黔西北桐梓水坝塘下奥陶统桐梓组碳酸盐岩微相

黔西北桐梓水坝塘剖面下奥陶统特马豆克阶桐梓组沉积时位于上扬子区陆表海极浅水区,抑或部分时段存在盐度异常的云坪,与下伏寒武系第四统娄山关群和上覆下奥陶统红花园组均呈整合接触。桐梓组岩性多为灰色、深灰色灰岩,后生动物大化石和陆源碎屑岩含量显然偏低。常见内碎屑滩相堆积,偶见含砾屑滩,砾屑边缘弱磨圆,属非强水流的近源搬运颗粒类型;少许鲕粒灰岩层。微生物岩颗粒丰富并作为碳酸盐岩的主要供物来源,部分微生物群落形成的细纹层粘结岩属小型叠层石礁相单元,表明在贵阳乌当和桐梓红花园剖面的叠层石可北延超过100km到达水坝塘地区,构建了狭长的叠层石礁相带。黔西北桐梓组部分碳酸盐岩层中出现单一而广适性的钙质微生物造礁群落,与特马豆克阶全球生物礁演化发展阶段相吻合,在奥陶纪生物大辐射的初期,生物礁具有地理分布局限、低生物多样性以及钙质微生物占主导地位的特征。     

黔东北石阡志留系雷家屯组的生物-岩相分异

黔东北石阡志留系兰多维列统埃隆阶上部的雷家屯组属灰岩和碎屑岩混合相,其分布限于滇黔桂古陆以北离岸几十千米范围内的浅海区。出露于枫香铺子沟、雷家屯、白沙龙口—筷子山、白沙均田和本庄岩门的5个剖面存在生物-沉积相的差异性。铺沟村粉砂岩—泥岩含量高于灰岩,达到4∶1,壳相化石丰度最低且无生物礁滩灰岩沉积;其它4个剖面的碎屑岩—灰岩比例约1∶1,雷家屯壳相化石最丰富,上部出现米级厚度的珊瑚-层孔虫点礁;筷子山剖面的雷家屯组下部开始出现雏形礁,之上的礁核灰岩现已被剥蚀掉,但从礁基的菲利普构造和礁翼塌积岩推测,点礁的正向隆起规模是很明显的;均田剖面仅出露雷家屯组上部约7m的地层,多为粉砂岩和生屑灰岩薄层;岩门剖面多为薄层细颗粒生屑滩相灰岩,近顶部出现钙质微生物形成的叠层石。偏西剖面的雷家屯组顶部展现侵蚀面、泥裂等现象,是桐梓上升期海底暴露的证据,唯有最靠东的铺沟村雷家屯组顶部未见暴露标志,与之上马脚冲组呈整合接触。     

苏、皖北部震旦纪叠层石及其沉积环境学意义

根据苏、皖北部震旦纪叠层石的外形、大小、颜色及纹层特征,结合其它沉积构造特征,探讨水体能量、光照及沉积作用等环境因素对叠层石生长的影响,以此找出叠层石上具有指相意义的某些生态学特征。目前发现,大型层状、层柱状叠层石往往与浅水低能沉积物共生;分叉不发育的大、中型柱状叠层石则与具周期性变化的浅水高能沉积物共生;分叉发育的柱状叠层石与变化不定的浅水高能沉积物共生;平行分叉的小型柱状叠层石则与中-次中等能量沉积物共生。值得注意的是,环境的变化使一些叠层石形态也随之变异,形成一系列过渡类型,引起分类上的混淆。叠层石纹层表面积与水深往往成反比梯变,这也许与藻席受光照的充足程度有关。叠层石及其围岩的原生色也可作为了解当时水体化学性质及生物对矿物的富集作用的线索。     

岩溶洞穴叠层石

岩溶洞穴叠层石系藻类等生物在洞穴弱光带富含钙质的岩溶水中,通过粘结捕获或同化作用沉淀碳酸盐面形成。油穴叠层石均显示双层叠置构造,即背光浅色层和向光深色层,并由细密纹层组成,。向光深色层疏松多孔,表面粗糙可见生物体;背光浅色层致密,表面光滑。因生长微环境的变化,其形态可分为３类：倾斜型,直立型和坝型。     

