Category Archives: Interpreting ancient environments

The provenance of detrital zircon

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detrital zircon

This post is part of the How To…series – using zircon geochronology to decipher provenance

Zircon is a common accessory mineral in igneous and metamorphic rocks so it’s not surprising that it is also a common constituent of sedimentary heavy mineral suites. Detrital zircon has assumed a remarkable popularity over the last 2-3 decades as a provenance indicator because:

  • crystals contain measurable amounts of uranium (U), lead (Pb) and thorium (Th) isotopes and can therefore be dated radiometrically,
  • zircon is resistant to chemical and mechanical change – crystals can survive multiple sedimentary cycles (i.e. episodes of erosion from source rocks, deposition, burial and uplift, whereupon the whole process begins anew), and
  • they commonly contain multiple stages of crystal growth that record magmatic, metamorphic and depositional episodes.

Continue reading

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Provenance and plate tectonics

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This post is part of the How To…series – Provenance, sedimentary basins and plate tectonics

Deciphering the history of sedimentary basins is one of the more exciting tasks geologists can undertake; the provenance of sandstones plays an important part in this adventure.

Sedimentary basins are crustal structures. They are regions of long-term subsidence, responding to tectonic and sediment loads, cooling in the crust and upper mantle, and tectonic dislocation along crustal structures like transform faults. The processes that create sedimentary basins and the sediments that fill them are inextricably linked to plate tectonics.

The idea that the composition of sedimentary rocks was related to large-scale crustal processes was acknowledged by Charles Darwin, Charles Lyell, Henry Sorby and others, but it wasn’t until the 1950s – 60s that sedimentologists began to develop empirical models of sandstone provenance. Robert Folk, Francis Pettijohn, Robert Dott and contemporaries observed links between sediment composition and tectonic domains (or provinces), such as stable cratons and geosynclines – this was the era before plate tectonics.

For geoscientists, the discovery and development of plate tectonic theory changed everything. This is where William R. Dickinson comes into the picture. Dickinson recognized the fundamental link between sedimentary basins and plate tectonics, particularly at plate boundaries. He developed models that relate the modal composition of sandstones to plate tectonic provinces such as collision orogens, magmatic arcs, forearc basins and stable cratons. It is important to remember that these models are based on empirical evidence – analysis of 1000s of thin sections that he and many others had recorded from diverse locations.

The models are based on ternary plots like those used by Dott to classify sandstones. Dickinson and his co-workers used different combinations of the quartz, feldspar and lithics end-members to emphasize certain characteristics of the sediment and the source rocks. Both models shown here use the full suite of minerals. Other plots used only the lithic components, or polycrystalline quartz and lithics.

The Qt-F-L plot combines all varieties of quartz (mono- and polycrystalline quartz, including chert) as a single category and as such emphasizes the maturity of the sediment. Deposits with greater volumes of quartz are generally considered more mature, where mechanical and chemical weathering during sediment transport and deposition have removed less- stable components like feldspar and lithics.

In the Qm-F-L plot, polycrystalline quartz is shifted to the lithic field and in so doing emphasizes the source rocks and production of rock fragments (the quartz component consists only of monocrystalline varieties). Lithics here are key indicators of reworked orogenic provinces along continent-continent and continent-magmatic arc collision provinces. Here, erosion of sedimentary cover and volcanic rocks tends to produce greater proportions of rock fragments.

The second set of diagrams shows typical plate tectonic configurations that correspond to the various QFL fields. The diagrams are highly simplified. In addition to pigeonholing sandstone compositions, the plots provide a useful means of documenting systematic changes as uplift and erosion expose deeper crustal rocks.

 

 

For example, unroofing a fold and thrust belt along a collision margin will yield an initial rush of lithics derived from the deformed sedimentary cover. Gradual exposure of a metamorphic core will yield increasing volumes of quartz (mostly polycrystalline) and a new suite of heavy minerals. Likewise, unroofing a magmatic arc complex (the “dissected arc” field in these plots) will provide abundant volcanic lithics followed by more felsic sediment from the deeper intrusive rocks. This is shown schematically in the cartoon below.

