Upper Cretaceous Sanandaj (UCS) units (K8–K12) in the northern Sanandaj-Sirjan Zone, west Iran, which consists of ~2,000 meters of shale, sandstone, limestone, conglomerate, and pyroclastic rocks, with interbedded basalticandesitic lava. The Rb–Sr whole-rocks isochron for nine shale samples shows an age of 125 ± 39 Ma with an initial 87Sr/86Sr = 0.7106 ± 0.0032. The chemical compositions of the shale samples show average values of 71.8 wt.% SiO2, 17.1 wt.% Al2O3, 4.8 wt.% Fe2O3, 0.3 wt.% CaO, and 1.7 wt.% Na2O. The SiO2–Al2O3 + Na2O + K2O and Th/Sc–Zr/Sc diagrams for samples reflect low sediment maturity, and the shale components were deposited by the first sedimentary cycle. The REEs Chondrite-normalized patterns reveal higher LREEs contain (La/YbCN = 10.5), flat HREEs, negative Eu anomaly (Eu/Eu*SN = 0.26–0.89), and Nb anomaly (Nb/Nb*SN = 0.14–0.82). Also, the ratios of Al2O3/TiO2 (18–22), La/LuCN (5.43–24.4), La/Sc (0.51–2.53), Th/Sc (0.42–0.68), and LREEs/HREEs (Nd/ErCN > 5), suggesting these rocks were dominantly derived from intermediate-acidic rocks in an active continental margin. Meanwhile, Ni/Co (5.6–9.9), V/(V + Ni) (0.64–0.90), and Mn* (− 3.6 to − 6.6) values reveal that shale layers were deposited in a sediment-starved basin under reducing conditions during the Late Cretaceous contemporaneous with volcanic activities. Upwelling of the anoxic water due to volcanic events has led to an oceanic anoxic event, the demise of micro-organisms, and the deposition of organic-rich black shale. The UCS shale geochemistry and the vertical stacking pattern of the K8-K12 units show that their tectonic settings have been changed from a trench to a fore-arc from bottom to top. These transitions are related to the continent-ward migration of the magmatic arc.
