Process Approach to Diagenesis of Reefs and Reef-Associated Limestone: ABSTRACT

Abstract

Geologic reasoning commonly is based on analogy rather than on process. In facies studies, geology by analogy generally works. Spatial distribution of sediment types is usually in accordance with a few general rules and is therefore repetitive in the geologic record. However, when one approaches problems, geology by analogy runs amuck. The modification of a carbonate sediment is a composite of numerous biological, physical, and chemical processes separated in time. Each limestone unit results from a combination of several or all of these processes operating in varying degrees and in varying sequence. It is therefore proposed that a more fruitful approach to problems of carbonate diagenesis is to identify the processes that produce important modification in the Holocene and Pleistocene where processes can be studied first hand, and in ancient rocks where processes can be strongly inferred. Then, systematically review this list of processes with regard to the rock units in question to discover which processes are potentially important in these rocks and which processes can likely e discounted for reasons of sedimentary facies, paleoclimatology, paleogeography, or sea level history. One process approach to problems in ancient rocks, is to study those processes which appear to produce significant modification in Holocene and Pleistocene materials. Marine cementation is becoming well documented as an important void-filling process in certain environments. Certain generalities concerning the marine environment may enable us to predict the location and importance of submarine cements. Although seawater commonly is saturated with respect to calcium carbonate, the amount of calcium carbonate available from any 1 batch of pore water is small. If cement is to grow in the pore space of the submarine sediment, water must either be pumped through the pore space or calcium and carbonate ions must diffuse into the pore space. These requirements for a pump or a diffusion mechanism may grossly limit the environments in which we shall expect to find submarine cementation to be an important process. The vadose environment (subaerial and above the water table) is the site of important solution and precipitation processes in Pleistocene rocks. The stabilization of aragonite and high-magnesium calcite to low-magnesium calcite provides a basic driving mechanism for both precipitation phenomena and selective solution. Availability and flow of water, combined with shape and mineralogy of sedimentary particles allow for a wide variety of fabrics to be formed within the environment. Further, carbonate equilibrium in this environment is a complicated composite of equilibrium between the rock and the water, the water and local PCO2, and the local PCO2 and a larger CO2 reservoir. Finally, seasonal variation allows solution and precipita ion phenomena to be superimposed although the sediment remains in essentially the same environment. The freshwater phreatic environment (pore space completely occupied by fresh water) has several unique features primarily related to the fact that mineralogic stabilization occurs more rapidly in this environment than in the associated vadose environment. Because of the differences in solubility of the 2 mineral phases, massive precipitation commonly occurs when water from an aragonitic vadose environment enters a calcite phreatic environment. Solution processes may operate in close proximity to the phreatic environment attendant to CO2 evolution as phreatic precipitation occurs. Caution should be exercised in ascribing observed modification to ill-defined late diagenetic processes where there are so many well-defined early processes from which to choose. End_of_Article - Last_Page 859------------