Journal of Sedimentary Research, 2018, v. 88, 495–515 Research Article DOI: http://dx.doi.org/10.2110/jsr.2018.26

U-PB ZIRCON AGES AND PROVENANCE OF UPPER CENOZOIC SEDIMENTS FROM THE DA LAT ZONE, SE VIETNAM: IMPLICATIONS FOR AN INTRA-MIOCENE UNCONFORMITY AND PALEO-DRAINAGE OF THE PROTO–MEKONG RIVER

1 1 1 1 2 2 3 JULIANE HENNIG, H. TIM BREITFELD, AMY GOUGH, ROBERT HALL, TRINH VAN LONG, VINH MAI KIM, AND SANG DINH QUANG 1Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, U.K. 2South Vietnam Geological Mapping Division, 200 Ly Chinh Thang Street, Ward 9, District 3, Ho Chi Minh City, Vietnam 3PetroVietnam University, 7th floor, PVU Building, Long Toan Ward, Ba Ria City, Ba Ria-Vung Tau Province, Vietnam e-mail: [email protected]

ABSTRACT: The present-day Mekong River is the twelfth longest river in the world. It drains the Tibetan Plateau and forms a large delta in south Vietnam. Remnants of upper Cenozoic fluvial to marginal marine proto-Mekong sediments are exposed in the Da Lat Zone in southeast Vietnam and are likely the on-land equivalents of large sediment packages offshore in the Cuu Long and Nam Con Son Basins. Provenance studies are used here to identify sources that allow reconstruction of sediment pathways. The Oligo-Miocene Di Linh Formation has a main Cretaceous zircon age population and subordinate Paleoproterozoic (c. 1.8-1.9 Ga) zircons, sourced mainly from Cretaceous plutons. In contrast, the early Pliocene to Pleistocene Song Luy Formation includes additional Permian– Triassic and Ordovician–Silurian age populations which are interpreted to be sourced from basement rocks in central and northern Vietnam. There is a significant change in provenance between these formations, interpreted as an intra- Miocene unconformity. The late Pliocene to Pleistocene Ba Mieu Formation shows a much greater abundance of Precambrian zircons, including a small age peak at c. 2.5 Ga, interpreted to be derived from old basement exposed in South China. Abundant Cretaceous zircons and a lack of Jurassic zircons in all three formations contrast with the present-day Mekong River delta. Therefore, the data are proposed to show a development of the proto–Mekong River progressing northward from the Da Lat Zone, to the Kontum Massif and northern Vietnam, to capturing of reversed rivers and/or reworking of sediments from old cratons in South China.

INTRODUCTION River, which can be used to reconstruct drainage patterns and give further insights into the geological evolution of the region. Furthermore, the The Da Lat Zone of SE Vietnam (Fig. 1) comprises abundant studied sediments are potential lateral equivalents of thick Oligocene to Cretaceous granitoid intrusions, assigned to the Dinh Quan, Deo Ca, and Miocene sediments in the adjacent offshore basins (e.g., Cuu Long and Ankroet–Cana Complexes, as well as lower Mesozoic sedimentary rocks of Nam Con Son basins), and therefore can provide valuable insights into the the Ban Don Group (Tran, V.T. et al. 1998a, 1998b, 1998c, 1998d, 1998e, origin and deposition of these important hydrocarbon reservoirs in the 1998f) (Fig. 2). In the same region, Neogene to Quaternary sedimentary South China Sea. rocks of the Ba Mieu Formation are widely exposed along the northeast boundary of the Mekong River delta (Tran, V.T. et al. 1995a, 1998b). REGIONAL GEOLOGY Several other formations of similar age (Nam Can, Can Tho, Phung Hiep, Ben Tre, Nha Be, and Binh Trung formations) were reported from Tectonic History of Indochina boreholes located in the Quaternary sedimentary delta successions of the West and East Nam Bo areas (Tran, V.T. et al. 1995a, 1995b). Farther to the Vietnam forms part of the Indochina–East Malaya block (Fig. 1A), east, upper Cenozoic sediments assigned to the Di Linh, Song Luy, and which separated from Gondwana in the Devonian and formed part of Mavieck formations (Fig. 2) are only locally exposed along small river Cathaysia, including West Sumatra, Indochina–East Malaya, South China valleys and in quarries (Tran, V.T. et al. 1998a, 1998c, 1998d). Their and West Burma, during the Permian (Metcalfe 2009, 2011). Collision of depositional ages are poorly known, and no provenance studies have been Cathaysia with North China occurred in the Late Permian to Early Triassic, conducted so far. followed by the accretion of the Sibumasu terrane and Sukhothai Arc to We have targeted a number of upper Cenozoic sedimentary rocks from Indochina in the Late Triassic to Early Jurassic (Sone and Metcalfe 2008; the Di Linh, Song Luy, and Ba Mieu formations in this study for analysis Metcalfe 2009, 2011). of light-mineral and heavy-mineral compositions as well as U-Pb zircon This accretion event was proposed by Carter et al. (2001) to be related to geochronology to identify sources and sediment pathways in the Neogene the Qinling Orogenic Belt formed during North–South China collision, to Quaternary. These can help to determine potential provenance changes leading to compression of Indochina, which caused high-temperature of a proto–Mekong River as well as shifts to the present-day Mekong metamorphism in Vietnam between c. 243 and 258 Ma. Triassic

Published Online: April 2018 Copyright Ó 2018, SEPM (Society for Sedimentary Geology) 1527-1404/18/088-495/$03.00 496 J. HENNIG ET AL. JSR

metamorphism was recorded in metamorphic basement in Vietnam, e.g., Nui Con Voi (Maluski et al. 1999; Lan et al. 2000a) and Kontum Massif (Lo et al. 1999) and related to an Indosinian orogeny (Deprat 1914; Fromaget 1932). Lepvrier et al. (2008) pointed out some similarities between the Kontum granulitic basement of Indochina and South China (Fig. 1A), both of which were probably affected by Caledonian collision with Ordovician–Silurian magmatism and similar Devonian facies in central and northern Vietnam (Indochina) and South China and therefore were likely connected, but they could have been separated afterwards and accreted again during the collision of Gondwana-derived terranes with Indochina in the Permo- Triassic. Various authors (e.g., Seng¨or et al. 1988; Faure et al. 2014; Hieu et al. 2015; Wang, S. et al. 2016) proposed a Triassic Indochina–South China collision based on the Ailoshan–Red River–Song Ma–Song Da suture zones (Fig. 1A) as well as abundant Triassic magmatic rocks found in both blocks (Carter et al. 2001; Nagy et al. 2001; Mao et al. 2013), but the nature and orientations of the proposed subduction zones vary in different reconstruction models. For example, Faure et al. (2014), Halpin et al. (2016), and Wang, S. et al. (2016) suggested a south- to southwest- directed subduction zone beneath Indochina, while Hieu et al. (2015) proposed a north-directed Paleo-Tethys subduction beneath South China– North Vietnam. These concepts are contradictory and explain Triassic magmatic rocks only in South China–northern Vietnam or in central Vietnam. The model of Lepvrier et al. (2008) tried to account for Triassic magmatic rocks in both regions by suggesting a paired subduction, including north-directed subduction beneath South China and another, south-directed subduction, located in Indochina. Carter and Clift (2008) argued that there is no definitive evidence for an Indochina–South China collision and interpreted supposed suture zones as linked to reactivation events related to the Sibumasu–Indochina collision. Alternatively, Triassic magmatism and metamorphism in South China (Taylor and Hayes 1983) and along the eastern margin of Indochina, including the Da Lat Zone, could have continued southwards to Borneo and may have been the result of long-lived westward subduction at the Paleo-Pacific margin from the Triassic to the Cretaceous (Breitfeld et al. 2017; Hennig et al. 2017).

Mesozoic to early Cenozoic Stratigraphy in Southeast Vietnam (Da Lat Zone)

Triassic volcaniclastic rocks in south-central Vietnam and the East Nam Bo area (Fig. 2) were assigned to the Chau Thoi, Buu Long, Dau Tieng, and Mang Yang formations (Tran, V.T. et al. 1995a, 1997, 1998e). Jurassic metasedimentary rocks are abundant in southern Vietnam (Srepok–Tay Nam Bo province and Da Lat Zone; Fig. 1B) and were assigned to the Ban Don Group (Fig. 2), which comprises the Dak Bung, Dak Krong, Ma Da, Song Phan, and Ea Sup formations (Tran, V.T. et al. 1995a, 1997; Tong- Dzuy and Vu 2011). Their depositional ages are based on stratigraphic relations and paleontological evidence (Tong-Dzuy and Vu 2011). The Triassic and Jurassic rocks are intruded by several plutons, which were subdivided into three magmatic complexes (Fig. 2). The Early to mid- Cretaceous Dinh Quan and Deo Ca complexes were dated to 110–112 Ma or c. 115 Ma (Dinh Quan) and 88–92 Ma or c. 118 Ma (Deo Ca) by U-Pb

al. 2004; Wang, Y. et al. 2007, 2017; Liu, R. et al. 2009; Wan et al. 2010; Chen, C.H. et al. 2011; Chen, Z.H. and Xing 2013; Li, J. et al. 2014; Zhao et al. 2015; Chen, FIG. 1.—A) Simplified tectonic map after Metcalfe (2011, 2013) and Hall (2012, Z.H. et al. 2016 and references therein; Sukhothai Arc: Charusiri et al. 1993; Barr et 2017), including selected magmatic ages (red crosses), as well as some metamorphic al. 2000; Qian et al. 2017). All ages shown are zircon U-Pb ages; except for a few Ar/ ages (in italics) and Hf or Nd model ages (in brackets) in the region (Borneo: Davies Ar ages which are indicated on the map. B) Tectonic map of Vietnam modified from et al. 2014; Breitfeld et al. 2017; Hennig et al. 2017; Vietnam: Lan et al. 2000b, Tran, V.T. and Vu (2011), showing the main tectonic units of Vietnam as well as the 2003; Carter et al. 2001; Nagy et al. 2001; Nam et al. 2003; Nguyen, T.T.B. et al. location of nearby offshore basins SE of Vietnam (based on Lee, G.H. et al. 2001). 2004, 2014; Shellnutt et al. 2013; Ishihara and Orihashi 2014; Hieu et al. 2015; The small inset map summarizes all offshore basins in the South China Sea and Gulf Hainan: Jiang, X.Y. et al. 2015; Li, X. et al. 2016; Yan et al. 2017; S China: Deng et of Thailand. JSR RT-EOG -BZRO GSADPOEAC FTED A OE EVIETNAM SE ZONE, LAT DA THE OF PROVENANCE AND AGES ZIRCON U-PB PROTO-MEKONG:

