BASAL PALEOGENE BIOSTRATIGRAPHY AND BIOCHRONOLOGY OF THE COLLISION-INDUCED PARALIC-FLUVIAL PACKAGE: INTERPRETATIONS FOR PALEOENVIRONMENTS AND PALEOBIOGEOGRAPHY OF A NARROWING TETHYS, NW HIMALAYA, INDIA

The present paper gives insight into the India-Asia collision-induced basal Tertiary sediments paralleling the Outer Tethyan palaeoshoreline of the Indian Northwestern Himalaya. Geoidal crustal anomalies resulting from Indian Plate's rotation in the Palaeogene led to widespread transgression in NW Himalaya from Shingar-Surghar Ranges (North Pakistan), Kohat Basin (Northwestern Pakistan) and Subathu Basin of NW India. Detailed biostratigraphic outcrop data from the Subathu Formation in Himachal Pradesh provides unique insight on fossil biotas, associated facies and varying time-space relations of the fluctuating Subathu Basin in the light of ongoing India-Asia collision. Models constraining Tethyan withdrawl and narrowing Tethys are presented.

An initial marine episode related to the rapid drift of the Indian Plate documents the basal deposition of the Subathu Formation with green mudstones, shales, carbonate lenses, minor sandstones and fine silts. Marine fish, crocodilian biotas are present in a lensoidal bone bed that is intercalated with basal mudstones. The bone bed shows phosphatic clasts, abraded oyster shells and bones churned up in lime mud indicating high energy shallow paralic sedimentation. A Late Palaeocene-Early Eocene age is suggested by the fossil biotas and stratigraphic positioning. Another basal facies is the crocodile,mammal- bearing alluvial facies, comprising redbeds of mottled siltstones, overbank conglomerates and channel sandstones. Paleosols with carbonate nodules are observed in interfluve surfaces. Continental vertebrate remains of turtles, crocodiles and mammals are present in silts and overbank conglomerates. Evidences of shoreline progradation are suggested in this restricted sand flat-channel bar complex.

Upsection in Uppermost Subathu, (Middle-?Upper Eocene), alluvial redbeds appear capping the marine facies. The facies comprise siltstones, pebbly sandstones, conglomerates, palaeosols with large carbonate nodules and quartzose sandstones. The pebbly sandstones representing the overbank sediments yield fossils of rodents and other large mammals with Central Asiatic affinities, being coeval to the Kuldana Formation of Kala Chitta Range of Pakistan. A prograded basin with continental facies was established, culminated from a regressive phase of Tethyan withdrawl and the India-Asia collision.