陕南川北志留系特列奇阶宁强组礁相中的微生物岩和钙藻

分布于扬子区西北缘宁强-广元地区志留系宁强组以约三千米厚的浅水相泥页岩,海相红层夹灰岩为特征,部分灰岩层段中发育生物礁,通过对礁灰岩的微相分析表明,菌藻类可通过四种方式参与造礁过程;1）叠层石;2）凝块岩;3）核形石;4）钙藻碎屑堆积,其中以叠层石和凝块岩最为常见,它们对灰泥基质起显著的粘结作用。而核形石和钙藻相对较少,有的叠层石出现于礁顶相并作为后生动物骨架岩造礁衰减的标志。     

淮南地区新元古代九里桥组叠层石成礁过程及其影响因素

淮南地区新元古代九里桥组中段灰岩中发育有形态多变的叠层石礁体,具有与显生宙生物礁相似的相分异特征,基底、礁核、盖层、礁前、礁后、礁翼等不同微相可以明确区分,定殖期、拓殖期、泛殖期和衰亡期等不同造礁阶段的叠层石柱体变化特征明显。该组叠层石礁体自下而上分别为分散分布的小型丘状礁体、连绵分布的大型丘状礁体和分散分布的小型丘状礁体或透镜状礁体,该变化趋势指示了九里桥组沉积时期海平面先升高后降低的变化趋势,结合该组沉积期沉积环境变化特征可将该组叠层石礁体划分为风暴环境型礁体、海进环境型礁体和海退环境型礁体三种类型。对九里桥组沉积学、古生物学等研究表明,该组沉积时期造叠层石生物与其它生物之间存在较强的生存竞争关系,但更能适应风暴沉积环境,叠层石在该组沉积晚期的消失很可能与以海平面变化为特征的沉积构造环境变化有关。     

陕南寒武纪早期仙女洞组生物礁灰岩微相序列

扬子地台北缘在寒武纪筇竹寺期—沧浪铺期出现海退,该时期的仙女洞组属浅海碳酸盐岩与碎屑岩的混合沉积单元,分布于陕西南部及四川北部。陕南南郑福成和碑坝剖面组成完整仙女洞组序列,其中包含8期生物礁相单元。单层礁体为米级厚度,形态特征显示丘状隆起或平缓延伸。水体的清澈度、深度以及水动力强度变化对各期礁相单元生长的控制力度因时而异,陆源碎屑数次侵入导致海水浑浊度增高,点断了礁体生长。礁灰岩微相类型包括钙质微生物粘结岩、古杯-钙质微生物绑结岩、古杯漂砾-砾屑灰岩、粒泥状-泥粒状灰岩以及泥状灰岩。早期礁灰岩中钙质微生物粘结岩含量丰富,之后古杯绑结岩在礁灰岩构建中具有丰度增长的趋势,且上部礁灰岩层位中不规则古杯占主要地位。仙女洞组沉积晚期海水变浅和水动力条件增强,常见漂砾灰岩和砾屑灰岩。     

叠层石衰减事件

“叠层石”这一术语是1908年Kalkowsky首先定义的,用来描述具有多少近似平面状、生物成因细纹层构造的灰岩体。叠层石这个词的使用和定义经历了逐渐的变化,现在大家一致认为叠层石是蓝细菌等微生物群体形成的一种层状的生物沉积构造,如下图所示。近代叠层石主要形成于清水环境、海边缘、潮间带的浅水环境。叠层石并不一定是碳酸盐岩,也可以是硅质岩、蒸发岩或磷酸盐。许多金属矿沉积都与叠层石有密切联系,尤其是叠层石礁。在叠层石形成过程中,微生物对于各种自生矿物的沉淀起到了重要作用,包括碳酸盐、磷酸盐、氧化物、硫化物和硒酸盐。微…     

