Challenges in deep basin sequence stratigraphy: a case study from the Early–Middle Cretaceous of SW Zagros

摘要

Thick Neocomian–middle Albian successions include the most significant hydrocarbon source rocks in the Zagros Basin. These successions, named the Garau Formation, are composed of about 700 m of limestone, shaly (argillaceous) limestone, and bedded cherts with high organic matter (OM) content. Considering several lines of evidence, including the presence of partially dissolved planktonic foraminifer shells, thin-bedded radiolarian cherts, and the absence of index palynomorphs and shallow- marine fauna and facies, the depositional environment is interpreted as a deep basin (probably up to 1,000 m deep). Due to its bathymetry, facies changes are subtle, which in turn makes it challenging to reconstruct the sequence stratigraphic framework. Identification of five microfacies related to the deep basin along with palynological factors (AOM ratio, AOM%, phytoclast ratio, and phytoclast %) and their variations in the stratigraphic column led to identification of the relative sea-level changes during this time span. Consequently, various sequence surfaces and ten third-order sequences are recognized. These sequences are clearly correlatable with the shallowwater sequences of the Arabian Plate in their numbers and ages. Using Rock-Eval pyrolysis, organic content of the Garau facies is determined. TOC data show an elevated OM content (1.648 %) with a marked increase during the Early Aptian, which could be ascribed to the OAE1a.