FOBEX 5: Ophiolite in Karangsambung, Kebumen, Central Java

Ophiolite is fragment of oceanic crust underlying above mantle. Ophiolite itself is divided into 4 parts ( from above – bottom ), the first is a rich marine sediments in silica, which will form chert. The second is the main composition of pillow lava basalt, which is formed when magma ocean rift zones that come out through the small gap will coagulate due to direct contact with water, which occurs in the deep sea zone around 3000-4000 meters where the pressure is very high . The third layer is a layer of basalt that has a muscular structure of sheeted dikes, then the bottom layer of the gabbro which has the main composition of the mineral red maroon garnet.



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Ophiolite in Karangsambung layer has a different pattern of ophiolite outcrop bedding pattern generally. Patterns found in this research area shows there is a layer of pillow lava on the top, and then chert is below. This outcrop is a rare phenomenon, especially in the ocean bedrock. This is due to the enormous tectonic, so deformed into a vertical outcrop. This is why as if it is above the pillow lava during the formation of chert. Pillow lava , though it comes first before chert, as in the central division of the ocean , cannot be found because of thin sediment. Therefore, the location of the ophiolite formation showed that Karangsambung is an ancient subduction zone trench.


Figure 1. Pillow lava in Karangsambung


Figure 2. Chert in Karangsambung

Chert outcrops found in the red due to the presence of iron minerals from the shells of marine animals which consist of SiO2. It shows that there is also the influence of the Carbonate Compaction Depth (CCD) which when it is currently under the CCD, where CaCO3 will disolved, and SiO2 will last , so that the silica -dominated chert. Chert structure is alternating with carbonate claystone, caused by ion transfer process , it’s when a rock is in certain circumstances at the time of the ion in the mobile condition , where similar ions will gather . The presence of the chert carbonate limestone may be the result of an unstable CDD condition, thus causing CaCO3 can also be precipitated and then form carbonate limestone. The red color is also caused by iron or Fe carried by limestone.

The presence of clay and sand are repeated or alternating turbidite deposition is a characteristic that is easily found in the area of Karangsambung. The existence of black clay and white sand alternating with each other, indicating compaction has occurred. Black clay is a product of deep-sea depositional environments. Black clay here as a wrapper for the larger fragments, where the pillow lava outcrops are found trapped here. This outcrop has low density.

Karangsambung has geological characteristics that are very interesting to learn. In this area there are a variety of rock types and structure and very complex geological structure. These complex geological conditions in the area formed because Karangsambung is a lifted meeting zone between the plates (subduction). The plates are colliding to form oval formations, each with a variety of rock types. Before the subduction trench lifted , many types of rocks deposited with the dominant form of  limestone as a wrapper . In this area also found rocks from the deep-sea, because the process of subduction zone trench lifting.

Created by :

Kadya Kirana

Vice-head Membership Division

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