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Controls on the nature of loess particles and the formation of loess deposits
Quaternary International xxx (2017) 1e5
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Quaternary International journal homepage: www.elsevier.com/locate/quaint
Controls on the nature of loess particles and the formation of loess deposits Ian Smalley a, *, Slobodan B. Markovic b a b
Giotto Loess Research Group, School of Geology, Geography & the Environment, University of Leicester, Leicester, LE1 7RH, UK Department of Geography, University of Novi Sad, Trg Dositeja Obradovica 3, RS-21000, Novi Sad, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2017 Received in revised form 13 July 2017 Accepted 7 August 2017 Available online xxx
The story (history) of a single loess particle can be told from the formation of the planet to the making of bricks. The modal loess particle chosen for study is a 30 mm quartz particle. Many factors bear on its complex history, starting with the formation of quartz units in the original granitic crustal rocks. Quartz units appear in the old granites as the product of a eutectic-like reaction. This controls particle size and ensures that sand is much the same wherever (and whenever) it is encountered. The eutectic reaction controls sand size; the silt size is controlled by crystalline defects which are introduced into the quartz by the high-low displacive transformation which the quartz undergoes as the system cools. Thus all quartz silt is much the same, as is the case for sand. The quartz clastic particles are surprisingly bi-modal, a more even size distribution might have been expected. The silt particles are largely formed by glacial grinding, by continental and mountain glaciers. These silt particles are distributed across the landscape by the action of large rivers, and deposited on floodplains. A critical stage in the story is the aeolian transportation of the particles which then form an airfall deposit. The glacially provided silt particles are quite close in size to the optimum for aeolian pick-up (~80 mm). At this stage, on wind-fall deposition, the characteristic metastability is introduced into the loess deposit. The chief post-depositional process is the development of collapsibility, which is largely due to the movement of clay minerals to the main particle contacts. This low clay content of most loess deposits makes loess an ideal material for brick production. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Loess particle Eutectic reactions High-low quartz transformation Stages in loess deposit formation Post-depositional changes
“It is so difficult to find the beginning. Or better: it is difficult to begin at the beginning, and not try to go further back.” Ludwig Wittgenstein ‘On Certainty’
1. Introduction A loess deposit is an airfall deposit, with an open metastable structure, largely composed of silt sized quartz particles (Fig. 1). There are many other constituents in a loess deposit, clay, carbonates, other minerals etc, but the predominant unit appears to be a silt sized quartz particle. The mode particle might be described as a particle of low-quartz (a-quartz) with a nominal diameter of around 30 mm. It is this particle which is considered in this paper.
* Corresponding author. E-mail addresses: [email protected] (I. Smalley), [email protected] (S.B. Markovic).
The story of a deposit is encapsulated in the story of a particle. The idea has been deployed before; Zalasiewicz (2010) explored the story of Earth's deep history via a single pebble of grey Silurian slate from a beach in Wales. The idea of ‘story’ has gained some traction in Earth Science scholarship since the study by Phillips (2012) of the eight basic plots in geo-accounts and geo-stories. Of the eight basic plots two at least may be relevant to the story of all the influences which combine to shape the basic loess particle and to deliver it into position in a contemporary loess deposit. The human ‘story’ of loess begins in the Rhine Valley with Von Leonard and Lyell (see Jovanovic et al., 2013) so it seems reasonable that a chosen representative loess particle might come from the Kaiserstuhl close to the original observations (Smalley et al., 1973). The two basic plots are ‘cause and effect’ and ‘genesis’. In the cause and effect section there are the factorial studies. “whereby the phenomenon of interest is described, modelled, or interpreted on the basis of multiple controlling factors, though the latter may include relatively static or passive controls as well as processes or external changes. The best known is Jenny's (1941) factorial model of soil formation, but the approach is common in
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Please cite this article in press as: Smalley, I., Markovic, S.B., Controls on the nature of loess particles and the formation of loess deposits, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.021
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2. Eutectic (and related) reactions
Fig. 1. The particles of loess; a view by Scheidig (1934, p.73, Abb.57). This is the loess dust (Loesstaub) from Honan. The scale bar intervals are 100 mm. A clear early view of loess material.
both implicit and explicit forms in several areas of Earth and environmental sciences.” (Phillips, 2012 p.156). This ’sequence of factors' approach fits rather nicely with the idea of studying loess deposit formation via a sequence of definable events (Smalley, 1966b) in which provenance (P) events were succeeded by transport (T) events and deposition (D) events. This led to the so-called ‘Hardcastle sequence’ of P1T1D1T2D2 which was seen as the very basic, simplest sequence of events which lead to the deposition of the main deposits on the South Island of New Zealand (Smalley and Fagg, 2014). The up-to-date sequence of events is defined and discussed in Smalley et al. (2014). The genesis section is relevant. “Genesis stories in Earth science are concerned with the origin of phenomena. The key research question involves the creation or origin of a specific feature, or a specific type or class of features. The most obvious examples are studies of well known but unexplained phenomena, locally or regionally anomalous features, or features whose origin is disputed or controversial.” (Phillips, 2012 p.156.) Applied to loess the genesis story pertains to the formation of a single loess particle, the formation of loess material in general, the formation of a specific deposit (at the Kaiserstuhl, or the brickworks), or the formation of loess systems all over the World. The aim of this paper is to consider the formation of loess; of loess material and loess deposits, and to identify the key stages in complex processes. It is necessary to explain the nature of loess, and the location and distribution of the loess deposits, and in doing so make sure that some of the earlier, hitherto neglected eventssuch as the eutectic reaction control of sand formation and the high-low quartz transition control of silt formation, are fully discussed.