FIG. 2.—Geological map of the Da Lat Zone and eastern Srepok–Tay Nam Bo Zone in SE Vietnam, modified from Tran, V.T. et al. (1995a, 1998a, 1998b, 1998c, 1998d, 1998e, 1998f) and Nguyen, X.B. (2000), including main cities and towns, provinces, and sample locations. 497 498 J. HENNIG ET AL. JSR zircon geochronology (Nguyen, T.T.B. et al. 2004; Shellnutt et al. 2013), the Oligocene Na Duong Formation (Pham et al. 2000) and Rinh Chua while the Ankroet or Cana complex has Late Cretaceous ages of 93–96 Ma Formation (Nguyen, D.D. et al. 1996) (Fig. 3). (Nguyen, T.T.B. et al. 2004) or c. 87 Ma (Shellnutt et al. 2013). Volcanic and volcaniclastic rocks of the Nha Trang, Don Duong, and Dak Rium Song Luy Formation formations are considered equivalents of the plutonic complexes. The youngest magmatic episode formed felsic and mafic dikes assigned to the The Song Luy Formation (Fig. 2) is exposed along the Song Luy and Phan Rang and Cu Mong complexes, of supposedly Paleogene age. Song Mao Rivers in the Thanh Chau (north of Luong Son) and Mo Thap areas (near Cho Lau) (Tong-Dzuy and Vu 2011). The Mo Thap Formation and the Mavieck Formation south of Phan Rang to the east are considered Cenozoic Ocean Basins equivalents of the Song Luy Formation (Tong-Dzuy and Vu 2011). A In the Late Cretaceous the Proto–South China Sea was an ocean basin succession 10 to 20 m thick in the valley of the Song Luy River comprises underlain by Mesozoic oceanic crust between the Indochina–East Malaya three members, including i) a gray, well-cemented silty sandstone, ii) a block and Borneo (Holloway 1982; Taylor and Hayes 1983; Rangin et al. well-cemented pebbly sandstone and gravelstone, and iii) medium-bedded 1990; Hinz et al. 1991; Hall 2002; Hall and Breitfeld 2017). Subduction claystones and sandstones that contain plant remains and tektites (Le 1982; beneath northern Borneo started in the Eocene and ceased in the early Nguyen, D.T. 1999). Miocene and eliminated the Proto–South China Sea. This resulted in K-Ar analysis carried out on tektites from the Mo Thap area suggested extension and led to the opening of the South China Sea from the an age of 0.6 Ma (Le 1982). However, this is not supported by diatoms Oligocene to early Miocene (Taylor and Hayes 1980, 1983; Briais et al. indicating a Pliocene age (Tong-Dzuy and Vu 2011). Sporomorphs 1993; Barckhausen and Roeser 2004; Zhu and Lei 2013; Barckhausen et collected from a borehole in the Mo Thap area have a Pliocene to early al. 2014). There is still much debate about the tectonic evolution of the Pleistocene age (Nguyen, D.T. 1999) (Fig. 3). South China Sea, the Proto–South China Sea and the timing of regional unconformities in the area. Subsidence of the South China Sea margins in Ba Mieu Formation the Paleogene to Neogene, related to further extension, formed large The Ba Mieu Formation is exposed mainly in the East Nam Bo region sedimentary basins, such as the Cuu Long and Nam Con Son basins, which (Fig. 2) and has been subdivided into three members by Tong-Dzuy and Vu include fluvial to marginal marine deposits (Matthews et al. 1997; Lee, (2011) with a total thickness of 125–150 m. Member 1 consists of G.H. et al. 2001) and may have extended into the Da Lat Zone. medium- to coarse-grained pebbly sandstones; Member 2 is a coarse- grained polymictic sandstone which is interbedded with siltstone; and Stratigraphy of Upper Cenozoic Sediments in the Da Lat Zone Member 3 comprises sandstones, siltstones, and claystones. Its deposi- Various Neogene clastic sedimentary formations are reported in the tional environment was interpreted by Kitazawa (2007) as macrotidal geological maps of Tran, V.T. et al. (1995a, 1998a, 1998c, 1998d) for estuarine to delta based on successions along the Dong Nai River (Fig. 2). southeast Vietnam. They include the Binh Trung, Nha Be, Ba Mieu, Di Nguyen, D.D. et al. (1996) suggested an upper Pliocene age for the Ba Linh, Song Luy, and Mavieck formations, as well as the Nam Can, Can Mieu Formation based on diatoms and plant fragments. More recently, Tho, Phung Hiep, and Ben Tre formations, which were identified only optically stimulated luminescence (OSL) dating of the poorly consolidated from boreholes in the eastern Mekong delta (West Nam Bo). In the study Ba Mieu and Thu Duc formations (Fig. 3) indicated a Pleistocene area of the Da Lat Zone the clastic Song Luy, Di Linh, and Ba Mieu depositional age of 176 6 52 ka and 97 6 27 ka, respectively (Kitazawa et formations are exposed in small localized outcrops (Fig. 2). al. 2006). These are younger than a floral assemblage identified in Member 3, which was correlated with other Pliocene formations in the area. The Ba Mieu Formation has a conformable contact with the underlying Nha Be Di Linh Formation Formation, of lower Pliocene age (Fig. 3), determined from flora, The Di Linh Formation is exposed in the Lam Dong province southwest sporomorphs, and foraminifera, and therefore was concluded to be upper of Da Lat (Fig. 2) and includes the formerly separated coal-bearing Bao Pliocene (Tong-Dzuy and Vu 2011). The Nha Be Formation rests Loc Formation and the bentonite-bearing Di Linh Formation, which extend unconformably on the upper Miocene Binh Trung Formation, dated from from Bao Loc in the west to Di Linh and Dinh Van in the east (Trinh 1981; palynological evidence (Tong-Dzuy and Vu 2011). Its lower boundary is Tran and Nguyen 1982; Tran, V.T. et al. 1998a, 1998d; Tong-Dzuy and Vu unknown. 2011). The formation has been subdivided into four or five members with a FIELD RELATIONS total thickness of 110–145 m based on outcrop and borehole observations Di Linh Formation (Nguyen, D.D. et al. 1996; Tong-Dzuy and Vu 2011). The first member contains ash-gray medium- to coarse-grained quartz sandstones, pebbly Description.—The Di Linh Formation crops out locally in Neogene to sandstones, siltstones, and claystones which are interbedded with basalt Quaternary deposits assigned to the Tuc Trung and Dai Nga formations layers. The second and third members are formed by coal- or bentonite- (Fig. 2) in the area between Dinh Van in the northeast to Bao Loc in the bearing sandstones, siltstones, and carbonaceous shales. They are followed southwest (Tran, V.T. et al. 1998a, 1998d). The formation was sampled by quartz gritstones grading upwards into sandstones with interbeds of (VDLZ_42.1) in an old bentonite quarry at Dai Ninh east of Di Linh (Fig. bentonite. The uppermost member is represented by fine-grained sandstone 2). A sequence of several subhorizontally bedded claystone and bentonite grading upwards into white-gray claystone, which is interbedded with layers overlain by basalt was observed (Fig. 4A). Poorly sorted mud-rich bentonite and is overlain by basalt (Tong-Dzuy and Vu 2011). sandstones to siltstones forming several lenticular bodies are interbedded Stratigraphically, the Di Linh Formation unconformably overlies the (Fig. 4B). The sandstone beds contain pebble-grade white mud clasts and Upper Cretaceous Don Duong Formation (Tong-Dzuy and Vu 2011). An black shale clasts (max. 1 cm in length), as well as some rootlets. A fining- Oligocene to Miocene depositional age was favored by Tong-Dzuy and Vu upward sequence from clay-bearing sandstones and siltstones to claystones (2011) based on interbedded basalts with K-Ar cooling ages of 9–16 Ma, and bentonite overlain by basalt may represent the uppermost member of and sporomorph and palynomorph assemblages which were correlated to the Di Linh Formation described by Nguyen, D.D. et al. (1996). JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 499

FIG. 3.—Summary of the Cenozoic stratigraphy of the on-land clastic sediments from the Da Lat Zone and units reported from northern Vietnam (Bac Bo zone), as well as their probable offshore age equivalents in the Cuu Long and Nam Con Son Basins (adapted from Matthews et al. 1997; Lee, G.H. et al. 2001; Cuong and Warren 2009; Tong- Dzuy and Vu 2011).

