June 2011

KARANGSAMBUNG

FIELD TRIP 2011

 

University of Indonesia – Student Chapter of Ameri­can Association of Petroleum Geologists held a Field Trip to Karangsambung, Kebumen, Central Java, Indonesia. This is our second field trip after going to West Java on November 2010. This field trip was held in order for us; Geophysical prospective experts to not only understand the methods and theories of geophysical methods, but also to have better understanding in geology. Because geophysics and geology are the main base knowledge needed to understand the environmental condition in explora­tion and earth studies. The field trip was leaded by Mr. Kris Hendardjo and Mr. Supriyanto and also two seniors as our assistant because the amount of par­ticipant of this field trip is too much to be handled by only two instructors on the field.

Not only geological features that we have studied in this field trip, but we also learned in using one from many geophysical methods, the Ground Penetrat­ing Radar (GPR) Method. This is to accommodate our main needs to apply the theories on geophysical methods in the real field. By studying the geologi­cal history of the site that we are doing the mea­surement and backing it up with exact data’s from the field, we can have a complete interpretation of whole condition of a site. We chose this site because firstly it has been well – known as a site that has fa­cilitate many research and studies for beginners like us. Beside that we can see the outstanding forma­tion of what used to be the bottom of the deep sea million years ago and now became a main land like it is today.

FIRST DAY (31TH MAY 2011): LEARN IN USING THE GEOLOGICAL COMPASS AND APPLYING IT ON THE FIELD

By understanding the way in using the geological compass, we were able to know the forces affecting the structure on the field and also capable of locat­ing the position of where we are. We applied this knowledge for the rest of our journey because in Ka­rangsambung there are so many geological features and sites that are combined at one place, and that is what makes this place rare.

Our first location was Banjarsari village where we can see the contact between the Karangsambung formation that was formed in the Tertiary time with the Totogan formation very clearly on the side of the river flowing through the formation.

Next we went to the top of Pasanggarahan village where we can see the panoramic view/amphitheater of the surroundings where we can see the whole formation from the eyes of a bird. After going to Pasanggrahan village, we went down the hill and go Mount Parang where we can see the congealment of magma that has intruded inside a formation and making columnar joints as it froze and cracks. Our next stop was the Mandala River where there were many fractures on the structure of the rocks in this area. We use these fractures to measure the dip and strike of the formation and put the data’s into the Ro­sette diagram to measure and to estimate the forces working on the area.

To end of the day, we visit one last loca­tion where we can see the sedimentary rocks of the Karangsambung formation. We were very lucky this year to be able to see this because usually a landslide and flooding occur on this site making us unable to see what is there on the location. Here we can see the baking effect of the Karangsambung formation and many big boulders that cam from on top of the small waterfall here.

SECOND DAY (1ST JUNE 2011): MELANGE AREA

Our second day was filled by an instructor from LIPI, Mr. Wawan. He was an expert on the Karangsam­bung area and took us to our first location, which is the Brengkok River where we can see the formation that was created in the Pre – Tertiary time. He ex­plained that the interpretation that has been made by the geologist is that this area contains the unifi­cation of three main plates in those days, forming mountains through the Javanese Island from north to south. Not only that, he reminded us again on how to differentiate between the three main division of rocks, igneous, metamorphic, and sedimentary rocks.

River, Seboro village. Here we were able to see very clearly the layering of the Rijang and red limestone rocks. On top of this steep layering formation we saw pillow lava which proves that this area was once where the magma flows into the sea. After Seboro, we went to Pucangan area where we see strange outcrop of serphentinite, metamorphic rock that shows up in the middle of a rice field. And in front of this was the back side horse shoe and saw the Mount Brujil and Mount Paras from behind the formation.

The last visit of the day was the marble rock out crop and also the LukUlo River. The marble formation may not be as big and wide formation at Tulungagung, maybe this is because it is still in the Melange area where the formation is very complex. At LukUlo River here we can see many types of rocks coming from many different source of the formation. Here Mr. Wawan explained to use the divisions of metamor­phic rocks and we can see all those types of division of the metamorphic rocks here at LukUlo River.

