2.2 Diagenesis

Diagenesis plays a crucial role in the fossilization process, altering sediments and remains after burial. It encompasses physical and chemical changes that occur before metamorphism, including compaction, cementation, recrystallization, dissolution, and mineral replacement.

Understanding diagenesis is key to interpreting the fossil record accurately. It can both enhance and limit fossil preservation, creating biases that affect our understanding of past life and environments. Recognizing diagenetic indicators helps reconstruct the history of sedimentary rocks and fossils.

Physical and chemical processes of diagenesis

• Diagenesis encompasses the physical and chemical changes that sediments and fossils undergo after initial deposition but before metamorphism
• These processes occur in the upper few kilometers of the Earth's crust and can significantly alter the original characteristics of the sediments and fossils
• The main processes of diagenesis include compaction, cementation, recrystallization, dissolution, and replacement of minerals

Compaction of sediments

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• Compaction is the process by which sediments are compressed and lose porosity due to the weight of overlying sediments
• Leads to a reduction in the volume of the sediments and an increase in their density
• The degree of compaction depends on factors such as the composition of the sediments, the rate of burial, and the presence of fluids
• For example, mudstones and shales are more susceptible to compaction compared to sandstones due to their finer grain size and higher clay content

Cementation of sediments

• Cementation involves the precipitation of minerals from pore fluids, which binds the sediment grains together
• Common cementing minerals include calcite, quartz, and iron oxides
• Cementation can occur at various stages of diagenesis and can significantly reduce the porosity and permeability of the sediments
• The type and extent of cementation depend on factors such as the composition of the pore fluids, the pH and temperature conditions, and the availability of nucleation sites

Recrystallization of minerals

• Recrystallization is the process by which the original minerals in the sediments or fossils are transformed into new minerals with the same chemical composition but different crystal structures
• Can occur in response to changes in temperature, pressure, or pore fluid chemistry during diagenesis
• For example, aragonite shells of mollusks can recrystallize into calcite, which is a more stable form of calcium carbonate under diagenetic conditions

Dissolution of minerals

• Dissolution involves the removal of minerals from the sediments or fossils by the action of undersaturated pore fluids
• Can create secondary porosity in the rocks and can also lead to the destruction of fossils
• The susceptibility of minerals to dissolution depends on factors such as their solubility, the pH and composition of the pore fluids, and the presence of organic acids
• For example, carbonate minerals such as calcite and aragonite are more soluble than silicate minerals and are more prone to dissolution in acidic pore fluids

Replacement of minerals

• Replacement occurs when the original minerals in the sediments or fossils are dissolved and simultaneously replaced by new minerals with a different chemical composition
• The replacing minerals are typically more stable under the prevailing diagenetic conditions
• Common examples of replacement include the replacement of calcite by dolomite, the replacement of aragonite by calcite, and the replacement of silica by pyrite
• The extent and selectivity of replacement depend on factors such as the solubility of the original minerals, the availability of the replacing ions in the pore fluids, and the permeability of the sediments

Stages of diagenesis

• Diagenesis can be divided into three main stages based on the depth of burial and the dominant processes operating at each stage
• These stages are early diagenesis, burial diagenesis, and late diagenesis
• The transitions between these stages are gradational and depend on factors such as the geothermal gradient, the sedimentation rate, and the tectonic setting

Early diagenesis

• Early diagenesis occurs near the sediment-water interface and is dominated by biological and chemical processes
• Includes processes such as microbial degradation of organic matter, bioturbation, and precipitation of authigenic minerals
• The pore fluids during early diagenesis are typically in equilibrium with the overlying water column and are characterized by oxidizing conditions
• For example, the formation of pyrite framboids in marine sediments is a common product of early diagenesis and reflects the activity of sulfate-reducing bacteria

