Li Xiang-hui

Upper Cretaceous oceanic red beds in southern Tibet: Lithofacies, environments and colour origin

Application of mineralogy, geochemistry, sedimentary petrology, and sedimentology methods result in better understanding of the genesis and paleoenvironmens of the Upper Cretaceous oceanic red beds exposed in southern Tibet. The red beds comprise the Chungde Formation. Nine lithofacies recognized within this formation are: red foraminiferal packstone/grainstone, red microfossils wackestone, red marlstone with microfossils, red marlstone, red to variegated floatstone and rudstone (debris flow), red shale, red radiolarite, red chert with radiolaria, and red chert. Sedimentary structures and textures, microfossils, and carbonate content show that the Chuangde Fm was deposited near the base of a continental slope in a deep oceanic basin environment, with the basin floor below the carbonate compensation depth (CCD). Red marlstones and limestones intercalated within red shales represent slides and slumps from the upper part of the continental margin. Debris flow and turbidity deposits consist of volcaniclastic, fossilliferous rudstone and floatstone, and very thin calcareous mudstone, intercalated with red shales.The Upper Cretaceous oceanic red beds in southern Tibet are characterized by high Fe2O3, low FeO, which indicates an oxic diagenetic environment, resulting in precipitation of hematite. The latter occurs as finely, disseminated ferric oxide giving the red color to the rocks. It is concluded that the red beds in southern Tibet were deposited under highly oxygenated bottom conditions in the deep ocean basin. Such conditions not only occurred in a deep ocean basin as indicated by the occurrence of pelagic red shale deposited below the CCD, but also extended up the continental margin as indicated by the presence of red colored marlstones and limestones embedded in the Chuangde Fm. The latter were deposited above CCD, most probably on the continental slope. The oxic bottom conditions are interpreted to be a result of a combination of climate cooling, active bottom ocean circulation, and change in the ocean-atmosphere oxygen budget.

Paleosols of the Mid-Cretaceous: A Report from Zhejiang and Fujian, SE China

Abstract An investigation on the mid-Cretaceous continental sediments was recently conducted in both SW Zhejiang and SW Fujian Provinces, SE China. It is the first time the Cretaceous paleosols in mainland China have been reported. Three types of paleosols are recognized, such as, argillisol, calcisol, and oxisol on the basis of observation on eleven cross sections and stops in the field. The argillisol is characterized by burrow and root trace within reddish (silty) mudrock, the calcisol marks by pedogenic carbonate concretion within pale purple calcareous mudrock, and the oxisol is the association of ferric tegument and concretion within reddish purple, grayish orange siltstone, and mudrock. According to climate direction of paleosol types, it is proposed that it could have been in aridity or semiaridity of subtropic climate during the Aptian-Albian epoch of the mid-Cretaceous in SW Zhejiang, where the calcisols were recorded in the Hengshan Formation, Zhongdai Formation, Jinhua Formation, Guantou Formation, and Chaochuan Formation; at the same time, it was in semiaridity of tropic climate in SW Fujian which was recorded by oxisols within the Bantou Formation, Junkou Formation, and Shaxian Formation. In the Cenomanian, it could become more moist in SW Zhejiang as evidenced by the production of the argillisol within the Quxian Formation with abundant organic activities, whereas the paleoclimate in SW Fujian in the Cenomanian might be similar with that in the Aptian-Albian even if the sediments were transformed to huge thick conglomerates (so-called “Danxia Landform”) by tectonic uplift processes.