On the Transition From Remnant Tethys Sea to Foreland Basin During the Late Paleogene in the Western Himalayas, Pakistan

The late Paleogene sedimentary succession exposed in the north Sulaiman fold‐thrust belt (SFTB), Pakistan, provides a critical record of the tectono‐sedimentary processes associated with the transition from the remnant Tethys Sea to a foreland basin. This study integrates sedimentary facies and biostratigraphy data of these sediments to unravel the dynamics of this transition, closely linked with the early stages of the Himalayan orogenic evolution. The Pirkoh Formation, characterised by three microfacies, indicates deposition in inner to open shelf settings. The overlying Drazinda Formation comprises six microfacies and two lithofacies, indicating deposition in semi‐restricted lagoon to middle shelf environments. The Chitarwatta Formation comprises five distinct lithofacies, with the lower unit reflecting deposition in tidal flat settings and the upper unit representing deposition in a fluvial floodplain. An assemblage and two taxon range larger benthic foraminiferal and two planktic foraminiferal assemblage biozones in the Pirkoh and Drazinda formations were identified. These biozones suggest a Lutetian to Bartonian (Middle Eocene) age for the Pirkoh and the lower part of the Drazinda formations, and a Priabonian (Late Eocene) age for the upper part of the Drazinda Formation. No age‐diagnostic fauna was retrieved from the Chitarwatta Formation. Therefore, an Oligocene age has been retained for it based on previous studies. The integration of sedimentary facies with biostratigraphic ages reveals a final transgression of the Tethys Sea in the SFTB in the Lutetian–Bartonian following the initial India–Asia collision in the early Eocene in the region. The Tethys Sea started to gradually retreat in the Priabonian with the deposition of dominantly semi‐restricted lagoonal shales in the upper part of the Drazinda Formation. The Tethys Sea further retreated at the start of the Oligocene with the deposition of the transitional marine tidal flat sediments in the lower part of the Chitarwatta Formation. The cessation of the Tethys Sea occurred in the late Oligocene, represented by the deposition of the fluvial sediments in the upper part of the Chitarwatta Formation in the SFTB. The typical Miocene Himalayan foreland basin molasse sediments overlie the Chitarwatta Formation, representing the establishment of the foreland basin sedimentation in the SFTB by the late Oligocene to Miocene. This transition from remnant Tethys Sea to Himalayan foreland basin sedimentation in the SFTB can be linked with the southward migration of the Himalayan deformation front.