On November 5th to 10th, 2014, some students from ITB SC AAPG went to Samarinda to have a field trip with TOTAL titled “From Turbidites, Trough The Shelf, to The Deltaic and Fluvial Deposit: Modern and Ancient Mahakam Delta(s)”
And here are the reports and stories written by the participants. Enjoy!
First observation held on November 5th about basin floor deposit.. At the time, we visited three outcrops that correlated each other and indicate basin floor deposit. This outcrops consist of;
- Interbedded sandstone and shales with sharp-base. This sandstones shows very dense sand in the bottom (massive/structure less) and changes into low dense sand at the top (lamination structure). From bottom to top, the sandstone succession are fining upward and thinning upward. In sandstone also found Ta, Tb, Tc that refers to Bouma sequence. The shales also contain organic matter.
- In the second outcrops, sandstones is thicker than first outcrops. At the bottom, shows a sharp-base (erosive) with pebble deposits. This sandstone is massive (structure less) with leaves fossils.
- At the third outcrops, we found very dense sand in the bottom and become low dense at the top (lamination). The sandstone also has leaves fossils.
From those outcrops, the correlation indicate lens geometry that refers to basin floor deposits.
After first observation, the second observation is about slope deposits. The outcrops consist of thick sandstone deposit and shales with sharp base. Sedimentary structure in sandstone is flute cast. The sandstone geometry is pinching out, with fining upward succession with pebbles in the bottom. Those pebbles indicate high energy during deposition (mass flow). In this outcrops also found slump structures and at the top found natural levee deposits that refers to the submarine channel.
The third observation is located in Batuputih. The objective of this observation is to know shallow marine deposits. In this location, we can found one shallow marine parasequences. The deposits consist of shales at the bottom that refers to offshore facies and continue with thick sandstones that refers shoreface sand facies, foreshore sand facies, and carbonate at the top of this parasequences. Shoreface sand facies characteristics are wave cross stratification, sand bed 3-5 cm thick, high bioturbation, hummocky cross-stratification, and sand bar thick is 1-5 m. This parasequences repeated continuously and had an observation key that is carbonate rocks.
On the second day, we visited 3 big outcrops located in Hutunan Village ( nearby Samarinda Medical Citra and also Batu Putih.The first outcrop is nearby Samarinda Medical Citra. This outcrop is about deltaic deposit which is distributary mouth bar as delta front facies. This outcrop shows NE-SW in strike and E in dip. There are 3 lithologies here which are shale, sandstone, and coal. Black shale can be identified by its dirty characteristic, dull, and could be distinguished with coal that’s totally black. In addition, what’s interesting enough for us is that we could observe a lot of foraminifera fossil in sandstone.
After that, we went to the second outcrop located near Hutunan Village Housing. This outcrop depicts us about turbidite deposit and is older compare to the first outcrop we just observed. This deposit has NE-SW strike direction and also E direction in dipping. This outcrop is compiled of sandstone and claystone with its firm contact in between. Furthermore, the more we go to the East, the clearer we could see that sandstone layer is thinning and also fining upward. In the sandstone deposit, we could also observed abundant of pebbles was deposited here and amazingly piles up of leaves fossil imprinted in the sandstone. Abundant of pebbles deposit here implies that there must be some catastrophic event that create all the deposit possible to get frozen at once, therefore some pebbles endured and didn’t fall to the bottom and this can be concluded as high density gravity flow influence. This catastrophic event was ventural and non-cyclic event. This turbidite deposit shows Boma Sequence with Ta (massive structure), Tb (pararel lamination), and Tc (cross lamination) features. Evolution of gravity flow in the bottom is signed by high energy deposit, thick sand, high density, and also massive in structure. On the other side, top of gravity mass flow deposit is characterized by low density, thin sand, and also fining upward succession.
