Sedimentological characteristics and new detrital zircon SHRIMP U–Pb ages of the Babulu Formation in the Fohorem area, Timor-Leste

Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin.


INTRODUCTION
The island of Timor is located in the southeastern part of the Indonesian archipelago ( Figure 1). It is a young accretionary fold-thrust belt formed by the underthrusting of Australian continental margin units beneath the Asian Banda forearc (Harris 2011 and references therein). The Banda arcÀcontinental collisional zone is divided into two main regional litho-tectonic stratigraphic successions representing fragments of continental crust that have Australian and Asian affinities, respectively. An understanding of these successions provides important insight into the stratigraphic relationships and tectonic development of the Banda Arc during arcÀcontinental collisions.
The Gondwana mega-sequence with Australian affinity exposed in a fold-and-thrust belt and m elange complex in Timor Island was previously divided into three principal Triassic lithostratigraphic units, the Niof, Aitutu and Babulu formations, and one predominantly Jurassic unit, the Wai Luli Formation (Charlton et al. 2009). Distinguishing between these stratigraphic elements is fundamental for understanding both the precollisional tectonic history of the Gondwana megasequence and the tectonic evolution of the Banda Arc. However their detailed sedimentological characteristics, including depositional processes and environments, remain unclear. Recent preliminary geochronological data from individual areas of the Gondwana sequence and Banda forearc units in the Timor region have contributed to the understanding of their origins (Zobell 2007;Standley & Harris 2009), but the systematic stratigraphic elements and their origins are remain unclear.
The Fohorem area, located at the southwestern extreme of Timor-Leste, has a thick succession consisting of the Aitutu and Babulu formations. Because of the adjacent type locality of the Babulu Formation (Babulu River; Bird & Cook 1991), detailed sedimentological study in the Fohorem area can provide valuable information for interpreting the lithological characteristics of the formation in Timor. In this paper, we present sedimentary characteristics, depositional environments and detrital zircon SHRIMP UÀPb ages of the Babulu Formation in the Fohorem area. These results have important implications for understanding the paleoenvironments and the definition of formal and6 or informal formations of the Australian continental margin successions in Timor.

GEOLOGICAL SETTING
The island of Timor is part of the outer arc of the Banda Arc ( Figure 1) and formed due to N-to NE-directed subduction of the northwestern Australian continental margin beneath the southeastern Banda Arc (Audley-Charles 1968;Barber et al. 1986;Charlton 1989;Harris 2006Harris , 2011Kaneko et al. 2007;Keep & Haig 2010). Recent estimations on the timing of this collision are between 9.8 and 5.5 Ma (Harris 1991(Harris , 2011Fortuin et al. 1997;Hall 2002;Audley-Charles 2004), followed by the emergence of the island above sea level at ca 3 Ma (Keep & Haig 2010). Despite differences in interpretations on the timing of the initial collision, the island clearly is one of the youngest arcÀcontinent collision zones on Earth with subsequent and ongoing rapid uplift (Kaneko et al. 2007;Keep & Haig 2010).
Timor-Leste is made up of four main tectonostratigraphic units: the Australian affinity sequence, the Banda affinity sequence, the synorogenic m elange and the synorogenic sedimentary sequence. These units represent the complex tectonic history of Timor, including continental rifting and spreading as well as subduction, collision and exhumation (Harris et al. 2011 and references therein).
The Australian affinity sequence, a volcanic to sedimentary succession of the subducting plate, is divided into the Gondwana mega-sequence and the Australian passive margin sequence. The Gondwana mega-sequence is composed of Carboniferous to Jurassic intracratonic volcanicÀsedimentary rocks predating the breakup of Gondwana (Audley-Charles 1968;Charlton et al. 2002Charlton et al. , 2009. The Mesozoic to Cenozoic Australian passive margin sequence is a post-breakup slope and rise succession characterised by pelitic rocks together with carbonates and silciclastic interbeds (von Rad & Exon 1983;Charlton 1989).
The Banda-affinity sequence overthrust the Australian-affinity sequence. It includes fragments of volcanic and sedimentary rocks, formed and metamorphosed as a part of the Great Indonesian Arc system (Audley-Charles 1968;Harris 2006Harris , 2011Standley & Harris 2009). The representative component of the sequence in Timor-Leste is the Cretaceous Lolotoi complex, which is a correlative of the Mutis metamorphic complex (Harris 1991(Harris , 1992Harris & Long 2000) in West Timor. It consists of metasedimentary rocks primarily of volcanogenic and pelitic origin, as well as metavolcanic rocks with basalt to basaltic andesite in composition (Harris 2006(Harris , 2011Kaneko et al. 2007;Standley & Harris 2009). The complex is overlain by Cretaceous to PaleogeneÀNeogene non-metamorphosed arcÀforearc succession (Harris 2006).
The synorogenic m elange is chaotic rock characterised by a variety of blocks within a scaly clay matrix (Audley-Charles 1965, 1968Barber et al. 1986;Harris et al. 1998;Harris 2011;Barber 2013). This unusual stratigraphic unit is mainly distributed around the boundary region between the Australian and the Banda terranes. Microfossils within the scaly clay matrix are reported to have originated predominantly from the Jurassic and Cretaceous pelitic rocks near the breakup unconformity (Harris et al. 1998;Harris 2011).
The synorogenic sedimentary sequence is composed of a pelagic chalk succession and carbonates with interbedded turbiditic sandstone, coral reefs and alluvial gravels (Audley-Charles 1968;Haig et al. 2007;Roosmawati & Harris 2009). The changing sedimentary lithofacies reflect the rapid uplift history of the island from a submarine to a terrestrial environment.

