The zeolite facies was defined to “bridge the gap” between diagenesis and metamorphism and was largely based upon the studies of Coombs (1954) from Taringatura, New Zealand. The lack of zeolites in many stratigraphic sections which were presumably subjected to similar Ps-T conditions as zeolite-bearing rocks has led to the definition of a clay-carbonate facies. On theoretical grounds it can be shown that T and fluid composition exercise a strong control on the mineral assemblages and this has been verified for active geothermal areas such as Wairakei and Broadlands, New Zealand.
Estimates of Ps, T and fluid composition attending zeolite facies alteration can be made from correlation of mineral assemblages with those predicted from experimental and computed phase equilibria. Such estimates encounter several problems: (1) at low Ps and T growth and persistence of metastable phases are more important than at higher Ps, T; (2) for P<sub>H<sub>2</sub>O</sub> < Ps equilibria involving zeolite dehydration are very strongly affected; (3) experimental studies often yield phases which differ chemically and structurally from the naturally occurring zeolites; (4) variables such as the activity of silica (a<sub>SiO<sub>2</sub></sub>) may be important since volcanic glass is involved in the production of many zeolites; (5) in teconically disturbed areas Zen and Oxburgh and Turcotte have shown that Ps-T conditions at the base of thrust sheets may place these rocks outside the stability field of many of the zeolites for significant periods of time; (6) factors such as distribution of porosity and permeability may strongly affect the mineralogy and these are often difficult to evaluate after the fact.
For a given area there is reasonable correlation between zeolite assemblages, coal rank and clay mineral assemblages and consideration of all of these types of evidence will lead to the best estimates of Ps, T and fluid composition.
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.