The Dickinson plots, like any scientific model, are highly simplified versions of the real world. No two collisional orogens are alike, no two magmatic arcs are exact duplicates. Finding exceptions to any of the models does not indicate their failure – quite the opposite. Their value lies in providing a direction for investigation.

Dickinson’s models have been through several iterations, but their basic structure has survived 40 years of intense scrutiny by geoscientists. They are still useful starting points for unravelling the links among sediment composition, sedimentary basins and plate tectonics.

Check out the companion article – Provenance of sandstones

 

Some useful texts and papers:

Petrology of Sedimentary Rocks, Sam Boggs Jr. 2012

Dickinson, W. R. and C. A. Suczek, 1979, Plate tectonics and sandstone compositions: American Association of Petroleum Geologists Bulletin, v. 63, p. 2164–2182.

W. R. Dickinson, 1988. Provenance and Sediment Dispersal in Relation to Paleotectonics and Paleogeography of Sedimentary Basins. In New Perspectives in Basin Analysis, Editors, Karen L. Kleinspehn & Chris Paola,  Springer-Verlag, pp 3-25.

R.V. Ingersoll, T. F. Lawton, and S.A. Graham, 2018. Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson. Geological Society of America, Special Paper v.540, 757 pages

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The provenance of sandstones

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Provenance – the origin of sand

This post is part of the How To… series

See the companion posts – Provenance and plate tectonics

The provenance of zircon

Provenance is an investigative process that attempts to find out where things originated; a piece of art, an ancient manuscript, a sandstone. In geology, provenance usually applies to sediments and sedimentary rocks – where did they come from? At its most basic, provenance determines the kind of source rock that supplied loose sediment. But provenance can also provide information about:

  • Paleoclimate, particularly weathering of rock where the physical agents of erosion and chemical agents of mineral alteration, determine the volumes of sediment produced and its composition. For example, warm humid climates favour the formation of deep soil profiles where minerals like feldspar, and ferromagnesians like amphiboles and pyroxenes are susceptible to dissolution and alteration.
  • Paleotopography – was there mountainous relief (e.g. orogenic belts or volcanic arcs), or low relief across an ancient craton?
  • Ancient sediment transportation corridors such as terrestrial drainage systems or sediment distribution across a marine shelf or platform to the deep ocean,
  • Possible tectonic dislocation between source areas and sites of sedimentation (sedimentary basins), for example shortening across a mountain thrust belt, or lateral displacement along strike slip or transcurrent faults? Such dislocations may even involve tectonic plates or terranes that have moved 100s of kilometres, separating source rocks from the sedimentary basin.

  • Changes in mineralogy resulting from unroofing. For example, the upper crustal levels of magmatic arcs (including volcanoes) will produce abundant lithics and plagioclase, and lesser amounts of quartz. As uplift and erosion exposes (unroofs) deeper intrusive rocks, there may be a change to lithic-poor sediment with concomitant increases in quartz and or feldspar. Likewise, uplift and erosion of an orogenic belt may produce an initial pulse of sedimentary lithics and recycled quartz, that with gradual unroofing of a metamorphic core produces sediment laden with polycrystalline quartz and a new suite of heavy minerals, particularly micas.

It is instructive to look at suites of sedimentary rocks. This is important because there may be an opportunity to gauge regional trends in composition and texture in relation to changes in the source area (e.g. uplift), drainage, and depositional paleoslope (i.e. the regional dip of sedimentary basins). Your first task is to identify the mineralogy and textural properties like grain sorting and angularity.

Some of the problems associated with the determination of provenance are nicely illustrated using quartz arenites as an example. Quartz arenites contain 95% and more detrital quartz grains. They tend to be well sorted; grains are well rounded. Whence all that quartz? Some useful questions and considerations to begin your determination might be (the questions apply to sedimentary rocks of any composition):