The story for the loess particle, as for the Silurian pebble of Zalasiewicz (2010), starts far back in Earth history. In fact the loess particle story starts further back in deep time than any Silurian pebble. A loess particle is a quartz particle and the quartz is formed as the primitive crust is forming. The processes that led to the formation of the planet caused there to be quantities of oxygen, silicon and aluminium in the planetary mass, and a proportion of these lighter elements, in the processes of chemical differentiation, found their way to the upper planetary region and formed the Earth's crust. Just a few kilometres thick but with a concentration of oxygen, silicon and aluminium, and serving as a source of sediments. The classic primitive crustal rock is granite. The simple view of granite is that it is a mixture of feldspars and quartz. A careful scientific view would reveal many other minerals, and other elements besides the essential three, but in terms of formative phase geochemistry it is the feldspars and the quartz which determine the essential nature of granite, and it is a eutectic-type reaction (Fig. 2) between the felspars and quartz, the last of the formative geochemical reactions, that produces the quartz material in the form in which it becomes sand and silt. In a fairly simple granite, perhaps similar to the granite of the batholiths in south-western England, one can imagine primary crystals of feldspar surrounded by a eutectic-type mixture of quartz and feldspar. In the Cornish granites the feldspar can weather to kaolinite (and be exploited) but there is a large amount of by-product sand. The nature of the sand is determined by the eutectic-type reaction in the granite. This is a somewhat neglected reaction but it provides the quartz sand for the beaches and deserts of the world. The key size control for quartz sand is the eutectic-type reaction in the crustal granites. The eutectic-type reaction deserves a closer look, and a somewhat more precise examination. It is the eutectic reaction at the low salt end of the water-salt (NaCl) system which is exploited by city councils when they spread salt on icy roads. It is the eutectic reaction in the lead-tin system which was used for many years by plumbers when joining lead pipes, and it now appears that the carbon-oxygen system may have an important eutectic reaction which influences the behaviour of white dwarf stars (Stevenson, 1980). The reaction between quartz and feldspar which is invoked in the granite forming process is very much a eutectic-like reaction; it is a eutectoid reaction rather than a true eutectic reaction and its function, from the point of view of the study of sand and loess, is to provide a size control. Its exact nature is not
Fig. 2. Phase equilibrium diagram for a typical two component eutectic system. The minimum in the liquidus is typical of eutectic systems. This is a very idealised picture, all real systems have some solid solution formation.
Please cite this article in press as: Smalley, I., Markovic, S.B., Controls on the nature of loess particles and the formation of loess deposits, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.021
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perceived but it is understood to operate in a way which produces a mixed feldspar-quartz product of restricted size. 3. The high-low transition in quartz The granites contain quartz, largely in the form of the product of a eutectic-type reaction between silica and feldspars. As the granite system cools the eutectic reaction defines the essential nature of the proto-quartz particles. As the system continues to cool these discrete quartz units undergo a crystalline transformation from high quartz (b quartz) to low quartz (a quartz, Figs. 3 and 4). The change of the bond angle at the oxgen ions allows for a slight structural contraction, a slight increase in density. This increase in density causes the development of widespread tensile stresses in the rock system. These tensile stresses cause defects to occur in the quartz structure, which will subsequently affect the formation of silt particles. (Smalley, 1966a). The quartz transition reaction has been studied by Pereira et al. (2014, Fig. 5). As a weathering front passes through the granite the sand sized quartz particles are released into the sedimentary system. The internal stresses in the rock system promote the production of sand. These sand sized particles carry internal stresses and defects which, if activated, lead to the formation of silt particles. The ‘sand’ particles in the granite are of a similar size and as they undergo the high-low transformation a similar level of stress is introduced into each particle. These largely determine the nature of the eventual silt and give it its ‘modal’ quality. The two fundamental processes operating in the granitic systems ensure that the quartz clastic particles eventually produced are distinctly bimodal in size; there is a sand mode and a silt mode. The shapes of these two modal populations are interesting and distinctive. The sand particles are fairly equi-axed, the eutectic reaction does not introduce a particular shape preference, but the silt particles tend to be flatter, to be blade shaped. The breakage process between sand and silt can introduce a shape factor (Rogers and Smalley, 1993). The eutectic factor controls the size of quartz sand particles and ensures that quartz sand is much the same wherever it is encountered. The size control on quartz silt particles needs more
Fig. 4. The high-low transition in quartz. On cooling the high quartz(b) converts into the slightly denser low quartz(a); the essential structure stays the same but the orientation of the silica tetrahedral changes. This is a displacive transformation. Most transitions in the silica system are reconstructive (e.g. tridymite to quartz). The structural re-arrangement in the displacive transformation appears trivial but the slight change in density has major consequences.
Fig. 5. The expansion of quartz and related minerals with respect to temperature. Note the remarkable variation in the quartz curve, this transition activity at around 600 C controls quartz silt production; after Pereira et al. (2014).
investigation and explanation. The quartz silt size control is as universal as the sand size control and ensures that there is a large quartz silt mode in terrestrial sediments. The contraction stress defects in quartz sand particles control silt formation; since all sand particles are more or less the same and the stress arrives via the universal high-low transformation. The same stresses in the same size sand particles give the same product; the control is effective. The mechanism is not fully explored but the output is universally observed. 4. Glacial action
Fig. 3. The p-T diagram for silica. From the point of view of silt formation the critical phase boundary is between high and low quartz. The transition temperature increases with pressure and would operate in the granite formation scenario.
