VOLCANIC ROCKS

External Form, Structure, and Origin of Volcanic Mountains. — Cones and Craters. — Hypothesis of “Elevation Craters” considered. — Trap Rocks. — Name whence derived. — Minerals most abundant in Volcanic Rocks. — Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks. — Similar Minerals in Meteorites. — Theory of Isomorphism. — Basaltic Rocks. — Trachytic Rocks. — Special Forms of Structure. — The columnar and globular Forms. — Trap Dikes and Veins. — Alteration of Rocks by volcanic Dikes. — Conversion of Chalk into Marble. — Intrusion of Trap between Strata. — Relation of trappean Rocks to the Products of active Volcanoes.

The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. In the diagram (Fig. 584) suppose a, a to represent the crystalline formations, such as the granitic and metamorphic; b, b the fossiliferous strata; and c, c the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through a and b, sometimes overlying both, and occasionally alternating with the strata b, b.

Plume Tectonics dan Kisah Terdapatnya Intan

Dibawah ini ada sebuah tulisan menarik dari Pak Awang H Satyanatentang Plume Tectonic dan keterdapatan Intan.

Banyak yang berpikiran bahwa Intan yang isinya Carbon merupakan proses metamorfose dari batubara tingkat tinggi. Mengharapkan bahwa bila mendapatkan antracite nantinya akan ketemu juga intan karena intan merupakan proses kelanjutannya. Namun kenyataannya keterdapatan intan berasosiasi dengan intrusive breccia. Atau dalam kehidupan sehari-hari merupakan sebuah cerobong gunung api.

Bagaimana kisah keterdapatan intan ini ? Silahkan diresapi dibawah ini.

Plume Tectonic

(1) Plume tectonics sebagai mantle plume hypothesis telah dikemukakan pertama kalinya oleh Wilson (1963 - A possible origin of the Hawaiian islands: Can. J. Phys, 41, 863-870) dan Morgan (1971 - Convection plumes in the lower mantle: Nature, 230, 42-43) saat menjelaskan hotspot volcanoes seperti di Hawaii dan Iceland. Saat itu, mantle plume didefinisikan sebagai massa ringan (buoyant) material mantel yang naik karena keringanannya secara densitas (buoyancy). Saat mencapai litosfer, dikenal yang namanya plume heads dengan diameter 500–3000 km, dan plume tails yang diameternya 100–500 km yang masuk jauh ke mantel atas. Pentingnya peranan mantle plume, terutama superplume, dalam evolusi geodinamika Bumi, pertama kali diajukan oleh Maruyama (1994 - Plume tectonics: J. Geol.

Gunung Benau (Benau Mount), Type of Sedimentation and Lithology

Consist of interbedded marl and limestone with intercalation of fine grained sandstone conformably overly shoreface sandstone unit. Marl characteristic by grey, limey, frequent calsitic, rich concretions, and pyrite nodules, bioturbated, fossils, minor mica and carbon, concretion. The marl interbedded with light grey, hard, massive, calcite veined, micritic limestone. This well bedded micritic limestone indicate a platform environment. Some build up reef limestone and minor sandstone were also found in this unit.

There also reef limestone with a great amount of fossils including coral, foraminifera, and gastropod. The reef limestone growth in shallow marine environment and interacted with sea level change. Distribution of the reef limestone is not too far and just only found in several spot. It means that the reef can not grow optimally because the sea level change that makes reef can’t catch up or give up to the sea level. The lack of sunshine and nutrient or large influx of terigeneous clastic sediments could also disturbing the growth of reefal limestone.

