glacially crushed quartz grains in loess as indicators of long-distance transport from major...

Glacially crushed quartz grains in loess as indicators of long-distance transport from major European ice centers during the Pleistocene WILLIAM C. MAHANEY AND WOLFGANG ANDRES

Mahaney, W. C. & Andres, W. 1991 (September): Glacially crushed quartz grains in loess as indicators of long-distance transport from major European ice centers during the Pleistocene. Boreas, VOI. 20, pp. 231-239. Oslo. ISSN 0300-9483.

Detailed mineralogical analysis of the fine and very fine sand fractions (63-250 pm) in the Dreihausen Loess Sequence shows the presence of numerous glacially crushed grains, including a significant amount of quartz. Previous theories regarding the origin of loesses in central Germany held that all sediments were derived locally, following short-distance transport from nearby sandstone outcrops. New data presented here show that many quartz and feldspar grains were glacially crushed, weathered, crushed again, transported, and emplaced by aeolian processes at Dreihausen. Crushing features observed are compatible with transport in both continental and alpine glaciers; some clasts show the effects of abrasion by both water and ice transport, while others are partly rounded presumably by Ruvial and/or aeolian processes. Stronger paleowind systems during the Pleistocene presumably contributed to the transport and emplacement of glacial grains from ice centers in both the Alps and the Baltic areas, and also possibly from the Rhine Basin.

William C. Mahaney, Geomorphology and Pedology Laboratory, Atkinson College, York University, 4700 Keek Street, North York, Ontario, Canaah M3J IP3; Wolfgang Andres, Fachbereich Geogrphie, Philipps Uniuersity, 0-3.5.50 Marburg, Germany; 5th March, 1990 (revised 25th February, 1991).

BOREAS

The Central European loess record has been discussed in depth by numerous workers (Demek & Kukla 1969; Lozek 1969; Kukla 1970,1975). The loess stratigraphy is both long and unrivaled for its complexity of buried paleosols (Fink et al. 1976; Fink & Kukla 1977; Morrison 1978). Indeed several loessic deposits and paleosols are older than the Brunhes/Matuyama boundary of 0,73- 0.79 my, and hence of early Pleistocene age. As at other localities, the Dreihausen loess record (Fig. 1) shows depositional/weathering ( paleosol formation) episodes representing glacial/ interglacial cycles, all of which appear to be of Brunhes age (<0.73my). Above each loess deposit in the sequence is a brownearth paleosol with an argillic horizon (Alfisol; Soil Survey Staff, 1975), presumably formed under mixed-decidu- ous forest (Morrison 1978). Each paleosol (Fig. 2) in the sequence represents a distinct weathering horizon, termed a ‘Leitgrenze’ or marker bed (Kukla 1969, 1970).

From textural and mineral content the assump- tion has been drawn that the loess was blown out from the more or less vegetation-free adjacent areas. These could have been the outwash plains in front of the glaciers in the case of the loess

layers of northern Germany and the prealpine region (Fig, 4). The up to 30 m thick loess cover of the Rhine Rift Valley may be derived from the broad seasonally dry river bed of the Rhine, which during Pleistocene cold periods had the character of a braided stream channel (Guenther 1987; Eitel & Blumel 1990).

The loewes of the basins and depressions of the Central German Uplands are believed by some workers to be derived from relict soils and frost- weathered debris of the surrounding mountain areas which rose above the tundra zone (Bremer 1989).

On the other hand, there is a predominant theory in the Russian literature that aeolian transport provided only a little contribution to the formation of loess at all and that, after different forms of transportation of fine material, the in situ ‘loessification’ (Russian: oblessoranie) was the most important process (Lozek 1965).

Although the loess sheets of the intramontane basins are very well investigated with regard to paleopedology and Quaternary stratigraphy no attention has been paid to the question whether aeolian grains could have been transported from far away, especially from the above-mentioned

232 William C. Mahaney and Wolfgang Andres BOREAS 20 (1991)

- - Maximum stoge (external moraines 1 of the Fenno . Scandion and Alpme glacwr of Wmichsel ( Wisconrin ) age

-.- Maximum extension of ?leislocenm glaciations c Direction of ice movement

0 ~0011 cover

Fig. 1. Map showing location of the Dreihausen section, distribution of loess, and the glacial Limits in the alpine foreland and in Northern Germany.