The ages of the basalts are poorly known. The uppermost basalt layer on Song Luy Formation top of the Di Linh Formation is likely related to an early Miocene to late Miocene (40Ar/39Ar: 17.6 to 7.9 Ma; Lee, T.Y. et al. 1998; Hoang and Description.—The Song Luy Formation is exposed near the coast Flower 1998) or middle Miocene to early Pliocene (K-Ar: 13.8 to 4.1 Ma; between Phan Thiet and Hon Rom (VDLZ_7.1), and east of Luong Son An et al. 2017) episode of basaltic volcanism that has been subdivided into (VDLZ_9.1) and Cho Lau (Fig. 2). Outcrops were studied along a small the Neogene to Quaternary Tuc Trung and Dai Nga formations and the river and a localized exposure on the hillside. The outcrops are c. 3–4 m in Quaternary Xuan Loc Formation (Fig. 2) by Tran, V.T. et al. (1998a, height and show horizontal to subhorizontal bedding (Fig. 4C, D). The 1998b, 1998d). rocks are gray to yellowish poorly sorted sandstones with grain sizes ranging from mud to coarse sand. The main components are quartz and Interpretation.—Thick bentonite beds with interbedded bentonite- feldspar, which are moderately well cemented. Locally, small carbonaceous bearing sandstone to siltstone layers, which are overlain by potentially mud lenses, as well as unfilled to partly mud-filled Skolithos burrows, were contemporaneous basalts, indicate a volcanogenic origin for parts of this observed with a maximum length of c. 3.5 cm (Fig. 4E, F). The rocks are formation. cut by thin quartz veins. The outcrops are moderately weathered, including Trough cross bedding in a lenticular sand body is indicative of channel limonitic alteration in places and unconsolidated rocks at the surface which structures (Reading 1996; Miall 2013). show some trough cross-bedding. 500 J. HENNIG ET AL. JSR

FIG. 4.—Outcrop and hand-specimen photos of the Di Linh (A, B) and Song Luy formations (C–F). A) Thick sequence of claystones which is overlain by basalt. B) The siltstone to sandstone beds form lenticular channelized sand bodies in the succession. C, D) Planar-bedded sandstones with some trough cross-bedding and lenticular channel structures. E, F) The sandstones show some local bioturbation, including mud-filled Skolithos. JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 501

FIG. 5.—Outcrop photos of the Ba Mieu Formation. A) A c. 10-m-thick sequence of planar-bedded sediments is exposed in an old quarry. It shows a fining-upward sequence consisting of pebbly sandstones and conglomerates, which are overlain by sandstone beds and a claystone unit at the top. B) Pebbly sandstones with tabular-cross- bedded conglomerate lenses and planar-interbedded thin claystone layers. C) The sandstone unit consists of several massive to planar-cross-bedded sandstone beds which are also interbedded with thin claystone layers. D) The uppermost unit is formed by a thick package of planar-bedded claystones which is interbedded with thin sandstone layers and lenses.

Interpretation.—The facies is composed of compositionally and succession of approximately 10 m comprises a fining-upward sequence of texturally immature material, indicating a fresh proximal source. pebbly sandstones and conglomerate lenses to sandstones and thick Predominantly massive sandstones with minor bioturbation were observed packages of claystones. The basal part consists of 1.0–1.5 m of red to dark in this formation. Skolithos ichnofacies may indicate a sandy shore (littoral pink pebbly sandstones (VDLZ_52.1) which contain conglomerate lenses zone) to shelf (sublittoral zone) environment (Bromley and Asgaard 1991; and show planar cross-bedding (Fig. 5B), which has a paleo-flow direction Benton and Harper 1997; Buatois and Ma´ngano 2011). The rocks show to the southeast (dip direction/dip angle: 118/34, averaged from three some poorly preserved trough cross bedding which can be interpreted as measurements). The beds are interbedded with several thin layers of white delta channels based on the observed bioturbation. The absence of fine to gray claystone (c. 5–10 cm) which show sharp boundaries. This is sediment preservation may indicate channel accretion. The sediments show stratigraphically followed by a c. 2–3-m-thick unit of yellow to orange or both poorly preserved and well preserved trough cross bedding which is red fine- to medium-grained sandstones that form massive (VDLZ_52.2) interpreted as the migration of bedforms under conditions of unidirectional or, subordinately, planar cross-bedded sets of c. 10–20 cm thickness (Fig. currents. Due to the presence of Skolithos these features are interpreted as 5C) and poorly preserved Skolithos burrows which are unfilled in the soft representing channels in a fluvio-deltaic environment, or as sandy bars in a sediment. The planar sets have an orientation of 140/36 (dip direction/dip well-established channel system (Vos and Tankard 1981; Nichols 2009). angle), averaged from three measurements. Locally, subhorizontal clay- stone layers are interbedded with the sandstones. Ba Mieu Formation At the top, a thick succession (c. 3–4 m) of gray to pink horizontally laminated to undulated claystones is exposed (Fig. 5D). The pink color Description.—Clastic sediments assigned to the Ba Mieu Formation occurs either along the bedding plane or is irregular and cuts through the were studied in an old quarry c. 9 km southeast of An Binh (Figs. 2, 5A). A beds, indicating that it is related to secondary alteration after deposition 502 J. HENNIG ET AL. JSR under atmospheric conditions. In a c. 30-cm-thick interval an undulated (opaque grains are excluded from the analysis). The mounts were carbon- irregular sand layer of c. 2–8 cm width, as well as several small sand lenses coated at a thickness of 5–10 nm and subsequently analyzed with an EDS/ with pinching-out structures, are abundant in the claystones. EDX (energy-dispersive spectroscopy/X-ray spectroscopy) SDD-X-Max50 At the uppermost part of this unit, c. 20–60 cm of weathered claystones on a Hitachi S3000N secondary electron microscope (SEM) to analyze the and sandstones are exposed, followed by a thin laterite horizon (max. 5 cm chemical properties of each grain. A Cu standard was used as reference, in width) which marks the boundary to an unconformably overlying and analysis was performed using the AZtec software (Oxford Instru- Pleistocene basal conglomerate of c. 1.5 m thickness. ments). This allows identification of opaque minerals as well as minerals that may have an opaque appearance, such as rutile, if grains are thick. Interpretation.—Planar cross bedding in coarse sediments usually Furthermore, comparison of the chemical properties with the optical represents the accretion of basal channel-confluence bars or lobate linguoid properties enables identification of aluminosilicate polymorphs. At least bars in fluvial systems (Steel and Thompson 1983; Khadkikar 1999). It can 240 translucent heavy minerals were analyzed for each sample (Supple- also represent the evolution of these bars into deltaic lobes in a marginal mental File 2). The majority of titanium dioxide (TiO2) grains were rutile. marine setting (e.g., Miall 2013), and this interpretation is supported by the Minor occurrences of brookite and anatase were included with rutile when ichnofacies present. point counting. Tabular cross-bedded sandstones represent the migration of dune forms at low flow velocities, sometimes in a subaqueous setting (e.g., Miall LA-ICP-MS Zircon U-Pb Geochronology 2013), possibly at the base of a delta channel (Johnson and Dashtgard 2014). Restricted bioturbation found in a few beds suggest stressed The heavy-mineral separates were further processed for zircon extraction using a Frantz Isodynamic Separator (. 1.0 A,˚ 258 tilt angle) environments in brackish water. Skolithos ichnofacies may indicate a sandy 3 shore (littoral zone) to shelf (sublittoral zone) environment (Bromley and and di-iodomethane (DIM) with a density of c. 3.3 g/cm . The hand-picked Asgaard 1991; Benton and Harper 1997; Buatois and Ma´ngano 2011). zircons were mounted in Araldite resin and polished to expose mid grain Massive claystones and thin interbedded claystones in the pebbly sections. Before analysis, photos of the zircons were taken in transmitted sandstones, conglomerates, and sandstones could represent coastal light as well as with a Deben cathodoluminescence (CL) detector to avoid floodplain deposits or tidal flats in a fluvio-deltaic environment (e.g., inclusions or cracks for the analysis spot selection, and to identify internal Hassan et al. 2016). morphological features of the zircons. Oscillatory zoning is associated with magmatic origin, while homogeneous, irregular convolute, or patchy structures are possibly related to a metamorphic origin (Corfu et al. 2003). ANALYTICAL BACKGROUND Laser ablation inductively coupled plasma mass spectrometry (LA-ICP- Analysis of Light-Mineral Modes MS) was carried out at University College London, Birkbeck College, on a 193 nm laser that is coupled to an Agilent 7700 mass spectrometer. The Thin sections stained for K-feldspar and plagioclase were prepared using Pleˇsovice zircon standard with an age of 337.17 6 0.37 Ma (Sla´ma et al. sodium cobaltinitrite, and barium chloride and amaranth solution for the 2008) and the NIST 612 silicate glass standard (Pearce et al. 1997) were five samples to analyze light-mineral compositions. Point-counting used for calibration. Analyses were carried out using a beam size of 25 lm. analysis was performed on a Zeiss Axiolab following the Gazzi-Dickinson Data reduction was performed using the GLITTER software (Griffin et al. method of Gazzi (1966), Dickinson (1970), and Gazzi et al. (1973). Five 2008) and the common Pb correction spreadsheet of Andersen (2002) hundred counts were made for each sample over an evenly distributed grid which is proposed for 204Pb common Pb-independent analysis. Concordant using an automatic stepping stage and the software PetrogLite to obtain at ages were determined by a difference of 10% between the 206Pb–238U least 400 framework grains (Supplemental File 1, see Supplemental and 207Pb–235U systems for ages , 1000 Ma and between the 206Pb–238U Material). and 207Pb–206Pb systems for ages 1000 Ma. Ages are reported at the The samples were named using the classification scheme of Pettijohn et 61r error level. Where cores and rims were observed in CL images, both al. (1987) and plotted on the QFL (quartz–feldspar–lithics) and QmFLt sites were targeted following the approach described by Zimmermann et al. ((undulose) monocrystalline and volcanic quartz–feldspar–total lithics) (in press) in order to detect all age peaks, which is important for provenance diagrams of Dickinson et al. (1983). provenance studies. The data are displayed in zircon age histograms with probability density plots which were created using a script written by I. Heavy-Mineral Separation Sevastjanova based on the approach of Sircombe (2004) for the R language (R Core Team 2012). The histograms are used to define the age The consolidated to unconsolidated sediments were crushed using a populations discussed in the text. The diagrams consist of two parts mortar and pestle for grain-size analysis. The samples were washed to separating Phanerozoic ages (0–500 Ma) from Precambrian ages (500– separate a grain-size fraction of 63–250 lm using disposable nylon tissue 4000 Ma) at different scales in order to get a better display of older ages, meshes to avoid any contamination. Heavy minerals were separated using a which are usually less abundant in a sample. Single histogram plots lithium heteropolytungstate solution (LST) with a density of c. 2.8–2.9 g/ showing all ages of the samples are compiled in Supplemental File 3. cm3. The proportions of heavy minerals were determined for each sample from the amount of sample material before and after the heavy-liquid RESULTS separation (Supplemental File 2). The heavy-mineral fraction was subsequently split into parts several times to retain a small representative Light-Mineral Modes amount for heavy-mineral analysis. Classification.—Samples VDLZ_7.1 and VDLZ_9.1 of the Song Luy Analysis of Heavy-Mineral Modes Formation are arenites to wackes based on matrix contents of 9% and 16% and are classified as subarkose and arkosic wacke (Fig. 6A) according to Heavy-mineral separates of the five samples analyzed were mounted on the QFL diagram of Pettijohn et al. (1987). Samples VDLZ_42.1 of the Di sticky tape and sealed in Araldite resin (refractive index 1.5798; Banerjee Linh Formation and VDLZ_52.1 and 52.2 of the Ba Mieu Formation have et al. 1970). The mounts were polished and photos were taken in less than 15% matrix contents and are classified as subarkose to transmitted light to create overview maps. Optical mineral identification sublitharenite (VDLZ_42.1) and (quartz-rich) sublitharenites (VDLZ_52.1, was performed on a Nikon Eclipse LV100 for all translucent grains VDLZ_52.2), respectively (Fig. 6A). JSR RT-EOG -BZRO GSADPOEAC FTED A OE EVIETNAM SE ZONE, LAT DA THE OF PROVENANCE AND AGES ZIRCON U-PB PROTO-MEKONG:

