Characteristics of Carbonate Facies and Depositional Environment of Tapak Formation in the Ajibarang Area, Central Java, Indonesia

The limestone of Tapak Formation is well exposed in the Ajibarang area and its surroundings, Central Java, Indonesia. Very intensive mining activity is a very serious threat to this limestone. Hitherto, the geological study on this limestone is still very limited. The objective of this study was to investigate the characteristics of these limestone facies and their sedimentation. This research consists of field and laboratory (petrography and micropaleontology) analysis. The results showed that the limestone of the Tapak Formation consisted of three facies: planktonic packstone, algal foraminifera packstone, and foraminifera algal packstone. Planktonic packstone facies are composed of bioclastic fragments in a micro matrix. The fragments were dominated by planktonic foraminifera consisting of a genus of Globigerina, Globigerinoides, Globorotalia, and Orbulina. Benthic foraminifera and radiolarians were also identified. Sedimentation from planktonic packstone facies was estimated to occur in the deep marine environment. Algal foraminiferal packstone facies are composed of bioclastic fragments in a micro matrix. Large algae and large foraminifera dominated the fragments. Algae were from the type of red algae, consisting of Lithophyllum, Corallina, and Rhodolite. Foraminiferal algal packstone and algal foraminiferal packstone that were observed in the studied area were characterized by the abundance of large foraminifera and interpreted to be deposited in the upper reef slope. We interpreted that the limestone in the study area was deposited in the upper reef slope to the lower reef slope environments. We hope that the information we provided here is useful for petroleum system study in Central Java, Indonesia.