Paleoclimate changes across the Cenomanian–Santonian transition at the southern Neo-Tethys margin (SW Iran): An integrated approach

Abstract

The Cenomanian–Santonian (C–S) carbonate successions include valuable petroleum reservoirs in the Zagros Basin. They also provide important information on the paleogeographic setting, paleoclimatic conditions, and tectonic evolution of the southern Neo-Tethys margin. This study focuses on the paleoclimatic changes across the C–S transition using the petrographical, petrophysical, paleontological, major and trace element and isotope geochemical data from three subsurface sections selected from the southern Dezful Embayment and northwestern part of the Persian Gulf. Remarkable differences in allochemical (skeletal and non-skeletal) elements indicate dissimilar paleoclimatic conditions during the Cenomanian–Turonian and Santonian. Paleoexposure events are related to diagenetic processes such as extensive meteoric dissolution (karstification) and the development of bauxitic–lateritic paleosols with high Fe, Al, and kaolinite concentrations indicating warm and humid (tropical) conditions. Increased values of Fe, Al, Rb, and 87Sr/86Sr isotopic ratios and depletion of Ca, Sr, δ18O, and δ13C considered together indicate meteoric diagenetic effects under this paleoclimatic condition. Spectral gamma measurements are used to investigate paleoclimatic conditions. Low content of Uranium (U) throughout the transgressive systems tract (TST) and high concentration in the regressive systems tract (RST) along with low contents of kaolinite within the Santonian sequence denote the predominance of arid conditions. The major fall in U concentrations at the Cenomanian–Turonian interface (CT-Exposure Surface) could be regarded as an outcome of the fall in the relative sea-level and a proxy of oxidizing conditions (Th/U > 7) that could be the consequence of a change in paleoclimate from humid/semi-humid to humid. Evidence for a 3–4 °C decrease in paleo-temperature and increase in aridity are recorded during the Cenomanian–Santonian transition in the Zagros Basin, which is in close agreement with global trends. This cooling pattern is attributed to a decline in atmospheric CO2 levels and alterations in oceanic circulation, specifically an increase in meridional circulation.