BOREAS 20 (1991) Glacial quartz in loess 233

CM 0

100

200

300

400

Fig. 2. Stratigraphy of the DRD section, Dreihausen, Central Germany. Unit 8 consists of fresh basalt exposures (lower) and regolith (upper). Unweathered loesses include units 7 and 5 . Paleosols are found in unit 6 and in the upper parts of units 4 and 3. Units 2 and 1 are weathered throughout. Paleosol nomenclature follows Birkeland, 1984. A horizons are composed of dark-colored humus-rich materials; AB is a transitional horizon; B horizons are composed of clay-rich (Bt, Bbt); and fresh, unweathered material (Cub).

500 . -

8 7

I I I I I A .. 3:. Loess unweathered

I I I I Older poleosol * Regolith

Bbt Polesol e o ~ Transported loess

glaciofluvial areas in front of the continental or alpine ice or the Rhine Rift Valley of south- western Germany.

As many as nine interglacials have been re- corded in the Brunhes Chron alone, ending with T1 (Holocene) and starting with T9 (c0.73 my) (Morrison 1978). Four of these paleosols are known at Dreihausen and some may record more than one interglacial cycle. Sabelberg et al. (1976) described altogether 13 pedogenic horizons from Dreihausen which they believed to be remnants of interglacial soils. But only four of these soils contained sediments considered to be autoch- thonous. Considering the fact that the whole sequence shows normal magnetic remanence (Mahaney et al. unpublished), one can assume that some of these 'soil cycles' involved renewed weathering of in situ interglacial soils, the parent material of which was originally emplaced during one or more glacial periods.

Thin loess deposits were emplaced during glacial cycles at Dreihausen. The oldest of these (unit

A AB Bt

Bbt

Cub

Bbt

Cub

Bbt

7, Fig. 2) overlies solifluction deposits formed by reworking of residual regolith formed from Miocene volcanic bedrock. These loess deposits contain clasts presumed to have a local provenance (Kallenbach 1965; Tillmanns & Windheu- ser, 1980). Analysis of the fine and very fine sand fractions (63-250 pm), however, shows the presence of glacially crushed quartz that must have originated from the Baltic or alpine ice centers, or from glacial sediments in the Rhine Basin.

Previous work on quartz microtextures Distinct glacial crushing microtextures on quartz particles is considered by some workers (Krinsley & Doornkamp 1973; Whalley & Krinsley 1974; Mahaney 1990a, b amongst others) as providing evidence of glacial transport. Among other microtextures cited, conchoidal fractures, subparallel fractures, arc-shaped steps, and deep grooves or


furrows are all collectively considered representative of transport by ice. Objections to these interpretations have been raised by Brown (1973), Setlow and Karpovich (1972) and Bull (1981). The main objections include the use of single microtextures to infer a glacial origin for a sedi- ment, principle of equifinality that different geological processes may produce the same assemblage of microtextures, and a lack of quantification presented by many workers.

To overcome the lack of quantification Mah- aney (unpublished) began a study of 500 till samples from different parts of the world. The main objective has been to catalog the types of microtextures found on particles of sand (63- 2,000pm) emplaced by thin mountain ice (<200 m thick), thicker expanded mountain ice ( ~ 2 0 0 - -500 m) and continental ice (>SO0 m). In short, the overall results indicate that mountain ice confined to cirques appears not to damage quartz fragments and acts to preserve grains during transport. Expanded mountain ice, reaching thicknesses of 500 & 300 m, appears capable of minor fracturing and abrasion on the suite of samples studied. Arc-shaped steps, for example, begin to appear on grains emplaced by expanded mountain ice; only 20-30% of the grain surfaces studied to date are damaged by fracturing and abrasion. The greatest amount of fracturing and abrasion occurs on grains emplaced by continental ice (>l,OOO m ice thickness). This includes tills emplaced by the Laurentide (Mahaney et al. 1988; Mahaney 1990a), Fenno-Scandian (this paper), and Antarctic glaciers (Mahaney 1991). From these samples the main conclusion to be drawn is that progressively increasing ice thickness inflicts a greater degree of damage on many quartz particles within the glacier system. Presumably, the degree of damage depends also on position within the ice (supraglacial, englacial or basal) and whether or not the ice is above or below the pressure melting temperature.

