The Rajang Unconformity: Major provenance change between the Eocene and Oligo-Miocene sequences in NW Borneo H. Tim Breitfeld1, Juliane Hennig1, Marcelle BouDagher-Fadel2 and Robert Hall1 1 Southeast Asia Research Group, Earth Sciences Department, Royal Holloway University of London, Egham, Surrey, TW20 0EX, United Kingdom 2Department of Earth Sciences, University College London, London, WC1H 0BT, UK

1. Introduction 2. Stratigraphy 5. U-Pb detrital zircon geochronology and provenance The research area is located in the Zone of , NW Borneo. The Miri Zone comprises Upper Paleogene to The offshore Sarawak Basin is a major hydrocarbon 9 Neogene fluvial, deltaic and marine sediments that are unconformably above Lower Paleogene turbidites of the Rajang 7 Cen K J T P C D S O Proterozoic Archean province in SE Asia. A very thick sedimentary 1.2 8 n= 69 n= 44 0.20 Group. The Rajang Group is widely exposed in the Zone, which is separated by the Bukit Mersing Line from the 6 7 sequence of Oligocene to ?Early Miocene age, 1.0 5 6 overlying Miri Zone (Haile, 1974). 0.15 0.8 named Cycle 1 and 2, is an important hydrocarbon umber 5 4 NY-05 4 source and reservoir. The stratigraphy and tectonic 0.6 modified from Haile (1974) and Hennig et al. (2017) 3 0.10 SOUTH 3 Probability 0.4 Setap Fm history are not very well understood. Analyses n 2 CHINA km 2 10˚N Indochina 6 º N 0.05 SEA South China Sea 0 100 200 1 0.2 1 0 0 Sabah The onshore Nyalau Formation is the supposed 0.0 PSCS Brunei 0.00 N South China 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 equivalent of the offshore Cycle 1 sequence. The 14 Sea 0.25 Crocker Cen K J T P C D S O 1.5 7 Proterozoic Archean Nyalau Formation is widely exposed in the Miri 12 fan n= 80 n= 41 4 º N 6 Zone. It conformably overlies the Buan and Tatau 0.20 offshore Sarawak basin 10 Miri Zone 5

1.0 Malay-Thai Formations, and in places unconformably the umber 8 NY−03 0.15 4 Tin Belt Nyalau Formation Belaga Formation. The Belaga Formation of the 6 3 Lupar Line Sarawak 0.10 Probability

0.5 0˚ º Sibu Zone is part of the Rajang Group that Analyses n 4 ?uplifted Rajang Sibu Zone 2 N 2 represents remnants of a large submarine fan 2 0.05 Group Kayan Kalimantan 1 sub- deposited in the Late Cretaceous to Eocene in 0 0 0.0 Ketungau 0.00 6°N basins Basin Mandai West Central Sarawak. The Bawang Member is the part of 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 E. Oligocene Basin Kutai º 14 Melawi Basin Basin 0 the Belaga Formation within the Miri Zone. 100˚E 110˚E - E. Miocene L. Miocene-Plio/ Tatau Fm Cen K J T P C D S O 1.4 Proterozoic Archean West Borneo 12 Kuching Zone n= 76 n= 55

1.2 10

Pleistocene - 0.20 10 9

1.0 8 province umber n 8 7 0.15 º Setap 0.8 6 TB200a SW Borneo Basement 2 S 6

0.6 5

Fm 0.10 Borneo Makassar 4 Probability

Nyalau Fm Analyses 4

? 0.4 Straits 3 2 2 0.05 Miocene E 0.2 1 Java Sea 0 0 4º S Buan Fm 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 major provenance signature change ASTER Global DEM V002 (https://search.earthdata.nasa.gov/search) GEBCO bathymetry 110 º E 112 º E 114 º E 116 º E 118 º E (https://www.gebco.net/data_and_products/gridded_bathymetry_data/) Nyalau

20 Cen K J T P C D S O 7 Proterozoic Archean 0.20

Formation 2.0 18 n= 109 6 n= 17 L 16 0.15 PROTO- There is a major change of depositional environments from deep marine turbidites of the Belaga Formation (Rajang Group) 5 14 1.5 Buan umber 10˚N SOUTH n 12 to the mainly deltaic and tidal-influenced Nyalau Formation which marks an important unconformity, interpreted to be Fm 4 TB199b CHINA Bukit Mersing Line 10 0.10 Oligocene 1.0 associated with the Sarawak Orogeny (Hutchison, 1996). We interpret the Rajang Unconformity (Hall & Breitfeld, 2017; Bawang Mbr 3 SEA ? Tatau 8 Probability Galin et al., 2017) to be caused by a major plate re-organisation associated with the onset of the Proto-South China Sea E Analyses 6 2 ? 0.05

Formation 0.5 4 1 subduction (Hall, 2013). 2

Rangsi Cgl. atau Formation 0 0 Bukit Mersing T 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 L Rajang Unconformity 14 Cen K J T P C D S O 7 Proterozoic Archean 1.5

3. Field relations n= 18 0.15 12 n= 106 6

Belaga Fm 10 5 0˚ uplifted Rajang umber Rangsi Conglomerate Nyalau Formation - deltaic and tidal-influenced Tatau Formation - fluvial conglomerates and cross- V n Group 1.0

8 4 TA−04 0.10 sandstone-mudstone alternations bedded sandstones Eocene Bawang M Mbr 6 3 Probability

BML Analyses

4 0.5 2 0.05 lower Early 1 m modified from Heng (1992) 2 1 Belaga 100˚E 110˚E Oligocene double channel 0 0 Formation Rajang 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 E Unconformity 27 5

