Diagenesis

Diagenesis refers to the physical, chemical, and biological changes that sediment undergoes after it is deposited. These processes change loose sediment into sedimentary rock and occur in the upper several hundred meters of the earth’s crust.

Distinguishing Diagenesis from Metamorphism

Diagenesis refers to the physical, chemical, and biological processes that occur in sediment after deposition as it is buried and transformed into sedimentary rock. These processes alter the texture, porosity, fabric, structure, and mineralogy of the sediment. Through diagenesis, sand is changed into sandstone, mud is changed into shale, and carbonate sediments are changed into limestone and dolomite. The processes and degree of alteration depend in part on the initial sediment composition and the depth of burial.

As sediment is buried to increasing depths, the temperatures and pressures increase, and diagenesis grades into metamorphism. Although the exact limits separating diagenesis and metamorphism are not strictly defined, diagenesis can be considered to occur under temperatures ranging from those at the earth’s surface up to nearly 300 degrees Celsius and under pressures ranging from atmospheric pressure to at least 1 kilobar (1,000 bars). These conditions occur at depths approximating 10 kilometers. Some classifications restrict the zone of diagenesis to about 0 to 1 kilometers and define zones of catagenesis (several kilometers deep, with temperatures of 50 to 150 degrees and pressures of 300 to 1,000 or 1,500 bars), metagenesis (up to about 10 kilometers deep), and finally metamorphism. Temperature is an important control on many diagenetic processes, because it influences chemical reactions such as the dissolution and precipitation of minerals, recrystallization, and authigenesis.

Compaction

The primary physical diagenetic process is compaction. Compaction presses sedimentary grains closer together under the load of overlying sediment, causing pore space to be decreased or eliminated and squeezing out pore fluids. In sandstones, compaction occurs by the rotation and slippage of sand grains, the breakage of brittle grains, and the bending and mashing of ductile (soft, easily deformed) grains. Brittle grains include thin shells, skeletal fragments, and feldspar grains. Ductile grains include clay or shale chips and fecal pellets. Compaction also causes some mineral grains to interpenetrate, producing irregular stylolitic contacts where the grains have intricately complex boundaries. In general, sands compact much less than muds. That is true since the average sandstone has a high percentage of hard grains, such as quartz; muds typically have a high initial water content, and water is squeezed out during compaction. The compaction of sands, however, is influenced by the nature of the sand grains present; sands with a large percentage of ductile grains are more susceptible to compaction.

Cementation, Authigenesis, and Replacement

Chemical diagenetic processes include cementation, the growth of new minerals (authigenesis), replacement, neomorphism (recrystallization and inversion), and dissolution. Cementation is the precipitation of minerals from pore fluids. These minerals glue the grains of sediment together, forming a rock. The most common cements are quartz (silica), calcite, and hematite, but such cements as clays, aragonite, dolomite, siderite, limonite, pyrite, feldspar, gypsum, anhydrite, barite, and zeolite minerals occur as well. The type of cement is controlled by the composition of the pore fluids.

Authigenesis refers to the growth of new minerals in the sediment and the transformation of one mineral into another. Some of the most common authigenic minerals in sandstones are calcite, quartz, and clay cements. Other than cements, authigenic minerals include glauconite, micas, and clay minerals. Glauconite is a green mineral that forms on the sea floor when sedimentation rates are low. Authigenic micas and clays typically form in the subsurface at higher temperatures and pressures. Often, one clay mineral is transformed into another as a result of dehydration (water loss) or chemical alteration by migrating fluids. Clays may also be formed from the alteration of feldspars or volcanic ash and rock fragments. Authigenesis also includes the alteration of iron-bearing minerals (such as biotite, amphibole, or pyroxene) to pyrite, under reducing conditions, or to iron oxide (limonite, goethite, or hematite), under oxidizing conditions.