GEOLOGY OF THE FOHOREM AREA
The study area is located in the boundary region between the terranes with Australian and Banda affinities and includes the four major tectonostratigraphic units (Figures 2, 3). The terrane with Australian affinity is represented by the Maubisse, Aitutu, Babulu and Makokon formations in ascending order, predominantly occurring in the Fatumean area. The Permian Maubisse Formation (Audley-Charles 1968) is surrounded by the Bobonaro M elange deposits and is composed of shallow shelf limestoneÀcalcareous shale successions and pillow to massive basalt to trachyandesite. The SHRIMP UÀPb zircon age of the trachyandesite is 270 § 3 Ma (Appendix I). The Triassic Aitutu and Babulu formations (Audley- Figure 1 Regional tectonic map of the South-East Asian region including Timor Island (modified from Kaneko et al. 2007). The eastern Indonesian region is made up of a chain of islands called the "Banda Arc", which is subdivided into inner volcanic and outer non-volcanic arcs. Timor Island is a component of the outer arc, which formed by north-northeast-directed subduction of the northwestern Australian continental margin beneath the southeastern Banda Arc. Charles 1968;Bird & Cook 1991;Charlton et al. 2009) are characterised by shallow marine limestoneÀshale and deep marine siliciclastic successions, respectively. The strata of the formations generally show NE-strike dipping to the NW, with some variability due to folding and thrusting. The strata of the Babulu Formation generally overlie the Aitutu strata, but some Aitutu strata abruptly overlie the Babulu strata as a result of repeated thrusting ( Figure 3). The Aitutu Formation is primarily composed of wackestone to packstone with minor amounts of thin-bedded shale and chert nodules, whereas the Babulu Formation is predominantly mudstone with subordinate amounts of sandstone, limestone and conglomerate. The newly defined Makokon Formation, exposed in the small creek of Makokon village, is described as pink to pinkish-grey massive limestone consisting predominantly of foraminiferan pelagite ( Figure 3).
The Banda affinity sequence is characterised by Mesozoic to Cenozoic arcÀforearc successions consisting of the Fohorem Formation, the Baer Formation (or Dartollu Limestone) and the Cablac Limestone, in ascending order (Figures 2, 3). The first two lithological units are newly classified from the Lolotoi Complex   Figure 3) because of poor exposure and lack of suitable fossils for age determination in the limestone. The need for further studies of the limestone to resolve controversies surrounding its depositional age and associated affinity, has been identified (Audley-Charles 1968;Harris 2006;Haig et al. 2007Haig et al. , 2008. The pre-orogenic assemblages are disrupted and mixed during arcÀcontinent collision including the development of the synorogenic Bobonaro M elange (Figures 2, 3). The Bobonaro M elange occupies most of the study area, and is composed of unmetamorphosed clay matrix and embedded blocks. The clay matrix is generally reddish to brown to greenish grey in colour, with variations caused by differences in the clay compositions. A pervasive polished and scaly fabric, typically with striation, is commonly developed in the clay. Blocks are unsorted and composed of various lithologies derived from the underlying Maubisse, Aitutu and Babulu formations; blocks range in size from a few centimetres to several tens of metres.
The synorogenic sedimentary rocks are composed of Cenozoic limestone successions consisting of the Viqueque and Baucau formations. The Pliocene Viqueque Formation (Audley-Charles 1968) occurs as an irregular blocks, unconformably above the Bobonaro M elange deposits. It consists mostly of white, massive marlstone and claystone with minor conglomerate and limestone interbeds. The lower Pleistocene to Holocene Baucau Formation is a sequence of terraced reef limestone (Audley-Charles 1968) and consists of white, hard and mostly recrystallised coral reef limestone. All the coral specimens collected from the formation have been identified as scleractinians. All the above lithologic units are partially covered by the Holocene alluvium and colluvium deposits.

