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POSSIBLE FORMATION MECHANISMS FOR THE MODAL COARSE-SILT QUARTZ P A R T I C L E S IN LOESS DEPOSITS

Ian Smalley Centre for Loess Research and Documentation, Leicester University, Leicester LE1 7RH, U.K.

The first major event in the formation of a loess deposit is the production of silt-sized quartz particles. Many particle production methods have been proposed; the bulk of loess particles appear to be produced by (a) glacial grinding (to give ice-sheet loess) or (b) cold weathering in high regions (to give mountain loess). There are close relationships between mountain loess and "desert" loess. Loess material can be seen as a product of the cold phase of the glacial cycle.

INTRODUCTION

Loess is characterized by particle size. It is a well- sorted sedimentary deposit with most of the particles falling into the size-range 10--60 gm; on many grade scales it can be described as coarse silt. Loess is also characterised by mineralogy. Most of the coarse silt particles are quartz; there may be a substantial amount of feldspar, a variable amount of clay minerals, some heavy minerals and often a considerable amount of carbonate. However, the typical particle mineralogy is quartz. Thus the modal loess particle can be seen as a quartz particle with a nominal diameter of around 30 gm.

This paper is devoted to a discussion of how these modal quartz particles are formed; it in effect reports on about 25 years of attention to this problem, since the topic was first raised in a serious scientific manner in 1966 (Smalley, 1966a). The question is - - what natural processes produce the large amounts of quartz silt which are required to form the loess deposits of the world? The Smailey (1966a) answer was that 'glacial grinding' was an efficient producer of coarse quartz silt and that many loess deposits could be seen to be directly associated with the large scale continental glaciations of the cold phases of the Quaternary period.

GLACIAL GRINDING

Boulton (1978) wrote "Vita- Finzi and Smailey (1970) have argued that a major proportion of the world's silt is produced by glaciers. I would support them in this, and go further to suggest that most of this is produced in the basal zone of traction which I believe to be a uniquely glacial environment in which large forces at non-inertial shear contacts produce fine- grained wear products". This is the basic contention of the glacial grinding school - - that glaciers produce coarse silt; that large continental glaciers represent sufficiently large zones of energy transfer for the production of vast amounts of silt. The Smalley (1966a) mechanisms and event sequences appear to still apply.

23

It has been proposed (Smalley, 1978) that loess produced by continental glaciations should be called 'ice-sheet' loess. Smalley and Smailey (1983) proposed that the North American loess "'is essentially a glacial material", i.e. ice-sheet loess, but we can see now that, in fact, there has to be an admixture of what might be called 'mountain' loess. The absolute predominance of glacial loess which was proposed by Smalley (1966a) is not observed. The world's deposits might be divided between 'ice-sheet' and 'mountain" loess, but the absolute distinction is becoming harder to make. The attempt made by Smalley and Smalley (1983) is still valid, but certainly requires modification and improve- ment.

MOUNTAIN LOESS AND THE DESERT LOESS PROBLEM

The desert loess problem persists. It is usual practice to assign the origin of the desert loess problem to Obruchev (1911); he created the division into 'cold' loess and "hot" loess. He postulated that loess formed from dust which was blown out of deserts. He started the association which has grown up between loess and deserts - - which may have obscured the solution of the problem of the origin of loess particles.

Smalley and Vita-Finzi (1968) discussed the desert loess problem, and came to the conclusion that there were no specifically desert processes which could produce the vast amounts of silt required to form a large loess deposit. They did propose that interparticle impacts could cause chipping which might produce a small amount of loess sized material but their main conclusion was that desert loess did not exist; in the sense that particle production on the large scale in sandy deserts did not seem possible. In this they were following Butler (1956) who, after a careful considera- tion of the Australian situation suggested that "consid- ering the vast areas of deserts in the world, and our relative ignorance of 'hot' loess, the latter may be more hypothetical than real". It was apparent to Butler that the Australian deserts were not producers of loess

24 I. Smalley

material, and this is true of the Sahara as well. Smalley and Krinsley (1978) attempted another

review of the desert loess problem, and came up with a tentative solution. It was essentially a logical solution; some large hot sandy deserts had associated loess deposits, and some did not. Therefore those that did were located near some source of loess particles, a source which could feed loess particles in to the desert. The desert acted as a sort of 'holding area' - - a storage region for loess particles which were in transit from production zone to deposition zone. This scheme fits very well to the observed occurrence of hot/desert loess. There is no obvious source of particles to supply the Sahara and Australian deserts, and these lack significant loess deposits. The Central Asian and North Chinese deserts have large (very large) associated loess deposits, but they also have nearby sources of loess particles - - the very high, very cold regions of High Asia.

