The Dynamics of Mahakam Delta

        

November 12-16^th, 2011      

   This fieldtrip is as a part of the American Association of Petroleum Geologist UGM Student Chapter Program. The event is planned to enhance undergraduate student’s knowledge about stratigraphy and sedimentation infilling in depositional system especially in deltaic environment. This program is also aimed to improve field skills of AAPG UGM SC’s and UPN SC’s member.

Summary of Activity

                        Day 1

            Day 1 is the departure day; participants arrived in Balikpapan at 15.30 WITA and stay at Mitra Amanah Hotel, Balikpapan.

            Day 2

The second day started with a visit to VICO facilities located in Badak, Samarinda. Participants were warmly welcomed by Mr. Abdul Qodir (VICO Indonesia), with coffee break and lunch session at VICO cafetaria. Before heading to the facilities, VICO gavea brief introductory about VICO Indonesia and also safety informations. Participants then heading to the pipelines of VICO, compressors, the first well of VICO in Badak (Badak-1), and also drilling rig located in the area. In the end of the visit, there was a closing and momento session. 

Day 3

The fieldtrip to Mahakam Delta wasconducted on the third day and guided by Geosain Delta Andalan (GDA) team consisted of 4 people, led by Dr. Ir. Andang Bachtiar, M.Sc. There are four stop sites, as follows:

Stop Site 3.1: Sanga-Sanga Head of Passes at sungai Mariam

Sanga-sanga head of passes revealed the fluvial environment of Mahakam Delta area. the width of Mahakam river in this stop is 1200 meters. From the surficial observation, the river shape does not indicate a meandering shape although the river has been confirmed in literatures as mature river. This is because the meandering can only be seen below water.

In this site, we conducted a grab sampling into 6 sites across the river. The interesting fact revealed was that the thalweg part of the river (the deepest part) contained mud (fine silt-clay), not the expected gravel sized sediments, although thalweg had the fastest current than any other part of the river. This can be explained by using Huljstrom diagram: the energy to erode gravel-sized sediments is actually less than clay, hence in the thalweg of Mahakam river, the gravel sediment has been all eroded whereas the clay sized sediments stay on the place.

For all other parts of the river, the increasing depth resulted in the increasing sediment size. Thus, the riverback sediment was also mud, but it differed with thalweg's mud in that thalweg's mud is more gel-like than riverback sediment.

Stop Site 3.2: Muara Bayur

Muara Bayur, located about 17 km from the Sanga-sanga head of passes in Southeast direction, is a part of distributary channel system. Here, we conducted echo sounding to show the river's profile. Although it was observed in situ that sea trucks were operated above the  Southern side (assumed to be the deepest part of the river), the echo sounding showed that the deepest part is actually in the Northern side.

From the observation, it can be concluded that this area is dominated by fluvial processes rather than tidal processes, marked by the wide and consistent flow course of the river and sediment characteristics (obtained from grab sampling).

If we noted on the type of vegetation during the journey from Stop Site 1 to Stop Site 2, it could be seen that the vegetation type changed markedly. This is due to the difference of salinity (salinity is higher down to the East).

B.1.3. Stop Site 3.3.: Datu Island

Datu Island is a mid-channel bar that is a part of upper delta plain of Mahakam delta. The sediment a layer (about 6 cm) of medium sand in the uppermost part followed by very fine, carbonaceous sand below that is interlaminated with clay. The type of sedimentary structure is wavy structure.

 The surface features were ripple marks of multi direction, but the major trend was bimodal SE-NW. This is very important because learning to the present condition, the tidal environment can result in multidirectional ripple mark, including directions opposing the river course. Hence if we find an outcrop that contains a ripple mark, just a single structure cannot be used to interpret the whole ancient mechanism of sedimentation; numerous statistical analysis has to be conducted to obtain an accurate paleocurrent direction.

 We observed many clay pellets are scattered, indicating that tidal processes is quite dominant in the area. The clay pellets were formed when sediments from the river bottom is scrolled up and down by tidal processes.

B.1.4. Stop Side 3.4: Muara Bujit

Muara Bujit is located 12 km in the SE direction of Datu Island and is a mouth bar that is strongly influenced by tidal processes. The area has intertwined by marine environments; water has salinity of 10.000 ppm NaCl and many marine shell fragments are found between the sediment. Even live cephalopods are found making burrows inside the sediment.

