DISCUSSION ON THE FORMING FACTORS OF DOG-TOOTH CRYSTAL AND STONE CORAL IN FURONG CAVE, CHONGQING CITY, CHINA

摘要

The forming factors of dog-tooth crystal and stone coral are discussed for the first time on the basis of comparative analysis of microscopic characteristics, chemical features of pool water, elemental contents of speleothems, and field work. Such conclusions are drawn: dedolomitization, which is functioned by gypsum in the overlying stratum, is of great significance for the forming of rare cave landscapes. Dog-tooth crystal and stone coral is closely related with it and different degrees of dedolomitization lead to different landscapes. In a dog-tooth crystal pool, the crush belt promotes dedolomitization, providing enough Ca ions and removing quantities of Mg ions, which can satisfy calcite to deposit continuously. Consequently, full grown crystals form in the shape of long columns or fibers. In the coral pool, dedolomitization is weaker because there is no fault and the pool is farther away from the gypsum source. Less Mg ions are removed, not enough Ca ions are provided. Meanwhile some of the Ca ions are consumed with the deposition of calcite, so Mg/Ca increases during crystallization of calcite. When it reaches a certain value, crystallation is interrupted, thus, stone coral is shaped by short columns and accumulates multiple layers. In other places away from the gypsum, almost no dedolomitization occurs, Mg/Ca maintains high levels and no calcite crystals can form. Only aphanitic CaCO_3 (stalactite, stone column, stalagmite) forms. All these explanations above conform wonderfully with the geological feature, microscopic characteristics and geochemistry of water and rock, which is of great importance to understand the evolution dynamics of landscapes, to learn their outstanding universal value, and to protect landscapes.