Petrography and geochemistry of structural limestones in the Pringkuku Karst area, Pacitan, East Java

Abstract

The Pringkuku karst area is located in the south-west of East Java. Pringkuku is a transition zone between Gunung Sewu karst hills in the west and Gerindulu structural areas in the east. Sample observations were carried out by considering four parameters: structural and non-structural reef limestone from Wonosari Formation (Tmwl) and structural and non-structural clastic limestone from Oyo Formation (Tmo). Wonosari Formation and Oyo Formation were traversed by the fault trending northeast-southwest in the Eastern Pringkuku region. Reef limestone units of Wonosari Formation consist of calcite and dolomite, while clastic limestone units of Oyo Formation are dominated by calcite. Both of limestone lithologies undergone process of diagenesis, in non-structural limestone, the crystal still has a euhedral form while the structural unit does not have an almost rounded shape due to dissolution. There are Ca, MgO, and SiO2 contents from clastic and reefs limestone units and dolomite minerals were found in several samples from reef limestone units. Based on petrographic studies, clastic limestone units can be classified as lime-packstone, while reef limestone units can be classified as lime-boundstone and lime-wackestone. Petrographic and geochemical analysis results on clastic limestones show an increase in its main constituent elements such as calcite and magnesium, whereas, in structural limestones samples, there are additional quartz minerals and periclase minerals. Meanwhile, petrographic and geochemical analysis results on reef limestones show that the main constituents in structural and non-structural limestone are calcite and dolomite minerals.