The Smalley-Krinsley hypotheses were rationalised and presented more logically by Smalley and Smalley (1983). The material produced by these high cold regions has been named 'mountain' loess (Smalley, 1978). The 'desert' loess of Obruchev is mountain loess. It comes from the desert to form a loess deposit, but it comes at a late stage in a complex sequence of loess forming operations, which begin in the high mountains of Central Asia.

How are the quartz particles of mountain loess formed? We lack the obvious grinding zones of the subglacial environment so we have to fall back on more traditional weathering mechanisms. Freeze/thaw appears the obvious one to choose, but before examin- ing breakage mechanisms too closely we should look at the source rocks again. The ice-sheet loess of North America and North Europe is probably largely derived from the old granitic shield rocks to the north. This fits nicely with the model sequence since the shield rocks contain quartz of the right size and in the stressed condition which makes breakout relatively easy (Smal- ley, 1966b).

The rocks of High Asia, from which the Chinese and Central Asian loesses are derived, are not of this simple igneous variety. We see sediments being remobilised in the region of Indian underthrust, the region of crustal doubling which for convenience is called High Asia. The quartz in the sediments is not in the same stressed condition as that in granites, but it is more accessible in general. The breakup of the source rocks and the release of the loess particles is driven by the tectonic energy available because of the magnitude and speed of the uplift, and by the freeze-thaw cycles occurring in the high cold terrain. Field observations in High Asia reveal that enormous quantities of sediment are pro- duced, and a substantial proportion of this is loess material.

It appears that the major deposits of 'desert' loess owe their existence to a tectonic accident. If India had not collided with Asia the uplifted region would not have formed and the zone of intense weathering would

not have existed; thus there would be no large scale desert loess.

CRYOGENIC PROCESSES: KONISHCHEV AND MINERVIN

According to Konishchev (1987) it has been estab- lished experimentally that 50 ~tm to 10 ~tm grain sizes are the limits for cryogenic disintegration of quartz. Minervin (1980, 1984) appears to reach a similar conclusion; he certainly emphasises the importance of cryogenic processes. Minervin claims that the experi- mental studies point to a decisive role for cryogenic weathering in the formation of the main loess fraction in orogenic source regions of Central Asia. The effec- tive breakage can be explained by the microstructure of quartz crystals which consist of individual units of a ditrigonal prism (Minervin term) separated by linear defects in the form of dislocation channels 2-3 ~tm in diameter and numbering 2 x 107 per square cm. The elementary units of natural quartz and feldspar are 100 to 10 ~tm in diameter. The disintegration of quartz and feldspar crystals as a result of cryogenic weathering is caused by the crushing of thin walls of dislocation channels less than 1 p.m in diameter by the pressure of freezing water. The fragmentation process reaches the level of micro-units, i.e. the stablest elements of the crystal structure (with a strength of 5000-8000 kg/cm 2) which did not break under the pressure of ice (ca. 2000 kg/cm2). Such, claims Minervin (1984), is the mechanism which governs the formation of the initial coarse-silt fractions of .loess under the severe climatic conditions of Pleistocene cold epochs in the mountains of Central Asia.

The process, in outline, looks very like that described by Smalley and Krinsley (1978) and Smalley (1980) for the Tashkent loess. Here is a cold weathering mechan- ism which exploits the defects in quartz crystals, and produces loess-sized material. The defects described by Minervin are presumably the same as those described by Moss and his co-workers (Moss and Green, 1975) and by Riezebos and Van der Waals (1974).

The defect structure of quartz crystals is a key factor in allowing the large scale production of loess material. It allows breakage under less-than-extreme conditions; it appears to control the size of the particles, and it may significantly affect the shape of the particles. Quartz is not isotropic, but it is not extremely anisotropic, the range of shapes in breakout particles could be close to the ideal (Smalley, 1966c).

