Introduction

Comminution of sediment grains is an important subglacial process. Many authors (Reference Dreimanis,, Vagners, and Goldthtwait,Dreimanis and Vagners, 1971, Reference Dreimanis,, Vagners,, Yatsu, and Falconer,1972; Reference Boulton,Boulton, 1978; Reference Haldorsen,Haldorsen, 1981) have attributed a mode in the silt-size range in till to such comminution, and Reference Drake,Drake (1972) inferred that a change in roundness of pebbles with increasing distance from their source in east-central New Hampshire was a result of it.

In studies of the size distributions of various mineral species in till. Reference Dreimanis,, Vagners, and Goldthtwait,Dreimanis and Vagners (1971) suggested that quartz and feldspar can be reduced to a terminal grain-size of 0.062-0.031 mm, while other minerals might have smaller terminal sizes. Reference DiLabio.Dilabio (1981) found that chalcopyrite in till in the Lac Mistassini-Lac Waconichi area of Quebec has a clast mode between 16 and 1 mm and a matrix mode from 0.063 to 0.016 mm. Other researchers have studied comminution of quartz in till with the use of a scanning electron microscope (e.g. Reference Krinsley, and Doornkamp,Krinsley and Doornkamp, 1973; Reference Mahaney,, Vortisch, and Julig,Mahaney and others, 1988), and have observed what they infer to be abraded surfaces and conchoidal fracture resulting from subglacial comminution.

In discussing comminution, a distinction between crushing and abrasion is sometimes made. The word “crush” implies a process in which forces are predominantly normal to a grain surface, whereas abrasion suggests forces parallel to the surface. Clearly, there must he a continuum between these end members. However, shear forces are likely to dislodge small particles from the surface of a larger one, whereas normal forces break grains into daughter particles of roughly equal size. Herein, mineralogical analyses of different grain-size fractions from a basal till and micro-fabric studies of impregnated samples of the till are used to clarify the role of crushing and abrasion in controlling the grain-size distribution of the till matrix.

Regional Setting

The area from which the samples were obtained is in the Halasi River catchment on the south slope of the Altay Mountains in China (48°30′−49°20′N, 86°50′−87°55′ E). Elevations range from ~1000 m in the river valley to 4374 m at the top of the highest peak, Youyifong. The modern snow line is 3000-3100 m, so valley glaciers are present. Halasi Glacier, the largest and longest in the region, is over 10 km long. The snouts of modern glaciers are at elevations of 2300-2400 m, while the lower limit of known Quaternary glacial deposits is at 1300 m.

Lateral moraines are present along the banks of Halasi Lake and three sets of end moraines are distributed over a distance of 3.5 km down-valley from the lake Fig. 1). Each set of end moraines consists of several minor moraines. The till is 20-30 m thick in these moraines. Near the outlet of Halasi Lake, there are two small patches of basal till (or ground moraine about 10 m thick, with little relief.

Fig. 1. Maps showing sampling sites. (a) Location map of the study area. (b) Geomorphological features of moraine in the vicinity of the outlet of Halasi Lake.

¹⁴C dating of a secondary carbonate coating on till gravels on the valley floor near site B (Fig. 1) yielded an age of 4040 ± 80 years BP. This is considered to be a minimum age for the deposits. Accordingly, consistent with data on moraine ages in the other Chinese mountains, this deposit is believed to have been formed during the Last Glaciation in China (equivalent to the Wiscousinan or Weichselian) (Reference Zhijiu,, Chaolu and Jinfu.Cui Zhijiu and others, 1992).

The bedrock in the glaciated area is predominantly granite and chlorite, chlorite-quartz and quartz-schists. Accordingly, over 90% of the mineral grains in the till are quartz, feldspar or chlorite (Table 1).

General Properties of the Till

Tills near the outlet of Halasi River contain a large number of spherical clasts, believed to have come from ancient lacustrine sediments (Reference Zhijiu,, Chaolu and Jinfu.Cui Zhijiu and others, 1992; Reference Yi. and Zhijiu.Yi Chaolu and Cui Zhijiu, 1994). At site A (Fig. 1), the till is massive and compact, and the matrix has a high silt content. The till fabric is strong, with a preferred orientation of long axes parallel to the ice-flow direction and dipping up-glaciwr.

