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法国博物学家Dolomieu[1]在1791年首次描述了白云岩,之后国内外地质学者对白云岩进行了广泛的研究。白云石是一种常见的碳酸盐矿物,大量出现在古代地层中,但在现代近地表环境即常温、常压实验条件下不能生成有序的白云石矿物[2-6]。目前对白云岩成因的认识主要分为无机成因和有机成因:1)无机成因:王茂林等[7]将现有的20余种白云石的成因划分为原生白云石和次生白云石化两种模式,并对其中几种经典的白云石化模式的真实性和适用性进行了探讨;白云石生成机理主要包括原生沉淀作用、同生白云化作用、毛细管浓缩作用——准同生白云化作用(蒸发泵作用)、回流渗透白云化作用、混合白云化作用、埋藏白云化作用、淡水白云化作用、热液白云化作用等[8-9]。2)有机成因:1928年Nadson[10]利用俄罗斯盐湖中分离出的硫酸盐还原菌在厌氧条件下沉淀出自生碳酸盐,提出白云石可能在类似的厌氧条件下由细菌协同沉淀。Vasconcelos et al.[11]通过实验在硫酸盐还原菌作用下沉淀出白云石,从而提出白云石的微生物成因模式。之后,很多学者对白云石的有机成因进行了大量研究[12-32],微生物白云石化模式受到越来越多的关注,逐渐成为新的主流模式[7]。
豹皮灰岩(豹斑灰岩)是指具有不规则白云质条带或斑块的石灰岩,通常认为斑块部分为白云岩,基质部分为石灰岩,也有学者将其描述为蠕虫状灰岩。豹皮灰岩广泛分布于华北板块寒武系、奥陶系地层中。目前对豹皮灰岩的成因主要有无机成因、有机无机混合成因两种观点。1)无机成因:翟淳[33]详细描述了寒武系、奥陶系的豹皮灰岩,并对其进行了分类,认为豹皮灰岩是白云石冻泥渗入(坠入)还未完全硬化的方解石冻泥中或是二者互相渗入搅动的沉积作用所形成。王尧等[34]认为豹皮灰岩是白云石充填泥裂裂隙所形成。贾振远等[35]把豹斑灰岩中的豹斑解释为重力作用而形成的负载构造和由于波浪扰动作用使泥云质条带破碎而成。李定龙等[36]认为豹皮灰岩在成因上与沉积岩溶的发育形成过程密切相关,其形成过程可分为局部暴露发育裂隙、发育沉积岩溶洞穴、海侵使洞穴充填和海平面升降导致充填物白云石化四个阶段。郝毅等[37]在对川西北中二叠统栖霞组豹斑灰岩的研究中也认为豹斑灰岩的形成与溶蚀作用有关,认为溶蚀作用形成的缝洞系统控制了豹斑灰岩的分布形态。龙刚等[38]认为豹皮灰岩的白云岩化作用发生在成岩作用早期,是在低盐度的混合水动力环境下经过回流渗透白云岩化作用所形成的。陈云峰等[39]在对北京周口店豹皮灰岩的研究中认为豹皮灰岩与构造作用有关,是一种变形构造。2)有机无机混合成因:陈战杰等[40]认为在碳酸盐沉积物的形成和改造过程中,生物起了巨大作用,豹斑的整体形状保留了遗迹化石的形态。豹斑的整体形态实际上是生物潜穴(虫迹)经白云石化而成。王起琮等[41]认为豹皮灰岩是早期以回流渗透方式进入生物潜穴的蒸发海水在埋深逐渐增加、温度不断升高过程中,交代潜穴中疏松灰质组分的结果,白云化流体的盐度和生物扰动强度是控制云斑构造发育程度的主要因素。许多学者的研究表明豹皮灰岩的形成与生物扰动作用有关[42-45]。国外学者对类似斑块状白云岩的研究也很多,并认为其主要与生物潜穴的选择性白云岩化作用有关[46-49]。
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寒武系关口剖面位于登封市东北部,实测露头主要为寒武系第二统朱砂洞组,其位于范家门村附近(图 1)。根据寒武纪古地理及古构造格局,沉积特征、岩性组合、岩相及沉积环境、生物群等综合分析,将华北地层区的寒武系划分为太行山分区、豫东平原分区和豫西分区三个大区,其中豫西分区进一步划分为灵宝—鲁山小区、叶县—确山小区、卢氏小区和渑池—登封小区四个小区,其中关口剖面位于渑池—登封小区[50]。
朱砂洞组源于张伯声在平顶山市朱砂洞村创建的“朱砂洞石灰岩系”,现在指辛集组之上、馒头组之下的一套灰岩、白云质灰岩[50]。实测剖面(关口剖面)朱砂洞组主要岩性为灰岩和白云岩(图 2)。朱砂洞组经历频繁的海退,沉积海水显示整体变浅的过程,继承了辛集组末期海退的趋势[51]。朱砂洞组为滨岸碳酸盐潮坪缓坡—潮间—潮上蒸发坪和潮间蒸发岩洼地以及潮下碳酸盐浅水缓坡的沉积环境[52]。对研究剖面产出的遗迹化石做了详细的属种鉴定,主要为Thalassinoides(海生迹),含有少量Planolites(漫游迹)。
图 2 河南登封寒武系第二统朱砂洞组和山西兴县中奥陶统马家沟组五5亚段地层柱状图
