Paleogene Nummulitid biostratigraphy of the Kohat and Potwar Basins in north-western Pakistan with implications for the timing of the closure of eastern Tethys and uplift of the western Himalayas

Abstract

The Paleogene larger benthic foraminifera (LBF) of the Kohat and Potwar basins, Pakistan are very useful in dating shallow marine sediments and cessation of marine sedimentation that provides constraint for crucial information on the initial age of India-Asia collision. We record important Paleogene LBF species in multiple sections of the two basins. We performed the biometric analysis of the nummulitid species useful for taxonomic purposes. We recognize six Larger Benthic Foraminiferal Zone (BFZ Zone 1-6) in the Kohat Basin. Among these Zones the first three (BFZ Zone 1-3) occur also in the Potwar Basin and the adjoining Trans Indus Ranges (TIR). The correlation of BFZ 1-6 Zones with the previous local and regional LBF biostratigraphic schemes in the Eastern Tethys (Pakistan-India) and Western Tethys (European Basins) resulted in recognition of useful index taxa for developing a regional stratigraphic framework during Paleogene. In the BFZ 1 Zone, The last occurrence (LO) of Miscellanea miscella and first occurrence (FO) of Assilina dandotica in the BFZ 2 Zone mark the Late Paleocene (Late Thenetian) Early Eocene (Lower Illerdiuan1) Boundary. The co-occurrence of A. pustulosa, Al. vredenburgi, Al. globula and Al. pasticillata in the BFZ 2 Zone characterizes the Early Eocene (Lower IlIerdian 1-Middle Illerdian 1) sediments. The synchronous FO of N. atacicus and N. globulus is an excellent global biostratigraphic marker of the Early Eocene (Middle Illerdian 1 Middle Illerdian 2) Boundary and the FO of O. complanatus is a useful biostratigraphic marker for the Early Eocene (Lower Cuisian 2-Middle Cuisian) Boundary in the BFZ 3 Zone. Mammalian bones found at the base of Koldana Formation in the Kohat Basin represent Early Eocene (Upper Cuisian), which is in agreement with the LBF biostratigraphy of the underlying Middle Cuisian strata. The FO of A. exponens in the BFZ 4 Zone record the Middle Eocene (Middle Lutetian 1) sediments while FO of N. beaumonti in the BFZ 5 Zone marks the Middle Lutetian 1Middle Lutetian 2 Boundary. The FO of A. cancellata in the BFZ 6 Zone marks Middle Lutetian 2Upper Lutetian Boundary. The implications of our LBF study are that cessation of marine sedimentation in both Pakistani basins occurred in Early Eocene (Middle Cuisian ≈BFZ 3) around 50-49.5 Ma. Notably, marine conditions returned in the Kohat Basin in Middle Eocene (Middle Lutetian 1 ≈ BFZ 4) due to an eustatic sea level rise. The final cessation of marine sedimentation, causing closure of the Eastern Tethys seaway in the Kohat Basin occurred in Middle Eocene (Upper Lutetian ≈BFZ 6) around 41.2 Ma, probably a result of some form of post-collisional stress.