事件地层学——建立地层界线的基本原则和方法

本文首先认为,事件地层学是建立地层界线的基本原则和方法,由此建立的界线是自然的、真实的。由于事件的等时性,这种界线也代表了特定范围内年代地层单位的界线。另一方面,认为地层界线的范围或级别应取决于相应地质事件的范围和强度;一般来说,全球性事件可用于建立全球性或级别较高的界线,区域性事件可用于建立区域性或级别中等的界线,地方性事件可用于建立地方性或级别较低的界线。据此,还建议将前寒武系——寒武系界线置于云南梅树村剖面的“A”点,即小壳化石Anabarites-Protohertzina组合带之底。

Event Stratigraphy-the Basic Principle and Method for Establishing Stratigraphic Boundaries

It has been recogoized that the biotic and abiotic evolution in the geologic history is in the pattern of gradual changes with interludes of rapid and saltational changes, and, in general, the latter played a major role in the evolution from an old geological stage to a new one and displayed specifically in the forms of geological events. As a result, a natural boundary should be established on the basis of the effective events. A chronostratigraphic boundary can also be determined particularly in terms of events because of their relatively optimal synchroneity. Thus it can be seen that the basic principle of boundary establish ment is implied in the event stratigraphy. With different events in a sequence, the boundary can be established in the method of bioevent stratigraphy, lithoevent stratigraphy, tectoevent stratigraphy, or other exent stratigraphy. But all of them are in the category of methodology of event stratigraphy. Furthermore, each of these event boundaries may represent a chronostratigraphic boundary for the reason mentionad above. Thus it can be concluded that the basic method for boundary establishment is the very event stratigraphy(see Table 1). On the other hand, stratigrtaphic units and, certainly, their boundaries should be termed as different ranks and limits, which mainly depend on the scales of the relevant events. Generally, a global or higher ranked boundary ought to be rested on a global event, a regional or intermediately ranked boundary on a regional event, and a local or lower ranked boundary on a local event. In addition, an analysis of the evolutionary diversity of small Shelly faunas at Meishucun section shows that the faunas were amounting gradually to their apex at Point B in the Meishucunian age, but were dwindling promptly and drastically(see Table 2 and Figure 1)since then. For this reason, the selection of this position as the Precambrian-Cambrian boundary really implies that the major part of the faunas＇history existed in the late Precambrian and that the beginning of the Cambrian was that of the faunas to decline. Point A, the base of Anabarites-Protohertzinazone, however, not only signals the first radiation event of the faunas but also has a better potential for correlation. Therefore, it should be regarded as a natural stratotype point of the PrecambrianCambrian boundary.