This agenda was held on Sunday, February 9, 2019. Along with first gathering of SEG UGM SC members, it took place in Batur Mountain and Watulumbung Beach, Gunung Kidul, Yogyakarta. 25 members joined this fieldtrip and was guided by SEG UGM SC former, Mradipta Lintang AM and Salma Difa Masti. According to regional geomorphology, this area is included in Southern Mountain Zone, specifically at Gunung Sewu Subzone. This area is composed by 2 formations, Wuni and Wonosari Formations. But Wuni Formation is dominating this area. The lithologies are pyroclastic, agglomerate interbedded with tuffaceous sandstone and sandstone. During observations, we find lithologies such as igneous rock (intrusions and lavas) and pyroclastic rocks which are intensely altered. Mount Batur was interpreted as dacite intrusions and altered characterized chlorite and epidote as the key minerals, besides, we can find clay and green-chlorite-like minerals in the outcrop. The alteration is intense because there are lots of fractures on the outcrop so that it became the way for hydrothermal fluid to interact with the wall rock. After doing observations in Mount Batur then we went down to Watulumbung Beach. During the way down, we observe the newly-cut slope which is more intensely altered than the outcrop in Mount Batur.
Altered rocks found in way down to beach has different characteristics. Rocks are red-coloured and white-spotted, which is interpreted as Kaolinite-hematite alteration.
There, we studied about basic alteration and how to differentiate weathering and alteration. Also, we studied about how to describe an altered outcrop.