Burial diagenesis

• Burial diagenesis occurs at depths of a few hundred meters to a few kilometers and is dominated by physical and chemical processes
• Includes processes such as compaction, pressure solution, cementation, and thermal maturation of organic matter
• The pore fluids during burial diagenesis are typically reducing and are modified by reactions with the sediments and the dissolution of unstable minerals
• For example, the transformation of smectite to illite in shales is a common product of burial diagenesis and reflects the increase in temperature and potassium availability with depth

Late diagenesis

• Late diagenesis occurs at depths of several kilometers and is dominated by chemical processes that are controlled by the elevated temperatures and pressures
• Includes processes such as recrystallization, replacement, and thermal degradation of organic matter
• The pore fluids during late diagenesis are typically highly saline and are modified by the expulsion of fluids from compacting sediments and the interaction with deeply circulating basinal brines
• For example, the formation of saddle dolomite cement in limestones is a common product of late diagenesis and reflects the precipitation from hot, saline fluids at depth

Factors influencing diagenesis

• The nature and extent of diagenetic alterations in sediments and fossils are controlled by a range of physical, chemical, and biological factors
• These factors can operate at different spatial and temporal scales and can interact in complex ways to produce the observed diagenetic features
• Understanding the key factors influencing diagenesis is essential for interpreting the diagenetic history of rocks and for predicting the potential impact of diagenesis on reservoir properties

Composition of sediments

• The mineralogical and chemical composition of the sediments is a primary control on the type and extent of diagenetic alterations
• For example, carbonate sediments are more reactive than siliciclastic sediments and are more susceptible to dissolution and recrystallization during diagenesis
• The presence of organic matter in the sediments can also influence diagenesis by providing a source of acids and complexing agents that can enhance mineral dissolution and by creating reducing microenvironments that can favor the precipitation of certain authigenic minerals

Porosity and permeability of sediments

• The porosity and permeability of the sediments control the flow of pore fluids and the transport of solutes during diagenesis
• High porosity and permeability facilitate the exchange of ions between the sediments and the pore fluids and can promote the precipitation of cement and the dissolution of unstable minerals
• Low porosity and permeability can limit the extent of diagenetic alterations by restricting the access of pore fluids to the sediments and by creating chemical microenvironments that are out of equilibrium with the bulk pore fluids

Temperature and pressure conditions

• The temperature and pressure conditions during diagenesis are major controls on the rates and products of diagenetic reactions
• Increasing temperature accelerates the kinetics of chemical reactions and can trigger the recrystallization and replacement of minerals
• Increasing pressure can promote the dissolution of minerals by increasing the solubility of the solid phases and can also drive the compaction and dewatering of sediments
• The geothermal gradient and the burial history of the sediments determine the temperature and pressure conditions experienced during diagenesis

Geochemistry of pore fluids

• The composition and pH of the pore fluids are important controls on the solubility and stability of minerals during diagenesis
• Pore fluids can be derived from a variety of sources, including seawater, meteoric water, and basinal brines, and can evolve in composition due to reactions with the sediments and mixing with other fluids
• For example, the presence of organic acids in the pore fluids can enhance the dissolution of carbonate minerals, while the presence of magnesium can promote the replacement of calcite by dolomite

Time and burial history

• The duration and rate of burial are important controls on the extent and timing of diagenetic alterations
• Slow burial rates allow more time for early diagenetic processes to operate and can result in more extensive cementation and recrystallization of the sediments
• Rapid burial rates can lead to the preservation of metastable minerals and the generation of overpressures in the pore fluids
• The burial history of the sediments, including any episodes of uplift and erosion, can also influence the diagenetic evolution by changing the temperature and pressure conditions and by exposing the sediments to different pore fluid chemistries

Diagenetic environments

• Diagenetic environments refer to the physical, chemical, and biological settings in which diagenetic processes operate
• These environments are characterized by distinct combinations of factors such as the source and chemistry of the pore fluids, the temperature and pressure conditions, and the nature of the sediments and fossils
• The main types of diagenetic environments are marine, meteoric, and burial environments, each of which can be further subdivided based on specific conditions and processes

