Depositional Fabric in Permian Reef Limestones

Abstract

Reef limestone in which primary textures have not been markedly altered by diagenetic changes is characterized by a distinctive fabric of both inorganic and organic constituents, and the more salient elements of the fabric are not primarily dependent on the particular kinds of organisms that make up reefs. The complex of reef-forming organisms has evolved markedly during geologic time, first one, then another group coming into prominence. For example, scleractinian corals and coralline algae, principal reef-formers today, were not responsible for formation of the Paleozoic reefs. In spite of this qualitative diversity among reefs of different geologic ages, all reefs are characterized by a common growth mode which is reflected in the fabric. Reference to the work on the Funafuti reef rock by Cullis (1904), the most complete study of its kind published to date, shows that there are many parallels betwen certain modern and Permian reef limestones. After formation of a reef frame by firmly attached calcareous organisms, relatively large primary voids are incrusted and even filled by deposits of fibrous calcium carbonate precipitated from sea water. Similar deposits form the cementing agent of beachrock. Probably these deposits are of great importance in consolidation of the reef rock. After burial, but before the voids are sealed, many of the incrusted cavities become filled with detritus. The sequential relationships of these late interstitial deposits demonstrate conclusively that the fibrous deposits are precipitated from freely circulating sea water. It appears, then, that the early history of reef limestone is marked by progressive reduction and loss of primary porosity. Secondary porosity is produced by selective leaching of aragonite and calcite constituents under gradually increasing pressures. It is this secondary porosity which is of interest in connection with petroleum reservoirs.