The limestone is various. In some place, found grey, massive, micritic, very fine-fine grained (calsilutite-calcarenite), no fossil, calsitic, up to 1.5 metres and has a good thin and thick bedding with the marl. The others is fossiliferous limestone, whitish-yellow, texture compact, largely calsite, shells, gastropods, algae, coral. This unit deposited in shelf and shallow water environment. We can interpret the limestone that the massive, micritic limestone is deposited around the outer shelf, near reef, or barrier reef. The major frame building colonies, heads of algae, gastropods, poorly sorted calsirudite, shells, and very rich cavities, hollows is part of reefal (Biohermal Limestone) as a lense or boulder and surrounded by marl and the pelletal packestone, bioclastic wackestone, and limestone is only a local reef development, and growth after shoreface sandstone unit deposited because of drop sea level phase. This local boundstone with limey sand bodies is part of lagoonal or mid-shelf environment.

The middle section contains a tiny section of minor mica, very fine-grained sandstone that interbedded with limestone and marls. This sandstone indicates deltaic/tidal influence in the platform environment. Probably, this environment just occurred in a short time. The whole unit represents platform sedimentation with some localized reef. Sea level change and tectonic process play role during the deposition. The base of this unit is conformably overlies the shoreface unit .The upper contact is an uncomformity with volcanic rock sequence because airfall tuff overlies the marl-limestone unit.

Tectonic and Structure Geology of Sumatra Island

Sumatra Island is located in the path of volcano (NW-SE). Sumatra volcanic arc was formed by the meeting of two plates, the Indo-Australian plate which plunge down into Eurasian plate. The converging between the two plates as more detailed formed tectonic elements as follow:

• Active subduction zone, manifested by the Java-Sumatra Trench.

• Non-magmatic arc as accretionary wedge that formed island of Nias, Simeule Island, Mentawai Islands, etc..

• Fore arc basin, manifested by Sibolga Basin and Bengkulu Basin.

• Magmatic arc, indicated by the Barisan Mountains. Volcanoes located in the Barisan Mountains including Mount Merapi, Mount Kerinci, etc..

• Back arc basin, manifested by the Malacca Straits.

• Continental shelf of Sundaland.

Structure Map of Sumatra Island (Darman & Sidi, 2000)

Important symptoms that occur in Sumatra, in addition to that described above is the presence of horizontal Sumatra fault, known as the Sumatra Fault System (SFS) which divides the island of Sumatra, and following the path of the Barisan Mountains from Aceh to the Sunda Strait. There are two thoughts about SFS:

• Allegedly as a consequence of oblique subduction occurred in Sumatera (Katili, 1985).

• The movement was done by collision between India-Eurasia plate which extruded blocks of Southeast Asia toward Southeast (Tapponier, 1982).

In general, the process of Barisan Mountains uplifting began in Late Miocene, probably reached its peak at the boundary between the Miocene-Pliocene. This uplifting process is not consistently going on until now as estimated by recent geological features followed by the pattern of tectonics in the Early Pleistocene. Tectonic activity along the island formed massive geanticlines that causing the temperature rise related to rapid intrusion of accumulated magma underneath. It is characterized by increasing of both volcanic activity and lateral movement along Sumatra Fault System. All active tectonic activity over the Sumatra region is considered as the main source of recent earthquakes.

Things are not what they seem in Indonesia

Peter Baillie, TGS-NOPEC Geophysical Co.

Sumatra forearc is unprospective; there is not sufficient sedimentary section in Bone Bay to have generated hydrocarbons; Cendarawasih Bay is underlain by oceanic crust and is not prospective. If you answered “true” to all of these, many experts would agree that you are correct. Until relatively recently, all of these statements were believed to be true.

In fact, there are many widely held ideas about the geological structure underlying Indonesia’s deepwater basins that are now being turned on their heads in the face of new geophysical and geological evidence. In a recent mega-study of Indonesia’s deepwater basins, evidence was found indicating that the Sumatra forearc may not be a forearc at all, Bone Bay has a potentially prolific (and previously unidentified) synrift section, and Cenderawasih Bay is underlain by something quite different than oceanic crust.