  • Have the distribution and age of sedimentary units and potential source rocks been mapped in sufficient detail?
  • Stratigraphic-sedimentologic evidence may help pin-point potential sediment sources? Particularly useful are any mineralogical changes that coincide with down-dip facies changes or vertical stratigraphy.
  • Can regional paleocurrent trends be linked to drainage of potential source rock areas?
  • Do the potential source rocks contain sufficient amounts of quartz; do they contain the right kinds of quartz?
  • What is the mineralogy of the quartz? You will need to determine the proportions of monocrystalline and polycrystalline varieties, and whether the quartz has strained (i.e. deformed crystal lattice) or unstrained extinction. Is there any volcanic quartz?
  • The suite of heavy minerals can be very instructive. Common minerals include the ferromagnesian groups of like pyroxenes, amphiboles, micas, and olivine, plus iron oxides such as magnetite and ilmenite, and tourmalines, garnets and zircon. Most of these minerals occur in several different rock types (intrusive, metamorphic, volcanic), and on their own may not be diagnostic. Ferromagnesians are prone to alteration by weathering and during diagenesis. But with careful observation you should be able to tease useful provenance information from the suite. Of the micas, muscovite is probably the most useful in that it does not occur in extrusive volcanics but is common in metamorphic and intrusive rocks. Minerals like sillimanite and kyanite are almost exclusively metamorphic. Olivine (although uncommon as a heavy mineral), is particularly prone to alteration and in most cases will be a first cycle product that pinpoints mafic volcanic and intrusive source rocks.
  • Zircon is extremely durable and can survive two or more cycles of deposition, burial, metamorphism and uplift. Over the last 2 decades, Zircon has assumed a position of importance because technology now allows the determination of radiometric dates not just from whole single crystals, but from different parts of a crystal. Knowing zircon ages helps  pinpoint source candidates.

  • Are there any trends in textural properties or mineralogy along depositional dip or stratigraphically? Are there down-stream changes in grain size and angularity with distance from source? Has the proportion of less stable minerals been reduced? For example, if the primary source rock is granite there will be an initial mix of monocrystalline quartz, potassium feldspar and plagioclase, and heavy minerals like biotite and muscovite. Mechanical abrasion and chemical weathering will preferentially reduce the feldspar population.  The micas, even though they are denser than quartz or feldspar, will be preferentially removed because they behave like hydraulically light minerals. There will be a tendency for quartz to become dominant even though it originally was volumetrically subordinate to the feldspars.From your thin section observations and having considered some of the above, what are the potential source rock types? In answering this question, you need to keep in mind the likelihood that the sedimentary rock under the microscope may look nothing like its progenitor.

Having decided on likely source rocks and source areas, it is now time to consider provenance in relation to tectono-stratigraphic domains associated with plate tectonics – are the source rocks-source terrain and adjacent sedimentary basin part of an orogenic belt and foreland basin, an accretionary prism, a magmatic arc-trench complex?  A companion post examines the rationale behind these questions.

 

 

Some useful texts and papers:

Petrology of Sedimentary Rocks, Sam Boggs Jr. 2012

R.V. Ingersoll, 1978, Petrofacies and Petrologic Evolution of the Late Cretaceous Fore-Arc Basin, Northern and Central California. The Journal of Geology Vol. 86, pp. 335-352

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The mineralogy of sandstones – matrix & cement

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SEM of quartz overgrowths

This post is part of the How To… series – How to identify matrix and cement in arenites

The primary architectural element of a sandstone is its framework of sand grains. Spaces between grains will be filled, to varying degrees by much finer sediment – or matrix. The amount of matrix initially deposited is strongly dependent on the energy of the depositional system and the degree of sediment reworking. Wind-blown dune sands and beach sands will have very little matrix at the time of deposition; those deposited farther out to sea will tend to accumulate more.

The proportion of matrix to framework is an important determinant of sandstone classification.  Arenites have less than 15% matrix, wackes have more than 15%. Sandstones with little matrix (commonly referred to as clean sands) have the potential to preserve some of their original porosity.

Detrital matrix generally consists of clay minerals mixed with silt-sized quartz and feldspar.  At the point of deposition there is a significant volume of interstitial water, most commonly seawater or freshwater.  The mix of solid and liquid phases means that matrix is highly reactive in both mechanical and chemical contexts. Compaction that begins soon after deposition and continues during burial, will physically compress the matrix and at the same time drive off some of the interstitial fluid. As burial proceeds, the increase in temperature and accompanying fluid flow will promote chemical reactions involving dissolution of some detrital minerals (particularly clays and feldspar) and precipitation of new minerals.  Thus, the matrix gradually changes from purely detrital to a mix of detrital components and diagenetic products. At some point in this transformation there may be no recognisable detrital matrix. Continue reading

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The mineralogy of sandstones: feldspar grains

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Identifying detrital feldspar.

