PANGGANG, sub-district that located in Gunung Kidul Regency, Daerah Istimewa Yogyakarta is an area that rich in limestone lithology. This area also has conical hills as typical karst morphological pattern. Panggang is the location where I became a porter of Geological Mapping Universitas Gadjah Mada 2013 that held on July 15 – 28, 2013.
Figure 1. Digital Map of Earth Surface in Indonesia (Panggang, Gunung Kidul – DIY)
The mapping area includes in Wonosari Formation. Distributing of this formation dilated nearly half the southern part of Southern Mountains Zone longwise toward the east, veer toward the north, eastern of the hills Panggang until it reaches the western part of the depression Wonogiri-Baturetno. This formation was formed in neritic zone (shallow marine) and the thickness of the formation is more than 800 meters. The whole of Wonosari formation is formed during the late Miocene ( N9-N18 ). The lower part of Wonosari formation interfingers with Oyo Formation and the upper part interfingers with Kepek Formation.
In the mapping area, we found a lot of variety of limestone, such as packstone, floatstone, and rudstone. The naming of limestone is based on classifications of Embry and Klovan, 1971. Based on the manner of occurrence, Embry and Klovan divided limestone as follows.
Figure 2. Embry and Klovan Clasification
Based on the classification above, packstone, floatstone, and rudstone that found in Panggang is allouchtonous limestone. Packstone contains lime mud (<0,02 mm), less than 10% > 2mm components, and grain supported. Floatstone contains greater than 10% >2mm components, and matrix supported. Then, Rudstone contains greater than 10% >2mm components, and >2mm component supported. In the mapping area, we also found limestone bedded between packstone, floatstone and rudstone. Beside that there are also limestone reefs, we even found core reef in our mapping area.
Beside having peculiar lithology, namely limestone, Panggang has unique and distinctive morphological namely conical karst. Conical Karst is a karst hill shapes cone, precipitous slope and surrounded by depression. This Conical Karst is a positive karst morphology. The formation process of this area is through elevation removal of the seabed shallow ( litoral zone ) cause the influence of endogenous or tectonic.
In this mapping area, geological structural that we found there are faults and joints. However, we just could observe this fault in the bedded limestone, but we had difficulties for observing it in the massive limestone. That matter caused by we had difficulties to divide what if that fracture is it cause of fault, joint or just product by process of the karstification.
Figure 3. Conical Karst in Panggang (Camera look towards West)
Geological potential that exists at the mapping area consist of positive potential such as entrenchment material class C namely limestone. Reserve of entrenchment material limestone in this mapping area is very wide enough in Wonosari Formation. Therefore, there are many outcrops on the hills opened by rock mining activities. While the negative potential is bad hydrological conditions because no surface water founded, most of the water that falls to the surface directly into the ground because the rock is porous.
Figure 4. Rock Mining (Camera look towards South)
Created by : Endah Sulistiani