The Gachsaran evaporate Formation is the caprock of the Asmari oil reservoir in the Zagros oil province. This Formation is a main detachment horizon in the Zagros. As an intermediate detachment horizon, this Formation separates rocks of distinct structural features and folding styles in its overlaying and underlying units. Around 1600 m thick Gachsaran Formation has seven Members (Gs.1–7). The Formation consists of salts, anhydrites, coloured marls, limestones and a single layer of bituminous shale. In this work, we interpret the different Members of the Gachsaran Formation and study the structures in the Chamshir area, located in the lowland basin of the Zagros belt, using surface and subsurface data in 2D sections and 3D models. The presence of a thick layer of the Gachsaran Formation has affected the structural style in the Chamshir area. In the Chamshir area, Members Gs.5–7 crop out. The salt members of the Gachsaran Formation are deformed to several secondary folds and faults. Based on surface and subsurface data (up to 3000 m deep), the area has two types of faults. The faults seen in the exposed part of the studied area often continue to Gs.4, such as the Dezh Soleyman Fault, Chamshir Fault, F4 and F3 faults. The second category of faults developed within the Asmari Formation up to the approximate depth of the Triassic-Jurassic Formations. In other words, a thick layer of the evaporate member of the Gachsaran Formation (Gs.4), as the main detachment layer, has caused the structural style to be separated across it. As a result, the folds above and below the Gachsaran Formation in the Zagros belt significantly different geometries. As the Gachsaran Formation is the main caprock of the Asmari reservoir in Zagros, understanding the effect of this detachment level on the geometry of the anticlines as oilfields is crucial for future exploration in the region.