This post is part of the How To… series

Quartz may be the most common mineral in sandstone, but feldspar is the most abundant mineral in pretty well every other rock type; in fact, it is the most abundant mineral in the Earth’s crust. Unlike quartz, feldspar is an essential ingredient in nearly all igneous rocks, felsic through ultrabasic. It begins to crystallize in magmas at temperatures about 1000oC – 200o warmer than quartz crystallization. Feldspars are also common in metamorphic rocks. As such, feldspar is an important (usually subordinate) component of most terrigenous clastics, reflected in its inclusion in QFL classification schemes.

The two major groups of feldspar are potassium feldspars and plagioclases. All have low relief in plain polarized light (similar to quartz). Both groups have two good cleavage planes at 90o to each other such that broken crystal fragments tend to be blocky. Twinning is common. Continue reading

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The mineralogy of sandstones: Quartz grains

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This post is part of the How To… series – quartz mineralogy in sandstones

Classification of terrigenous sandstones depends on the identification of two main components: framework grains and matrix. Frameworks are represented by a QFL triad – quartz, feldspar and lithic fragments, where the proportion of each grain type is determined from thin section.  Most classification schemes aggregate all types of quartz, feldspar and lithics into each end-member. This approach is sensible and easy to use.

But simply naming a sandstone (or any rock type for that matter) is not enough. We also want to know about its provenance, the sediment source or sources – was it a stable continent or active mountain belt, volcanic arc or ocean basin, perhaps a far-travelled terrane or tectonic sliver for which the only evidence is the collection of grains that have survived multiple cycles of attrition.

Teasing this information from the rocks requires us to delve into the mineralogy in greater detail. The simplest and cheapest way to do this is with thin sections and a polarizing microscope. We begin with the most common terrigenous component – quartz. Continue reading

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Classification of sandstones

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sandstone classification

This post is part of the How To… series – how to classify sandstones using QFL plots

In science, classification of things is one of those tasks we readily identify as a crucial component of knowledge but prefer that someone else does it. Classification schemes don’t just name things, they organize them according to their properties, appearance, structure, composition.  If I wish to talk about a particular rock or fossil, then the people who are interested in such things will have a frame of reference to understand and contribute to the discussion, based on whatever classification scheme applies.

The classification of sandstones matured in the 1940s through 1960s; many publications were devoted to the subject; some of the key players were Robert Folk, Harvey Blatt, Francis Pettijohn, Raymond Siever, P. Krynine, E. McBride, H. William, F Turner, C Gilbert, Robert Dott, and R. Fisher. Several schemes were proposed and debated; few were accepted. One of the central topics of discussion was the relative importance of sandstone texture versus sandstone composition. A classification based on texture alone was deemed inadequate; if the rock or sediment had >50% sand, then it was a sandstone, or arenite. Qualifications such as pebbly, silty or muddy might be applied, but this said nothing about the variability of mineral types. However, textural properties such as the percentage of matrix (clay plus silt) did provide grounds for distinguishing between ‘clean’ sandstones (i.e. those lacking significant matrix) and wackes – those rocks containing significant matrix. Continue reading

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Some controls on grain size distributions

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poorly sorted conglomerate

This post is part of the How To… series

What processes determine the size distribution of clasts in clastic sediments? Why is it that dune sands tend to be very well sorted, river deposits less so, and at the other end of the spectrum, turbidites on a submarine fan are left with no sorting at all? In this article we will look briefly at three determinants of grain size distributions: inheritance, depositional hydraulics, and post-depositional changes.  The three determinants are discussed in greater detail by R.L. Folk under the heading Textural Maturity (a PDF of the 1980 issue can be downloaded here)