Glacial grinding has been proposed as the chief method for producing loess particles (Smalley, 1966b). The concept has evolved and changed a little since it was first proposed. Initially glacial grinding was an idea attached to the action of large continental glaciers; loess deposits considered in relation to ice sheets. It was initially imagined that the vast geo-energy supplied by the very
Please cite this article in press as: Smalley, I., Markovic, S.B., Controls on the nature of loess particles and the formation of loess deposits, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.021
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large ice sheets might be needed to crush the very hard resistant quartz crystals in the crustal rocks. Now it is realised that the initial quartz particles, far from being strong and adamant, are in fact full of defects which render them relatively weak and easily comminuted. More modest sources of geo-energy can supply abundant loess particles, which allows the more modest mountain glaciers to become the chief suppliers. This has vast stratigraphic and palaeolclimatological implications because the mountain glaciers have a continuous existence, as distinct from the extreme periodicity of the continental glaciers, and can supply a variable amount of loess material- depending on the climatic conditions prevailing. It should be noted that the glacial grinding concept did not actually originate with Smalley (1966b) but was well described and delineated by Hardcastle (1889) in New Zealand. The Hardcastle contribution has been discussed elsewhere (Smalley and Fagg, 2014). Some preliminary model studies on glacial grinding have been carried out by O'Hara-Dhand et al. (2015) using a modified Bromhead ring-shear machine as a model glacier. The crushing events are shown in Fig. 6: stage 1 is initial dilatant expansion as the quartz sand test material is initially stressed, then particle deformation begins. Stage 2 is the activation of macro-defects in the sand particles, and stage 3 is the key stage where the Moss defects are activated and large scale breakage occurs. Stage 3 represents the transition from sand to silt. Further crushing is difficult to achieve and the system heads for the comminution limit. (see also Smalley, 2014). If low quartz was an isotropic material then the crushing and grinding to produce silt particles might be considered as a random process. The application of simple probability theory allows the expected shapes of silt particles to be calculated (Smalley, 1966c; Rogers and Smalley, 1993; Howarth, 2010, 2011). The mode particle turns out to be a very flat blade shaped particle with an 8:5:2 aspect ratio. So the typical quartz particle is likely to be a very flat particle. Krinsley and Smalley (1973) did suggest that observations indicated a preponderance of very flat particles, and the calculations support this. Allowance should be made for the low quartz not being isotropic, but it is almost isotropic, the veering into anisotropy is very slight. The shape of loess particles is important because it contributes to the metastable nature of the loess soil structure
and the collapsibility which is one of the key defining factors for loess at large. 5. Transportation and deposition Rivers transport loess material before it is in position for that final aeolian movement to form the characteristic loess deposit (Smalley et al., 2009). The Rhine is a loess river, so is the Danube. There are many great rivers which have contributed to the supply of loess material and the location of loess deposits. It might be argued that the position of major loess deposits depends on the actions of large rivers delivering loess material into the required vicinity. Rivers supply the T1 stage of the Hardcastle sequence. They might deliver loess material into a holding area where it is stored for a long time before being moved on for the next stages of the process. A lot of the confusion over ‘desert’ loess was caused by the failure to appreciate the importance of these storage zones. In a desert situation such as Central Asia the P1 material from High Asia is carried out into the deserts for initial long-term deposition. Eventually it is raised by the wind and carried to the formation region for a loess deposit (Smalley et al., 2006). In the Hardcastle sequence P1 and T1 are succeeded by D1. The D1 event has not received much attention or been the subject of much discussion. It sits between two very significant events; T1 which is the widespread distribution of loess material by big rivers, and T2 which is the all-important aeolian transportation to form the open structured loess deposit. By pure chance the ideal size for particles to be picked up by the wind is not far from the actual size of loess particles as demarcated by the early high-low transitions in the granitic quartz. Many years ago it was pointed out that the optimum size for a particle to be picked up by the wind was around 80 mm (Bagnold, 1941). This optimum size is a compromise between cohesive forces and weight forces; the smaller particles stick together, the larger particles become too heavy to lift into suspension (although they can travel by saltation). The interaction between the cohesive and weight forces can be examined (e.g Smalley, 1964) and the size at the flowstick transition determined. The fact that loess particles have a mode size of around 30 mm suggests that the wind is selecting from a not-totally homogeneous deposit. Presumably if all particle sizes were available loess deposits would consist of 80 mm particles. It can be argued that the mixed ground is dominated by two particle sizes, two modes in the sand and silt ranges. For transportation in suspension, such a key part of the loess formation process, the wind chooses the available particles nearest to the 80 mm optimum. Previous events mean that this is around 30 mm; the available particles are transported and form metastable loess deposits. Material falls. There are other deposits where material has been formed by aeolian action but loess is the classic deposit where stuff falls from the sky. Same for volcanic dust clouds but not the same for sand dunes where the material is blown about but can hardly be said to fall. Loess falls, and the falling produces the characteristic nature of loess. Loess has an open structure, it is a highly porous deposit, each individual particle plays its part in the formation of an impressive three-dimensional network. 6. Post-depositional events
Fig. 6. Breakage of quartz particles in a modified Bromhead ring-shear machine, acting as a model glacier (from O'Hara-Dhand et al., 2015). The sample is effectively crushed and the diminution in height indicates stages of the crushing process. 1, initial dilatant expansion; 2, activation of macro-defects in sand sample particles; 3, crushing proper, this is the critical stage where Moss defects are activated and the sand to silt reduction takes place; 4e7 relatively trivial further deformation and compaction. After the production of the silt particles large stresses are required to produce further significant deformation.
The day after the aeolian deposit is formed it could be called loess. A geologist would call it loess; a pedologist might hesitate, might call it ‘parent material’ and consider the subsequent formation of a soil. Some classic soil formation operations take place in loess parent materials. The most obvious and impressive of these is ‘chernozemization’ the formation of chernozems (now incorporated into the mollisol group is USDA Soil Taxonomy). A chernozem
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is essentially loess þ organic material, perhaps with some carbonate movement involved. Fragipans are another pedological favourite which forms in loess (see Smalley and Davin, 1982). A claim that fragipans are exclusive to loess would be disputed but there can be no doubt that they prefer to form in loess ground. This is a significant statement because it relates to a specific mechanism that has been proposed for fragipan formation (Bryant, 1989). 7. Commentary The first detailed look at actual loess particles came when scanning electron microscopy became available (Cegla et al., 1971; Smalley and Cabrera, 1970; Smalley, 1970). We can produce a set of generalisations about loess particles which are probably still reasonable. The mode size at about 30 mm; a narrow distribution of size- loess is a modal material. The popular mineralogy is quartz. The shape of the ideal loess particle is still being discussed (Howarth, 2010). The shape is important because it contributes to the openness of the airfall deposit, and thus to the characteristic metastability, and to collapsibility. A truly collapsible material has to have somewhere to collapse into- this available collapsing space is provided by the combination of the airfall deposition mechanism and the extreme blade shaped particles. Rogers and Smalley (1993) calculated that the default shape of a loess particle was a Zingg 3 m blade shape with axial proportions of 8:5:2- a remarkably flat, tabular particle. Earlier, using simple probabilistic methods Smalley (1966c) had calculated that 72% of the particles should have blade shapes, and Krinsley and Smalley (1973) had gone some way towards verifying this observationally. 8. Conclusions A scientifically satisfying account of loess would explain all the factors that led to the nature of the material, its transportation and deposition in the landscape, and the significant post-depositional events which are reflected in its current status. Reasonable explanations now appear to be available for (1) the provision of quartz material for the default quartz particles, (2) the formation of sandsized quartz particles in granitic crustal rocks via eutectic-like reactions, (3) the operation of tensile stresses caused by the high-low quartz transition to control silt particle size, (4) the distribution of loess material in the landscape by rivers, (5) the aeolian deposition of silty material to form the metastable loess deposit, (6) the developments of collapsibility, (7) the formation of pedological features such as chernozem soils and fragipans in loess ground. There may be other features which will require to be identified and investigated, but the seven listed tell the basic loess story. For the future: investigate the ways in which the temporal signal is incorporated into loess materials and loess deposits; develop a further understanding of loess as a ‘climate register’.