Figure 2. Lithostratigraphy of the Cambrian epoch 2 Zhushadong Formation in the Dengfeng area and the Ordovician Majiagou Formation (submember 5) in the Xingxian area
奥陶系关家崖剖面位于山西省吕梁市兴县蔚汾镇关家崖村一带。剖面地处兴县县城东北部山区,中奥陶统马家沟组马五5亚段沿蔚汾河北岸支流即关家崖村006乡道两侧分布,实测剖面位于006乡道东侧河谷内(图 1)。从大地构造来看,研究区位于华北地台的西南缘、鄂尔多斯盆地东缘,次一级构造单元属陕甘宁盆地,三级构造单元属陕甘宁盆地东缘晋西挠褶带,四级构造单元属兴县—石楼南北向褶皱带[53],实测剖面位于背斜的右翼。中奥陶统马家沟组自下而上可分为六段,其中马五段自上而下划分为十个亚段,其中马五5亚段主要为微晶灰岩、含灰云岩(图 2)。对研究剖面产出的遗迹化石做了详细的属种鉴定,中奥陶统马家沟组五5亚段中遗迹化石较丰富,识别出的遗迹化石共7个遗迹属,分别为Arenicolites、Cylindricum、Helminthopsis、Lorenzinia、Planolites、Teichichnus、Thalassinoides,其中Thalassinoides占主导地位。研究剖面产出的遗迹化石组合属于Cruziana遗迹相,主要分布在滨海、潮坪或潟湖沉积环境[54-55]。结合剖面岩石学特征、沉积相标志以及遗迹化石特征,兴县关家崖中奥陶统马家沟组马五5亚段总体反映了一套蒸发台地相或局限台地相的沉积环境[53]。
评估生物扰动程度遵循标准的遗迹化石惯例,使用生物扰动指数(BI)[56]。BI显示原始沉积结构可见程度。在该方案中,BI=0(0%)为没有生物扰动,原生组构保存完好。BI=1(1%~4%)有少量生物扰动,有不连续的遗迹局部覆盖,沉积组构保存的较好。BI=2(5%~30%)代表中低等生物扰动,沉积构造保存的仍然较好。BI=3(31%~60%)是有不连续遗迹化石的遗迹组构,中等生物扰动,仍然可以辨认出地层界面。BI=4(61%~90%)代表强烈的生物扰动,遗迹化石密度高,遗迹化石和原始沉积构造几乎全部被抹掉。BI=5(91%~99%)特征为层面沉积物被完全扰乱和强烈的生物扰动。BI=6(100%)是完全生物扰动和重新沉积,与遗迹化石重复覆盖有关。
对研究剖面采集的标本磨制薄片,并进行偏光显微镜下观察,发现生物潜穴内部和围岩的矿物成分及晶粒大小都有很大差别。用场发射环境扫描电镜观察生物潜穴内部与围岩的微观结构特征。扫描电镜样品制备:将切割好的岩石标本用蒸馏水清洗烘干后用导电胶固定在样品台上,放入金属离子溅射仪中喷金60 ~80 s,然后使用场发射扫描电镜进行观察(场发射环境扫描电镜(ESEM,FEI Quanta 250 FEG)和布鲁克能谱仪(EDS,Bruker Quantax 200 XFlash 6|30),河南省生物遗迹与成矿过程重点实验室,河南理工大学,焦作)。
通过偏光显微镜对寒武系第二统朱砂洞组和奥陶系马家沟组五5亚段的生物扰动白云岩进行了微观结构特征的观察,发现生物潜穴内部主要成分为白云石,围岩部分主要成分为方解石。通过Image J软件来测量晶体大小,并确定其晶粒级别。
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翟淳[33]将豹皮灰岩分为豹皮作不规则状和杂乱分布的豹皮灰岩、豹皮垂直层面分布的豹皮灰岩、豹皮作波线状分布的豹皮灰岩、豹皮作平行带状分布的豹皮灰岩—泥质条带灰岩、豹皮作角砾状的豹皮灰岩、似角砾状的豹皮灰岩6个类别。通过对登封关口剖面寒武系第二统朱砂洞组和兴县关家崖剖面中奥陶统马家沟组五5亚段进行详细的分析研究认为两个研究剖面的生物扰动白云岩/生物扰动灰岩(豹皮灰岩)为遗迹化石Thalassinoides潜穴经过不均匀白云岩化的结果。
登封关口剖面寒武系第二统朱砂洞组主要为中厚层到巨厚层的生物扰动白云岩/生物扰动灰岩(豹皮灰岩),潜穴(豹斑)部分与围岩界线清晰,其中潜穴填充物颜色为深灰色、围岩为浅灰色,在风化面上十分明显(图 3a,b)。由于潜穴内部充填物成分与围岩部分不同(潜穴内部为白云岩、围岩部分为石灰岩),故二者抗风化能力有差别,在野外露头中潜穴部分向外凸出,围岩部分向内凹陷。寒武系第二统朱砂洞组中的Thalassinoides在垂直方向上较短小,但丰度很高,生物扰动很强烈,在层面上显示出网状结构,其T-型分支和Y-型分支清晰可见,还能够看到一些垂直方向的潜穴管。
图 3 河南登封寒武系第二统朱砂洞组生物扰动白云岩野外特征
Figure 3. Bioturbated dolomite in the Cambrian epoch 2 Zhushadong Formation in the Dengfeng area