Inheritance
All those grains of sand, pebbles and cobbles come from somewhere, from other sediments or source rocks. Erosion and weathering of rock generally results in a wide range of clast sizes – from large blocks to silt and finer. During sediment transport, particularly in fluvial, alluvial and high energy coastal systems, the larger fragments are whittled to smaller fragments. However, there are situations where the source contains a limited range of clast sizes which means that sediment sourced from these deposits will also have similar grain size limitations. The example of modern beach and dunes sands illustrated in the previous post is a case in point. The siliciclastic component of the modern sands is derived from consolidated and weakly lithified Pleistocene deposits that have similar, if not identical grain size distributions. The grain size characteristics of the modern sands are inherited from the older deposits.

Hydraulics
Environments of deposition are strong determinants of grain size and grain size distributions in clastic sediments. The primary control here is hydraulics – the strength and longevity of water-wind currents and waves acting on a sediment bed. The strength of a flowing medium determines the size of clasts being moved; in general the stronger the flow, the coarser or heavier the clasts that will be transported (note that size here depends on mass and density).

However, the strength of the flowing medium alone is not the only determinant; the longevity and/or repetition of flow is also critical. For example, turbidity currents and debris flows are basically single depositional events. For contrast, compare sediments in river channels where flow is continuous (albeit fluctuating). Sediment movement along the length of a river commonly produces a down-stream increase in finer sediment fractions, where much of the coarser material (especially gravel) remains upstream. Wave washing along coasts results in repetitive sediment movement, such that grains may travel many kilometres in the swash and backwash and yet never leave the beach.  In both these examples the potential for episodic sediment movement is high. We refer to this process as reworking.

There is a direct correlation between sediment sorting and the degree of reworking; well sorted sands have generally been subjected to high degrees of reworking (notwithstanding the possibility of inheritance). For any given flow velocity there is a maximum grain size that can be moved across a sediment surface; smaller or lighter grains will move across the bed (or in suspension), coarser or heavier grains will not move. Thus, sediment is sorted according to size and mass; lighter or smaller grains are separated or winnowed from coarser-heavier grains. The more frequently this process occurs, the greater the degree of grain sorting.

Movement of sediment also results in mechanical wear and tear of clasts (abrasion). Prolonged abrasion during reworking will ultimately reduce the size of clasts. This process depends primarily on the mechanical strength of clasts. Quartz and feldspar (the two most common components of terrigenous clastic rocks and sediments) react differently during prolonged reworking; quartz is mechanically stable and although grain sizes may become smaller over time, grains survive several cycles of deposition and reworking. Feldspars on the other hand tend to break along crystal cleavage planes. Thus, a sediment that that originally contains equal amounts of quartz and feldspar can, following prolonged reworking, become a well sorted quartz sand with little or no feldspar.

Post-depositional changes
Diagenetic changes that specifically effect the size distribution of grain populations can involve either dissolution (size reduction) or precipitation (enlargement) of the more common rock-forming minerals such as quartz, feldspar and carbonate.

Pleistocene shallow marine deposits in northernmost New Zealand contain bivalve moulds and casts but no calcium carbonate. Post-depositional leaching of the original coarse carbonate size fraction has created deposits that now are as well-sorted as the associated ancient dune deposits.

Calcium carbonate is not the only mineral component affected by leaching in these Pleistocene deposits; feldspar grains too show a remarkable degree of dissolution, resulting in size reduction and even complete removal. Thus, diagenetic changes have skewed the overall grain size distribution towards that of the surviving quartz grain population.

Grain size enlargement also occurs in more lithified deposits.  This commonly takes the form of crystal overgrowths on quartz and feldspar grains.  Mineral overgrowth not only changes the size of clasts, but also their textural properties such as shape and angularity. Care needs to be taken when observing disaggregated sands to distinguish these post-depositional changes from original depositional textural attributes. The most reliable way to do this is using thin-sections and a polarizing microscope.