华南板块古生代生物礁及其古地理控制因素

华南板块在古生代处于中、低纬度,碳酸盐岩类型多样并形成不同时空背景下的多种生物礁建造,生物礁发展序列基本吻合于全球古生代生物礁的宏演化趋势,寒武纪生物群和古生代动物群演化过程中重要造礁生物门类的起源、辐射、灭绝与复苏事件是塑造礁群落基本生态结构的历史因素。寒武纪早期的古杯-藻礁和继之的微生物礁生长区域相当局限;早—中奥陶世的苔藓虫礁、藻礁以及瓶筐石-硬海绵礁群落分异明显;晚奥陶世珊瑚-层孔虫礁以及藻丘建造见于浙赣局限台地及台缘带,而扬子区志留纪兰多维列世生物礁的生长频繁受陆源碎屑岩覆盖;中泥盆世的珊瑚-层孔虫-藻礁群落结构相对稳定,晚泥盆世法门期—密西西比亚纪的微生物礁、苔藓虫-珊瑚礁、宾夕法尼亚亚纪—早二叠世的苔藓虫-海绵-藻礁、中—晚二叠世的珊瑚-苔藓虫-海绵-藻礁可诠释为与生物灭绝事件相关的幕式群落演替。区域构造活动导致的岩相分异和海平面变化显著制约生物礁的时空分布。中—晚奥陶世的偏深水环境、志留纪兰多维列世—早泥盆世早期扬子区整体抬升的古地理格局造成适宜于生物礁生长海域的缩减;泥盆纪较长的温室期促进了生物礁发展,而宾夕法尼亚亚纪—早二叠世偏凉的海洋水体对生物礁的规模影响力度明显。从华南板块古生界整体的视角看,海相碳酸盐岩具有量值优势,海水在时间尺度和空间展布上多维持较高的清澈度,陆源碎屑岩沉积在特定的时间段可视为生物礁生长的主控因素;海平面变化因其幅度有限可在单剖面或区域上控制生物礁群落的纵横迁移,碳酸盐岩沉积区多见基底沉降与沉积补偿速率基本均衡,具备不同规模的浅海相沉积空间,因此水深变化并非起到决定性作用。特定时段碳酸盐岩台地海水的盐度异常可造成大规模白云岩沉积可排除生物礁发育。     

黔北桐梓志留系石牛栏段近岸相灰岩

黔北石牛栏组上部的石牛栏段灰岩时代为志留纪兰多维列世埃隆晚期,是上扬子区浅海底栖壳相后生动物繁盛阶段的产物。桐梓代家沟剖面处于黔中古陆以北的近岸带,石牛栏段灰岩序列呈现海退过程,微相特征表明这里频繁出现各类近岸浅海带清澈环境中常见的生屑滩沉积。石牛栏段下部出现风暴浪基面附近腕足类Zygospiraella和Pentamerus密集的介壳滩,大多数的滩相灰岩以细颗粒生屑堆积为主,夹含薄层腹足类生屑滩和鲕粒滩,仅见少量珊瑚薄层,可以排除此地为后生动物礁的栖居区;石牛栏段上部见数期叠层石生长,形态呈穹窿状或平缓席状,每期厚度不超过1m,伴生的暗色泥质夹层中产出的腕足类Lingullela-Eospirifer群落指示澙湖相;潮间带特有的薄层灰岩交错层理以及石牛栏段顶部的喀斯特面进一步昭示区域性逐渐发生的桐梓上升事件结束了近岸沉积。     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FROM MODERN FRESHWATER SETTINGS1

Some modern laminated and calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolies growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventunally become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. Katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a property they share with cyanobacterial stromatolite builders.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

Stromatolitic communities in Mediterranean streams: adaptations to a changing environment

A stromatolitic microbial mat extensively covers La Solana streambed, a calcareous Mediterranean stream. This stromatolite shows remarkable biological and physiological diversity. It is mainly composed by cyanobacteria, with Rivularia and Schizothrix as the most abundant taxa. The stromatolite is photosynthetically adaptated to the high irradiances reaching the streambed. Photosynthetically active chlorophyll is present even in the lowest layers of the stromatolite, indicating the presence of well-preserved cyanobacteria in that part. Diffusion of gases and nutrients within the stromatolite can be possible because of the high porosity of the crust. It has been experimentally established that the stromatolite recovers heterotrophic and autotrophic activities in a few hours, after being desiccated for long periods. Recovery after desiccation is indicative of the high resilience of this community to environmental extremes, which are common in Mediterranean climatic regimes. The stromatolitic community is adapted to nutrient limitation, both to low availability of inorganic phosphorus and nitrogen (that constrain growth of primary producers), and to low dissolved organic carbon (mainly affecting heterotrophs). Stromatolitic heterotrophs mainly rely on the organic carbon stored in the crust as the main organic carbon source. These strategies are the direct response of the stromatolite to oligotrophy, and justify the restricted occurrence in stream systems affected by organic pollution.     

