Tethys a very old seaway formed with the breaking up of the ancient supercontinent Pangea that existed during Permo-Triassic time (286-208 million years ago). The Tethys seaway separated ancient continental masses of Laurasia on the northern hemisphere and Gondwanaland on the southern hemisphere. At about 75 million years ago (Cretaceous), the Tethys Ocean extended across the globe between latitude l0'N and 25'N, and covered southern Europe, the Mediterranean, North Africa, Iran, and the Himalayan region, possibly extending into Burma and south-east Asia. It was the site of the alpine orogeny.

The present Mediterranean Sea has long been considered a remnant of the old Tethyan basin. The Tethys seaway was originally recognised and named because of a remarkable homogeneity of Mesozoic marine fauna. The rich assemblages include a greater diversity of species of corals, sponges, ammonoids and other molluscs than to the south and north. All these fossils occur within a vast expanse of thick carbonate rocks, which were deposited in shallow shelf environment on the margin of the supercontinent facing the Tethys. The Tethyan Ocean closed by mid-Cenozoic time with the great Alpine-Himalayan orogenic upheaval resulting from the collision of Indian and Afro-Arabian Plates collided with Eurasian Plate.

The Himalayan Mountain on the north of Bangladesh and the Indo-Burmese Mountain Range on the east is a part of a super orogenic complex known as Tethysides that descended from Tethys. A remarkable feature of the Tethys (Tibetan) Himalayan is the practically unbroken sedimentary sequence from Cambro-ordovician (570-438 million years ago) to Lower Eocene (around 55 million years ago). Sedimentary gaps are minor eg Upper Carboniferous (around 300 million years ago). Furthermore, this thick pile of mostly platform-type of sediment is conformable with surprising facies continuity from east to west for over 2,000 km along the entire range. All along the northern side of the Himalayan, the Tethyan platform sediments change into a flysch facies with all the indications of deeper and pelagic sediments. [Mahmood Alam]