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Tanaka G.,Gunma Museum of Natural History | Komatsu T.,Kumamoto University | Saito Y.,Geological Survey of Japan | Nguyen D.P.,Vietnam Institute of Geosciences and Mineral Resources VIGMR | Vu Q.L.,Ai Mo 1
Marine Micropaleontology | Year: 2011

Eighty-five species belonging to 46 genera of Ostracoda were recovered from 169 samples in three sediment cores (VN, NB, and GA) from the Red River delta plain, northeastern Vietnam. From a Q-mode cluster analysis, we recognized three biofacies (Biofacies A, B, and C). Biofacies B is subdivided into Biofacies BI and BII. The environment of Biofacies A is interpreted as open marine, those of Biofacies BI and BII as marine to brackish water and brackish water, respectively, and that of Biofacies C as oligohaline to mesohaline brackish water. The Holocene sediments consist of estuarine sediments deposited under relatively high sea levels following transgression, deposited during 11-8.5cal kyr BP, overlain by deltaic sediments deposited under relatively low sea levels following regression, and the ostracod assemblages reflect these changes. The estuarine sediments of the early Holocene transgression are characterized by ostracod Biofacies BI. An erosional surface marks an abrupt change to the shelf environment, accompanied by sediment starvation. The interval around this surface, identified as the maximum flooding surface, is characterized by ostracod Biofacies A. During the subsequent delta progradation, the sedimentary environment changed from a prodelta to delta front, and then to a delta plain, and the corresponding ostracod assemblages changed from Biofacies A to BI, BII, and then C in ascending order. Taken together, our results show that the temporal changes in ostracod assemblages since 10. cal kyr BP are closely related to the evolution of the Red River delta system and can serve as good indicators of paleoenvironmental changes. © 2011 Elsevier B.V.

Dupret V.,Lille University of Science and Technology | Dupret V.,Uppsala University | Phuong T.H.,Vietnam National University, Hanoi | Thanh T.-D.,Vietnam National University, Hanoi | And 4 more authors.
Journal of Vertebrate Paleontology | Year: 2011

The acanthothoracid Hagiangella goujeti Janvier, 2005, has been described exclusively on the basis of isolated thoracic plates from the Lochkovian (Lower Devonian) Khao Loc Formation of Tung Vai, Ha Giang Province, northern Vietnam. It is characterized by a very high, triangular median crest on the median dorsal plate, and has been referred to the Acanthothoraci on the basis of the morphology of its fused anterolateral, spinal and anterior ventrolateral plates, and the characteristic stellate ornamentation of the group. Isolated plates of H. goujeti are relatively abundant at Tung Vai and no other placoderm taxon from this locality seems to share the same type of ornamentation. However, the skull of this species remained elusive. Here we report two well-preserved skull roofs from Tung Vai, which we refer to H. goujeti. They display the same stellate ornamentation and small size as the previously described plates of the thoracic armor of this species. This new material shows that the head of H. goujeti is surprisingly short (i.e., possibly lacking dermal rostral and pineal elements), in contrast to the elongate and narrow skull of all other acanthothoracids. The combination of unique characters (e.g., presence of two pairs of posterior pit lines, two pairs of central and paranuchal plates, etc.) suggests a possible sister group relationship to the placoderm assemblage Petalichthyida + Ptyctodontida + Arthrodira. © 2011 by the Society of Vertebrate Paleontology.

Komatsu T.,Kumamoto University | Naruse H.,Kyoto University | Shigeta Y.,National Museum of Nature and Science | Takashima R.,Tohoku University | And 7 more authors.
Sedimentary Geology | Year: 2014

