243:-supported with angular fragments commonly 2 to 75 cm (0.79 to 29.53 in) across with some blocks up to 3 m (9.8 ft). Fragment imbrication within the breccias records diverging west and southwest flow from a source in the northeast part of the Batu Agung Escarpment. There is no evidence of any marine influence, and we interpret the Nglanggran Formation as terrestrial deposits formed by sector or repeated dome collapse, marking the end of arc activity in the Southern Mountains. The top and base of the Nglanggran Formation forms topographic breaks in slope, because the andesitic breccias are much more resistant than the underlying and overlying units, and the feature and the formation can be easily mapped across the area over a distance of 46 km (29 mi).
256:-rich sandstones of Early Miocene age exposed in East Java that may be Semilir products but have not been precisely dated. One such sandstone is the Jaten Formation, exposed near Pacitan, 80 km (50 mi) east of the Batu Agung Escarpment. The location and character of this formation are described in detail in Smyth et al. (2008). Zircons from the Jaten Formation have a weighted mean SIMS age of 19.6 ± 0.5 Ma and this is consistent with them being reworked products of the Semilir eruption.
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large slump folds indicating an unstable marine slope. Smyth et al. (2011) interpret these observations to indicate the
Semilir Formation was erupted in a subaerial setting in which pyroclastic flows travelled across vegetated slopes picking up and baking plant fragments, with some of these flows entering the sea. Some of the primary material would have been redeposited as lahars in distal settings.
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In East Java the base of the
Semilir Formation can be identified by the abrupt termination of bioturbated reworked deposits characteristic of the Kebobutak Formation. The volcanic rocks of the Semilir Formation are characterised by features typical of terrestrial air-fall, pyroclastic surge and flow
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Overlying the
Semilir Formation is a 200 to 500 m (660 to 1,640 ft) thick sequence of massive, resistant, monomict andesitic volcanic breccias, crystal-rich sandstones and minor lava flows of the Nglanggran Formation. The breccia beds are 1 to 10 m (3.3 to 32.8 ft) thick and can
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Based on examined exposures, measured sections and published geological maps, the combined thickness of the
Semilir and Nglanggran Formations is between 250 and 1,200 m (820 and 3,940 ft), exposed over 800 km (310 sq mi), and the total volume of volcanic material at least
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structures, crystal layering, well-sorted granular laminations, diffuse bedding, breccias (with metre-scale pumice blocks), thick mantling ashes, and abundant fragments of charcoal. Locally there are water-laid deposits, with scoured irregular bases, flame, traction and suspension structures, and
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to Early
Miocene volcanic arc in East Java, the Southern Mountains Arc, is parallel to and south of the modern arc. Thick volcanic and volcaniclastic deposits related to this arc are well preserved in East Java and identified volcanic centres are separated by similar distances to volcanoes of the
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The volcanic deposits are fresh and dip uniformly towards the south at approximately 30°. The sequence is divided into three formations which are, from base to top, the
Kebobutak, Semilir and Nglanggran Formations (note that Kebobutak Formation is not a same eruption event with Semilir and
189:-type assemblage. The formation is interpreted to be air-fall deposits reworked in a shallow marine sublittoral setting. Above the Kebobutak Formation lies at least 600 m (2,000 ft) of
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indicate initiation of the arc, and subduction beneath Java, by the Middle Eocene (42 Ma). The arc was active until the Early
Miocene (18 Ma) and volcanic activity was most abundant during the
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164:-type. The erupted products range from andesite to rhyolite and formed lava flows, lava domes, volcanic breccias and extensive pyroclastic deposits including flow, air-fall and
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modern arc. New field investigations and stratigraphic studies in East Java have identified the most important features of arc activity in the
Southern Mountains. The oldest
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deposits. Volcanic activity along the
Southern Mountains Arc culminated in the Early Miocene in a climactic phase of eruptions preserved in the Batu Agung Escarpment, near
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be mapped for several kilometres. Interbedded crystal-rich sand- stones are much thinner. The breccias are both clast- and
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Smyth, HR (2008). "Significant volcanic contribution to some quartz-rich sandstones, East Java, Indonesia".
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Smyth, HR (2011). "A Toba-scale eruption in the Early
Miocene: The Semilir eruption, East Java, Indonesia".
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flows). It has incredible thickness (at least 1,200 m (3,900 ft)). In contrast, The
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and Nglanngran Formation. These two geological formations are in the Southern Mountains of
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and continental rocks. In the overlying sedimentary rocks volcanic debris and detrital
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rocks are conglomerates and sandstones that lack volcanic material and were deposited
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was a major volcanic event which took place in Indonesia during the Early
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172:. It was during this climactic phase that the Semilir Eruption occurred.
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500 km (120 cu mi) (DRE). Widely distributed volcanic
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to Early Miocene. Volcanic activity was commonly explosive and of
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75:. The eruption created two formations which consist of mostly
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Nglanggran). The top part of Kebobutak Formation are intensely
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91:. The estimated Semilir eruption age date by
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87:only formed 600 m (2,000 ft)
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212:assigned to the Semilir Formation.
67:. This eruption formed the Semilir
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95:method is 21 million years ago.
531:Volcanic eruptions in Indonesia
382:Volcanic eruptions in Indonesia
321:Journal of Sedimentary Research
234:Facies of Nglanggran Formation
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306:10.1016/j.lithos.2011.07.010
148:on a basement that included
216:Facies of Semilir Formation
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247:Thickness and distribution
418:19th & 20th century
398:(21 million years ago)
113:Indonesian archipelago
119:subducts beneath the
117:Indo-Australian Plate
111:at the centre of the
404:(c. 74,000 years BP)
341:10.2110/jsr.2008.039
27:21 million years ago
333:2008JSedR..78..335S
298:2011Litho.126..198S
221:deposits including
99:Geology Background
85:Toba supereruption
536:Plinian eruptions
526:Miocene volcanism
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166:pyroclastic surge
123:. There has been
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292:(3–4): 198–211.
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176:Eruption Product
131:since the Early
103:Java is between
61:Semilir eruption
19:Semilir eruption
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121:Eurasian Plate
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38:Ultra-Plinian
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502:Mount Marapi
472:Mount Merapi
465:21st century
455:Mount Marapi
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79:rock (minor
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486:(2017–2019)
484:Mount Agung
443:Mount Agung
389:Before 1800
196:, volcanic
183:bioturbated
142:sedimentary
129:Java Trench
77:pyroclastic
520:Categories
260:References
170:Yogyakarta
150:ophiolitic
125:subduction
115:where the
89:ignimbrite
158:Oligocene
109:Australia
73:East Java
69:Formation
52:Indonesia
48:East Java
437:Krakatoa
227:antidune
210:breccias
187:Cruziana
133:Cenozoic
44:Location
408:Samalas
396:Semilir
329:Bibcode
294:Bibcode
202:crystal
191:dacitic
162:Plinian
154:zircons
127:at the
93:isotope
65:Miocene
504:(2023)
498:(2021)
496:Semeru
492:(2018)
480:(2014)
474:(2010)
457:(1979)
451:(1979)
449:Sinila
445:(1963)
439:(1883)
433:(1871)
427:(1815)
410:(1257)
286:Lithos
254:quartz
241:matrix
204:-rich
198:lithic
194:pumice
137:Eocene
478:Kelud
431:Ruang
206:tuffs
185:by a
135:. An
402:Toba
225:and
223:dune
208:and
200:and
107:and
105:Asia
81:lava
59:The
33:Type
24:Date
337:doi
302:doi
290:126
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325:78
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268:^
50:,
374:e
367:t
360:v
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