【作者】 赵贵生；

【导师】 史晓颖；

【作者基本信息】 中国地质大学（北京） ， 古生物学与地层学， 2011， 硕士

【摘要】 雾迷山组是华北地台中元古界分布最广的地层单位之一,以环潮坪白云岩为主。其沉积极富韵律性,主要由凝块石白云岩、含硅质条带纹层石白云岩和泥质微晶白云岩层构成的向上变浅的副层序有序堆叠而成。地层中含有丰富多彩的微生物岩,尤其以凝块石最为特征,其次为层状、丘状、以及锥状叠层石。在潮下泻湖相或深潮下带中还发育有黑色纹层石与微指状叠层石。大量保存良好的微生物席以及发育的微生物岩表明,中元古代华北陆表海存在活跃的微生物群落。它们与沉积环境相互作用形成了多种形态的微生物岩。此外还发育一些与微生物作用相关的沉积组构,包括纹层石、鲕粒、泥晶、板刺和MISS等构造。研究表明,雾迷山组不同类型的微生物岩与古水深存在良好的对应关系:黑色纹层石与微指状叠层石主要产出于潮下泻湖带或深潮下带;凝块石主要产出在浅潮下带偶尔达到深潮下带;低丘状叠层石等多种形态叠层石主要产出于浅潮下带上部至潮间带下部;含硅质条带纹层白云岩主要产出于潮间带;而泥质微晶白云岩则产出在潮间带上部至潮上带。雾迷山组多种微生物岩中含大量纤维状文石假形。这是中元古代大气高CO2浓度、海洋贫氧条件下,有机质细菌硫酸盐还原作用导致的CaCO3过饱和从而诱发的海底自生碳酸盐岩沉淀。对雾迷山组各类碳酸盐岩的显微观察发现:深潮下带或潮下泻湖相黑色纹层石与微指状叠层石由针状文石等厚层与微生物席层交互堆叠而成;潮下带凝块石白云岩由针状文石形成的环带或扇状集合体包裹有机质团块所组成,少见碳酸盐微粒。浅潮下带上部—潮间带下部纹层白云岩与叠层石以自然沉淀的碳酸盐微粒为主导,含少量等厚层和扇状、葡萄状文石集合体,以及以孔洞充填形式产出的针状文石。而潮间带上部—潮上带泥晶白云岩则完全由碳酸盐微粒组成,不含针状文石沉淀。从总体上看,从深潮下带至潮上带,纤维状文石假晶的产出比例逐渐减少,表明了细菌硫酸盐还原作用从潮下带非常活跃转变为潮间带仅在微生物席覆盖之下存在而至潮上带不活跃。这种变化,可能反映了不同水深环境中,水体氧化还原状态的转变。因此,可以反映古海洋的氧化条件特征。更多还原

【Abstract】 The Wumishan Formation is one of the most widely distributed lithostratigraphic units in the Mesoproterozoic of North China Platform and dominated by various dolomites of tidal flat origin, showing clearly rhythmic deposition. The succession is made up of numerous parasequences which are typically composed of thrombolitic dolomite, biolaminitic dolomite with cherty bands and micritic dolomite, showing a clear tendency of shallowing upward. Abundant microbilites have been recognized from this formation, including thrombolite of most characteristic, followed by stratiformed, mound-shaped, and conical stromatolites. In the subtidal lagoon or deep subtidal zone there outputs black biolaminite and microdigital stromatolites.The presence of abundant well-preserved microbial mats and various microbialites suggests strong activity of microbial communities in the Mesoproterozoic epicontinental sea of North China. Their interaction with sedimentary environments resulted in the microbialites with morphology highly variable. In addition, some fabrics which may derive from microorganism etabolisms have also been identified, such as biolaminitic, oolitic, micritic components, as well as microbially coated slender spins and MISS （microbilly induced Sedimentary structures）. The study also shows that different microbialites in the formation are in good coincident to the water depth of deposition. The black biolaminite and microdigital stromatolite developed mainly in subtidal lagoon or deep subtidal zone, thrombolite largely in shallow subtidal, a variety forms of stromatolite more often in shallow subtidal to lower intertidal zone, laminated dolomite with chert bands largely in intertidal zone. While dolomicrite with rare mats is commonly restrictic to upper intertidal to supraltidal zone.Abundant fibrous aragonites have been identified from the microbialites. Due to the high CO₂ in atmosphere and low oxygen in ocean during Mesoproterozoic, bacterial sulfate reduction（BSR） mihght have resulted in oversaturated CaCO3 in the water, thus induced the precipitation of authigenic carbonate on sea-floor directly. The present study of microstructures of various carbonates in this formation shows that the black biolaminite and microdigital stromatolites in deep subtidal or subtidal lagoon are typically composed of alternated microbial mats and isopachous acicular aragonites. Subtidal thrombolitic dolomite is mainly composed of organic clots encrusted by fan-shaped or ring-shaped acicular aragonites, but rare micritics. Laminated dolomite and stratiform stromatolite in shallow subtidal to the lower intertidal zone are mainly composed of micritic carbonate or cements precipitated naturally, with a few isopachous, fan-shaped, or botryoidal aggregate of aragonites that might be generated as pore-fillings. However, dolomite in upper intertidal to supratidal zone is almost completely composed of carbonate micritics, with no acicular aragonites.In general, from deep subtidal to supratidal, fibrous aragonites show a clear tendency of decline, suggesting that BSR was active in deep subtidal, locally in upper subtidal and lower intertidal in the sediments underlying microbial mats, but absent from supratidal zone. This gradient may reflect the variation of redox in environments of various water depth. Thus aragonite sea-floor precipitation may have a potential to be used an indicator for redox condition in carbonates.更多还原