Siliciclastic Diagenesis

Introduction

Siliciclastic sedimentary rocks begin as loose, uncemented accumulations of gravel, sand, or mud. Through a suite of physical and chemical changes collectively called diagenesis, these unconsolidated deposits are progressively transformed into hard sedimentary rock. Diagenesis begins at the sediment surface and continues through deep burial - and in some cases through uplift and erosion. Understanding diagenesis is essential not only for reconstructing the provenance and transport history of sedimentary rocks, but also for evaluating their economic significance as petroleum and groundwater reservoirs.

Newly deposited siliciclastic sediments are characterised by loosely packed, uncemented fabrics, high porosities, and high interstitial water contents. As sedimentation continues in subsiding basins, older sediments are buried beneath younger ones to depths that may reach tens of kilometres. This burial is accompanied by increasing pressure from the weight of overlying sediment, rising temperature, and changes in pore-water composition - changes that together drive compaction and lithification. Unconsolidated gravel is eventually lithified to conglomerate, sand to sandstone, and siliciclastic mud to mudstone (shale). ^[1]

The lithification process involves physical, mineralogical, and chemical changes. Loose grain packing gives way with burial to more tightly packed fabrics with greatly reduced porosity. Porosity may be further reduced by cement precipitation into pore spaces. Minerals stable at low surface temperatures become altered at higher burial temperatures and changed pore-water compositions, and may be dissolved, partially replaced, or completely replaced by new minerals. ^[1]

Diagenesis also has economic significance because it can adversely affect the ability of siliciclastic rocks to store and transmit fluids - a subject of considerable importance to petroleum and groundwater geologists. ^[1]

Temperature and Pressure Conditions

Diagenesis takes place at temperatures and pressures above those of the weathering environment but below those that produce metamorphism. There is no sharp boundary between diagenesis and metamorphism, but diagenesis is commonly considered to occur at temperatures below about 250°C. Diagenesis can begin almost immediately after deposition - while sediment is still on the ocean floor - and may continue through deep burial and eventual uplift. ^[1]

With increasing burial depth, geostatic (rock) pressure, hydrostatic (fluid) pressure, and temperature all rise. Pore-water salinity commonly increases with burial depth, and pore-water chemistry changes in ways that vary from basin to basin. Important mineral-forming ions - including Si⁴⁺, Al³⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, and HCO₃^- (bicarbonate) - often increase in abundance with depth, concomitant with the salinity increase. ^[1]

The Three Stages of Diagenesis

Most workers recognise three stages of diagenesis. The terminology of Choquette and Pray (1970) remains widely cited; simplified equivalents are also in common use. ^[1]

| Stage (Choquette & Pray) | Simplified name | Setting | |---|---|---| | Eodiagenesis | Eogenesis | Very shallow burial (few m to tens of m); near-depositional conditions | ^[1] | | Mesodiagenesis | Mesogenesis | Deep burial; rising T and P, changed pore-water chemistry | ^[1] | | Telodiagenesis | Telogenesis | Uplift; meteoric water incursion, oxidising and low-salinity conditions | ^[1] |

Some authors now use eogenesis, mesogenesis, and telogenesis exclusively as the simplified equivalents of the three original terms. ^[1]

Sedimentary rocks that remain deeply buried in depositional basins have not yet undergone telodiagenesis. ^[1]

Key Diagenetic Processes (Summary)

The most important diagenetic processes, summarised by stage, are:

• Eogenesis: Bioturbation destroying primary sedimentary structures; precipitation of pyrite (reducing environments) or iron oxides (oxidising environments); early cementation by quartz, feldspar, carbonate, kaolinite, or chlorite. ^[1]
• Mesogenesis: Physical compaction; pressure solution; cementation by carbonate and silica; dissolution of framework grains and cements creating secondary porosity; mineral replacement; clay mineral authigenesis. ^[1]
• Telogenesis: Dissolution or alteration of framework grains and previously formed cements by meteoric waters; oxidation of iron carbonate minerals to iron oxides; oxidation of pyrite to gypsum if calcium is present; alteration of feldspars to clay minerals. ^[1]

See the dedicated pages - Eogenesis, Mesogenesis, Pressure Solution, Cementation, Intrastratal Solution, Mineral Replacement, Clay Mineral Diagenesis, and Telogenesis - for detailed treatment of each process.

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