FACIES ASSOCIATIONS
The Babulu Formation in the study area is predominantly composed of mudstones with subordinate amounts of sandstone, limestone and conglomerate ( Figure 4). These lithologies differ markedly from the typical Babulu Formation elsewhere in Timor, which is described as consisting predominantly of sandy deposits (Charlton et al. 2009). The deposit crops out mostly along the riversides of Mota (river) Maubui and Mota Haliboe in the study area ( Figure 3). To interpret the depositional processes and environments of the formation, detailed columnar logs were measured at different scales, particularly where well-exposed thick sandstones were encased within the mudstones (Figure 4). Based on lithology, grain size, primary sedimentary structures and bedding features, six lithofacies were defined (Table 1). These facies were grouped into three facies associations (FAs), representing basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III) ( Figure 5).

Facies association I: Basin plain deposits
This association comprises the majority of the Babulu Formation, and is composed of homogeneous mudstones (Mh) with subordinate amounts of limestone and thin sandstone layers (Figures 5, 6a). The mudstones are laminar-to thin-bedded, massive to stratified in appearance and purple or dark grey to blackish in colour. Each layer is laterally persistent with distinct lower and upper boundaries. The mudstones may contain numerous shale, sandstone and limestone blocks up to 3 m in diameter ( Figures 5, 6b, c) and highly distorted internal stratification (Figure 6b) in the different stratigraphic levels. Orthoconic nautiloids are also found in the mudstone (Figure 6d). The limestone occurs as beds or nodules within the mudstone. Under the microscope, this limestone consists of diverse fossil allochems, including fragments or bodies of miliolid foraminiferans, crinoids, echinoids, calcareous chaetetid sclerosponges, sphinctozoans, calcimicrobes, bivalves, gastropods, ostracods, brachiopods and unidentifiable skeletons as well as nonskeletal allochems such as oncoids, peloids and quartz ( Figure 7). The intercalated sandstone layers (a few centimetres thick) within the mudstones, consist of moderate-to well-sorted, fine-to medium-grained sand and have distinct lower and upper boundaries and lenticular or wedge-shaped geometries.
The thinly bedded mudstones with good lateral continuity are indicative of suspension settling of finegrained, dilute turbidity currents on the outer lobe, or in a basin plain in a predominantly low-energy setting     Table 1 for brief descriptions of lithofacies.