In the An Chau basin in northeastern Vietnam, the Olenekian (Lower Triassic) stage comprises mixed carbonate and siliciclastic shallow marine to marginal basin deposits that extend into the southern Nanpanjiang basin in South China. The upper Lang Son Formation, which is dominated by siliciclastic facies, is composed of storm- and wave-influenced shallow marine and slope deposits. The overlying and interfingering Bac Thuy Formation consists of tidal flat, wave-influenced carbonate platform, slope, and marginal basin plain deposits. The tidal flat deposits are composed mainly of lenticular, wavy-bedded, or thin flat-bedded carbonates containing desiccation cracks and ripples that are indicative of bidirectional paleocurrents. The platform carbonates are characterized by wave ripples, ooids, and abundant marine mollusks and microfossils; the molluscan fossil assemblage primarily contains Smithian ammonoids such as Owenites koeneni. The slope deposits are characterized by limestone breccia and slump beds. The marginal basin plain deposits consist of classical turbidite beds, minor limestone breccia, and hemipelagic mudstone and marl containing the Spathian ammonoids Tirolites and Columbites. The turbidite sets may represent frontal splay environments in the marginal basin plain. These Olenekian carbonate and siliciclastic facies are typical of a transgressive succession.The Smithian-Spathian boundary and the anoxic to dysoxic facies of the latest Smithian are characterized by organic-rich dark gray limestone and mudstone deposits intercalated in the succession of slope to marginal basin plain facies, which are composed mainly of gravity flow and hemipelagic deposits. The organic-rich deposits show no evidence of bioturbation and are characterized by low-diversity fauna. Clastics in the marginal basin plain deposits were apparently supplied from the west; hence, the paleocurrent direction of the siliciclastic gravity flows was approximately eastward. © 2013.

Komatsu T.,Kumamoto University | Takashima R.,Tohoku University | Shigeta Y.,National Museum of Nature and Science | Maekawa T.,Kumamoto University | And 5 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2016

The Smithian-Spathian boundary is indicated by the first occurrence of the ammonoid Tirolites cf. cassianus in the Olenekian Bac Thuy Formation, northeastern Vietnam. The boundary is intercalated within organic-rich dark gray mudstone that accumulated under anoxic to dysoxic conditions in the An Chau and Nanpanjiang Basins on the South China Block. In Lang Son area, three conodont zones, Novispathodus ex gr. waageni, Novispathodus ex gr. pingdingshanensis, and Icriospathodus collinsoni, are recognized in the formation. The Smithian-Spathian boundary is intercalated within N. ex gr. pingdingshanensis conodont Zone. The positive excursion inδ13C with values increasing from around -2.3‰ to +5.7‰ was recorded in the uppermost Smithian Xenoceltites variocostatus ammonoid beds and N. ex gr. pingdingshanensis conodont Zone. The δ13C values decrease across the Smithian-Spathian boundary. These δ13C isotopic patterns are correlated with well-known positive excursions around the Smithian-Spathian boundary globally. © 2016 Elsevier B.V.

Ho H.H.,Catholic University of Leuven | Ho H.H.,Vietnam Institute of Geosciences and Mineral Resources VIGMR | Swennen R.,Catholic University of Leuven | Cappuyns V.,Catholic University of Leuven | And 4 more authors.
Science of the Total Environment | Year: 2012

Since contaminated river-bed sediments in the Cam River-mouth (Vietnam) are regularly dredged and disposed on land, an understanding of the influence of time, pH and oxidation on the leaching behavior of heavy metals (Cd, Cu, Mn, Pb and Zn) and arsenic is necessary for the management of these dredged materials. A 96h pHstat-leaching test to examine the leaching behavior of elements at pre-set pH values (2, 4, 6, 8 (natural), 9 and 11) and a BCR 3-step extraction to clarify the element fractionation, were performed on a freshly-collected wet suboxic sediment and a dry oxidized sediment. All heavy metals and arsenic display a V-shaped pH-dependent leaching pattern with important releases at pHs 2 and 11. At the investigated pH values, the release of As, Mn, Pb and Zn from the oxidized sediment is slower and lower if compared with the suboxic sediment while the opposite trend is found for Cd and Cu at pHs 2-8. The transfer from the acid-soluble (exchangeable and carbonate-bound) fraction to the reducible (Fe and Mn hydr/oxide-bound) fraction is consistent with the lower leachability of As, Mn and Zn at pHs 2-8 and Pb at pHs 4-8 after oxidation, while the transfer from the oxidizable (organic matter and sulfide-bound) fraction to the reducible fraction relates to the higher leachability of Cd and Cu at pHs 2-8. The lower leachability of all elements at alkaline pHs 9-11 is due to lower leached concentration of organic matter from the oxidized sediment. Sulfides only play a minor role in controlling the leachability of heavy metals and arsenic. © 2012 Elsevier B.V.

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