旋回地层──一个正在发展中的理论

地层中的旋回性很早就引起了人们的注意。Ｍ． Ｍｉｌａｎｋｏｖｉｔｃｈ对地球轨道参数的定量计算和对冰期的解释是旋回地层发展的重要里程碑。６０～７０年代开展的ＤＳＤＰ项目，在深海沉积记录中发现了支持米兰科维奇理论的强有力证据，使该理论得以复活。８０年代，层序地层的发展和全球地质观的兴起又重新激发了人们对地层旋回性研究的热情，并形成了一个新的地质学分支──旋回地层学。

CYCLOSTRATIGRAPHY:A DEVELOPING THEORY

The stratigraphical cyclicity has long been recognized in geological records for centuries. The classical works by G. K. Gilbert in the Niobrara Chalk under the Rocky Mountains, and by W. B. Bradley in the oil shale varves of the Green River Formation bridged between the stratigraphical cyclicity and the orbital motion of the Earth. The quantitative calculation of orbital parameters done by Milutin Milankovitch and the interpretation for the glacial / interglacial periods laid the milestone in the development of cyclostratigraphy. This theory, however, also provoked much debate and became silent for nearly half a century. In 60' s and 70' s, more and more evidences for orbital forcing cycles were discovered in the deep-sea sediments, supporting and revitalizing the Milankovitch theory. The rejuvelation of this theory was benefited by the continuous C, O isotopic stratigraphical records and paleogeomagnetic data in the deep-sea sediment columns. In 70's and 80' s, the development in sequence stratigraphy further stimulated the interests in high--frequency cyclicity in geological records. Extensive work on stratigraphical cyclicity in more diversified environments covering a vast range from seas to conhnents, spaning from the Cenozoic to the Pre--Cambrian gathered much more evidences of the orbital controls on the formation of variable-scale depositional cycles. The great enthusiasm for the study of the stratigraphical cyclicity from the academia and industry gave a big push to the development of the cyclostratigraphy. Great progresses have been made in the understanding of the climatic and depositional response to the orbital forcing, and the orbital evolution of the Sun-Earth-Moonsystem in the geological past However the poor timing of stratigraphical intervals makes much difficulties and uncertainties in cyclic analysis, although in some specific circumstances, cyclostratigraphy is expected to calibrate the accuracy of the geological time--scale. Therefore, how to extract orbital signals from the incomplete stratigraphical records filled with 'noises' such as tectonic and volcanic interruptions become the most challenging work in sedimentology and stratigraphy. However, we still need to learn to choose suitable tools and methods of data collection and processing.