The interesting feature of Muara Bujit is that sand had the same size and even slightly coarser than Datu Island, with less portion of clay. As for the explanation of the coarser sand size, it is because the environment is a head of passes, in which the grain size is due to the energy of the channel. It resulted in a repeated series of lateral fining trend of sediment. Hence in the upper delta plain (Datu Island) we may observe a finer grain than in lower delta plain (Muara Bujit).

The surface was also covered by ripple marks. Ripple marks shown a sequence of current ripple NW-wavy-current ripple SE, indicating tidal environment in which ripple trend is bimodal.  carbonaceous and organic materials are concentrated behind the stoss side of ripple mark. This feature can explained where the interbedded clay came from, which is, from the breakdown of carbonaceous and organic materials. This will finally build a flasser structure in which streaks of clay is located between sand, as shown in the coring profile. We also observe cross bedding of the sediment.

Day 4

Another fieldtrip will be conducted on the fourth day with destination to the Samarinda-Balikpapan surroundings. After the participants learned about the present day mechanism of Mahakam sedimentation, now it is the time to apply the concept “the present is the key to the past” towards the outcrops of ancient Mahakam.

Stop Site 4.1: Batu Putih Limestone

Batu Putih Limestone is an oldest Samarinda’s outcrop of limestone (Eocene) located in Desa Air Putih, Samarinda. It is a limestone hill that is also the highest point of Samarinda. From the site, we can observe the city of Samarinda that is actually is the flood plain of Mahakam delta and a part of Samarinda anticlinorium, marked by the NE-SW lineanment trend of hills.

In the lower part of the hill, the limestone was bioclastic reefal limestone, whereas succeding upward the limestone changed into muddy limestone with reef fragments. Furthermore, the reef is bigger with more complex shape in the upper part than lower part. This indicated a shallower environment upward. It can be explained that in Eocene until Oligocene there is a transgression event, followed by basin inversion in Miocene.

Stop Site 4.2.: Perjuangan Quarry

Perjuangan Quarry is located in the easat flank of Separi Anticline. The cliff trends from NE to SW, facing NW with a total traverse of 250 meters. The height reach up to 40 meters. an outcrop of coal in the lowest part, followed by cross-bedding sandstone, followed by conglomerates with erosional contact with sandstone, followed by upper sandstone. The coal is papery and continous. The cross-bedding sandstone is actually a  lateral accretion deposit, marked by the presence of lateral accretion surfaces than contains cross-bedding inside.

In this site, we tried to infer the environment of the outcrop. It showed an overall coarsening upward trend. The environment cannot be delta front or delta mouth bar since it involves no erosional features. The environment cannot also be distributary channel or delta plain because we have to observe flasser or burrowing in such site. Hence, the accurate interpretation is that this is fluvial environment of point bar deposit. It appears that the large amount of the relief has been scoured by the channel so it is worth considering as sequence boundary.

Stop Site 4.3: By Pass Paralan

Outcrop of by Pass Paralan are located along high way of Samarinda-Tenggarong. Geologically if strikes Ne-SW and dips SE. Outcrops are exposed 200 meters width and 15 meters height. It consists of interbedded sandstone and shale intercalated by thin layers of coal or coaly shale. Sedimentary structure such as cross-bedding and parallel lamination are found in the outcrop.

From West to East, the outcrop shown a story of succession from channel, mid channel bar, upper delta plain, lower delta plain, delta front, until marine. the western most part is sandstone succeeded by erosional contact with crossbedded sandstone with clay drapes in the lower part, indicating a scouring channel. The next layer is sandstone with many burrowing, indicating a start of delta plain environment. Next is a shale of wavy sedimentary structure, indicating lower delta plain. It is then followed by a layer of coal that marked the transition zone to marine environment. A bed full of oyster (cephalopod) is found in the East, marking the marine environment.

Gunung Pancur oil seep is located in a river on the east side of Jl. Batam-1 Gunung Pancur area, Balikpapan. The Klandasan Sandstone developed in this location shows a thick bedding of fine grained to medium grained sandstone with some indication of cross bedding. The river showed a very clean, rounded quartz sand deposits and interpreted to be fluvial deposition in a braided river complex. An interesting feature is the oil dripping from the river cliffs that might be related to a faulting event. This site showed the lithology of the reservoirs explored in all over Balikpapan.

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