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 resistivity 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 fragments during the olistostrome deposition, transported from its original location.

Tectonic Geomorphology of the Karangsambung Area, Central Java, Indonesia

“…The Melange Luk Ulo complex is covered with unconformities by the Karangsambung Formation and the Totogan Formation. The Karangsambung and Totogan formations generally consist of sedimentary mixtures of fragments and blocks (olistolite), such as sandstones, siltstone, conglomerates, nummulitic limestone, lava in the clay basement and are interpreted as olistostrome deposits (Asikin, 1974;Handayani et al, 2021). Furthermore, the younger Neogene rocks are deposited, namely the Waturanda Formation and the Penosogan Formation.…”