REGIONAL SETTING: DIAPIRS AND MUD VOLCANOES OF JAVA

Mud diapirs and mud volcanoes are found along the depressions of Java to Madura in Bogor-North Serayu-Kendeng-Strait of Madura Zone Several of them are: Ciuyah in Kuningan area, North Serayu, Bledug Kuwu & nearby mud volcanoes, Sangiran Dome, LUSI, Porong, Pulungan, Gunung Anyar, Kalang Anyar and submarine mud diapirs and mud volcanoes beneath the Madura Strait.

Ciuyah Mud Volcano, Kuningan Area

Ciuyah means salted water. Ciuyah is located 5 kms to the south of Kuningan area in the district of Ciniru Geologically, it is the eastern part of the Bogor Depression. Here, hot salted mud and water flows continuously from several springs. The local people called the springs as Ciuyahleutik and Ciuyahgede, relative to the size of water flow coverage (“leutik” is small, “gede” is big). As water evaporated, salt crystalls are deposited. The Ciuyah complex of salted mud flow and hot water springs is considered as mud volcano of post-eruptive phase or called as dormant and “gryphon-salsa” periods (Akhmanov and Mazzini, 2007). It represents the stage-4 mud volcano due to decrease in subsurface pressures, indicated by partial subsidence beneath the mud volcanoes. The surface morphology is a combination of swamp-like area and crater muddy lake.

The eastern part of the Bogor depression is slightly convex to the north, with intensive folding and northward upthrusts. The core of the anticlinorium consists of Miocene strata and its flanks are formed by Pliocene and Lower Pliocene deposits. The total thickness of the Neogene sediments in this depression is more than 6000 meters (van Bemmelen, 1949). The sediments are dominated by turbiditic volcanic-clastic sediments rapidly deposited into the depression (Miocene-Pliocene aged Rambatan, Halang, Pemali, Kumbang formations). To the northwest of Ciuyah area is Quaternary Mount Ciremai magmatic volcano. The north and west flanks of the volcano are slightly bulged due to draping overlying the diapirs made of plastic Pemali clay-shales and Early Pliocene plastic Kaliwangu clay marls.

In the post-eruptive phase of mud volcano, fluid migration through more permeable zones in the channel constructs small seeping features at the mud volcano surface. Sealed pathways to the surface turn the system back from open to closed physicochemical condition. This phase follows an eruptive phase and starts preparation for new paroxysm. In closed environment the elisional processes regenerate high water-saturation and overpressure of deep strata. Delicate equilibrium can be broken and new eruptive phase can be triggered by earthquake or tectonic reactivation, being followed with new cycle.

North Serayu Diapirs

North Serayu Depression is an eastward continuation of the Bogor Depression. It has been uplifted to become the North Serayu Anticlinorium/ Range. Geology of the North Serayu Range records the history of basin subsidence with deepwater sedimentation and ended with uplift (Satyana and Armandita, 2004).

The Eocene Worowari siliciclastic beds are the oldest sedimentary rocks in this area. These were transgressively covered by Early Miocene coarse conglomerates and quartzitic sandstones of the Lutut Beds and reef limestones called Sigugur Beds. Significant subsidence of the North Serayu Basin started thereafter. Thick succession of the turbiditic deposits of the Early to Middle Miocene marly clays, quartz sandstones and tuff-sandstones were deposited rapidly. This flysch-like series comprise the Merawu and Penyatan formations in the central and eastern section, and Pemali Formation in the western section of the North Serayu Depression.

The uplift of the southern Central Java in the Middle-Late Miocene was volumetrically compensated by a sudden increase of subsidence of the floor of the North Serayu Depression (van Bemmelen, 1949; Satyana and Armandita, 2004; Satyana, 2007). This sudden increase of orogenic relief has not only caused the gravitational sliding movements from south to north, but caused also that portions of the northern flank of the basin slid down toward the deepest part. The Mio-Pliocene succession of strata, filling the trough of the North Serayu Zone, began with volcanic deposits, alternated with conglomerates, and ended with the soft clay marls and tuffaceous sandstones of the Kalibiuk Formation.