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References Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265p. Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck, N.E., Ciolkosz, E.J. (Eds.), Fragipans: Their Occurrence, Classification & Genesis. Soil Science Society of America Special Publication 24. Cegla, J., Buckley, T., Smalley, I.J., 1971. Microtextures of particles from some European loess deposits. Sedimentology 17, 129e134. Hardcastle, J., 1889. Origin of the loess deposits of the Timaru plateau. Proc. Trans. N. Z. Inst. 22, 406e414. Howarth, J.J., 2010. The shape of loess particles reviewed. Central Eur. J. Geosci. 2, 41e44. Howarth, J.J., 2011. A commentary on the shape of loess particles assuming a spatial exponential distribution for the cracks in quartz. Open Geosci. 3, 231e234. Jenny, H.A., 1941. The Factors of Soil Formation. McGraw-Hill, New York, 224p. Jovanovic, M., Gaudenyi, T., Smalley, I.J., 2013. Karl Caesar von Leonhard (1779-1862) and the beginnings of loess research in the Rhine valley. Quat. Int. 334/5, 4e9. Krinsley, D.H., Smalley, I.J., 1973. Shape and nature of small sedimentary quartz particles. Science 180, 1277e1279. O'Hara-Dhand, K., McLaren, S., Frost, M., Smalley, I.J., Bentley, S.P., 2015. Breakage of Quartz Sand Particles Controlled by Internal Defects: Model Glacier Studies. Scribd.com upload 25 May 2015. Pereira, A.H.A., Miyaji, D.Y., Cabrelton, M.D., Medeivos, J., Rodrigues, J.A., 2014. A study about the contribution of the a-b phase transition of quartz to the thermal cycle damage of a refractory used in fluidized catalytic cracking units. Ceramica 60 (355). http://dx.doi.org/10.1590/S0366-69132014000300019. Phillips, J., 2012. Storytelling in Earth sciences: the eight basic plots. Earth Sci. Rev. 115, 153e162. Rogers, C.D.F., Smalley, I.J., 1993. The shape of loess particles. Naturwissenschaften 80, 461e462. Scheidig, A., 1934. Der Loess und seine geotechnischen Eigenschaften. Theodor Steinkopff, Dresden u. Leipzig, 233p. Smalley, I.J., 1964. Flow-stick transition in powders. Nature 201, 173e174. http:// dx.doi.org/10.1038/201173a0. Smalley, I.J., 1966a. Formation of quartz sand. Nature 211, 476e479. Smalley, I.J., 1966b. The properties of glacial loess and the formation of loess deposits. J. Sediment. Res. 36, 669e676. Smalley, I.J., 1966c. The expected shapes of blocks and grains. J. Sediment. Res. 36, 626e629. Smalley, I.J., 1970. Calcium carbonate encrustations on quartz grains in loess from the Karlsruhe region. Naturwissenschaften 57, 87 only. Smalley, I.J., 2014. Glacial Loess: Revisited, Reviewed, Reconsidered. Loess Ground blog 26 September 2014. Smalley, I.J., Cabrera, J.G., 1970. The shape and surface texture of loess particles. Geol. Soc. Am. Bull. 81, 1591e1596. Smalley, I.J., Davin, J.E., 1982. Fragipan Horizons in Soils: a Bibliographic Study and Review of Some of the Hard Layers in Loess and Other Materials, vol. 30, p. 122. New Zealand Soil Bureau Bibliographic Report. Smalley, I.J., Fagg, R., 2014. John Hardcastle looks at the Timaru loess; Climatic signals are observed, and fragipans. Quat. Int. 372, 51e57. Smalley, I.J., Krinsley, D.H., Vita-Finzi, C., 1973. Observations on the Kaiserstuhl loess. Geol. Mag. 110, 29e36. Smalley, I.J., Mavlyanova, N.G., Rahkmatullaev, Kh.L., Shermatov, M.Sh., Machalett, B., O'Hara-Dhand, K., Jefferson, I.F., 2006. The formation of loess deposits in the Tashkent region and parts of Central Asia: and problems with irrigation, hydrocollapse and soil erosion. Quat. Int. 152/153, 59e69. Smalley, I.J., O'Hara-Dhand, K., Wint, J., Machalett, B., Jary, Z., Jefferson, I.F., 2009. Rivers and loess: the significance of long river transformation in the complex event sequence approach to loess deposit formation. Quat. Int. 198, 7e18. Smalley, I.J., O'Hara-Dhand, K., Kwong, J., 2014. China: materials for a loess landscape. Catena 117, 100e107. Stevenson, D.J., 1980. A eutectic in carbon-oxygen white dwarfs. J. de Phys. Colloques 41, c261ec264. Zalasiewicz, J., 2010. The Planet in a Pebble: a Journey into Earth's Deep History. Oxford University Press, 234p.