SOME WEATHERING POSSIBILITIES

Nahon and Trompette (1982) suggested that chemi- cal weathering, particularly in tropical areas, is an active agent of silt formation. Weak minerals disappear by incongruent dissolution and quartz is fragmented by partial chemical dissolution.

Goudie et al. (1979) investigated the formation of silt from quartz dune sand by salt-weathering processes in

Quartz Particles in Loess Deposits 25

deserts. They found that experimental salt weathering of typical quartzose aeolian dune sand from the deserts of Southern Africa, using carefully controlled cycles characteristic of a hot desert, and using a common desert salt, sodium sulphate, indicated that even after a relatively small number of cycles there was measurable disintegration of sand grains and identifiable changes in grain textures. This suggests that salt weathering can cause a reduction in the size of quartz grains and can produce fine, angular quartz particles.

Smith et al. (1987) investigated silt production by weathering of a sandstone under hot arid conditions. Their study, and the mechanisms they outlined, can be seen as complementary to the studies conducted by Goudie et al. (1979) and Pye and Sperling (1983); they suggest that their mechanisms could, under suitable conditions of rock type and salt availability, provide a viable source of ioessic silt.

PARTICLE IMPACT

It seems feasible that two quartz sand particles impacting in a sand-storm should generate chips of quartz, possibly of loess size. It is unlikely that enough energy would be available at the moment of collision for total grain breakage to occur - - but it should be possible for quartz chips to be generated. This idea was explored by Smalley and Vita-Finzi (1968) who pro- posed that a small amount of potential loess material might be formed in this way.

The mechanism has been championed more vigor- ously by Whalley et al. (1982, 1987) who propose that it is effective enough to provide the material for signifi- cant loess deposits. They stated that it seems that an aeolian abrasional origin for some loessic material is quite possible. It is unlikely that much European loess is produced in this way even though long distance travel from the Sahara is possible. Most Saharan produced silt lies not on the South-Eastern Mediterranean land surface but out in the Atlantic Ocean or even South America in the case of fine dust.

It would seem, claim Whalley and coworkers, that a mechanism may exist whereby weathered grains in the Gobi desert could produce a substantial amount of China's loess. It may also be possible that silts in the south-west of the United States and Northern Mexico were produced by an aeolian abrasion mechanism, perhaps coupled with silt production by weathering mechanisms.

SILT PRODUCTION: PYE AND BLATT

Pye (1987) stated that "the balance of distributional, stratigraphic, and sedimentoiogical evidence presently available strongly suggests that most of the silt in the world's major loess deposits is of cold weathering and/ or glacial origin". This is probably true; the major loess deposits are extraordinary natural phenomena and it required extraordinary events to produce them. The two extraordinary events were (1) Pleistocene glacia-

tions - - essentially the source of ice-sheet loess (predominant in North America and North Europe), and (2) the impact of India on Asia and the tectonic consequences - - this produced High Asia, high, cold and energetic - - and the source of the Chinese and Central Asian loess.

Pye (1987) also produced a table showing some possible mechanisms of silt particle formation sug- gested in the literature (Table 1). All of these are interesting and worthy of study, and possibly all contribute material to loess deposits but the major contributors are probably glacial grinding and cold weathering.

Blatt (1987) also listed modes of quartz silt forma- tion; he proposed five sources:

(1) unfractured, fine-grained quartz from low-grade metamorphic rocks;

(2) fine quartz produced during weathering and soil formation by fracturing of coarser grains:

(3) quartz fragments generated bv grain-to-grain im- pacts during transport:

(4) authigenic quartz formed as a by-product of clay diagenesis;

(5) crystallisation of tests of siliceous organisms depo- sited in fine-grained rocks.

Presumably source (2) is the most relevant to loess. Blatt also pointed out that the mean size of quartz grains at the start of their sedimentary history was about 600 ~tm, with about 40% monocrystalline grains. Some of this quartz becomes loess, which forms part of the 1033 particles of silt size which Blatt estimates are in the sediments of the world.

