Age, duration, and geochemical signatures of paleo-exposure events in Cenomanian–Santonian sequences (Sarvak and Ilam formations) in SW Iran: Insights from carbon and strontium isotopes chemostratigraphy

Abstract

This study addresses the timing implications of chemical stratigraphy and chronostratigraphic correlation of unconformity-related shallow water (neritic) carbonate rock units. Cenomanian–Santonian sequences (i.e., Sarvak and Ilam formations) host considerable volumes of hydrocarbon in the Zagros region of SW Iran, which represent good examples of unconformity-related reservoirs worldwide. Geochemical traits including carbon, oxygen, and strontium isotopes of these sequences are measured in three wells from the Abadan Plain and one well from the Dezful Embayment. The altered intervals below the exposed erosional surfaces are characterized by remarkable variations in both elemental and isotopic compositions. These variations are marked by increasing in Mg, Al, Fe, Mn, and Rb elemental contents as well as decreasing in Sr and Ca elemental contents. The altered intervals show a positive shift in 87Sr/86Sr isotopic values and a negative trend in δ13Ccarb and δ18Ocarb isotopic values. The paleontological age that is available from previous studies provide a valuable basis for numerical dating of the studied intervals using the rubidium-corrected 87Sr/86Sr isotopic values. The duration of the stratigraphic hiatuses at the Cenomanian–Turonian and mid-Turonian paleo-exposures are calculated as 0.53 Myr and 2.7 Myr, respectively. The calculated durations corroborate the results of previous studies of these paleo-exposures within the Zagros region of western Iran. The concluded (carbon and Sr-isotope) chemostratigraphic correlation of the Cenomanian–Santonian sequences in the Abadan Plain and other parts of the Zagros region can provide a more reliable chronostratigraphic framework of larger scale. The latter can be used in regional correlation and interpretation of the geological-structural evolution of the Zagros region during Cenomanian–Santonian, particularly in western Iran.