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Abstract

By and large, reasoning in sedimentary geology is based on analogy rather than on process. In facies studies, geology by analogy usually works. Spatial distribution of sediment types is usually in accord with a few general rules and is therefore repetitive in the geologic record. However, in dealing with diagenetic problems, geology by analogy may have serious shortcomings.

Whereas the sedimentation of a stratigraphic interval happened only once, the diagenetic modification of those sediments is a composite of numerous processes separated in time. A limestone unit is therefore the product of sedimentation, diagenetic processes, and sequence of diagenetic processes. For this reason, it may prove more useful to attempt to understand the various diagenetic environments and processes in order to unravel the diagenetic problems of particular rock units. Toward this purpose, this paper reviews the basic chemistry of carbonate diagenesis and the diagenetic environments and processes which occur near the earth’s surface.

Marine cementation is becoming well documented as an important void-filling process in certain environments. Although sea water is commonly supersaturated with respect to calcium carbonate, the amount of calcium carbonate available from any one batch of pore water is small. If cement is to grow in the pore space of the submarine sediment, either water must 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 severely limit the environments in which submarine cementation is 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-magne- sium 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, allows for a wide variety of diagenetic fabrics to be formed within this diagenetic 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 local Pco2 and a larger CO2 reservoir. Finally, seasonal variation allows solution and precipitation phenomena to be superimposed one on the other while the sediment remains in essentially the same environment.

The fresh water phreatic environment (pore space completely occupied by fresh water) has several unique features primarily related to the fact that mineralogical stabilization occurs more rapidly in this environment than in the association vadose environment.

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