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

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.