Facies association II: Distal fringe lobe deposits
This association within the FA I mudstones (Mh) is characterised by the alternation of sandstone and mudstone ( Figures 5, 8). The >0.5 m-thick sandstone units are composed of massive sandstones (Sm), low angle to planar stratified sandstones (Ss) and ripple cross-laminated sandstones (Sr). These sandstones consist of moderate to well-sorted fine-to medium-grained sand of a light grey to greenish grey colour. Each sandstone bed is laterally persistent over several tens of metres with sharp and flat bases and tops. Some of these beds have wedge-shaped or lenticular and channelised geometries. The thinly bedded and laterally persistent massive to stratified sandstone, alternating with mudstones with sharp and flat bases, is interpreted as sand-laden turbidity flows on the distal fringe of lobes (Pr elat & Hodgson 2013). The ripple cross-laminated sandstone is interpreted as traction movement with fallout processes during waning turbidity flows (Mulder & Alexander 2001;Mulder et al. 2003). Some channelised sandstone beds are interpreted as a filling of minor channels or scours influenced turbidity flows.

Facies association III: Medial to distal lobe deposits
This association is the most interesting feature of the formation and is characterised by the frequent and abrupt presence of thick-to very-thick-bedded sandstones (up to 10 m thick) within the FA I mudstones (Figures 5, 9a). It is composed of massive sandstones (Sm), low angle to planar stratified sandstones (Ss), minor ripple cross-laminated sandstones (Sr), homogeneous mudstones (Mh) and disorganised conglomerates (Cm). Each sandstone bed is several tens of centimetres to up to 3 m thick, and some are amalgamated into several to tens of metres thick units with sharp and nonerosional lower surfaces that are laterally persistent over several tens of metres with sheet to tabular geometries. The massive sandstones (Sm) commonly include either one or more discontinuous (>5 m long) stringers or thin streaks of mud chips and blocks (Figures 5, 9b). The elongated mud chips are generally aligned parallel to the bedding plane or may be imbricated. The sandstones (Sm) are concave-upward, with erosional to flat lower surfaces and relatively flat or slightly undulatory tops. The stratified and ripple cross-laminated sandstones (Ss, Sr; Figure 9c) commonly occur above and below the massive sandstones with either sharp or gradational contacts ( Figure 5). Some stratified sandstones are also overlain by ripple cross-laminated sandstones and vice versa. The disorganised conglomerate (Cm) is present only in the Fatumea I section and is composed of clast-to matrix-supported, pebble-sized intraformational mud chips set in a muddy sand matrix (Figure 9d). It is about 0.4 m thick and laterally persistent (>5 m in lateral extent) with non-erosive lower surfaces.
The extremely thick-bedded sandstones are interpreted as resulting from the deposition by turbidity flows as evidenced by basal erosive surfaces and moderate to good sorting indicating flow turbulence, and parallel and6 or ripple laminated top divisions showing layerby-layer deposition (Hodgson 2009). Therefore, these sandstones, together with the internal variation of depositional structures and repeated mud chip layers, are interpreted as a gradual aggradation of a highly concentrated, sustained quasi-steady turbidity flow (Kneller & Branney 1995;Mulder & Alexander 2001;Mulder et al. 2003;Petter & Steel 2006;Plink-Bj€ orklund et al. 2001). Good preservation of the mud chips and blocks within the massive sandstones indicate an origin in the upper parts of the turbidity currents (e.g. Postma et al. 1988) that have not disaggregated sufficiently in the body of the flow to increase the clay content and suppress turbulence. Lack of significant internal erosion surfaces in