Experimental crushing of Brazilian (alpha) quartz and silica spheres by H. H. Schloessin (Schloessin-Mahaney, unpublished) at the Uni- versity of Western Ontario shows that considerable crushing may be expected at the sole of glaciers resulting in the production of deep grooves, conchoidal and subparallel fractures, arc-shaped steps and sharp edges. These grains, grown in an ice cylinder, were subjected to uni- axial stresses equivalent to 3.0 and 4.0 km of ice. The resulting damage to grains in ice above and

below the pressure melting temperatures is very similar to damage features observed on grains emplaced by continental ice.

Field area The field area is situated on the northwestern border of the Miocene volcanic Vogelsberg area near Marburg, Germany. Surrounding prevol- canic bedrock formations are of Lower Neogene age, primarily consisting of sandstone, which is believed to be one of the sources for Pleistocene aeolian deposits. Because the local volcanic rocks contain relatively little quartz, we originally believed that most of the quartz sand came from nearby sandstone outcrops, which were never glaciated. Subsequent analysis of the microtextures on these grains, however, showed that some have a glacigenic origin quite far removed from the periglacial area.

The climate in the field area is subject to mari- time influences from the west. Mean annual temperature approximates 9°C; mean annual precipitation is about 600 mm.

Methods Samples were collected from the Dreihausen Loess Sequence following the nomenclature of the Soil Survey Staff (1951, 1975) and Birkeland (1984). Whole soil samples were air dried and wet sieved to separate the fine (<63 pm) from the coarse (63-2,OOO pm) fractions. The sands (63- 2,000 pm) were oven-dried to 110°C for 1 h. The pretreatment for particle size analysis included oxidation of organic matter using H2O2, defloc- culation with Na-pyrophosphate (Day 1965) and sonification at low energy to remove adhering clay particles.

Scanning electron microscopy analysis was car- ried out using JEOL JSM-840 and CAMSCAN-4 scanning electron microscopes. The sands were subsampled from loess and paleosol deposits and only the fine and very fine fractions (63-250 pm) were analyzed. The quartz grains were identified by X-ray microanalysis (energy-dispersive spec- trometry). For an expanded discussion of the subsampling and analytical routines see Mahaney et al. (1988) and Vortisch et ul. (1987).


The range of glacially crushed microtextures shown on grains from Dreihausen included almost all the features representative of transport by ice, including linear and conchoidal crushing features (high and low frequency types; Mahaney et al. 1989), and deep-furrow crushing, abrasion features, arc-shaped steps, crescentic gouges and variations in grain edge sharpness. Other features, such as rounded edges and V-shaped percussion cracks, relate more to water transport (with the former also attributed to aeolian trans-

Overprinted grains are plentiful in all the loess deposits, indicating complex weathering and transport histories. In general, overprinted grains show crushing + weathering + crushing +

aeolian transport + emplacement. Because these overprinted grains are widespread in all the

port).

Results The analysis of several dozen grains in each loess deposit and paleosol showed the presence of numerous glacially crushed grains (Figs. 3-8) (Krinsley & Takahashi 1962; Hamilton & Krinsley 1967; Krinsley & Doornkamp 1973; Mahaney et al. 1988), along with grains showing only fresh and weathered fracture faces, but with- out glacigenic features.

In general, quartz grains increase in frequency from unit 7 to the top of the section. No quartz is present in unit 8. Other grains showed surface rounding, presumably inherited from transport by aeolian or fluvial processes. A small fraction of the grains studied showed V-shaped percussion cracks inherited from collision during water transport (Krinsley & Doornkamp 1973).

