从准层序到米级旋回——层序地层学与旋回地层学相互交融的纽带

层序地层学的基本单位是层序。准层序和准层序组构成了层序的基本结构单元,它们在层序内有规律的空间叠置样式是识别体系域的客观标准。准层序被定义为以海泛面为界的向上变浅序列,但是准层序是一个比较松散的概念,存在问题是对其成因机制研究不足。米级旋回作为旋回地层学的基本单位,定义为数厘米至数米厚的向上变浅的岩相序列,是高频率海平面升降变化的产物,其形成时限为相应的米兰柯维奇天文旋回周期。作为旋回地层学的基本单位,米级旋回以其明确的沉积学涵义和有规律的天文成因机制,是对层序地层学概念体系中的"准层序"的良好补充,对其进行深入研究具有重要的理论意义和应用价值。从准层序到米级旋回这一概念体系的演变,代表了层序地层学与旋回地层学相互交融的纽带。最后介绍旋回地层学的最新进展亚米兰柯维奇旋回,它对精确数字定年具有重大意义。

FROM PARASEQUENCES TO METER-SCALE CYCLES:THE CONNECTION BETWEEN SEQUENCE STRATIGRAPHY AND CYCLOSTRATIGRAPHY

Sequence stratigraphy is the study of genetically related facies within a framework of chronostratigraphically significant surfaces.The depositional sequence is the fundamental stratal unit of sequence stratigraphy.Parasequence and parasequence set are the building blocks of the sequence,and their stacking patterns in a sequence are the objective criteria with which system tracts can be identified.A parasequence is defined as a relatively conformable shallowing upward succession of genetically related beds or bed sets bounded by flooding surfaces or their correlative surfaces.However,the definition of parasequence is imprecise,and the underlying mechanism for parasequence formation has not been well understood.Meter-scale cycle(<10m thick)is the fundamental unit of cyclostratigraphy,defined as a shallowing upward rock succession separated by surfaces marked by abrupt change to deeper facies.Meter-scale cycle is the stratigraphic product of autocyclicity superimposed on an allogenic mechanism with frequencies of 400ky to 20ky.The so-called allogenic mechanism is high-frequence sea-level changes which are genetically related with the Milankovitch cycles.As the fundamental stratal unit of cyclostratigraphy,meter-scale cycle can be considered as a valid supplement to the parasequence in sequence stratigraphy because of its sedimentary meaning and the orbit cycle related mechanism.Therefore,the evolution of the concept system from parasequence to meter-scale cycle indicates possible connection of sequence stratigraphy and cyclostratigraphy.Sub-Milankovitch cycles are briefly introduced as the latest advancement in cyclostratigraphy.