Please cite this article in press as: Smalley, I., Markovic, S.B., Controls on the nature of loess particles and the formation of loess deposits, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.021
Contents lists available at ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Controls on the nature of loess particles and the formation of loess deposits Ian Smalley a, *, Slobodan B. Markovic b a b
Giotto Loess Research Group, School of Geology, Geography & the Environment, University of Leicester, Leicester, LE1 7RH, UK Department of Geography, University of Novi Sad, Trg Dositeja Obradovica 3, RS-21000, Novi Sad, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2017 Received in revised form 13 July 2017 Accepted 7 August 2017 Available online xxx
The story (history) of a single loess particle can be told from the formation of the planet to the making of bricks. The modal loess particle chosen for study is a 30 mm quartz particle. Many factors bear on its complex history, starting with the formation of quartz units in the original granitic crustal rocks. Quartz units appear in the old granites as the product of a eutectic-like reaction. This controls particle size and ensures that sand is much the same wherever (and whenever) it is encountered. The eutectic reaction controls sand size; the silt size is controlled by crystalline defects which are introduced into the quartz by the high-low displacive transformation which the quartz undergoes as the system cools. Thus all quartz silt is much the same, as is the case for sand. The quartz clastic particles are surprisingly bi-modal, a more even size distribution might have been expected. The silt particles are largely formed by glacial grinding, by continental and mountain glaciers. These silt particles are distributed across the landscape by the action of large rivers, and deposited on floodplains. A critical stage in the story is the aeolian transportation of the particles which then form an airfall deposit. The glacially provided silt particles are quite close in size to the optimum for aeolian pick-up (~80 mm). At this stage, on wind-fall deposition, the characteristic metastability is introduced into the loess deposit. The chief post-depositional process is the development of collapsibility, which is largely due to the movement of clay minerals to the main particle contacts. This low clay content of most loess deposits makes loess an ideal material for brick production. © 2017 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Loess particle Eutectic reactions High-low quartz transformation Stages in loess deposit formation Post-depositional changes
“It is so difficult to find the beginning. Or better: it is difficult to begin at the beginning, and not try to go further back.” Ludwig Wittgenstein ‘On Certainty’
1. Introduction A loess deposit is an airfall deposit, with an open metastable structure, largely composed of silt sized quartz particles (Fig. 1). There are many other constituents in a loess deposit, clay, carbonates, other minerals etc, but the predominant unit appears to be a silt sized quartz particle. The mode particle might be described as a particle of low-quartz (a-quartz) with a nominal diameter of around 30 mm. It is this particle which is considered in this paper.
* Corresponding author. E-mail addresses: [email protected] (I. Smalley), [email protected] (S.B. Markovic).
The story of a deposit is encapsulated in the story of a particle. The idea has been deployed before; Zalasiewicz (2010) explored the story of Earth's deep history via a single pebble of grey Silurian slate from a beach in Wales. The idea of ‘story’ has gained some traction in Earth Science scholarship since the study by Phillips (2012) of the eight basic plots in geo-accounts and geo-stories. Of the eight basic plots two at least may be relevant to the story of all the influences which combine to shape the basic loess particle and to deliver it into position in a contemporary loess deposit. The human ‘story’ of loess begins in the Rhine Valley with Von Leonard and Lyell (see Jovanovic et al., 2013) so it seems reasonable that a chosen representative loess particle might come from the Kaiserstuhl close to the original observations (Smalley et al., 1973). The two basic plots are ‘cause and effect’ and ‘genesis’. In the cause and effect section there are the factorial studies. “whereby the phenomenon of interest is described, modelled, or interpreted on the basis of multiple controlling factors, though the latter may include relatively static or passive controls as well as processes or external changes. The best known is Jenny's (1941) factorial model of soil formation, but the approach is common in
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2. Eutectic (and related) reactions
Fig. 1. The particles of loess; a view by Scheidig (1934, p.73, Abb.57). This is the loess dust (Loesstaub) from Honan. The scale bar intervals are 100 mm. A clear early view of loess material.
both implicit and explicit forms in several areas of Earth and environmental sciences.” (Phillips, 2012 p.156). This ’sequence of factors' approach fits rather nicely with the idea of studying loess deposit formation via a sequence of definable events (Smalley, 1966b) in which provenance (P) events were succeeded by transport (T) events and deposition (D) events. This led to the so-called ‘Hardcastle sequence’ of P1T1D1T2D2 which was seen as the very basic, simplest sequence of events which lead to the deposition of the main deposits on the South Island of New Zealand (Smalley and Fagg, 2014). The up-to-date sequence of events is defined and discussed in Smalley et al. (2014). The genesis section is relevant. “Genesis stories in Earth science are concerned with the origin of phenomena. The key research question involves the creation or origin of a specific feature, or a specific type or class of features. The most obvious examples are studies of well known but unexplained phenomena, locally or regionally anomalous features, or features whose origin is disputed or controversial.” (Phillips, 2012 p.156.) Applied to loess the genesis story pertains to the formation of a single loess particle, the formation of loess material in general, the formation of a specific deposit (at the Kaiserstuhl, or the brickworks), or the formation of loess systems all over the World. The aim of this paper is to consider the formation of loess; of loess material and loess deposits, and to identify the key stages in complex processes. It is necessary to explain the nature of loess, and the location and distribution of the loess deposits, and in doing so make sure that some of the earlier, hitherto neglected eventssuch as the eutectic reaction control of sand formation and the high-low quartz transition control of silt formation, are fully discussed.
