Late Paleocene to Early Eocene integrated biostratigraphic framework, chronostratigraphy and Paleocene/Eocene boundary in the Salt Range, Pakistan

Upper Paleocene to Lower Eocene mixed carbonate siliciclastic system of the Salt Range monocline, located at the southern border of the oil bearing Potwar Depression, is comprised of a contemporaneous lagoonal to basinal facies. The interfingering of these facies provides an excellent opportunity to study shallow to open marine biota, apply and test the globally accepted zonation schemes and define the P/E boundary. The synopsis is based on the analysis of 350 samples from 11 outcrops and subsurface locations and review of existing data from additional locations. The global planktonic foraminiferal zones (P4c-E7) and larger foraminiferal zones (SBZ3-SBZ10) were recognized based on 45 species of planktonic foraminifera and 40 species of larger foraminifera. The results were also tied with the existing data and new analysis of calcareous nannofossil (NP6-?NP12/13) from the same locations. Around the P/E boundary, our data indicates a significant departure from the global scenario. A modified integrated zonation scheme is therefore proposed based on the Salt Range data. The Paleocene/Eocene boundary is defined at the lowest occurrence (LO) of Acarinina esnehensis (proposed here as a marker for Zone e1) which is also associated with the appearances of A. pseudotopilensis and A. wilcoxensis prior to the LO of Pseudohastigerina wilcoxensis (marker of global Zone E2). The base of Zone e1 coincides with the first appearance of Alveolina vredenburgi that defines the base of SBZ5, which has been found coeval at the base of carbon isotope excursion (CIE) onset in the Kala Chitta Range. These events correlate with the upper most part of Zone NP9a. Two events of larger foraminiferal extinction and origination (LFEO) have been revealed. The first lies in the middle of SBZ5 whereas the second lies at the SBZ5/6 boundary. A gradual increase in the larger benthic foraminiferal shell size is observed from Zone SBZ4-SBZ6 and onward. Integration of different biostratigraphic zones has enabled a chronostratigraphically constrained section that displays major facies change and progradation patterns. Four depositional hiatuses are defined attributed to eustatic sea level change and down-wrapping of a gently dipping ramp in the center of the study area that was triggered by the India-Asia collision. It is also considered that certain barriers between the oceanic environments of the eastern and western Neotethys are a likely reason for the difference in faunal assemblages and stratigraphic ranges of various taxa of all fossil groups.