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Abstract

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.

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