The story for the loess particle, as for the Silurian pebble of Zalasiewicz (2010), starts far back in Earth history. In fact the loess particle story starts further back in deep time than any Silurian pebble. A loess particle is a quartz particle and the quartz is formed as the primitive crust is forming. The processes that led to the formation of the planet caused there to be quantities of oxygen, silicon and aluminium in the planetary mass, and a proportion of these lighter elements, in the processes of chemical differentiation, found their way to the upper planetary region and formed the Earth's crust. Just a few kilometres thick but with a concentration of oxygen, silicon and aluminium, and serving as a source of sediments. The classic primitive crustal rock is granite. The simple view of granite is that it is a mixture of feldspars and quartz. A careful scientific view would reveal many other minerals, and other elements besides the essential three, but in terms of formative phase geochemistry it is the feldspars and the quartz which determine the essential nature of granite, and it is a eutectic-type reaction (Fig. 2) between the felspars and quartz, the last of the formative geochemical reactions, that produces the quartz material in the form in which it becomes sand and silt. In a fairly simple granite, perhaps similar to the granite of the batholiths in south-western England, one can imagine primary crystals of feldspar surrounded by a eutectic-type mixture of quartz and feldspar. In the Cornish granites the feldspar can weather to kaolinite (and be exploited) but there is a large amount of by-product sand. The nature of the sand is determined by the eutectic-type reaction in the granite. This is a somewhat neglected reaction but it provides the quartz sand for the beaches and deserts of the world. The key size control for quartz sand is the eutectic-type reaction in the crustal granites. The eutectic-type reaction deserves a closer look, and a somewhat more precise examination. It is the eutectic reaction at the low salt end of the water-salt (NaCl) system which is exploited by city councils when they spread salt on icy roads. It is the eutectic reaction in the lead-tin system which was used for many years by plumbers when joining lead pipes, and it now appears that the carbon-oxygen system may have an important eutectic reaction which influences the behaviour of white dwarf stars (Stevenson, 1980). The reaction between quartz and feldspar which is invoked in the granite forming process is very much a eutectic-like reaction; it is a eutectoid reaction rather than a true eutectic reaction and its function, from the point of view of the study of sand and loess, is to provide a size control. Its exact nature is not
Fig. 2. Phase equilibrium diagram for a typical two component eutectic system. The minimum in the liquidus is typical of eutectic systems. This is a very idealised picture, all real systems have some solid solution formation.
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perceived but it is understood to operate in a way which produces a mixed feldspar-quartz product of restricted size. 3. The high-low transition in quartz The granites contain quartz, largely in the form of the product of a eutectic-type reaction between silica and feldspars. As the granite system cools the eutectic reaction defines the essential nature of the proto-quartz particles. As the system continues to cool these discrete quartz units undergo a crystalline transformation from high quartz (b quartz) to low quartz (a quartz, Figs. 3 and 4). The change of the bond angle at the oxgen ions allows for a slight structural contraction, a slight increase in density. This increase in density causes the development of widespread tensile stresses in the rock system. These tensile stresses cause defects to occur in the quartz structure, which will subsequently affect the formation of silt particles. (Smalley, 1966a). The quartz transition reaction has been studied by Pereira et al. (2014, Fig. 5). As a weathering front passes through the granite the sand sized quartz particles are released into the sedimentary system. The internal stresses in the rock system promote the production of sand. These sand sized particles carry internal stresses and defects which, if activated, lead to the formation of silt particles. The ‘sand’ particles in the granite are of a similar size and as they undergo the high-low transformation a similar level of stress is introduced into each particle. These largely determine the nature of the eventual silt and give it its ‘modal’ quality. The two fundamental processes operating in the granitic systems ensure that the quartz clastic particles eventually produced are distinctly bimodal in size; there is a sand mode and a silt mode. The shapes of these two modal populations are interesting and distinctive. The sand particles are fairly equi-axed, the eutectic reaction does not introduce a particular shape preference, but the silt particles tend to be flatter, to be blade shaped. The breakage process between sand and silt can introduce a shape factor (Rogers and Smalley, 1993). The eutectic factor controls the size of quartz sand particles and ensures that quartz sand is much the same wherever it is encountered. The size control on quartz silt particles needs more
Fig. 4. The high-low transition in quartz. On cooling the high quartz(b) converts into the slightly denser low quartz(a); the essential structure stays the same but the orientation of the silica tetrahedral changes. This is a displacive transformation. Most transitions in the silica system are reconstructive (e.g. tridymite to quartz). The structural re-arrangement in the displacive transformation appears trivial but the slight change in density has major consequences.
Fig. 5. The expansion of quartz and related minerals with respect to temperature. Note the remarkable variation in the quartz curve, this transition activity at around 600 C controls quartz silt production; after Pereira et al. (2014).
investigation and explanation. The quartz silt size control is as universal as the sand size control and ensures that there is a large quartz silt mode in terrestrial sediments. The contraction stress defects in quartz sand particles control silt formation; since all sand particles are more or less the same and the stress arrives via the universal high-low transformation. The same stresses in the same size sand particles give the same product; the control is effective. The mechanism is not fully explored but the output is universally observed. 4. Glacial action
Fig. 3. The p-T diagram for silica. From the point of view of silt formation the critical phase boundary is between high and low quartz. The transition temperature increases with pressure and would operate in the granite formation scenario.