24 Cen K J T P C D S O Proterozoic Archean 0.15 n= 100 3.0 21 4 n= 11 2.5 18 PROTO- 10˚N 0.10

umber 3 SOUTH

2.0 TB199a 4. Petrography 15 CHINA 12 1.5 2 SEA 9 Qm The Nyalau and Tatau Formations are Calcareous beds of the upper Nyalau Formation yielded large benthic Probability 0.05 Analyses n 1.0 6 mainly composed of quartz and various foraminifera, which indicate a Late Aquitanian to Burdigalian (Early Miocene) 1 lithic fragments including polycrystalline age and a forereefal environment of deposition. 3 0.5 ? Rajang Group fan 0 0 ripples, trough 0.0 coal layers and 0.00 (Belaga Fm) quartz and chert, indicating a recycled 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 Malay-Thai cross-bedding West fragments 5 Tin Belt 3.0 orogenic provenance in the QmFLt 1 mm Cen K J T P C D S O Proterozoic Archean 0.14 Borneo

n= 133 n= 8 0.12 diagram. The Bawang Member has 20 2.5 4 18 0˚ 0.10

generally more feldspar, which results in 16 2.0 Bawang Member 1 m umber 3 14 a magmatic arc/mixed provenance. TB47 0.08 Schwaner granites

12 1.5

10 2 0.06 (modified from Galin et al., 2017) There is a significant overlap between Probability

8 1.0

Nyalau Fm Analyses n

6 0.04 Middle to polymict trough cross- the three analysed formations. 1 4 0.5 100˚E 110˚E

T 0.02 Late Eocene conglomerate bedding atau Fm 2 bioturbated tidal heterolithics 0 0 0.0 0.00 Bawang Mbr Light mineral analysis does not 0 100 200 300 400 500 500 1000 1500 2000 2500 3000 3500 4000 show significant differences in 1 mm Age, Ma Age, Ma Recycled orogen composition and provenance. Continental block Austrotrillina asmariensis Lepidocyclina stillafera 6. Conclusions Magmatic arc 1 mm 1 mm Rangsi Conglomerate thick sandstone beds ŸThere was a major change in depositional environment from the turbiditic deep marine Rajang Group (Belaga Formation) to the fluvial, deltaic and tidal Tatau and Nyalau Formations in the Late Eocene to Early Oligocene. Bawang Member (Belaga Formation) - steeply dipping turbidites F Lt ŸThe Rajang Group submarine fan was sourced by the Schwaner granites and Sundaland (Malay-Thai Tin belt, West Borneo) in the Late Cretaceous to early Paleogene. 10 cm Rangsi Conglomerate Clastic sediments yielded ŸThe probably Lower Oligocene Rangsi Conglomerate directly above the Rajang Unconformity has an almost identical (Tatau Fm) abundant zircons, which were zircon age signature, indicating reworking of the Rajang Group. separated for U-Pb LA-ICP-MS ŸThere was a major change in provenance in the Lower to Upper Oligocene Tatau and Nylau Formations, suggesting a provenance studies. main source shift towards Sundaland and Indochina (Malay-Thai Tin belt or ?Vietnam). 1 mm Heterostegina (Vlerkina) borneesis Spiroclypeus tidoenganensis ŸThere was little to no sediment supply from Borneo in the Oligocene to Early Miocene. ŸAlmost no zircon ages were recorded close to the depositional age of the Tatau and Nyalau Formations. This implies Rajang there was only limited magmatism in Borneo in the Paleogene to lower Neogene. Therefore the successions were not Unconformity References deposited at an active subduction margin. Galin, T., Breitfeld, H.T., Hall, R., Sevastjanova, I. (2017). Provenance of the Cretaceous–Eocene Rajang Group submarine fan, Sarawak, from light and heavy mineral assemblages and U-Pb zircon geochronology. Gondwana Research 51, 209-233. Bawang Mbr Haile, N. S. (1974). Borneo. Mesozoic‐Cenozoic Orogenic Belts. A. M. Spencer. 4: 333‐347. Hall, R. (2012). Late Jurassic–Cenozoic reconstructions of the Indonesian region and the Indian Ocean. Tectonophysics, Vol. 570-571, 2012, p. 1-41. (Belaga Fm) Hall, R. (2013). Contraction and extension in northern Borneo driven by subduction rollback. Journal of Asian Earth Sciences 76, 399-411. Acknowledgements Bawang Mbr Hall, R., Breitfeld, H.T. (2017). Nature and demise of the Proto-South China Sea. Geological Society of Malaysia Bulletin 63, 61-76. We thank the Minerals and Geoscience Department Malaysia – Sarawak (JMG) for collaboration in carrying out work in the Miri Zone, especially Heng, Y.E. (1992). Geological Map of Sarawak, 1:500,000. Geological Survey of Malaysia. (Belaga Fm) Hennig, J., Breitfeld, H.T., Hall, R., Nugraha, A.M.S. (2017). The Mesozoic tectono-magmatic evolution at the Paleo-Pacific subduction zone in West Borneo. Gondwana Research 48, 292-310. Thomson Galin. Andy Carter and Martin Rittner from the University College London is thanked for assisting with the LA-ICP-MS analysis and Inga 50 cm modified from Galin et al. (2017) modified from Galin et al. (2017) Hutchison, C.S. (1996). The 'Rajang Accretionary Prism' and 'Lupar Line' problem of Borneo. In: Hall, R., Blundell, D.J. (Eds.), Tectonic Evolution of SE Asia, Geological Society London Special Publication, 106, 247-261. Sevastjanova for creating the histogram routine are thanked. This project was supported by the SE Asia Research Group.

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