Algal stromatolites from the Late Precambrian of Sweden

Stromatolitic structures from the Late Precambrian Visingö Beds are described. The mode of development of these sedimentary-biogenic structures and the problems related to their characterization in a system of formulas are discussed. The study of the stromatolitic structures and their associated sediments proved that the stromatolitic structures LLH (vertically arranged, lateral-linked hemispher-oids) and SH (discrete, vertically stacked hemispheroids) could develop together on the same stromatolite dome, depending on different degrees of exposure to the action of water currents in an intertidal environment. Superimposed LLH and SH structures in the Visingsö Beds may indicate periodical changes in current activity depending on repeated periodic events such as tidal and/or storm waves.     

‘Stromatolites’ built by sponges and microbes – a new type of Phanerozoic bioconstruction

Two ‘stromatolites’ from Carboniferous and Triassic carbonates previously regarded as microbial bioconstructions are analysed and reinterpreted as sponge‐microbial build‐ups. The automicritic aggregations in these build‐ups are similar to the previously reported fossils of keratose demosponges in showing moulded anastomosing filamentous structures. All the studied columnar or domal constructions were formed in turbulent water with high sedimentation rate. The Carboniferous build‐ups were constructed in the shallow subtidal zone of an open shelf or a ramp. The laminations within the stromatolite‐like columns are composed of alternating dark micritic laminae of sponge fossils and pale laminae of neomorphic microspars. The accretion of these columns is probably related to the repeated cycles of sponge growth, rapid lithification after burial, re‐exposure and erosion, and settlement of new generations. The Triassic rocks are presumed to have been precipitated in a slightly evaporitic environment based on lithological features. They show a transition from planar laminae, which were formed under the influence of microbial mats, to stromatolitic columnar or domal build‐ups, which are dominated by stacked micritic clumps of probable sponge fossils. The sponge–microbe alternation may have been controlled by variation of salinity. Comparable with a recent study, this work shows that sponge‐related bioconstructions can be morphologically similar to microbialites in the level of mega‐ and mesostructures.     

Modern freshwater microbial carbonates: thePhormidium stromatolites (tufa-travertine) of southeastern Burgundy (Paris Basin,France)

Summary The relationships of microstructure and vegetal remains, obtained by decalcification, were studied in Modern tufa from Burgundy, in order to try to link a given species with a particular crystal habitus. The edifices have various shapes (coatings on floors; encrusted pebbles, shells, vegetal shoots, mosses; oncolites; hydrodynamically shaped tufts). The biological content is rich in algae and animals, mainly at the proximity of springs, even ifPhormidium incrustatum is the predominant species. It is associated with several species ofGongrosira, Schizothrix, andOocardium stratum, the latter only known by its specific crystallizations. Among the animals, we point the galleries ofPsychomiidae (Trichoptera= Phrygan) larvae. The algae and animals are associated within a “biological felt” (in the sense ofForel, 1901). Some species are encrusted by calcite crystals of typical habitus (micrite:Phormidium incrustatum, Gongrosira andSchizothrix, ssp; sparite:Oocardium andBatrachospermum), and there are very little diagenetic modifications. The fabric results in an alternation of seasonal light laminations composed of juxtaposed bundles ofPhormidium incrustatum α, and dark laminations due to parallel filaments ofPhormidium incrustatum β. The influence of other algal species on shape and the internal fabric of the laminations is negligible.Phormidium incrustatum tufa are common in Western Europe, and probably have some fossil analogue in the Upper Cretaceous and Tertiary; the strongly differ from most older stromatolitic microstructures. Half of the studied tufa can suffer summer exposure and winter frost but related particular features do not seem to be preserved in the stromatolitic edifices.     

A New Stromatolitic Mat Builder Discovered from Late Precambrian in Northern China

Abundant and preserved chain algal microfossils have been discovered in cherty stromatolitic mats from the second member of Gaoyuzhuang Formation (about 1,500 Ma old), Ling-qiu county of Shanxi province, China. They are described as Veteronostocale moniliforme Xu et Gao sp. nov.. The small diametral trichomes resemble the Family Nostocaceae in possessing specilized cells resembling the classic heterocysts and akinetes. This paper gives emphasis to the following remark: (1) The Gaoyuzhuang stromatolitic mats were products of microbial (Veteronostocale moniliforme) activity and the plant Nostocaceae was one of the main stromatolitic builders as both modern and Precambrian stromatolite-forming microorganisms; (2) Based on the fact that the chain fossils are preserved perpendicularly to the laminations the rates of sedimentation and algal growth were probably equal and each small sedimentary rhythm could be completed in about one or two months; (3) According to the environment and habits of living Nostoc Precambrian hydrosphere might be of a fresh water type.     

