Geology and lignite-organic petrology in Ngalang area, Gunung Kidul regency, special province of Yogyakarta, Indonesia

The Ngalang Area is located in the centre of Gunung Kidul Regency and is a good geological complex area, and there is still lack of detail geological information. The detail geological map with larger scale has been created through geological mapping processes. The geomorphology of Ngalang area was classified in to undulating hills landscape, escarpment valley, and flat landscape, and these also can be related with the dendritie, parallel and rectangular drainage patterns together with the lithologies units. There are 8 lithologies units; the massive tuffaceous sandstone, tuffi pumice breccia, and sandstone units that correlates with the Semilir Formation; lava unit that correlates with the Nglanggran Formation; interbedded volcanic breccia and sandstone, interbedded sandstone and claystone, and calcareous sandstone units correlates with Sambipitu Fonnation. The structural analysis indicates that the main compressive force (0l) is from the north and south direction, and all the findings were plotted in the geological map. Besides, the specification objective is to determine the depositional environment of coal based on the organic petrology analysis using Diessel’s Diagram and ternary diagram. Three different types of coals from Early Miocene were found in the study area, and it indicates the different depositional environment. The fusaainous coal fiom the pumice breccia unit which deposited in the shallow marine but the charcoal woods were from the wildfire in the swampy environment. The brown coal (unconsolidated) from the sandstone unit that deposited in the upper deltaic enviroment in reed moor, while the lignite coal from sandstone unit deposited at the lower deltaic that influenced by the brackish water. Framboidal pyrite that observed in the lignite were the evidence for the mixture of freshwater and seawater.