Replacement is the molecule-by-molecule or volume-for-volume substitution of one mineral for another. Replacement generally involves the simultaneous dissolution of an original mineral and precipitation of a new mineral in its place. Fossils that were originally calcium carbonate may be replaced by different minerals, such as quartz, pyrite, or hematite. Many minerals are known as replacement minerals, including calcite, chert, dolomite, hematite, limonite, siderite, anhydrite, and glauconite. Factors controlling replacement include pH, temperature, pressure, and the chemistry of the pore fluids.

Neomorphism and Dissolution

Neomorphism is a term meaning “new form”; it refers to minerals changing in size, shape, or crystal structure during diagenesis. The chemical composition of the minerals, however, remains the same. Neomorphism includes the processes of recrystallization and inversion. Recrystallization alters the size or shape of mineral grains without changing their chemical composition or crystal structure. Recrystallization can occur in any type of sedimentary rock, but it is most common among the carbonates. Limestones are commonly recrystallized during diagenesis, producing a coarsely crystalline rock in which original sedimentary textures and structures may be fully or partially obliterated. The reason that minerals recrystallize is not well understood, but it may be related to energy stored in strained crystals or to a force arising from the surface tension of curved crystal boundaries. Inversion is a process in which one mineral is changed into another with the same chemical composition but a different crystal structure. The two minerals involved are called polymorphs, meaning “multiple forms.” Aragonite and calcite are polymorphs. Both have the same chemical composition, but each has a different crystal structure: Aragonite is orthorhombic and calcite is rhombohedral. Aragonite, with time, will become calcite by inversion. Inversion may occur along a migrating film of liquid, causing the simultaneous dissolution of one mineral and precipitation of its polymorph, or by solid-state transformation (switching of the positions of ions in the crystal lattice).

Dissolution refers to the dissolving and total removal of a mineral, leaving an open cavity or pore space in the rock. This pore space may persist, or it may become filled by another mineral at a later time. Some of the more soluble minerals are the carbonates and the evaporites, such as halite and gypsum. Large-scale dissolution of limestone leads to the formation of caves and caverns. Pressure solution is the dissolution of minerals under the pressure of overlying sediment. Stylolites, a common result of pressure solution, commonly occur in carbonate rocks. Stylolites are thin, dark, irregular seams with a zigzag pattern that separate mutually interpenetrating rocks. The dark material along the seam is a concentration of insoluble material such as clay, carbon, or iron oxides. Pressure solution can result in a 35 to 40 percent reduction in the thickness of carbonate rocks. The carbonate removed by pressure solution is frequently a source of carbonate cements.

Finally, minerals in solution may be deposited on existing grains. This phenomenon is very common with quartz grains. Silica in solution builds on the existing crystalline structure of the grain, causing new quartz to form. Often the sand grain develops actual crystal faces as a result. Such new mineral growths are called overgrowths.

Biological Diagenetic Processes

Biological diagenetic processes occur soon after sediment is deposited and consist of the activities of organisms in and on the sediment. Bacteria are particularly important to the chemical diagenetic processes. Bacteria living in the sediment control many chemical reactions involving mineral precipitation or dissolution; they are involved in the breakdown or decomposition of organic matter (one of the steps in the formation of oil and gas), and can cause the pH of the pore fluids to increase or decrease, depending on the kinds of microorganisms, organic matter, decomposition products, and availability of oxygen. For example, in aerobic environments (those where oxygen is present), decay of organic matter generally causes decreasing pH (increasing acidity), which may lead to the dissolution of carbonate minerals such as calcite. Under anaerobic conditions, organic decay generally raises the pH and may lead to the precipitation of calcite cement. The formation of pyrite is also influenced by the activity of bacteria. Sulfate-reducing bacteria in anoxic environments change sulfate into hydrogen sulfide. If iron is present, it reacts with the hydrogen sulfide to form iron sulfides, such as pyrite.

Bioturbation is the disturbance of the sediment by burrowing (excavation into soft sediment), boring (drilling into hard sediment), the ingestion of sediment and production of fecal pellets, root penetration, and other activities of organisms. Bioturbation generally occurs shortly after deposition and causes mixing of sediment that was originally deposited as separate layers, destruction of primary sedimentary structures and fabrics, and breakdown or clumping of grains. In some cases, chemical alteration of the sediment accompanies bioturbation. For example, light-colored halos may form around burrows or roots, particularly in red or brown sediments, because of the reduction of iron.

