微生物岩的概念和重庆老龙洞剖面P-T界线地层微生物岩成因*

微生物岩的原始定义指底栖微生物主导形成的沉积体或岩石体。笔者对该概念进行了扩充,认为微生物岩除了包括叠层石、凝块石、纹层石、核形石、均一石之外,还应该包括微生物骨架岩、微生物粘结岩、非钙化浮游或漂浮微生物形成的模铸岩、矿化浮游或漂浮微生物形成的颗粒岩和泥粒岩。P-T界线地层微生物岩的特征是具有由较粗矿物晶体(主要是方解石,其次是白云石)组成的斑点状、树枝状、网状结构的灰岩。这3种结构分别称为斑点状体、树枝状体、网状体,在露头上呈暗色,在薄片中呈浅色,一般由无定形的亮晶充填体和其间的微亮晶组成。亮晶充填体是指无定形的孔洞被不同成岩期形成的矿物充填形成的结构体,因成岩作用各异,造成不同层位、不同地点的亮晶充填体内部的矿物类型和充填顺序存在差异;所有的亮晶充填体都不具有壁,故不是钙化化石。通过形态、大小和生态比较,以及形成演化分析,认为亮晶充填体的前身是漂浮蓝细菌微囊菌,胶鞘是微囊菌形成模铸化石的关键因素。亮晶充填体是表层水漂浮生活的微囊菌沉入海底后,被泥晶沉积物掩埋或者被早期海底胶结物胶结,在泥晶沉积物半固结或固结之后腐烂留下的孔洞被后期成岩作用形成的矿物充填形成的。P-T界线地层微生物岩段顶部遭受成岩作用程度高,树枝状体和网状体中的亮晶充填体的轮廓基本都被破坏,变成微亮晶和亮晶,以前被学者解释为凝块石;但斑点状体、树枝状体、网状体是成岩流体沿着亮晶充填体或其他化石丰富的地方运移形成较粗的晶体而造成的,并不符合凝块石的定义。同时,少数学者把树枝状体本身当成底栖生物,也是没有充分认识树枝状体的矿物组成而做的解释。该微生物岩段含有钙化的小球状化石和同心层状化石,但它们不是组成微生物岩的主体。

Conceptions of microbialites and origin of the Permian-Triassic boundary microbialites from Laolongdong,Chongqing, China

Microbialites are carbonate bodies formed by microbial actions of benthic microbial communities or mainly composed of skeletons of planktonic microbes,and include the old types such as stromatolites,thrombolites,microbial laminated rocks,oncolites,leiolite,and the new types: Microbial framestones,microbial bindstone,and microbial caststone mainly composed of cast fossils of microbes,microbial grainstone,and microbial packstone. The Permian-Triassic boundary microbialites in South China include three types: Those with patchy coarse-mineral structures,those with dendroid coarse-mineral structures,and those with net-like coarse-mineral structures. The three types of structures are composed of spar-filled structures and the microsparry filling the space between the spar-filled structures. The spar-filled structure is defined as caves left by decay of some non-calcified organisms and filled by various minerals formed in diagenesis,and varies in inner fillings and fabric due to difference in diagenetic history. The rocks between the coarse-mineral structures are micrites with few fossils and scattered spar-filled structures. The shapes of the spar-filled structures in the PTB microbialites are very similar to those of the colonies of Microcystis,an extant cyanobacterial genus,in shape and size and are the cast fossils of the latter. The PTB microbialites are not thrombolites. As colonies of Microcystis sank to the floor and were buried by micrites,their sheaths did not decay until the micrites were slightly lithified. After their decay,the caves they left were firstly cemented by needle-like aragonite,and then altered by dolomitization forming dolomites,which mostly were altered to calcite and were dissolved by fresh water in surface environment. The caves finally were filled by blocky calcite. Diagenesis has destroyed the outlines of most spar-filled structures,making it difficult to recognize. Diagenetic fluids migrated along the sediments with abundant spar-filled structures,destroying them,forming coarse minerals,which resulted in the patchy,dendroid and netlike appearance of the PTB microbialites. Since the formation of the patchy,dendroid and netlike bodies is controlled by diagenesis rather than microbes,and are not composed of micrites but coarse minerals,they are not thrombolites. There is no calcified wall in the spar-filled structures,and they are not calcimicrobes. Small coccoid fossils occur in the upper part of the microbialite interval,but not the lower part,in small content,and is not responsible for formation of PTB microbialites.