Fig. 3. 0 A. Glacially crushed subangular quartz particle from DRI3-3 loess (Illinoian, Saale) showing extensive older (0) and younger (y) linear and conchoidal crushing features. 0 B. Enlargement of grain shown in A; fresh crushing features (to right, arrow) have fewer adhering clay particles.

Fig. 4. 0 A. Quartz grain in DRI3-3 loess (ILLinoian, Saale) showing mainly fresh low frequency conchoidal and linear crushing features, dissolution weathering on remainder of grain surface, and adhering clay particles. 0 B. Deep-furrow type crushing in quartz (extensively modified by dissolution etching).


investigated size are glacially crushed), it is appar- ent that many of the airfall-influx clasts had possible sources at least several hundred kilometers to the north and south. The nearest ice centers in the Alps and in the Baltic areas (Fig. l ) , as well as outwash in the Rhine Basin, presumably provided the bulk of the loess, which was previously considered to be mainly of local provenance (Tillmanns & Windheuser 1980; Guenther 1987). Because this study centered on the fine sand fraction (63-250 pm), we must assume the paleowind system had at least a speed of 5 m/sec (11 mph) (Bagnold 1941). Assuming wind updrafts were 1/ 5 of this minimum speed (Easterbrook 1969), sand with an average diameter of <250 pm could have been winnowed from coarser grains and maintained in motion. While wind speeds in the westerly wind belt are lower over land relative to the ocean, they are known to be relatively higher during winter (Donn 1965). Presumably the

Fig. 5. 0 A. Quartz grain in the DRI3-3Bt horizon showing older crushing surfaces (extensively weathered). 0 B. Fresher surface with mainly low frequency subparallel (arrow) crushing features affected by minor dissolution etching.

samples studied representative examples are shown here (Figs. 3-8) along with representative crushing features.

Adhering particles (Fig. 3) that may contain important information on mechanical alteration of grains by cryostatic pressure and/or neo- formation of clay minerals during interglacial cycles were not studied due to time constraints. Mechanically upturned plates on quartz grain surfaces were not observed during this study, but are known to occur on glacially crushed grains (Krinsley & Doornkamp 1973). Presumably such features might be lost by low velocity impact collisions during aeolian transport.

Discussion Because of the prevalence of glacigenic grains in

Fig. 6. 0 A. Glacially crushed quartz grain with deepfurrow type features in DR13 5 loess. n B. Enlargement showing old weathered subparallel linear crushing features (arrow) in deep-

the Dreihausen loess (>SO% of all grains of the furrow like troughs.


paleowind had a speed greater than 5 m/sec and was of sufficient strength to entrain and transport sand grains for considerable distances (-300 km) from glaciated areas.

The range of microtextures observed on the full suite of samples studied indicates a range of source areas. Many grains show the presence of deep-furrow type crushing (Fig. 1A), arc-shaped steps and crescentic gouges. All are considered to result from continental glaciation (Mahaney et al. 1988; Mahaney 1990a, 1991). Other clasts show considerably less crushing and in some cases <20% of the grain surface is crushed (Fig. 6). These grains may be derived from alpine areas, presumably originating in the European Alps. In some cases particles with both fresh and weathered fracture faces lacking glacial-crushing features were observed in the group of grains studied.

Because crushing microtextures have high and low frequency wave forms (Mahaney 1991) giving linear and conchoidal shapes, we paid considerable attention to documenting the range observed. The distance between wave crests ran- ged from <0.05 pm to 5 pm, and grains with both linear and conchoidal forms tended on the whole to be extensively crushed. We consider the degree of crushing to increase with increasing ice thickness and distance of transport (Mahaney et al. 1988), so that increasing damage to particle surfaces probably reflects increasing ice thickness. Indeed, many of the heavily fractured grains could have a source area in Fenno-Scandia. Less extensively crushed grains (especially those with lowfrequency microtextures) may have originated in the Alps, and some particles may have been transported through the Rhine Basin. Some loesses contained quartz particles with only minor crushing features similar to those reported by Mahaney (1990~) from the Virunga Mountains of East-Central Africa.

