Pressure Solution

Introduction

Pressure solution - also called chemical compaction - is a mesogenetic diagenetic process in which increased pressure at grain contact points elevates the local solubility of the mineral, driving partial dissolution of the grains at those contacts. The dissolved material is removed in pore fluids, reducing both porosity and bed thickness. Pressure solution is therefore one of the two major compactional mechanisms acting during deep burial, the other being physical (mechanical) compaction.

Mechanism

Increased pressure at the contact point between grains raises the solubility of the grains there, leading to their partial dissolution. This process is referred to as pressure solution or chemical compaction. Chemical compaction further reduces porosity and increases bed thinning beyond what physical compaction alone achieves. Pressure solution also releases silica into pore waters - and this silica is an important source for quartz overgrowth cementation elsewhere in the rock. ^[1]

Increased pressure during deep burial causes an increase in solubility of minerals at point contacts, resulting in partial dissolution of the minerals. This process releases silica into pore waters and is an important mechanism for furnishing silica that can later precipitate as new silicate minerals. ^[1]

Textural Expression

Pressure solution is identifiable in thin section by the grain contact types it produces. Simple tangential contacts indicate minimal compaction. Long contacts result from grain flattening. Concavo-convex contacts form where one grain is pressed into another. Sutured contacts - highly irregular, interlocked grain boundaries - represent the most advanced stage of pressure solution and are the diagnostic indicator of chemical compaction (as distinct from purely physical compaction, which produces long and concavo-convex contacts). Compaction also causes bending of flexible grains such as micas and squeezing of soft grains such as rock fragments. ^[1]

Depth Relationships

Mechanical compaction and pressure solution together cause porosity loss in quartzose sandstones mainly at burial depths less than about 2 km. The combined effects of compaction, pressure solution, and a small amount of quartz cement produce stable grain-packing arrangements at those depths. Porosity loss beyond 2 km is primarily the result of quartz cementation rather than continued compaction. Some compactional porosity loss can, however, continue to depths of at least 5 km. ^[1]

In quartzose sandstones, porosity is reduced from approximately 50 percent at the surface to virtually zero at a burial depth of about 5000 m, through the combined action of mechanical compaction, intergranular pressure solution, and cementation. ^[1]

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