摘要:
华北地台在晚寒武世为一个较为典型的缓坡型台地。在其北部地层中,以发育风暴砾屑灰岩及生物丘灰岩闻名,包括崮山组、长山组和凤山组。在该套地层中,深缓坡相泥岩和泥灰岩与浅缓坡相颗粒灰岩和泥粒灰岩一起构成若干潮下型碳酸盐米级旋回,这些米级旋回本身即为有序的异成因岩相序列而成为基本工作单元。在长周期三级层序中,米级旋回形成有序垂直叠加形式,反映了三级层序本身是一个与三级相对海平面变化相关的、环境加深和变浅过程中所形成的沉积相序列。根据这些特征,在华北地台北部晚寒武世地层中可识别出4个三级层序。从北向南,相变为一套白云岩,其中三级层序的高水位体系域则发育较多的潮坪相白云岩以及由其构成的环潮坪型米级旋回。这些层序的典型特征是:层序界面为淹没不整合面,层序的相序序列以"深缓坡相-浅缓坡相"为特征,从而构成特殊的"CS(?)+HST"序列;而在华北地台南部,层序界面多为暴露间断面,层序的相序组构多以"缓坡相-潮坪相"为特征。从北向南的变化,形成了一个富有规律性的层序地层格架。从旋回到层序、从岩相序列到沉积相序列的层序地层研究表明:作为基本工作单元,米级旋回实际上是由"间断-加积作用"旋回机制所形成的岩相序列,由米级旋回的有序叠加形式所构成的三级层序则是一个沉积相序列。由各剖面点的古水深变化曲线,可以定性地综合出华北地台晚寒武世的海平面变化曲线。与北美晚寒武世海平面变化曲线相对比,既存在相似性更存在差异性,说明了长周期海平面变化既受全球性海平面变化的控制,同时也受区域因素的影响。
Abstract:
The North China platform appeared as a typical ramp platform during the Late Cambrian. The strata in the northern part of the platform consisting of the Gushan and Changshan Formations are known for the storm calcarenite and biohermal limestone, of which the deep ramp mudstone and marl and shallow ramp grainstone and packstone constitute a number of meter-scale subtidal carbonate cycles. The vertical ordered stacking patterns of these meter-scale cycles within the long-term third-order cycles have reflected that the third-order (sequences) represent, in turn, the sedimentary facies sequences deposited during the deepening and shallowing processes and constrained by the third-order sea-level changes. Four third-order sequences may be identified for the Late Cambrian strata in the study area, in which the tidal-flat dolostones and their circum-tidal flat carbonate meter-scale cycles are well developed in the highstand systems tract. These sequences are characterized by the drowned unconformity as sequence boundary and "deep to shallow ramp facies sequences" consisting of the "condensed sections (?) + highstand systems tract" ("CS(?) + HST") sequences. In the southern part of the North China platform, the sequence boundaries are mostly represented by the exposed hiatal surfaces. The facies sequences comprise the "ramp facies-tidal-flat facies". The strata in the northern and the southern parts are both involved a regular sequence stratigraphic framework. The sequence stratigraphic examination based on the cycles and sequences, and lithofacies and sedimentary facies shows that the meter-scale cycles as the basic units actually may be interpreted as the lithofacies sequences resulted from the "hiatal-aggradational" cyclic mechanisms. The sea-level curves for the Late Cambrian strata on the North China platform may be qualitatively and synthetically constructed on the basis of palaeobathymetric curves for various sections. The similarities and differences of the sea-level curves for both the study area and the North America platform have mirrored that the long-period sea-level changes are controlled by both the global and local changes in the study area.
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