Glacial grinding has been proposed as the chief method for producing loess particles (Smalley, 1966b). The concept has evolved and changed a little since it was first proposed. Initially glacial grinding was an idea attached to the action of large continental glaciers; loess deposits considered in relation to ice sheets. It was initially imagined that the vast geo-energy supplied by the very
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large ice sheets might be needed to crush the very hard resistant quartz crystals in the crustal rocks. Now it is realised that the initial quartz particles, far from being strong and adamant, are in fact full of defects which render them relatively weak and easily comminuted. More modest sources of geo-energy can supply abundant loess particles, which allows the more modest mountain glaciers to become the chief suppliers. This has vast stratigraphic and palaeolclimatological implications because the mountain glaciers have a continuous existence, as distinct from the extreme periodicity of the continental glaciers, and can supply a variable amount of loess material- depending on the climatic conditions prevailing. It should be noted that the glacial grinding concept did not actually originate with Smalley (1966b) but was well described and delineated by Hardcastle (1889) in New Zealand. The Hardcastle contribution has been discussed elsewhere (Smalley and Fagg, 2014). Some preliminary model studies on glacial grinding have been carried out by O'Hara-Dhand et al. (2015) using a modified Bromhead ring-shear machine as a model glacier. The crushing events are shown in Fig. 6: stage 1 is initial dilatant expansion as the quartz sand test material is initially stressed, then particle deformation begins. Stage 2 is the activation of macro-defects in the sand particles, and stage 3 is the key stage where the Moss defects are activated and large scale breakage occurs. Stage 3 represents the transition from sand to silt. Further crushing is difficult to achieve and the system heads for the comminution limit. (see also Smalley, 2014). If low quartz was an isotropic material then the crushing and grinding to produce silt particles might be considered as a random process. The application of simple probability theory allows the expected shapes of silt particles to be calculated (Smalley, 1966c; Rogers and Smalley, 1993; Howarth, 2010, 2011). The mode particle turns out to be a very flat blade shaped particle with an 8:5:2 aspect ratio. So the typical quartz particle is likely to be a very flat particle. Krinsley and Smalley (1973) did suggest that observations indicated a preponderance of very flat particles, and the calculations support this. Allowance should be made for the low quartz not being isotropic, but it is almost isotropic, the veering into anisotropy is very slight. The shape of loess particles is important because it contributes to the metastable nature of the loess soil structure
and the collapsibility which is one of the key defining factors for loess at large. 5. Transportation and deposition Rivers transport loess material before it is in position for that final aeolian movement to form the characteristic loess deposit (Smalley et al., 2009). The Rhine is a loess river, so is the Danube. There are many great rivers which have contributed to the supply of loess material and the location of loess deposits. It might be argued that the position of major loess deposits depends on the actions of large rivers delivering loess material into the required vicinity. Rivers supply the T1 stage of the Hardcastle sequence. They might deliver loess material into a holding area where it is stored for a long time before being moved on for the next stages of the process. A lot of the confusion over ‘desert’ loess was caused by the failure to appreciate the importance of these storage zones. In a desert situation such as Central Asia the P1 material from High Asia is carried out into the deserts for initial long-term deposition. Eventually it is raised by the wind and carried to the formation region for a loess deposit (Smalley et al., 2006). In the Hardcastle sequence P1 and T1 are succeeded by D1. The D1 event has not received much attention or been the subject of much discussion. It sits between two very significant events; T1 which is the widespread distribution of loess material by big rivers, and T2 which is the all-important aeolian transportation to form the open structured loess deposit. By pure chance the ideal size for particles to be picked up by the wind is not far from the actual size of loess particles as demarcated by the early high-low transitions in the granitic quartz. Many years ago it was pointed out that the optimum size for a particle to be picked up by the wind was around 80 mm (Bagnold, 1941). This optimum size is a compromise between cohesive forces and weight forces; the smaller particles stick together, the larger particles become too heavy to lift into suspension (although they can travel by saltation). The interaction between the cohesive and weight forces can be examined (e.g Smalley, 1964) and the size at the flowstick transition determined. The fact that loess particles have a mode size of around 30 mm suggests that the wind is selecting from a not-totally homogeneous deposit. Presumably if all particle sizes were available loess deposits would consist of 80 mm particles. It can be argued that the mixed ground is dominated by two particle sizes, two modes in the sand and silt ranges. For transportation in suspension, such a key part of the loess formation process, the wind chooses the available particles nearest to the 80 mm optimum. Previous events mean that this is around 30 mm; the available particles are transported and form metastable loess deposits. Material falls. There are other deposits where material has been formed by aeolian action but loess is the classic deposit where stuff falls from the sky. Same for volcanic dust clouds but not the same for sand dunes where the material is blown about but can hardly be said to fall. Loess falls, and the falling produces the characteristic nature of loess. Loess has an open structure, it is a highly porous deposit, each individual particle plays its part in the formation of an impressive three-dimensional network. 6. Post-depositional events
Fig. 6. Breakage of quartz particles in a modified Bromhead ring-shear machine, acting as a model glacier (from O'Hara-Dhand et al., 2015). The sample is effectively crushed and the diminution in height indicates stages of the crushing process. 1, initial dilatant expansion; 2, activation of macro-defects in sand sample particles; 3, crushing proper, this is the critical stage where Moss defects are activated and the sand to silt reduction takes place; 4e7 relatively trivial further deformation and compaction. After the production of the silt particles large stresses are required to produce further significant deformation.
The day after the aeolian deposit is formed it could be called loess. A geologist would call it loess; a pedologist might hesitate, might call it ‘parent material’ and consider the subsequent formation of a soil. Some classic soil formation operations take place in loess parent materials. The most obvious and impressive of these is ‘chernozemization’ the formation of chernozems (now incorporated into the mollisol group is USDA Soil Taxonomy). A chernozem
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is essentially loess þ organic material, perhaps with some carbonate movement involved. Fragipans are another pedological favourite which forms in loess (see Smalley and Davin, 1982). A claim that fragipans are exclusive to loess would be disputed but there can be no doubt that they prefer to form in loess ground. This is a significant statement because it relates to a specific mechanism that has been proposed for fragipan formation (Bryant, 1989). 7. Commentary The first detailed look at actual loess particles came when scanning electron microscopy became available (Cegla et al., 1971; Smalley and Cabrera, 1970; Smalley, 1970). We can produce a set of generalisations about loess particles which are probably still reasonable. The mode size at about 30 mm; a narrow distribution of size- loess is a modal material. The popular mineralogy is quartz. The shape of the ideal loess particle is still being discussed (Howarth, 2010). The shape is important because it contributes to the openness of the airfall deposit, and thus to the characteristic metastability, and to collapsibility. A truly collapsible material has to have somewhere to collapse into- this available collapsing space is provided by the combination of the airfall deposition mechanism and the extreme blade shaped particles. Rogers and Smalley (1993) calculated that the default shape of a loess particle was a Zingg 3 m blade shape with axial proportions of 8:5:2- a remarkably flat, tabular particle. Earlier, using simple probabilistic methods Smalley (1966c) had calculated that 72% of the particles should have blade shapes, and Krinsley and Smalley (1973) had gone some way towards verifying this observationally. 8. Conclusions A scientifically satisfying account of loess would explain all the factors that led to the nature of the material, its transportation and deposition in the landscape, and the significant post-depositional events which are reflected in its current status. Reasonable explanations now appear to be available for (1) the provision of quartz material for the default quartz particles, (2) the formation of sandsized quartz particles in granitic crustal rocks via eutectic-like reactions, (3) the operation of tensile stresses caused by the high-low quartz transition to control silt particle size, (4) the distribution of loess material in the landscape by rivers, (5) the aeolian deposition of silty material to form the metastable loess deposit, (6) the developments of collapsibility, (7) the formation of pedological features such as chernozem soils and fragipans in loess ground. There may be other features which will require to be identified and investigated, but the seven listed tell the basic loess story. For the future: investigate the ways in which the temporal signal is incorporated into loess materials and loess deposits; develop a further understanding of loess as a ‘climate register’.
