Abstract:

Oncolites are formed through the capturing of sedimentary particales by blue algae and the sticking by viscous liquid substance of algal mats.Therefore,in the Middle Triassic of Guizhou province,they often have alga mats as the base on whieh they develop,and are always closely associated with them.The core and the laminate are two essential components of an oncolite,with the former being its foundation of formation and the latter being a record of its evolution.The form of an oncolite depends on a lot of factors,so it may be different within the same energy zone.The microcrystallized laminate and the condensed laminate represent two characteristically different modes of formation,namely,the dynamic and the static.Based on this,oncolites can be divided into three types:1)the dynamic type;2)the static type;and 3)the composite dynamic and static type.The third type can be,according to the character of the last generation of lamination,subdivided into two subtypes:the dynamically terminated composite one and the statically terminated composite one.This subdivision is suitable because the forms of oncolites of the composite type depend to a large extent on the character of the last generation of lamination.The form of an oncolite is controlled by three factors:1).The form and size of core:This factor obviously affects only the dynamic type of oncolites,particularly when the thickness of initial laminate in comparing with the radius of core is small,and is not thick enough to counteract or compensate the influence of the form of core,with the thickening of laminate,the controlling effect of the core would decrease and,finally,disappear.2).The hydrodynamic intensity:This factor may influence to a large degree the configuration of an oncolite.The disturbed environment is favorable for the formation of microcrystallized lamination and configuration similar to those of ooids;the static environment is favorable for the growth of algae and the formation of condensed lamination and configuration similar to those of algal nodules.3).The abundant viscous liquid substance of algal base:Under the influence of the coherent force of this substance,oncolites in the same hydrodynamic environment may repeatedly roll here but keep still there.This substance can thus disturb and destroy the results made by hydrodynamic process.It is due to this influence that oncolites on two different reef bodies of a major barrier reef are distinctly different,namely the oncolites formed in Longtou are of a dynamic type,but in Nidang they are of a static type or a composite dynamic-static type.