The Spiti Basin in Himachal Pradesh preserves a remarkably complete and richly fossiliferous Palaeozoic sedimentary sequence, representing a passive continental margin setting of the Indian Plate prior to Himalayan collision.

The Palaeozoic succession, spanning from the Cambrian to the Permian, is characterized by distinctive lithologies and faunal assemblages:

1. Cambrian: The basal Kunzam La Formation comprises predominantly quartzites, slates, and phyllites, often thinly bedded. It is rich in trace fossils like Cruziana and Diplocraterion, alongside early trilobites (e.g., Redlichia) and diverse brachiopods.
2. Ordovician: This period is marked by the Thango Formation (quartzites and conglomerates) followed by the lower part of the Muth Quartzite. Fossils include characteristic brachiopods (Orthis), graptolites, and trilobites.
3. Silurian: Lithologically, this includes slates, quartzites, and greywackes, often overlain by limestones. Dominant fossils are brachiopods (Atrypa), crinoids, and trilobites.
4. Devonian: The prominent Muth Quartzite characterizes this period – a thick, often white, highly pure quartzite, indicating shallow marine to terrestrial conditions. Overlying it are thin limestones and shales. Fossil content includes brachiopods, crinoids, and occasional fish remains.
5. Carboniferous: The sequence begins with the Lipak Formation, comprising dolomitic limestones, shales, and calcareous shales. It contains abundant brachiopods, bryozoans, crinoids, and early ammonoids. This is followed by the Po Formation, consisting of dark shales and diamictites, often with plant fossils and brachiopods, signaling glacially influenced sedimentation.
6. Permian: The Permian is represented by the Ganmachidam Formation (diamictites) and the Kuling Formation (black shales and sandstones). These units are highly fossiliferous with diverse Productid brachiopods (Productus, Marginifera), ammonoids (Cyclolobus), and bivalves, providing excellent biostratigraphic markers for the Late Permian.

The Spiti Basin's Palaeozoic sequence thus provides an invaluable window into the ancient Tethyan realm, offering critical insights into stratigraphy, palaeontology, and palaeogeography of the Indian subcontinent before its northward journey.