These are just a few examples demonstrating that the geology and petroleum potential of Indonesia’s deepwater basins are very poorly understood and that many preconceptions about prospectivity are based on lack of data rather than rational analysis. TGSNOPEC Geophysical Co. (TGS) and the Indonesian Ministry of Energy and Mineral Resources (MIGAS) are bringing new insights into the realities of these areas with the IndoDeep exploration program, a megastudy over Indonesia’s deepwater basins covering nearly 610,000 sq miles (1 million sq km) of the region. The IndoDeep program integrates 2-D seismic data and gravity and magnetic data with core data and multibeam sonar technology that can help detect oil seeps and other active geological processes on the ocean floor.

TGS began acquisition of data for the IndoDeep program in late 2006 and completed the project in early 2008. The dataset includes 21,924 line miles (36,000 line km) of high-resolution, long-offset 2-D seismic data; 154,440 sq miles (400,000 sq km) of multibeam sonar (bathymetry and backscatter) data; 85,260 miles (140,000 km) of gravity and magnetic data; 50 heat flow probes; and the collection and analysis of 1,150 core samples. Now that the data have been acquired and the study is underway, theories such as the previously held notions about Sumatra, Bone Bay, and Cenderawasih are being challenged as the data reveals new clues about the geological evolution and prospectivity of the archipelago.

Theories thwarted

The IndoDeep program has already challenged some of the widely held beliefs about several of the deepwater regions.

One example is the Sumatra “forearc” area west of the island of Sumatra. The study indicates that the very notion that this represents a forearc area may not be accurate. The results of the seismic, multibeam, and coring surveys indicate that the oblique collision of the Australo-India and Eurasian plates offshore of Sumatra has resulted in the formation of a series of wrench basins. In addition, a synrift section overlain by Miocene and Pliocene carbonates has been identified on the seismic data. This is significant because a similar synrift section has generated billions of barrels of oil in nearby onshore areas and throughout much of southeast Asia. Furthermore, core analysis indicates the presence of thermogenic hydrocarbons in the study area. An additional 5,664 miles (9,300 km) of 2-D seismic data is currently being acquired in the Sumatra region.

Another region that has been shown to be geologically interesting is Cendera-wasih Bay. An emergent and previously unknown fold-and-thrust belt has been found on the eastern side of the bay. In addition, a significant sedimentary section was found underlying a substrate of sedimentary and metamorphic rocks. These rocks include reefs that are apparently similar to producing reefs in western parts of the island of New Guinea.

 Potential proven

A similar synrift trend was found in Bone Bay, which was once believed to have neither significant sedimentary section nor hydrocarbons.

In addition to the spectacular results from the seismic data, the coring geochemistry was an unprecedented success. All areas showed the presence of thermogenic hydrocarbons, with 46% of cores returning thermogenic gas hits and 13% showing oil seeps. These results are double the world average for this type of survey, and that success is attributed to a combination of factors. First of all, the targets were found using backscatter, or the intensity of the returning sonar “ping” which gives an indication of the hardness or lithology of the seabed. Other key components of the project’s success include the direct navigation of the piston corer onto the target and the active tectonics of the Indonesian archipelago.

Another interesting result of the project is the repeated evidence of gas hydrates in the region. The data acquired over the Sunda area between the islands of Java and Sumatra has revealed the existence of significant amounts of gas hydrates, which may well be an energy source for the distant future. Other instances of gas hydrates were found in the Makassar Straits between the islands of Borneo and Sulawesi, in eastern Indonesia in the Seram Trough near the island of Waiego and the westernmost end of New Guinea.

IndoDeep insight

Indonesia is still a significant hydrocarbon producer with more than 950,000 boe/d from eight basins. Output, however, has been declining at an average annual rate of 5%, and potentially large stores of hydrocarbons remain to be discovered. Up until now, the 30 frontier sedimentary basins were lacking modern geophysical data to indicate their potential.

