ALBERT

Description: This paper provides a brief outline of the current, detailed inter-disciplinary work on the Xiaoyangquiao section, trying to expose all the aspects for reference tied to the Global Single Stratigraphic Point (GSSP) Concept for defining the Cambrian–Ordovician Boundary. The 45 m critical interval of this section outcrops very well along the steep bank of a stream and is free from folding, faulting, intrusions, and has not been affected by weathering. Colour alternation of conodonts and acritarchs, and crystallinity indices of illite all indicate a maximum thermal grade of 100 °C. The lithofacies, being of great lateral persistence for a hundred kilometres, consists mainly of a rhythmical sequence of lime mudstone and shales deposited in a moderately deep outer shelf environment of quiet water, well below the normal storm wave base. Chemical investigation of the rocks demonstrates strong positive correlation between A12O3 content and those of K2O, MgO, Fe2O3, TiO2, Be, Cr, Co, Ni, Zn and Ba, indicating these components are tied to the clay fractions. A stable depositional environment is demonstrated by the uniform chemistries through the boundary interval. Close to the boundary itself P2O5 contents are low, indicating continuous sedimentation at fairly substantial rates.The major biological events, the biostratigraphic framework, and the stratigraphic range of conodont, graptolite, trilobite, and acritarch taxa are illustrated briefly with diagrams. Following the majority views of the Calgary Plenary Session, the boundary level is to be chosen at a point marked by the First Appearance Datum (FAD) of the selected conodont taxon or taxa in the vicinity of the level close to, but below, the first influx of nematophorous graptolites. The following four points marked by the incoming of conodont taxon or taxa are recommended for consideration of the ‘Golden Spike'‘: (1) FAD of Cordylodus intermedius at 5.28 m below the first influx of nematophorous graptolites; (2) FAD of Hirsutodontus simpler–Cordylodus drucei–Albiconus postcostatus at 5.23 m; (3) FAD of Semiacontiodus lavadamensis–Utahconus utahensis–Monocostodus sevierensis at 3.85 m; (4) FAD of Cordylodus lindstromi at 2.23 m. For the following reasons the FAD of H. simplex–C. drucei–A. postcostatus is favoured: (1) the taxa are all distinct and widely dispersed; (2) intensive evolutionary change took place in conodonts, graptolites, trilobites and acritarchs prior to or after this point; all the fossil groups occur together, providing correlation with many regions throughout the world; (3) the point is in a position between the previously widely accepted boundary levels based on graptolites and trilobites; (4) the proposed point lies within a thin, laterally persistent, rhythmical sequence. The FAD of Cordylodus lindstromi is also a favourable point, sharing many advantages mentioned above. But this point is less satisfactory in being defined by the FAD of a single taxon C. lindstromi which also has an extremely small population size.An isochron age of 500.7 ± 7.4 Ma is determined from clay fractions of mudstones 8.5 m below the proposed point by means of the Rubidium–Strontium method.The ɛND signature determined from conodonts, trilobites hyolithids and acrotretid brachiopods has a mean value of −6.7, comparable with that of the coeval oceanic water mass occupying southeastern North America and Europe, and indicating that northeastern China bordered the same ocean. The mean Tdm model age determined was 1.26 Ga at the time of sedimentation, compatible with the mean Tdm model age of approximately 1.1 Ga for the Pacific Ocean today. The relatively low value of the Tdm model age indicates a substantial input from young orogenic volcanic island arcs and terranes.