测老庙坳陷早白垩世巴音戈壁组叠锥构造特征及其环境解释

Characteristics and Environmental Interpretation of Cone-in-Cone Structure,Early Cretaceous Bayingebi Formation,Celaomiao Depression

为了全面了解测老庙坳陷早白垩世叠锥构造的沉积特征,并探讨其成因及形成环境等问题,运用野外露头实测、光薄片观察、X射线衍射分析、阴极发光照相、扫描电镜照相等方法,发现叠锥构造发育于钙质结核外侧,覆盖所有方向,呈放射状,直圆锥锥顶指向结核中心;叠锥矿物成分主要为纤维状低镁方解石,纤维状方解石呈锥形套叠,具波状消光;阴极发光显微镜下呈均一的棕红色,不具微量元素分异和析出的成岩色带;叠锥结核的发育规模明显受制于原生沉积环境.结果表明叠锥结核的发育具有明显的阶段性;其发育的原生沉积环境应为水深在5-30m、水体能量较高的滨浅湖沉积环境.推测叠锥构造的形成可能是同沉积期生物沉积作用和胶结作用,以及后期上覆岩层静压力共同作用的结果。

To better understand the sedimentary characteristics of the cone-in-cone structure in the outcrop areas, and to explore the causes of the formation and the environmental issues of the cone-in-cone structure, we employ methods such as outcrop measure, polished thin section observation, X-ray diffraction analysis, cathode-ray luminescence, electronic scanning in this study. It is discovered that cone-in-cone structure develops radially around the calcareous concretions, with conical apex point- ing straight to the concretion center; the major mineral composition of cone-in-cone structure is fibrous low magnesium calcite, with fibrous calcite crystals being cone-shaped nest, with wavy extinction which appears uniform red-brown under cathodolumi- nescence microscopy without diagenetic ribbon with trace element fractionation and precipitation. The developing scale of cone- in-cone concretions is in the control of primary sedimentary environment obviously. It is considered that the growth of cone-in- cone concretions has distinct stages; the original sedimentary environment should be shallow lake sedimentary environment with about 5-30 m depth and relatively higher energy. It is concluded that the formation of cone-in-cone structure is a result of synsedimentary biological deposition, cementation and later overburden static pressure.