EAST JAVA BASIN

EAST JAVA BASIN

Location of the East Java Basin

GEOMORPHOLOGY

This zone includes the northern coast of Java which stretches from Tuban to the east via Lamongan, Gresik, and almost the entire Madura Island. It is a plain area that is undulated with hilly terraces that run west-east and intersect with alluvial plains. The average width of this zone is 50 km with the highest peaks of 515 m (Ivory) and 491 (Mounds). Carbonic lithology dominates this zone. Accessibility is quite easy and the land character is hard.

The Rembang line consists of folds shaped like the Anticlinorium which extends west - east, from Purwodadi City through Blora, Jatirogo, Tuban to Madura Island. The morphology in the area can be divided into 3 units, namely Low Morphology Unit, corrugated hills and Morphological Unit steep hills, with hilly ridges generally extending West-East direction, so the river flow pattern is generally almost parallel (sub-parallel) and partly patterned dendritic. The main river that passes through the investigation area is S. Lusi, which flows towards the Southwest, through the City of Blora and empties into Bengawan Solo.

STRATIGRAPHY

According to Sutarso and Suyitno (1976), physiographically the research area is included in the Rembang Zone which is part of the North East Java Geosyncline sedimentation basin. This basin was formed in the Late Oligocene, which traversed East - West, almost parallel to Java (Van Bemmelen, 1949). According to Koesoemadinata (1978), the northern part of the East Java basin is more geosyncline with Tertiary sediment thickness may exceed 6000 meters. One thing that is typical of the northern part of the East Java basin is East-West and is seen as a Young Tertiary tectonic phenomenon.

Three stages of orogenesa have been known to influence the deposition of the Cenozoic rock series in Indonesia (Van Bemmelen, 1949). The first occurred between the Late Cretaceous - Middle Eocene interval, the second in the Middle Eocene (Intramiocene Orogeny) and the third occurred in Plio-Pleistocene. Orogenesa that occurs in the Middle Miocene is characterized by important events in the distribution of sediments and the spread of flora and fauna, especially in the western part of Indonesia and also causes a regression phase (sea shrinkage) that occurs in a short time in Java and the Java Sea area. The Middle Miocene orogenesis phase is also characterized by a hiatus in the Cepu region and is characterized by facies changes, from transgression facies to regression facies throughout the Rembang Zone. In addition to the above, the orogenesa phase is characterized by the emergence of several Pre-Tertiary bedrock in the island of North Java (Van Bemmelen, 1949).

A striking difference regarding the lithological properties of sediments in the Kendeng Mandala, Rembang Mandala, and Java Sea Exposure is sediment. Kendeng Mandala is generally filled with turbidite current deposits which always contain pyroclastic rocks with napal interludes and carbonate rocks and are deep sea deposits. In general, these sediments are folded firmly and have a hood to the north, while the Mandala Rembang shows rocks with high levels of sand in addition to increasing carbonate levels and the disappearance of pyroclastic deposits. Mandala Rembang sediments give the impression of shallow sea deposits not far from the coast with a non-uniform depth of the seabed. This is caused by the existence of block faults which result in facies changes and form a high or low area. The area off the coast of the Java sea is generally occupied by exposure deposits which consist almost entirely of carbonate deposits.

The Mandala Rembang according to the Tectonic system can be classified into the retro arc back arc (Dickinson, 1974) which is filled with thick and continuous Kenozoic sediments ranging from Eocene to the Pleistocene. Eocene age deposits can be seen from the wellbore data (Pringgoprawiro, 1983).

Tertiary Litostratigraphy in the northern part of the East Java Basin has been studied by geologists including Trooster (1937), Van Bemmelen (1949), Marks (1957), Koesoemadinata (1969), Kenyon (1977), and Musliki (1989) and many experiencing development in its stratigraphic arrangement. The confusion of the lithostratigraphic unit has been discussed in detail by Pringgoprawiro (1983) where the sediment deposits in the North East Java Basin are included in the Mandala Rembang stratigraphy from the order of the young, namely the Ngimbang Formation, Kujung Formation, Prupuh Formation, Tuban Formation, Tawun Formation, Tawun Formation The Bulu Formation, the Ledok Formation, the Mundu Formation, the Tongue Formation and the youngest sediment are referred to as the Undak Solo deposit. The Ngrayong Formation Tawun member from Pringgoprawiro (1983) was upgraded to the Ngrayong Formation by Pringgoprawiro, 1983. Members of the Selorejo Mundu Formation (Pringgoprawiro, 1983) were upgraded to the Selorejo Formation by Pringgoprawiro (1985) and Djuhaeni and Martodjojo (1990). While the Lidah Formation has three members, namely the Tambakromo Member, Malo Member (commensurate with Dander Members from Pringgoprawiro, 1983) and Turi Members (Djuhaeni, 1995).

Picture of the Mandala Rembang Stratigraphy Column (Harsono Pringgoprawiro, 1983)

Details of the stratigraphy of the North East Java Basin from the Rembang Zone compiled by Harsono Pringgoprawiro (1983) are divided into 15 (fifteen) units namely Pre-Tertiary Rocks, Ngimbang Formation, Kujung Formation, Prupuh Formation, Tuban Formation, Tawun Formation, Ngrayong Formation, Bulu Formation, Wonocolo Formation, Ledok Formation, Mundu Formation, Selorejo Formation, Paciran Formation,

Tongue Formations and Solo Steps. The discussion of each unit from old to young is as follows:

1. Tawun Formation

The Tawun Formation has a aligned position above the Tuban Formation, with the Tawun Formation boundary characterized by soft rock (claystone and marl). The lower part of the Tawun Formation, consists of claystone, sandstone, sandstone and lignite limestone, while at the top (Ngrayong member) consists of sandstones which are rich in mollusks, lignite and upward found quartz sand containing mica and iron oxide. The naming of the Tawun Formation was taken from the village of Tawun, which was first used by Brouwer (1957). The Tawun Formation has a wide spread in West Mandala Rembang, from the type location to the East to Tuban and Rengel, while to the West the rock unit can still be found in South Pati. Deposition environment of the Tawun Formation is a protected shallow exposure, not too far from the beach with a depth of 0 - 50 meters in the tropics. The Tawun Formation is the main oil reservoir in the Rembang Zone. Based on the existing fossil content, the Tawun Formation is estimated to be from the Upper Early Miocene to the Middle Miocene.

