We have finished all schedule in this field trip and everything ran well. Field trip: “From Turbidites, Through The Shelf, To The Deltaic Deposits: Miocene Deposits of Ancient Mahakam Delta” was attended by 20 participants which come from Geological Engineering Department Universitas Diponegoro. The field trip was held in Samarinda, East Kalimantan. The field trip leader is Mr. Irfan Cibaj, sedimentologist at TOTAL E&P INDONESIE.
Kutei Basin is located in the eastern part of Kalimantan. It’s limited to the south by Meratus mountains, Patenosfer Platform and the Adang flexure zone, to the north by the Sangkulirang Fault, Mangkalihat Ridge, and to the north west by the Central Kalimantan Range Mountains. All of the sedimentary sources most come from the Neogene sediments. The development of Kutei Basin itself started with the forming of the basement on Upper Cretaceous, occurred syn rift phase from Middle until Late Eocene. During Oligocene there were sag phase and East-dipping extensional fault system – half graben infill. The next big geological history occurred from Early until Middle Miocene, Kalimantan was deformed and uplifted – shelf deposits in lower Kutei Basin. On Late Miocene started continuous structural inversion which related to Sulawesi collision and depocenter moved from its point. On Early Pliocene, there was active uplift in Sulawesi – sedimentary wedge thinning towards Kalimantan.
The Mahakam Delta is one of well known delta that shown ancient product of sedimentary process and also modern one. This delta is located on the east coast of Kalimantan, Indonesia. This delta is an active delta system which has formed in humid tropical environment under condition of relatively high tides, low wave-energy, and large fluvial input. Tidal processes control the sediment distribution patterns in the delta mouth and responsible for the flaring estuarine-type inlets and numerous tidal flats. This delta consist of many islands that are formed by deposition of sediment from Mahakam River that estuary on Makassar Strait.
This field trip was instructed by Mr. Irfan Cibaj, sedimetologist from TOTAL E&P INDONESIE. He knew much about sedimentology and stratigraphy of Mahakam Delta, either the old or the modern one. It needs one day to explore this field. The trip started Monday, September 23th 2013. We leaved from Semarang to Adisutjipto International Airport, Jogjakarta at 09.00 WIB after all participants registering. We held the opening ceremony, there was speech from chairperson and Head Department of Geological Engineering Universitas Diponegoro. After that, we pray before we’re leaving. We arrived at Sepinggan Airport, Balikpapan at 17.30 WITA.
The first day (25/9) was started by lecture about Regional Geology of Kutei Basin. It takes place near Samarinda Medical Center. Mr. Irfan showed and told us about channel-levee complexes in a slope turbidities series of lower Kutei Basin. Some of these outcrops have been previously described as slope turbidites. The new outcrops show cahnnel-levee complexes. The sandstone bodies are mostly constituted by mass flows and debris flow which pinch out in short distance to thin bedded turbidites and shilt shale alternations. Shales are very dark and rich in vegetal organic matter. This is interpreted as the lateral transition from proximal to distal leeves. Channel are filled in general by thick mass flow deposits (Irfan Cibaj, 2011). They are usually constituted by two distinct facies: thick, medium-grained sands with clay pebbles and thinly laminated fine grained sands extremely rich in vegetal organic matter (Irfan Cibaj, 2013). At the end of the day, we visited the Batu Putih area. There, Mr. Irfan explained about sea level change and showed us the outcrop of shallow marine deposit. Because it was dark, we will continue the observation tomorrow. At the night, we held a discussion in the hotel to review the outcrops we visited during the day.
