Sedimentological attributes of the Middle Jurassic peloids-dominated carbonates of eastern Tethys, lesser Himalayas, Pakistan

Abstract

The carbonate factories of the tropical to sub-tropical regions characterize a number of facies, attributable to biological and oceanographic variables. For this purpose, the Middle Jurassic carbonate rock unit (Samana Suk Formation) of the Upper Indus Basin of Pakistan was studied to understand the various Tethyan platform attributes. Four sections from Hazara area were chosen which were also compared with a section from the Nizampur Basin. The resultant microfacies were placed in their paleoenvironmental settings to reveal the facies distribution and geometry of the platform. The platform represents a gently dipping ramp, dominated by peloidal–bioclastic–ooidal grainstones and peloidal lime mudstones. The allochems and fauna show some contrasting signatures regarding paleoclimate. These microfacies infer about existence of a relatively extended tropical climatic belt in the paleo Tethys during the Middle Jurassic. The notable presence of planktonic lime mudstone facies corresponds with global marine transgression, occurring in the middle of rock unit. The nature of ooids and types of fauna of sub-tropics to tropics advocate an extended warm climatic zone in comparison with today. The dominance of gently dipping inner-ramp shoals accompanied by lagoons to steep dipping outer-ramp planktonic lime mudstones restricted the development of least encountered middle-ramp facies. The ramp platform was struck by waves and tides of different strengths including low-middle amplitude storms and the overall absence of evaporites and primary dolostones designate a humid region as manifested by euryhaline conditions. The study has drawn paleoenvironmental and palaeoceanographic similarities with sections in Tanzania, western India, NW Iran, NW Afghanistan and Western Australia. The oceanic parameters compare with present day Trucial Coast of Persian Gulf, reflecting an almost similar nature of paleoceanography in the paleo Tethys.