Fault control on distribution of sag-ponds in Imogiri, Bantul, Yogyakarta, and Its implications for the existence of the Opak active fault

Sag-ponds, a prominent geological feature, may be found near the summit of Gagak Hill in Bantul Regency, Yogyakarta. These depressions, which form within volcaniclastic rocks, are strongly tied to the Opak Fault's activity. The site is made up of Semilir Formation rocks from the Western Southern Mountains Zone, which were exposed along the Mengger Hills escarpment. The purpose of this study is to determine the location and orientation of sag-ponds governed by the Opak Fault. The method is based on structural geology analysis, which is backed by field observations such as lithological descriptions, sag pond dimensions, and documenting of structural elements such fracture planes, lineaments, shear and gash fractures. The sag-ponds range in shape from round to rectangular and elongated, with lengths of 1 to 15 meters and widths of 1 to 9.2 meters. Their depths, which are often filled by rainfall, range between 0.8 and 3 meters. The development of these sag-ponds corresponds to the Opak River's east-west diversion north of Watu Gagak Hill, which caused isolated depressions along the fault zone. Overall, the presence and position of these sag ponds indicate that the examined area is being impacted by the active Opak Fault.