Monthly Archives: May 2019

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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Describing sedimentary rocks – some basics

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This post is part of the How To… series

The formation of sediment and sedimentary rock involves many physical, chemical and biological processes, sometimes operating separately but more commonly in concert. The journey from loose sediment to hammer-ringing rock is one of the marvels of the geological world. Deciphering this journey requires us to delve into the rock record. To do this we need a starting point – we need to know what it is we’re looking at and be able to communicate this information to others.

Asking how one describes sedimentary rocks is a bit like questions about bits of string; there is a bewildering array of them, each deserving a rational description. Fortunately, the guidelines for this task are well established. We have a century and a half of sedimentology icons to thank for this: Sorby, Wentworth, Twenhofel, Folk, Pettijohn, Potter, and Bathurst, to name a few.

Decades of collective experience allows us to place sedimentary rocks into four broad categories:

  1. Siliciclastic (or terrigenous) deposits contain fragments derived by erosion or weathering of older rocks. The fragments range in size from house-sized blocks to micron-sized clays – conglomerate and breccia (gravel in an unconsolidated state), sandstone-siltstone (sand and silt), and mudrocks (mixtures of silt, and clay-sized fragments or crystals). They represent the recycling of material across Earth’s surface. Some sediment components can survive more than one cycle of deposition, burial and induration (hardening), uplift, and erosion – quartz grains fit this category well, as do grains of zircon because of their chemical stability and hardness that enables them to withstand the rigours of erosion and deposition.
  2. Sediments derived primarily by biogenic processes (biological and biochemical) include carbonates (limestones and dolostones), peat, coal, and other accumulations of organic matter such as oil shale, and phosphates. These deposits may also contain fragmental debris, such as bioclastic fragments in limestones (e.g. coral, molluscs), but unlike multicycle siliciclastics the primary constituents are directly associated with organic processes; they are mostly first-cycle deposits.
  3. Chemical deposits include those formed directly by precipitation from saline water. The most common deposits in this category include evaporites (halite, gypsum), iron formations (particularly those in the Precambrian), and siliceous deposits that form chert (in Precambrian iron formations iron oxide precipitates and chert are commonly found together). As a general rule the environments of precipitation are low energy where the influx of siliciclastic sediment is low.
  4. Volcaniclastics represent fragmental debris derived from volcanic eruptions. This includes air-fall ash (tephras) and pyroclastic flow deposits (e.g. hot and cold ignimbrites) that are derived directly from eruption events, and material that is redeposited by lahars or subaqueous sediment gravity flows (turbidites, debris flows).

Like most classification schemes, particularly one as all-encompassing as this, there are exceptions – in fact exceptions are quite common. Mixed siliciclastic-carbonate deposits are common in stratigraphic intervals that are transitional between carbonate-dominated and siliciclastic-dominated environments.  Precambrian iron formations, like the Gunflint Formation in southern Ontario, may contain coarse-grained fragmental deposits and stromatolites that indicate high energy shoreline or shallow marine environments. Volcaniclastic debris that is redeposited may incorporate siliciclastic or biogenic carbonate material. We could deal with all these exceptions with a bit of fine tuning, subdividing each category into any number of variants, but I’ll leave that for others to do (I’m more a lumper than a splitter).

The posts that follow deal with sedimentary rock description at the hand-sample – outcrop scale. The descriptors were invented not just for the sake of description, but to provide fundamental links to depositional processes such as hydraulics, and the mechanical winnowing or sorting of grains based on shape and density. Our descriptions make use the following physical sediment properties:

  • Colour. This may seem pretty basic but colour provides valuable information about environmental conditions such as REDOX chemistry (i.e. the reduction-oxidation potential of chemical constituents), particularly the oxidation state of iron. For example, iron is in a reduced 2+ state in green mudrocks, and an oxidized state (3+) in terrestrial red beds.
  • Texture. This is a broad term that includes qualities like clast framework, and quantities like grain size and shape, and grain size distributions from which we can estimate statistical measures of central tendency, and standard deviations that sedimentologists refer to as sorting. Note that the term ‘grain’ is used here to mean any kind and size of clast, such as siliciclastic sand and gravel, or carbonate bioclasts and oolites.
  • Fabric, is a textural property that describes the orientation of clasts (including fossils). Common fabrics include parting lineation that develops from the alignment of individual sand grains, and imbrication of platy clasts in gravels. Sedimentary fabrics like these record paleocurrent flow directions.
  • Porosity is a measure of the void, or pore space between adjacent grains. At the point of deposition, all sediments have porosity. Weakly indurated rocks tend to retain some of their original porosity, but this decreases as compaction and cements develop.

Sediment framework and matrix. The literature on sedimentology is replete with references to attributes like “clast-supported framework”. What does this mean? Frameworks in this context refer to the essential ingredients that make something a sandstone, a conglomerate, a coquina – in other words the sand grains, pebbles cobbles and boulders, and bioclasts, respectively. Sediment framework is basically all the large bits.

Matrix refers to the fine-grained sediment, commonly clay, silt and fine sand sized material that occurs between or separates framework clasts. Matrix is a depositional product; it accumulates with the framework. Matrix clays are prone to alteration during burial diagenesis.

          

These definitions may seem a bit vague. We tend to make the distinction between framework and matrix only in ‘coarse-grained’ deposits. Trying to decide what is framework in mudrocks is next to impossible – they are mostly matrix.  Given this seemingly arbitrary distinction, why do we bother? The main reason is that identifying these textural properties helps us decipher depositional processes. Consider the two conglomerates shown above. The one on the left consists of clasts that are in direct contact such that the framework is clast-supported; the interstitial matrix here is a small amount of mud and sand. The second example also contains pebbles and cobbles but in this case they are separated by muddy sediment; they appear to ‘float’ in the matrix – this is a matrix-supported framework. In each case, the hydraulic conditions associated with their deposition were very different. On the left, flowing water was energetic and continuous enough to move gravel and remove most of the finer sediment.  On the right, the flow was energetic enough to move the gravel but not continuous enough to remove the matrix. The clast-supported conglomerate was deposited by a braided river where sediment was washed and reworked by strong currents, and the matrix-supported conglomerate by a submarine debris flow in which pebbles and cobbles were thoroughly mixed and transported as a muddy torrent – in this case there was no mechanism to remove the matrix.

Keep in mind that all the above properties can change during sediment burial. Diagenetic features are usually determined by examining thin sections with a polarizing microscope. Physical compaction and chemical diagenesis can:

  • Change the organization of grains one to the other (i.e. their framework),
  • Change the shape of softer grains,
  • remove some grains by dissolution (e.g. fossil fragments, or feldspars) – this is one mechanism that creates secondary porosity,
  • result in an increase in grain size as a result of precipitation; common examples of this are quartz overgrowths on quartz grains, and
  • produce cements that replace matrix and in some cases framework clasts.

Some other useful posts in this series:

Grain size of clastic rocks and sediments

Analysis of grain size distributions

Some controls on grain size distributions

Measuring a stratigraphic section

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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