Evolution of Larger Benthic Foraminifera during the Paleocene-Early Eocene Interval in the East Tethys (Indus Basin, Pakistan)

Paleocene-Eocene stratigraphy of the Indus Basin is revised and a modern stratigraphic nomenclature is presented. Forty-five species of larger benthic foraminifera (LBF) are described from newly collected sections. Eight Tethyan foraminiferal biozones (SBZ1-SBZ8) and a stable carbon isotopic stratigraphy spanning the Paleocene to Early Eocene interval are established. The evolution of shallow marine communities (especially LBFs) in the east and comparison with west Tethys is discussed. In contrast to coralgaldominated west Tethys (particularly during the SBZ3 Biozone), the Late Paleocene carbonate biotas in the Indus Basin were dominated by LBFs, and corals are scarce. This difference is interpreted here as a product of local palaeoecological conditions and/or the Tethyan latitudinal temperature gradient. The responses of the shallow marine biota and ecosystem to earliest Eocene environmental changes in the east Tethys were remarkably different from those of west Tethys. The larger foraminifer turnover and rapid radiation of typical Eocene LBFs (e.g. Alveolina, Nummulites, Orbitolites) during this time were not observed in the east Tethys. During the earliest Eocene, inner ramp environments of the Indus Basin were still characterized by typical Late Paleocene LBF assemblages (mainly miscellanids, ranikothalids). Mid ramp settings were occupied by nutrient-tolerant heterotroph-dominated communities (mainly encrusting foraminifera) with very rare or no LBFs. These biotic differences were possibly created by the superimposition of initial India-Asia collision on PETM related stresses at the P-E boundary, generating biogeographical barriers and severe environmental conditions (increased continental run off, elevated temperatures) in the east. High sea temperatures (32°C to 33.5°C) and increased productivity/nutrient levels compared to the west Tethys are suggested by δ18O and δ13C isotope data from wellpreserved carbonates. Later, in the Early Eocene (SBZ7-SBZ8 biozones), oligotrophic LBFs (nummulitids, orthopragminids) started to dominate ramp environments, indicating stable environmental conditions.