The till at site B is also massive but less compact and coarser. It contains lenticular layers of sand and silt, 0.180-0.50 m thick, with individual laminae that are 1-2 mm thick. The preferred orientation of gravel-sized clasts is perpendicular to the trend of the moraine.

Methods

Grain-size an mineralogical analyses

Fourteen pairs of samples were collected, six from ground moraine near site A, and eight from an end moraine near site B (Fig. 1). One sample from each pair was loose and was used for grain-size analysis, whereas the second was an oriented, impregnated sample which was thin-sectioned and used for microfabric studies.

Size distributions of grains >0.039 mm were determined by sieving. Fractions >0.125 mm were separated at 1ø intervals (ø = log₂ d where d is the grain-size in millimeters) and fractions between 0.125 and 0.039 mm were separated at ¼ø intervals.

The fine-sand (0.1-0.2 mm) fraction was divided into heavy- and light-mineral components with the use of tribrothane (specific gravity: 2.89-2.93). Ferromagnetic minerals were further separated magnetically from the heavy fraction. The rest of the heavy fraction and the light fraction were sorted into three or four parts electromagnetically. The mineral constituents were then identified by using a binocular microscope, and occasionally with a polarizing microscope using oil immersion. The variation in mineralogy among the 14 samples collected is small (Table 1). Thus, four representative samples were selected for detailed mineralogical analysis of the respective grain-size separates.

Table 1. Mineral components in glacial till (grain-size: 0.1-0.2 mm)

Samples from grain-size fractions >0.039 mm were impregnated and thin-sectioned. Mineral species were identified using a polarizing microscope. Grain counts were made using more than 500 grains per sample.

The >0.039 mm grain-size fractions were further separated at 1ø intervals by pipette. The separate grain-size fractions were then ground into a fine powder and the mineral components were determined quantitatively by infrared spectrometer.

Microfabric studies

Sets of mutually perpendicular thin sections were prepared from the oriented impregnated samples. Microfabrics and characteristics of the comminuted mineral grains were studied by using a polarizing microscope. Sizes of over 900 primary grains and over 2700 daughter grains were measured (Table 2).

Table 2. Number of grains measured

Some samples from the local bedrock, a schist, were also sectioned in order to determine mineral grain-sizes in the parent rock.

Results

Size distribution of quartz, feldspar and chlorite grains

The variation in mineral content among the various size fractions is similar in the four samples (Fig. 2). Quartz dominates in the 0.125-0.031 mm size range. In contrast, feldspar and particularly chlorite concentrations increase relative to quartz in the smaller- and larger-size fractions.

Fig. 2. Variation in mineral composition with grain-size for the four samples.

Fig. 3 shows the grain-size distribution in the bulk till samples and Fig. 4 shows the grain-size distributions for quartz, feldspar and chlorite, individually, calculated from the data in Fig. 2 and 3. Quartz has a preferred mode at 0.063-0.032 mm and in some samples a secondary mode at 0.25-0.125 mm. Feldspar and chlorite commonly have weak modes at 0.25-0.125, 0.063-0.032 and 0.016-0.004 mm.

Fig. 3. Grain-size distributions of bulk samples.

Fig. 4. Grain-size distributions of quartz, feldspar and chlorite.

Crushed and abraded minerals in thin sections from oriented samples

In thin sections prepared from the oriented impregnated samples, many grains of quartz, feldspar and chlorite appear to have been broken into two or more smaller fragments (Fig. 5a - g). In some cases, small quartz fragments are inferred to have been abraded from larger ones. The shapes of the edges of these grains mirror that of the larger presumed parent (Fig. 5c) and, in some places, they remain weakly connected to the parent (Fig. 5d).

