微生物碳酸盐岩分类体系的修订:对灰岩成因结构分类体系的补充

微生物沉积作用在前寒武纪地层中普遍发育,在显生宙的一些地层中也较为发育。在碳酸盐岩地层之中,以叠层石为代表的微生物岩尤为引人注目。经过长期研究,2000年Riding曾经将微生物碳酸盐岩分为叠层石、凝块石、树形石和均一石4大类型。实际上,核形石以其较为广泛的发育和特殊的微组构也应该作为一种典型的微生物碳酸盐岩类型而纳入微生物碳酸盐岩的分类体系之中,而不能简单地作为球状叠层石。而那些纹理石灰岩,较厚的纹理和较深的产出沉积环境与叠层石形成明显的区别,也应该作为一种微生物碳酸盐岩的类型。生物沉积作用所形成的碳酸盐岩,以生物礁岩最为典型,在20世纪70年代曾经被Embry和Kloven归为骨架岩、障积岩、粘结岩三大类型,后来又增加了胶结岩,这是对20世纪50年代Folk、Dunham关于灰岩成因结构分类体系的良好补充。这些生物礁岩石以其高能量形成环境而有时又几乎见不到颗粒而与"颗粒含量越高沉积环境的能量越高"的基本理念不相符,所以Wright在1992年将它们归为生物作用类岩石,从而将灰岩划分为沉积作用、生物作用、成岩作用三大类。根据该分类,Folk和Dunham所描述的分类则属于沉积作用类灰岩,而Embry和Kloven所描述的生物礁岩石则归为生物作用类灰岩。微生物碳酸盐岩,总体上构成生物作用类碳酸盐岩中的粘结岩类,以其明显的微生物作用特点而具有自己的分类体系;它不但作为生物礁岩石的主要类型,而且也常常以生物礁、生物层和生物丘三种形式发育在地层之中。因此,上述概念和认识的进步,在强调微生物沉积作用的重要性的同时,有必要将微生物碳酸盐岩重新分为6大类:叠层石、凝块石、核形石、树形石、纹理石和均一石。

Revised classification of microbial carbonates: complementing the classification of limestones

Microbial sedimentation was widespread in the Precambrian and was also developed in stromatolitic carbonate horizons of the Phanerozoic,which constitute one type of microbial carbonates.Through long-term research,Riding(2000)grouped microbial carbonates into four types,(1)stromatolite;(2)thrombolite;(3)dendrite;and(4)leiolite.Oncolite is widespread in both the Precambrian and the Phanerozoic and its particularly microscopic fabric should be grouped under one category of microbial carbonates and not be simply be regarded as one type of spherical stromatolite.For micrites with laminated structures,both the thicker laminae and their sedimentary environment compared to that of stromatolites makes them as one type of microbial carbonates.Reef rocks are carbonates that form by biosedimentation,which in the 1970s Embry and Kloven has subdivided into three types:framestones,bafflestones and boundstones.They subsequently added another type,called.cementstones.The classification of reef rocks is a complementary to the classification of limestones,as proposed by Folk and Dunham in the 1950s and 1960s.The basic concept that the higher content of grains the higher energy of the sedimentary environment does not fit to reef rocks.Reef rocks were grouped into the biological textural groups by Wright(1992);thus,limestones are subdivided into three textural groups:biological,depositional and diagenetic.In terms of this revised classification of limestones,as proposed by Folk and Dunham should belong to the depositional textural group.Generally,microbial carbonates belong to boundstone of reef rocks and owe their particular classification because of their biosedimentary features,resulted from microbial action.Thus,microbial carbonates should be classified within the limestones.Similar to reef rocks,microbial carbonates are frequently developed in strata from the Precambrian through to the Phanerozoic,in three forms:reef,biostrome and bioherm.Considering the above changes,the author proposes a revised classification of microbial carbonates into six categories:stromatolite,thrombolite,oncolite,dendrite,laminite and leiolite.