Correlation of the Upper Devonian and Lower Carboniferous Depositional Sequences in the Different Facies Belts on the Southern Margin of the Upper Yangtze Platform
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摘要: 层序的空间分布是否具有全球性，地史时期海平面变化是否具有全球等时一致性，是层序地层学的根本问题。上扬子地台南缘不同沉积相区上泥盆统和下石炭统沉积层序研究表明，不同级别的层序具有特定的空间分布和对比范围，虽然形成各种级别层序的海平面变化基本上是全球性的。本文把空间分布，特别是在不同沉积相区的分布作为定义和识别各种级别层序的一个标准。正层序（ 三级层序） 的分布可能是全球性的，至少可以在同一大陆边缘的不同相区辨认，并能进行远距离对比，有些甚至可以进行全球对比。一般来说，亚层序（ 四级层序，时间间隔0.5 M a～ 1.5 M a） 在不同的沉积相区对比比较困难，虽然有些亚层序的空间分布也可以是全球性的。亚层序至少应当在一个沉积相带内可以普遍识别和对比。更高级的层序，包括副层序（ 五级层序，即Wang 和 Shi 1996 年使用的亚层序） 和小层序（ 六级层序） 的分布是区域性的或地区性的，它们可能分别反映米兰柯维奇旋回的长周期和短周期。正层序和亚层序一般可在不同沉积相区识别，而副层序和小层序一般仅能在浅海相区识别，在斜坡和盆地相区则不易辨认。Abstract: A fundamental problem in sequence stratigraphy is whether the sequences are global in extent and whether the sea level changes are eustatic in nature. Study on the Upper Devonian and Lower Carboniferous depositional sequences in the different facies belts on the southern margin of the Upper Yangtze Platform shows that different orders of sequences possess of distinctive distribution and correlation in space, although the sea level changes in the various orders of sequences may be primarily global in nature. The present paper regards the distribution in space, especially in different facies belts, as a criterion for defining and recognizing the various orders of sequences. The orthosequence (third-order sequence) is probably global in nature, which may be discerned in various depositional facies belts on the continental margin and can be correlated over long distances, sometimes may be worldwide. Commonly, correlation of the subsequences (fourth-order sequences with time interval of 0.5 to 1.5 Ma) is difficult in different facies belts, although some of them are probably also worldwide in distribution. A subsequence should be distinguishable and correlatable within at least one facies belt. The number of subsequences in one orthosequence varies in different facies belts.Only the number of subsequences recognized in shallow marine facies represents the correct subdivision of subsequences within one orthosequence.The higher-order sequences.including parasequences (fifth-order sequences, i. e. subsequence of Wang and Shi) and microsequences (sixth-order sequences), are regional or local in distribution. Generally their correlation are difficult even within a single facies belt.Parasepuences and microseunces may have reflected the longer and shorter Milankovitch cycles,respectively. Orthosequences and subsequences are usually recognizable in different facies belts, while parasequences and microsequences may be distinguished only in shallow marine deposits,but not in slope and basin facies deposits.The widely used meter-scale cyclothems probably include various orders of sequences and may represent parasequences (fifth-order sequences)and microsequences (sixth-sequences),even orthosequences,in different depositional facies belts.
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Key words:
- facies belt /
- orthosequence /
- subsequence /
- parasequence /
- microsequence /
- meter-scale cyclothem /
- Milankovitch cycle
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