Blatt (1987) has produced a very useful survey of the 6~SO values of quartz of various types and from various sources. He found that published data indicate that quartz from plutonic igneous rocks has an average 6~O value of about +9%0 with metamorphic rocks +13-14%o, sandstones +11%o, shales + 19%o, quartz overgrowths +20%o, and cherts +28%0. This suggests that there might be a discernible difference in the 6180 values for quartz particles found in ice-sheet loess deposits in Europe and North America and for those found in mountain loess deposits in China and Central Asia. The ice-sheet loess, derived directly from the igneous rocks of the northern shield should have lower b 1~0 values.

TABLE 1, Mechanisms for loess particle formation

Mechanism Best reference

Release from phyllosilicate rocks Glacial grinding Frost weathering Fluvial comminution Aeolian abrasion Salt weathering Chemical weathering Biological origin Clay pellet aggregation

Kuenen, 1969 Smailey, 1966a Zeuner, 1949 Moss et al., 1973 Whalley et al., 1982 Goudie et al., 1979 Pye, 1983 Wilding et aL, 1977 Dare-Edwards, 1984

26 I. Smalley

CONCLUSIONS REFERENCES

Some factors to be considered when the origin of the modal quartz particles in loess is being investi- gated:

(1) The quartz crystals in the shield rocks of Northern Europe and North America are in a stressed condition (Smalley, 1966b). They emerge after glacial grinding as sand/silt particles. The 8 t80 values could be close to those for quartz in plutonic igneous rocks.

(2) The quartz particles in the rocks of High Asia are not so well defined as those in the northern shield rocks. The ib 180 values could be significantly different from the ice-sheet loess material, prob- ably higher. More studies are needed on source type and breakage/release mechanisms.

(3) The range 10--60 Ixm may contain the effective comminution limit for plutonic(?) quartz. With other processes operating perhaps the High Asia loess has a slightly different mode size, possibly smaller via the multiple event nature of the deposition process (Smalley and Smalley, 1983); i.e. sorting plays a larger part in China than in North America.

(4) Blatt (1987) puts the mode quartz particle breakout size at 600 ~tm, and 40% of the particles emerge as single crystals. Is there a distinction into broken/ not broken? Does the bimodal distribution of quartz particles as shown by Smalley and Vita-Finzi (1968) actually exist? This would require a sand mode (out unbroken, mode size 600 ~tm) and a silt mode (original particles thoroughly broken, approaching comminution limit, mode size around 30 ~tm).

(5) We might consider and contrast two loess systems: (a) the loess derived by glacial grinding from the shield

rocks of the northern latitudes; classic ice-sheet loess. Particle production materially assisted by the stressed condition of the quartz particles in the igneous rocks (Smalley, 1966a, b). Without large scale continental glaciations this loess would not exist;

(b) the loess derived by cold weathering/tectonic de- formation from the High Asian rocks; classic mountain loess, which passing through North Chinese and Central Asian deserts emerges as desert loess. Particle production driven by temper- ature variations and tectonic energies, possibly with silt production mechanism as described by Minervin (1980) operating. Without the production of 'High Asia' by India impacting on Asia this loess would not exist.

There are varieties of loess, but the two systems listed above include most of the world's loess material. Two major production mechanisms account for the bulk of the quartz silt particles; other mechanisms produce strictly minor amounts.

Blatt, H. (1987). Oxygen isotopes and the origin of quartz. Journal of Sedimentary Petrology, 57. 373-377.

Boulton. G.S. (1978). Boulder shapes and grain size distribution of debris as indicators of transport paths through a glacier and till genesis. Sedimentology, 25. 773-799.

Butler, B.E. (1956). Parna - - an aeolian clay. Australian Journal of Science, 18, 145-151.

Dare-Edwards, A.J. (1984). Aeolian clay deposits of south-eastern Australia: parna or loessic clay. Transactions of the Institute of British Geographers, 9, 33%344.

Goudie, A.S., Cooke, R.U. and Doornkamp. J.C. (1979). The formation of silt from quartz dune sand by salt processes in deserts. Journal of Arid Environments, 2, 105-112.

Konishchev, V.N. (1987). Origin of loess-like silt in Northern Jakutia, USSR. GeoJournal, 15, 135-139.

Kuenen, Ph .H (1969). Origin of quartz silt. Journal of Sedimentary Petrology, 39, 1631-1633.

Minervin, A.V. (1980). Modelling of the conditions of formation of coarse dust particles of loess rocks, lnzhenernaya Geolog(va, 1, 51- 60 (in Russian).