兴县关家崖剖面中奥陶统马家沟组五5亚段主要为薄层到中厚层生物扰动灰岩、白云岩,在风化面上潜穴部分与围岩界线较清晰,但相对朱砂洞组稍差,潜穴内部颜色较围岩部分略浅,潜穴部分多为灰黄色、围岩部分多为青灰色。兴县关家崖剖面中奥陶统马家沟组五5亚段的生物扰动灰岩在野外露头中较平整,没有类似寒武系剖面的凹凸情况。中奥陶统马家沟组五5亚段中Thalassinoides的T-型分支、Y-型分支和垂直潜穴管亦清晰可见,在垂直方向上较寒武系更深,潜穴宽度更大,层面上一些部分可连接成块状,在一些层位发现生物潜穴的多期次覆盖,扰动强烈的层位难以区分层序界面(图 4)。一些层位可见到大颗粒充填的潜穴(图 4c),表明沉积环境较为动荡。马家沟组五5亚段的生物扰动灰岩较为破碎,可见大量裂隙。
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寒武系第二统朱砂洞组的生物潜穴中主要为细粉晶—粗粉晶、半自形—他形/半自形—自形、直面—曲面白云石,接触关系多为直面接触或镶嵌接触。朱砂洞组中的Thalassinoides潜穴与围岩边界明显,较少出现白云石由潜穴内部向围岩扩散的潜穴边界白云岩化。潜穴内部白云石发育晶间孔、残余晶间孔,但各潜穴中差异较大,一些潜穴很少发育晶间孔(图 5a,e,f),一些潜穴晶间孔、残余晶间孔较发育(图 5b~d)。在一些潜穴中可以见到较大的方解石斑晶(图 5e),方解石斑晶开始逐渐向白云石转变;以及后期的方解石脉体,方解石脉体同时切穿潜穴和围岩部分(图 5f)。围岩部分主要为泥晶方解石。
图 5 登封寒武系第二统朱砂洞组生物扰动白云岩微观结构特征
Figure 5. Microstructural characteristics of bioturbated dolomites from the Cambrian epoch 2 ZhushadongFormation in the Dengfeng area
奥陶系马家沟组五5亚段的生物潜穴中主要为粗粉晶、半自形—自形、直面—曲面(直面较多)白云石,接触关系为点—直面接触(生物潜穴边界白云岩化,白云石由潜穴内部向围岩扩散)、直面接触或镶嵌接触(生物潜穴内部)。马家沟组五5亚段的Thalassinoides潜穴与围岩边界有一些是渐变式(图 6a,c),也有十分清晰的边界(图 6b,e)。潜穴内部白云石晶间孔发育较差,在一些渐变式潜穴边界发育晶间孔。潜穴中发现一些方解石斑晶(图 6d)和沿潜穴发育的微裂隙(图 6c),也存在一些后期切穿潜穴及围岩的方解石脉体(图 6e)。在一些潜穴的边界及围岩部分发现存在生物碎屑(图 6a,b),在图 6f中发现围岩中有方解石充填的石膏假晶,表明沉积环境可能为炎热的蒸发环境。围岩部分主要为泥晶方解石,偶见孤立散落的、自形程度较高的白云石。在岩芯中多发育沿潜穴边界分布的缝合线构造,例如XX井马家沟组五5亚段中生物潜穴与围岩边界常发育一些裂隙,裂隙存在油浸现象,在潜穴边界周围多发育缝合线构造(图 7)。
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通过场发射环境扫描电镜及能谱对寒武系朱砂洞组和奥陶系马家沟组五5亚段的生物扰动白云岩进行了超微结构观察,二者中Thalassinoides的潜穴与围岩边界均清晰可见(图 8a~d、图 9a~c)。从图像中可以发现Mg集中分布于生物潜穴内部,Ca集中分布于围岩部分,表明潜穴内部主要为白云石,围岩部分主要为方解石。
图 8 登封寒武系第二统朱砂洞组生物扰动白云岩超微结构特征
Figure 8. Ultramicro characteristics of bioturbated dolomites from the Cambrian epoch 2 Zhushadong Formation in the Dengfeng area
图 9 山西兴县中奥陶统马家沟组生物扰动白云岩超微结构特征
Figure 9. Ultramicro characteristics of bioturbated dolomites from the Middle Ordovician Majiagou Formationin the Xingxian section
