It is increasingly apparent that temperature effects have been overemphasized in evaluating many diagenetic reactions associated with burial. The classic concept of burial metamorphism is far too simplistic to explain the wide variation in reactions and reaction sequences observed in many diagenetic terranes. This is particularly true when considering the complex problem of diagenetic alteration of volcanogenic sandstones. For example, changing the geothermal regime from area to area cannot explain the common observation that individual mineral ranges broadly overlap and are not related to stratigraphic position.
The diagenetic reactions of interest in volcanogenic sandstones such as: glass→clay, glass→zeolite, zeolite→zeolite, plagioclase→clay, or zeolite, among others, involve a fluid phase and commonly ionic species in the fluid phase. Consider for example the typical diagenetic reaction of heulandite→laumontite (Ca++ + Ca3K2 Al<sub>8</sub>Si<sub>28</sub> O72-24H20→Ca4 Al<sub>8</sub>Si16048- 16H20 + 2K+ + 12SiO2 + 8H20). Obviously the problem is not one of just thermal stability, but is one of chemical or ionic stability as well. Such factors as fluid flow and composition are as significant as depth of burial in controlling the distribution of diagenetic mineral phases in volcanogenic sandstones. Variations in fluid flow, and more importantly fluid composition, can explain many of the perplexing questions that previously were inadequately explained by thermal variations alone. Fluid effects are most pronounced in the early stages of diagenesis when the fluid/solid ratio is high, and in the later stages during fracturing and/or dewatering.
Figures & Tables
Aspects of Diagenesis
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.