Gravitational sliding movements from south to north in the North Serayu Depression and rapid deposition of volcanic-clastic sediments during Mio-Pliocene coeval with compressional tectonics had resulted in formation of diapirs in this area. Seismic data acquired to the south of Tegal (Figures 4, 7) show the presences of these diapirs (called as “Tegal disturb zone” by Kartanegara et al., 1987). The presences of these diapirs may affect the formation of folds and thrusts in this area. These diapirs represent stage-2 of the development of diapir to mud volcano.

The North Serayu Zone is a good example of development of a depressional trough into a fold and thrust belt. The elevation of the depressional deposits caused gravitational reactions and the crest of the fold and thrust belt was pierced by diapirs and also volcanic magma. Gravitational tectogenesis interrupted the subsidence of the North Serayu depression at the end of Miocene by a revolutionary phase of gravitational tectogenesis. It is possible that this gravitational settling was promoted by an impulse of uplift of the adjacent belt (South Serayu Range), and concomitant further subsidence of the depression. This tectogenesis gave rise to the overthrusts converging towards the axis of subsiding trough. The plastic Merawu flysch pierced as diapirs and were intensively folded. At the end of the Neogene, the subsidence of depression came to an end. The Neogene formations, thousands of meters thick and partly consisting of plastic, unconsolidated clays and marls, were arched up by diapirism.

Bledug Kuwu Mud Volcano Complex

“Bledug” means the sound like cannon-fire. Complex of Bledug Kuwu mud volcanoes are located to the east of Semarang city and 20 kms to south of Purwodadi town, Central Java (Figures 4, 6, 7). Geologically, they are located at the boundary area between North Serayu and Kendeng Depressions.

There are some mud volcanoes in this area (Bledug Kuwu, Bledug Kesongo, Bledug Kropak, and several others). Kuwu is the largest of a number of active mud volcanoes in the area. The morphology is a set of vents in a flat area of quick-sand like clay, with a dried mud crust. Eruptions vary in frequency but generally occur more than once a minute, as a burst of warm gas spraying mud in all directions. It is assumed that mud flows more slowly and gradually, but is kept liquefied by the escaping fluids and gases (Burgon et al., 2002).

The morphology type is a combination of swamp-like area extending for large territory and crater muddy lake. The phase of mud volcano is eruptive to post-eruptive (stage 3 to 4). The presence of quick-sand indicates the subsidence of mud volcano area. The Kuwu mud volcano cluster covers about 45 hectares. The biggest vent can erupt materials as high as 5.3 meters with expelled mud reaching out for a diameter of about 9 meters. At the main Kuwu site the big mud volcano usually erupts four or five times a minute.

The salinity and turbidity of the mud volcanoes mean that there is no vegetation or animal life in the area of the eruptions. The gas expelled is non­flammable, reported as CO2, with traces of H2S. Local people commercially process the expelled water to extract salt for cooking. It is also reported that the mud sometimes carries rock fragments and fossils, but no details are available. Samples of the mud taken by Burgon et al. (2002) proved to be barren of any calcareous micro or nannofossils suggesting mildly acidic conditions.

The temperature of the big mud volcano ranges from 28-30ºC, while the small mud volcano is reported to have a temperature of only 15-16ºC, a surprising feature hard to explain or verify. Small mud volcano has more water content (lower density), which might account for its more frequent eruptions and lower temperature.

Seismic sections across these mud volcanoes show disturbed zones from top of the Kujung Formation up section through the top of Wonocolo Formation to the surface. Bledug Kesongo is obviously characterized by collapse structure with upward concave horizons along the disturbed zone indicating the stage-4 of mud volcano subsidence. The Bledug Kuwu disturbed zone is chaotic mixture of upward convex and concave reflectors indicating stage-3 to stage-4. The mother beds of diapirs and mud volcanoes are considered the lower part of Late Miocene Wonocolo shales. This is confirmed by fossil content of mud materials. However, seismic sections show that the source of mud may also come from Early Miocene Tuban shales. Some diapirs also occur in this area, they are generally below the top of Wonocolo Formation. Folds in this area are considered to form relating to underlying diapirs as suggested by some seismic sections (Figure 7).

