Transform Fault at Ngalang

Alutsyah Luthfian | Contributor

Geological Background of Ngalang River

Ngalang River was our second field trip location. It is located in Ngalang Village, Gedangsari District, Wonosari Regency. Using motorcycle, we could reach it from Javan Cultural Centre of Yogyakarta for about, uhm... 2 hours.

What's unique about Ngalang River? In its north - south traverse, the river encompasses four formations respectively (Semilir, Nglanggran, Sambipitu, and Oyo), and the inter-formation contact can be clearly seen. This means, amateur geologists like me didn't waste much sweat there :-D. Have you read my previous post about Watuadeg Pillow Lava? If haven't so, please read it too :-D just to make the whole geological story complete.

The Semilir Formation is composed of tuff, dacitic pumice breccia, tuffaceous sandstone, and shale. This was resulted from a mega-colossal Early to Middle Miocene volcanic eruptions at Parangjoho and Songputri area in Wonogiri, about 30 km to the southeast. Why mega-colossal? The first thing to note is this formation's lateral spread. Unlike a usual volcanigenic geological formation (which can be drawn in one or two quadrangles of geologic map) this Semilir Formation needs four sheets (Yogyakarta, Surakarta, Pacitan, and Ponorogo Sheet), equivalent to > 1000 km² spread. Another thing to note is this formation's dacitic to andesite rocks, a sign that this formation's magma source was mildly enriched in silica. Those enriched magma might been in the process of cooling (represented by dacite rock), thus it releases gaseous fluid into the chamber and ---- ULTRA PLINIAN ERUPTION!

Beside Semilir Formation, there was deposited Nglanggran Formation. Same with his friend Semilir, this formation is composed of andesite rock. The difference is at the genesis; Nglanggran was deposited by a rather "quiet" volcano, refer to its composition: thick lavas plus some breccias (no tuffs and pumices). The breccias are deposited in deep sea environment, due to its visible reverse bedding. Reverse bedding is a behavior of sedimentation process in which softer sediments are deposited below the coarser one due to turbidite current - a rapid current resulting from very muddy and stony marine landslide. Asking about the source volcabo's dimension ? Never compare it to today's Merapi, it might be as huge as 4,000 or even 5,500 meters (seeing the case of Mount Kenya, prior to its carving by glaciation,the volcano's height was predicted to be 7,000 m). Nglanggran Formation was deposited alternately with Semilir Formation - thus Nglanggran Formation is also from Early to Middle Miocene.

Above the Nglanggran Formation...(Note, just Nglanggran, not including Semilir), deposited the Sambipitu Formation. Sambipitu Formation seems to be deposited in coastal environment, due to its abundance of sand, clay and mud. Some features we can found in Sambipitu Formation is burrow (a hole cutting accross lamination which was dig by animals), mud cracks, and convolute bedding. It was deposited around Middle Miocene.

The latter formation is Oyo Formation. The Oyo Formation is interesting, the lower part is composed of tuff and tuffaceous marl but as you trace it upward it would gradually changes into bedded limestone with some rounded andesite fragments. It might be deposited in continental shelf at Middle Miocene, after Sambipitu. Lack of eruptional-origin materials at Sambipitu and Oyo Formation is a sign of retreating volcanic activity during the course of Middle Miocene.

A crop of Eastern Yogyakarta stratigraphy, made by Surono (1992).

Faulting at Ngalang River

Besides good quality outcrops and inter-formation contacts, other thing can't be missed in Ngalang River is the fault. Based on geological map (released 1992) made by Surono, B. Toha, and Ignatius Sudarno (the latter is my lecturer for structural geology), there is a sinistral transform fault at Ngalang River. Here is the crop of the map.

Dextral transform fault at Ngalang. North is to above.

This is the geologic cross section of Ngalang River made by Anindyo Widiasworo (2011) in his undergraduate thesis. This geologic cross section runs from northwest to southeast and cutting Ngalang River at its mid (see the fault and the first 'S' - stands for 'sungai' or river).

In those cross section, we could see that the eastern part of the sinistral fault is uplifted.
Up to this section, everything is fine. But when going into the realm, my mind = kicked.
Why those should happen ?...

The contact between Nglanggran and Sambipitu Formation. The Nglanggran Formation is dominated with breccias and sandy siltstone . The sandy siltstone is deposited below breccia - a clear example of reverse bedding. The Sambipitu Formation is dominated with softer sediments like tuffaceous siltstone. Photo taken by Annisa Sofhia Pratiwi and delineation by Alutsyah Luthfian. North is to your left.

Explanation from Structural Geology's Eye

The image below may have explained what has happened in Ngalang River.

Another thing to note is the presence of micro-reverse and micro-normal faults at Sambipitu Formation. I will talk about this very later, so stay tune! :-D