2. Ngrayong Formation

The Ngrayong Formation has a position aligned above the Tawun Formation. The Ngrayong Formation is composed of quartz sandstones with claystone silt, silt, lignite and bioclastic limestone. In its sandstones sometimes it contains shells of marine mollusks. Deposition environment The Ngrayong Formation in a shallow area near the coast which is increasingly above its environment becomes a littoral, lagoon to sublittoral edge. The thickness of the Tawun Formation reaches 90 meters. Because it consists of quartz sand, the Tawun Formation is a potential oil reservoir rock in the North East Java basin. Based on the existing fossil content, the Ngrayong Formation is estimated to be Middle Miocene.

3. Bulu Formation

The Bulu Formation is harmoniously above the Ngrayong Formation. The Feather Formation was originally known as the 'Platen Complex' with a stratigraphic position located in harmony above the Tawun Formation and the Ngrayong Formation. The lithological features of the Fur Formation consist of interlacing between limestone and calcarenite, sometimes with claystone inserts. In thin-layered limestone sometimes shows large-scale cross-structure and shows the presence of napal. In the sandstone limestone shows a quartz mineral content of up to 30%, large foraminifera, algae, Bryozoa and echinoids. This formation is deposited in shallow marine environments between 50 - 100 meters. The thickness of this formation reaches 248 meters. The Fur Formation is estimated to be the upper Middle Miocene.

4. Wonocolo Formation

The location of the Wonocolo Formation was not stated by Trooster, 1937, possibly from the village of Wonocolo, 20 km northeast of Cepu. The Wonocolo Formation is located in harmony above the Fur Formation, consisting of sandy marlene with calcarenite inserts and sometimes claystone. In the marl, it often shows a parallel laminate structure. The Wonocolo Formation is deposited in open sea conditions with a depth of between 100 - 500 meters. The thickness of this formation is between 89 meters and 339 meters. The Wonocolo Formation is estimated to be of the late Late Miocene to the late Middle Miocene.

GEOLOGICAL STRUCTURE

In the present (Neogen - Resen), a tectonic pattern that develops in Java and its surroundings, especially the North East Java Basin is a convergent zone, between the Eurasian plate and the Indian plate - Australia (Hamilton, 1979, Katili and Reinemund, 1984, Pulonggono, 1994).

Tectonic evolution in East Java can be followed starting from the Late Cretaceous (85-65 million years ago) to the present (Pulonggono, 1990). In summary, the East Java basin experiences two periods of time which cause the relative direction of the magmatic pathway or its tectonic pattern to change, namely in the Paleogene (Eocene-Oligocene) era, which is oriented towards the Northeast - Southwest (in the direction of the Meratus pattern). This pattern causes the North East Java Basin, which is a back arc basin, experiencing strain tectonic regimes as indicated by Pre-Tertiary bedrock lithology showing North-West-Southwest trending accretion pattern, which is indicated by fault fault orientation in bedrock, horst or faults factoring and graben or fault ladder. And in the Neogen era (Miocene - Pliocene) changed to relatively East - West (in line with the length of Java Island), which is a compression tectonic regime, resulting in fold geological structures, factoring faults and causing elevated North Java basins (Orogonesa Plio - Pleistocene) (Pulonggono, 1994). Especially in the North East Java Basin, data that supports both tectonic patterns can be seen from seismic data and from revealed structural data.

According to Van Bemmelen (1949), the North East Java Basin, namely the Kendeng Zone, the Rembang Zone - Madura, the Java Sea Exposure Zone and the Randublatung Depression Zone. The condition of folding structures in the northern part of the East Java Basin is generally West - East, while the fracture structure is generally in the Northeast - Southwest direction and there are several upward faults in the East - West direction.

 Regional structure of the East Java Basin

The Rembang - Madura mountain zone (Northern Java Hinge Belt) can be divided into 2 parts, namely the North (Northern Rembang Anticlinorium) and the South (Middle Rembang Anticlinorium).

The northern part has experienced lift which is stronger than in the south so that erosion occurs until the Tawun Formation, sometimes even down to the Lower Kujung. In the southern part of this area lies the Banyubang, Mojokerep and Ngrayong structures.

The southern part (Middle Rembang Anticlinorium) is marked by two positive paths that are clearly adjacent to Cepu. On the positive north, there are important oil fields in East Java, namely the fields: Kawengan, Ledok, Nglobo Semanggi, and also include the anticline - the Ngronggah, Banyuasin, Metes, Divinity and Tambakromo anticlines. In the southern positive lane there are anticlinal-anticlinal / Gabus, Trembes, Kluweh, Kedinding-Mundu, Balun, Tobo, Ngasem-Dander, and Ngimbang High structures.

Along the Rembang Zone path forms a folding structure that can be divided into 2 parts, namely:

1. The East, where the general direction of the anticline axis extends from the North West to the Southeast.
2. The western part, where each axis has a West-East direction and in general the anticlines subdue either westward or eastward.

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