REFINED STRATIGRAPHICAL AND BIOCHRONOLOGICAL FRAMEWORK OF PALEOGENE SUCCESSIONS IN THE NORTHWEST ARABIAN PLATE

Abstract

This study updates the lithostratigraphic and chronostratigraphic framework of the marine carbonate-dominated Paleogene strata of northwestern Saudi Arabia within the Sirhan–Turayf Basin by integrating detailed field observations and high-resolution calcareous nannofossil biostratigraphy. Previous studies assigned these strata to the Middle–Upper Eocene Rashrashiyah Formation but lacked precise chronostratigraphic constraints. New biostratigraphic and lithostratigraphic data from three measured sections (Rash1–Rash3) reveal that these strata comprise two distinct formations: the Middle–Upper Eocene Rashrashiyah Formation and the newly defined Lower Oligocene Qurayyat Formation, separated by a prominent erosional disconformity at the Eocene–Oligocene boundary (EOB). The revised framework highlights the role of local tectonics, including block faulting and differential subsidence, in preserving the Oligocene strata. Calcareous nannofossil assemblages suggest that the Rashrashiyah Formation corresponds to Zones NP17–NP20, while the Qurayyat Formation is assigned to Zone NP21, collectively spanning the Bartonian–Rupelian interval. The documentation of marine Oligocene strata in NW Saudi Arabia for the first time, with erosional disconformity at the base, challenges previous interpretations of continuous sedimentation across the Eocene-Oligocene transition. The findings suggest that depositional architecture and stratal distribution in the study area are significantly controlled by regional tectonics and global sea-level fluctuations. The proposed lithostratigraphic scheme and chronostratigraphic constraints improve regional correlations across the Arabian Plate.