| Cone-in-cone structures are among the most spectacular and enigmatic geological features. The origin and mechanism of their formation have been debated for more than two centuries and are still essentially unresolved. The cone-in-cone structure consists of coaxially nested right circular cones forming more or less densely packed sets of columns. This structure is found in practically all regions of the globe in different sedimentary rocks ranging from the Riphean to the Cretaceous. The most frequently described cone-in-cone structures are those built of calcite. Calcite cone-in-cone structures have been observed in the lacustrine fine grained sediments of several sedimentary basins in China, such as Tuha Basin, Qaidam Basin and Ordos Basin, Qinshui Basin, but little research of cone-in-cone structures has been done. To focus on the northeastern Ordos Basin and the western Celaomiao Depression as the study areas, studied by the methods such as outcrops measure, polished thin section observation, XRD analysis, cathode-ray luminescence, electronic scanning, studied with the internal structure and distribution of the cone-in-cone concretions, have discussed the relationship between the development scale and the morphological characteristics of cone-in-cone and the synsedimentary environment, have speculated the formation mechanism of cone-in-cone structure, and have demonstrated the important indication significance of cone-in-cone structure in sedimentary environment.In the study area of the northeastern Ordos Basin, cone-in-cone structure develops in Jurassic Yan’an group and Zhiluo group, which develops in the coal-bearing formation of Yan’an group is especially stable of volume, horizon, morphology and well preserved. In the Jurassic Yan’an group, the host rock of cone-in-cone structure is mainly calcareous concretions and thin calcareous layers in mudstone and sandy mudstone, usually develops discontinuously in a stable horizon. Cone-in-cone concretions of varying sizes appear discontinuously in the same layer of dark mudstone the second small sequence of the second sedimentary unit of Yan’an group, Kaokaowusu, Shenmu area. The thickness of cone-in-cone layers ranging from 1 cm to 0.4 m, their maximum size correlates with the total thickness of the host interlayer. The cone-in-cone concretions are lenticular and honeycomb after weathering, large volume, have clear boundaries with the surrounding mudstone.The largest cone-in-cone concretion is 3m in length,1.5m in width,0.9m in thickness; The section diameter of the smallest concretion is about 0.4m,0.25m in thickness. Internal cone-in-cone concretion presents obvious heterogeneity, the cone-in-cone layer consists of coaxially nested right circular cones forming more or less densely packed sets of columns, the cones may be individual or combined into separate small groups of sevearal cones contacting with each other. The height of cone is about 5cm, the bottom has irregular circle shape section like tubaeform or convex round,2-3cm in diameter. The apex angle of cone changes between 30° to 60°, coaxially nested right circular cones have good inheritance. There are frame shaped annular ridges and annular depressions on the medial aspect of the cones, longitudinal ladder shaped. The annular ridges and annular depressions are most significant near the bottom of the cones and become very thin and not clear near the top. In addition there are vertical parallel lines on the outside of the cones, parallel to the axis of the cones. Cone-in-cone structure not only develops on the surface of the calcareous concretions, but also on the surface of the thin calcareous layers, the axis of cones is perpendicular to the rock layer, the apex angle downwards. The scale of cone-in-cone structure develops in the lacustrine fine-grained sediments of Zhiluo group is smaller than Yan’an group, shape is irregular, but it is found that cone-in-cone structure develops on the surface of Calcified wood. In the study area of Celaomiao Depression, calcareous cone-in-cone concretions have different size, the largest concretion is 275 cm in length,156 cm in width,126cm in height; The smallest concretion is less than 0.5m in diameter. Cone-in-cone structure develops in the outer parts of concretions, radiated, the vertex of cones point to the core of concretions. The apex angles of cones change between 20°-40°. The bottom has irregular circle shape section like tubaeform or convex round, there are frame shaped annular ridges and annular depressions on the medial aspect of the cones, vertical parallel lines on the outside of the cones. It can be observed on the section of the cone-in-cone concretions that quite a few angular granite fragments have been wrapped in concretions. Compared with the cone-in-cone structure of northeastern Ordos Basin, the thickness of cone-in-cone structure of western Celaomiao Depression is smaller, calcite content is relatively lower, clastic particles content is relatively higher, the internal structure is relatively clutter. Cone-in-cone structure is also found in shore shallow lacustrine fine grained sediments of Sangonghe group in Yili Basin, its macromorphology is similar to the cone-in-cone