Folding style of the Dezful Embayment of Zagros Belt: Signatures of detachment horizons, deep-rooted faulting and syn-deformation deposition

Abstract

Defining the governing factors on the geometry of the Dezful Embayment folds, a major oil province in the Zagros Fold-Thrust Belt in SW Iran, reduces the risks inherent in hydrocarbon exploration. The goal of the current study is to find a reasonable relationship between folding and faulting in the northern part of the Dezful Embayment. The role of major and minor detachment horizons and deep-rooted faults on structural style is also a matter of investigations. To achieve these goals, geological surface information and the available well and 2D and 3D seismic data were used to construct a balanced structural cross-section. The area is subdivided into NE and SW sectors based on dissimilar physiographic features of the surface and structural and stratigraphic characteristics. In the both sectors, the Miocene Gachsaran Formation acted as the upper detachment horizon. In the NE sector, the role of the Gachsaran Formation as major detachment horizon in decoupled folding styles of overlying and underlying units is prominent. The Gachsaran Formation becomes less effective as a detachment horizon towards the SW as its thickness decreases and its facies change. The Triassic Dashtak Formation is most probably a major detachment horizon in the study area and evidence of another deeper detachment horizon (such as the pre-Cambrian-Cambrian Hormuz Series) is not apparent based on available data. The Lower Cretaceous Garau, Upper Cretaceous Gurpi and Paleocene-Eocene Pabdeh formations and the Oligocene Kalhur Member are subordinate detachment horizons. The role of major faults in the area, such as the Mountain Front Fault in driving thick-skinned deformation is taken into account along with the dominant thin-skinned deformation in the study area. The Kuh-e Asmari Anticline is the only emergent anticline exposing the Paleocene strata due to the activity of the deep-rooted Lahbari Thrust. This study reveals the commanding role of the upper detachment Gachsaran Formation and the overlying syn-deformation fluvial clastics of the Aghajari (Miocene) and Bakhtyari (Pliocene) formations on disharmonic folding styles.