basin forms not only the largest succession of the Tethyan sequences in the Himalaya, but also exposes one of the best developed sections. The complete successions of rocks ranging in age from Precambrian to Eocene are exposed in the Zanskar area of Ladakh Himalaya. The Cambrian sediments in this basin are exposed in Tangzee yogma, Tangzee Kogma, Kuru, Purni – Phuktul, Kurgiakh sections of the Suru valley and Karsha section of the Zanskar valley. On the basis of the faunal studies of icnofossils and on polymerid as well as agnostid trilobites, various faunal assemblage zones have been worked out from these sections. The Himalayan Cambrian successions were whitherto considered to be deficient in agnostid trilobites. During recent years a variety of agnostid taxa have been reported from the Cambrian succession of the Zanskar basin. Agnostids constitute the most important index fossils for the global correlation of Cambrian successions. In the Zanskar Himalayan belt, the agnostid fauna is well preserved in the Middle Cambrian succession of Tangzee-KuruPurni-Phuktul and in the Kurgiakh sections of Lingti and Suru valleys.Preliminary studies reveal the presence of Baltagnostus, Clavagnostus, Peronopsis, Hypagnostus, Diplagnostus, Lejopyge and Goniagnostus a characteristic taxa of Hsuchuangian to Changhian stages of the Middle Cambrian. In Spiti valley the agnostid fauna collected from the Parahio section (Parcha 1999) is represented only by Peronopsis and Baltagnostus, which indicates the Hsuchuangian stage of Middle. However, the agnostid fauna is not as widely present as in the Zanskar and Kashmir regions.