基质围岩部分主要成分为方解石,结晶颗粒较小且排列紧密,晶间空隙较少,围岩中会存在一些散落、独立分布的白云石晶体,通常结晶程度较好多为自形白云石呈菱形(图 8c、图 9d)。生物潜穴内部主要成分为白云石,结晶颗粒较大,发育一些晶间微孔(图 8e、图 9e)。白云石颗粒表面由于溶蚀作用会出现一些晶内次生溶孔(图 8f、图 9f),随着溶蚀作用的增强,原有的晶内次生溶孔相互连通,使得白云石颗粒表面越来越破碎(图 8g,h),直至形成残余的片状白云石颗粒(图 9h白色三角所指部位)。在奥陶系马家沟组五5亚段的Thalassinoides潜穴中还发现了一些球状白云石(图 9g白色三角所指部位)。
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目前被普遍认同的白云石生成机理主要有原生沉淀作用、同生白云化作用、毛细管浓缩作用——准同生白云化作用(蒸发泵作用)、回流渗透白云化作用、混合白云化作用、埋藏白云化作用、淡水白云化作用、热液白云化作用等[8-9]。前人对寒武系、奥陶系豹斑灰岩/豹斑白云岩进行了大量研究,很多学者都认为豹斑灰岩/豹斑白云岩的形成与生物作用密切相关[42-49]。豹斑灰岩/豹斑白云岩中的白云石斑块并不是杂乱分布的,而是有一定的规律性,白云石斑块沿生物潜穴分布,潜穴内部为白云石,围岩部分为方解石,白云石斑块保存了完整的生物潜穴形貌,表明生物潜穴内部优先发生了白云石化作用,因此生物扰动作用是豹斑灰岩/豹斑白云岩形成的重要因素。登封寒武系第二统朱砂洞组的生物扰动白云岩和兴县中奥陶统马家沟组五5亚段的生物扰动白云岩均与遗迹化石有关,生物潜穴在野外露头颜色与围岩不一,生物潜穴内部为白云岩,基质围岩为灰岩。Thalassinoides是潜穴壁光滑的无衬壁潜穴,通常产生于硬底质上[55],基质围岩已经脱水较坚固,从而形成完好连通的三维潜穴系统。一般认为Thalassinoides为甲壳动物十足目虾或类虾动物的居住兼觅食潜穴[55],古生代的Thalassinoides造迹生物也可能为三叶虫、海葵和蠕虫类[57-58]。Thalassinoides是开放式潜穴,其三维潜穴系统为白云石化流体提供了良好的通道,大大促进了高镁盐水的回流渗透作用。生物潜穴内部充填物经过生物扰动作用后较疏松,由于Thalassinoides为硬底质上的潜穴,因此潜穴系统对内部充填物起到一定的保护和支撑作用,潜穴内部充填物承受的压力较围岩更小,从而较好的保护潜穴内部充填物的孔隙,增加了潜穴内部充填物质的孔隙度、渗透率,增强了高镁盐水的回流渗透作用,是生物扰动白云岩形成至关重要的一步(图 10)。高镁盐水沿着潜穴通道运移,并通过潜穴向围岩渗透,从而形成了一些白云石由潜穴内部向围岩过渡的渐变式潜穴边界。
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登封寒武系第二统朱砂洞组的生物扰动白云岩潜穴填充物颜色为深灰色、围岩为浅灰色,在风化面上十分明显,潜穴部分向外凸出,围岩部分向内凹陷;中奥陶统马家沟组五5亚段的生物扰动白云岩在野外露头中较平整,没有类似朱砂洞组的凹凸情况。马家沟组五5亚段的Thalassinoides较朱砂洞组的Thalassinoides潜穴直径更大,潜穴分支处也略有不同,马家沟组五5亚段的Thalassinoides在潜穴分支处连接成片状。登封寒武系第二统朱砂洞组的生物扰动白云岩和兴县中奥陶统马家沟组五5亚段的生物扰动白云岩均与遗迹化石Thalassinoides潜穴系统有关,Thalassinoides的开放式三维潜穴系统为白云石化流体提供了良好的运移通道,使潜穴内部产生优先白云石化作用,而基质围岩部分仍为方解石。生物潜穴内部为白云岩,基质围岩为灰岩,从而形成了豹斑状的生物扰动白云岩。
Formation Mechanism of Cambrian-Ordovician Bioturbated Dolomites in North China
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摘要: 华北地区寒武系和奥陶系发育了大量的生物扰动白云岩/生物扰动灰岩,选择两个代表性剖面——河南登封寒武系剖面和山西兴县奥陶系剖面进行碳酸盐岩系生物扰动成因研究。河南登封寒武系第二统朱砂洞组中的遗迹化石较单一,主要为Thalassinoides、含有少量Planolites;山西兴县中奥陶统马家沟组五5亚段中遗迹化石较丰富,识别出的遗迹化石共7个遗迹属,分别为Arenicolites、Cylindricum、Helminthopsis、Lorenzinia、Planolites、Teichichnus、Thalassinoides,其中Thalassinoides占主导地位。朱砂洞组主要为滨岸碳酸盐潮坪相,中奥陶统马家沟组五5亚段主要为局限台地相。通过野外地质考察及室内相关实验手段及方法,利用偏光显微镜、扫描电镜对生物潜穴及围岩进行了宏观和微观结构分析,发现生物潜穴内部主要为白云石,基质围岩主要为方解石,生物潜穴在野外露头上表现为颜色不一的斑块状。两个剖面大量发育遗迹化石Thalassinoides(海生迹),Thalassinoides的三维潜穴系统为白云石化流体提供了良好的运移通道,为回流渗透作用提供了有利条件,促进潜穴充填物白云石化,从而形成了大量豹皮状的生物扰动白云岩。
