VII. BANDA, LESSER SUNDA ISLANDS

This chapter of the bibliography contains 139 pages and about 1111 titles on the Banda Sea and the area of the Lesser Sunda Islands, including Sumba and Timor. The key feature of this southeastern part of Indonesia is the East Sunda- Banda volcanic arc, with a young  'back-arc' marginal oceanic basin to the North (Banda Sea) and a 'non-volcanic outer arc' to its S and E.

The outer arc is a fold-and-thrust belt, mainly composed of sediments scraped off the subducting Indian Ocean and distal Australian continental margin. Where Australian continental crust has arrived at the subduction zone (Roti-Timor and farther East), the collision zone includes uplifted parts of the forearc of the overriding Banda Arc plate ('Banda Terrane').

It is subdivided into five chapters:

  1. VII.1. Banda Sea, East Banda Arc (incl. Tanimbar, Kai, Aru)
  2. VII.2. E Sunda- W Banda Volcanic Arc ( Lombok- Wetar)
  3. VII.3. Sumba, Sumbawa, Savu, Savu Sea
  4. VII.4. Timor, Roti, Leti, Kisar (incl. Timor Leste)
  5. VII.5. Timor Sea, Indonesian Sahul Platform

Download pdf - Chapter VII. BANDA SEA, LESSER SUNDA (1.93 MB)


VII.1. Banda Sea, East Banda Arc (incl. Tanimbar, Kai, Aru, etc.)

There have been several theories on the origin of the deep Banda Sea. Verbeek (1908) noticing major extensional features along the islands surrounding the Banda Sea, saw it as a collapsed structure. Abendanon (1919), struck by the presence of crystalline schists in much of E Indonesia, saw it as a sunken part of a large old Paleozoic continent that he named Aequinocta.

In the 1980's it was suggested to be underlain by oceanic crust, but this was interpreted by most authors as a trapped piece of Indian Ocean plate, of Cretaceous age (Bowin et al. 1980, Lee & McCabe 1986, Hartono 1990). Hamilton (1978) was the first to suggest a Tertiary age for the oceanic crust of the Banda Sea, formed by backarc spreading, a view shared by Norvick (1979) and Nishimura and Suparka (1990).

Particularly since the work of French groups in the 1990's, the interpretation of the creation of the Banda Sea basins by Late Miocene- Pliocene spreading is prevalent today (Rehault et al. 1994, Hinschberger et al. 1998, 2001, 2003). The driving force of the extension is rollback of old, N-ward subducting Indian Ocean slab (Milsom 1999, 2000, 2001, Hinschberger et al. 2003, Harris 2006, Spakman & Hall 2010). Indian Ocean crust has now been consumed completely between Sumba and New Guinea.

Remnants of small blocks of continental crust (some with Late Triassic limestones) and Late Miocene arc volcanics have been documented in the Banda Sea oceanic basin (Banda Ridges, Lucipara, Sinta and Rama Ridges, etc.)

  W-E cross section through Kai Besar  
 
W-E cross-section through Kai Besar, showing weakly folded, mainly 10° W-dipping, Eocene marly limestone, overlain by uplifted Miocene and younger reefal limestone terraces (Verbeek 1908).

Two rows of islands surround the Banda Sea, the Banda volcanic 'inner arc' and a non-volcanic 'outer arc'. The size of the Banda volcanic arc islands appears to diminish in an easterly direction, but it should be remembered that these islands are built on progressively deeper sea floor and the vertical relief of the volcanoes is as high as, or higher than, the volcanoes on Java, Sumatra, etc.

The 'non-volcanic outer arc' includes the islands of the Tanimbar and Kai groups. These are relatively little studied. They are mainly a continuation of the forearc-accretionary system of Timor, with Triassic-Jurassic and younger sediments, folded and thrusted towards the Australian craton. On the islands closest to the Banda Sea metamorphic and ultrabasic rocks are common, a pattern similar to that seen on  the Banda Sea sides of Timor and Seram.

The eastern side of the Kai islands (Kai Besar) was not involved in thrusting. Instead, it appears to be a fragment of the Australian continental margin that experienced large scale extensional faulting and underwent 2km of uplift in the last 10 My (Van Marle & De Smet, 1990). It looks like a rift shoulder at the W side of the Aru Trough, which is a very young and deep extensional basin.

