Provenance of Oligocene–Miocene sedimentary rocks in the Cuu Long and Nam Con Son basins, Vietnam and early history of the Mekong River

Abstract

The offshore Cuu Long (CLB) and Nam Con Son (NCSB) basins of SE Vietnam are two important Cenozoic hydrocarbon-bearing basins in the southern South China Sea (SCS), which can contribute to understanding the evolution of major SE Asian river systems, in particular the Mekong River. The Oligocene to Early Miocene basin fill of the Cuu Long Basin is dominated by sediment sourced locally from the Da Lat Zone basement on land. Sandstones have abundant Cretaceous detrital zircons and heavy mineral assemblages dominated by apatite and epidote. The Bach Ho Unconformity at c. 16 to 20.5 Ma marks a major provenance change and the overlying Middle to Late Miocene Con Son and Dong Nai formations were sourced by a large river system, which drained Indochina or even the Himalaya, resembling the present-day Mekong River. These formations have heterogeneous detrital zircon populations dominated by Triassic ages and zircon–rutile–tourmaline-dominated heavy mineral assemblages. The Oligocene Cau and Early Miocene Dua formations of the Nam Con Son Basin have a similar provenance to the CLB Con Son and Dong Nai formations, indicating a comparable drainage history of a large proto-Chao Phraya. At the Dua Unconformity at c. 15.4 to 17 Ma the Indochina provenance signature changes to a predominant Sibumasu signature.The early Mekong River evolution is suggested to have involved two river systems. A proto-Mekong 1 initially filled the CLB, while at the same time a proto-Chao Phraya River with a capture area into the Himalaya filled the Malay Basin and/or the NCSB. At the end of the Early Miocene drainage was completely reorganised, and the proto-Mekong 1 captured the headwaters of the proto-Chao Phraya and became the proto-Mekong 2, while the NCSB was filled by sediment from the Malay–Thai Peninsula. This major change marks a very important drainage reorganisation in the southern SCS region.

Syn‐ and post‐rift lower crustal flow under the Sunda Shelf, southern Vietnam: A role for climatically modulated erosion

“…The sedimentation patterns tell us something about the history of the Mekong River, which implies sediment delivery from the north and is consistent with the detrital zircon data (Breitfeld et al, 2022). The first indication of rapid sedimentation in the north of the surveyed area comes in the Late Miocene.…”

“…Drainage systems have been flowing from north to south in the Gulf of Thailand since the Paleogene (Alqahtani et al, 2017; Morley & Morley, 2013; Morley & Racey, 2011) as well as towards the northeast on the Sunda Shelf offshore southern Vietnam. Recent work on detrital zircon U–Pb dates from the Cuu Long Basin indicates sediment largely from neighbouring Indochina basement until the Early Miocene, followed by supply from a paleo‐Mekong (Breitfeld et al, 2022). The Nam Con Son Basin appears to have been supplied from erosion flux from the north as well as from the rift flanks until the Early Miocene (Hall & Morley, 2004) followed by a change to supply by erosion of the Malay and Thai peninsula (Breitfeld et al, 2022).…”

Phylogeny and biogeography of Indochinese freshwater mussels in the genus Pilsbryoconcha Simpson, 1900 (Bivalvia: Unionidae) with descriptions of four new species

“…The area around the Middle Mekong is was noted for its high species-level diversity of unionids, especially for the Rectidentini, Contradentini, and Pseudodontini 3 , 4 , 12 , 16 , and thus undoubtedly represented one of the ancient evolutionary hotspots for unionid radiation 59 . The transcontinental Mekong River, as in its present course, probably did not exist before 17–15 Mya, in the middle Miocene 53 , 54 but instead existed as a proto-Mekong River with restricted drainages in the middle reach of the present-day Mekong River since for at least 25 Mya 53 , or even earlier, since the Early Cretaceous 60 .…”

“…The river systems in Indochina have undergone a series of complex geomorphological changes 53 , 54 that certainly affected species diversification and population structure of freshwater taxa in the region 55 – 57 . In the case of Pilsbryoconcha , time-calibrated phylogeny and ancestral area reconstruction suggested its origin likely occurred in the vicinity of the present-day Khorat Plateau around 43.1 Mya in the Eocene (Fig.…”

Early Cretaceous volcanic-arc magmatism in the Dalat-Kratie Fold Belt of eastern Cambodia: implications for the lithotectonic evolution of the Indochina terrane

“…Frontiers in Earth Science frontiersin.org (336 ± 3 Ma) to Permian (267 ± 3 Ma) mafic and silicic rocks known throughout Indochina. The magmatic rocks may be related to an earlier period of subduction of the Paleotethys and back-arc extension prior to the amalgamation between the Sukhothai and Indochina terranes or subduction of the Paleo-Pacific plate (Hall and Sevastjanova, 2012;Burrett et al, 2014;Wang et al, 2018;Cheng et al, 2019;Kasahara et al, 2021;Shi et al, 2021;Waight et al, 2021;Breitfeld et al, 2022). The Triassic zircon of O05c is contemporaneous with rocks in Cambodia as Cheng et al (2019), Kasahara et al (2021) and Waight et al (2021) reported zircon U-Pb ages from 201.88 ± 0.36 Ma to 238.21 ± 0.31 Ma.…”