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关键词:
- 寒武系 /
- 奥陶系 /
- 生物扰动 /
- 白云岩 /
- Thalassinoides(海生迹)
Abstract: A large number of bioturbated dolomites/bioturbated limestones were developed in the Cambrian and Ordovician in North China. Two representative stratal sections, Dengfeng (Cambrian) and Xingxian (Ordovician), were selected for study. Rocks are the cause of bioturbation. The trace fossils in the Cambrian series2 Zhushadong Formation in the Dengfeng Henan are relatively simple, mainly Thalassinoides along with a small amount of Planolites. Abundant fossil traces are present in submember 55 of the Middle Ordovician Majiagou Formation in the Xingxian section. Seven trace fossil ichnogenera were identified: Arenicolites, Cylindricum, Helminthopsis, Lorenzinia, Planolites, Teichichnus and Thalassinoides, of which Thalassinoides is dominant. The Lower Cambrian Zhushadong Formation from the Dengfeng section mainly occurs in the carbonate tidal flat and in the 55 submember of the Middle Ordovician Majiagou Formation from the Xingxian deposit in the carbonate platform. Field geological investigation and laboratory analysis by polarizing microscope and scanning electron microscope established the macro-and microstructure of the bioturbation in the surrounding rocks. Dolomite was predominant in the burrows, and calcite was the main material in the matrix. The burrows are seen in the outcrops as patches of different colors. The abundant three-dimensional burrowing system of Thalassinoides in both sections benefited the transport of dolomitization fluids and created favorable conditions for reflux osmosis, thus dolomitizing the burrow infill and forming a large number of bioturbated dolomite plaques.-
Key words:
- Cambrian /
- Ordovician /
- bioturbation /
- dolomite /
- Thalassinoides