By combining modern geophysical data with geochemistry from carefully mapped oil and gas seeps, the IndoDeep project could effect a change in the speed and efficiency with which exploration companies find hydrocarbons in frontier basins and assess their commercial potential.

The bathymetric data generate spectacularly beautiful images of the seafloor and illustrate active geological processes. The combination of multibeam bathymetry and 2-D seismic data provides a virtual 3-D image to help unravel the geological history of these very complicated areas while costing significantly less than 3-D. A deepwater basin can now be fast-tracked from an area of little or no exploration interest to a hydrocarbon exploration hot spot.

As anticipated, the IndoDeep program is generating a huge amount of new data and fresh ideas about Indonesia’s geology. Early indications are that the complete dataset will challenge previously held concepts of the hydrocarbon potential of Indonesia’s frontier basins, which will prove valuable to explorers and the people of Indonesia for many years to come.

Deadly Java Quake Highlights "Ring of Fire" Dangers

Richard A. Lovett
for National Geographic News

The magnitude-6.3 earthquake that shook the Indonesian island of Java this weekend has so far killed about 5,700 people and left more than 100,000 homeless.

The devastating quake is at least the fourth geological disaster to strike Indonesia in 18 months (Indonesia map, music, and profile), highlighting the constant threat faced by residents of the Pacific Ocean zone known as the Ring of Fire. This tectonically active region rims the Pacific and includes Mount St. Helens, now erupting in Washington State.

The event also puts a spotlight on scientists' efforts to understand how and why quakes can cause different kinds and levels of damage.
On December 26, 2004, the Indonesian island of Sumatra was hit by the largest earthquake the world had seen in 40 years—a magnitude 9.3.
The massive quake triggered a tsunami that left about 131,000 dead in Indonesia alone (news reports: "Tsunami in Southeast Asia: Full Coverage").
That event was followed on March 28, 2005, by another enormous earthquake, with a magnitude of 8.7, sparking fears of a second tsunami. But the deadly wave didn't materialize, and the death toll was about 320.
More recently one of Indonesia's many volcanoes, Mount Merapi, began erupting only a few dozen miles from the epicenter of this weekend's quake (related photo: Volcano Eruption Countered With Dance, Food).
The latest quake occurred on Saturday at 5:54 a.m. local time and had a magnitude of 6.3, the U.S. Geological Survey (USGS) reported on its Web site.
 

Same Plates, Different Results

Earthquakes and volcanoes are produced by the forces of plate tectonics, which cause the vast plates that form the Earth's crust to slowly but steadily collide.
Just south of Java, for example, the Australian plate is moving northward at about two and a half inches (six centimeters) a year.
When the Australian plate collides with the Sunda plate, which includes Java, the Australian plate is forced beneath the Sunda plate in a process called subduction.

Earthquakes result when the subducting crust gets stuck, then lurches back into motion. Volcanoes are formed when subducted rock melts and returns to the surface as magma. (Learn more about how and why earthquakes happen.) The large Indonesia earthquakes of December 2004 and March 2005 were caused by a similar plate collision off the island of Sumatra.

But the Java earthquake has some important differences, Mark Leonard, senior seismologist with the government organization Geoscience Australia in Canberra, wrote in an email.
Not only was the epicenter of the Java quake several hundred miles away from Sumatra, but the motion was of a type geologists refer to as strike-slip, Leonard says.
Strike-slip earthquakes involve sideways motion along a fault.
The 2004 and 2005 earthquakes were "thrust" earthquakes, in which the two sides of the fault were rammed more directly toward each other.
Also, Leonard says, this weekend's earthquake originated only 18.6 miles (30 kilometers) beneath the surface.

Usually subduction-zone earthquakes in this region occur 40 to 60 miles (70 to 100 kilometers) deep.
"This all suggests that this earthquake was possibly not on the main subduction zone," he wrote, "but on a shallower [unmapped] strike-slip fault [in the overlying crust]."
"I am speculating," he added, "but if this is the case, [the shallow nature of the quake] would explain why the damage is greater than for other magnitude 6.0 to 6.5 earthquakes in the last couple of decades."
 