Furthermore, we went to the third outcrop located nearby Samarinda street which consist of sandstone and claystone deposit. This outcrop shows W in dipping that we just interpreted as a flank of anticline. Sandstone and claystone deposit has pinch out feature and also massive pebble. This pinch out sandstone deposit compensate each other which means that through its depositional time, the sandstone is getting concentrate because of high energy. In this sandstone deposit, we also found flute cast dan slump structure that can be interpreted as under the sea slope channel deposit.
Lastly, we went to our fourth outcrop which is Batu Putih. In this location, we found open marine shelf deposit that consist of offshore shale, shoreface sandstone, foreshore sandstone and carbonate rock. These four deposits are called shallow marine parasequence. In addition, we found some cycloclypeus fossil that belongs to deep shelf deposit; as an open marine characteristic. In foreshore sand, we found coarser sandstone, thickening and also bioturbated (we presume it was ophiomorpha). What surprise us is that we virtually found hummocky cross stratification for the first time. This sedimentary structure was made by storm and mostly formed in inner neritik. When the storm came in, it might forms dome. This dome structure is called as Hummocky. We can find it not only in the outcrop scale but we also can find it in core scale as well. This deposit usually has a sharp base on the bottom and we also commonly find pebble, paralel lamination, and bioturbation. After that, we also found nodul in foreshore sandstone facies that might be a cement formed because of early diagensis . Last but not least, we found same keybeds as the summary which are coal in the first outcrop and also carbonate in the last outcrop.
Location 1: Mahakam River, near Islamic Center (Mosque)
At 07.30 am (WITA) we were shown part of the Mahakam River with a width less than 1050 feet (800m-1000m). The river is located approximately 50 miles from shoreline in the east. River flow direction is from the south to the north. At this location we have to pay attention to the water level of the benchmark (border wall). In the afternoon at 01:30 pm the water level is going down from the benchmark. It shows that, the tides also affects the flow of surface level of the Mahakam River. This pattern of tidal is sinusoidal showed high tide in the morning and low tide in the afternoon. Why water flowing is in south-north direction? While in general Mahakam River flow is west to east. That is because the type of the Mahakam river meandering. Spot that we saw is the curve of the river so that it was as though the south-north direction of flow.
Location 2: Kunjang River
At 8:05 am we were shown to a large outcrop with oblique bedding dip about 70 degrees to the east. Bedding consists of black shale, sand, and carbonate. Dark thick shale with 5 m thick, sandstone with 3.5 thick with parallel lamination structure at the bottom, ripple lamination, hummocky crosstratification, and bioturbation structure on top of it. The upper rock is carbonate with 60cm thick with many fossils like cycloclipeus, gastropods, branching, rugose, and head coral. There is a similar loop 3 times. It shows that there are four times parasequence sets.
Location 3: Behind of Samarinda Medika Citra Hospital
Here, we were shown mouth bar sediment. These deposits form oblique dip bedding about 40-50 degrees from bottom to top in the form of shale (dark, flake), then silt (whitish color), and sand with very fine sand, with a total thickness is 2 m. Sandstone has parallel lamination structure and the top of the (sandstone) there are many bioturbasi. The grain size of the sandstone is very fine because it’s mouth bar located far from the delta front (approximately 30 miles), so that energy which work is not too big and still allows very fine sand grains deposited. This sandstone is also showing a parallel lamination and bioturbation structure. Bioturbation shows a living beings which can growing. It is because the mouth bar here has not received the sediment supply that living beings could live here. The maximum thickness of the mouth bar is 5 m and sandstone is 2 m.
In the first outcrop, we observed 4 cycles of deltaic cycle which showed 4 repetitions of shale-sandstone-carbonate rocks. Shales were thick, followed by sandstones that were coarsening upward, and overlaid conformably by carbonate rocks which were mostly marl with mollusk fossils. This section showed regression which was then followed by transgression of a deltaic environment. A syn-sedimentary fault was also observed, obtained by the difference of layer’s thickness in the hanging wall and footwall side of the fault.
