Determining the origin of volcanic rocks in the mélange complex of karangsambung based on the electrical resistivity imaging

Abstract

An ENE-WSW-trending localized basalt-diabase outcrop along the SE margin of Luk Ulo Mélange Complex has been suggested as intrusive rocks cut through the Paleogene Totogan and Karangsambung formations. However, the absolute dating of the volcanics is older than the inferred relative age of the sedimentary formations, hence the in-situ intrusion theory is less likely. A subsurface imaging should delineate the possibility of the in-situ nature of volcanic rock by looking at the continuation of the rocks to the depth. In this study, we did a subsurface imaging by electrical resistivity method. The electrical resistivity surveys were conducted at 3 (three) lines across the ENE-WSW trend of the volcanic distribution. From those three measurements, we obtained three inversion models that present the distribution of the resistivity. We could differentiate between the high resistivity of volcanic rocks and the low resistiv-ity of the clay-dominated sediments. Instead of the deep-rooted intrusions, the geometry of the volcanic rocks is concordant with the sedimentary strata. Since we do not observe any spatial continuity of the bodies, both laterally and vertically, the volcanic rocks might be part of broken intrusive rocks. Furthermore, the size and the sporadically distributed of the rocks also indicated that they are more likely as frag-ments during the olistostrome deposition, transported from its original location.