Shaken Temples

The damage has been significant, with many villages destroyed and two of Indonesia's major cultural sites affected.
Borobudur, the largest Buddhist temple on Earth (see photo), was built 1,200 years ago about 25 miles (40 kilometers) north of the royal capital of Yogyakarta (pronounced JOG-jakarta), the major city in the affected region.

The temple lay abandoned for centuries before being rediscovered and restored in the early 1900s. The recent quake damaged the structure, but it appears to be mostly intact.
Prambanan, the oldest Hindu temple in Indonesia, was not so fortunate. This complex, built about 1,150 years ago, appears to have taken significant damage, according to newspaper reports.
The epicenter for Saturday's earthquake is also only a few dozen miles from the erupting Merapi volcano. But this doesn't mean that the eruption triggered the quake.
In a statement posted to its Web site, the U.S. Geological Survey notes that magma movements at volcanoes can produce shallow earthquakes.

"In the cases of many earthquakes that occur in the general vicinity of volcanoes, however, there are not obvious links to volcanic eruptions," the agency said.
Leonard, of Geoscience Australia, was more certain.
Though Merapi and the recent earthquake were produced by the same large-scale tectonic forces—the slow collision of the Sunda and Australian plates.
The fact that the quake occurred during the eruption "is a coincidence," he wrote.

Pacific Ring of Fire

The Ring of Fire is a zone of frequent earthquakes and volcanic eruptions that encircles the basin of the Pacific Ocean. It is shaped like a horseshoe and it is 40,000 km long. It is associated with a nearly continuous series of oceanic trenches, island arcs, and volcanic mountain ranges and/or plate movements. It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt.
90% of the world's earthquakes and 81% of the world's largest earthquakes occur along the Ring of Fire. The next most seismic region (5­6% of earthquakes and 17% of the world's largest earthquakes) is the Alpide belt which extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. The Mid-Atlantic Ridge is the third most prominent earthquake belt.
The Ring of Fire is a direct consequence of plate tectonics and the movement and collisions of crustal plates.

The eastern section of the ring is the result of the Nazca Plate and the Cocos Plate being subducted beneath the westward moving South American Plate.
A portion of the Pacific Plate along with the small Juan de Fuca Plate are being subducted beneath the North American Plate. Along the northern portion the northwestward moving Pacific plate is being subducted beneath the Aleutian Islands arc.

Further west the Pacific plate is being subducted along the Kamchatka - Kurile Islands arcs on south past Japan. The southern portion is more complex with a number of smaller tectonic plates in collision with the Pacific plate from the Mariana Islands, the Philippines, Bougainville, Tonga, and New Zealand.
Indonesia lies between the Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor.
The December 2004 earthquake just off the coast of Sumatra was actually a part of the Alpide belt. The famous and very active San Andreas Fault zone of California is a transform fault which offsets a portion of the East Pacific Rise under southwestern United States and Mexico. The motion of the fault generates numerous small earthquakes, at multiple times a day, most of which are too small to be felt.
Major volcanic areas in the Ring of Fire

• In South America the Nazca plate is colliding with the South American plate. This has created the Andes and volcanoes such as Cotopaxi and Azul.
• In Central America, the tiny Cocos plate is crashing into the North American plate and is therefore responsible for the Mexican volcanoes of Popocatepetl and Paricutun (which rose up from a cornfield in 1943 and became a instant mountains).
• Between Northern California and British Columbia, the Pacific, Juan de Fuca, and Gorda plates have built the Cascades and the infamous Mount Saint Helens, which erupted in 1980.
• Alaska's Aleutian Islands are growing as the Pacific plate hits the North American plate. The deep Aleutian Trench has been created at the subduction zone with a maximum depth of 25,194 feet (7679 meters).
• From Russia's Kamchatka Peninsula to Japan, the subduction of the Pacific plate under the Eurasian plate is responsible for Japanese islands and volcanoes (such as Mt. Fuji).
• The final section of the Ring of Fire exists where the Indo-Australian plate subducts under the Pacific plate and has created volcanoes in the New Guinea and Micronesian areas. Near New Zealand, the Pacific Plate slides under the Indo-Australian plate.

