华北地台下中寒武统碳酸盐与陆源碎屑混合沉积──以山东张夏地区为例

Mixed siliciclastic-Carbonate Sediments during the Lower-Middle Cambrian in the North China Platform

华北地台下中寒武统馒头组—徐庄组为碳酸盐岩与陆源碎屑岩的互层沉积或同一岩层中碳酸盐组分与陆源碎屑组合混合沉积。通过对山东张夏地区地层沉积特点、地球化学特征以及沉积环境演化分析。表明混合沉积的主要控制因素是海平面变化。不同级次的海平面变化以及它们之间的相互叠加造成了类型多样的混合沉积。

Mixed siliciclastic-carbonate deposits from the Mandouian Formation to the Xuzhuangian Formation during the Early-Middle Cambrian in the North China Platform were interbeds of siliciclastic and carbonate rocks with high frenquency and mixed sediments with both siliciclastic and carbonate components. The former indicated interbeds of the mudstone and dolostone or the grainstone and silty shale, the latter formed the argillaceous dolostone, dolomitic mudstone, quartz calcarenite etc.The study on geochemistry of elements indicated the content of calcium gradually increased from the Mandouian to Xuzhuangian Formation with the increasing of limestone. But the content of magnesium decreased gradually with the decreasing of dolostone. Mg/Ca ratios were higher in Mandouianu Formation than in Maozhuangian and Xuzhuangian Formation.The content of trace element Sr was higher in limestone, 150-300×10-6 ;medium in argillaceous limestone, 100 ×10-6±;lower in argillaceous dolostone, mudstone and silty shale, <100 × 10-6. Sr/Ba>1 in limestone, <1 in most mudstone and argillaceous dolostone. Mn=4000-13000 ×10- in limestone,<2500 ×10-6 in dolostone, <1000 × 10-6 in mudstone and argillaceous dolostone. The contents of Ti, Cr, Ni, V and etc. were higher in mudstone and silty shale, medium in argillaceous dolostone and dolostone, lower in limestone.The stable carbon and oxygen isotopic data of carbonate rocks indicated that the variant scale of isotopic values was greater in the Mandouian and in the lower part of Maozhuangian Formation, δ13C: - 2. 59‰─+1.14‰ (PDB) and δ18O:-14. 89‰── 4. 71‰ (PDB); while they were smaller in the upper part of Maozhuangian and in the Xuzhuangian Formation, ─2.25‰──0. 37‰ (PDB),─ 10. 07‰── 7. 95‰ (PDB) respectively. It could be deduced that the sea water was relatively shallow during the Mandouian and the early Maozhuangianstage,thus minor scale sea level changes could cause the changes of salinity, reduction-oxdation conditions, temperature and circulation of sedimentary environment, till the late Maozhuangian and Xuzhuangian, the sea water became relatively deep and sedimentary environment was stable.It was considered that transgression expanded gradually from the Early to Middle Cambrian in the North China Platform. The terrigenous deposits and dolostone decreased but limestone deposits increased from the Mandouian to Xuzhuangian. During Mandouian the transgressive cycles were deposits of parasyngenetic dolostone and regressive cycles were mud deposits, from Maozhuangian to Xuzhuangian the transgressive cycles were grainstone and regressive cycles were silty shales. The third sea level changes showed a model of rapid transgression and slow regression.