Marine diagenetic environments

• Marine diagenetic environments occur in settings where the sediments are deposited and altered under the influence of seawater
• Include shallow marine environments such as tidal flats, reefs, and carbonate platforms, as well as deep marine environments such as continental slopes and abyssal plains
• Marine pore fluids are typically characterized by high salinity, high pH, and oxidizing conditions near the sediment-water interface
• Common marine diagenetic processes include micritization of carbonate grains, seafloor cementation, and glauconite formation

Meteoric diagenetic environments

• Meteoric diagenetic environments occur in settings where the sediments are exposed to freshwater derived from precipitation or groundwater
• Include coastal and inland settings such as beaches, rivers, and karst terrains
• Meteoric pore fluids are typically characterized by low salinity, low pH, and oxidizing to reducing conditions depending on the degree of water-rock interaction
• Common meteoric diagenetic processes include dissolution and reprecipitation of carbonate minerals, formation of calcrete and silcrete, and kaolinite precipitation

Burial diagenetic environments

• Burial diagenetic environments occur in settings where the sediments are buried beneath younger sediments and are altered under the influence of compaction, heating, and fluid flow
• Can be further subdivided based on the depth of burial and the dominant diagenetic processes, such as shallow burial, deep burial, and hydrothermal environments
• Burial pore fluids are typically characterized by increasing salinity, decreasing pH, and reducing conditions with increasing depth
• Common burial diagenetic processes include compaction, pressure solution, thermal maturation of organic matter, and precipitation of burial cements such as quartz and dolomite

Diagenetic alterations of fossils

• Fossils can undergo a variety of diagenetic alterations that can modify their original composition, structure, and appearance
• These alterations can have important implications for the preservation and interpretation of fossils in the rock record
• The main types of diagenetic alterations of fossils include permineralization, replacement, dissolution, and distortion

Permineralization of fossils

• Permineralization is the process by which the pore spaces within a fossil are filled with mineral matter precipitated from pore fluids
• Can occur in both the hard and soft tissues of fossils and can preserve fine anatomical details
• Common permineralizing minerals include calcite, silica, and pyrite
• For example, the permineralization of wood by silica can preserve the cellular structure of the wood and create petrified wood

Replacement of fossils

• Replacement involves the dissolution of the original mineral components of a fossil and their simultaneous replacement by new minerals
• Can occur at various scales, from the molecular level to the entire fossil
• The replacing minerals are typically more stable under the prevailing diagenetic conditions and can include calcite, dolomite, silica, and iron oxides
• For example, the replacement of aragonite shells by calcite is a common diagenetic alteration in mollusks

Dissolution of fossils

• Dissolution is the process by which the mineral components of a fossil are removed by the action of undersaturated pore fluids
• Can lead to the partial or complete destruction of fossils and can create molds and casts in the surrounding sediments
• The susceptibility of fossils to dissolution depends on factors such as the mineralogy of the fossil, the pH and composition of the pore fluids, and the presence of organic acids
• For example, the dissolution of aragonitic fossils is more common in meteoric diagenetic environments due to the lower pH and calcium concentration of the pore fluids

Distortion and compression of fossils

• Fossils can be subjected to physical deformation during diagenesis due to the effects of compaction and tectonic stress
• Distortion involves the plastic deformation of fossils without significant volume loss and can result in the flattening, stretching, or bending of fossils
• Compression involves the volume reduction of fossils due to the collapse of internal cavities and the crushing of skeletal elements
• The degree of distortion and compression depends on factors such as the original morphology and mineralogy of the fossil, the grain size and composition of the surrounding sediments, and the magnitude and orientation of the stress field

Diagenesis and fossil preservation

• Diagenesis plays a crucial role in the preservation and alteration of fossils in the rock record
• The complex interplay of physical, chemical, and biological processes during diagenesis can lead to a range of outcomes for fossil preservation, from exceptional preservation to complete destruction
• Understanding the effects of diagenesis on fossil preservation is essential for reconstructing the paleobiology and paleoecology of ancient organisms and for recognizing potential biases in the fossil record

