华北地台北缘奥陶系亮甲山组的碳酸盐复合海平面变化旋回层序

COMPOSITE SEA-LEVEL CHANGE CYCLIC SEQUENCES OF THE CARBONATES IN THE ORDOVICIAN LIANGJIASHAN FORMATION ON NORTHERN NORTH CHINA PLATFORM

关键词:  旋回层序  复合海平面变化  亮甲山组  奥陶系

Key words:  cyclic sequence  composite sea-level change  Liangjiashan Formation  Ordovician

摘要:

华北地台北缘亮甲山组地层中发育有六种碳酸盐米级旋回层序:a型和b型属潮下型,其下部单元为钙质泥岩(a型)及薄层泥晶灰岩(b型),上部单元为颗粒泥晶灰岩(a与b相同);c-f型属环潮坪型,它们的上部单元均为薄层泥晶自云岩及泥质白云岩,其下部岩石单元:c型为厚层及块状颗粒泥晶灰岩,d型为局部白云石化颗粒泥晶灰岩。e型为白云岩化颗粒泥晶灰岩,f型为中厚层含少量颗粒的泥晶结晶白云岩。从a型到f型正好构成一个米级旋回层序的环境变化谱系。
根据米级旋回层序的有序垂直叠加形式,可以建立亮甲山组地层构成的复合海平面变化旋回层序:1个三级旋回层序包含3个四级旋回层序,13-15个五级旋回层序,60余个左右的米级旋回层序。
通过这项研究表明,在浅水地层中进行露头旋回层序研究时,米级旋回层序是基本工作单元,其有序垂直叠加形式是识别长周期旋回层序的关键。

Abstract:

Six types of the metre-scale carbonate cycles are recognized in the Ordovician Liangji-ashan Formation on northern North China platform.Types a and b are assigned to the sub-tidal types,which consist of calcareous mudstone (Type a)and thin-bedded micritic lime-stone (Type b) in the lower units,and of grainy micritic limestone(the same in Types a and b).Types c to f are ascribed to the tidal flat types,which are all composed of thin-bed-ded dolomicrostone and muddy dolostone in the upper units,whereas in the lower units, Type c consists of the thick-bedded and massive grainy micritic limestone;Type d is built up by the locally dolomitized grainy micritic limestone;Type e is made up of dolomitized grainy micritic limestone,and Type f represents the moderately thick-bedded micritic crys-talline dolostone with a small amount of grains.It follows that Types a to f do well to form the hierarchy of the environmental changes of the metre-scale cyclic sequences.
Based on the ordered vertical stacking patterns of the metre-scale cyclic sequences,the composite sea-level change cyclic sequences may be constructed for the strata in the Liangjiashan Formation.One third-order cyclic sequence comprises three fouth-order cyclic sequences,thirteen to fifteen fifth-order cyclic sequences,and sixty and more metre-scale cyclic sequences.
The results herein have revealed that the metre-scale cyclic sequences may be used as the basic working units whose ordered vertical stacking patterns are believed to be the key to the identification of long period cycles in the study of outcrop cyclic sequences in the shallow-water strata.