Kontak Formasi Nanggulan dan Andesit Tua di G. Mujil, Kulon Progo-Yogyakarta

Abstrak

Spread of Nanggulan Formation is very limited on eastern side of Kulon Progo Mountains. Nanggulan Formation is below Old Andesite Formation especially around Mujil Mount. Mujil Mount has formed a bulge known as Nanggulan tumor. Nanggulan Formation can be exposed to the surface due to 2 (two) factors, namely the first rigid and heavy presses are breccias on sedimentary plastic rocks, and the second is presence of thrust faults. Hypothesis in this study is that contact between Nanggulan and Old Andesite Formation could be in the form of a stratigraphic or a fault contact. Purpose of research is to identify pattern of contact between Nanggulan and Old Andesite Formation, with an approach to surface and sub-surface data, especially on Mujil Mount. Methods used are geological studies and gravity measurements. Gravity measurement uses a 19 point looping system. Instruments used are Gravitimeter LaCoste & Romberg G-118 MVR, and Global Positioning System (GPS) Trimble Navigations 4600 LS. Field identification results show that contact between Nanggulan and Old Andesite Formation is in the form of a stratigraphic vertical. Two formations have a wavy boundary. Density value of Nanggulan Formation is 2.5 gr/cc and Old Andesite Formation is 2.7 gr/cc. Mujil Mount anomaly is due to lithological differences and contact between Nanggulan and Old Andesite Formation is a stratigraphic contact.

Keywords: Formation contact, Gravity, Mujil Mount, Tumor