Sangiran Dome

Sangiran Dome is located 12 kms to the north of Surakarta/ Solo city (Central Java) (Figures 4, 7). Geologically, it is located between the Zone of Central Depression and the southern border of the Kendeng Zone (van Bemmelen,1949). The Solo Zone is a generally low, flat area in which the oldest outcropping beds are Late Pliocene (Sangiran Dome). Most of the zone is covered by young volcanics. Subsurface geology is poorly known. The thickness of the sedimentary section may reach 9000 meters (Kadar et al., 1989). Almost all modern (Pleistocene-Recent) subduction-related andesitic volcanoes are situated in this zone. Average height of the major peaks is 3000 meters.

The Sangiran Dome is a unique feature in Javanese geology. It is set on the southern margin of an area of intense folding and faulting (the Kendeng Zone) where structures are dominantly east-west in orientation and elongation, yet it is a dome with, if anything, slightly northern elongation (Lunt et al., 1989).

The hilly area around Sangiran exhibits some excellent outcrops of the Late Pliocene and Pleistocene of the so- called Central Depression (Solo zone, Ngawi Subzone) of Java. Good examples of young shallow marine, lacustrine, fluvial and volcanic deposits like lahars and tuffs can be studied. Other features of geological interest are its structural expression, mud volcanoes (with exotic blocks of Miocene, Eocene and basement rock). The area has been famous since early of 1900s for its rich hominid and vertebrate fossil faunas. The Sangiran location was the second site where Homo erectus fossils were found in the eastern Java, after the river section at Trinil, 50 kilometers to the east. It remains one of the most active sites for Plio-Pleistocene research and is also an excellent location to study fluvial and volcanic sedimentation.

Structurally the area is a dome, somewhat elongated in NNE-SSW direction. Structuring is very young

(0.5 Ma or younger – Kadar et al., 1989 ). Van Bemmelen (1949) considered its origin as compressive related to volcano-tectonic collapse of the Old Lawu volcano; other opinions are diapiric shale flow, a wrench-related fold, an incipient volcano, or due to a basement related fault. Itihara in Watanabe & Kadar (1985) explained that Sangiran Dome related to basement involved fault, perhaps with a north - south orientation, occurred first and produced a fold perpendicular to the regional orientation. This fault acted as a focus for overpressured Early Miocene muds which could then rise and, along with overthrusted, fractured rock from the fault plane, escape to the surface as mud volcano. The dome shape and presence of several small saline seeps may confirm that the Sangiran Dome is an extinct mud volcano (Lunt et al., 1998). All exotic blocks are thought to have come from subsurface as a result of the upward piercement.

The oldest beds in the center of the dome are relatively soft clays and now form a topographic depression, surrounded by a ring of hills composed of the harder sands and conglomerates of the Kabuh and Notopuro Formations. The oldest beds are a shallowing-upward marine sequence (Kalibeng Formation, Late Pliocene), successively overlain by brackish bay to fresh water lacustrine black clays (Pucangan Formation), fluviatile sandy beds (Kabuh Formation) and "cold lahars" (Notopuro Formation). Thin tuff beds are found throughout the section. Reworked Pliocene deep-water marine microfauna are locally abundant in the non-marine Pucangan and Kabuh Formations, probably Kendeng zone erosional products. The andesitic volcanic clasts in the Kabuh and Notopuro Formations must have had a southern origin.

Based on the schematic profile of the Sangiran Dome mud volcano of Watanabe and Kadar (1985), the mud volcano morphology type of the Sangiran Dome is collapsed synclinal depression. The development of the mud volcano is stage-4 showing some subsidence underlying the mud volcano.

