Paleoenvironmental Shifts Across the Upper Cretaceous-Palaeocene Succession in Eastern Tethys, Pakistan

The Cretaceous-Paleogene (K/Pg) transition is a global mass extinction event that affected the paleoenvironment, palaeogeography, and biota of the Earth. In this study, we investigated the sedimentary record of the K/Pg transition in the Hazara Basin, a part of Eastern Tethys in Pakistan, using an integrated approach of sedimentology, micropaleontology, geochemistry, and mineralogy. We identified eight biozones based on benthic and planktonic foraminifera, ranging from Middle Cenomanian to Thanetian in age. We also recognised 10 microfacies, reflecting different depositional settings from middle-outer ramp to inner ramp and shoreface environments across the K/Pg transition. We used geochemical proxies and indices to infer the paleoredox conditions, paleosalinity, paleotemperature, detrital input, and paleoproductivity of the basin. We found that oxygenated conditions prevailed across the K/Pg transition, with normal salinity in marine settings and lower salinity in continental settings. The Sr/Ca and Mn/Ca ratios indicated moderate paleotemperature and low terrigenous input, except for shale intervals with higher detrital input. The chemical weathering proxies showed moderate to intense weathering in the source area. The organic matter was mainly of type-III and type-IV, with low total organic carbon and hydrogen index values. The clay mineralogy was dominated by kaolinite, indicating warm and humid conditions, followed by smectite, chlorite, and illite. The K/Pg boundary could not be constrained by the fossil record due to the absence of Late Maastrichtian and Danian fauna in the Hazara region, which may be attributed to tectonic uplift, erosion, and non-deposition of sediments during the collision of the Indian Plate and Kohistan Island Arc. However, the boundary could be recognised by the facies change corresponding to lateritic and oolitic haematite at the base of the early Palaeocene Hangu Formation.