Preliminary Gravity Study of Pre-Tertiary and Tertiary Rock in Northern Kebumen, Central Java, Indonesia

Luk Ulo Melange Complex is a Pre-Tertiary rock formation consisting of various lithological background and complicated geological structure. Located in Northern Kebumen Regency, this rock is originally interpreted as products of tectonic process caused by paleo-subduction between Indian Oceanic Plate beneath Eurasian Plate in Cretaceous age. According to previous study on geological surface investigation shows that the melange complex is tectonically overlain by Tertiary sedimentary formation. On the contrary, geophysical study of the subsurface condition of the area is still under discussion due to limited of measured data. In order to obtain detailed subsurface image, gravity profiling was measured and interpreted across both Tertiary and pre-Tertiary rock. Frequency domain filter and designed moving average filter were used to separate residual and regional anomaly. The results denote gravity value of residual anomaly varying from -5 to +5 mGal, while regional anomaly is ranging from 150-180 mGal respectively. The preliminary interpretation was developed by taking into account the geological information and also the data that was collected during field campaign. Overall, high gravity anomaly is interpreted as diabase intrusion in the middle of research area. Meanwhile, low gravity anomaly indicated clay rich rock material related to matrix content of Luk-Ulo Melange Complex in the northern area.