| Summary: | This study was about the geology and analysis of planktonic foraminifera assemblages of Oyo Formation in Nglipar, Yogyakarta, Indonesia. The purpose of this research were to produce a geological map of the study area, to analyse the foraminifera assemblages based on planktonic foraminifera species and to mark and update the data of biozonation based on stratigraphy chart. Field observation, sampling and measuring section were done during geological mapping activity. The Oyo Formation is Well exposed at Nglipar district of Special Province of Yogyakarta, Indonesia. The unit rock of marl intercalated with grainstone, which is part of Oyo Formation consist of well-preserved planktonic foraminifera is significant for microfossils study for petrography and biostratigraphy of this formation. The depositional environment for this formation is neritic, Where these planktonic foraminifera lived in Warm and humid neritic environment. The Oyo Formation is unconformably underlain by Wonosari Formation. While the Sambipitu Formation is overlain by Oyo Formation, found to have a sharp contact in Ngalang river. Based on planktonic foraminifera range chart and assemblages, it is suggested the age of Oyo Formation is Early Miocene to Late Miocene based on foraminifera zonation analysis. In this study, assemblages planktonic foraminifera fossils found within the study area been compiled, classified and determined for the age of Oyo Formation, which is to be found within N4-N17, from Early to Late Miocene
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