Tong Jinnan

Carbon isotope composition of the Lower Triassic marine carbonates, Lower Yangtze Region, South China

Studies on three Lower Triassic sections located on the shallow water platform, the deep water slope and in the deep water basin in the Lower Yangtze Region, South China, show the similar trend of carbon isotope evolution. Biostratigraphic correlations among the Lower Triassic sections on the basis of standard conodont zones indicate that three negative shifts occurred in the Griesbachian, the Smithian and the late Spathian stages respectively, and one distinctly positive shift occurred in the early Spathian stage. Trend of carbon isotope evolution of the Lower Triassic reflects some significant changes in the global carbon cycle. Moreover, δ13C background values are intensively controlled by palaeogeographic environment. In general, δ13C values from deep-water slope carbonates are lighter than those from carbonate platform and heavier than those from deep-water basin carbonates. The positive carbon isotope excursion may be induced by a significant amount of organic carbon burial in marine sediments and increase in primary productivity. The large negative carbon isotope excursions during the Early Triassic in Lower Yangtze Region are interpreted to relate to volcano eruptions based on tuffaceous claystone interlayers observed near the Permian-Triassic boundary, the Induan-Olenekian boundary and the Lower Triassic-Middle Triassic boundary.

Cyclostratigraphy of the Induan (Early Triassic) in West Pingdingshan Section, Chaohu, Anhui Province

The Induan sequence in the West Pingdingshan Section, Chaohu, Anhui Province, displays a series of superimposed mudstone/limestone cycles. The lithological character of the cycles, combined with power spectral and wavelet analysis of magnetic susceptibility readings, reveals 12 short eccentricity and 56 precession Milankovitch cycles — obliquity cycles are not apparent. The uniformity of cycle thicknesses indicates a stable depositional setting making this section ideal to perform various geological studies. Accordingly, the Induan Stage is estimated to have lasted 1.1 Ma, and the depositional rate for this part of the section is about 3.7 cm/ka. This places the Induan-Olenekian boundary in the West Pingdingshan Section at about 251.5 Ma based on an age of 252.6 Ma for the Permian-Triassic boundary.

An Update of Conodonts in the Induan-Olenekian Boundary Strata at West Pingdingshan Section, Chaohu, Anhui Province

Abstract The Lower Triassic in Chaohu area, Anhui Province, China, is well developed and its sequence is typical in South China. After a brief introduction of the Induan-Olenekian boundary of Chaohu, this article presents some new data on conodonts. More than ten times of conodont samplings and investigations have recovered thousands of conodont specimens, which are especially rich in the Induan-Olenekian boundary strata at the West Pingdingshan Section in Chaohu City, Anhui Province. The most distinctive forms are the conodonts of the Neospathodus dieneri group and N. waageni group. The first occurrence of N. waageni eowaageni , which is regarded as the indicator of the Induan-Olenekian boundary, is situated at 40.49 m above the base of Yinkeng Formation. Some key conodonts and seven new specimens are introduced.

Proposed new Lower Triassic stages in South China

As the global Lower Triassic stages are not properly defined, a Chinese scheme of the Lower Triassic subdivision was recently proposed to meet the insistent need of the general investigation in China. The Lower Triassic sequences of South China are briefly summarized and the twofold Lower Triassic subdivision is discussed. The type sections of the two Chinese Lower Triassic stages, Yinkengian and Chaohuan, and their definitions are stated.

The Middle Triassic environstratigraphy of central-south Guizhou, southwest China

Abstract Environstratigraphy is proposed to produce a direct stratigraphic correlation for the Middle Triassic sequences of entirely different lithologic and palaeontologic compositions in various palaeogeographic environmental gradients (facies) of southern Guizhou. As environstratigraphy is based on environmental analysis, various indicators of environments can be used to complete the environstratigraphic sequence. The Middle Triassic environstratigraphic sequence in the terrigenous clastic basin of southern Guizhou is established mainly by the study of sedimentology and community ecology. Microfacies analysis is very useful for the environmental reconstruction in carbonate sedimentary areas, for example the carbonate platform and carbonate buildup in front of a platform. The method of geochemical facies analysis has been developed for a comprehensive evaluation of relative environmental changes, and it is very suitable for environstratigraphic study. Environstratigraphic correlation is based on the regional significant environmental events and their effects at different palaeogeographic gradients. The alternations of tectonic environments indicated by sedimentation are of great importance for the direct correlation of the events at different environmental gradients. The six correlation lines divide the Middle Triassic development of the basin into five intervals, which exactly express the periodicity of sedimentation in various areas and the evolutionary process of tectonic and palaeogeographic patterns.

Application of ecostratigraphy to sequences tratigraphy

The results of ecostratigraphy can directly serve sequence stratigraphy. The habitat type curve is useful not only in the analysis of sequences and parasequences, but also in demonstration of the process of regional sea level change. The various biological surfaces usually coincide with or relate to the boundaries of sequences or system tracts. The ecostratigraphic framework composed of coenozones, community sequences and ecotracts with good timing completely corresponds to the sequence stratigraphic framework of the sedimentary basin. Therefore, through establishment of the habitat type curve in individual section, recognition of the various biological surfaces, regional ecostratigraphic correlation and the formation of an ecostratigraphic framework of the sedimentary basin, ecostratigraphy plays an important role in the study of sequence stratigraphy and the reconstruction of regional and even global sea level changes.