Diagenesis of Sedimentary Rocks Stratigraphy & Sedimentology

A. Diagenesis Defined:

The AGI Glossary defines diagenesis as: “The physical, chemical, and biological changes, modifications, or transformations undergone by a sediment after its initial deposition.”

B. Implications for Diagenetic Changes:

Changes that occur after a sediment has been deposited can significantly alter the composition and behavior of sediments and sedimentary rocks. For example, deposition of cement in the pore spaces of a highly porous sandstone can significantly reduce porosity and affect permeability. Replacement of original mineral material may significantly change the composition of a sedimentary rock from its parent sediment. For example, Iron oxides which replace minerals and which becomes a cementing agent may come to comprise a large percentage of a sedimentary rock. In Minnesota, some of the world-famous iron “ores” are actually sedimentary in origin and alteration. Knowing how alteration occurs and what transformations are likely, a geologist can often predict or interpret characteristics of rocks that have important bearing on resource recovery opportunities and environmental issues.

C. Identification of Diagenetic Changes:

From beginning Physical Geology and in Petrology, you have been acquainted with the general changes that can occur in sediments and sedimentary rocks. What are the broad categories that you have studied in the past?

1. compaction

2. dewatering

3. cementation

4. replacement

5. recrystallization

D. Diagenetic Changes:

1. Compaction:

Compaction is the compression of sediments in response to the increased weight of accumulation of overlying sediments. Compaction occurs in all sediments, detrital and non-detrital. The impact of compaction is most significant in sediments that are granular (detrital grains, carbonate shell fragments, plant pieces) although compaction can impact sediments of evaporitic origin (compaction usually initiates recrystallization in evaporites that will be discussed later). Compaction results in the reduction of pore space, and it can promote the alignment of grains (such as the alignment of platy clay grains parallel to bedding). Sediments rich in clays may have the porosity reduced and the permeability almost completely impaired as clay grains are flattened and align and become very cohesive. In general the implications of compaction and nature of sediments and sedimentary rocks are:

a. reduction of porosity

b. reduction of permeability

c. almost complete destruction of permeability

d. dewatering

e. the older the sedimentary rock the greater compaction likely

f. sedimentary rocks associated with orogenic regions likely to be have undergone greater compaction

2. Dewatering:

Dewatering accompanies compaction. Basically compaction creates physical changes in sediments, but dewatering and the movement of water through sediments can cause chemical changes. Mineral matter can be dissolved, moved in solution or in a colloidal state, and then be redeposited elsewhere in the sediment or to be removed from the sediment entirely. Dewatering can be important in development of cements, but cements will be discussed under a separate category.

3. Cementation:

Cementation involves the deposition or precipitation of mineral matter in open spaces of sedimentary rocks. Mineral materials may accumulate in pore spaces, fractures, interiors of fossil shells, etc. Deposition of cement in open spaces depends upon the chemistry of water moving through the sediments after burial. Fluid may be:

• Pore water from compaction and dewatering

• Fluids introduced after original pore fluids have been removed by dewatering (such as freshwater flushing through marine sediments that have been uplifted after their initial position or deep burial of fresh water sediments exposed to marine briney of marine sediments)
  

• Fluids introduced by heating and fluid migration associated with igneous

a. Primary Cementation: Generalities:

In general, as pore water is removed by compaction, some of the mineral matter of pore fluids may be precipitated or deposited in pore spaces. Early cementation, known as primary cementation, may nearly completely cement a rock. In quartz arenites, silica that is dissolved, can be precipitated as a quartz megacrystalline cement or as a microcrystalline cement (chert). Silica precipitation is normally slow because concentrations of silica in pore waters is low and silica is “slowly soluble” so oftentimes pores spaces in quartz arenites are not completely filled. Only with long periods of time do quartz arenites become well cemented. Carbonate grainstones may cement quite readily because solution of the grains (fossils fragments, etc.) can be rapid because calcium carbonate is readily soluble in warm marine waters. Pore space of a fossiliferous grainstone may be completely infilled. Implications are that grainstones may be expected to be completely cemented by calcite “spar”. Sandstones may not be completely cemented by primary cementation if quartz is the microcrystalline quartz variety or quartz “overgrowth” variety. Some quite old (Cambrian and Ordovician) quartz arenites have quite high porosity and permeability because of low amounts of cement, and they serve as important groundwater aquifers.

b. Primary Cementation: Cement Mineralogy:

Cements can be comprised of any mineral species that is dissolved or carried in a colloidal state in pore waters. The most common primary cements include:

1. silica (quartz and microcrystalline chert)
2. iron oxides (hematite and limonite)
3. calcite
4. dolomite (Ca, Mg Carbonate)
5. siderite (Fe Carbonate)
6. clay (although clay is a detrital grain, the surfaces of the platy clay grains are cohesive and act as as cement to other clay grains and other detrital fragments even thought they are not "precipitated" cements)

The composition of the primary cements depends mainly upon the nature of the original pore fluids of the sediment; so therefore the nature of primary cements depends upon the environment of deposition. Marine pore waters are rich in calcium, magnesium, “salts” (such as halite and gypsum) and some silica and iron. Terrestrial pore waters are richer in iron and silica. The differences in pore waters are primarily due to the differences in cations that influence the “acidity”/pH of the pore fluids. Marine pore waters are more “basic” and and terrestrial pore waters tend to be more “acidic”

(reviewed nature of acidity and alkalinity and pH. Defined pH as, “the negative logarithm of the effective concentration of hydrogen in grams per liter. A high pH number value means a high number of decimal places which means a low hydrogen concentration = basic/alkaline. A low pH number value means a low number of decimal places which means a high concentration of hydrogen = acidic)

A chart circulated compares the eH and pH of pore waters from a variety of depositional settings. In general:

• Marine pore fluids are basic and are richer in calcite, salts (such as halite and gypsum), siderite, and some silica -- elements and compounds that go into solution easily and at lower acidity (at higher pH); also these are elements and compounds that neutralize acidic solutions readily; so they also contribute to lower acidity and raiser alkalinity

• Terrestrial pore fluids are much more acidic so that slowly soluble cautions like iron and compounds such as silica are in higher concentrations (more acidic partly because of the lack of ocean cautions and more acidic because of the acids released into pore water from decaying plant matter)

Therefore, as compaction occurs, terrestrially formed rocks tend to have cements rich in silica and iron oxides and illitic clays. Rocks from marine settings tend to have more cements dominated by calcite, dolomite, sulfates, siderite, and illitic and glauconitic clays.

c. Conditions that Promote Cementation:

Cementation occurs when pore fluids become saturated and supersaturated. The geological conditions that can promote saturation and supersaturation include:

• Desiccation of sediments -- If sediments are desiccated, especially shallow sediments, then the solutions become saturated with respect to mineral compounds, and precipitation or deposition can occur. Some desert sediments and soils have cementation of the shallower horizons in response to evaporation and dessication.

• Changes in the pressure of sediments -- If sediments are deeply buried in the center of a basin, the compression of overlying layers can cause dewatering; the pore fluids migrate into shallower parts of the basin where pressure is less. Pressure release can promote precipitation.

• Changes in the temperature of sediments -- If sediments are deeply buried in the center of a basin, not only do they become compressed, the temperature of the sediments and pore fluids increases. As compression drives the pore fluids into shallower parts of the basin where temperatures are less, the pore fluids can become supersaturated and precipitation can occur. Association with mountainbuilding or hydrothermal conditions may also contribute heat and increase temperature conditions that put compounds in solution. As pore fluids migrate into cooler settings, precipitation can occur.

• Changes in pore fluid chemistry -- changes in concentration, eH and pH. Increases in concentration or changes in pH can "push" pore fluids into conditioins of "oversaturation" and can result in precipitation of mineral cements in pore spaces.

d. Implications for Primary Cementation:

Margins of depositional basins tend to be areas of concentration of cementing processes. Sedimentary rocks "ringing" depositional basins tend to be better cemented and have lower porosities and permeabilities than sedimentary sequences in the centers of depositional basins. Some of the most productive of petroleum oil fields are in the centers of deep sedimentary basins (if the oil has not migrated significantly) and in the zones between the center of the basin and the basin margins (where there has been some migration of petroleum). There is almost a "halo" effect or "bulls eye" effect in the pattern of petroleum production from sedimentary basins.

Some of the United States' most important groundwater aquifers ring the margins of sedimentary basins. The Jordan Sandstone, a quartz arenite, in the Forest City Basin (Iowa, Nebraska, Missouri, Kansas) supplies much of the drinking water across Iowa. On the northern flank of the basin the Jordan is tapped by hundreds of municipal wells. Toward the center of the Forest City Basin, the water becomes so high in sulfate that the water is "not potable" = not drinkable because drinking the water results in a "mild" laxitive effect.

Some important mineral deposits are found in the "ringing" areas of depositional basins. The lead and zinc deposits of the tri-state mining district may have originated deep in sedimentary basins in pore fluids and have been precipitated or deposited in the pore and fracture spaces in the shallower ringing areas of the Forest City Basin on its southern margine in the Tri-State Mining District (Missouri, Oklahoma, Kansas state lines junction).

