Cambrian Microfossils of the Tethyan Himalaya
- Author(s): Gilbert, Ian Ross
- Advisor(s): Hughes, Nigel C.
- et al.
The Parahio Valley hosts the type section of the Parahio Formation, a unit that occurs widely in the Tethyan Himalaya. Recent revision and recollection of trilobites from multiple horizons within this unit show that it spans the latest part of State 4 and much of Stage 5 of the Cambrian Period. Broadly coeval Parahio Formation deposits occur in the Zanskar Valley some 150 km to the northwest. A variety of small shelly fossils have been recovered by acid dissolution of carbonate beds within the Parahio Formation from both these sections, and these are described herein. They include tetract and pentact sponge spicules belonging to hexactinellid sponges, chancelloriid spicules belonging to Chancelloria sp. and new species of Archiasterella, A. n. sp., shells of a helcionelloid comparable to Igorella maidipingensis, the hyolithid-like Cuptittheca, sp., a poorly preserved hyolithid, a meraspid trilobite that may belong to ptychopariid, and an assortment of spinose microfossils of uncertain affinity. No conodont or paraconodont specimens were recovered, in contrast to an earlier report of microfossils from one horizon at this locality, and inspection of those earlier reported micofossils suggest that they may have been detached chancelloriid spicules. These newly recovered microfossils are consistent with the trilobite-based middle Cambrian age determination for these formations and are inconsistent with the late Cambrian age for the top of the Parahio Formation in its type section by some earlier workers. Individual small shelly fossils taxa show variable ranges within the section, and lie within known stratigraphic ranges for comparable forms known from sections elsewhere in the world. The newly described species A. n. sp. provides the first biostratigraphic link for the Parahio Formation between the Parahio and Zanskar Valleys.