We moved to the second section, which were very thick sandstones everywhere. In the base of the thick sandstones there were conglomerates, followed by fining upward sequences of sandstones. Thin layer of white shale was observed. This shale consisted of kaolinite, a kind of clay mineral that marks continental environment. On the top of the shale there was brown shale which was thought as equivalent with coal regarding a sedimentary cycle (it is said that every cycle starts and ends with coal layer). By seeing the big picture of the whole other sections that had been observed in the previous day, we could obtain that this was fluvial channel deposit. The white shale was probably flood plain deposit.
Tens of meters from there, we could see another very thick sandstone which laterally changed into siltstone with climbing ripples structure. The siltstone layer wasn’t more than 1 meter thick. Since we already interpreted this place as fluvial channel, then this layer of siltstone could be interpreted as crevasse splay deposit.
Heading to the last section of the day, we could see another cycles of fluvial channel which from bottom until top consisted of: conglomerate-thick sandstone-shale-coal. One thick sandstone can be consisted of a few channels which eroded one another. The channels were marked by thick cross-bedding structures.
In this day was different and became the most special day because riding a boat around the delta to see what it is called distributary channel, tidal channel, also walking in the top of delta mouth bar deposits were so rare moment that not every person get the same chance like us to do all of those things.
Like usual, we took off by bus around 7.00 to main gate of delta Mahakam to get on to the boat, for your information that day was raining when we get on to the boat, but luckly when we arrive at the delta the rain was stop so we could made so measurement and made some discussions with mr. Irfan Cibaj effectively.
First, we stopped at the distributary channel delta to make a depth measurement, made the curve section of the depth, took some sediment sample of the distributary channel with dredging method, and some discussions were held to prove theory that we got from books and campus is true.
Second, we stopped far from the Distributary Mouth Bar because the level of sea was only 1 meter high so the boat was stucked and we must got off from the boat to walk to reach the observation spot in TUNU-20 (location is named based on TOTAL Indonesie). Before that, we took a core sample of the mouthbar sedimentation below then we discussed it together. After arrived to the mouth bar we had some observation about recent bioturbation and recent current ripples, then we didn’t forget to get some pictures to memorize all of this great experiences.
Third, we were going back to the delta by crossing tidal channel, there we made a depth measurement and curve section, also took some sample by dredging method. The sample that we got is a black shaly thing, a lot of organic matter, more finer than the sample from the distributary channel so we called it shale. That proven theory of tidal channel were discussed to give more understanding about the differences for tidal channel and distributary channel.
After that we continued to go back, then we got to the bus soon after we arrived in the main gate. We went back to the hotel and arranged many thing because tomorrow morning we must check out on 7.00.
Birth day cake stop
In this stop we saw the layer of sand and conglomerate above sand what eroding in the top of sand. This succession show us that in the past time this place is river. And there is some river here what overlay. In the sandstone we found wood fossil and in conglomerate we found a lot of big fragmen. This fluvial parasequence show us sharp base because there is a erosion there.
In this stop we saw big succession. There is a lobe of delta in the bottom one proved by its stratigraphy shale-sand-shale-sand and we found sand dike here. And followed by shallow marine and found kind of slump so there is a structurless bed, kind of conglomerate and coming finer to up. This bed show there is a sudden flooding what makes its like debrite. And also there is slump in this section, and a lot of micro fault. Then we found thick sand and form cross bedding, so this is a fluvial channel, in the fluvial channel the sand is medium sand. And in this part we think that this is regression phase.
Top Valley stop
We found maybe 5 till 7 of lobe delta sequence. What characterize by layering of sand-shale-coal. And the thickness around 40 meters. The sand is fine grain sand. And in the top of each boundary layer we found a lot of bioturbation. And Pak irfan said if we continue our walk we will find batu gamping.
In the last meeting in the Total office in Balikpapan.
We had opportunities to meet some total employee, and they share some job in geology what still needed by oil company. Like goescience in well drilling, operation and geopressure.