The Nanggulan Formation sediments form a weakly undulating hill and range in age from the Middle Eocene to the Late Oligocene. In Samigaluh and Sermo areas, a series of sedimentary rocks with small dimensions resembling fragments and forming a steep hilly morphology is exposed. Outcrops are mainly made up of sandstone and claystone interbedded with lignite intercalation. An outcrop in Sermo-2 found andesite intruding claystone. Claystone is found near rock contact, where it is an alteration to metasediment and chlorite is formed. Petrographic data showed the presence of chlorite, pyrite, and kaolinite. Andesite dike is one factor that contributes to the Nanggulan Formation being exposed to the surface. The Nanggulan Formation is a roof pendant above a dike. The implications of the roof pendant are related to the propylitic to argillic of hydrothermal alteration, which produce chlorite, pyrite, illite, and kaolinite minerals.