Overprinted grains yield important information on previous crushing and preweathering episodes (Mahaney 1991). The overwhelming extent to which many grains in the Dreihausen sequence are overprinted attests to the immense recycling of clasts during each glacial cycle. Many grains show a complex sequence of crushing+ weathering + crushing + aeolian transport and emplacement. Sometimes the first stage of crushing appears to have been more extensive followed by weathering and renewed crushing of a less extensive nature. This gives the impression that some grains may have been crushed by con-

Fig. 7. Enlargement of crushing features on grains in DRI3-4 loess showing mainly low frequency conchoidal crushing features.

Fig. 8 . Glacial crushing features in the DRI3-Bt horizon. 0 A. Quartz fragment with mainly old deep-furrow type features (arrows) extensively modified by weathering. 0 B. Overprinted (arrow) grain with extensive linear and conchoidal crushing features heavily modified by weathering. Fresher conchoidal crushing is present in upper right comer of the micrograph.


tinental ice, weathered, and later crushed by thin- ner ice, possibly in the.Alps or near the terminal areas of continental ice. In almost all overprinted cases quartz weathering appears to involve stages of chemical dissolution leaving cleanly etched grain surfaces. In other cases, coatings of Fe-rich material and neoformed clay minerals seem to have developed after etching ceased.

The greatest number of glacially crushed grains was found in the DRI3-3 parent material and in

surfaces - suggests both alpine and continental origins.

Rounding of some grains suggests also aeolian and/or fluvial transport following crushing and weathering episodes. Also, overprinting of many grains shows complex transport and weathering histories that provide information on at least two episodes of glacial crushing separated by weathering prior to emplacement. The data discussed herein suggest that the Pleistocene glacial paleo-

the 3Bt paleo-argillic horizon (Figs. 3-5). The wind systems were sufficiently strong to transport parent material contained particles that had been fine and very fine sand size material (63-250 pm) extensively weathered prior to transport and over several hundred kilometers. Intense cyclonic emplacement at the site. Both the loess and over- storms moving over the central European peri- lying paleosol contained grains extensively weath- glacial zone presumably provide the most logical ered by dissolution etching. Some even had means of explaining the northward and southward adhering clay particles and Fe-rich stains attesting transport of quartz sand from the main regional to high intensity weathering during Pleistocene interglacial periods. Analysis of quartz particles in older (units 4 and 5) and younger horizons (unit 1) showed considerably fewer glacially crushed clasts. In almost all cases these grains were crushed, weathered extensively, transported to and emplaced at DRU. In some cases it is possible that grains in the younger beds could have been reworked locally from older beds at DR13 that were exposed to congeliturbation (frost heaving, solifluction, frost wedging) that might disturb surface sediments exposing them to the action of the wind. Indeed the presence of deep ice wedges (circa 2m) at Dreihausen supports this hypothesis.

Conclusions A number of glacially crushed quartz grains in all loesses and paleosols studied suggests that aeolian sediments in the Dreihausen loess record have partly undergone long-range transport (approx. 200-300 krn) from ice centers in the Alps and northern Germany. However, quartz grains may also have been blown from the Rhine Valley after having been transported by the Rhine (these grains should not show fresh fractures). The long- standing hypothesis that airfall sediments in central Germany have predominantly local origins should be re-examined in light of this new evidence. The presence of a variety of grains - some with mainly low frequency type crushing over small percentages of clast surfaces, and others with high and low frequency crushing features covering large (>SO%) areas of the grain

ice centers.

Acknowledgements. - We gratefully acknowledge use of the scanning electron microscopes (assistance of the late Klaus Fecher, Institute of Geology and Paleontology, Marburg and George Gomolka, Toronto) and energy dispersive X-ray facil- ities at the University of Toronto and Philipps University. K. McSweeney and an anonymous reviewer provided critical reviews of the paper. We acknowledge funding from the joint Natural Sciences and Engineering Research (NSERC) - Deut- xhe Forxhungsgemeinschaft (DFG) program to WCM. Samples were analyzed at the Institute of Physical Geography (Philipps University), and Geomorphology and Pedology Laboratory (York University). David Hinbest (York) assisted with the sample analysis. John Dawson (DIAR-York) prepared the photomicrographs and Janet Allin drafted some of the figures.

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