Suggested Reading Banda Sea region:
Banda Sea region

Umbgrove (1948), Bowin et al. (1980), Jongsma et al. (1989),  Rehault et al. (1994), Villeneuve et al. (1994), Milsom (2000, 2001), Hinschberger et al. (1998, 2003, 2005), Cornee et al. (2002).

Circum Banda Sea Islands

Brouwer (1923, Schluter & Fritsch (1985), Van Marle & De Smet (1990) Charlton et al. (1991), Milsom et al. (1996), Honthaas et al. (1997)


VII.2. East Sunda- West Banda Volcanic Arc ( Lombok- Flores - Wetar)


The islands of the East Sunda- W Banda 'inner arc' East of Bali represent a young, active volcanic arc system, mainly of latest Miocene- Recent age. Most or all of the volcanics in this eastern part of the Sunda- Banda Arc system formed on oceanic crust.

The westernmost of these islands (southern parts of Bali- Lombok- Sumbawa- Flores) are underlain by an Early Miocene arc system, which is the continuation of the 'Old Andesites' arc of the S coast of Java, but continues from here towards W Sulawesi instead of the E Banda Arc.

Volcanism and shallow earthquakes are no longer active in the segment North of Timor, probably because the subduction zone is locked there, after the collision of the Australian continental margin and the Banda Arc. This resulted in a belt of N-directed thrusting N of the Banda Arc, that can be traced West to N of Flores, Bali, then into Madura Straits and East Java

This also resulted in widespread young uplift, with Pleistocene reef  terraces up to 700m and the exposure of commercial ore bodies like the large porphyry copper-gold deposit of Batu Hijau on SW Sumbawa (probably formed at ~5 km depth around 7- 3.7 Ma; Garwin 2002) and stratiform sulfide-barite-gold deposits on Wetar (formed on flanks of submarine volcano at ~2 km depth around 4.8 Ma; Sewell & Wheatley 1994, Scotney et al. 2005).

Suggested Reading E Sunda- Banda Arc:
General

Brouwer (1940), Nishimura et al. 1981, Abbott & Chamalaun (1981), Silver et al. 1983, 1986,.Elburg et al. (2005).



VII.3. Sumba, Sumbawa, Savu, Savu Sea

S-N cross sections of S Sumba Island
S-N cross sections of S Sumba island, showing intensely folded Cretaceous, unconformably overlain by less deformed Eocene and Miocene, and intruded by granite. (Laufer & Kraeff, 1957)

The origin of the Sumba as a detached micro-continental fragment from SE Sundaland has been suggested by numerous authors, including Hamilton (1978), Djumhana & Rumlan (1992), Simandjuntak (1993), Wensink (1994, 1997), Soeria-Atmadja et al. (1998), Satyana (2003), Prasetyadi et al. (2006), etc. This was mainly based on the stratigraphic succession that is very similar to that of SW Sulawesi (granitoids, Late Cretaceous flysch, Paleo-Eocene arc volcanics, Eocene-Miocene shallow water carbonates, etc.).

A frequently quoted presence of a Jurassic Aegoceratid ammonite from West Sumba by Roggeveen (1928) has never been substantiated, and is suspected to be a Cretaceous species.

An Oligocene angular unconformity separates Late Eocene- earliest Oligocene (zone Tb-Tc) limestones, with dips of 30° or more, from more horizontal earliest Miocene(Te5) sediments (Caudri 1934). This is also observed in the 'Banda Terrane' of Timor and may be related to the mid-Oligocene faulting-erosional event in the Tonasa Lst of SW Sulawesi (Wilson et al. 2000, etc.).

Paleomagnetic work suggests clockwise rotation of Sumba (60° between Jurassic and Miocene, Nishimura et al 1981, 90° since Cretaceous, Wensink 1997).

Several authors (mainly Australian) suggested Sumba was part of the Australian NW shelf (Chamalaun et al. 1982, Harrowfield et al. 2003, etc.), but there are no similarities with Australian NW margin stratigraphy.