LUSI and Other Mud Volcanoe

An unexpected eruption of mud and fluids took place on 29^th May 2006 in the Porong area, Sidoarjo, East Java (Figures 4, 6, 7). The eruption site was named LUSI (abbreviation of “Lumpur Sidoarjo” or Sidoarjo mud). Based on the characteristics of eruption and related geologic data, LUSI was concluded as eruption of mud volcano. The eruption is still taking place at the time of this writing (March 2008). The volumes of erupted mud increased from the initial 5000 m³/day in early stage to 120,000 m³/day in August 2006 and peaked to 170,000 m³/day in September 2006 and reached the record-high level of 180,000 m³/day in December 2006 (Mazzini et al., 2007). LUSI was still active in December 2007 expelling more than 80,000 m³/day. As of December 2007 the total volume of expelled mud was estimated at 1 billion cubic feet, covering an area of 2.5 square miles, burying eleven villages and displacing at least 16,000 people. Transportation and power transmission infrastructure has been damaged extensively in the area. It is expected that the mud eruption will last for years to come and the area will experience a significant depression, forming a large caldera.

LUSI is one of numerous mud volcanoes in the eastern part of the Kendeng Zone/Depression. Mud volcanoes occur at many locations in the Kendeng Zone from Bledug Kuwu at the western border of the zone to the submarine mud volcanoes in the Madura Strait. Other recognized mud volcanoes close to LUSI (variably active, extinct, or dormant) are Porong, Kalang Anyar (Sidoarjo), Gunung Anyar (near Surabaya), Socah (Bangkalan, Madura), Wringin Anom (border of Gresik-Mojokerto), Semolowaru, Pulungan, and Sedati (Sidoarjo). Based on historical chronicles, folklore, and geologic data, recent paper by Satyana (2007) elaborated the presence of mud volcanoes complex erupted in historical time during the periods of Jenggala and Majapahit Kingdom in Indonesia (12^th-15^th century). He indicated the presence of mud volcanoes in this period in a zone called Tunggorono – Jombatan – Segunung – Canggu - Bangsal in Jombang-Tarik, mostly in Sidoarjo area, as long as 25 kms.

The Kendeng Zone is one of the youngest tectonic features in the eastern Java area. It was formed virtually at the early to late Pliocene (Lunt et al., 1996). It was part of a continuously subsiding basin from Miocene to the end of the middle Pleistocene. Late Miocene and older sediments in the Kendeng Zone are typically thick, interbedded mudstones and volcaniclastic sands. The marls and limestones of the Lower and Upper Kalibeng Formation were deposited in almost entirely marine environment during the Pliocene. At the time, volcanic activity probably began in the volcanic arc to the south (Wilis and Lawu volcanoes). This activity influenced the western part of East Kendeng Zone at the beginning of the Pleistocene. Here early Pleistocene Pucangan volcaniclastic sediments conformably above the Upper Kalibeng. Above this in conformable contact occur volcanic sandstones of the middle Pleistocene Kabuh and then the Notopuro Formation. During the early Pleistocene, marine blue clays of Pucangan were deposited in the east where finally volcanic deposits prograded (Duyfjes, 1936). The volcanic material was initially deposited in a marine environment in the west, but filled the basin very fast, so that sediment input near the volcanic centers soon exceeded the accommodation. In the east, where volcanic sediments arrived later, the volcaniclastic input was not sufficient to fill the basin until the late Pliocene.

The gravity data of the Kendeng Zone shows strongly negative anomalies indicating considerable depth to the basement in the Kendeng Zone. The Kendeng Zone is the deepest part of the Java’s depression from Bogor-North Serayu-Kendeng-the Madura Strait. The Kendeng Zone is strongly folded and sometimes heavily faulted in the western part. Structuring is very recent and is probably still active. The sudden change of the Brantas River in the time of King Airlangga (11^th century) was due to the Recent deformation of fold underlying the river (Satyana, 2007). Fold axes in this area are oriented in E-W direction; an indicator that the adjacent and parallel volcanic chain is, at least in part, is responsible for compression. In the east, south of Surabaya where numerous mud volcanoes occur, the folds are nearly lost under recent alluvium and even Pleistocene rarely crops out.

