Sedimentary budget of the Southwest Sub-basin, South China Sea: Controlling factors and geological implications

Calculation of the sediment budget in the South China Sea abyssal basin lacks constraints from drilling data and reinterpretation of seismic data. On the basis of six multichannel seismic profiles across the Southwest Sub-basin (SWSB) and the drilling data from International Ocean Discovery Program Expedition 349, we divided the Cenozoic sediments into four sedimentary units. The sedimentary budget of the abyssal basin at different geological times was calculated. The previous works in the Mekong continental shelf and slope areas were integrated to calculate the sediment budget of the whole SWSB. Our researches show that the sediment budget in the SWSB increased during the Palaeogene and reached its first peak because of intensified erosion, which might have resulted from the continued uplift of the Tibetan Plateau and accelerated southeastward extrusion of the Indo-China Peninsula. Since the Late Miocene, the sediment budget was mainly influenced by the East Asia monsoon, that is, the intensified winter monsoon decreased the sediment budget of the entire area during the Late Miocene, whereas the strengthened summer monsoon increased the sediment budget during the Pliocene and reached a peak during the Pleistocene. The sediment budgets of the Mekong continental shelf, the Mekong continental slope, and the abyssal basin have distinct characteristics, relating to the infilling sequence of the terrestrial sediments in different regions. The sediment provenance of the SWSB was mainly from the Indo-China Peninsula, the Nansha area, and the Palawan before the Late Miocene. After that time, sediments were mainly transported from the modern Mekong River.

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Fossilized feature records Moon’s slow retreat from Earth

New research provides insight into the Moon’s excessive equatorial bulge, a feature that solidified in place over four billion years ago as the Moon gradually distanced itself from the Earth. A new study sets parameters on how quickly the Moon could have receded from the Earth and suggests the nascent planet’s hydrosphere was either non-existent or still frozen at the time, indirectly supporting the theory of a fainter, weaker Sun that at the time radiated around 30 percent less energy than it does today.

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An ichthyosaur from the UK Triassic–Jurassic boundary: A second specimen of the leptonectid ichthyosaur Wahlisaurus massarae Lomax 2016

The holotype of Wahlisaurus massarae is known only from a partial skull and postcranial skeleton from the Lower Jurassic, collected near Normanton on Soar, Nottinghamshire, UK. It is diagnosed relative to other ichthyosaurs on the basis of autapomorphies of the coracoid and a unique combination of characters. Here, we report a second specimen of W. massarae. The new specimen comprises a practically complete right coracoid that clearly shows the unique morphology of W. massarae. This specimen was collected in situ from a quarry in Somerset, from the base of the Blue Lias Formation, which corresponds to the Triassic–Jurassic boundary (uppermost Rhaetian or lowermost Hettangian), extending the geographic and stratigraphic range of the species. Furthermore, the coracoid of the new specimen is 20% anteroposteriorly longer than in the holotype, representing a larger individual. In light of preparation and the identification of additional fragments of the holotype, a redescription of the skull morphology of the holotype, as well as comparison with the skull roof of Leptonectes tenuirostris, is included.

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At the southern limits of the Devonian reef zone: Palaeoecology of the Aferdou el Mrakib reef (Givetian, eastern Anti-Atlas, Morocco)

Devonian reefs of north-western Gondwana represent the southernmost record of shallow-water coral reefs in the Palaeozoic. However, few studies have attempted to date palaeoecological reconstructions of these high-latitude reefal buildups. This study provides the first detailed palaeoecological analysis of Aferdou el Mrakib, an isolated, Givetian coral-stromatoporoid reef, which developed in a semirestricted basin in the south-eastern part of the Rheic Ocean. The study documents spatial facies variability and succession of faunal replacements accompanying progressive reef accretion towards the sea surface. The investigations included both autochthonous communities found at the base of the reef and, partially, within the reef core, and allochthonous deposits of reef-derived skeletal debris that accumulated in the fore-reef setting. Contrary to some previous suggestions, the study shows that the Aferdou reef shared many characteristics of classical Middle Devonian coral-stromatoporoid buildups, including the ecological succession, limited role of calcareous algae, and development within the range of the euphotic zone, but likely below the zone of regular water agitation. Critical factors in the facies development and temporal changes in the character of reef building were the palaeobathymetry, dominant sedimentary and circulation regimes, level of wave energy, and, possibly, light availability. Distinctive features of the palaeoecology of Aferdou el Mrakib are the dominance of massive colonies of heliolitid tabulates and a subordinate role of massive stromatoporoids, both explained here primarily as a result of increased water turbidity in the high-latitude sedimentary basin. The growth of the high-latitude coral-stromatoporoid reefs in the south-eastern Rheic Ocean was favoured by a combination of the exceptionally warm climate and plate tectonic configuration typifying the Devonian. Of critical importance appears the palaeogeographic position of the Rheic, which resulted in the seawater circulation in the ocean being dominated by tropical water masses, with restricted inflow of cold water from the circumpolar oceanic circulation.

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Identifying Early Carboniferous bimodal volcanic rocks and geochemical characteristics in the Atengtao Mountain, Yili Block (Chinese western Tianshan)

The Early Carboniferous bimodal volcanic rocks are firstly recognized in the Atengtao Mountain, Yili Block (Chinese Western Tianshan). The bimodal volcanic rocks are composed of dominant rhyolite and subordinate basaltic andesite. The SiO2 contents of basaltic andesites span a range of 52.08 to 60.44 wt.%, whereas those of the rhyolites range from 72.50 to 76.78 wt.%, showing a sharp gap between 60.44 and 72.50 wt.% for SiO2 content. Zircon U–Pb dating of rhyolitic sample yielded crystallization age of 340 ± 6 Ma, which is interpreted as the extrusive age of the bimodal volcanic rocks. Basaltic andesite samples belong to the calc-alkaline series, whereas rhyolites are peraluminous. The basaltic andesite samples display enrichment in LREE ((La/Yb)N = 3.16–6.27), and nearly no obvious Eu anomalies (δEu = 0.86–1.21), with relative enrichment in large ion lithophile elements (Rb, K, Sr, U) and depletion in high field strength elements (Nb, Ta). Compared to the basaltic andesites, the rhyolites show enriched LREE patterns ((La/Yb)N = 7.05–9.27) and significantly negative Eu anomalies (δEu = 0.36–0.62), with remarkably negative Nb, Ta, P, Ti, and Sr anomalies, which is consistent with A-type granites. The rhyolites have positive εHf(t) values from 1.5 to 4.8 and TDM2(Hf) values from 1,036 to 1,247 Ma. The basaltic andesites are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components, whereas the rhyolites were derived from basaltic melt-induced reworking of Proterozoic juvenile crustal material. Based on our data, and taking into account the regional geology, a model of back-arc setting was proposed for the Early Carboniferous formation of the bimodal volcanic rocks in the Atengtao Mountain. Therefore, we suggest that the volcanic rocks were likely induced by the northward subduction of the South Tianshan Ocean. The subduction resulted in opening of a back-arc basin in Atengtao Mountain, leading to upwelling of mantle that had produced the basaltic andesites. Moreover, this process provided heat to remelt the crustal materials and form the rhyolites.

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