Fig. 5. (a) Quartz grains with day rims, and two recently crushed 0.045-0.062 mm quartz grains (left side mid upper right corner), Under crossed nicols. The clay was formed in situ. (b) Enlargement of the crushed quart-grain in upper right corner of (a). Arrow points to clay. (c) Crushed and abraded quartz grains. Under crossed nicols with gypsum plate. One quartz grain has been split into two smaller ones. On the right side, eight 0.04-0.05 mm quartz grains are inferred to hare been abraded from a large one. One of the abraded grains is still weakly attached to its parent. Clay has formed on the rims of the grains. Qc, crushed quartz grain: Qa, abraded quartz grain: Cl, clay. (d) Small quartz grains, 0.03-0.0.5 mm in diameter, are inferred to have been abraded from a larger one. Note that in the lower right corner, the edges of two small grains mirror the shape of the corresponding edge of a larger one. Qa, abraded quarlz grain. Arrow points to abraded daughters. Under crossed nicols with gypsum plate. (e) Except for several large quartz grains, most of the grains are feldspar. Large ones appear to have been crushed into finer grains 0.016-0.004 mm. Under crossed nicols with gypsum plate. Q. quartz. Arrows point to crushed feldspar grains. (f) Silt grains in fine matrix. Feldspar and chlorite were still being crushed into even smaller sizes. Single arrows point ta chlorite grains and double arrows to feldspar grains. Under crossed nicols. (g) Crushed small chlorite grains (Ch). Under crossed nicols with gypsum plate. (h) Part of a mineral aggregate caught in the process of being crushed into individual minerals of chlorite, mica, quartz and so forth. Under crossed nicols with gypsum plate

Polymineralic fragments were sometimes found to be broken into monomineralic grains along mineral boundaries (Fig. 5h). Similar clasts, inferred to have resulted from crushing, were also observed in the field (Reference Zhijiu,, Chaolu and Jinfu.Cui Zhijiu and others, 1992). The results that follow, however, come from analysis of monomineralic grains.

Modes in the distribution of parent grains are 0.063-0.125 mm for quartz and feldspar and 0.016-0.063 mm for Chlorite (Fig. 6a). The mode for feldspar is skewed toward smaller grain-sizes compared with quartz. The average sizes of parent grains of quartz, feldspar and chlorite are 0.129, 0.078 and 0.059 mm, respectively, while the smallest parent grains observed were about 0.016, 0.006 and 0.002 mm, respectively. Although the shapes of the size distributions of daughter grains are similar to those of the respective parent grains, there is a clear shift of about a factor of 2 (1 ø) toward finer grain-sizes (Fig. 6b). This reflects the fact that parent grains tend, most commonly, to split into two daughter fragments. The average grain-sizes thus decrease to 0.068, 0.041 and 0.024 mm, respectively. Observed daughter fragments can be as small as 0.005 mm for quartz, 0.003 mm for feld spar and 0.0014 mm for chlorite.

Fig. 6. (a) Grain-size distribution of parent particles. (b) Grain-size distribution of daughter particles. (c) Crushing factors. C_f. (d) Size distribution of parent particles that split into exactly two daughter particles (i.e. C_f = 2).

Fig. 7 shows the changes in size distribution during comminution, expressed as the percentage of the total area of a thin section occupied by grains of different sizes. Transitions from net decrease to net increase occur at 0.125, 0.063 and 0.25 mm for quartz, feldspar and chlorite, respectively.

The largest increases are 3.6% at 0.063-0.032 mm for quartz, 10.9% at 0.063-0.032 mm for feldspar and 10.0% at 0.032-0.016 mm for chlorite.

Fig. 7. Increments of grain-size percentage after crushing.

Let us define the crushing factor, C _f, as the number of daughter grains resulting from crushing a parent. A crushing factor of 2 was found for 65.6% of the quartz grains, 47.2% of the feldspar grains and 40.7% of the chlorite grains (Fig. 6c). Crushing factors greater than 5 are most common for crushing events involving chlorite. In addition, for chlorite crushing factors tend to be larger for larger grain-sizes, whereas this is not so clearly the case for quartz and feld-spar. The correlation coefficient for the relation between C _f and grain-size is 0.392 (n =322) for chlorite (significant at the 0.1 % level), whereas those for quartz and feldspar are only 0.150 (n =270) and 0.167 (n =333), respectively.