The Kendeng Depression/Zone is the best elisional basin in Indonesia therefore, numerous mud diapirs and mud volcanoes occur here. Young tectonic feature, subsided basin, compressed, very thick young sediments deposited rapidly in relatively short period, and thermally significant due to nearby volcanic arc make the Kendeng Depression to be elisional. High sedimentation rate initiated during Upper Miocene – Early Pliocene time causing deposition of a very thick highly overpressured sedimentary succession. Clayey and silty sediments interbedded with sand beds contain great amount of fluids. The presences of overpressured sediments, less dense plastic shale succession underlying more dense beds and saturated with the fluids, and high tectonic activity favors the mud diapirs and mud volcanoes development in the region. Mud diapirism and mud volcanism apparently plays important role in the regional geology of the area. They also critical in the formation of folds at shallow depth. Seismic sections show that many folds in the Kendeng Zone are cored by diapirs.

Presently erupting-LUSI mud volcano provides good opportunity to know the origin of mud volcanoes in the Kendeng Depression. All other active, extinct, or dormant mud volcanoes in the Kendeng Depression might occurred several hundreds, thousands, or several million years ago. Historical chronicles (Kitab Pararaton – Book of Kings) during the Majapahit Kingdom from 13^th to 15^th century show the occurrences of natural disasters may be interpreted as mud volcanoes eruption (Satyana, 2007). The main Kendeng Depression has existed since 5 Ma, it has been compressed since then. It can be expected that mud volcanoes in the Kendeng Depression were triggered by tectonics and seismicity as most mud volcanoes in the world originated. LUSI mud volcano, began erupting on 29 May 2006 however, can not provide straightforward explanation on its origin due to possibility that LUSI may relate to drilling of exploration well located 200 meters away from LUSI.

The origin of LUSI mud volcano has been a matter of debate. Three trigger mechanisms have been proposed : (1) tectonic re-activation by the May 27^th 2006’s Yogyakarta earthquake, (2) well drilling operations (Banjar Panji-1 well by Lapindo Brantas) in progress near the initial eruption site at the time of the eruption, and (3) a combination of earthquake and drilling operations. Earthquake trigger was argued among others by Mazzini et al. (2007) and Svensen et al. (2007). Drilling operations trigger was argued among others by Davies et al. (2007). Earthquake trigger was challenged by Brumm et al. (2007). Mori et al. (2007) indicated that the trigger of LUSI may a combination of both the 27^th May 2006’s Yogyakarta earthquake and drilling of Banjar Panji-1. The controversy on the origin of LUSI has complicated the legal aspect of LUSI in the court.

Based on geochemical and field results, Mazzini et al. (2007) proposed a mechanism that LUSI mud volcano eruptions started following the May 27^th 2006’s Yogyakarta earthquake due to fracturing and accompanied depressurization of > 100º C pore fluids from > 1700 meters depth.

This resulted in the formation of a quasi-hydrothermal system with a geyser-like surface expression and with an activity influenced by the regional seismicity. Brumm et al. (2007) challenged this idea since the earthquake is too distant and too small to trigger the LUSI eruption, and plotting of LUSI to cross plot of Manga and Brodsky (2006) which is a statistic cross plot between occurred earthquakes and related liquefaction and mud volcanoes show that LUSI plotting is out of general trend. Brumm et al. (2007) also argued that in the past 35 years, tens to hundreds of other earthquakes caused stronger ground shaking at the site of the eruption but why did not trigger an eruption.

Davies et al. (2007) suggested that LUSI eruption appears to have been triggered by drilling of overpressured porous and permeable limestones at depths of ~2830 m below the surface.

They proposed that the borehole provided a pressure connection between the aquifers in the limestones and overpressured mud in overlying units. As this was not protected by steel casing, the pressure induced hydraulic fracturing, and fractures propagated to the surface, where pore fluid and some entrained sediment started to erupt.

