Sedimentary Geology, 46 (1986) 33-47 33

Elsevier Science Publishers B V, Amsterdam - Printed m The Netherlands

P R O C E S S E S OF DEBRIS C O M M I N U T I O N IN THE GLACIAL E N V I R O N M E N T A N D IMPLICATIONS FOR QUARTZ SAND-GRAIN M I C R O M O R P H O L O G Y

M A R T I N SHARP and BASIL GOM E Z

Department of Geography, Unwerslty of Cambridge, Downing Place, Cambridge CB2 3EN (Great Britain) School of Geography, Umverstty of Oxford, Mansfwld Road, Oxford OXI 3TB (Great Bmtmn)

(Received September 20, 1984, revised and accepted July 10, 1985)

ABSTRACT

Sharp, M and Gomez, B, 1986 Processes of debns comimnution in the glacial environment and

imphcanons for quartz sand-gram mlcromorphology Sediment_ Geol , 46 33-47

Studies of till composition, rock crushing and abrasion expenments , and detatled conslderatton of the

mechanics of the comrmnutton processes which occur in the subglaclal environment suggest that

monormneial quartz sand grains are mostly produced by the brittle fracture of larger parttcles_ Abrasive

wear is an inefficient mechamsm for producing and modifying quartz sand grams because of the relatively

great hardness of quartz. Viewed under the scanning electron microscope, surface textures of subglaclally

derived quartz sand grains are typically those assocmted with brittle fracture of quartz Since the release

of quartz sand grmns from bedrock by mechanical weathering may also revolve brittle fracture, grains

which have been passively transported through a glacier may exhibit slrmlar textures to those actively

produced in the glacial environment Hence the exaimnauon of the surface textures of quartz sand grains

under the scanning electron imcroscope is unlikely to be a satisfactory technique for the d]scnrmnation of active and passive transport paths through glaciers

I N T R O D U C T I O N

Recent work on debris transport by glaciers makes the distinction between "supraglaclal", "englacial" and "subglaclal" transport paths (Whalley and Krlnsley, 1974; Reheis, 1975, Boulton, 1978). Supraglacial and englaclal transport Is believed to be largely "passive", resultmg in httle modification of debris characteristics whtch are "mhented" from source matenal, while subglacial transport provides an active environment m which new particles are produced and grains are modified by crushmg and gnnding as a result of inter-particle and particle-bed contacts Al- though these processes substantially modify the particle-size distribution of debris and the shapes of large clasts (Rehets, 1975, Boulton, 1978), examination of surface textures of quartz sand grams under the scannmg electron microscope (SEM) has

0037-0738/86/$03.50 © 1986 Elsevier Science Publishers B V.

34

A B

C D

E F-

F~g 1 Typical morphologies and surface textures attributed to quartz sand grams from glacial enwron-

ments (A) Angular outline, (B) hLgh-rehef surface, (C) concholdal breakage patterns, (D) stepped surface

note discontinuities and surface debris, (E) breakage blocks, (F) edge abrasion Grams shown here

were transported by Swtss glaciers

35

failed to reveal major differences between actively and passively transported debris (Whalley and Krinsley, 1974, Eyles, 1978, Gomez and Small, 1983)

Whilst quartz sand grains from actively transported debris may show shghtly greater edge rounding than those from passively transported debris (Whalley, 1978, Dowdeswell, 1982) grams from both environments are charactensed by their angular form, irregular breakage patterns, conchoxdal fractures, sharp edges and flat or stepped cleavage surfaces (Krmsley and Smalley, 1972; Krmsley and Doornkamp, 1973, Whalley and Krmsley, 1974, see Fig_ 1). If supraglacial transport is indeed truly passive, th~s suggests that many surface textures which have been considered mdxcatlve of the glacial environment (Krmsley and Funnell, 1965; Krinsley and Smalley, 1972, Krinsley and Doornkamp, 1973, Table 1) may also be produced by non-glacial processes such as the mechanical weathenng of bedrock (Whalley and Krmsley, 1974). If this is the case, then it seems doubtful whether SEM examination of quartz sand gram surface textures is an appropriate means of investigating the

ongm of glaclgemc sediments At least part of the difficulty m interpreting the results of SEM studies ~s that

most of these studies have been concerned w~th the relauonships between the occurrence of particular surface textures and individual sedimentary environments rather than wxth the relatlonshaps between surface textures and the specific processes which produce them. Although most workers have assumed that "crushing" and "grinding" are the predominant commmution processes m the (sub)-glaclal environ- ment (Krmsley and TakahashL 1962, Krinsley and Smalley, 1972; Whalley, 1978; Lister, 1981), these processes have not been defined m terms of recogmsed wear processes (e.g. Rabinowlcz, 1976) or considered m terms of the role that each might play m the production and modification of quartz sand grams m the glacial environment. Since not all of the commmution processes active m the subglacial environment operate on all size fractions of the debris assemblage, it ~s quite possible that they will not all significantly influence the surface textures of quartz sand

grams In this paper we seek to describe the commlnutlon processes which occur m the

subglacml environment and the pattern of rock breakdown associated w~th each m order that we can identify those specific processes which are likely to influence the morphology and surface texture of quartz sand grams We then compare the surface textures which have been experimentally produced by these processes w~th those reported from quartz sand grains sampled m the modern glacml environment This approach allows us to suggest mechanical reasons for the fmlure of SEM investiga- tions to successfully distinguish between acuvely and passively transported debris, and to advocate potentially more profitable approaches to the problem