Some other useful links

Describing sedimentary rocks; some basics

Analysis of sediment grain size distributions

The hydraulics of sedimentation: Flow Regime

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Analysis of sediment grain size distributions

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modern beach and foredunes

This post is part of the How To… series

The graphical display and statistical analysis of sediment grain size became a popular pursuit of sedimentologists in the 1950s and 60s, particularly those who studied modern sediments. The science of grain size distributions developed in concert with rapidly evolving concepts of sedimentary facies and a more sophisticated approach to interpreting ancient depositional environments. As such, grain size analysis was seen as a possible addition to a sedimentologist’s toolbox. It didn’t quite work out as planned – but that’s a tale for the next blog.

Clast sizes in gravels and conglomerates can be measured directly. For semi- and unconsolidated sands and silts, mechanical sieving is still the preferred method.  Wire-mesh sieves are stacked, coarsest mesh on top; mesh sizes are commonly spaced at 1.0, 0.5 or 0.25 phi intervals depending on the range of size classes in your sample. Each sieve will retain sediment that is coarser than the mesh size; grains with a minimum diameter less than the mesh diameter will pass through to the next sieve.  A dry sample of known weight is placed in the top sieve and the sieve stack is placed in a mechanical shaker for 10 to 15 minutes. Sieve shakers are notoriously noisy so a sound-proof cupboard is a good idea. Sieve mesh openings range from about 40 microns (4.5ϕ – coarse silt) to 125mm (cobbles).  The USGS has a couple of short videos demonstrating this method.

sediment sieves

Once shaken, the contents of each sieve are weighed. The cumulative weight from all sieves including the pan should be within 1-2% of the original weight. The weight from each phi interval is converted to a percent of the total weight.

Graphical presentation of the data provides a visual picture of size distribution in each sample and (importantly) the interpolated phi values used to calculate statistical measures. The simplest plot is a histogram of frequency and phi. Examples from modern beach and foredune sands are shown below. The changes in size distribution from low tide to foredunes are nicely represented – there is an obvious (and not unexpected) coarsening towards the high energy surf zone.

grain size of beach and foredune sands

The data can also be plotted on either arithmetic or log templates as cumulative weight percent against the phi values of the corresponding sieve. In most sediments the bulk of the grains tend to cluster round a narrow range of size classes. Log plots emphasize this central tendency and are therefore the preferred graphing method. In the examples below, cumulative weight percents are plotted on the log scale and phi values on the linear x-axis (recall that the phi values themselves are log transformations). Each curve shows a dominant straight-line segment that represents the log-normal distribution of grain sizes for about 95% of each sample. The remaining 5% in the ‘tails’ departs from log normal.

log plots of grain size

Some basic statistics for each sample can now be calculated: the mean and median grain size (which are measures of central tendency), sorting (that is an expression of standard deviation), and skewness which describes the asymmetry of frequency curves or histograms. The formulae use phi values corresponding to the specified percentile as shown in the diagram above. The most commonly used formulae, developed by R.L. Folk and W. Ward (Journal of Sedimentary Research (1957) 27 (1): 3-26) are:

Median     The phi value at the 50 percentile (ϕ50)

Mean        Mϕ = [ϕ16 + ϕ50 + ϕ84] /3

Sorting      σϕ = (ϕ84 – ϕ16 /4) + (ϕ95 – ϕ5 / 6.6)

Skewness Sk = [(ϕ16 + ϕ84 – 2 ϕ50) /2(ϕ84 – ϕ16)]  +  [(ϕ5 + ϕ95 – 2 ϕ50) / 2(ϕ95 – ϕ5)]

An additional measure of central tendency is the mode, which is the phi value of the most abundant size class.  Note that median, mode, mean and sorting have units of phi; skewness is a dimensionless number.

Median and mode are useful descriptors of sediments but they do not convey as much information about the conditions of deposition as mean and sorting.  Mean values represent the most common sizes classes in a sample and may give an indication of the prevalent current strength.

Sorting measures the spread of size classes about the mean. In the beach-foredune example shown above, wind strength is strong enough to move sand but not the coarser shell material that has been selectively removed. Folk and Ward also devised a sorting scale based on calculated phi values.

sorting and skewness scales

The foredune and upper beach samples in our example are very well sorted, whereas the lower beach samples are moderately well sorted.

grain size skewness

Note that when quoting a mean, it is important to also include the sorting value (standard deviation). The example above shows three samples that have the same mean grain size but clearly are very different sediments.