Diagenesis may decrease or increase the porosity and permeability of the sediment. Porosity is decreased by compaction, the precipitation of cements in pore spaces, and bioturbation. Porosity is increased by dissolution. Zones of increased porosity are particularly favorable for oil and gas accumulations.

Study Techniques

Diagenesis is primarily studied using sedimentary petrography, which is the microscopic examination of thin sections of sedimentary rocks. Thin sections are slices of rock, typically 30 micrometers thick, bonded to glass slides, which are examined with a petrographic microscope. In this way, minerals can be identified based on their optical properties, and textural relationships can be studied, such as the size, shape, and arrangement of grains; the geometry of cements and pore spaces; the character of contacts between grains; the presence of dissolution features; and mashing or fracturing of grains. Thin sections may be enhanced, to allow easier identification of minerals, with various staining and acid etching techniques. In addition, acetate peels may be prepared from etched and stained rock surfaces for examination with the microscope.

There are a number of other techniques that can be used in conjunction with petrography to obtain more specific diagenetic data. Cathodoluminescence microscopy can provide information about the spatial distribution of trace elements in rocks. Luminescence is the emission of light from a material that has been activated or excited by some form of energy. Cathodoluminescence works by activating various parts of a polished thin section with a beam of electrons. The electron beam excites certain ions, producing luminescence. This technique can reveal small-scale textures and inhomogeneities of particles and cements through differences in their luminescence, which are related to differing concentrations of trace element ions.

Scanning electron microscopy can magnify images 70,000 times or more, permitting detailed study of extremely small particles that cannot be adequately examined using a petrographic microscope. Scanning electron microscopy involves reflections of an electron beam from a rock or mineral surface. Fine details of cements and grains may be readily observed and photographed.

X-ray diffraction is used to determine the mineralogy of sedimentary rocks, particularly fine-grained rocks such as shales. The technique is based on reflections of X rays from planes in the crystal structure of minerals. Each mineral has a characteristic crystal structure and produces a distinctive X-ray diffraction pattern consisting of peaks of different position and intensity, which are plotted on chart paper by the X-ray diffractometer.

Fluid inclusions are extremely small droplets of fluid encased within crystals or mineral grains. The fluids are a small sample of the original pore fluids from which the mineral was precipitated. By examining fluid inclusions using heating and freezing devices attached to a microscope, the geologist can determine the composition of the original pore fluids and the temperature at which the mineral was precipitated. This technique reveals that many minerals and cements were precipitated from hot, saline pore fluids.

Stable isotopes of oxygen and carbon are commonly used to determine the chemistry of the pore fluids and the temperatures under which precipitation of cements or authigenic minerals occurred. Studies of the stable isotopes of microfossils have also provided information on past climatic changes. Isotopes are different forms of elements that vary in the number of neutrons present in the nucleus; hence, the various isotopes of an element have different atomic weights. By comparing the ratios of oxygen-16 and oxygen-18 or carbon-12 and carbon-13 in minerals such as calcite, it is possible to determine whether the minerals precipitated from fresh water or marine water, or to determine the temperature of the fluid from which the mineral precipitated.

Principal Terms

bar: a unit of pressure equal to 100 kilopascals and very nearly equal to 1 standard atmosphere

carbonate: a mineral with CO₃ in its chemical formula, such as calcite (CaCO₃)

lithification: the hardening of sediment into a rock through compaction, cementation, recrystallization, or other processes

pore fluids: fluids, such as water (usually carrying dissolved minerals, gases, and hydrocarbons), in pore spaces in a rock

porosity: the amount of space between the sedimentary grains in a rock or sediment

sediment: loose grains of solid, particulate matter resulting from the weathering and breakdown of rocks, chemical precipitation, or secretion by organisms

sedimentary rock: a rock resulting from the consolidation of loose sediment that has accumulated in flat-lying layers on the earth’s surface

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