A Student’s t-test indicates that the average sizes of parent grains of feldspar and chlorite with C _f = 2 are significantly smaller than the average size of the total population of parent particles of these mineral species (Fig. 6d). This property is more obvious in chlorite (mean = 0.033 mm, t = 4.89, DF = 451, level = 0.1%, mean size of parent = 0.059 mm) than in feldspar (mean = 0.064 mm, t = 2.87, DF = 478, level = 5%, mean size of parent = 0.078 mm). In other words, smaller particles are more likely to split into only two daughter particles, whereas larger grains may split into three or more daughter particles. This is not true of quartz. On the other hand, the average sizes of daughter grains of the three minerals resulting from crushing events yielding exactly two particles are not significantly different from the average sizes of all daughter grains of these respective minerals.

Discussion

The process of comminution in till probably starts with breakdown of rocks along zones of weakness, eventually resulting in monomineralic grains (Fig. 5h). These grains are then split into smaller ones (Fig. 5a - g), perhaps eventually reaching a terminal grade or crushing limit.

The common secondary modes at 0.25-0.125 mm in the grain-size distributions of quartz and feldspar (Fig. 4) come from the parent schists which have crystals of this size. Similarly, chlorite is relatively abundant in the 1.0-0.125 mm size range because chlorite crystals are large in the schists.

For quartz, the greatest increase in the number of grains, expressed in per cent by area, occurs in the size category 0.032-0.063 mm (Fig. 7). Consistent with the work of Reference Dreimanis,, Vagners, and Goldthtwait,Dreimanis and Vagners (1971), this results in a mode in the grain-size distributions in this size range (Fig. 4). This mode, however, is not the lower limit of crushing or abrasion; rather it is a preferred daughter grain-size.

Feldspar is weaker than quartz, in part because it has three cleavage directions. Thus, it is crushed more readily, as shown by the larger changes in size distribution compared with quartz (Fig. 7). Furthermore, crushing of feldspar yields a wider range of grain-sizes with more fine material and no clearly dominant mode below 0.063 mm (Fig. 4).

Chlorite is quite weak and has a well-developed cleavage. Thus, it is readily crushed. The decrease in chlorite grains in the 0.5-1.0 mm size range is the largest observed (Fig. 7) and, consistent with grinding experiments summarized by Reference Drewry,Drewry (1986), the daughter grains vary widely in size (Figs 6b and 7). The strong positive relation between C_f and grain-size of chlorite, mentioned earlier, reflects the fact that larger chlorite grains are likely to be crushed into many small fragments rather than fewer large ones.

For feldspar and chlorite the maximum increases in Fig. 7 appear to be in the 0.032-0.063 and 0.016-0.032 mm size categories, respectively. However, the lack of distinct modes at these sizes in Fig. 4 suggests that these are not preferred daughter sizes. Rather, the preferred sizes may be as low as 0.016-0.004 or even 0.002 mm (Fig. 4), and thus not detectable given the limits of resolution of thin sections and the optical microscope.

Some failures involve detachment of one or several quite small fragments from the rims of a parent (Fig. 5c and d). These surficial failures are most common on larger grains. It is inferred that they are due to particle-particle interactions that are dominated by shear forces rather than by normal forces. The term abrasion may be appropriate for this process, although any distinction between abrasion and crushing is likely to be arbitrary. Micro-shears that could result in such failures have been observed in till in scanning electron-microscope studies (Reference Love,, Derbyshire, and Forde.Love and Derbyshire, 1985) and in optical microscope studies using thin sections Reference Van der Meer,van der Meer, 1990: Reference Yi.Yi Chaolu, 1992). Because such failures are most common on larger grains, they are most commonly observed on quartz.

Conclusions

Mineral properties determine grain-size distributions of individual minerals in till. During comminution, parent grains of quartz and feldspar tend to break into two daughter particles of roughly equal size, whereas chlorite grains, particularly the larger ones, break into more than two daughter particles. There appears to be a preferred daughter grain-size for quartz of about 0.05 mm. In contrast, for feldspar and chlorite the preferred daughter grain-size is less distinct, and may be quite small as these weaker minerals are more abundant than quartz in the fine-silt and clay fractions.

Failures resulting in a small number of daughter particles of roughly equal size are far more common than ones yielding several very small daughter particles and a large residual parent. To the extent that the former process is associated with normal stresses and the latter with shear stresses, crushing appears to be more common than abrasion. This has implications for understanding deformation mechanisms in till.