FIG. 6.—QFL (quartz–feldspar–lithics) classification diagrams based on A) Pettijohn et al. (1987) and B) Dickinson et al. (1983), and C–F) selected photomicrographs of light-mineral analyses of the Di Linh, Song Luy, and Ba Mieu formations. Fk, K-feldspar; Fp, plagioclase; Qp, polycrystalline quartz; Qm, monocrystalline quartz; Qmu, undulose monocrystalline quartz; Lm, metamorphic lithic fragment; Ls, sedimentary lithic fragment. 503 504 J. HENNIG ET AL. JSR

FIG. 7.—Summary of heavy-mineral analyses of the Di Linh, Song Luy, and Ba Mieu formations with selected photomicrographs of the predominant heavy minerals. As, aluminosilicate, Cr-spl, chrome-spinel; Ep, epidote; Mnz, monazite; Rt, rutile; Tur, tourmaline; Xen, xenotime; Zr, zircon. JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 505

All five samples have high quartz contents and plot in the upper part of determine ages and provenance of these rocks. All analyses are presented the QFL diagram of Dickinson et al. (1983), indicating a recycled-orogen in the Supplemental File 4. character, except for sample VDLZ_9.1, which plots into the continental- block field based on a lower abundance of lithics (Fig. 6B). Samples VDLZ_42.1: Di Linh Formation.—The zircons analyzed are c. 60 to VDLZ_7.1 and 42.1 could also be classified as mixed because they plot 260 lm in length and show a wide range of euhedral and subhedral to (sub- between the recycled-orogen and magmatic-arc fields, according to the ) rounded crystals or anhedral fragments which are colorless, yellowish, or QmFLt diagram of Dickinson et al. (1983), while both samples of the Ba subordinately orange. Mieu Formation have slightly higher lithics contents and lower feldspar 114 concordant analyses were acquired from 113 zircons from the Di contents, therefore plotting into the quartzose recycled-orogen field (Fig. Linh Formation (Fig. 8A). The majority of the analyses form a main age 6B). population in the Cretaceous (91 6 2 Ma to 142 6 2 Ma) and a small age peak at c. 1.8 Ga in the Paleoproterozoic (1836 6 10 Ma to 1882 6 8 Ma). Composition.—The samples analyzed are composed predominantly of Eight zircons have Jurassic, Triassic, Permian, and Carboniferous ages (undulose) monocrystalline quartz and contain subordinately volcanic and (198 6 2 Ma to 327 6 4 Ma), and there are a few ages that are greater than polycrystalline quartz, chert, feldspar, as well as sedimentary, volcanic, and c. 2 Ga (2181 6 6 Ma; 2267 6 8 Ma; 2484 6 7 Ma). metamorphic lithics (Fig. 6C–F). K-feldspar is the dominant feldspar in the The Paleoproterozoic zircons are often yellowish to orange and Di Linh and Song Luy formations but is absent in the Ba Mieu Formation, subrounded to rounded with oscillatory zoning or irregular and where only plagioclase is present. The Ba Mieu Formation also differs homogeneous internal structures. In contrast, Paleozoic to Mesozoic from the others by a slightly higher proportion of metamorphic lithics, zircons are colorless to rarely yellowish and have euhedral to subhedral or which reveal a schistose to gneissic metamorphic character. Subordinate subordinately subrounded to rounded shapes. Their internal morphologies lithics in the Di Linh and Song Luy formations show varying degrees of vary from oscillatory zoning to irregular convolute structures and bright or metamorphic grades ranging from sericite-rich phyllitic fragments to dark CL reflectance. cleaved schist and dynamically recrystallized gneissic fragments. The high abundance of monocrystalline quartz in all five samples, as VDLZ_7.1: Song Luy Formation.—The zircons are euhedral to well as some volcanic quartz and feldspar, suggest a predominantly rounded or anhedral fragments, c. 90 to 360 lm in length, and colorless, magmatic source for the Di Linh, Song Luy and Ba Mieu formations. yellowish, or red to pink. Subordinate polycrystalline quartz is present in all five samples, as well as 153 concordant analyses were acquired from 149 zircons of sample metamorphic lithics with a slightly higher abundance in the Ba Mieu VDLZ_7.1 from the Song Luy Formation (Fig. 8B). They reveal dominant Formation, indicating a contribution from a metamorphic source. age populations in the Cretaceous (85 6 3 Ma to 128 6 2 Ma), Triassic– Permian (223 6 5 Ma to 276 6 4 Ma), and Ordovician–Silurian (416 6 7 Heavy-Mineral Modes Ma to 472 6 9 Ma). One zircon has a core with an age of c. 428 Ma which is overgrown by a dark rim with a Triassic age (c. 241 Ma). A small Heavy minerals were analyzed from the Di Linh Formation number of zircons have Jurassic (180 6 6 Ma; 191 6 8 Ma), Devonian (VDLZ_42.1), the Song Luy Formation (VDLZ_7.1, VDLZ_9.1) and the (374 6 6 Ma; 400 6 6 Ma), and Proterozoic to Archean (712 6 9Mato Ba Mieu Formation (VDLZ_52.1, VDLZ_52.2). The results are summa- 2631 6 15 Ma) ages. Three of these ages (c. 725 Ma; 1375 Ma; 1383 Ma) rized in Figure 7, and heavy-mineral counts as well as ZTR (zircon– are from cores which are overgrown by Silurian and Triassic oscillatory tourmaline–rutile) and ZTi (zircon–tourmaline) indices (Hubert 1962; zoned zircons (c. 428 Ma; 249 Ma) and an Ordovician dark zircon rim (c. Mange and Wright 2007) are provided in Supplemental File 2. The main 454 Ma), respectively. opaque mineral for all samples analyzed is ilmenite. The majority of Mesozoic and Paleozoic zircons are euhedral to Heavy minerals of sample VDLZ_42.1 of the Di Linh Formation are subhedral and show oscillatory zoning. A few of them are rounded grains predominantly rutile (~ 68%) and tourmaline (~ 27%). Subordinate zircon which are colorless or red to pink, and reveal oscillatory zoning, patchy is present at 3.4%, and other accessory heavy minerals include andalusite structures, or dark CL reflectance. Proterozoic to Archean zircons are (0.8%), monazite (0.8%), and epidote (0.8%). rounded to subhedral, and show oscillatory zoning, irregular convolute Both samples of the Song Luy Formation differ from the Di Linh structures, or dark CL reflectance, partly due to a pervasive pink color. Formation by a smaller proportion of rutile (~ 37–41%) and a larger proportion of zircon (~ 19–25%), tourmaline (~ 21–22%) and aluminosilicate (~ 15–16%). The aluminosilicate polymorphs are VDLZ_9.1: Song Luy Formation.—The zircons are c. 70 to 390 lm approximately 60% sillimanite, 25–40% kyanite, and 2–12% andalusite. in length and euhedral or anhedral to rounded with colors ranging from Subordinately, monazite (1.2–1.9%) and chrome-spinel (0.7–1.6%) were colorless and yellowish to red or pink. detected. 130 concordant analyses of 125 zircons from this sample of the Song The Ba Mieu Formation has a heavy-mineral composition similar to that Luy Formation (Fig. 8C) yielded dominant age populations in the of the Song Luy Formation. The majority of heavy minerals are rutile (~ Cretaceous (83 6 1 Ma to 120 6 2 Ma) and Triassic (216 6 3Mato 55–59%), zircon (~ 15%), and tourmaline (~ 15–21%), and there are 250 6 4 Ma), and minor peaks in the Silurian (422 6 5 Ma to 432 6 6 small amounts of monazite (5.4–7.4%) and aluminosilicate (2.2–3.3%) at Ma) and Paleoproterozoic (1783 6 28 Ma to 1948 6 22 Ma). There were proportions different from those in the Song Luy Formation. The subordinate Jurassic (176 6 3 Ma), Permian (257 6 3 Ma to 290 6 4 aluminosilicates are much rarer in the Ba Mieu Formation and include Ma), Devonian (384 6 7 Ma), Ordovician (442 6 8 Ma to 483 6 6 Ma), variable amounts of sillimanite, kyanite, and andalusite. Similarly to the and Proterozoic (553 6 8 Ma to 2223 6 36 Ma) ages. One zircon showed Song Luy Formation, chrome-spinel is accessory (0.4–0.6%), as well as an irregular patchy core with an age of c. 1079 Ma which was overgrown xenotime (0.4–0.9%), which was identified only in the Ba Mieu Formation. by a dark rim of c. 461 Ma. Cretaceous and Triassic zircons are euhedral to rounded and show LA-ICP-MS Zircon U-Pb Geochronology oscillatory zoning or dark CL reflectance. Permian zircons are very similar but also include elongate crystals with oscillatory zoning. In contrast, the Zircons were separated from the Song Luy, Di Linh, and Ba Mieu few Jurassic and Devonian zircons are subrounded with irregular internal formations and dated by U-Pb geochronology using LA-ICP-MS to structures. Silurian, Ordovician, and Proterozoic ages were predominantly 506 J. HENNIG ET AL. JSR JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 507 obtained from colorless or yellowish (sub-) rounded zircons showing The youngest age of c. 47 Ma was obtained from a euhedral zircon with oscillatory zoning or patchy to homogeneous internal structures, or pink oscillatory zoning. zircons with dark CL reflectance. DISCUSSION VDLZ_52.1: Ba Mieu Formation.