structure of northeastern Ordos Basin. The host rock of cone-in-cone structure is usually calcareous concretions and thin calcareous layers in mudstone, sandy mudstone, usually develops discontinuously in a stable horizon, and has clear boundaries with the surrounding rock.Internal cone-in-cone concretion presents obvious heterogeneity. In the study area of northeastern Ordos Basin, cone-in-cone concretion A can be divided into the three layers as upper, middle and lower, the middle layer is massive limestone which appears extremely rules in the microscopic morphology, lightgray fibrous calcite and black clay fillings distribute disorderly; The microscopic morphology of cone-in-cone layer has obvious regularity. Internal cone-in-cone concretion presents obvious heterogeneity on morphological characteristics, but ont on the mineral composition and component content. Cone-in-cone concretion is mostly composed of calcite, argillaceous fillings are mainly chlorite and illite, mixed with mineral particles such as quartz and feldspar. There are obvious "V" or "W" shaped laminas on the longitudinal section of cone-in-cone layer, The calcite crystals directionally arranged, about 50 u m in diameter, 0.5～1mm long. Black material is clay fillings, mainly chlorite and illite, which can be observed under the scanning electron microscopy that the plate shaped chlorite clusters together. At the edges of the cone, orientation of calcite crystals oblique or nearly same with the conical surface. On the transverse section, it is seen that the unequal-sized concentric circles structure is spumescent. Internal argillaceous fillings not only form the dark laminas, but also form relatively bulky, visible to the naked eyes argillaceous fillings, serrated edge is very regular. The size of argillaceous fillings varies greatly and usually with traces of rupture, which contains mineral particles such as quartz and feldspar. In the study area of western Celaomiao Depression, from macromorphology view, cone-in-cone concretion C1 can be divided into four periods. From the center to the outside, the period Ⅰ is brown micrite, which is divided into two parts, the structure is uniform density, see brown yellow spots, calcite content is relatively higher which is around 60%. The period Ⅱ-Ⅳ are cone-in-cone limestone, calcite content is relatively lower than the period I, is about 50%. The period Ⅱ is maroon limestone, the structure is more compact, cone-in-cone structure is invisible to the naked eyes, microscopically fibrous calcite has obvious directionality, see "V" shaped laminas, some debris such as quartz, feldspar and lithic, was wrapped in the laminated material; The period Ⅲ is similar to period Ⅱ, maroon limestone, structure is relatively looser than period Ⅱ, cone-in-cone structure is invisible to the naked eyes, microscopically fibrous calcite is inhomogeneity, feather collection and "V" shaped laminas appear alternately. The period Ⅳ are maroon cone-in-cone limestone, structure is quite loose, cone-in-cone structure is visible to the naked eyes, microscopically jagged crack evenly spaced, fibrous calcite has excellent directional property. The samples of each period of the concretion is uniform in cathodoluminescence microscope besides a small amount of quartz, feldspar and cuttings particles, without diagenetic ribbon because of the precipitation and differentiation of trace elements. It can be speculated that the formation of cone-in-cone concretion is gradually from inside to outside, in stages, and the formation of cone-in-cone concretion should be later than the formation of granite weathering crust.The development of Cone-in-cone structure was closely related to synsedimentary environment, and the depth of water had very obvious influence on the scale of its development. The primary sedimentary environment of cone-in-cone structure should be shore shallow lake environment and inner distributary bay, abandoned distributary channel, delta front environment connected to lake, with a depth of water for about 5-30 m. Under the condition of same water depth and salinity, the relatively closed and closer to terrestrial environment such as inner distributary bay was more favorable to the development of cone-in-cone structure than relatively open environment such as shore shallow lake environment. In addition, in the synsedimentary stage, the turmoil of water, the increase of debris particles in water would have certain inhibition on the development of cone-in-cone structure, relatively quiet, clean water in the synsedimentary stage might be more beneficial to the growth of cone-in-cone concretions. Thus it can be seen that calcareous cone-in-cone structure indicates a kind of synsedimentary environment covered by shallow water and the water body was relatively low energy and clean.Base on the pre-researches and the authors’ own study, this paper puts forward the formation mechanism of cone-in-cone structure interpretation. It is considered that the formation of cone-in-cone structure took place in the synsedimentary-early diagenesis stage and can be divided into three stages of development evolution. |
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