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图 3 河南登封寒武系第二统朱砂洞组生物扰动白云岩野外特征
(a)垂直层面的Thalassinoides;(b)平行层面的Thalassinoides,Tb.T型分支,Yb.Y型分支,VS.垂直潜穴;(c)(d)多期次的垂直层面的Thalassinoides殖居
Figure 3. Bioturbated dolomite in the Cambrian epoch 2 Zhushadong Formation in the Dengfeng area
(a) Thalassinoides in cross⁃sectional view; (b) Thalassinoides on bedding surface (Tb:T⁃branch; Yb:Y⁃branch; VS:vertical burrow); (c)(d) Thalassinoides in the cross⁃section representing multi⁃layer colonizers
图 4 山西兴县中奥陶统马家沟组五5亚段生物扰动白云岩野外特征
(a)垂直层面的Thalassinoides;(b)垂直层面的Thalassinoides(发育裂隙);(c)平行层面的Thalassinoides潜穴内大颗粒充填;(d)平行层面的Thalassinoides,Tb. T型分支,Yb. Y型分支,VS.垂直潜穴;(e)平行层面的Thalassinoides,潜穴分支处连接成网状;(f)平行层面的,多级潜穴覆盖,T1.第1期潜穴,T2.第2期潜穴
Figure 4. Bioturbated dolomites in the submember 55 of the Middle Ordovician Majiagou Formation in the Xingxian area
(a) Thalassinoides in cross-sectional view; (b) Thalassinoides in cross-sectional view, with fracture; (c) Thalassinoides on bedding surface filled with large grains; (d) Thalassinoides on bedding surface (Tb:T-branch; Yb:Y-branch; VS:vertical burrow); (e) Thalassinoides network on bedding surface; (f) two generations of Thalassinoides distinguished by their cross-cutting relationships, indicating two different colonization surfaces (T1:early generations of Thalassinoides; T2:later generations of Thalassinoides)
图 5 登封寒武系第二统朱砂洞组生物扰动白云岩微观结构特征
(a)(b)(f)生物潜穴内充填为白云石,基质为微晶灰岩;(c)~(e)潜穴内部充填的白云石
Figure 5. Microstructural characteristics of bioturbated dolomites from the Cambrian epoch 2 ZhushadongFormation in the Dengfeng area
(a)(b)(f) matrix consisting of micrite and Thalassinoides filled with dolomite; (c)-(e) dolomite dominant inside burrow
图 6 兴县中奥陶统马家沟组生物扰动白云岩微观结构特征
(a)(b)潜穴围岩边界,边界处可见生物碎屑;(c)沿潜穴发育的微裂隙;(d)潜穴中发现一些方解石斑晶;(e)方解石脉切穿潜穴和围岩;(f)潜穴围岩边界,围岩中有破碎的方解石脉体
Figure 6. Microstructural characteristics of bioturbated dolomites from the Middle Ordovician MajiagouFormation in the Xingxian section