—The zircons are euhedral to rounded or anhedral fragments, c. 60 to 410 lm in length, and colorless, Depositional Environment yellowish, or red to pink. Limited exposures of late Cenozoic sedimentary rocks assigned to the 130 concordant analyses of 125 zircons were analyzed from sample Di Linh, Song Luy, and Ba Mieu formations were sampled and analyzed VDLZ_52.1 of the Ba Mieu Formation (Fig. 8D). This sample has main for sedimentary provenance. The formations include conglomerates, age populations in the Cretaceous (92 6 3 Ma to 129 6 4 Ma), Triassic pebbly sandstones, sandstones, siltstones, and claystones. The latter (210 6 3 Ma to 245 6 5 Ma), and Paleoproterozoic (1753 6 17 Ma to contains tuffaceous bentonite, which indicates volcanic activity, possibly 1982 6 15 Ma). An oscillatory zoned zircon revealed a Triassic core age in the Oligocene to Miocene, as determined from K-Ar dating of of c. 213 Ma and a Cretaceous rim age of c. 106 Ma. contemporaneous interbedded and overlying basalts and stratigraphic The sample contains a relatively high proportion of Precambrian ages relations (Tong-Dzuy and Vu 2011). This is partly reflected in the QmFLt (approximately one third of all analyses) and includes small peaks in the diagram (Dickinson et al. 1983), where this sample plots in the mixed area Neoproterozoic (768 6 10 Ma to 818 6 10 Ma) and at the Proterozoic– between the magmatic-arc and recycled-orogen fields because of its higher Archean boundary (2407 6 7 Ma to 2596 6 15 Ma). A Proterozoic zircon feldspar and lithic contents. was analyzed with oscillatory zoned core and rim yielding ages of c. 2470 All three formations include i) planar or trough cross-bedding in and 1944 Ma, respectively. A small number of zircons recorded Cenozoic (pebbly) sandstones or conglomerates, ii) claystone beds of varying (13 6 1 Ma; 50 6 1 Ma), Jurassic (171 6 3 Ma; 179 6 5 Ma), Permian thickness (c. 5 cm to 4 m) and iii) localized bioturbation, indicating tidal (252 6 4 Ma; 272 6 5 Ma; 290 6 11 Ma), Devonian to Cambrian (387 6 flats with isolated channels in a fluvio-deltaic environment. Based on the 5 Ma to 534 6 6 Ma), Proterozoic (966 6 12 Ma to 2341 6 10 Ma), and locations of the small-scale localized exposures they were probably part of Archean (3333 6 7 Ma) ages. a large-scale river- to tidal-flat-dominated delta that extended from west of Two Cenozoic ages were obtained from bright oscillatory zoned or Ho Chi Minh City in the west to Phan Rang in the east, where similar homogeneous zircons. Mesozoic to late Paleozoic zircons are predomi- deposits have been assigned to the Mavieck Formation, and to Tuy Hoa in nantly euhedral to subordinately subrounded, showing oscillatory zoning the northeast where the Kon Tum Formation is exposed just to the or rare patchy structures. Early Paleozoic zircons include similar internal southeast of the Kontum Massif (Tran, V.T. et al. 1997, 1998d). Limited structures but are mainly subrounded to rounded. The majority of these foresets show paleo-flow directions to the southeast, which is similar to the zircons are colorless; only a few colored (yellowish, red, and pink) zircons present-day Mekong delta, suggesting that an earlier delta, interpreted to were identified. In contrast, Proterozoic to Archean ages are mainly represent the proto–Mekong delta, extended farther to the east during the subrounded to rounded with various internal structures, including Neogene and migrated southwestward in the Quaternary. The lithologies oscillatory zoning, patchy, or homogeneous to dark crystals. They show observed in the late Cenozoic clastic formations are similar to those a wide range of colors, including abundant yellowish grains, and pink observed in the present-day Mekong River delta, including clay, silt, sand, zircons which are characteristic of the c. 1.8 Ga peak. pebble, and coarser gravels (Tran, V.T. et al. 1995a, 1995b).

VDLZ_52.2: Ba Mieu Formation.—The zircons are euhedral to Provenance Studies rounded or anhedral fragments, c. 60 to 300 lm in length, and colorless, yellowish, or red to pink. Light-Mineral and Heavy-Mineral Mode Analyses.—Light-mineral 107 concordant analyses of 105 zircons were analyzed from sample analysis of all five samples identified a major proportion of monocrys- VDLZ_52.2 (Fig. 8E). They give an age distribution very similar to that of talline quartz, and a minor proportion of polycrystalline quartz and sample VDLZ_52.1, including main peaks in the Cretaceous (96 6 2Ma metamorphic lithics, indicating a significant contribution from igneous to 127 6 3 Ma), Permo-Triassic (213 6 3 Ma to 259 6 4 Ma), and rocks and subordinately from metasedimentary and metaigneous sources. Paleoproterozoic (1791 6 13 Ma to 1893 6 20 Ma), as well as minor All samples are quartz-rich, dominated by monocrystalline quartz; thus peaks in the Neoproterozoic at ~ 700–850 Ma and ~ 900–1000 Ma, as provenance interpretations of the samples as proposed by Dickinson et al. well as at ~ 2.5 Ga (2447 6 5 Ma to 2574 6 9 Ma). A few other ages (1983) are difficult using QFL diagrams. Chemical weathering under analyzed are Cenozoic (47 6 2 Ma), Jurassic (147 6 3 Ma; 171 6 2 Ma), humid climate conditions promotes feldspar dissolution and breakdown of Carboniferous to Cambrian (332 6 5 Ma to 513 6 6 Ma), and Archean unstable lithic fragments, which cause alteration of the original (3097 6 10 Ma). Several Proterozoic ages were analyzed from zircon compositions (e.g., Suttner et al. 1981; Sevastjanova et al. 2012; van cores which have homogeneous bright or dark CL reflectance. A zircon Hattum et al. 2013; Galin et al. 2017). core of c. 644 Ma is overgrown by an oscillatory zoned rim of c. 246 Ma. Magmatic and metamorphic sources are also reflected in the heavy- The sample is similar to VDLZ_52.1, including Mesozoic and Paleozoic mineral assemblages, which are predominantly zircon, tourmaline, and zircons which show a wide range of shapes from euhedral to rounded with rutile in all three formations, and characteristic of a wide range of igneous some color variations and oscillatory zoning, homogeneous or irregular to and metamorphic rocks, as well as minor occurrences of aluminosilicates patchy internal structures. Older Proterozoic and Archean zircons have the (kyanite, sillimanite, and andalusite), monazite, xenotime, chrome-spinel, same variety of internal structures but are often subrounded to rounded and and epidote. While monazite and xenotime can be accessory minerals in have a more colorful appearance of yellowish, red, pink, and red-brownish. both igneous and metamorphic rocks, the aluminosilicates are index