THIRD DAY (2ND JUNE 2011): GPR METHOD AT JATIBUNGKUS

Here we saw the Jatibungkus formation where we considered this as an out crop of the Karangsambung formation. We also saw rocks that were once corals of the formation, we studied the Dunham Classification that were based on the com­pactness of the rocks. After studying the geological of this formation, we learn in measuring with one of the geophysical methods, which is Ground Penetrat­ing Radar method. We measured 4 long lines in this area, hoping to be able to see what is beneath the surface of this formation.

At night of the discussion, Mr. Supriyanto explained to us in how to process the data’s that we obtained from the measuring in the field on the afternoon us­ing a program called Reflexw. From this processing, Mr. Kris Hendardjo and Mr. Supriyanto tried to inter­pret based on this geophysical data and geological too.

Those interpretations are:

1. a) There is a similarity when the depth of the layer is 5 m and the bottom layered is looks like it breaks down into small pieces. The boundary line of the Jatibungkus formation is located in 20 m depth of the layer.

2. b) There is a small river beside the measure­ment area that contains with the clastic lime­stone as the result of the buried rocks in the deep sea. This fault is considered as the bound­ary line between the Jatibungkus formation and the Karangsambung formation.

3. c) The shape of this fault is looked like an anti­cline, but in small size anticline. It was cause of the north side of this river has northern dip and the south side of the river has southern dip.

4. d) There are two basic theories about these formations. First, the bottom side of the soil in Jatibungkus formation is found before the Jatibungkus region itself. So the formation age is older than the Jatibungkus region itself and second theory said that Jatibungkus formation has the same age with the Jatibungkus region itself and they are pressed by fault force togeth­er in the same time.

5. e) Kind of limestone in Jatibungkus is different in every place. For example, kind of limestone in Jatibungkus River is clastic limestone and the regular limestone outside the river.

FOURTH DAY (3RD JUNE 2011): PANASOGAN HILLS, WATURANDA FORMATION, AND LOK IDENG RIVER

Panasogan Hills is the youngest in the forma­tion of Karangsambung and is located on top of the Waturanda formation. Here we saw tufa marl rocks and also travertine that is still very young aged of rocks that has lifted up to the surface. After Pana­sogan Hills, we went to see the Waturanda forma­tion. Here we saw large sized spheroidal weathering on the side of the formation that is located on the side of the main road.

After the Friday prayers and had lunch at camp, we went to our last location of the day, the LokIdeng River. This is our first location of where we had to take our shoes of and go inside of the river. As the other rivers, it contains many types of rocks coming from different sources.

We can differentiate them from the grain size on the rocks, the color of the rocks, and many other speci­fications that can differentiate from one rocks with another. Our main finding in this area is gabbro rock, which is a type from the igneous rock.

FIFTH DAY (4TH JUNE 2011): WAGER SAMBENG

This is our last day on the field of this event. We went hiking up the Wager Sambeng in order to see the amphitheater view where we are able to see both Mount Brujul and Mount Paras. Here we learn to look from an eagle eye view to see the horse shoe shape from above as the big picture. Again, we also studied on how to use the geological compass to locate of where we are standing on the geological map, by shooting the arrow of the compass as two Mount Brujul and Mount Paras.

On the way up of this hill, we saw an outcrop of the Rijang and red limestone again like the one that we saw at Seboro. The difference here is that the color of the rocks are more darker showing that it has faced weathering a lot maybe because of the location that is straight facing up to the sky’s. And also what makes it different from the one at Seboro is that it doesn’t have any pillow lava around this area. This shows that this area is the spreading zone of the lava as it hits the bottom surface of the sea bed. After having lunch and drinking fresh coconut on the top of the hill we went down and head back to campus passing by the LukUlo River. While passing through this river, we manage the time to have a short photo session together as it is

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