Geochemical expression of sequence stratigraphic surfaces: A case from Upper Cretaceous shallow-water carbonates of southeastern Neo-Tethys margin, SW Iran

Providing a chronostratigraphic framework often is problematic in shallow-marine carbonates with poor biostratigraphic resolution. In such cases, an integration of geochemical, sedimentological, and paleon-tological data can be used to construct a reliable sequence stratigraphic framework. This study focuses on elemental concentrations and isotopic ratios of Cenomanian-Santonian (C-S) neritic carbonates in SW Iran used to construct a sequence stratigraphic framework, asses sequence ages and durations of hiatus. The diagenetic and geochemical expression of two paleoexposure surfaces, representing type-I sequence boundaries (SBs), are discussed. Increase in Fe and Rb concentrations and decrease in Ca content are recorded at and/or below distinct erosional surfaces. Mg content shows a facies-dependent response with former LMC (low magnesium calcite) or aragonitic facies showing increase while HMC (high magnesium calcite) indicates decrease in response to SBs. Mn shows considerable changes apart from sequence surfaces that are strongly facies dependent. Sr concentrations show an increase in HMC and decrease in LMC in samples which experienced semi-closed meteoric diagenesis, far below the SBs. However, in samples characterized by open system diagenesis, directly underlying the SBs, Sr tends to decrease in all components. The Sr/Rb ratio is a reliable tool in detecting erosional SBs, shown as con-spicuous negative excursions. C (carbon) and O (oxygen) isotopes show distinct negative excursions at erosional SBs and a slight increase at or around the MFSs. The 87Sr/86Sr ratio shows clear positive ex-cursions at SBs probably due to 87Rb decay in incorporated aluminosilicate detritus, which makes it as the most reliable proxy for the detection of SBs in the studied sections. Two main erosional SBs are marked by 87Sr/86Sr ratios including the CT-ES (Cenomanian-Turonian boundary exposure surface) and mT-ES (middle Turonian exposure surface) with hiatus durations of 0.53 and 2.7 Myr, respectively. These paleoexposure surfaces are correlated with other neritic sections of C-S sequences in SW Iran. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Providing a chronostratigraphic framework often faces challenges in shallow-marine carbonates with poor biostratigraphic resolution. This study demonstrates the integration of geochemical, sedimentological, and paleontological data to construct a reliable sequence stratigraphic framework in Cenomanian-Santonian neritic carbonates in SW Iran. The study highlights the diagenetic and geochemical expressions of paleoexposure surfaces and provides insights into the identification of erosional sequence boundaries using elemental concentrations, isotopic ratios, and Sr/Rb ratios. The study also reveals the durations of hiatuses and their correlation with other neritic sections in the region.