Geology of the Kebasen Paleovolcano in Banyumas Regency, Central Java Province, Indonesia

Abstract

The new interpretation on geology of Kebasen district in Central Java, Indonesia which is located in the middle parts of Sunda Arc indicates the existence of a paleovolcano. This conclusion is achieved through 1:25,000 geological mapping with a focus on volcanic facies, characterization of lithology types, mineralogy, and fossil analysis, to clarify its volcanic history and paleo-environment. Laboratory analysis works on representative rock samples are done using petrography and fossil identification on calcareous sedimentary members. Our research results conclude that Kebasen paleovolcano was a composite volcano with a diameter of about 8 km, elongated in NW-SE orientation, with Kebasen city located at the SW rim of the old volcanic edifice. The volcanic history and resulting volcanic rocks consisted of two contrasting volcanic stages. The first one was explosive caldera-forming eruptions that produced thick and partly welded pyroclastic rocks, dominated by pyroclastic breccia at the lower parts and finer tuff members at the upper parts. After caldera-forming eruptions, volcanic activity was followed by effusive eruptions of basaltic to andesitic lavas that covered the caldera floor. The lavas have abundant pillow structures indicating a submarine environment, intermediate to basaltic composition, and porphyritic texture, with plagioclase and pyroxene as the most abundant phenocryst minerals. Available rock geochemistry data demonstrate that lavas have mafic to intermediate compositions, with magma affinity at the transition of calc-alkaline to tholeiitic series. Results of geomorphological and lithology association analysis conclude that the research area was part of the central to distal facies of a paleovolcano complex, with the paleo depositional environment in the open marine environment. The results of fossil analysis on calcareous sedimentary rocks between volcanic layers indicate that the age of volcanism was Early Pliocene (N18) with the paleobathymetry transitioning from upper-middle bathyal to outer–middle neritic, which were parts of the continental margin. It is concluded that Kebasen paleovolcano is a newly identified submarine caldera-type Pliocene volcano within the middle parts of the Sunda Arc in Central Java, Indonesia.