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References Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265p. Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck, N.E., Ciolkosz, E.J. (Eds.), Fragipans: Their Occurrence, Classification & Genesis. Soil Science Society of America Special Publication 24. Cegla, J., Buckley, T., Smalley, I.J., 1971. Microtextures of particles from some European loess deposits. Sedimentology 17, 129e134. Hardcastle, J., 1889. Origin of the loess deposits of the Timaru plateau. Proc. Trans. N. Z. Inst. 22, 406e414. Howarth, J.J., 2010. The shape of loess particles reviewed. Central Eur. J. Geosci. 2, 41e44. Howarth, J.J., 2011. A commentary on the shape of loess particles assuming a spatial exponential distribution for the cracks in quartz. Open Geosci. 3, 231e234. Jenny, H.A., 1941. The Factors of Soil Formation. McGraw-Hill, New York, 224p. Jovanovic, M., Gaudenyi, T., Smalley, I.J., 2013. Karl Caesar von Leonhard (1779-1862) and the beginnings of loess research in the Rhine valley. Quat. Int. 334/5, 4e9. Krinsley, D.H., Smalley, I.J., 1973. Shape and nature of small sedimentary quartz particles. Science 180, 1277e1279. O'Hara-Dhand, K., McLaren, S., Frost, M., Smalley, I.J., Bentley, S.P., 2015. Breakage of Quartz Sand Particles Controlled by Internal Defects: Model Glacier Studies. Scribd.com upload 25 May 2015. Pereira, A.H.A., Miyaji, D.Y., Cabrelton, M.D., Medeivos, J., Rodrigues, J.A., 2014. A study about the contribution of the a-b phase transition of quartz to the thermal cycle damage of a refractory used in fluidized catalytic cracking units. Ceramica 60 (355). http://dx.doi.org/10.1590/S0366-69132014000300019. Phillips, J., 2012. Storytelling in Earth sciences: the eight basic plots. Earth Sci. Rev. 115, 153e162. Rogers, C.D.F., Smalley, I.J., 1993. The shape of loess particles. Naturwissenschaften 80, 461e462. Scheidig, A., 1934. Der Loess und seine geotechnischen Eigenschaften. Theodor Steinkopff, Dresden u. Leipzig, 233p. Smalley, I.J., 1964. Flow-stick transition in powders. Nature 201, 173e174. http:// dx.doi.org/10.1038/201173a0. Smalley, I.J., 1966a. Formation of quartz sand. Nature 211, 476e479. Smalley, I.J., 1966b. The properties of glacial loess and the formation of loess deposits. J. Sediment. Res. 36, 669e676. Smalley, I.J., 1966c. The expected shapes of blocks and grains. J. Sediment. Res. 36, 626e629. Smalley, I.J., 1970. Calcium carbonate encrustations on quartz grains in loess from the Karlsruhe region. Naturwissenschaften 57, 87 only. Smalley, I.J., 2014. Glacial Loess: Revisited, Reviewed, Reconsidered. Loess Ground blog 26 September 2014. Smalley, I.J., Cabrera, J.G., 1970. The shape and surface texture of loess particles. Geol. Soc. Am. Bull. 81, 1591e1596. Smalley, I.J., Davin, J.E., 1982. Fragipan Horizons in Soils: a Bibliographic Study and Review of Some of the Hard Layers in Loess and Other Materials, vol. 30, p. 122. New Zealand Soil Bureau Bibliographic Report. Smalley, I.J., Fagg, R., 2014. John Hardcastle looks at the Timaru loess; Climatic signals are observed, and fragipans. Quat. Int. 372, 51e57. Smalley, I.J., Krinsley, D.H., Vita-Finzi, C., 1973. Observations on the Kaiserstuhl loess. Geol. Mag. 110, 29e36. Smalley, I.J., Mavlyanova, N.G., Rahkmatullaev, Kh.L., Shermatov, M.Sh., Machalett, B., O'Hara-Dhand, K., Jefferson, I.F., 2006. The formation of loess deposits in the Tashkent region and parts of Central Asia: and problems with irrigation, hydrocollapse and soil erosion. Quat. Int. 152/153, 59e69. Smalley, I.J., O'Hara-Dhand, K., Wint, J., Machalett, B., Jary, Z., Jefferson, I.F., 2009. Rivers and loess: the significance of long river transformation in the complex event sequence approach to loess deposit formation. Quat. Int. 198, 7e18. Smalley, I.J., O'Hara-Dhand, K., Kwong, J., 2014. China: materials for a loess landscape. Catena 117, 100e107. Stevenson, D.J., 1980. A eutectic in carbon-oxygen white dwarfs. J. de Phys. Colloques 41, c261ec264. Zalasiewicz, J., 2010. The Planet in a Pebble: a Journey into Earth's Deep History. Oxford University Press, 234p.
Please cite this article in press as: Smalley, I., Markovic, S.B., Controls on the nature of loess particles and the formation of loess deposits, Quaternary International (2017), http://dx.doi.org/10.1016/j.quaint.2017.08.021