Lithofacies and Biostratigrafics of Olistostrome Deposits of the Karangsambung Formation, Kebumen, Central Java

Abstract

The scarcity of biostratigraphic investigations conducted on the Karangsambung Formation has resulted in limited knowledge regarding its lateral boundaries and age. Consequently, there are notable uncertainties surrounding the formation process of the “Eocene sediments” within this formation. The study site is located along the Mendek River route in Karangsambung, Kebumen, Central Java. The objective of this study is to ascertain the lithostratigraphic nature, with the aid of biostratigraphy, in order to interpret age and paleobathymetry. The employed methodology entails the utilization of stratigraphic cross-sectional measurements and biostratigraphic analysis, focusing on the examination of planktonic foraminifera and benthonic foraminifera as the subjects of study. The findings of the lithostratigraphic analysis indicate that the lithostratigraphic unit consists of three distinct categories of fragmented mudstone. The age of P12 - P19 has been determined through biostratigraphic analysis, utilizing foraminifera as the primary indicator. There exist a total of seven distinct species of index foraminifera, which include Truncoratoloides topilensis, Globorotalia cerroazulensis, Globigerina senni, Globigerina selli, Globigerapsis Mexicana, and Globigerapsis. The findings derived from the paleobathymetric analysis indicate that the Upper Batial and Lower Battial exhibit the occurrence of benthonic foraminifera, namely Bulminela grata, Rectuvigerina striata, Cibicidoides crebbsi, Cibicidoides alazanensis, and Bigerina nodosaria. The deposit under consideration is an olistostrome, which was formed during the Middle Eocene to Lower Oligocene epochs in a marine trench setting.