MICROBIOTA FROM EARLY CENOZOIC SEDIMENTARY SEQUENCES OF INDUS SUTURE ZONE, LADAKH HIMALAYA

Abstract

The zone of collision between Indian and Eurasian plate is known as Indus Suture Zone. Indus Suture Zone exposes thick succession of lower Cenozoic sedimentary sequences, which holds important clues to understand the early history of India-Asia collision and the final closure of the intervening Neo-Tethys. These sedimentary successions are broadly divided into two stratigraphic units, (a) Older stratigraphic unit, formally known as Indus Flysch/Indus Tar Group ,and (b) Younger Ladakh Molasse, formally designated as Indus Molasse/Indus Group. These sequences spread in a long arcuate belt along the southern margin of the Trans-Himalayan volcanic arc, extending eastwards from Kargil township through Nyoma in eastern Ladakh. Stratigraphically, these sedimentary rocks are folded, faulted and rest unconformably over the granitoids of the Ladakh. At places, these sequences also overlie unconformably over the Indian passive margin units, ophiolitic mélanges etc. However, in some areas, such as along Zanskar river, these horizons are believed to have a conformable relationship with the underlying Nindam Formation. Investigation of microbiota from these sedimentary sequences may provide useful information regarding change in landscape, regional biotic events. As a step in this direction, a putatively early Cenozoic sedimentary package, particularly terminal marine to early terrestrial sequences was studied. During the course of this work, several forms of larger bethic foraminifera were recovered and represented by different species of Assilina, Nummulites, Discocyclina, Alveolina etc. Systematic biostratigraphy of larger benthic foraminifera reveals that Sumdo Formation was deposited in earliest Eocene, while Nummulitc milestone was deposited in late Yepressian. Sumdo Formation can be correlated with the Upper part of Ranikot Formation of Pakistan, upper part of Patala Formation, while Nummulitic Limestone Formation can be correlated with upper part of Kong Formation of Zanskar Basin, Renging Formation of Arunachal Pradesh etc. Overall faunal assemblage (Nummulite-Assilina dominant and coral absences) of Sumdo Formation suggests a warmer climatic condition, in which deposition has taken place from fore to inner platform setting, while Nummulitic Limestone Formation is represented by Assilina, Nummulites escheri, Alveolina sp., micro gastropoda genus: Turritella sp., coral: Coleilonus elongates etc., suggest a backreef to lagoonal environment of deposition. Recovered bio assemblages from basal Molasse deposits of Ladakh includes vertebrates: fish teeth, fish scale and vertebra, representing Cyprinidae: Barbus sp., Schizothorax sp., Cyprinus sp., Squalius sp., rodents, crocodilian tooth ii etc. Hence, the microfossils and their association suggests a typical fresh water environment of deposition for basal molasse horizons, which militates changes in landscape from marine to freshwater regime, a complete disappearance of Neo-Tethys due to the Himalayan tectonics, and final suturing between Indian and Asian plates. The occurrence of these fresh water fossil assemblages from basal molassic stratigraphic unit in eastern (Nyoma and Liyan areas), central (Taruche, Saspochey and Basgo areas) and western (Kargil area) Ladakh, along the Indus Suture Zone, also shows close similarity with stratigraphical arrangement, that suggests that fossiliferous molasse deposits of these areas may be homotaxial in nature. Presence of mammals Juxia, Hyoboops, lberomeryx rodents, charophytes, ostracods and palm leaf points that terrestrial link was established between the Indian and Asian landmasses by the time of deposition of Ladakh Molasse, that facilitated the dispersal of the above taxa from Asia to the Indian subcontinent.