Characteristics of the crystalline basement beneath the Ordos Basin:Constraint from aeromagnetic data

Aeromagnetic anomaly zonation of the Ordos Basin and adjacent areas was obtained by processing high-precision and large-scale aeromagnetic anomalies with an approach of reduction to the pole upward continuation. Comparative study on aeromagnetic and seismic tomography suggests that aeromagnetic anomalies in this area are influenced by both the magnetic property of the rock and the burial depth of the Precambrian crystalline basement. Basement depth might be the fundamental control factor for aeromagnetic anomalies because the positive and negative anomalies on the reduction to the pole- upward-continuation anomaly maps roughly coincide with the uplifts and depressions of the crystalline basement in the basin. The results, together with the latest understanding of basement faults, SHRIMP U-Pb zircon dating of metamorphic rock and granite, drilling data, detrital zircon ages, and gravity data interpretation, suggest that the Ordos block is not an entirety of Archean.

An important reference criterion for the selection of GSSP

A study on the relationship between biostratigraphy and seguence stratigraphy in several designated global boundary stratotypes shows that the best way may be to take the GSSP at a point coincident with the base of the first widespread Leading Group biozone above the first flooding surface (FFS) of the relevant third-order seguence. it is suggested that the first flooding surface of the sequence should be an important reference criterion for the selection of GSSP. As the base of the first widespread Leading Group biozone chosen for the definition of GSSP could not be lower than the first flooding surface of the referred seguence, the latter surface may be an important criterion for the recognition and correlation of the chronostratigraphic boundaries.

Cambrian Ichnofossils from the Zhoujieshan Formation(Quanji Group) Overlying Tillites in the Northern Margin of the Qaidam Basin, NW China

The Quanji Group is composed of siliciclastics and carbonates and was deposited on a relatively stable block, in the northern margin of the Qaidam Basin, NW China. It is one of the research hotspots in the Qilian-Qinling-Kunlun tectonic zones. However, it has long been argued whether the up- per Qnanji Group should be assigned to the Precambrian or the Cambrian for lack of convincing fossil evidence. The discovery of ichnofossils, including Rusophycus, Cruziana, Dimorphichnus, Treptichnus, Skolithos, Arenicolites, Palaeophycus, and Helminthopsis, indicates that the upper member of the Zhou- jieshan Formation （top Quanji Group） is Cambrian in age, and at least above the Treptichnus （Phycodes） pedum Biozone, the lowermost biozone in the Cambrian. The lower member of the Zhoujieshan Forma- tion should belong to the Cambrian. During the time when the upper member of the Zhoujieshan For- mation was deposited, the northern margin of the Qaidam Basin was dominated by the Cruziana Ichnofacies, characterizing a lower-energy shelf （shallow） sea environment with moderate-rich oxygen con- tents. In addition, the conglomerates in the Hongtiegou Formation of the Quanji Group underlying the Zhoujieshan Formation were generally regarded as tillites. However, the dolostones of the lower member of the Zhoujieshan Formation are, in lithology and geochemistry, different from the typical cap-dolostones of the Doushantuo Formation in South China, so it is necessary to further study the origins of the lower member of the Zhoujieshan Formation and the conglomerates of the Hongriegou Formarion.

Macroalgal holdfasts and their interaction with environments from the Neoproterozoic Doushantuo Formation in Guizhou,South China

Numerous macro-organisms of the Doushantuo macrobiota,which were found in the black carbonaceous mudstone of the upper Neoproterozoic Ediacraan Doushan-tuo Formation in Jiangkou County,Guizhou Province,China,are considered to live on sea floor by their holdfasts.The appearance and preserved forms of the macroalgal holdfasts may provide some data to the study of the living and buried environments of macrobiota.They lived in the lower energy and clear environment,and fixed on a soupground with rich water(about 79%water).Currents,possibly ocean currents,could pull out the macroalgal holdfasts from the soupground and break off the macroalgal foliations.After such events,the corpses of macro-organisms would be covered in a reduced environment by the deposits.Afterwards,a new community,including regenerating and undying macro-organisms,lived continuously on a new deposit.