UÀPB GEOCHRONOLOGY
Handpicked zircon grains were mounted in an epoxy disk with the FC-1 zircon standard (1099 Ma; Paces & Miller 1993). The surface was ground using abrasive paper and polished with a diamond suspension to expose the grain interiors. The mount was then photographed at 80£ magnification in reflected and transmitted light to reveal internal structures within the zircons. For further investigation of zonation microstructures, cathodoluminescence (CL) and backscattered electron images were also obtained using a scanning electron microscope (JEOL 6610LV). After whole-image analysis, the mount was ultrasonically cleaned in petroleum ether and ethyl alcohol, rinsed in Millipore water and then dried in an oven at 60 C. It was then evaporatively coated with high purity Au prior to SHRIMP analysis. UÀPb ages were measured using a SHRIMP-IIe6 MC installed at the Korea Basic Science Institute (KBSI) in Ochang. Analytical procedures for the SHRIMP dating largely followed those of Williams (1998) and Williams et al. (2009). UÀPb isotopes of zircon were collected using a primary oxygen ion (O 2 ¡ ) beam of 3.0 to 4.0 nA intensity at 10 KeV with a diameter of approximately 25 mm. Secondary ions, accelerated to 10 KeV, were analysed by cycling of the magnet through five scans. The mass resolution at 1% of the 238 U 16 O C peak height was greater than 4500, and the total Pb sensitivity was within the range of 14 to 19 counts6 s6 ppm Pb6 nA O 2 ¡ . Prior to each analysis, the surface of the analysis site was pre-cleaned by rastering the primary beam for 3 minutes to reduce surface common-Pb. The measured 206 Pb6 238 U ratio was calibrated using the FC1 zircon. Concentrations of U and Th were calculated with reference to standard SL13 (Sri Lankan gem zircon, U D 238 ppm). Ages were calculated and concordia diagrams produced using the Squid 2.50 and Pb6 238 U ages for younger zircons. The 207 Pb6 206 Pb age is more reliable for the older zircon having large amounts of radiogenic Pb, whereas 206 Pb6 238 U age is ideal for younger zircon due to low radiogenic Pb content and uncertainty of common Pb correction (Anderson 2007). Ages with less than 15% discordance were used to avoid analytical bias owing to Pb loss or common Pb contamination.
The zircon grains (50À300 mm in diameter) from three sandstones (BA21, BA6 and BA12) occurred as subhedral to euhedral crystals showing oscillatory and sector zoning in CL images (Figure 10aÀc). Many grains had rounded to sub-rounded terminations, indicating abrasion during sedimentary transport and reworking. The analysed zircon grains had a range of U contents and Th6 U ratios as follows: (1)  In three sandstone samples (BA21, BA6 and BA12), the estimated deposition ages are inferred from the major  In contrast, sandstone (B13) from the Babulu Formation in the Bobonaro M elange in the western part of the Fatumean area contains zircon grains (50À400 mm in diameter) with a euhedral to subhedral crystal shape ( Figure 10d) and are oscillatory and sector-zoned under CL. The grains showed low to moderate U contents (18À656 ppm) and a wide range of Th6 U ratios (0.02À2.18) (Appendix II). Estimated ages were concentrated between 400 and 236 Ma, but ages of 62 SHRIMP spot analyses ranged from 2804 to 236 Ma. This suggests a detrital origin of zircons from this rock and sedimentation after the early Upper Triassic (Figure 11d). The provenance of the B13 sandstone from the Bobonaro M elange is likely similar to the provenances of the BA6 samples from the Babulu Formation because the probability density diagrams of the three specimens are similar, with the exception of the upper limit ages (Figure 11).  (Figures 11, 12). Predominant age pluses between the Paleozoic and Late Triassic were identified in these samples. Previously reported UÀPb ages of detrital grains from the Babulu Formation sandstone of the Savu and East Timor (Zobell 2007), using laser ablation (LA)-inductively coupled plasma mass spectrometry (ICP-MS), have produced age ranges from ca 2543 Ma to ca 256 Ma, with major peaks at ca 1878À1857 Ma and ca 329À256 Ma (Figure 12). The combined zircon geochronological data indicate that zircon source regions of the Babulu sandstones should be from the peripheral Australian continent ( Figure 12). Although sedimentation of the Babulu Formation was likely initiated at6 after ca 256 Ma in some regions ( Figure 12; Zobell 2007), the young major detrital zircon age groups of ca 256À238 Ma from most sandstones of the Babulu Formation in the Fohorem Quadrangle, reflect prolonged sedimentation after the early Upper Triassic (Figure 12).

