摘要
北京西山寒武系-奥陶系出露良好,根据岩性组合特征,结合沉积结构、古生物、地球化学等指相标志,可区分出的副层序类型主要有6种,它们形成于不同的环境,其厚度一般几米,少数几十厘米或十几米,每类副层序中岩性可达4-6种,但单个副层序可仅由两种岩性构成,每一副层序反映的水深总是自下而上持续变浅,而在每两个副层序之间总是由浅水到深水的相跳跃。根据这些副层序的特征,可以推断在每两个副层序之间有一次海水突然加深过程。正是突然加深的副度、持续稳定的时间以及缓慢加深与沉积速率平衡关系等因素,决定了副层序各单元的缺失与否及它们之间的堆叠关系。根据同一类型副层序由下向上的变化趋势,可以确定副层序组的类型,进而划分体系域和层序。此外,笔者探讨了导致海水突然加深的原因,认为仅仅气候的周期性变化难以导致副层序界面的形成。海平面变化可能仍是主要与构造运动有关。
In the Western Hills, Beijing, Cambrian-Ordovician strata are well developed. Based on the characteristics of the rock association coupled with sedimentary structures and paleontological and geochemical indicators, 6 major types of parasequences may be distinguished. They were formed in different environments. Their thicknesses are commonly a few meters and, in a few cases, in the range from several dozens of centimeters to dozens of meters. Each type of parase-quence is composed of 4-6 kinds of rocks, but a single parasequence may be composed of only two kinds of rocks. Within each parasequence, the depth of water always becomes pregressively shallower upward, and between two parasequences, there always is a f acies jump from shallow to deep water. From the characteristics of these types of parasequences, it may be inferred that there is an abrupt deepening process of sea water between each two parasequences. And the presence or absence and stacking pattern of the parasequences depend on the abrupt deepening amplitude, the relation between the steady deepening and the deposition rate and the lasting time of the steady deepening. The system tract and sequence can be determined by the type of the parasequence group (regressive, progressive and accumulative), which is recognized from the trend of upward change of the same type of parasequence at the section. In addition, it is estimated that the abrupt deepening of sea water could not result from ice melt. So the Milankovich cycle could hardly lead to parasequence formation.
出处
《地质论评》
CAS
CSCD
北大核心
1997年第2期148-154,T001,共8页
Geological Review
基金
国家攀登附加项目
关键词
副层序
层序地层
形成机制
寒武纪
奥陶纪
parasequence, sequence stratigraphy, Cambrian, Ordovician, Beijing, Western Hills