Skewness describes the symmetry of grain size distributions, or more commonly the departure from a symmetry. Skewness takes the ‘tails’ of the frequency curve into account, such that a surplus of fine material produces a positively skewed histogram (or smoothed curve), and an excessive coarse tail a negative skew – the classification of skewness proposed by Folk and Ward is shown in the chart.

Some other useful links

Describing sedimentary rocks; some basics

Grain size of clastic rocks and sediments

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Grain size of clastic rocks and sediments

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grain size of beach sediment

This post is part of the How To… series

You have collected a sandstone that has been described as coarse-grained.

  • What does ‘coarse’ refer to?
  • What do we mean by ‘grain size’? and
  • What parameters do we measure to make such a determination?

Grains, whether they are silt, sand or gravel, assume myriad shapes. Grain shape is determined by several factors, perhaps the most important being:

  • Crystal properties such as habit and cleavage in the case of common minerals like quartz, feldspar or mica,
  • Original texture or fabric in the parent rock from which the grains are derived,
  • The mechanical and chemical stability of grains during transport, and
  • Post-depositional changes like compaction (important in soft lithic fragments), size reduction by mineral dissolution or size enlargement by precipitation.

The problem of defining grain size boils down to one of defining the most useful shape. The image of a beach deposit at the top of the page serves to illustrate this.  Clasts here include those that approximate spheres, are elongate, rod-like, blocky cubes, and those that are more flattened. To determine their size, do we measure maximum, intermediate or minimum diameters? Do we calculate their surface areas? Or do we assume they all approximate spheres and use a diameter that best fits each grain?

The answer – all of the above. Years of experience and experimentation have shown show that the most useful grain size measures depend on what it is we want to study (see the generalized list of analytical methods appended to the USGS Chart, below).

For example, if we are interested in how the sedimentary particles behave hydraulically (e.g. in channel flow or across a wind-blown sand dune) then we could choose a measure that reflects resistance to flow or drag. If we want a more descriptive measure for loose or disaggregated sand and gravel, then we might apply a sieve diameter (the minimum diameter that will pass through a particular sieve aperture). Measurement of very fine particles (clays) that uses pipettes or settling columns, relies on particle attributes such as settling velocity through a column of water, or with the advent of new laser technology, the light-scattering properties of clay-sized particles in a dispersion (with water) or solid state (laser diffraction and laser particle sizer).

In 1922 Chester Wentworth devised a grain size scale based on a geometric progression – this scale is still the most popular (The Journal of Geology, Vol. 30, No. 5 1922, pp. 377-392). Wentworth followed the reasoning advanced earlier by J. Udden, where a geometric progression based on ‘2’ (rather than a linear scale) is sensible because:

  1. It allows a useful subdivision of grain sizes in silts and sands (the most common clastic rock types),
  2. The grain size in samples that represent single grain populations tend to plot as straight lines on log and semilog graphs.
  3. In addition, we now know that small changes in the grain size of sands and silts have hydraulic significance and hence it is very useful to incorporate these size classes into our general classification. In comparison, small changes in the size of cobbles and boulders have little hydraulic significance.

For the Wentworth scale, each successive size class is twice the size in inches of the previous class – hence the geometric progression 1/16, 1/8, ¼, ½, 1, 2, 4 and so on (these days we work in millimeters).

Note that ‘clay’ in this context refers to a size class, and not a mineral class.

A modification of this scale was devised by Krumbein – the well-known Phi Scale (ϕ), that is calculated using the expression

Φ = -Log2 of the grain size in millimeters

The Phi scale simplifies size measurement (sieve mesh sizes usually quote a Phi interval), the graphical representation of grain size populations, and calculation of statistical measures like mean and sorting. The handy USGS chart above shows a comparison of grain size classes in relation to sieve sizes, settling velocities, and threshold velocities for initiation of grain movement.

We now have the tools to describe the grain size of clastic rocks where size classes like coarse-grained sand(stone) have corresponding size ranges expressed in millimeters and Phi values.

Some other useful links

Describing sedimentary rocks; some basics

Analysis of sediment grain size distributions

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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