FIG. 8.—U-Pb zircon age histograms and probability density plots for the five samples analyzed in this study. The samples are arranged in stratigraphic order. A) The Di Linh Formation has a dominant Cretaceous age population and minor Carboniferous to Jurassic and Paleoproterozoic ages. B, C) Samples of the Song Luy Formation have additional Permo-Triassic and Ordovician–Silurian age populations, as well as a few scattered Neoproterozoic and Mesoproterozoic ages. D, E) The Ba Mieu Formation shows age populations similar to those of the Song Luy Formation but contains a much higher proportion of Precambrian zircons, including a large Paleoproterozoic peak at c. 1.8–1.9 Ga and a small Paleoproterozoic to Archean age population at c. 2.5 Ga. 508 J. HENNIG ET AL. JSR

FIG. 9.—Sketch maps of east and Southeast Asia showing A) potential source regions (based on Nguyen, T.T.B. et al. 2004; Wang, Y. et al. 2007; Liu, R. et al. 2009; Wan et al. 2010; Chen, C.H. et al. 2011; Liu, J. et al. 2012; Wang, W. et al. 2012; Chen, Z.H. and Xing 2013; Mao et al. 2013; Shellnutt et al. 2013; Ishihara and Orihashi 2014; Hieu et al. 2015; Jiang, X.Y. et al. 2015; Chen, Z.H. et al. 2016; Halpin et al. 2016; Wang, S. et al. 2016; Yan et al. 2017) for zircon age populations identified in the upper Cenozoic sediments of the Da Lat Zone, as well as the present-day Mekong River (Clift et al. 2006; Clift 2016) with sediment supply from the Qiangtang block, Sibumasu or west Indochina, e.g., Permo-Triassic rocks assigned to the Lampang Group and Chanthaburi area (Sone et al. 2012; Burrett et al. 2014), and east Indochina (e.g., Kontum Massif), and B) development of drainage from the Oligo-Miocene (Phase 1) to the early and late Pliocene to Pleistocene (Phase 2 and 3) of the proto–Mekong River. The simplified map of east Asia was modified from Clift et al. (2006), showing the course of main rivers (blue) and reversed flows (arrows) of the Middle Yangtze River (Clark et al. 2004) and the Pearl River (Clift et al. 2008). The drainage pattern shows a northward progression with main sediment supply coming from Cretaceous rocks of the Da Lat Zone (Phase 1), basement rocks in central and northern Vietnam (Phase 2), and old continental crust of South China which may have been brought in by capturing of reversed rivers, or reworking of their deposits by the Upper proto–Mekong River (Phase 3). minerals for high-temperature, low-pressure metamorphism (andalusite, Sources for Detrital Zircons.—Zircon U-Pb age dating reveals sillimanite) or medium-temperature, medium-pressure metamorphism dominant Cretaceous age populations in all five samples with ages (kyanite, sillimanite). Subordinate chrome-spinel was identified in the ranging from 83 6 1 Ma to 142 6 2 Ma that were predominantly from samples of the Song Luy and Ba Mieu formations, indicating a minor input oscillatory zoned euhedral magmatic zircons. The ages resemble reported from ultrabasic rocks, but it is missing in the Di Linh Formation. A small U-Pb zircon mean ages of c. 87 to 118 Ma for the widespread Cretaceous amount of epidote was recognized in the Di Linh Formation, which is plutons assigned to the Dinh Quan, Deo Ca, and Ankroet magmatic related to retrograde metamorphism of Ca-bearing minerals (e.g., complexes in the Da Lat Zone (Nguyen, T.T.B. et al. 2004; Shellnutt et al. plagioclase). No apatite was found in any of the samples which could be 2013) and can also be correlated with Cretaceous volcanic-arc and post- an indication of intense chemical weathering (Morton and Hallsworth collisional magmatism in West and SW Borneo at c. 72 to 130 Ma (Davies 1994). Furthermore, garnet and staurolite are also absent, although et al. 2014) also related to the Paleo-Pacific subduction at the Indochina– metamorphic rocks were sources, indicating they were probably dissolved East Malaya margin (Breitfeld et al. 2017; Hennig et al. 2017). Other by acid leaching (Morton 1984; Mange and Maurer 1992). This means that Cretaceous intrusions have been reported from Hainan (Jiang, X.Y. et al. heavy-mineral ratios are strongly altered by weathering and stable to 2015; Yan et al. 2017) and SE China (Li, J. et al. 2014; Wang, Y. et al. ultrastable heavy minerals are predominantly preserved. 2017) (Fig. 9A). Due to the wide Cretaceous age ranges of intrusions along K-feldspar is another mineral that is prone to dissolution under acid the Paleo-Pacific margin it is not possible to further distinguish the source leaching conditions (Wilkinson et al. 2001) and therefore abundant K- of Cretaceous zircons. However, given that the Di Linh Formation consists feldspar would be an indication for first-cycle erosion. The Di Linh and predominantly of Cretaceous zircons, proximal sources are considered Song Luy formations contain c. 14–16% of K-feldspar, in contrast to the which result in a bimodal age distribution reflecting the magmatic Ba Mieu Formation, which lacks K-feldspar. These could be related to complexes in SE Vietnam. Although the Song Luy has a unimodal chemical weathering but may also reflect minor input from granitoid distribution, the age range is very similar and is therefore interpreted not to sources. represent a different source but a more restricted local input from mid- JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 509