PROCESSES RESPONSIBLE FOR DEBRIS COMM1NUTION IN THE GLACIAL ENVIRONM ENT

Rabinowicz (1976) identified five principal wear processes, all of which might be expected to occur m the glacml enwronment (Table 2). We assume here that the

TA

BL

E 1

Sum

mar

y, o

f pr

inci

pal

surf

ace

text

ures

obs

erve

d on

qua

rtz

sand

gra

ms

from

con

tem

pora

ry g

laci

al e

n',l

ronm

ents

de

sign

ed t

o si

mul

ate

glac

ial

grin

ding

an

d of

th

ose

prod

uced

bv

exp

erim

ent,

,

Ref

eren

ce

Env

iron

men

t A

ngul

ar

Hig

h C

onch

olda

l S

tepp

ed

Bre

akag

e

sam

pled

ou

thne

re

hef

brea

kage

,,u

r fat

_e,,

bl

ock,

, G

roov

es

Rou

ndin

g

stri

ae

+ "e

dge

lnd

enta

tlo

n~

ab

rasi

on

"

Krm

sle,

, an

d S

mal

ley

(197

2)

Krl

nsle

~ an

d D

oorn

kam

p (1

973)

Krm

sle,

~ an

d M

argo

hs (

1969

)

Wha

lle~,

and

Krm

slev

(197

4)

Evl

e~ (

1978

)

Wha

lle?

(19

78)

Dow

desv

, ell

(198

2)

Wha

llev

and

Lan

gwa.

v (1

980)

Gom

ez a

nd S

mal

l (1

983)

Krl

nqev

and

gak

aha,

,hl

(196

2)

Gla

cial

Gla

cial

Gla

oal

Sup

ragl

acla

l ×

Eng

lacl

al

×

Sub

glao

al

Sup

ragl

acia

l

Su b

glao

al

Sub

glac

lal

×

Exp

erim

enta

l ,.

Sub

glao

al

×

Sub

glac

tal

Hig

h le

vel

×

Low

lev

el

Exp

erim

enta

l

y

x

w w )t

,(

X x"

x x

37

TABLE 2

Classification of wear processes (after Rablnowicz, 1976)

Adhesive wear Abrasive wear Corrosive wear Bnttle fracture

Surface fracture Fatigue fracture

Temporary adhesion A hard asperity Shdlng removes Wear involves the Cracks form in or between points on ploughs a groove the corrosive formation of sur- below the sliding the sliding sur- through a softer product, allow- face cracks during surfaces as a face leads to surface by lng corrosion to shdmg result of subsur- shear at some plastic defor- continue face stress point away from matlon variations the original interface

" g r i n d m g " process referred to by Kr lns ley and Takahash i (1962) and Wha l l ey (1978)

lS eqmvalen t to ab ra swe wear, and that "c rush ing" is a combina t ion of surface and

fat igue fracture. We thus make an ~mportant distmct~on between wear by processes

of plas t ic de fo rma t ion and wear by br i t t le fracture. Corros ive wear and adheswe

wear are not cons idered further here.

Abras ive wear is the process mode l led by exist ing theories of glacial ab ras ion

(Boul ton, 1974, 1979, Hallet , 1979, 1981), and it is respons ib le for the fo rmat ion of

smooth, con t inuous str iae on g lac ia ted bed rock surfaces. There are two dis t ract ive

forms of abras ive wear, namely " t w o - b o d y " and " t h r e e - b o d y " wear (Burwell, 1957)

In the former, wear results f rom the p loughing of one surface th rough the other,

whale in the la t te r it ~s p roduced by par t ic les t r apped be tween the two surfaces

Boul ton (1974) argued that " t w o - b o d y " wear p r o d u c e d striae, while " t h r e e - b o d y "

wear p roduced glacial pohsh. In the glacial env i ronment surface f racture wear is

respons ib le for the fo rma t ion of chips, gouges and arcuate fractures (Hal let , 1979;

Metcalf , 1979). This was the d o m i n a n t mode of wear observed dur ing a t t r l t twty tests

on andesl tes (Metcalf , 1979) and dur ing s a n d s t o n e - s a n d s t o n e shdmg exper iments

(Riley, 1982). Wea r processes may ac twely ini t ia te the p roduc t ion of sand grams from coarser

par t ic les and f rom bedrock, and modi fy the surface textures of exist ing sand grains.

