The range of physical and chemical conditions included in diagenesis is 0-200° C, 1-2000 kg/cm2, and water salinities from fresh to brines twice as concentrated as the Dead Sea. Numerical values of these parameters vary not only with depth, but areally at any single depth. In addition, gross differences in heat flow in different tectonic settings can cause the average temperature at a given depth in a eugeocline to be double that in a miogeocline.
Despite the large variation in physical and chemical conditions during diagenesis, it is striking that calcite and quartz are the dominant chemical precipitates in pore spaces of sandstones. To a first approximation, this reflects two facts. (1) Most preserved sands were deposited in shallow marine environments and sea water is at least saturated with respect to calcium carbonate. (2) The average sand contains about 65% quartz and rates of flow of subsurface waters are so slow that quartz is the main buffer for the silica content of these waters.
The slow rate of movement of subsurface waters places important restrictions on when cementation of a sand can occur. To lithify a sand, the circulating water must be supersaturated with respect to the solid to be precipitated in the pore space and the number of pore volumes of water that must flow through the sand must be very large. If the sand is areally widespread, calculations for well-sorted pure quartz sand indicate that cementation of the unit by quartz by horizontal flow of subsurface water is impossible within geologically reasonable periods of time. Plugging of pore spaces by quartz must result from vertical circulation, probably when the depth of sand burial does not exceed a few hundred meters. This cementation can occur either soon after deposition of the sediment or at any subsequent time when tectonic forces elevate the sediment to a shallow depth.
Figures & Tables
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.