It is not the aim of this paper to examine in detailed the mechanism responsible triggering the LUSI mud volcano. However, there are facts indicating that the Yogyakarta earthquake was significant triggering the LUSI mud volcano eruption. The facts are hereafter :

(1) drilling operations sequence show that partial loss of drilling mud in Banjar Panji-1 well occurred ten minutes after the earthquake,

(2) total loss in the well occurred after several the earthquake’s aftershocks,

(3) mud erupted two days after the earthquake,

(4) early eruptions of hot mud and salt water occurred in several localities forming the SW-NE trend parallel with the main structural trend of faults or fractures in this area,

(5) mud eruption never expel from the well, it is 200 meters to the southwest of the well and engineering test showed that there is no communication between the site of mud eruption and the wellsite,

(6) LUSI lies within major faults (Watukosek Fault) trending SW­NE from Penanggungan volcano to the Strait of Madura, the fault becomes the sites of extinct or dormant mud volcanoes of Kalang Anyar, Pulungan in Sidoarjo area, Gunung Anyar in Surabaya area, and Socah in Bangkalan, western Madura area,

(7) the occurrence of a fracture in the wellsite hundreds meters long and tens of centimeters wide trending NE-SW a few days after the eruption

(8) there was a decrease in gas flow rate at nearby Carat well at about the time of the Yogyakarta earthquake took place,

(9) the Yogyakarta earthquake re-activated the Semeru volcano three days after the earthquake by increasing its surface temperature and erupting volcanic ash (NASA’s Terra satellite data),

(10) the Yogyakarta earthquake was recorded 34 seconds after the main shock in Ujung Pangkah waters to the northwest of Surabaya and its energy affected the seismic recording which was being surveyed at the area at the time of earthquake occurred in Yogyakarta,

(11) the energy propagation of the earthquake was mainly eastward and northeastward as shown by the trend of its aftershocks and the areas affected,

(12) regional satellite data show the presences of major faults and fractures from Yogyakarta to Sidoarjo area in a right-stepping pattern,

(13) there is a positive correlation between mud eruption rates of LUSI and swarms of earthquakes measured 300 kms around the site, high rates followed occurrences of earthquakes,

(14) recent deuterium isotope data of erupted water showing a mixing with static magmatic fluids from depth deeper than 20,000 feet, indicating the presence of deep basement faults or fractures within the site as the conduits, (15) the sudden bending of railway to south of the eruption site occurring after the May 27^th earthquake is exactly at the crossing between the Watukosek Fault and the railway and in line with the bending of the Porong River, confirming that the fault was re-activated by seismic activity.

On the other hand, argument of Davies et al. (2007) based on the assumption that Banjar Panji-1 total depth in porous Kujung aquifer limestone is not supported by the well data. Well logging data show no direct evidence that the Kujung Formation has been intersected in the borehole.

The deepest cuttings did not reveal the presence of any carbonate, and calcimetry data indicate only 4 % calcite with no significant increase or changes.

The presence of Oligo-Miocene Kujung Formation in this area is also mis-conception. Based on the available data and facts explained above, the idea that LUSI eruption is triggered entirely by drilling is inconclusive. Earthquake-related tectonic re­activation of fault where the well is located and the elisional condition which has been critical for mud volcanism in the area may play significant role for the birth of LUSI mud volcano.

In terms of morphology, LUSI mud volcano is of the type of combination between swamp-like area and crater muddy lake. It is obviously stage-3 development (on eruptive phase). Similar type of morphology is shown by Kalang Anyar mud volcano.

Whereas, Pulungan, Gunung Anyar, and Socah mud volcanoes show morphology of classic conic volcanic edifice. The stages of those mud volcanoes are as follows : Kalang Anyar (dormant, ­stage 3 to 4), Pulungan (extinct –stage 4), Gunung Anyar (extinct –stage 4), Gresik (dormant-stage 3), Socah (dormant –stage 3 to 4), Porong (extinct – stage 4). Collapse of mud volcano crater of Porong is obviously shown by successive concave horizons (Figure 7).