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

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 pointing 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 cathodoluminescence 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.