As lndwldua l sand grams may only reasonably be expected to provide a de ta i led

p ic ture of the comminu t lon processes which have p r o d u c e d and modi f i ed them, successful in t e rp re ta t ion of their surface textures d e m a n d s an apprec ia t ion of how

the different size f ract ions of glacial debr is assemblages are p roduced .

EFFECTS OF COMMINUTION PROCESSES ON THE PARTICLE-SIZE DISTRIBUTION OF DEBRIS

One of the most comprehens ive a t t empts to s imula te the effects of d i f ferent

commmut~on processes on the size d i s t r ibu t ion of a debr is assemblage and on the

38

mineralogy and geochemistry of the different size fractions is that ol Haldorsen

(1981, 1983) Haldorsen carried out a series of ball-mill experiments under both wet

and dry conditions which were designed to investigate the effects of crushing by

percussion and of abrasion by particle-particle contact on samples of a Norwegian sandstone

In the crushing experiments, the gram s~ze of the rock fragments was gradually

reduced untd a stable log-normal size distribution was achieved and no further

commlnutlon occurred During the initial stages, the crushing process appeared to

follow Rosln's Law (Klttleman, 1964), but the end product was a sertes of mono-

mineral fragments whose size reflected that of minerals in the parent material,

suggesting that fracture took place primarily along lntergranular boundartes_ The

abrasion experiments produced silt-sized rock flour, much of which was smaller than

the mineral grams in the parent rock. Feldspars and sheet silicates were selecttvely

commlnuted, but the size of quartz grains was not significantly reduced

Whilst there may be some doubt about the extent to which these expertments replicate naturally occurring conditions, Haldorsen was able to show that the

characteristics of the commmuted debris assemblages were very similar to those of

tills derived from similar bedrock in Astadalen, southeast Norway Basal meltout

tills in this area were characterlsed by a low silt content and appeared to have been

produced by crushing, whereas lodgement tills contained significant amounts of silt, which, smce it lacked any significant quartz component, was probably formed b'v

abrasion (Haldorsen, 1983) The bulk geochemistry of the meltout tills appeared to

preclude the possibility that their stlt-deficlency was a product of selecttve winnow-

ing of fines by meltwater Haldorsen's results suggest that most sand grains were

produced by the crushing of gravel clasts, whereas abrasive wear produced Slit-sized

debris which was derived primarily from cobble and boulder-sized clasts (Fig 2)_

CRUSHING ABRASION

G .... i j ~

Y /

Cobbles I

\ \ -<

Fig 2 Processes leading to the commmut lon of debris den'ced Irom a resistant, relatl,,el_~ ~.oarse-gramed

bedrock in the glacial environment (after Haldorsen, 1981) Sohd lines indicate major relations and dotted

hnes minor relations

39

Once formed, monomineral sand grams were remarkably stable, apparently suffering

very little further commmuuon by either abrasion or crushing. Haldorsen's results suggest that m the subglaclal environment quartz sand grains

are likely to be produced primarily by brittle fracture associated with crushing and that they are unlikely to be significantly modified by abrasive wear. These conclu- sions are, however, dependent upon satisfactory explanation of four principal results

of Haldorsen's experiments' (1) brittle fracture produces selective commlnutlon of large particles; (2) brittle fracture occurs primardy along mtergranular boundaries;

(3) abrasive wear does not produce significant comrmnutlon of quartz particles; and

(4) abrasive wear is concentrated on cobble and boulder size clasts. It is therefore necessary to consider each of these conclusions In terms of the mechanics of brittle

fracture and abrasive wear.

1 Why ts brittle fracture concentrated on large parttcles?

At the low temperatures and stresses characteristic of the subglacial environment, fracture ,s controlled by pre-existing cracks which can propagate at stresses much

lower than those required to induce slip or twinning on any crystallographic system (Atkinson, 1982). Fracture occurs at a critical stress"

Or = ( EGc/Tr C),/2 (1)

where E is Young's modulus, C Is the half-length of the pre-existing crack and Gc is the critical strata energy release rate, which for plane strata Is given by.