Biomineralization at hot springs and mineral springs, and their significance in relation to the Earth's history]

Recently, there is strong interest on microbe-mineral interactions. This is related also to recent expanded knowledges on extremely severe environments in which microbes live. Interaction between microbes and minerals contains biomineralization processes. Varieties of biomineralization products are found not only in various geologic materials and processes in the earth's history but also in present surface environments. Some hot springs represent such environments similar to those of unique and extremely severe environments for life. In this short review, the author briefly shows some examples of biomineralizations at some hot springs and mineral springs, Japan. In such environments, iron ore was formed and some varieties of growing stromatolites were found. The varieties of stromatolite are siliceous, calcic and manganese types. Cyanobacteria and the other bacteria are related to form the stromatolite structure. In the Gunma iron ore, sedimentary iron ores were mineralogically described in order to evaluate the role of microorganisms and plants in ore formation. The iron ore is composed of nanocrystalline goethite. Algal fossils are clearly preserved in some ores. Various products of biomineralization are found in the present pH 2-3, Fe2(+)- and SO4(2-)-rich streams. Bacterial precipitation had variations from amorphous Fe-P-(S) precipitates near the outlet of mineral spring, to Fe-P-S precipitates and to Fe-S-(P) precipitates. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. The Gunma Iron Ore can be said as Biologically Induced Iron Ore. At Onikobe and Akakura hot springs, growing stromatolites of siliceous and calcareous types, were found, respectively. At Onikobe, The stromatolites grow especially near the geyser. Cyanobacterial filaments in stromatolite were well preserved in the siliceous and calcic stromatolites. The filaments oriented in two directions which form the layered structures were found. At Yunokoya hot spring, black and brittle stromatolitic structures which were composed of amorphous Mn minerals are growing. The form of these structures are hemispherical. Many bacteria that were coated with amorphous Mn minerals were found on these structures. Furthermore, Precambrian (Proterozoic : Wittenoom-Chichester region, western Australia) manganese stromatolite was briefly shown in comparison. The black stromatolite has been clarified to be composed of todorokite. Small spotty and donuts-like shaped todorokite aggregates which are very similar to biologically induced Mn-precipitates were found in massive dolomite layers.     

Growth of modern branched columnar stromatolites in Lake Joyce,Antarctica

Modern decimeter‐scale columnar stromatolites from Lake Joyce, Antarctica, show a change in branching pattern during a period of lake level rise. Branching patterns correspond to a change in cyanobacterial community composition as preserved in authigenic calcite crystals. The transition in stromatolite morphology is preserved by mineralized layers that contain microfossils and cylindrical molds of cyanobacterial filaments. The molds are composed of two populations with different diameters. Large diameter molds (>2.8 μm) are abundant in calcite forming the oldest stromatolite layers, but are absent from younger layers. In contrast, <2.3 μm diameter molds are common in all stromatolites layers. Loss of large diameter molds corresponds to the transition from smooth‐sided stromatolitic columns to branched and irregular columns. Mold diameters are similar to trichome diameters of the four most abundant living cyanobacteria morphotypes in Lake Joyce: Phormidium autumnale morphotypes have trichome diameters >3.5   μm, whereas Leptolyngbya antarctica, L. fragilis, and Pseudanabaena frigida morphotypes have diameters <2.3   μm. P. autumnale morphotypes were only common in mats at <12 m depth. Mats containing abundant P. autumnale morphotypes were smooth, whereas mats with few P. autumnale morphotypes contained small peaks and protruding bundles of filaments, suggesting that the absence of P. autumnale morphotypes allowed small‐scale topography to develop on mats. Comparisons of living filaments and mold diameters suggest that P. autumnale morphotypes were present early in stromatolite growth, but disappeared from the community through time. We hypothesize that the mat‐smoothing behavior of P. autumnale morphotypes inhibited nucleation of stromatolite branches. When P. autumnale morphotypes were excluded from the community, potentially reflecting a rise in lake level, short‐wavelength roughness provided nuclei for stromatolite branches. This growth history provides a conceptual model for initiation of branched stromatolite growth resulting from a change in microbial community composition.