Cementation (Diagenesis)

Introduction

Cementation refers to the precipitation of minerals into the pore space of sediment, reducing porosity and bringing about lithification. Carbonate and silica cements are most common, but clay minerals, feldspars, iron oxides, pyrite, anhydrite, zeolites, and many other minerals can also form as cements. ^[1]

Carbonate Cements

Calcite is the dominant carbonate cement. Aragonite, dolomite, siderite, and ankerite are less common. Carbonate cementation is favoured by increasing concentration of calcium carbonate in pore waters and increasing burial temperature. Precipitation is inhibited by elevated CO₂ partial pressure in pore waters, which can result from organic matter decomposition during burial - increased CO₂ makes pore waters acidic and corrosive to carbonate minerals. ^[1]

Cement distribution within a sandstone can be highly variable. It may be restricted to small areas, or it may fill most of the pore space. In some cases, cement concentrates around a nucleus - such as a fossil or fossil fragment - building up a globular mass called a concretion. In rare cases, calcite, barite, and gypsum can crystallise as large crystals that envelop numerous sand grains, forming so-called sand crystals. ^[1]

Silica Cements

Quartz precipitated as syntaxial overgrowths around existing detrital quartz grains is the most common type of silica cement. Quartz overgrowth cements are particularly abundant in many quartz arenites. Less commonly, silica precipitates as microcrystalline quartz (chert) cement or as opal. Quartz cementation is favoured by high concentrations of silica in pore waters and by low temperatures. ^[1]

Silica for quartz cementation is supplied locally by pressure solution at grain contacts or by dissolution of siliceous fossil skeletal material such as diatoms and radiolarians. Silica may also be imported from elsewhere in a basin during episodes of fluid flow related to deep-basin mineral dehydration or tectonic activity. Quartz cementation is particularly likely in basins where deep, hot pore waters rise along basin edges, cool, and deposit their dissolved silica load. ^[1]

Porosity Reduction with Depth

The cumulative effect of compaction and cementation in quartzose sandstones reduces porosity from approximately 50 percent at the surface to virtually zero at a burial depth of about 5000 m. Quartz cementation - not compaction - is the primary cause of porosity loss at depths greater than about 2 km. ^[1]

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