VI. NORTH MOLUCCAS

This chapter of the bibliography contains about 349 titles on the northernmost part the Indonesian Archipelago, a geologically complex region with a number of active volcanic arcs, non-volcanic 'outer arcs', fragments of remnant arcs and deep basins floored by oceanic crust.

It is subdivided into three chapters:

  1. VI.1. North Moluccas (Halmahera, Bacan, Waigeo, Molucca Sea)
  2. VI.2. Banggai, Sula, Taliabu, Obi
  3. VI.3. Seram, Buru, Ambon

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VI.1. North Moluccas (Halmahera, Bacan, Waigeo, Yapen, Molucca Sea)

  Halmahera_Verbeek 1908  
 
Early geologic map of Halmahera- Bacan (Verbeek 1908)
 

This area of N Indonesia is in the realm of the Pacific Ocean (Philippine Sea Plate). The western part is the Molucca Sea complex, where Molucca Sea Plate oceanic crust is subducting in two directions, under Halmahera in the East and the Sangihe arc in the West. The S side is bordered by the Sorong Fault zone, a major strike slip zone separating the W-moving Pacific from a N-moving Australia- New Guinea plate.

Islands are composed of fragments of Late Cretaceous- M Eocene and younger island arc volcanics, intruded into and overlying collisional complexes with Jurassic or Cretaceous-age ophiolites. With the exception of parts of islands in the Sorong fault zone complex, no Pre-Tertiary sediments or continental crust material have been reported.

The K-shaped island of Halmahera may be characterized as a smaller and younger copy of Sulawesi, with the W arms forming a classic volcanic island arc and the central region and eastern arms containing ophiolite complexes and interlayered sediments.

Suggested Reading Hamahera region:
General

Verbeek (1908), Wanner (1913), Brouwer (1923), Sukamto et al.(1981), Hall (1987), Hall et al. (1988, 1990, 1995), Sukamto (1989), Ali et al. (2001), Sodik et al. (1993).



VI.2. Banggai, Sula, Taliabu, Obi

Early geologic map of Banggai- Sula islands (Verbeek 1908)
Early geologic map of Banggai- Sula islands (Verbeek 1908)

This group of islands west of the Birds Head of W Papua is generally believed to represent a set of microcontinental plates, sliced off the N margin of New Guinea. Basement consists of Paleozoic metamorphic rocks, overlain by thick Triassic arc volcanics (Mangole Fm) and intruded by co-magmatic granite batholiths (Banggai granite; K-Ar ages around 225 Ma). These form part of a long Permo-Triassic arc system that continues East to New Guinea Birds Head (Netoni, Anggi granites), to PNG (Idenburg, Kubor, Strickland granites; all ~220-240 Ma) and all along the E Australian active margin.

Historically, the Sula Islands have been famous for its marine Jurassic- Cretaceous marine sedimentary sequence, with probably the richest Jurassic ammonite, belemnite and mollusc faunas in Indonesia. Classic paleontological monographs on Sula Mesozoic fauna include Boehm (1904-1912), Kruizinga (1921, 1926, M-L Jurassic belemnites, ammonites) and Challinor & Skwarko (1982; Jurassic belemnites).

The western edge of the Banggai-Sula plate collided with East Sulawesi, probably in Late Miocene time. The foredeep created during this collision set up favorable conditions for the Miocene oil play in Miocene carbonate buildups of the Tomori Basin of East Sulawesi.

Suggested Reading Banggai-Sula region:
General

Garrard et al. (1998). Also: Boehm (1904-1912), Brouwer (1921-1926), Kruizinga (1921, 1926), Sukamto (1975), Pigram et al. (1985).


VI.3. Seram, Buru, Ambon

N-S cross sections through NW Seram
N-S cross-sections through NW Seram, showing N-directed folding and thrusting of metamorphics-granite complex over folded Mesozoic sediments (mainly Late Triassic) (Rutten & Hotz, 1920).

Seram and the chain of islands continuing in E/ SE direction all share a very complex fold-thrust belt geology, with N-directed thrusting and with fragments of continental blocks, metamorphic rocks and ophiolite complexes. Deformation is less intense West of Seram (Buru island). Large ophiolite bodies and metamorphics are present in SW Seram and on Buru.

Paleozoic metamorphics are overlain by M-L Triassic Kanikeh Fm flysch-type clastics, overlain by Late Triassic reefal and deepwater limestones (commonly interpreted as Jurassic, but paleontological evidence appears to exclusively suggest a Late Triassic age; e.g. Martini et al., 2004). This series has been interpreted as a Late Triassic intra-cratonic rift sequence.

The Triassic is overlain by a highly condensed Early-Middle Jurassic limestone (e.g. Wanner & Knipscheer 1951) or may locally be missing completely, and by Late Jurassic Kola Shale, a sequence that may represent continental breakup and onset of spreading. The Nief Fm pelagic limestone sequence of latest Jurassic (calpionellids), Cretaceous (Globotruncana) and Paleo-Eocene ages represents the oceanic drift or very distal passive margin stage of the Buru- Seram microplate.

Similarities in stratigraphy and structure between Seram and Timor have been noticed by many authors. There are also similarities with the Triassic stratigraphy of nearby Misool, but the Jurassic- Paleogene of Seram- Buru is in more distal facies, and lacks the rich macrofossil faunas of Misool. There is also evidence of consumed oceanic crust between Misool and Seram (see below), so the present-day proximity is not necessarily the same as the paleo-position.

Widespread folding and thrusting of Eocene and older rocks, with the formation of the 'Salas Block Clay' olistostome or melange, suggests a major collisional event, but the exact age of this remains uncertain. It is probably related to ophiolite obduction at the S/SW side of Seram, which have a Late Miocene onset of exhumation age (around 8 Ma; Linthout et al. 1996)

This young fold and thrust belt outcrops on Seram and continues N of Seram up to 100 km offshore (e.g. Teas et al. 2009), where it looks like a continuation of the Banda Arc accretionary complex. This foldbelt is commonly described as merely a zone of young thrusting within the 'Birds Head' part of the Australian continental margin between Misool and Seram Islands (e.g. Granath et al. 2011), but the width of this belt requires 100's of km of shortening. Also, a S-dipping subducted slab is clearly imaged below Seram by tomography and by earthquake distributions, and remnants of a Plio-Pleistocene volcanic arc are present S of Seram (Ambon). In my opinion it is difficult to not view Seram as a continuation of the (now mostly extinct) Banda Arc subduction complex.

Deep water facies marls as young as Early Pleistocene outcrop on Seram island and suggest about 2 km of Pleistocene uplift in SW Seram (De Smet et al., 1989).

Oil has been produced from Plio-Pleistocene sands in NE Seram since 1897 (Bula Field), and is believed to be sourced from Late Triassic bituminous shale. Much later oil was also discovered in fractures in Late Triassic limestones in the Oseil Field.

Suggested Reading:
General

Wanner (1907, 1923), Rutten & Hotz (1918-1920), Rutten (1927), Germeraad (1946), Valk (1945), Van der Sluis (1950), Wanner & Knipscheer (1951), Wanner et al. (1952), Zillman & Paten (1975), Audley-Charles et al. (1979), Linthout et al. (1989, 1996), Kemp and Mogg (1992), Sopaheluwakan (1994), Kemp et al. (1996), Martini et al. (2004).


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