In order to investigate the sedimentation history of the Gachsaran Formation, a surface section of this formation on the Khorramabad-Pol-Zal freeway (southern Lorestan) was selected and sampled. This section is 380 meters thick and is located 46 km southwest of Khorramabad in the folded Zagros zone. Petrographic studies of microscopic thin sections led to the identification of 10 microfacies: gypsum-anhydrite microfacies, carbonate mudstone, dolomitized carbonate mudstone, fenestral mudstone, bioclastic wackestone, wackestone to packstone, bioclastic grainstone, and oolitic grainstone. The lithological evidence showed that gypsum-anhydrite facies and fenestral mudstone are related to the sabkha facies belt (above the intertidal zone); carbonate mudstone, dolomitized mudstone, and wackestone to pelloidal packstone are related to the intertidal facies belt; microfacies of wackestone and packstone, bioclastic wackestone, and oolitic wackestone are related to the lagoon facies belt; and bioclastic grainstone and oolitic grainstone microfacies are related to the barrier facies belt. The presence of marl and evaporite hills, along with gypsum-anhydrite facies, confirms the upper part of the tidal zone environment (sabkha) for the Gachsaran Formation in the study area, indicating that these facies belts were deposited in the inner part of a carbonate ramp. The dominant diagenetic processes in this formation include micritization as a thin coating around grains and excessive micritization of grains; increased neomorphism due to recrystallization of fine micrite crystals to microasparite; decreased neomorphism in the shell of allochems; formation of equal-thickness marginal cement as first-generation cement around grains; formation of coarse-crystalline cement of the next generation between grains; formation of drusy cement within grains; formation of geopetal fabric; dissolution and formation of intragranular,