Raised Quaternary coral reef terraces up to 475m above sea level suggest very young uplift of Sumba (Jouannic et al. 1988, Pirazzoli et al. 1993)

Suggested Reading:
General

Caudri (1934), Von der Borch (1983), Djumhana & Rumlan (1992), Fortuin et al. (1997), Wensink (1997), Soeria-Atmadja et al. (1998), Lytwyn et al. (2001), Rutherford et al. (2001), Satyana (2003)


VII.4. Timor, Roti, Leti, Kisar

The complex geology and many unique rock types and rich fossil assemblages of Timor and adjacent islands have attracted numerous researchers since the early 1900's, resulting in some 480 papers in this bibliography for the combined territories of Indonesian West Timor, Timor Leste, and the adjacent smaller islands.

This group of islands is part of the Sunda-Banda 'non-volcanic outer arc', where Australian margin continental crust collided with the Banda Arc subduction system. It is a fold-and thrust belt that continues West into the Java accretionary prism and to the East into the Moluccas chain of Babar, Tanimbar, Kai, and eventually Seram-Buru.

SW-NE cross-section N Central Timor from Marks (1961)
SW-NE cross-section N Central Timor from Marks (1961)

Timor island was first recognized as an alpine style fold-and-thrust belt by Wanner (1913) and Molengraaff (1913, 1915). Although on an Indonesia-scale map it may look like a relatively small system, the length of Timor island is ~500km, making it of similar size as the French and Swiss Alps combined.

The question of how much of the Timor rock record is nappes of Asian origin ('allochtonous') and how much of it represents the folded sedimentary cover of the Australian continental margin ('para-autochtonous') has been debated for almost 100 years and is still not entirely settled.

Imbricated thrust slices along the S coast of Timor are composed of deep water Triassic- Cretaceous sediments, and may reasonably be interpreted as distal slope sediments scraped off the N-ward subducting Australian continental margin (e.g. the Kolbano fold-thrust belt of SW Timor)

  Diagrammatic stratigraphy of 'Mutis Overthrust Unit (= Banda terrane) of W Timor from Marks (1961)  
 
Diagrammatic stratigraphy of 'Mutis Overthrust Unit (= Banda terrane) of W Timor from Marks (1961);

Scattered across the central zone of W Timor and E Timor are 15 complexes of the so-called 'Banda Terrane' (the ‘Schist-Palelo complex’ of Brouwer (1942) = ‘Mutis Unit’ of De Waard (1957) = Lolotoi Complex of Audley Charles (1965)). These are outcrops of metamorphic rocks  (Mutis, Boi, Mollo, Lolotoi, etc.) often associated with ophiolitic rocks, stratigraphically overlain by Palelo Group Upper Cretaceous- Eocene arc volcanics and 'flysch-type' sediments. There are also Eocene shallow water carbonates with SE Asian Pellatispira forams, unconformably overlain by latest Oligocene- Early Miocene shallow water Cablac Limestone. This 'active margin' series does not tie to any rocks or events on the NW Australian margin, but it is very similar to the stratigraphy of SW Sulawesi and SE Kalimantan (Meratus). These complexes have therefore long been interpreted as 'allochtonous'.

Banda terrane stratigraphy suggests mid-Cretaceous metamorphism in a subduction zone, presumably at the SE Sundaland margin, followed by exhumation and onset of Cretaceous marine sedimentation. It formed part of the Late Cretaceous- Eocene 'Great Indonesian volcanic arc' of Harris (2006), probably at the Sundaland margin. It was affected by an Oligocene folding-uplift event during a period of carbonate deposition (also seen on Sumba and SW Sulawesi), and must have broken away from the Sundaland margin in Miocene-Pliocene time, to end up in the Banda forearc after opening of the S Banda Sea (see also Barber 1979, 1981, Earle 1979, etc.).

It may be noted that radiometric ages of the Banda Terrane metamorphics vary widely. Some are Early Cretaceous, but most of them appear to be of Eocene ages (~35-45 Ma), some even younger. Given its stratigraphic position below Late Cretaceous sediment, the post-Cretaceous ages must be too young and possibly reflect an Eocene heating event from arc volcanism. Whichever of these ages age is favored, this metamorphism is much older than the present-day Australia- Banda Arc collision and could not have taken place along the NW Shelf passive margin.

Along the North coast of Timor Island (and continuing East on Kisar, Leti, Moa, Sermata, W Tanimbar, and probably all the way to Seram) are remnants of the large 'Aileu Complex' ophiolites and associated metamorphic sole rocks. The ultrabasic complex was described by a.o. Molengraaf and Brouwer (1915; Leti), Berry & Grady (1981), Berry & McDougall (1986) (Late Miocene cooling ages ~8-6 Ma), Harris (1991) and Kaneko et al. (2007).

