Taihu crater: New evidence of impact metamorphism in sedimentary rocks, Taihu Lake, Jiangsu, China

doi:10.13745/j.esf.sf.2024.8.30

Abstract

Abstract:

This paper summarizes the results of fifteen years of research on the area surrounding Taihu Lake and its inner islands. Taihu Lake exhibits diagnostic features of an impact crater, including shatter cones, impact-metamorphosed breccias, and evidence of impact metamorphism in quartzite (PDFs and PFs) and impact melting. The ideal profile of an impact crater, comprising a vaporized melt zone, an impact metamorphic zone, impact-metamorphosed breccias, and a fracture zone, is fully present, confirming the existence of the Taihu impact crater. A schematic map of the distribution of impact metamorphism in Taihu has been compile based on these diagnostic features. For the first time, the Taihu impact crater, previously speculated from satellite images, has been confirmed in the Taihu Lake area, laying a solid foundation for further research. The distribution of impact metamorphism in Taihu can be divided into two concentrated zones and one ejecta distribution zone. These zones are located in the interior and edges of the lake area, primarily concentrated in the central and southern parts of Taihu Lake, indicating a relationship between the impact crater and the formation, evolution, and development of Taihu Lake. The Taihu impact crater, located in a fold belt at the margin of a continental platform with target rocks of sedimentary limestone and porous quartz sandstone, represents a new type of impact crater with many novel impact metamorphic phenomena and characteristics. The paper introduces the diversity, new features, and new types of shatter cones; reveals new characteristics of impact metamorphic PDFs in porous quartz sandstone; and describes new phenomena such as silica glass formed by intergranular shear friction-local melting and solidified melt inclusions. These findings enrich and advance the understanding of new characteristics and theories related to impact metamorphism in sedimentary rocks.

Key words: Taihu Lake, impact crater, shatter cones, impact metamorphism, impact melting, target rock

CLC Number:

P588.3

WANG Henian, CHEN Yang, SHENG Xuefeng, QIAN Handong. Taihu crater: New evidence of impact metamorphism in sedimentary rocks, Taihu Lake, Jiangsu, China[J]. Earth Science Frontiers, 2024, 31(6): 320-330.

Figures/Tables 13

Fig.1 Remote sensing image of Taihu Lake

Fig.2 Structural map of Taihu Lake dome. Adapted from [10].

Fig.3 Schematic diagram of the distribution of impact-metamorphism effects in Taihu Lake area

Fig.4 A small shatter cone in quartzite

Fig.5 Characteristics of shatter cones in Xishan. (a) A cm-size micro shatter cone was produced by clusters. (b) A medium-size shatter cone with conical striations radiating from an apex of cone, has a bifurcation feature. (c) A natural section of the Xishan shatter cone shows the internal structural features of the cone. Secondary litter cones and striations occur inside of the cone, with fracture and porosity development. (d) A new type of shatter cone in Xishan displays mesh structure on the surface of the cone.

Fig.6 Shatter cones in limestone target rocks with standard “horse-tail” striations features

Fig.7 Impact melting in limestone, residual clasts with impact induced calcite twinning

Fig.8 Shatter cones in target rock limestone appearing in groups. (a) Medium-small shatter cones with two different cone types.(b) Larger shatter cones with radiating bifurcated cones from top to bottom in the middle section.

Fig.9 Conical structures. A—fractured cone; B—conical rock block; C—fan-shaped fracture cone; D—impact metamorphic quartzitic breccia lava.

Fig.10 PDFs in quartz sandstone. Three groups of fine, dense PDFs can be seen in the monocrystalline quartz grains in quartz sandstone; 2—3 groups of PDFs also appear in another quartz crystal at the lower left. PDFs disappear at the regenerative edge at the junction with the upper grain. In the picture, many bright yellow molten glass can be seen between the grains.

Fig.11 PDFs in monocrystalline quartz grains. One to two groups of dense PDFs appear in the middle and upper parts of quartz crystal, with tiny black dots densely distributed along the PDFs.

Fig.12 Cross-shear microfissures shear quartz crystal into fine particles

Fig.13 Impact metamorphism in quartz sandstone. Bright yellow glass (silicate glass) and gas-solid mineral inclusions formed by shear friction melting are widely distributed in impact metamorphic quartz sandstone.

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