中上扬子地台中寒武世古地理及白云岩成因

Paleogeography of Middle Cambrian and Origin of Dolomites in the Middle-Upper Yangtze Plate

通过中上扬子地区寒武系野外露头实测、踏勘、前人研究成果整理以及盆地腹地露头缺乏地区的钻井资料的分析,研究了中上扬子地区中寒武世的古地理背景。白云岩的成因、平面上以及垂向上的分布特征都受控于古地理。中寒武统整个台地区为局限台地,在台地发育咸化澙湖、潮坪、局限潮下、浅滩等次一级的古地理单元。中寒武统的膏岩与白云岩不同程度互层。向台地内部,滩相发育愈少,咸化澙湖、潮坪以及局限潮下越发育;越往台地的边缘,滩相越发育。研究区中寒武统白云岩按照晶体大小可以分为泥粉晶白云岩和砂糖状白云岩。泥粉晶白云岩为潮坪准同生白云岩,机理为蒸发泵作用,白云化流体来自于澙湖浓缩的海水。砂糖状白云岩绝大多数为回流渗透白云化成因,白云化流体主要来自于蒸发泵机理富余的高镁钙比流体。回流渗透白云化过程缓慢,白云石结晶较好。寒武系岩性在垂向上有很好的叠置关系,反映古地貌随地质时代的变迁。

Using data of measured and observed outcrop successions,drilling information and predecessors' research achievements,lithofacies palaeogeography and the origin of dolomites of Middle Cambrian in the Middle-upper Yangtze plate were studied. lithofacies paleographic map was plotted based on comprehensive study on outcrops,wells and a series of contour maps,such as thickness of Middle Cambrian,thickness percentage of micritic dolomites,thickness of gypsum rocks and so on. In Middle Cambrian of Middle-upper Yangtze plate,from ancient land to deep sea basin( northwest to southeast) developed shallow fine-grained clastic continental shelf,restricted carbonate platform,slope and deep-sea basin. As the ancient land was low and gentle in Middle Cambrian,the sediments were fine-grained, such as sand,silt and mud. Restricted carbonate platform could be divided into different secondary paleogeography units,such as tidal flats,lagoons,particle beaches and so on. The slope in the east of the study area was gentle and the sediments were mainly micrite carbonate sediments and mud,and sediments in deep sea basin were black chert and mud. Sediments in restricted carbonate platform were gypsum and microcrystalline carbonate sediments. Origin of dolomites,sequences in plane and vertical successions were controlled by paleogeography. Layers of gypsum were Interbedded with dolomites,and little particle beach developed in interior platform,while lagoon,tidal flat,restricted subtidal zone developed. At the margin of Middle-Upper Yangtze platform,more particle beaches developed and layers of gypsum decreased. According to crystal sizes of dolomites,micritic dolomites were separated from sugar shaped dolomites( fine-grained dolomites,medium dolomites and coarse-grained dolomites). Micritic dolomites were formed in penecomtemporaneous diagenetic stage in tidal flat environments,with mechanism of evaporative pumping. High salinity fluid with abundant magnesium that dolomitization needed came from concentrated seawater in lagoon and normal salinity water of open platform. Sugar shaped dolomites were formed with the mechanism of seepage reflection,the high salinity fluid of which came from redundant fluid of evaporative pumping and lagoon. Seepage reflection went on slowly and coarse-grained dolomites were formed. In addition,mechanisms of dolomitization combined in three-dimensional space,and vertical sequence changed regularly. From top to bottom,micritic dolomites with no gypsum crystals,micritic dolomites with gypsum crystals,sugar shaped dolomites formed successively,which was affected by paleogeographic changes.