Clay Mineral Diagenesis
Introduction
The relative proportions of clay mineral groups change systematically with increasing burial temperature and age. These diagenetic processes are believed to account for the trend of changing clay-mineral relative abundance with age. [1] Clay mineral diagenesis is also important as a geothermometer: the specific transformation that has occurred constrains the burial temperature range a rock has experienced.
Key Transformation Reactions
In addition to mineral replacement of silicate framework grains, one clay mineral can alter to another during diagenesis. The principal transformation reactions and their temperature ranges are: [1]
- Smectite → illite: Occurs at temperatures ranging from about 55-200°C, with concomitant release of water. This process is particularly common in shales and is referred to as shale dewatering. [1]
- Smectite → chlorite: Occurs within approximately the same temperature range as the smectite-to-illite transformation. [1]
- Kaolinite → illite: Typically occurs at temperatures between about 120-150°C. [1]
Shale Dewatering
The smectite-to-illite transformation involves concomitant release of water. This process is particularly common in shales and is referred to as shale dewatering. [1]
Age Trend in Clay Mineral Assemblages
These diagenetic clay mineral transformations explain an otherwise puzzling observation: the relative proportions of clay mineral groups change systematically with burial temperature and rock age. In rocks older than the Mesozoic, the proportion of illite and chlorite increases at the expense of kaolinite and smectite. [1]
The pattern is not driven by changing source rocks or depositional environments across geological time. It is a burial effect: older rocks have, on average, been buried longer and to greater depths, and therefore further along the smectite-to-illite and kaolinite-to-illite transformation pathways. The clay mineral assemblage in a shale thus encodes its burial history.
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References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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