Cretaceous plutons of the Da Lat Zone. The overlying Ba Mieu Formation is a slightly smaller Cretaceous age population, and a much larger number shows the same bimodal distribution as the Di Linh Formation. of Precambrian ages, forming a dominant Paleoproterozoic (1.8–1.9 Ga) In addition to the dominant Cretaceous age peak, the Di Linh Formation age population and a small peak at c. 2.5 Ga, which differs from the Song shows a minor zircon age population at c. 1.8–1.9 Ga and a few Jurassic to Luy and Di Linh formations. This implies a greater contribution from the Carboniferous ages (Fig. 8A). Paleoproterozoic magmatism at c. 1.8 to 1.9 metamorphic basement rocks, including rocks with Proterozoic Hf and Nd Ga has been reported from various locations in the Wuyishan terrane in SE model ages from the Kontum Massif (Lan et al. 2000b; Hieu et al. 2015), China (e.g., Li, X.H. 1997; Liu, R. et al. 2009; Liu, Q. et al. 2014), for and Proterozoic and Archean Hf and Nd isotope ages as well as zircon U- example, from the Xiaochuan metagranites or the Longzhu rhyolite Pb ages from S and SE China (Xu et al. 2005; Liu, R. et al. 2009; Wan et porphyry, where the Paleoproterozoic zircon ages were interpreted as al. 2010; Chen, Z.H. and Xing 2013; Chen, Z.H. et al. 2016). In addition to crystallization ages. The igneous rocks form part of a possibly widespread the chemical weathering, this may further explain the absence of K- Paleoproterozoic crystalline basement underneath Cathaysia which may feldspar in the samples as more old continental crust has been reworked represent remnants of the Columbia supercontinent (Chen, Z.H. and Xing into the Ba Mieu Formation with a decreased input from the plutonic rocks 2013; Chen, Z.H. et al. 2016). K-Ar, inherited zircon U-Pb, and Hf model of the Da Lat Zone. ages of c. 1.8 Ga reported from the Kontum Massif (Hutchison 1989; Hieu The three youngest zircons analyzed from the Ba Mieu Formation et al. 2015) support a continuation of this basement into central Vietnam yielded two Eocene ages (47 6 2 Ma; 50 6 1 Ma) and a middle Miocene and therefore into Indochina. age (13 6 1 Ma). The Eocene ages may support a Paleogene magmatic In addition to the Paleoproterozoic zircons, the Di Linh Formation event forming the felsic and mafic dikes of the Phan Rang and Cu Mong contains a small number of late Paleozoic to early Mesozoic zircons which complexes, but the number of these youngest zircons is too small to be may be derived either from overprinted Paleoproterozoic metamorphic confident about their origin. Alternatively, they may represent Cretaceous basement rocks (Zhao et al. 2015) or early Mesozoic volcaniclastic rocks zircons that were affected by Pb loss. locally exposed in SE Vietnam (Tran, V.T. et al. 1995a). A metamorphic The significant differences in zircon age populations between the Di origin for some of the zircons is supported by subordinate metamorphic Linh and Song Luy–Ba Mieu formations indicate an important change in lithics recorded in the light-mineral fraction. provenance which is probably related to a regional intra-Miocene The zircon age histograms of the samples VDLZ_7.1 and VDLZ_9.1 unconformity that is likely to continue offshore (Matthews et al. 1997; from the Song Luy Formation show a similar minor peak at c. 1.8–1.9 Ga Lee, G.H. et al. 2001). The Di Linh Formation is only poorly dated; as observed in the Di Linh Formation, but reveal a significant change in therefore the precise age of the unconformity is uncertain. Further studies provenance indicated by a larger Permian–Triassic age population and an of the Di Linh Formation and the offshore sedimentary sequences in the additional Ordovician–Silurian age population, as well as a few zircons Cuu Long and Nam Con Son basins are required to establish the age of the with Cambrian, Neoproterozoic, and Mesoproterozoic ages (Fig. 8B, C). unconformity. The Permian–Triassic zircons are probably related to magmatism and metamorphism during the Triassic Indosinian orogeny, supporting an Implications for the Paleo-Drainage of the Proto-Mekong River during overprint of the Cathaysian basement rocks as proposed by Zhao et al. the late Cenozoic (2015), and may account for oscillatory as well as irregular disturbed Permian and Triassic zircons observed in CL images. Sources related to the West and southwest of Hanoi, the present-day Mekong River yields early Mesozoic Indosinian orogeny with Permian–Triassic zircons are concordant detrital U-Pb zircon age populations at approximately 200–300 possibly the Chau Thoi and Mang Yang formations, which are Ma and 350–420 Ma, discordant zircon ages between 500 and 650 Ma, and predominantly exposed in the Srepok–Tay Nam Bo province (Tran V.T. Concordia upper intercept ages of 0.8–1.0 Ga, 1.9–2.0 Ga, and 2.2–2.3 Ga et al. 1995a, 1997, 1998e), the Kontum Massif (Carter et al. 2001; Nagy et (Bodet and Sch¨arer 2000). Another study farther to the south of the al. 2001; Hieu et al. 2015), NE Vietnam (Ishihara and Orihashi 2014; present-day Mekong River delta reports similar zircon age populations at Halpin et al. 2016), the Truong Son belt (NW Vietnam) (Liu, J. et al. around 200–300 Ma, 400–500 Ma, Paleoproterozoic and Archean (c. 1.7– 2012), Hainan Island (Jiang, X.Y. et al. 2015; Yan et al. 2017), or South 1.8 Ga and 2.5 Ga), as well as an additional Jurassic age population (Clift China (Wang, Y. et al. 2007; Chen, C.H. et al. 2011; Mao et al. 2013) (Fig. et al. 2006). The age populations have been related to drainage from the 9A). Very localized ophiolite-style ultramafic-mafic complexes were Tibetan Plateau (Fig. 9A, B), in particular from the Qiangtang terrane (Clift reported in the melange of the Song Hien Tectonic Zone east of the Nui et al. 2006), which has abundant Permian–Triassic (Yang et al. 2011; Peng Con Voi complex in North Vietnam (Halpin et al. 2016) which could et al. 2015), c. 500 Ma (Liu, Y. et al. 2016), and c. 1.1 Ga zircons (Yuan et represent a source for the subordinately identified chrome-spinels in the al. 2017). Bodet and Sch¨arer (2000) concluded that the Proterozoic and samples. Ordovician–Silurian ages (Nagy et al. 2001; Wan et al. 2010; Archean zircons are similar to sources in China, and not explained by Jiang, X.Y. et al. 2015) were associated with a Caledonian or Guangxi present-day drainage patterns, therefore implying multiple recycling and/or event reported across Cathaysia (Ehiro and Kanisawa 1999; Wu 1999), possible drainage changes. Jurassic zircons reported by Clift et al. (2006) while early Paleozoic ages resemble age populations identified by Usuki et from the Mekong delta could have been derived from the Khorat Group, al. (2013) which were linked to the Indochina and Qiangtang blocks based exposed on the Khorat Plateau, which also includes 1.8 Ga and 2.5 Ga on U-Pb zircon ages and Hf isotopes. zircons (Carter and Moss 1999). Cretaceous zircons are absent in the N The early Pliocene to Pleistocene Song Luy Formation reveals a Laos samples and only rare in the Mekong delta. Although identified from significantly larger content of zircons and aluminosilicates in the heavy- a small number of samples only, the new zircon data suggest three phases mineral assemblages than recorded in the Di Linh Formation. Kyanite and in the development of the proto–Mekong River (Fig. 9B). sillimanite are the predominant aluminosilicate polymorphs, which must be The Oligo-Miocene Di Linh Formation is dominated by Cretaceous derived from metamorphic rocks, therefore suggesting a greater input from zircons with very minor Permo-Triassic and Paleoproterozoic (c. 1.8-1.9 metamorphic sources (Fig. 9A). This could indicate that the majority of Ga) zircons. This contrasts with the present-day Mekong River zircon age aluminosilicates and monazites recorded in the Song Luy Formation are populations, which lack a Cretaceous component and include several other directly linked to the sources of the Ordovician–Silurian zircons, both of age populations (Fig. 10). The Permo-Triassic and older zircon populations which are absent in the Di Linh Formation. of the Plio-Pleistocene Song Luy and Ba Mieu formations are similar to The late Pliocene to Pleistocene Ba Mieu Formation shows a zircon age those of the present-day Mekong River, but they differ significantly by distribution similar to that of the Song Luy Formation. The main difference additional main Cretaceous age populations, which are missing in the 510 J. HENNIG ET AL. JSR JSR PROTO-MEKONG: U-PB ZIRCON AGES AND PROVENANCE OF THE DA LAT ZONE, SE VIETNAM 511

present-day Mekong River sediments. Furthermore, the number of Jurassic from older sedimentary basins in Indochina and the catchment area of the zircons detected from the Song Luy and Ba Mieu formations is much proto-Mekong (Fig. 9B). smaller compared to the Mekong delta sample of Clift et al. (2006); this The Yinggehai Basin in the Gulf of Tonkin formed in the middle to late implies that the Khorat Group and its potential equivalent Jurassic Ban Eocene (Clift and Sun 2006) and continued to subside during movements Don Group were not important sources and thus drainage from the NW is on the Red River Fault in the Oligocene (Sch¨arer et al. 1994) and unlikely. reactivation during the Miocene and later (e.g., Clift and Sun 2006). A major source for all the Cenozoic sediments analyzed in this study are During the middle Miocene to Pliocene the Red River carried sediments Cretaceous rocks of the Da Lat Zone, which are part of a magmatic belt into the basin from Hainan, Central Vietnam, and the Red River Fault Zone along the Indochina–South China margin, which formed due to west- (Jiang, T. et al. 2015; Cao et al. 2015). Detrital-zircon age populations (Fig. directed Paleo-Pacific subduction. The Da Lat Zone can account for almost 10) are similar to those of the upper Cenozoic sediments of the Da Lat all the zircon ages present in the Oligo-Miocene Di Linh Formation (proto- Zone, but there is a much larger Neoproterozoic age population in the Mekong Phase 1; Fig. 9B), which is dominated by Cretaceous zircons with Yinggehai Basin, and the Permo-Triassic age peak is much more very small contributions of late Paleozoic to early Mesozoic zircons, as prominent (Jiang, T. et al. 2015). Small Cretaceous and Jurassic age populations are present in the Yinggehai Basin sediments and are well as Paleoproterozoic zircons. These are interpreted to be derived from interpreted to be related to Mesozoic magmatism and metamorphism local sources: either basement rocks similar to the Kontum Massif, or along the Paleo-Pacific subduction zone (Taylor and Hayes 1983). The Triassic volcanic and volcaniclastic rocks of the Chau Thoi or Mang Yang heavy-mineral assemblages show varying proportions of garnet but no formations south of the Kontum Massif (Tong-Dzuy and Vu 2011). aluminosilicates (Jiang, T. et al. 2015). Garnet was likely removed from the The lower Pliocene to Pleistocene Song Luy Formation has zircon Da Lat Zone sediments by chemical weathering; however, the Yinggehai populations similar to those of the Oligo-Miocene Di Linh Formation, sediments are barren of aluminosilicates, which are generally more stable indicating similar sources and/or reworking of zircons from the Di Linh and therefore contrast to the Da Lat Zone sediments, suggesting partly Formation. In addition, the Song Luy Formation has Permo-Triassic and different source regions and drainage. Ordovician–Silurian populations, and a few Neoproterozoic to Mesopro- The Salween River is located west of the Mekong River but resembles terozoic zircons, which were most likely derived from the Kontum Massif its present-day drainage from the Tibetan Plateau (West Qiangtang) to the north of the Da Lat Zone (proto-Mekong Phase 2; Fig. 9B). Permo- southeastwards into the Gulf of Thailand (Fig. 9B). Lin et al. (2017) Triassic zircons could have also come from the Truong Son belt in North demonstrated a major change in the river’s drainage system since the Vietnam (Liu, J. et al. 2012), and Paleoproterozoic to Archean zircons from Triassic based on a much greater abundance of Proterozoic to Archean nearby small areas of orthogneisses SE of the Nui Con Voi terrane (Nam et zircons (2.1–3.3 Ga) in a Triassic sample compared to recent river al. 2003), and small numbers of chrome-spinels from the Song Hien sediments, which have a main zircon age population at c. 2 Ga. Tectonic Zone (Halpin et al. 2016) (Fig. 9A). Furthermore, Hf isotopes indicated that the recent deposits are dominated

