Discussion and Supplement of Limestone Classification Schemes

In the more than 120 years since Amadeus William Grabau (1870-1964) first proposed the classification of carbonate rocks in 1904, geologists have conducted in-depth research on this issue and made multiple modifications, forming the currently relatively comprehensive and systematic limestone classification scheme. Among them, Folk (1959, 1962) and Dunham (1962) classified limestones based on the perspective of structural maturity, which has become the most widely used classification scheme to this day. It is worth noting that Mei Mingxiang supplemented Wright VP’s (1992) classification scheme in 2001 by adding types of limestones formed by event sedimentation that cannot be explained by the perspective of structural maturity, such as seismolithic limestone, turbidite limestone, etc.However, there are also problems with previous classification schemes. First, limestone types similar in structure to clastic rocks formed by terrigenous carbonate particles under abnormal depositional processes are not included in the carbonate rock classification system; Second, travertine precipitated carbonate rocks are not included in the carbonate rock classification system. Based on studies of carbonate rudite in the Bengbu depression, conglomerate deposits of the Guanzhuang Formation in the Pingyi Basin, and travertine, as well as years of field work experience, the author has further modified the limestone classification system by adding a terrigenous limestone class formed under abnormal depositional conditions and a purely chemical limestone class.In view of the well-developed characteristics of terrestrial carbonate rocks in China, a terrigenous clastic limestone subclass was added under the abnormal deposition subclass, and a purely chemical limestone class with mud crystal components was added under the diagenetic class. The following changes have been made to the cause-structure classification table of limestones in this study: (1) The classification method of dividing limestones into depositional, biological and diagenetic classes remains unchanged, but the skeletal components are used as the basis for further subdivision. Compared with the previous scheme, more terrigenous clastic and mud crystal classes have been added to the skeletal components. (2) For limestones formed by terrigenous carbonate particles under abnormal depositional conditions, they are subdivided into rudite limestone, arenite limestone, mudstone limestone and their transitional types according to the particle size of the skeletal grains. (3) In the diagenetic class, the added purely chemical limestone refers specifically to travertine or travertine cemented collapse limestone (cave limestone), with mud crystal as its skeletal component.