Biostratigraphy of the Upper Cretaceous Gurpi Formation: Enhancing correlation potential by identification of abundance events of planktonic foraminifera as auxiliary indicators

Abstract

The Upper Cretaceous Gurpi Formation represents a pelagic interval composed of shale and marly limestone rich in planktonic foraminifera. This study focuses on the analysis of planktonic foraminifera from the Gurpi Formation in the Zard-Shehneh Anticline section in southwestern Iran. The formation is of significant importance in parts of the Zagros foreland basin, as it acts as both a cap rock and a source rock. The presence of the Cretaceous/Paleogene (K/Pg) boundary at the upper boundary of this formation further emphasizes the significance of studying it. Therefore, the study of its biozonation and bioevents is crucial. Through the recovery of planktonic foraminifera from 125 samples in thin sections and washed residues, 30 genera, 88 species, and 10 biozones were identified for the Gurpi Formation, as well as 7 genera, 12 species, and one biozone for the base of the overlying Pabdeh Formation. In addition, high accuracy was achieved in the placement of bioevents, including: 1) the occurrence of Globotruncana linneiana at the base of the Gurpi Formation, indicating the Santonian age; 2) the highest occurrences of Dicarinella asymetrica and most species of marginotruncanids, marking the Santonian/Campanian boundary; 3) the earlier occurrence of Globotruncana aegyptiaca; 4) the occurrence of the Campanian/Maastrichtian boundary in the lower part of Gansserina gansseri Zone; 5) the occurrences of five abundance events including: V. eggeri in the upper half of the Dicarinella asymetrica Zone, benthic foraminifera in the middle to upper half of the Radotruncana calcarata Zone, G. linneiana, G. lapparenti, and G. esnehensis in the lower part of Globotruncana falsostuarti Zone, G. esnehensis in the upper part of Globotruncana falsostuarti Zone, and G. gansseri at the end of the Pseudoguembelina hariaensis Zone; and 6) the disappearance of most keeled planktonic foraminifera species during the late Maastrichtian, starting from the middle parts of the Abathomphalus mayaroensis Zone. In this study, the K/Pg boundary, which is considered the upper boundary of the Gurpi Formation, exhibits unconformity and condensation. The high-resolution biozonation and bioevents framework not only allows for accurate dating of the Gurpi Formation in the Zard-Shehneh section, but also enhances the ability to precisely correlate sedimentary sequences and paleoenvironmental intervals at regional or global scales in future studies.