# The Geology of Indonesia/Timor
## Regional Setting

The Timor Sea region occupies a large area in SE Indonesia, between Timor Island and the Australian continent. The region can be subdivided into several distinct structural provinces. These are basically the result of two intersecting structural trends. The early Paleozoic to Middle Jurassic structural grain in northwest southeast but Late Jurassic to Holocene trends are predominantly northeast-southwest. Post Middle Jurassic structural trends are complicated by the original northwest southeast Paleozoic component (Laws and Kraus, 1974). The structural provinces are components of the bordering craton are depicted in Fig. 1. The Bonaparte Basin, which is a fan shaped depression with its apex near the shoreline, situated northeast of the Browse Basin offshore Northwestern Australia. It is bounded to the west by the Londonderry High and to the east by the Darwin Shelf (Precambrian Sturt Block). The south and north boundaries are the Precambrian Kimberley Block and Timor Trough respectively (Laws & Kraus, 1974). The prominent tectonic elements in this basin are: the Petrel Sub-basin, the Sahul Platform, the Sahul Syncline, the Ashmore Platform, the Vulcan Sub-Basin, the Malita Graben, and. (Fig. 1).
The northwest-trending Petrel Sub-basin was initiated during rifting in the Late Devonian to Early Carboniferous (Gunn, 1988, Lee & Gunn, 1988), and more than 15000 meters of sediment thickness was reported (Gunn, 1989). The Sub-basin is overprinted by the NE to ENE-trending Mesozoic tectonic provinces, which are considered to be related to the rifting and ultimate break-up of Gondwanaland in the Middle Jurassic (O’Brien et al., 1993). The ENE-trending Malita Graben intersects and separates the Petrel Sub-basin from the Sahul Platform to the north. The graben contains a significant thickness of Tertiary, Cretaceous, and perhaps Late Jurassic sediments.
The Sahul Platform was a structurally high feature during the Late Jurassic. Sediments on this platform are relatively thin due to little deposition or erosion. To the west of the platform is the Flamingo High which contains approximately 7000 meters of sediment ranging from Triassic to Tertiary (Botte, P., and Wulff, K., 1990).
The Sahul Syncline separates the Sahul Platform trend and the Vulcan Sub-basin (located between Londonderry High and Ashmore Platform). This syncline was formed as a continuation of a Permo-Triassic to Early Triassic northwest to southeast rift trend. Sediments in this syncline are about 11,000 meters (Botten, P. and Wulff, K., 1990).
The Ashmore Platform is a large elevated block that lies west of the Vulcan Sub-basin and north of the Browse Basin. An arcuate fault zone, concave to the west, divides the platform into two major segments: a western terrain with mainly west-dipping faults, and an eastern terrain with both east-dipping and west-dipping faults. Faults swing toward a more easterly orientation along the southern margin of the platform and mark its boundary with the Browse Basin. A set of northeast-trending faults is taken as the eastern edge of the Ashmore Platform. Late Tertiary faults delineate its western limits. On the platform, flat-lying Cretaceous strata unconformably overlie up to 4500 m of Triassic sedimentes that were faulted during the Middle Jurassic to form an extensive terrain of tilted blocks prior to peneplanation (Daniel, 1988). The Vulcan Sub-Basin is a northern extension of the Mesozoic Browse Basin and has some affinities to the Paleozoic Bonaparte basin to the east. The sub basin is filled with Mesozoic and Tertiary sediments and encompasses three main depocenters: the Cartier trough, Swan graben , and Skua trough (Smith & Lawrence, 1989).
A broad northeasterly trending depocenter in the east of Timor Sea is called the Malita Graben. The graben is bordered by the Sahul Platform to the northwest and the Bathurst Terrace, a basin ward extension of the Darwin Shelf to the southeast. The Malita Graben is estimated to contain a post-Paleozoic sedimentary pile exceeding 10 km (West & Miyazaki, 1994) The Timor trough is also considered as a structural element located in the south of Timor Island and discussed in the Banda Arc chapter.

