中国科学院南京地质古生物研究所机构知识库

The type section of the Parahio Formation in the Parahio Valley, Spiti, is the best litho- and biostratigraphically documented Cambrian section in the Indian subcontinent. Fundamental issues of the geology of the type section, including its location, thickness, definition as a lithostratigraphical unit, nomenclature, and the stratigraphic position of fossil collections within it, are clarified herein. Our direct estimate of the section thickness made by measurement with tape and Jacob's staff is confirmed by satellite imagery, and reconfirms that H. Hayden, who published the first major monograph on the section in 1904, significantly underestimated the thickness of the body-fossil bearing part of the formation. That estimate has been the root of several subsequent misconceptions about the unit that we also clarify. In particular, we stress that shelly fossils span most of the type section and are not confined only to the upper part, and that there are trace fossils and carbonate layers throughout it. The formation can be traced regionally, and its distribution is used to understand aspects of early Palaeozoic tectonism in the Himalaya. The formation stratotype remains the Parahio Valley section, but a new stratotype is proposed in the section opposite Kuru in the Zanskar Valley for the base of the formation that conformably overlies it, the Karsha Formation. A suitable basal boundary stratotype for the Parahio Formation has yet to be identified. The term Kunzam La Formation is rejected as a junior synonym without superior merit.