Mesogenesis

Introduction

Mesogenesis (mesodiagenesis) is the deep-burial stage of siliciclastic diagenesis. It differs from eogenesis in one fundamental respect: the conditions governing it are no longer set by the depositional environment but by progressive burial. As sediments are buried to greater depths, temperature rises, geostatic and hydrostatic pressures increase, and the composition of pore waters changes dramatically. These changes collectively drive the most drastic transformations of siliciclastic sediments - the ones that determine whether a sandstone retains useful porosity or is sealed by cement, and whether its original mineral assemblage survives or is replaced.

A 10°C rise in temperature during burial can cause chemical reaction rates to double or triple. Mineral phases stable in the depositional environment may become unstable at depth. Higher temperatures favour the formation of denser, less hydrous minerals and increase the solubility of most common minerals except carbonate minerals. Silicate minerals therefore show an increasing tendency to dissolve at greater burial depths, whereas carbonate minerals such as calcite are more likely to precipitate. A decrease in pH (increase in acidity) of pore waters with depth - driven by CO₂ released by organic matter decomposition - can, however, drive carbonate dissolution. ^[1]

The most important chemical and mineralogical diagenetic processes during deep burial are cementation, dissolution (intrastratal solution), mineral replacement, and clay-mineral authigenesis. Each is covered in detail on dedicated pages. ^[1]

Physical Compaction

The load pressures imposed by deep burial bring about a significant increase in grain packing with concomitant loss of porosity and thinning of beds. Physical compaction causes bending of flexible grains such as micas and squeezing of soft grains such as rock fragments. Mechanical compaction and pressure solution together cause porosity loss in quartzose sandstones mainly at burial depths less than about 2 km, because the combined effects of compaction, pressure solution, and a small amount of quartz cement produce stable grain-packing arrangements. Porosity loss at greater depths is primarily the result of quartz cementation. Some porosity loss from compaction can, however, continue to depths of at least 5 km. ^[1]

Summary of Mesogenetic Processes

The principal diagenetic processes active during burial, their effects, and their diagenetic stage are summarised in Table 8 of the source. ^[1]

| Process | Effect | |---|---| | Physical compaction | Tighter grain packing; porosity reduction; bed thinning | ^[1] | | Pressure solution | Partial dissolution at grain contacts; further porosity reduction | ^[1] | | Cementation | Precipitation of carbonate and silica cements; porosity reduction | ^[1] | | Dissolution | Removal of carbonate cements and unstable framework grains; secondary porosity creation | ^[1] | | Mineral replacement | Partial or complete substitution of one mineral by another at constant volume | ^[1] | | Clay mineral authigenesis | Alteration of smectite to illite or chlorite, and kaolinite to illite | ^[1] |

See the individual pages for Pressure Solution, Cementation, Intrastratal Solution, Mineral Replacement, and Clay Mineral Diagenesis for full treatment of each process.

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