## Tectonics

The Bonaparte Basin development was initiated by rifting, which began in the Devonian and ceased at mid-Carboniferous time (Gunn, 1988a). The rifting phase was followed by subsidence and accompanying sedimentation. Tectonism in the Mid-Triassic produced fault trends sub-parallel to the current shelf edge. Rifting again occurred along the same trend in the Mid to Late Jurassic, prior to the onset of seafloor spreading. Callovian uplift and erosion is associated with this rifting phase. The Miocene collision of the Australian northwest shelf with Timor (Mory, 1988) resulted in reactivation of the Mesozoic fault systems and development of a further fault system caused by transpressional movements. The later faults are sub-parallel to the Timor Trough, which is an expression of the collision of Australia and Timor. The collision in the Miocene resulted in normal and reversed faulting into the shallow section. The structures show across-faulted geometry with a graben overlying a horst, producing the typical ‘hourglass’ configuration. Studies of the structures (Woods, 1988) suggested that some of the early major faults were not reactivated during the collision. It was further concluded that some significant collision-stage faults did not directly relate to older faults. (This section has been slightly modified after Williamson & Lavering, 1990)

## Stratigraphy

The stratigraphic units of the Northern Bonaparte Basin range in age from Precambrian to Quaternary and summarized by Williamson & Lavering (1990) as the following. The oldest sediments encountered (figure 3) are Permian age and indicate that a carbonate platform occupied the northern rim of the basin during the Late Permian (MacDaniel, 1988a). The Permian Hyland Bay Formation in the northwest contains fossiliferous carbonates similar to Late Permian fossiliferous limestones on Timor, and in the northeast consists of calcilutites. Marine siltstones and shales of the Mount Goodwin Formation were deposited during the latest Permian and Early Triassic. This was overlain in Middle to Late Triassic by the Cape Londonderry Formation, which is composed of shallow marine to fluvio-deltaic sediments. They are represented by a marine shelf carbonate sequence on the Sahul Platform. The carbonate sequence passes south and westwards into the Londonderry Formation both in the Malita Graben and Sahul Syncline, where respectively mixed clastic and carbonate sequences were deposited. From Late Triassic to early Jurassic time, a fine-grained red-bed sequence, the Malita Formation, was deposited. This was overlain during the Early Jurassic by the Plover Formation, which was composed of fluviodeltaic siliciclastics with variable proportions of shale and minor coal. Uplift accompanies Mid to Late Jurassic aged rifting and resulted in erosion, which removed some of the earlier sequence from the higher parts of the Sahul Platform and similar elements such as the Ashmore Block. During the Late Jurassic and Early Cretaceous, sedimentation occurred in grabens and troughs which remained after Jurassic rifting. The Flamingo Group was deposited during this time and consists predominantly of shales (Swan Formation in the Vulcan Sub-basin and Frigate Shale over the Sahul Platform) with subordinate undifferentiated sandstones.
Shales and calcarenites of the thick (up to 2000 m) and laterally extensive Bathurts Island Group were deposited in the Cretaceous. In general, the calcarenites were deposited over the more distal platforms and the shales deposited shoreward, except for some sandstone deposition at the proximal margin in the Vulcan Sub-basin.
During the Paleocene to Oligocene, carbonate sediments of the Hibernia Formation retreated oceanward and were fringed landward by a thin proximal zone of siliciclastic sedimentation. Shelf carbonate sediments were re-established in the Miocene with associated reef growth through to the Holocene. The carbonates of this phase are unnamed, but are up to 1000m thick.

## References:

Daniel, R. P. Mac, 1988, The geological evolution and hydrocarbon potential of the western Timor Sea region, Petroleum in Australia: The first century, APEA.
Laws, R. A. & Kraus, G. P., 1974, The regional geology of the Bonaparte Gulf- Timor Sea area: APEA Jour., v. 14, pt. 1, p. 77-84.
O’Brien …. 1998, An evaluation of hydrocarbon seepage in Australia’s Timor Sea (Yampi Shelf) using integrated remote sensing technologies, SEAPEX Exploration Conference proceedings.
Puspoputro, B., 1995, Mesozoic exploration in the northern Bonaparte Basin; in The Mesozoic in the eastern part of Indonesia proceeding of Pertamina symposium and workshop.
Smith, B. L., & Lawrence, R. B., 1989, Aspects of exploration, development of Vulcan sub-basin, Timor Sea, Oil & Gas Jour. Special edition, Oct. 30, p. 33-46.
West, B. G. & Miyazaki, S., 1994, Evans Shoal petroleum prospectivity, New opportunities, Petromin, July 1994, p. 34-43.
Williamson, P. E. & Lavering, I. H., 1990, The Mesozoic petroleum prospectivity of area A of the Timor Gap Zone of Cooperation, IPA 19th Annual Convention Proceedings.