Abstract: Diagenesis of Volcanogenic Sandstones

Ronald C. Surdam, James R. Boles

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 in many diagenetic terranes, particularly when considering the complex problem of diagenetic alteration of volcanogenic sandstones. For example, different geothermal regimes cannot explain the fact that individual mineral ranges broadly overlap and are not related to stratigraphic position.

The diagenetic reactions of interest in volcanogenic sandstones such as glass^rarrclay, glass^rarrzeolite, zeolite^rarrzeolite, plagioclase^rarrclay involve a fluid phase and, commonly, ionic species in the fluid phase. If the typical diagenetic reaction of heulandite^rarrlaumontite is considered, it is obvious that 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 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.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado