Geological Digressions

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; cements

Mineralogy of carbonates; diagenetic settings

Mineralogy of carbonates; seafloor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates: Stromatolite reefs

References

W.H.Berger. 1970. Planktonic foraminifera: selective solution and the Lysocline. Marine Geology, v. 8, p.111-138.

B.P.Boudreau, J.J. Middleburg, and F.J.R. Meysman, 2010. Carbonate compensation dynamics. Geophysical Research Letters, v. 37, L03603.

W.S. Broecker. 2008. A need to improve reconstructions of the fluctuations in the calcite compensation depth over the course of the Cenozoic. Paleooceanography, v. 23, PA1204.

J.W. Morse and F.T. Mackenzie 1990. Geochemistry of sedimentary carbonates. Developments in Sedimentology 48. Elsevier, Amsterdam, 707 p.

Mineralogy of carbonates; Beachrock

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Early lithification of beach sand. This is part of the How To…series on carbonate rocks

One of the most recognizable products of seafloor cementation is beachrock; lithified beach sand. Modern beachrock is common on tropical coasts, those that are wave-washed and quieter shores sheltered by reefs and island barriers. It is less likely to be found bordering cooler seas. Beachrock forms in clean carbonate and siliciclastic-volcaniclastic sands. Lithification occurs so rapidly (months, years) that one can find all sorts of interesting relics entombed – shells, fish skeletons, coconuts, the flotsam and jetsam of wars, boats that have come to grief, or the refuse strewn by centuries of ocean travelers.

Beachrock affords a domicile for algae and invertebrates that would not normally enjoy living on a soft sand beach; thus, its formation may change existing biotas. It also provides a protective carapace to a beach, reducing the impact of waves. Formation of beachrock potentially changes the beach dynamics. Continue reading →

Mineralogy of Carbonates; Sea floor diagenesis

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Diagenesis on the sea floor – This is part of the How To…series on carbonate rocks

Seafloor carbonate diagenesis encompasses the vadose intertidal-supratidal, the phreatic shallow subtidal lagoon and platform, deeper water slopes below the photic zone, to bathyal ocean floors. This post deals with the shallow end of the action.

Physical and chemical changes to carbonate sediment begin almost immediately; shells are bored by sponges and fungi, reef corals are grazed by sea urchins (what a lovely Dickensian expression), aragonite and calcite precipitate, skeletal debris dissolves. Biological processes go hand in hand with the abiotic. I like James and Choquette’s description of this important beginning to diagenesis – (paraphrased) loose sand that becomes cemented as a hardground changes the living conditions for a myriad critters and plants. Thus, the formation of beachrock or subtidal hardgrounds displaces a bunch of gastropods that prefer to live on soft sand.

Diagenesis at the seafloor involves fluids that are the same or similar composition to surface waters: unmodified seawater beneath open platforms, brackish fluids in seawater-fresh groundwater mixing zones, and hypersaline fluids in supratidal and sabkha environments. Fluid drive at the surface and shallow subsurface is provided by: Continue reading →

Mineralogy of carbonates; cements

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This is part of the of How To…series… on carbonate rocks

The diagrams and images of carbonate crystal habits and cements are descriptive and intended to provide essential background to other posts that detail the different diagenetic environments.

Carbonate diagenesis is like a game of two halves: one part involves mineral dissolution, the other precipitation. The two commonly go hand-in-hand; it all depends on the changing fortunes of thermodynamic stability and interstitial fluid flow as the game progresses.

Cements precipitate in available pore space: intergranular, intragranular (like the whorls of gastropods, the septa of corals, or the chambers of foraminifera), larger voids like those developed in reef frameworks, and microporosity such as pore throats between grains. Neomorphism and mineral replacement involve dissolution and precipitation that change existing cement fabrics and sediment frameworks and hence are not confined to pore space.

Carbonate cements are as varied as the diagenetic environments in which they form – the sea floor, meteoric, deep burial and everywhere in between. The crystal shapes of CaCO₃, it’s polymorphs and chemical variants range from needle and whisker-like, to blocky spar. The transitions from one crystal form to another, their growth in open pores, and replacement by stable carbonate phases is what makes carbonate petrography so fascinating. Continue reading →

Mineralogy of carbonates; diagenetic settings

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Carbonate diagenesis; How limestones form.

This is part of the of How To…series… on carbonate rocks

Of all the common rock-forming minerals, carbonates are the most reactive chemically. The transformation of loose sediment to hard limestone involves chemical reactions that, depending on the conditions (ionic concentrations, pH, degree of saturation, temperature) promote precipitation or dissolution of minerals, most commonly calcite, Mg-calcite, aragonite and dolomite. These reactions take place at the surface (e.g. sea floor) and at all stages during sediment burial and uplift. Limestone diagenetic pathways are complicated; this is part of the attraction for those who study them (notwithstanding the opportunity to conduct field work in places like Bahamas).

Some general requirements for diagenesis to proceed are:

The thermodynamic stability and metastability of precipitating phases are determined by pressure, temperature and chemical composition of the fluids (including partial CO₂ pressures, pH, and Mg/Ca ratios).
Reactions take place in water: sea water, modified sea water, fresh water and saline brines. These fluids are never static; they flow, delivering new solute (ions in solution) to sites of precipitation, and removing dissolved solids to other sites in the permeable sediment.
Carbonate diagenesis at all burial depths is, like any other rock type, governed by subsurface fluid flow and evolving fluid compositions. Fluid flow itself is governed by hydraulic gradients that are generated by topography, sediment compaction and tectonic loads.
If the conditions change, any phase that becomes thermodynamically unstable will dissolve. This applies particularly to metastable phases like aragonite and high Mg-calcite, where changes in the fluid environment can render them unstable and prone to dissolution. Fluid composition will change when, for example, shallow sea floor carbonates are exposed to meteoric fresh water as sea level falls, or during burial where original seawater is modified by reactions involving carbonates, siliciclastics and, importantly organic matter.

Geologists like to subdivide things and the carbonate diagenetic realm is no different (it helps to simplify a complex world). Three diagenetic environments are frequently cited, with a fourth located at the transition to metamorphism (each environment will be treated in greater detail in separate articles), and depicted in the diagram below:

Seafloor environments (almost syndepositional), including the first few centimetres or metres of burial.
Shallow burial – meteoric environments
Deep burial environments

Sea floor diagenesis

Precipitation of aragonite and high-Mg calcite cements on the sea floor or in the first few centimetres to metres beneath it, is common in tropical settings, less so in cooler waters. It is the region influenced by ambient seawater compositions; it is also referred to as the marine phreatic zone.

This, the earliest stage of carbonate diagenesis is promoted by low sedimentation rates, high levels of mineral supersaturation in seawater and rapid exchange of atmospheric CO₂ with aerated seawater. Microorganisms like bacteria and photosynthesizing algae also contribute, either as mediators or by direct precipitation. Some microorganisms such as endolithic bacteria and algae have a dual role in that their substrate boring activities tend to destroy primary grains but leave micrite rinds and infills in their wake.

Seafloor cementation can take place from the supratidal-intertidal zone (marine vadose zone) (e.g. crusts, beachrock), reef and shallow subtidal platform (mainly within the photic zone), to the outer platform and slope (beyond the photic zone). At greater depths, first aragonite (the aragonite compensation depth – ACD – is 2-3km) then calcite (CCD is 4-6km) begin to dissolve because of high CO₂ partial pressures.

Meteoric environments

Platform and reef deposits exposed by a fall in relative sea level will be subjected to an influx of fresh water. Carbonate deposits subjected to meteoric conditions undergo significant changes to their mineralogy and texture.

In the meteoric environment, groundwater flow through permeable aquifers is driven largely by gravitational potential energy, usually referred to as topography-driven flow. Hydrogeologists have long recognized three distinct zones within meteoric settings; James and Choquette (reference at bottom of the page) included these zones in their early model of carbonate diagenesis:

The watertable, below which all porosity is completely saturated; this is the phreatic zone.
Above the watertable is the unsaturated or vadose zone where pore spaces are mostly air-filled but are periodically wetted by rising watertables (may be seasonal) and infiltration of surface water.
Where aquifers intersect the coast there is a zone of fresh water and seawater mixing. The location of the phreatic mixing zone in relation to the shoreline depends on the hydraulic gradient (or hydraulic head) in the aquifer and how far the aquifer extends offshore. It is not uncommon for fresh water to flow 100s, even 1000s of metres offshore, beneath a platform or shelf.

Aragonite and high-Mg calcite are unstable in fresh water which means that any clasts (skeletal fragments, ooids, foram tests) and cements that contain these minerals will begin to dissolve. In some cases, secondary porosity will form. Also common is the replacement of clasts and cements by low-Mg calcite.

Karst landscapes form during prolonged exposure to meteoric conditions where even the low-Mg calcites dissolve.

Deep burial environments

Deep burial usually refers to the interval below the influence of marine phreatic and meteoric fluids (10s to 100s of metres deep) to several 1000m depth (depending on the geothermal gradient). The principle physical process is compaction that rearranges sediment frameworks and drives interstitial fluids to other parts of the sedimentary basin. The influence of temperature on the promotion and rates of chemical reactions becomes increasingly important with depth.

Fluid composition is primarily modified seawater, modification that can eventually produce saline brines. Many different reactions come to play as burial depth and temperature increase. Compaction enhances pressure solution where significant volumes of rock are dissolved and the solute transferred to other parts of the sedimentary basin; what remains are stylolites. Other reactions involve dehydration of clays, clay mineral transformations (e.g. kaolinite-smectite), and perhaps most significantly, the diagenesis of organic matter. All these reactions contribute to changes in pH and alkalinity that effect calcite and dolomite stability.

Carbonate diagenesis is dominated by precipitation of calcite and ferroan calcite that replace aragonite and high-Mg calcite. Recrystallization of calcite spar tends to mask and even obliterate original depositional fabrics. Dolomitization is also common during burial diagenesis and like calcite overprints earlier grain and cement fabrics (i.e. those formed in the sea floor and meteoric environments.

Companion diagenetic environments

It is important to recognize that the diagenesis of carbonates during burial is not divorced from broadly similar processes taking place in siliciclastic rocks. Precipitation of quartz and clays, and dissolution of feldspar are important determinants for evolving fluid compositions that significantly effect carbonate mineral stability (mostly low Mg-calcite). Organic matter in carbonates and siliciclastics has a profound effect on deep burial diagenetic pathways. Complex organic compounds like kerogens begin to break down at about 60^o C. The rate of organic diagenesis increases markedly at 80^o C – the lower burial temperature limit of the oil-generation window. Important byproducts of these reactions are organic acids that modify pH and control alkalinity. Note too that pH buffering and alkalinity are also influenced by silicate transformations, particularly those involving smectite, kaolinite and illite. The diagram above (from Surdam and others, 1989) summarizes the progression of diagenetic reactions commonly observed in siliciclastic rocks, in relation to organic maturation, organic acids and pH buffering.

Links to other posts in this series:

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

Mineralogy of carbonates; Pressure solution

There is a vast, and for the most part excellent literature on carbonate diagenesis. Here are a few classic and more recent texts that provide much more detail on the subject.

Robin G.C. Bathurst, 1976. Carbonate Sediments and their Diagenesis. Elsevier, Developments in Sedimentology, 12. 658p. An example of the longevity and utility of one of the best on this topic. Now also as an ebook.

Noel James and Phillip Choquette.1984. Diagenesis 5. Limestones; Introduction and subsequent articles on sea floor, meteoric and burial diagenesis. The Canada Geoscience Series on Carbonate Diagenesis is available from the CGS archive.

Noel James and Brian Jones. 2015. The origin of carbonate sedimentary rocks. American Geophysical Union, Wiley works, 464p.An excellent recent update.

Peter Scholle and Dana Ulmer-Scholle, 2003. A colour guide to the petrography of carbonate rocks: grains, textures, porosity, diagenesis. AAPG Memoir 77. Loaded with images.

R.C. Surdam, L.J. Crossey, E.S. Hagen, & H.P. Heasler. 1989. Organic-inorganic interactions and sandstone diagenesis. AAPG, v.73, p. 1-23.

SEPM Strata. Diagenesis and porosity. Part of SEPM’s online stratigraphic web contructed originally by Christopher Kendall. An excellent resource for pretty well anything sedimentological and stratigraphic. Continually updated.

Erik Flugel. 2010. Microfacies of carbonate rocks: Analysis, interpretation and application. Springer. The ebook is cheaper

The Lake District – on Titan

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The Lake District, sunlight glinting, ruffled by a thin breeze, guarded by icy ridges. Idyllic? Its cousin, 1.2 billion kilometres away conjures images of Wordsworth’s daffodils adorning dry-walled dale and hill. It is often stated that Titan, Saturn’s largest moon bares closest resemblance to Earth – it is the only other body in the Solar System to have liquid seas and lakes on its surface, fed by flowing rivers in channels and canyons. But Titan’s appeal as a possible holiday destination pales when one discovers the average surface temperature there is a chilly -179^oC (-290^o F). It is so cold that water-ice has the hardness of feldspar (6 on the Moh scale – hard enough to use as an abrasive to polish mirrors).

One of Cassini’s tasks during its long voyage to Saturn (via Jupiter) was to collect data on Titan’s atmosphere, surface, and layered interior (e.g. radar, spectral, gravity data). The data continues to unlock surprises. Continue reading →

Ceres; promoted to dwarf planet

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Asteroid Ceres has been promoted to dwarf planet

There is a disturbance in the symmetry of the solar system. Between the orbits of Jupiter and Mars there should be a planet. Instead, there is a belt of rubble, big rocks, little rocks, and lots of dust. In 1801 Giuseppe Piazzi discovered Ceres, in one of those fortuitous accidents that litter the history of science, (apparently he was looking for a star) and surmised, not unreasonably, that this was the missing planet. However, several other largish planet-like bodies were discovered some years later, all having similar orbits to Ceres. By 1850 so many objects had been discovered that Alexander von Humboldt coined the term Asteroid Belt, and by 1863 Ceres and its cousins were recognised as asteroids.

The asteroid count as of 25 September 2019 was 796,990! Expect this number to increase.

In 2006 Pluto was demoted to dwarf planet by the International Astronomical Union, but on the upside Ceres’ status improved setting it apart from all the other asteroids – it became a dwarf planet. According to the IAU, a dwarf planet “… is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite”.

Ceres is now regarded as an embryonic or proto-planet, one that began accreting 4.6 billion years ago but kind of lost interest; perhaps it liked its neighbours. Continue reading →

Mineralogy of carbonates; basic geochemistry

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Some basic geochemistry of carbonates, carbonic acid and carbon dioxide

This is part of the of How To…series… on carbonate rocks

Sedimentary carbonate petrology is concerned first and foremost with the precipitation and dissolution of mineral phases; principally calcite, aragonite and dolomite. Both processes involve chemical reactions and the two primary requirements for these reactions are:

thermodynamics – there needs to be sufficient energy to drive the reactions, and
an excess or deficit of dissolved mass that, for the minerals of interest, includes Ca²⁺_(aq), Mg²⁺_(aq), and CO₃^2-_(aq) (aq = aqueous)

All sedimentary carbonate reactions at the surface or during sediment burial take place in water: fresh water, sea water, or concentrated brines. Furthermore, the chemical composition of these fluids can evolve, for example during burial (sea water to brine), or uplift and exposure (sea water to fresh meteoric water). Such changes are commonly manifested as cement stratigraphy (e.g. high-Mg calcite overlain by low-Mg calcite), or where clasts, matrix and early cements are replaced by new generations of calcite or dolomite.

We are also interested in carbonate reactions because of the present trend towards climate change and the possibility that ocean chemistry may change; ocean acidification tops the list here. To get beyond the hyperbole, to determine whether this is a real possibility or not, we need to understand some basic chemistry. Some introductory concepts are outlined herein.

To delve deeper into the complexities of the carbonate system, have a read of the contributions cited below. Continue reading →

Mineralogy of carbonates; carbonate factories

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Carbonate sediment is produced in carbonate factories.

This is part of the of How To…series… on carbonate rocks

Most carbonate sediments are precipitated locally then redistributed across the sediment-water interface and through the water column; a kind of in-house production. Carbonate sediment is produced by biological and abiotic processes under conditions determined primarily by light intensity (the photic zone), temperature, salinity, nutrients, biologically available oxygen and water chemistry. The concept of carbonate factories was introduced in the late 1990s to focus attention on regions of benthic carbonate production where these environmental determinants reach a kind of nexus (some important contributions are listed at the end of this article).

The contribution to carbonate production by biotic and abiotic processes is shown in the diagram below. Abiotic production involves direct precipitation of calcite and aragonite and if truly abiotic, depends only on thermodynamic conditions such as temperature, ion activity and the degree of saturation with respect to each mineral, and free energy. It includes sea floor cements, whitings and ooids, although it is becoming increasingly apparent that the last two also involve biotic processes.

The involvement of biotic processes in carbonate production includes:

Biological activity that influences or mediates carbonate precipitation, but the minerals are not part of the organism’s structure. Thus, the metabolism of some bacteria, cyanobacteria and red and green algae can modify the local chemistry (e.g. alkalinity, pH, ion activity) and promote precipitation.
Organisms that produce calcium carbonate in shells, skeletons and rigid frameworks that are fundamental to their existence. In Tropical Factories, this includes a very large group of molluscs and other invertebrates like corals, bryozoans. All these critters are heterotrophs that derive their energy by consuming other organisms. The other important group are the autotrophs that manage their energy pathways using external sources like sunlight (i.e. photosynthesis) by creating molecules like carbohydrates that store energy. The autotrophic group includes bacteria, photosynthetic algae such as Halimeda and Penicillus and robust calcareous algae like Lithothamnion which, as we have seen, are important produces of lime mud.

The production lines
The rationale for introducing the concept of carbonate factories is based on the recognition of geologically and geographically recurring facies and associated biotic and abiotic production systems. An obvious advantage of the concept is to focus attention on the dominant environmental and biochemical controls on carbonate precipitation in ancient systems. One disadvantage is that subordinate processes might be sidelined.

An important contribution to the factory concept was published by Wolfgang Schlager (2000) who identified three benthic factories plus a planktic realm. A modified version of Schlager’s carbonate production scheme is shown here.

Tropical factories occupy shallow, warm waters where the primary environmental determinants are temperature and sunlight; biotic processes dominate. This factory is limited to the photic zone. Photosynthetic algae (photoautotrophs) play a critical supply role, either as primary producers or as mediators for heterotrophic producers like corals (their symbiotic algae). Invertebrates like molluscs and bryozoans also play an important but subordinate role.

Abiotic and biotically mediated precipitation is a significant component of the Tropical Factory – ooid shoals form platform-wide subaqueous bars and dunes, and whitings form extensive blankets of lime mud (the debate between biotic and abiotic processes already noted). Seafloor cementation is abiotically controlled and dominated by relatively rapid precipitation of aragonite and high-Mg calcite cements. Seafloor cements form in a range of water depths, from the intertidal zone to outer platform and slope. Intertidal cementation is commonly manifested as beachrock that can form in a matter of months.

Juxtaposition of Tropical and Cool-water factories is also possible. This can occur where cool-water biotas become dominant at depths and water temperatures where tropical biotas do not thrive, particularly below the thermocline.

Cool-water factories in modern oceans extend from about 30^o latitude to the polar regions, and at lower latitudes below the thermocline. They are dominated by heterotrophic biotas and can therefore extend to much greater water depths than their Tropical counterparts. Cool-water factories within the photic zone can also produce quantities of calcareous red algae. Deposits tend to be dominated by fragmented molluscs, barnacles, echinoderms, bryozoans, calcareous algae and some solitary Scleractinia corals. Benthic foraminifera are also common. Most deposits are distributed hydrodynamically.

There are no coral reefs, but bryozoans and oysters may accumulate as buildups and banks; good examples of oyster buildups are presented in the Oligocene Te Kuiti Group (New Zealand).

Sea floor cementation is far less common than in Tropical factories and where it does occur is dominated by low-Mg calcite.

Mud mounds are dominated by micrite precipitated biotically and abiotically. The name applies to three-dimensional mounds and to platform-wide buildups. Precipitation is commonly mediated by algae, bacteria and cyanobacteria and therefore is restricted to the photic zone. There is a cohesiveness to this sediment that prevents hydrodynamic distribution across the sea floor. Evidence in the rock record also indicates that some cementation took place in concert with deposition. Although cohesive and lithified, mud mounds are susceptible to erosion by storms, a common product of which is intraclast conglomerate.

There are several celebrated examples in the Phanerozoic, but it was in the Precambrian that cryptalgal mud buildups produced an extraordinary array of structures – from simple three-dimensional laminated mounds, to complex branched stromatolites and platform-wide buildups having several metres of synoptic relief (relief on the growing cryptalgal mat surface).

Planktic factories, the fourth in Schlager’s scheme, produce carbonate muds and oozes via ocean water masses. The primary producers here are phytoplankton and zooplankton that secrete calcite or aragonite tests. Common examples include coccoliths and foraminifera. They tend to accumulate in deep water, on parts of the sea floor that are devoid of terrigenous sediment. One example, celebrated in song, is the Cretaceous chalks of Dover.

The four factories here are perhaps the most basic. There is no doubt that some or all of them could be split into subfactories if you are inclined to splitting. It is also the case that there can be significant variation within any of the four, for example where a shallow platform is subjected to submarine freshwater discharge (springs) such that seawater is diluted locally, or where the opposite occurs in hypersaline conditions. In both examples the saturation state of calcite and aragonite may change drastically such forcing changes in the composition of benthic communities and abiotic cements.

Other posts in this series

Mineralogy of carbonates; classification

Mineralogy of carbonates; cements

Mineralogy of carbonates; diagenetic settings

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates: Stromatolite reefs

Some Important contributions

James, N.P., and Kendall, A.C., 1992. Introduction to carbonate and evaporite facies models. In Walker, R. G., and James, N. P. (eds.), Facies Models – Response to Sea Level Change. St. John’s: Geological Society of Canada, pp. 265–275.

Reijmer, J.J.G. 2016. Carbonate Factories. In: Encyclopedia of Marine Geosciences, Springer

Schlager, W. 2000. Sedimentation rates and growth potential of tropical, cool-water, and mud mound systems. In. E. Insalaco, P.W.Skelton, and T.J.Palmer (eds.). Carbonate Platform Systems: Components and Interactions. Geological Society London, Special Publication 178, p. 217-227.

Tucker, M.E. and Wright, V.P. 1990. Carbonate Sedimentology. Blackwell Science.

The mineralogy of carbonates; classification

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A post in the How to… series on carbonate mineralogy – limestone classification

The classification of carbonate rocks, like that of sandstones, has gone through a few iterations. Two schemes have stood the test of trial and error, in the field and through microscopes; both were compiled in the late 1950s – early 60s, each serves a slightly different purpose, both are still popular. They are the classification schemes of R. Folk (1959, 1962), and R. Dunham (1962). These two schemes form the backbone of modern carbonate classification and are summarized here, but keep in mind that quite a few iterations have been published.

Folk’s classification scheme

Folk’s scheme for carbonates is in some respects like the one he devised for terrigenous sandstones. It is based on the proportions of matrix, in this case lime mud, and framework components that are mostly allochems (grains that have been subjected to some degree of transport). At one end if the spectrum there is pure mud, or micrite; at the other end clast-supported frameworks with no mud. The spectrum of textures also reflects the energy of the depositional system. The pore volumes in clast-supported limestones are filled with cements, commonly sparry calcite mosaics, some with micritic cements. The degree of grain sorting in these sparites also increases. Grain size follows a relatively simple size-class like the Wentworth scale:

Lutites, 0.004 – 0.062 mm, that are approximately equivalent to mud (silt and clay)
Arenites, 0.062 – 1.00 mm (very fine- to coarse-grained sand)
Rudites, 1.00 mm to boulder range

Folk’s limestones are classified as either micrites or sparites with qualifiers added for the kinds of allochems (ooids, pelloids, fossils and intraclasts) and grain size. Identification of the allochems usually requires a microscope. Hence, Folk’s scheme is best applied to thin sections.

Folk extended this basic classification to include the percentages of micrite and spar cement (diagram below). The cut-off percentage between pure micrite and a micrite with allochems is 1%, 1-10% skeletal fragments is a fossiliferous micrite, 10-50% a sparse biomocrite, and >50% a packed biomicrite. Other qualifiers like pelloids and ooids use the same designations. Thus, an oosparite may be an unsorted oosparite, or if matrix-supported a sparse oomicrite.

Rigid structures like reefs and bioherms were placed in a category of their own – biolithite. The category dismicrite refers to micrite that has been disturbed by burrowing or erosion where voids have been filled by sparry calcite (i.e. disturbed micrite). However, it may be difficult to distinguish between a dismicrite and a micrite that has undergone partial recrystallization to calcite spar.

Dunham’s classification scheme

This scheme is more applicable to outcrop, hand specimen and drill core. It too is based on textural attributes but only those acquired during deposition (which means cements are excluded). His scheme uses three basic components:

The proportion of mud that differentiates between muddy limestones and grainstones (the latter having no mud),
The percentage of grains giving us a mudstone, wackestone or packstone, and
The presence of binding agents (mostly biological) giving us a boundstone.

Both Folk and Dunham apply a separate catch-all category of biolithite and boundstone respectively, for limestones containing fossils and inorganic structures bound by algal mats and more rigid frameworks like corals, stromatoporids and bryozoans. One could apply a qualifier, such as stromatoporoid boundstone, but this gives little paleoenvironmental information on the kinds of structures involved. While working on some Devonian reefs in the Canadian Arctic, Embry and Klovan realized that additional classification categories were required to fully describe the limestone structures they encountered. Their classification became an expansion of Dunham’s scheme, the basics of which have also stood the test of time, albeit with the odd modification.

Embry and Klovan added three new categories:

Bafflestone where organisms trap sediment. By their own admission, Embry and Klovan note that identification of organisms responsible for trapping is equivocal.
Boundstone where material is bound by encrusting and binding organisms, such as calcareous algae and cyanobacterial mats, and
Framestone that is constructed by framework-building organisms like corals and stromatoporids.

So which scheme does one use? The popularity of either scheme depends on which text or journal paper you read – some say Dunham’s scheme, others Folk’s scheme. It is also the case that the two are used interchangeably. If your study focuses on field and outcrop, then Dunham is the logical choice; if the focus is petrographic and thin section then Folk. And if you incorporate outcrop and microscope work, then perhaps use both schemes with cross-referenced rock names. To some extent it depends on personal preference.

Carbonate students should also look at an evaluation of these two popular schemes by Stephen Lokier and Mariam Junaibi. (2016). This paper (open access) looks at some of the modifications to both Folk and Dunham schemes. They find Dunham’s scheme (or modifications thereof) is the most popular. Their own modified scheme is shown below; note that Bafflestone is no longer used here.

*From Sedimentology, v 63, p. 1843-1885, Figure 12 – see link above)

Other post in this series:

Mineralogy of carbonates

Mineralogy of carbonates; carbonate factories