(a)(b) bioclastics in the boundary of Thalassinoides burrow; (c) a microfracture developed along the burrow; (d) some calcite phenocrysts inside Thalassinoides burrow; (e) calcite veins intersecting burrow and matrix; (f) fragmented calcite veins at boundary of Thalassinoides burrow
图 7 XX井中奥陶统马家沟组生物扰动白云岩微观结构特征
(a)~(c)潜穴围岩边界发育裂隙;(d)潜穴围岩边界发育缝合线
Figure 7. Microstructural characteristics of bioturbated dolomites in the Middle Ordovician Majiagou Formation in well XX
(a)-(c) Cracks developed in the boundary of Thalassinoides burrow; (d) stylolites developed along the boundary of Thalassinoides burrow
图 8 登封寒武系第二统朱砂洞组生物扰动白云岩超微结构特征
(a)电镜中识别的潜穴围岩边界;(b)为(a)中方框部分面扫描图像,蓝色为Mg元素;(c)围岩中的方解石晶体;(d)为(a)中方框部面扫描图像,其中绿色为O元素、蓝色为Mg元素、红色为Ca元素;(e)~(h)潜穴内部的白云石晶体
Figure 8. Ultramicro characteristics of bioturbated dolomites from the Cambrian epoch 2 Zhushadong Formation in the Dengfeng area
(a)SEM image of Thalassinoides burrow boundary; (b) mapping image of inset in fig. (a) with Mg element in blue; (c) calcite crystals in surrounding rock; (d) mapping image of inset in fig. (a) (green=O; blue=Mg; red=Ca); (e)-(h) dolomite crystals inside burrow
图 9 山西兴县中奥陶统马家沟组生物扰动白云岩超微结构特征
(a)电镜中的潜穴围岩边界;(b)为(a)中方框部分面扫描图像,蓝色为Mg元素;(c)为(a)中方框部分面扫描图像,其中绿色为O元素、蓝色为Mg元素、红色为Ca元素;(d)围岩中的方解石晶体;(e)~(h)潜穴内部白云石晶体
Figure 9. Ultramicro characteristics of bioturbated dolomites from the Middle Ordovician Majiagou Formationin the Xingxian section
(a) SEM image of Thalassinoides burrow boundary; (b) mapping image of inset in fig.(a), with Mg element in blue; (c) mapping image of inset in fig. (a) (green=O; blue=Mg; red=Ca); (d) calcite crystals in surrounding rock; (e)-(h) dolomite crystals inside burrow
图 10 生物扰动对白云岩回流渗透的促进作用(棕色部分为沉积物,蓝色部分为水体)
(a)造迹生物在沉积物中掘穴;(b)造迹生物的掘穴作用形成三维连通的潜穴系统,白云岩化流体沿潜穴迁移;(c)潜穴充填物发生白云岩化作用
Figure 10. Promoting effect of bioturbation on reflux osmosis of dolomite
(a) organism burrows in sediment; (b) three-dimensional burrow system established; dolomitized fluids migrate along the burrows; (c) dolomitization in burrows
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