Late Cretaceous Paleo-depth Variations in the Zagros Foreland Basin (Iran): Insights into Basin Evolution and Regional Paleoenvironment

Upper Cretaceous sedimentary successions of the Gurpi Formation in the eastern Lurestan Zone (Zagros Basin, Iran) provide a valuable record of paleo-depth variations during the early development of the Zagros foreland basin associated with Neo-Tethyan closure. This study presents an integrated sedimentological and micropaleontological analysis of the Campanian–Maastrichtian Gurpi Formation exposed in the Zangul anticline in a proximal foreland setting.

Lithofacies observations combined with quantitative planktonic–benthic foraminiferal data and morphotype-based paleoecological analysis were used to reconstruct relative bathymetric changes and evaluate controls on carbonate preservation. Planktonic foraminiferal biostratigraphy constrains the succession from the Globotruncanita elevata Partial Range Zone (middle–late Campanian) to the Gansserina gansseri Interval Zone (early Maastrichtian). A thin fossiliferous marl interval equivalent to the Seymareh Member records a short-lived late Campanian shallowing event related to forebulge development. This phase is followed by progressive deepening into the Maastrichtian, expressed by increasing proportions of deep-dwelling foraminiferal morphotypes.

The upper Gurpi Formation shows a marked decline in foraminiferal abundance and preservation, accompanied by radiolarian-rich micrites, glauconite, and pyrite. These features indicate deposition under low-oxygen and carbonate-corrosive conditions and are interpreted as the result of local shoaling of the lysocline and an effectively shallower carbonate compensation depth in a tectonically active foreland basin, rather than extreme bathymetric deepening. The transition to flysch sedimentation of the Amiran Formation and the presence of slump structures record foredeep development and basin steepening.

Regional comparison across Lurestan demonstrates that Campanian–Maastrichtian depth changes were diachronous and primarily controlled by flexural subsidence and forebulge migration, highlighting the dominant role of tectonics over global eustatic signals in shaping Late Cretaceous basin evolution of the Zagros Basin.