The late Pliocene to Pleistocene Ba Mieu Formation is similar to the by juvenile components with high and positive eHf(T) values and a larger Song Luy Formation but contains many more Precambrian zircons. There number of Mesozoic and Cenozoic isotope ages with peaks at c. 95–125 is a greater contribution from (meta-) igneous rocks of 1.8 Ga, indicating Ma and 220 Ma, which are missing in the Triassic sample. Therefore, Cathaysian crust and Paleoproterozoic to Archean rocks (Chen, Z.H. et al. Triassic sources were interpreted to have been from farther east in the 2016) (Fig. 9A), suggesting drainage from basement in South China, northeastern Songpan–Ganzi Terrane and the western South China block, including the Yangtze and Cathaysia blocks (Fig. 9A), due to river captures whereas modern drainage comes from the Tibetan Plateau (Lin et al. 2017). and drainage reversals (Phase 3; Fig. 9B). Neoproterozoic to Mesoproter- This would indicate a comparable change in direction, although at an ozoic zircons are also more abundant in the Ba Mieu Formation than in the unknown time, as proposed in this study for the proto-Mekong. Di Linh and Song Luy formations, and may be sourced from the Yunkai A more northern to northeastern source for the sediments of this study is terrane (Wan et al. 2010) or other parts of South China (Li, W.X. et al. supported by interpretations of drainage based on sediment cores from the 2005). South China Sea covering the last 17 Myr, which concluded that before c. The Middle Yangtze River and Upper Pearl River have experienced 8 Ma the Mekong was sourced mainly from Indochina, which is similar to reversal of flow (Clark et al. 2004) and were proposed to have been drainage of Phases 1 to 2 in this study, and the larger catchment of the captured by the Red River in the Eocene–lower Miocene (Clift et al. 2008), proto-Mekong in Phases 2 to 3 (Fig. 9B) was established between c. 8 and 4 Ma (Liu, C. et al. 2017). Causes for the final shift towards the present- which then drained the Yangtze craton. Similarly, the Upper Mekong was day Mekong delta to the west might be related to the widespread extrusion proposed to have been captured by the paleo–Red River (Clark et al. 2004), of Cenozoic basalts in SE Vietnam. implying a relatively small-scale proto-Mekong draining only Indochina (Clift et al. 2008). Although this can explain Phase 1 and 2 draining local sources for Cretaceous, Permo-Triassic, and Ordovician–Silurian zircons, it CONCLUSIONS cannot account for the large number of Precambrian zircons in the Ba Mieu (1) Clastic sediments of upper Cenozoic age in the Da Lat Zone are Formation (Phase 3) that were likely transported from South China interpreted as tidal flats with isolated channels in a fluvio-deltaic although the exact routes are speculative. Reversal of Yangtze River environment, suggesting that they represent deposits of the proto- drainage or other rivers could have supplied detrital grains from SE China Mekong River, which formed a wide delta from west of Ho Chi or areas farther to the north (Yangtze craton) or may have been reworked Minh City to Phan Rang in the east.

FIG. 10.—Comparison of zircon age histograms from the A) Yinggehai Basin and B) the present-day Mekong River to the upper Cenozoic sediments analyzed in this study (C–E). A) Zircon age histogram of all concordant ages (errors at 1-sigma uncertainty) from middle Miocene to Pliocene sediments reported by Jiang, T. et al. (2015). B) Zircon age histogram including data of Bodet and Sch¨arer (2000) from i) the Middle Mekong River (all data included; no errors were reported by the authors, therefore were calculated at 4% following the approach of Kl¨otzli et al. (2009)), and ii) of Clift et al. (2006) including all reported preferred ages quoted at 1-standard-error uncertainty. C–E) Zircon age histograms from the upper Cenozoic sediments analyzed in this study. They differ from the present-day Mekong River by dominant Cretaceous age populations and only a few Jurassic zircons. This suggests that the Khorat Plateau and its equivalent of the Jurassic Ban Don Group were not a source, and the drainage of the proto- Mekong lay farther away, from a region to the east or the northeast. 512 J. HENNIG ET AL. JSR

(2) The Oligocene to Miocene Di Linh Formation has a dominant BREITFELD, H.T., HALL, R., GALIN, T., FORSTER, M.A., AND BOUDAGHER-FADEL, M.K., 2017, Cretaceous zircon age population which is interpreted as derived A Triassic to Cretaceous Sundaland–Pacific subduction margin in West Sarawak, Borneo: Tectonophysics, v. 694, p. 35–56. from Cretaceous plutons in the Da Lat Zone, indicating only a small BRIAIS, A., PATRIAT,P.,AND TAPPONNIER, P., 1993, Updated interpretation of magnetic catchment area for the proto-Mekong (Drainage Phase 1). anomalies and seafloor spreading stages in the South China Sea: implications for the (3) In contrast, the early Pliocene to Pleistocene Song Luy Formation Tertiary tectonics of Southeast Asia: Journal of Geophysical Research, v. 98, p. 6299– records a significant change in provenance, with additional Permo- 6328. BROMLEY, R.G., AND ASGAARD, U., 1991, Ichnofacies: a mixture of taphofacies and Triassic and Ordovician–Silurian age populations, and a few biofacies: Lethaia, v. 24, p. 153–163. scattered Neoproterozoic to Mesoproterozoic zircons. Sources were BUATOIS, L.A., AND MA´ NGANO, M.G., 2011, Ichnology: Organism–Substrate Interactions in likely basement rocks of the Kontum Massif and possibly northern Space and Time: Cambridge, UK, Cambridge University Press, 366 p. Vietnam (Drainage Phase 2). BURRETT, C., ZAW, K., MEFFRE, S., LAI, C.K., KHOSITANONT, S., CHAODUMRONG,P., UDCHACHON, M., EKINS, S., AND HALPIN, J., 2014, The configuration of Greater (4) The youngest, late Pliocene to Pleistocene Ba Mieu Formation is Gondwana: evidence from LA ICPMS, U-Pb geochronology of detrital zircons from the similar to the Song Luy Formation but has abundant Neoproterozoic Palaeozoic and Mesozoic of Southeast Asia and China: Gondwana Research, v. 26, p. to Mesoproterozoic zircons, a dominant peak at c. 1.8–1.9 Ga, and a 31–51. small Paleoproterozoic to Archean age population at c. 2.5 Ga. This CAO, L., JIANG, T., WANG, Z., ZHANG,Y.,AND SUN, H., 2015, Provenance of Upper Miocene suggests that sources were old continental crust in South China that sediments in the Yinggehai and Qiongdongnan basins, northwestern South China Sea: evidence from REE, heavy minerals and zircon U-Pb ages: Marine Geology, v. 361, p. was overprinted by Caledonian and Permo-Triassic magmatic and 136–146. metamorphic events. River captures and drainage reversals provided CARTER, A., AND MOSS, S.J., 1999, Combined detrital-zircon fission-track and U-Pb dating: detrital grains from cratons in South China or reworked sedimentary A new approach to understanding hinterland evolution: Geology, v. 27, p. 235–238. rocks derived from them (Drainage Phase 3). CARTER, A., AND CLIFT, P.D., 2008, Was the Indosinian orogeny a Triassic mountain building or a thermotectonic reactivation event?: Comptes Rendus Geoscience, v. 340, p. (5) The abundant Cretaceous zircons and few Jurassic zircons differ 83–93. from zircon age populations of the present-day Mekong River CARTER, A., ROQUES, D., BRISTOW, C., AND KINNY, P., 2001, Understanding Mesozoic sediments, suggesting that the Mekong did not drain across the accretion in Southeast Asia: significance of Triassic thermotectonism (Indosinian Khorat Plateau during the Oligocene to Plio-Pleistocene but likely orogeny) in Vietnam: Geology, v. 29, p. 211–214. CHARUSIRI, P., CLARK, A.H., FARRAR, E., ARCHIBALD,D.,AND CHARUSIRI, B., 1993, Granite flowed from farther to the east or northeast. belts in Thailand: evidence from the 40Ar/39Ar geochronological and geological syntheses: Journal of Southeast Asian Earth Sciences, v. 8, p. 127–136. CHEN, C.H., HSIEH, P.S., LEE, C.Y., AND ZHOU, H.W., 2011, Two episodes of the Indosinian SUPPLEMENTAL MATERIAL thermal event on the South China Block: constraints from LA-ICPMS U-Pb zircon and electron microprobe monazite ages of the Darongshan S-type granitic suite: Gondwana Supplemental files 1–4 are available frim JSR’s Data Archive: http://sepm. Research, v. 19, p. 1008–1023. org/pages.aspx?pageid¼229. CHEN, Z.H., AND XING, G.F., 2013, Petrogenesis of a Palaeoproterozoic S-type granite, central Wuyishan terrane, SE China: implications for early crustal evolution of the Cathaysia Block: International Geology Review, v. 55, p. 1445–1461. ACKNOWLEDGMENTS CHEN, Z.H., XING, G.F., AND ZHAO, X.L., 2016, Palaeoproterozoic A-type magmatism in northern Wuyishan terrane, Southeast China: petrogenesis and tectonic implications: This research was supported by the SE Asia Research Group, funded by a International Geology Review, v. 58, p. 773–786. consortium of oil companies. We are grateful to Talisman/Repsol Vietnam for CLARK, M.K., SCHOENBOHM, L.M., ROYDEN, L.H., WHIPPLE, K.X., BURCHFIEL, B.C., ZHANG, sponsoring the field trip, and in particular to Kevin Meyer, Jeffrey Reinprecht, X., TANG, W., WANG, E., AND CHEN, L., 2004, Surface uplift, tectonics, and erosion of Terrence Lukie, and William Schmidt for help and support in the organization eastern Tibet from large-scale drainage patterns: Tectonics, v. 23, TC1006. CLIFT, P.D., 2016, Assessing effective provenance methods for fluvial sediment in the South of the field trip and insightful discussions about the regional geology. The South China Sea, in Clift, P.D., Harff, J., Wu, J., and Qui, Y., eds., River-Dominated Shelf Vietnam Geological Mapping Division is thanked for permission for sampling Sediments of East Asian Seas: The Geological Society of London, Special Publication and shipment of rocks. 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