The Geology Java Island

Java, with a backbone comprising a subduction-induced volcano-plutonic arc, is considered classically as the southernmost leading edge of the continental Sunda Plate, overriding the oceanic Australia-Indian plate। In fact, the structural configuration is that of alternating highs and transverse depressions related to a more complex pattern, where discrete crustal blocks can be interpreted as pieces separated from the original monolithic

Fig। 1( taken from book An Outline of The Geology of Indonesia)

 

Two dynamic processes interact: • Collision of blocks in Pre-Tertiary times by closing of oceanic gaps is recorded or marked by roughly east-west ophiolitic belts (Ciletuh in West Java, Lok Ulo in Central Java) but the colliding pieces are not clearly identified. • Lateral displacement between blocks in Tertiary times is made by transcurrent faulting, components of large-scale strike-slip movement in response to the plate-convergence process itself.
Those mechanisms are part of extensional and convergent global geotectonic events to which are related platform, fore-and back-arc basin sedimentation, and occurrence of volcanism. Offshore North Java, some extensional, half-graben and graben-like, transverse depressions, which are among the richest oil-provinces in the country (Sunda Basin, Arjuna Depression), locally extend to the land area where they merge into east-west back-arc basins. 

 

The Java Island and the adjacent Java Sea is divided into two major provinces West and East Java. The dividing line between these two areas is chosen as a meridian-line, roughly joining the Karimun-Jawa Islands to Semarang continuing southwards on land (Fig. 1).

WEST JAVA TECTONIC SETTINGThe West Java region currently marks the transition between frontal subduction beneath Sumatra, to the west. However, the region has been continuously active tectonically since rifting in the Eocene. The Eocene rifting, as throughout SE Asia, was probably related to the collision between India and Asia (e.g. Tapponier et al. 1986) and involved a significant influx of coarse clastic sediments. The Oligocene-Recent history is more dominated by subduction-related volcanism and limestone deposition. In general, West Java may be subdivided into the following tectonic provinces: (see Figure 4.2; modified after Martodjojo, 1975; Lemigas, 1975, and Keetley et al, 1997) • Northern basinal area: A relatively stable platform area, part of the Sundaland Continent, with N-S trending rift basins offshore and adjacent onshore, filled with Eocene-Oligocene non-marine clastics, overlain by Miocene and younger shallow shelf deposits. • Bogor Trough foreland basins composed of Miocene and younger sediments mostly deeper water sediment gravity flow facies. Young E-W trending anticlines formed during a recent episode of north-directed compressive structuring; • Modern Volcanic Arc: Active andesitic volcanism related to subduction of Indian Oceanic Plate below Sundaland Continent (Gede-Panggrango, Salak, Halimun, etc., volcanoes). • Southern slope regional uplift: mainly Eocene-Miocene sediments, including volcanic rocks belonging to the Old Andesite Formation. Structurally complex, N-S trending block faults, E-W trending thrust faults and anticlines and possible wrench tectonism. South-West Java contains a number of sedimentary basins that formed within the axial ridge and in the area between the volcanic arc and submerged accretionary prism associated with the northward subduction of the Indian Oceanic Plate. • Banten Block: The most western part of Java Island which may be subdivided into Seribu Carbonate Platform in the north, Rangkas Bitung sedimentary sub-basin, and Bayah High in the south. In the west there are minor low and highs so called Ujung Kulon and Honje High, and Ujung Kulon and West Malingping Low (Lemigas, 1975; Keetley et al, 1997).

