鄂尔多斯盆地马五_5亚段白云岩化成因模式

Genetic Model for M5-5 Sub-Member Dolomitization of Majiegou Formation in Ordos Basin

用地球化学法分析探讨了鄂尔多斯盆地马五5亚段的成岩机制。兴县关家崖剖面马五5亚段岩性自上而下主要由粉晶白云岩、灰斑白云岩及白云斑灰岩构成,属一完整的沉积旋回,白云石的含量、粒径、有序度自上而下逐渐减小。粉晶白云岩的碳氧同位素明显偏负,白云斑以及灰岩基质的δ18O偏负,但δ13C与奥陶纪海水接近,87Sr/86Sr值明显升高。研究表明,马五5亚段的原始沉积序列为泥晶灰岩、含粒及泥粒灰岩、生物扰动灰岩、砂屑灰岩,最后到泥晶白云岩,随着海平面的降低,蒸发浓缩海水依次从潮上、潮下带,最后到浅埋藏环境进行各类白云岩化作用,粉晶白云岩和白云斑灰岩成因为浓缩海水的回流渗透作用。

This paper analyzes and discussed the diagenetic mechanism for M5-5 sub-member of Majiagou formation in Ordos basin by geo-chemistry approaches. The M5-5 sub-member in Guanjiaya section is composed of powder crystal dolomite,calcareous dolomite and dolo-mitic limestone,belonging to a complete sedimentary cycle where the content,particle size and degree of order are gradually reduced fromtop to bottom. The power crystal dolomite＇s carbon-oxygen isotope ratio is obviously negative,the δ18O of calcareous and limestone matrix ispartially negative,but their δ13C values are close to the Ordovician water,and the87Sr/86 Sr ratio is significantly increased. The results showthat the original sedimentary sequence of this sub-member is characterized by micrite,packstone,disturbed biomicrite,calcarenite,and do-lomicrite. With the loss of thee sea level,evaporated and concentrated seawater started various dolomitizations in turn from supratidal zoneto subtidal zone until shallow burial environment. The powder crystal dolomite and dolomitic limestone originate in the reflux percolation ofconcentrated seawater.