Features of geological relics and genesis of the Chanjiayan Tiankeng group in Ningqiang, Shaanxi Province

The Tiankeng group in Hanzhong, Shaanxi Province is located in the northernmost of the humid tropics and subtropics between 32°N to 33°N, adjacent to the Hanzhong basin and the Qinling orogenic belt in the north and the central Sichuan uplift belt in the south. It is a unique and typical window karst type with complete karst system, good ecological environment and unique landscape. It is praised as "the great geographical discovery of the 21st century" by National Geography of China. This geologic feature has important scientific values for the comparative analysis of paleogeographic environments and climate change in north and south China and even in the world.The Tiankeng group of Chanjiayan in Ningqiang , Shaanxi Province is one of the four relatively independent karst systems in the Tiankeng group of Hanzhong, which lies in the northern margin of the Yangtze block. Bounded by the Dazhuba-Xinji fault (F1), in the north it belongs to the foreland thrust fold zone of Longmen mountains in Ningqiang; and in the south, it is part of the thrust syncline fold belt with nearly EW-trending Liping fault in the Hannan-Micangshan uplift area, which hosts various geological relics in complex geomorphology. Through a lot of field investigations, this paper sorts out the distribution characteristics of the geological relics and analyzes the basic characteristics and the causes of the main geological relics to reveal the formation and evolution mechanism of karst under the canyon landform in the north-south transitional zone. Results show that， (1) The geological relics of the Tiankeng group in the Chanjiayan area can be classified into 2 major groups, 4 types and 5 sub-types, which are characterized by Tiankeng, platforms, karst caves, canyons and secondary chemical sediments. (2)The karst landform is typical, in which Tiankeng and karst cave developed very well. (3) Affected by deep canyons, there are four surface karst platforms in the study area, all of which have relatively independent karst hydrogeological characteristics, and the Tiankeng and shaft have entered the degradation period. (4)Tiankeng was generated during the Neotectonics under the action of strata, lithology and structure.