Paleoenvironments of the Babulu sedimentary rocks in the Fohorem area
The Babulu Formation in Timor was previously interpreted as ranging from a non-marine (based on the absence of marine fauna) (Barkham 1993) to an outer fan marine environment where water depths exceeded over 200 m (based on the trace fossil assemblage) (Bird & Cook 1991). The detailed depositional processes, however, still remain to be studied. The predominant occurrence of laminated to thin-bedded mudstone successions, and the lack of hummocky cross-stratification and wave ripples in the intercalated sandstones, indicate that the deposition of the Babulu Formation in the study area was not influenced by storm waves (Pickering et al. 1986). The mudstones (FA I) containing large clasts further indicate that they were deposited near or beyond a high gradient depositional surface, with a slope likely produced by a progradation or aggradation of fan-delta systems (Nemec & Steel 1988;Hwang et al. 1995;Kim et al. 1995;Sohn et al. 1997;Sohn 1999Sohn , 2000Hwang & Chough 2000). The above interpretations suggest that the Babulu Formation was deposited in a relatively deep marine environment that is commonly developed in a Submarine lobes show complicated vertical variation in lobe successions such as thickening-and6 or thinningupward successions resulting from forward and6 or backward stacking of depositional lobes together with the migration of lobe systems (Macdonald et al. 2011;Pr elat & Hodgson 2013;So et al. 2013). The predominant occurrence of the basin plain mudstones (FA I), together with the distal lobe sandstones (FA II), suggests that the paleodepositional environment of the study area was very calm and low-energy setting with slightly basinward input of the distal part of the depositional lobes. The bimodal sedimentary materials from siliciclastic to carbonate materials are indicative of differently sourced submarine lobes from silicic to carbonate shelf Figure 13 Cartoon illustrating the depositional environment of the Babulu Formation. The thick-bedded sandstones of the formation are caused by the collapse of upslope sediments in association with new or renewed fault activity during basin evolution. Approximate location of the study area is indicated. environments (Charlton et al. 2009). The discrete and abrupt occurrence of thick-bedded sandstones (FA III) within the mudstones (FA I), in particular, shows a lack of systematic vertical variation of the depositional lobe. The non-cyclic architecture of the deposits indicates that the origin of the relatively large submarine lobes is closely related to the collapse of upslope sediments rather than a progressive basinward progradation of distributary depositional lobes in association with deltaic turbiditie (e.g. Bird & Cook 1991;Charlton et al. 2009) (Figure 13). These abrupt and intermittent collapses of the basin-fill sediments are interpreted to be the result of new or renewed fault activity during basin evolution (Charlton et al. 2009). The repeated occurrence of thin mud chip layers in the thick-bedded sandstones suggests that the depositional lobe was composed of multiply pulsed turbidity flows, resulted from a retrogressive slope failure of upslope sediments (Mastbergen & van Den Berg 2003;van Den Berg et al. 2002).

ACKNOWLEDGMENTS
We thank Prof. Ian S. Williams and Prof. Ron Harris for constructive reviews. This work was supported as a Basic Research Project (GP2011-004; Tectonic evolution of the western Gyeonggi Block and construction of geologic DB system) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Knowledge Economy, Korea. This study also contributes to the Project 'Geological Mapping of the Suai District in Timor-Leste.' The Project was implemented by the Korea International Cooperation Agency (KOICA), a government agency carrying out grant aid programs of the Republic of Korea and the Secretariat of State for Natural Resources (SERN) on behalf of the Democratic Republic of Timor-Leste.

SUPPLEMENTAL PAPERS
Appendix I Concordia plot of SHRIMP UÀPb isotopic analysis of zircon from early Permian trachyandesite in the Maubisse Formation.
Appendix IISHRIMP UÀPb data of detrital zircons from sedimentary rocks of the Babulu Formation in the Fohorem area.