Fig 2. Summary of west Java tectonic map ( from different sources)
 
NORTHWESTERN BASINAL AREA TECTONIC FRAMEWORK
The Northern offshore and adjacent onshore basinal area comprises two major basins so called North West Java Basin and Sunda-Asri Basinal area (Fig.3). The northern part of this area is dominated by extensional faulting with very minimum compressional structuring. The basins were dominated by rift related fault which contain several depocentres. In the NW Java Basin the main depocentres are called the Arjuna Basin North, Central and South and the Jatibarang Sub-basin. The depocentres are dominantly filled with Tertiary sequence with thickness in excess of 5,500 meters. The significant structures observed in the northern basinal area consist of various type of high trend area associated with faulted anticline and horst block, folding on the downthrown side of the major faults, keystone folding and drape over basement highs. Rotational fault blocks were also observed in several areas. The compressional structuring were only observed in the early NW-SE rift faults. These faults were reactivated during Oligocene time forming several series of downthrown structure associated with transpresional faulting in the Sunda area.

Although the Northwest Java basin area is currently positioned in a back arc setting, the West Java Sea rift systems did not form as back-arc basins. Extension direction fault patterns and basin orientation of the Northwest Java basins suggest that the sub-basinal areas are pull-apart basins at the southern terminus of a large, regional, dextral strike-slip system; i.e. the Malacca and Semangko fault zones propagating down to the west flank of the Sunda craton. Through both Eocene-Oligocene rift phases, the primary extension directions were NE-SW to E-W. Two observations support the interpretations that these basins are not back-arc related; 1) the extension direction for the WJS rifts is nearly perpendicular to the present subduction zone, 2) a thick continental crust is involved (Hamilton, 1979).

The NW Java depression is asymmetrical, with its deepest Arjuna Sub-basin lies at the foot of the Arjuna Plateau, separated by a major N-S trending fault. The basin opens southward into the onshore Ciputat, Pasir Putih and Jatibarang Sub-basins, separated by the Rengasdengklok and Kandanghaur – Gantar Highs, respectively. The sub-basins are characterised by the presence of alternating highs and lows bounded by extensional deep-seated faults which were active during sedimentation.

The Jatibarang Sub-basin( fig. 3) is bounded by the Kandanghaur - Gantar- horst-block to the west, and the Cirebon fault, east and north-eastwards. This major growth-fault is responsible for an important accumulation of Tertiary rocks including the Jatibarang volcanics, in the Jatibarang Sub-basin.
The Vera Sub-basin is a deep Mesozoic and Tertiary depression NE of Arjuna Sub-basin. This sub-basin is bounded by some major faults, especially to the south. The structures orientation is SW and SSW, similar to the direction of the Billiton Basin where Mesozoic (?) sediments are also known.

The Sunda-Asri basinal area consists of Sunda and Asri basin. This structural element is the westernmost basin of the northern basinal area of West Java. The Sunda Basin is a roughly northsouth depression with its main depocenter, the Seribu half graben, at its eastern edge, separated from the Seribu platform by steep flexures and faults. To the west, the basin is bounded by the Lampung High, to the south by the Honje High and to the north the Xenia arch separates the Sunda Basin from the Asri Basin. The Sunda Basin is the deepest basin in the northern basinal area of Java, where the basement is more than 3.8 second TWT, in the downthrown block of the Sunda/Seribu fault. A series of normal faults dissect the area in small horst and graben features.

The Asri Basin, located to the northeast of the Sunda Basin, is the second deep basin in the region with basement as deep as 3.0 sec. TWT. It is limited from the Sunda platform eastwards by a major normal fault. To the northwards and westwards, it is bordered by steep gradients and is dissected by normal faults.


Fig. 3. Half graben sub basin/depocenters within the Sunda, Asri
and NW Java BAsin areas ( Kohar et al, 1996)