Minervin. A.V. (1984). Cryogenic processes in Loess Formation in Central Asia. In: Velichko, A.A. (ed,), Late Quaternary Environ- ments of ttle Soviet Union, pp. 133-140. Longman, London.

Moss, A,J. and Green, P. (1975). Sand and silt grains: predetermina- tion of their formation and properties by microfractures in quartz. Journal of the Geological Society of Australia, 22, 48,5--495.

Moss, A.J.. Walker, P.H. and Hutka, J. (1973). Fragmentation of granitic quartz in water. Sedimentology, 20, 489-511.

Nahon, D. and Trompette, R. (1982). Origin of siltstones: glacial grinding vs. weathering. Sedimentology, 29, 25-35.

Obruchev, V.A. (1911). The question of the origin of loess - - in defence of the aeolian hypothesis. Izv~stfya Tomskago tekhnolo- gicheskago instituta lmperatora Nikolaya 11, 33 (in Russian).

Pye, K. (1983). Formation of quartz silt during humid tropical weathering of dune sands. Sedimentary Geology, 34. 267-282.

Pye. K. (1987). Aeolian Dust and Dust Deposits. Academic Press, London.

Pye, K. and Sperling, C.H.B. (1983). Experimental investigation of silt formation by static breakage processes: the effect of tempera- ture, moisture and salt on quartz dune sand and granitic regolith. Sedimentology, 30, 49--62.

Riezebos, P.A. and Van der Waals, L. (1974). Silt-sized quartz particles: a proposed source. Sedimentary Geology, 12, 27%285.

Smalley. l.J. (1966a). The properties of glacial loess and the formation of loess deposits. Journal of Sedimentary Petrology, 36, 669---676.

Smalley, l.J. (1966b). Formation of quartz sand. Nature, 211, 47(~ 479.

Smalley, l.J. (1%6c). The expected shapes of blocks and grains. Journal of Sedimentary Petrology. 36, 626-629.

Smalley, l.J. (1978). The New Zealand loess and the major categories of loess classification. Search, 9, 281-282.

Smalley, I.J. (1980). The formation of loess materials and loess deposits: some observations on the Tashkent loess. Geophys. Geol. Geophys. VerOff d. KMU Leipzig, 2, 247-257.

Smalley, 1.J. and Krinsley, D.H. (1978). Loess deposits associated with deserts. Catena, 5, 53--66.

Smalley, l.J. and Smalley, V. (1983). Loess material and loess deposits: formation, distribution and consequences. In: Brook- field, M. and Ahlbrandt, T.S. (eds), Eolian Sediments and Processes, pp. 51-68. Elsevier, Amsterdam.

Smalley, l.J. and Vita-Finzi, C. (1%8). The formation of fine particles in sandy deserts and the nature of 'desert" loess. Journal of Sedimentary Petrology, 38, 766-774.

Smith, B.J., McGreevy, J.P. and Whalley, W.B. (1987). Silt production by weathering of a sandstone under hot arid conditions: an experimental study. Journal of Arid Environments, 12, 199--214.

Vita-Finzi, C. and Smalley, l.J. (1970). Origin of quartz silt: comments on a note by Ph.H.Kuenen. Journal of Sedimentary Petrology, 40, 1367-1369.

Whalley, W.B., Marshall, J.R. and Smith, B.J. (1982). Origin of desert loess from some experimental observations. Nature, 300, 433--435.

Whalley, W.B., Smith, B.J., McAlister, J.J. and Edwards, A. (1987). Aeolian abrasion of quartz particles and the production of silt-size fragments, preliminary results and some possible implica- tions for loess and silcrete formation. In: Reid, I. and Frostick, L.

Quartz Particles in Loess Deposits 27

(eds), Desert Sediments Ancient and Modern, pp, 129~138. Blackwell, Oxford.

Wilding, L.R.. Smeck, N.E. and Drees, L.R. (1977). Silica in soils: quartz, cristobalite, tridymite and opal. In: Dixon, J.B. and Weed,

S.B: (eds), Minerals in Soil Environments, pp. 471-552. Soil Science Society of America, Madison.

Zeuner, F.E. (1949). Frost soils on Mount Kenya. Journal of Soil Science, 1, 20-30.