Evolution of Paleocene to Early Eocene larger benthic foraminifer assemblages of the Indus Basin, Pakistan

The Paleocene-Early Eocene carbonate successions of the Indus Basin in Pakistan formed on the northwestern continental shelf margin of the Indian Plate in the eastern Tethys Ocean. Based on larger benthic foraminifera (LBF), eight Tethyan foraminiferal biozones (SBZ1-SBZ8) spanning the Paleocene to Early Eocene interval are identified. The base of the Eocene is identified by the first appearance of Alveolina sp. Other stratigraphically important LBFs that are characteristic of the earliest Eocene are Ranikothalia nuttalli, Discocyclina dispansa and Assilina dandotica. Stable isotope analysis through the Paleocene-Eocene (P-E) boundary interval identifies more positive delta C-13 values for the Late Paleocene (+3.4 parts per thousand to +3.0 parts per thousand) and lower values (+2.7 parts per thousand to +1.6 parts per thousand) for the earliest Eocene. However, there is insufficient sampling resolution to identify the maximum negative delta C-13 excursion of the Paleocene-Eocene Thermal Maximum. During Late Paleocene times LBF assemblages in the Indus Basin were taxonomically close to those of west Tethys, facilitating biostratigraphic correlation. However, this faunal continuity is lost at the P-E boundary and the earliest Eocene succession lacks typical west Tethys Nummulites, while Alveolina are rare; LBFs such as Miscellanea and Ranikothalia continue to dominate in the Indus Basin. The absence of Nummulites from the earliest Eocene of Pakistan and rarity of Alveolina, elsewhere used as the prime marker for the base of the Eocene, may imply biogeographical barriers between east and west Tethys, perhaps caused by the initial stages of India-Asia collision. Later, at the level of the Eocene SBZ8 Biozone, faunal links were re-established and many foraminifera with west Tethys affinities appeared in east Tethys, suggesting the barriers to migration ceased.