The second day (26/9), we back to the Batu Putih area. We did exercise there, measuring stratigraphy. The Batu Putih carbonate has been dated as NN14 – NN5 nanoplancton zone (Seravallian 13 – 14 Ma) (Wilson, 2005). The series is constituted by shallow marine deposits organized in prograding – retrograding parasequences. Two complete parasequences are observed in the outcrop. From bottom to top a parasequence shows the following lithologic succession:
- Open marine dark shale (10 – 20 m thick) rich in well cemented carbonate nodules
- Coarsening and thickening upwards fine to medium to coarse-grained sand bars (prograding shelf bars, 5 – 20 m thick)
- Carbonate build-ups in form of patch reefs (transgressive carbonates)
This vertical succession is typical of prograding retrograding shallow marine parasequences with open marine shales overlain by prograding shelf bars which on their turn are covered by the transgressive carbonate build-ups during retrograding phase of the parasequence. Carbonate build-ups show platy corals followed by well developed corals, covered at the top by benthonic large foraminifera (Irfan Cibaj, 2013). After we finished with the measuring startigraphy, we moved to Samarinda Medical Center to observe the deltaic deposits. The outcrops showed us the deltaic progrdaing – retrograding parasequences. We found thick dark shales, very rich in marcoforaminifera and other bivalve; siltstone and fine-grained sandstone lenses and layers, exhibiting a coarsening and thickening upwards trend; hummocky and swaley cross-stratification; and coal layers associated with delta front mouth bars and delta plain distributary channels. Many deltaic parasequences are vertically stacked in a regressive pattern followed by a transgressive one. This pattern is interpreted as forming a Genetic Sequence.
Figure 1 The outcrop near Samarinda Medical Center
At the end of the day, we took photo session with all participants in front of the outcrop and then back to the hotel. At the night, we held a discussion in the hotel to review the outcrops we visited during the day.
The third day (27/9), we visited the fluvial sandstones in the Harapan Baru section. Thick red shales and thick white sandstones constituting the studied series, dominate this new landscape in Samarinda city and surrounding area. Coarse grained sandstones and conglomerates form lenticular bodies, surrounded by a mixture of shale and silts. They show sharp erosive base, and overall fining upwards grain size evolution. Through cross bedding is the dominating sedimentary structure. The sandstone bodies are 30 – 50 m thick and show a lateral extension of many hundred meters. They are interpreted as fluvial channels (Irfan Cibaj, 2013). We observed a complete fluvial cyclothem at Hapan Baru section. We break for a moment for Jum’atan pray and then we took lunch. At the same place, we discuss about the outcrops visited during the day. After that we go to find “oleh-oleh” before we come back to the hotel.
The fourth day (28/9), we went to Palaran. In this area, the outcrops are very good and interesting, Mr. Irfan discuss and talk about the sedimentology process of the outcrop. The Outcrops are exposed 200 meters width and 15 meters height. The outcrops consists of interbedded sandstone and shale that intercalated by thin layers of coal or coaly shale. Sedimentary structures such cross-bedding and parallel laminations are found in 6 meter sand body layer of this outcrop. Parasequence sets, showing as well regressive – transgressive stacking of sediments, are interpreted to represent a 4th order of stratigraphy. They are constituted by a regressive stacking of individual deltaic cycles, parasequences corresponding to 5th order of stratigraphy, capped by a short and thin transgressive event at the top (Irfan Cibaj, 2013). We took photo session with all participants in front of the slump outcrop. Then we had lunch and break. After that we continue the Stadion Utama section and visited the carbonate build-ups. Sequence boundary at 10.5 My and fluvial channels overlying it. After we finished the observation, we held a discussion for a moment to conclude the whole section from the first day until the last day.
Sunday (29/9), on 05.00 WITA we moved to Sepinggan Airport. We arrived there on 07.30 WITA. We took photo session with Mr. Irfan there. Our head department, Mr. Hadi Nugroho gave a memento to Mr. Irfan before we entered the boarding pass. In this field trip, there is a hope to increase our experience and knowledge about sedimentology as one of subject that must be understand to be a geologist, especially in petroleum interest. And after that, we hope one of the participant in this field trip can make a new method in future that will end this oil crisis.