Need for re-apprehension of basin tectono-depositional set-up during initial stage of Himalayan orogeny from pedogenic evidences

摘要

The collision and suturing of Indian and Eurasian plates about 50 Ma ago closed the Tethys Ocean, initiated Himalayan orogen and a foreland basin developed towards its south. The last phase of marine sedimentation in Indian sub-continent during Paleocene-Eocene is represented by Subathu Formation overlain by continental Dagshai sediments and nature of this transition, named as 'Passage beds' (sense Bhatia and Mathur, 1965), is controversial. Further, it is poorly understood both in terms of depositional environment and age. Surprisingly, these beds show evidences of pedogenesis but no such palaeopedological studies have been reported so far. The degree and nature of pedogenesis is widely used in solving diverse geological problems in complex geological settings. Therefore, pedogenesis of these beds exposed in the Shimla hills of Subathu sub-basin has been worked out. Morphological and micromorphological studies divide them into older green/greenish-grey pedofacies (GP) and younger red/reddish-brown pedofacies (RP) units. Pedons are up to tens of centimeters thick. Structure largely comprises sub-angular to sub-rounded blocky peds. A suite of pedogenic features include burrows, root traces, innings, mottling, cutans, segregations, and nodules which vary in dominance from GP to RP unit. Greenish-grey pedons are characterized by mottles, Fe-Mn oxide segregations whereas reddish-brown pedons by root traces commonly filled with carbonates, presence of carbonate segregations and cutans. Detailed field and laboratory studies at various local and regional exposures can reveal the nature and extent of pedogenesis which will be significant to enact depositional environment as well as stratigraphic position, of Passage beds co relatable at different localities throughout length of the Himalaya from Pakistan to Nepal because of their distinctive lithology and stratigraphic position. Such studies bear global significance in demonstrating the use of palaeosols as a geologic archive in complex geological settings.