The Aileu metamorphic complex appears to be metamorphosed 'Maubisse complex' Permian sediments and basic volcanics (Molengraaf & Brouwer 1915, Barber & Audley Charles 1977). Radiometric ages would suggest the North coast Aileu complex metamorphics/ ophiolites are younger than similar rocks of the Banda terrane of the central zone.

Another 'suspect terrane' is the Maubisse/ Sonnebait series of authors. Like the Banda terrane it is one of the higher 'nappes' and contains Permian- Cretaceous rocks and faunas that are different from the Australian NW Shelf. The Permian is composed of marls and reddish limestones (very rich in crinoids, blastoids and solitary corals of much higher diversity and more tropical aspect than nearby NW Shelf Permian), interbedded with pillow basalts, and overlain by thin, condensed, Triassic  cephalopod limestones of 'Tethyan' affinity and Jurassic- Cretaceous deep sea clays and pelagic marls. It may be viewed as an oceanic seamount assemblage that formed during a Permian breakup event, then drifted in oceanic setting until Cretaceous or Eocene collision with a subduction complex.

Miocene and younger tectonics on Timor are primarily normal faulting and significant late uplift, as demonstrated by the presence of Pleistocene coral reef terraces up to 1300m elevation.

Timor has been famous for over 100 years for its rich Permian- Triassic marine macrofossils. Beautifully illustrated paleontological studies include those on ammonites (Welter 1922, Diener 1922), corals (Gerth 1921), crinoids and blastoids (Wanner 1916-1949), brachiopods (Broili 1916), molluscs (Krumbeck 1921), etc.. Unfortunately much of this material was collected from loose blocks or obtained from villagers, so stratigraphic context is often unclear.

Oil and oil and gas seeps on the island have been tied to bituminous limestones of the Triassic Aituti Formation. Due to the structural complexity of Timor island preservation of significant commercial hydrocarbon traps is unlikely and exploration is extremely challenging.

Suggested Reading:
General, Tectonics

Wanner (1913), Brouwer (1942), De Waard (1954-1957), Audley Charles (1965), Carter et al. (1976), Brunnschweiler (1978), Hamilton (1979), Barber (1981), Bowin et al. (1981), Charlton (1989, 2002),Sawyer et al. (1993), Reed et al. (1996), Harris et al. (2000), Villeneuve et al. (2005), Harris (2006), Keep et al. (2009), Keep & Haig (2010)

Permian- Mesozoic stratigraphy and faunas

many historic papers summarized in Charlton et al. (2002, 2009), Bird et al. (1989), Bird & Cook (1991), Haig et al. (2007, 2010)

Banda terrane/ Aileu metamorphic complexes

Molengraaff & Brouwer (1915), De Roever (1939), Tappenbeck (1940), De Waard (1954, 1957), Berry & Grady (1981), Earle (1981, 1983), Sopaheluwakan (1990), Audley Charles & Harris (1990), Harris (2006), Kaneko et al. (2007), Standley & Harris (2009)

Australian margin accretionary prism/ Kolbano fold-thrust belt along S coast

Charlton (1987, 1989), Charlton & Suharsono (1990), Harsolumakso et al. (1995), Keep et al. (2005).

Roti

Brouwer (1922), Krumbeck (1922), Roosmawati & Harris (2009)

Leti

Molengraaff & Brouwer (1915), Kaneko et al. (2007)


VII.5. Indonesian Timor Sea, Sahul Platform

The area south of the Timor- Tanimbar islands comprises the South Timor accretionary prism (partly onshore, mainly offshore), the Timor Trench and the distal part of the Australian continental margin.

Parts of the Australian continental margin are within the Indonesian Economic Zone and in the Timor Leste- Australia ZOCA joint operating zone. Significant gas fields are present in Middle- Late Jurassic and basal Cretaceous reservoir sands (Abadi, Bayu-Undan, Sunrise, Troubadour fields), similar to the 'Plover play' elsewhere in the Bonaparte Basin of the NW Shelf.

The downwards bending of the downgoing plate causes widespread normal faulting (Harrowfield et al. (2003).

Suggested Reading Timor Sea
General

Hardjono et al. (1996), Nagura et al. (2003), Seggie et al. (2003), Barber et al. (2004), Zushi et al. (2009)


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