In the writer’s previous classification of limestones, rocks were divided into three major families. A more sensitive division can be made into eight groups forming a complete spectrum of textural types, representing deposition in environments of different physical energy. Basis for the classification is (1) relative proportion of allochems and carbonate mud, (2) sorting of allochems, and (3) rounding of allo-chems. A complete parallel exists between these limestone types and the sequence of textural maturity in sandstones, even to the existence of textural inversions. However, rounding appears to be accomplished best in environments where the energy level is too great for good sorting.
Figures & Tables
Classification of Carbonate Rocks—A Symposium
Limestones and dolomites form the economically important and exceedingly complex family of carbonate rocks. They are set distinctly apart from related rock families by their intrabasinal and highly local origin, their genetic dependence upon organic activity, and their extreme susceptibility to post- depositional modification.
The successful classification of carbonate rocks requires detailed knowledge of their multiple com-ponents and genetic processes. Such knowledge has been greatly increased during a period of accelerated investigations since 1940, with the result that the modern classifications are marked improvements over their predecessors.
Most of the newer classifications utilize a practical blending of descriptive and genetic parameters. The parameters most commonly used are depositional fabric, particularly the relative abundance of coarser carbonate particles (grains) as compared with the finer grained particles (matrix or micrite); the size and genetic types of the grains or of in-place biotic constituents; the mineralogy; and the nature and degree of post-depositional modification. Secondary parameters include porosity, cementation, the degree of abrasion or rounding of the grains, admixtures of noncarbonate material, and a host of others.
The symposium classifications of carbonate rocks and two allied articles of this volume are briefly reviewed and compared. Despite the differences in approach, purpose, and experience among the various authors, the resulting classifications show strong similarities and therefore indicate that a basis of mutual