Reviewing the chemical nature of cementing agents can also suggest/imply original depositonal environments -- marine depositional systems are more likely to be associated with hydrocarbon reserves. Terrestrial depositonal systems such as deltaic systems are more likely to be associated with coal and lignite resources.

e. Secondary Cementation:

Secondary cementation is, as the name implies, cementation that occurs after the initial cement precipitation. Secondary cements can form when:

• If initial cements are readily soluble, they can be partially or fully removed by later solution. That later pore fluid may have have minerals in solution that can be precipitated forming a second stage of cementation (known as a second "generation" of cementing)

• If some initial pore space remains and a second cementing medium moves thought the residual pore space, a second "layer" of cement can be deposited on-top-of or surrounding the original cementing material.

Secondary cements can be comprised of essentially any mineral species present in primary cements.

f. Implications for Recognition of Secondary Cements:

Recognition of secondary cements allows a geologist to perceive changes in the geological history of a sedimentary rock sequence. For example, a sedimentary rock may have a siliceous and iron-oxide primary cement indicative of terrestrial conditions but a secondary cement of carbonate. The carbonate indicates later pore fluid was possibly related to marine fluids/marine brines; so a geologist can infer a terrestrial area may have been submerged even though the overlying rocks may not bear witness to the event. Recognition of secondary cements also has implications in terms of rock recognition. Some sandstone formations may have distinctive cement signatures -- literally layers of cements that are unique because of the unique sequence of precipitation events that formed them. It is possible, sometimes, to distinguish very similar sandstones based on different cement signatures. Formations that might be hard to correlate and develop fence or panel diagrams can be discerned and equated based on their cement signature.

4. Recrystallization

Recrystallization will be limited to include grain changes that are physical in nature and not chemical in nature. Essentially recrystallization will be viewed as a change in grain size, shape, and orientation. Recrystallization results primarily from pressure changes in sedimentary sequences. Pressure may be due to deep burial or may be due to compression related to mountainbuilding. Where grains touch, pressure results in pressure solution. Dissolved compounds may be carried in solution only a short way (on order of fraction of mm) and precipitated almost immediately or may be carried away. Immediate precipitation causes mineral grains to enlarge and often become more equidimensional. The well rounded quartz grains in many quartz arenites have a quartz "halo" which is known as a quartz overgrowth, resulting from solution and some pressure solution and then precipitation of the silica as a cement. In effect the quartz grains become larger. Carbonate (calcite) grains are even more responsive to pressure solution than silicates and commonly carbonate grains in limestones have been recrystallized. Fossils fragments very commonly have overgrowths of calcite. The consequences of recrystallization are primarily in the nature of changes in porosity and permeability -- usually a reduction of both.

5. Replacement

Replacement will refer to the actually replacement of an original mineral species with another mineral species -- such as calcite replaced by silica or calcite (or calcite replaced by pyrite or an iron oxide mineral). Replacement involves the molecule-by-molecule replacement of original mineral material by other minerals in solution and is a result of changes in chemistry of pore solution. If a sediment is in equilibrium with its pore fluids, then the grains remain unchanged. If the chemical composition of the pore fluids change, then the grains and fluid are not longer in equilibrium. If the fluid is unsaturated with respect to the mineral species, then solution occurs. If the sediment is a limestone, then calcite is dissolved. If the solution is oversaturated with some mineral such as iron oxide, then the iron oxide precipitates in the space left by the dissolved calcite. The iron oxide replaces calcite in a molecule-by-molecule fashion. If long periods of non-equilibrium exist, then large masses of sediments and sedimentary rocks can be altered by replacement. Replacement, then, can occur when non-equilibrium conditions exist between rock and pore fluids. Some of the situations that can produce non-equilibrium conditions (i.e. pore fluid differences):

• Change in geological setting -- For example, a sedimentary rock may have formed in a carbonate Bahama type of setting and may be a fossiliferous limestone. If sea level drops, fresh water can percolate downward. Often times fresh water is more acidic because of plant acids in plant residues. Because of high acidity, silica and iron may be in solution and colloidal suspension. The fluids percolating through the underlying grainstones or wackestones is undersaturated with respect to calcite, so calcite begins to dissolve. Dissolved calcium reduces the acidity (raises the pH). Reductions in acidity mean that silica and iron can become supersaturated and precipitate. The calcite can be replaced by iron oxides or silica. A classic example of grainstones being replaced by iron oxides is in the iron quarries of northern Minnesota at the Hill Annex Mine. Thick sequences of fossiliferous limestones have been replaced by hematite – hematite gastropods, bivalves, etc.

• Changes in stratigraphy -- For example, where sedimentary strata change abruptly, such as the transition from sandstone to silty shale at the base of the local Hartshorne Sandstone and the underlying Atoka Formation, the lithologic change may promote chemical changes in the pore fluids that can result in replacement of existing minerals.

• Where fresh water and salt water/brines mix.