Regionally and stratigraphically, the Nanggulan Formation is the oldest in the Kulon Progo Region. The research area covers Kalibawang, Kulon Progo Regency, which is dominated by the lithology of the Nanggulan mudstone unit, which has variations based on physical features and characteristics. The distribution and lithological variations of the mudstone units in the Nanggulan Formation have never been studied by researchers, especially in the study of age and depositional environment, as well as stratigraphic relationship which make the research area interesting to study. This research aims to determine the relative age, depositional environment, and stratigraphic relationship for each variation of mudstone lithology in the study area. Research methods include paleontological analysis to determine age and depositional environment based on foraminifera fossil content. Stratigraphic relationships are interpreted based on detailed geological mapping and observations of rock contacts in variations in mudstone lithology. Based on the results of detailed geological mapping, the distribution of mudstones with variations in lithology is obtained, including the Nanggulan carbonate mudstone unit, the Nanggulan mudstone unit, the Nanggulan fossiliferous mudstone unit, the Nanggulan tuffaceous mudstone unit, and the Nanggulan black mudstone unit. Based on paleontological analysis, it is interpreted that the mudstone deposition in the study area is P15 (Middle Oligocene) in the Nanggulan to N11 (Middle Miocene) in the Nanggulan unit in the Middle Neritic to Outer Neritic environments. The stratigraphic relationship between each mudstone lithology in the study area shows a mutually interlocking relationship.