Enhancing fossil preservation

• Certain diagenetic processes can enhance the preservation of fossils by promoting the early mineralization of tissues and the rapid burial of organisms
• For example, the early precipitation of carbonate or silica cements around fossils can create a protective envelope that shields the fossils from further alteration
• The formation of concretions around fossils can also enhance preservation by creating localized chemical microenvironments that inhibit the degradation of tissues
• Rapid burial of organisms by sediments can minimize the exposure of fossils to destructive processes such as scavenging and microbial decay

Limiting fossil preservation

• Other diagenetic processes can limit or destroy the preservation of fossils by promoting the dissolution, replacement, or deformation of skeletal elements
• For example, the dissolution of carbonate fossils by acidic pore fluids can lead to the complete loss of fossil material and the formation of molds and casts
• The replacement of original skeletal mineralogy by more stable minerals can obscure the fine morphological details of fossils and make them difficult to identify
• The distortion and compression of fossils by compaction and tectonic stress can alter the original shape and orientation of fossils and limit their paleoecological and taxonomic utility

Selective preservation of fossils

• Diagenesis can lead to the selective preservation of certain fossil groups or skeletal elements based on their original composition and structure
• For example, fossils with robust, heavily mineralized skeletons (such as brachiopods and echinoids) are more likely to be preserved than fossils with delicate, poorly mineralized skeletons (such as worms and jellyfish)
• Fossils composed of stable minerals (such as calcite and silica) are more likely to be preserved than fossils composed of metastable minerals (such as aragonite and high-magnesium calcite)
• The selective preservation of fossils can create biases in the fossil record and affect the interpretation of past biodiversity and ecological patterns

Biases in fossil record due to diagenesis

• Diagenetic processes can introduce biases in the fossil record that can affect the interpretation of past environments, ecosystems, and evolutionary patterns
• For example, the preferential dissolution of aragonitic fossils in meteoric diagenetic environments can lead to the underrepresentation of certain taxonomic groups (such as mollusks) in the fossil record
• The selective preservation of fossils in certain depositional environments (such as rapid burial in fine-grained sediments) can lead to the overrepresentation of certain fossil assemblages relative to their original abundance and diversity
• The alteration of fossil morphology and chemistry by diagenetic processes can complicate the taxonomic identification and phylogenetic analysis of fossils
• Recognizing and accounting for potential diagenetic biases is essential for accurate paleobiological and paleoecological reconstructions based on the fossil record

Diagenetic indicators in rocks and fossils

• Diagenetic processes leave behind a variety of physical, chemical, and mineralogical indicators in rocks and fossils that can be used to reconstruct the diagenetic history of a sedimentary succession
• These indicators provide valuable insights into the nature and timing of diagenetic alterations and can help to constrain the interpretation of past depositional environments and burial conditions
• The main types of diagenetic indicators include cement types and textures, diagenetic minerals and fabrics, geochemical signatures, and cathodoluminescence patterns

Cement types and textures

• Diagenetic cements are mineral precipitates that fill the pore spaces between sediment grains or within fossils
• The type, texture, and distribution of cements can provide information about the diagenetic environment and the sequence of diagenetic events
• For example, early marine cements (such as fibrous and bladed calcite) are indicative of cementation in shallow, high-energy environments, while late burial cements (such as blocky and poikilotopic calcite) are indicative of cementation in deep, low-energy environments
• The textural relationships between cements and other diagenetic features (such as compaction fabrics and dissolution surfaces) can help to establish the relative timing of diagenetic events

Diagenetic minerals and fabrics

• Diagenetic minerals are formed by the precipitation, replacement, or alteration of pre-existing minerals during diagenesis
• The presence and abundance of certain diagenetic minerals can provide information about the chemistry an