G~ = K,2(1 - VZ) /E (2)

where 3' is Polsson's ratio. K,, the stress intensity factor , is a measure of the strength of the singularity in the crack-tip stress field which produces tensde mode crack propagation (Paris and Sih, 1965). Neglecting interactions between adJacent

cracks and treating rock as a linear elastic medium:

K, = (~rZC/2)'/Z(P + o) (3)

where P is the pressure inside the crack, and o is the applied normal stress perpend,cular to the crack plane Crack propagation often occurs at sub-critical velocities because the presence of liquid water, water vapour or other reactive species m the crack-tip environment promotes weakening reactions which facilitate crack propagation (cf. Wlederhorn, 1967; Scholz and Martin, 1971; Scholz, 1972). This phenomenon, known as stress corrosion, may have occurred in Haldorsen's (1981) experiments, since wetting of the samples was found to accelerate the crushing process, and ~t is also likely to occur in temperate glaciers, where water if freely available at the glacier bed and ,n the basal ice. For stress corrosion, K, can be related to the crack velooty, V, by Charles' (1958) power law (Atkinson, 1982):

V = V 0 exp( - diH/RT)K," (4)

4O

where V 0 is a p ropor t iona l i ty factor character is t ic of the material , A H ~s an

act ivat ion enthalpy, R is the gas constant , T is the absolute temperature , and n ~s a

ma te r i a l -dependen t cons tant termed the stress corros ion index.

F rom eqn. (3) it is apparen t that for a given value of K,, there is an inverse

re la t ionship between ( P + o ) and the initial crack length 2C. The smaller the crack,

the greater the stress required to make it p ropaga te at a given velocity_ Large cracks

will therefore tend to p ropaga te more easily than small ones, and since crack size

must in some way be l imited by the s~ze of sed imenta ry particles, it follows that for a

given mater ia l the stresses required to cause cracks to p ropaga te at a given velocity

will tend to increase as par t ic le size decreases (cf Charles, 1957). Hence one would

expect bri t t le fracture to be concent ra ted on larger par t ic les in the debr is assem-

blage

2 Why doe3 jracture occur prlmartly along lntergranular boundaries ~

Haldorsen ' s crushing exper iments indicate that the end p roduc t of debris com-

mlnut lon by bri t t le fracture is an assemblage of monomlne ra l fragments, and this

suggests that fractures tend to p ropaga te selectively a long grain boundar ies These

may be boundar ies between grains of the same mineral or between different minera l

species, and their s trength will depend upon the coherence between the structures of

the two phases (Lawn and Wllshaw, 1975). Where violat ions of the d l rec t lonah ty

and charge reqmrements of covalent- ionic bonds occur there are subs tant ia l reduc-

tions In lnterfacxal cohesion and this is reflected in a reduc t ion m the force required

to p romo te crack extension This effect may, however, be coun te rac ted by the

increased reststance to fracture which results from devta t lon of the local crack pa th

f rom the main crack p lane This is par t icu la r ly impor t an t at large grain s~zes since

the force driving crack growth will tend to decrease with increasing dis tance from

the mare crack p lane

Where stress corrosion makes an impor t an t con t r ibu t ion to crack growth, Inter-

g ranular cracking may be common, par t icu lar ly at low crack velocit ies (Atk lnson

and Rawhngs, 1981)_ Penet ra t ion of mois ture into the rock wall occur p r imar i ly

a long grain boundar ies , and at low values of K, these will tend to stress cor rode

before the more highly stressed mte rgranu la r cracks There will therefore be a

tendency for mlcrocracks to follow gram boundar ies when K, is small

3 Why does abrastve wear not cause stgntftcant commmutton of quartz grams '~

Haidorsen ' s (1981, 1983) abras ion exper iments p roduced selective commlnu t ion

of fe ldspars and sheet slhcates, while quar tz was relat ively unaffected_ Raley (1982)

gives a model of abras ive wear in which the rate of surface lowering_

V, - K ( t a n O / ~ r ) ( o V / H ) (5)

41

where K is a measure of the effectiveness of the abrading point, o is the applied

normal load, V is the particle velocity, H the indentation hardness of the wearing

surface, and 8 the angle between the wearing surface and the sides of the asperity

According to Torrance (1981a) a point can indent a surface when

K a > K m [1 + ( 7r /2) ] / [1 + (~r/2 - 0)] (6)

where K a and K m are the shear yield strengths of abrasive and wearing surface,

respectively. Assuming that the yield strength can be related to the hardness by a

function of the form H = cK and that c is the same for both abrasive and wearing

surface, eqn (6) can be rewritten:

, ~ > H m [1 + (~r /2 ) ] / [1 + (~r/2 - 0)] (7)

Thus when 0 = 20 ° abrasive wear begins when H a = 1.16 H m and when 0 = 30 °, it

begins when H a = 1.28 H m The implicanon is that as H~ approaches /4, the

stresses on the tool tip increase until it starts to collapse, reducing 0 and the

hardness differential required for abrasion. Observation suggests that abrasive wear ceases when 0 < 20 °, implying that a 16% hardness differential is required for it to

occur (Torrance, 1981b). This requirement may explain Haldorsen's experimental results and the paucity of

quartz silt In the Norwegian lodgement tills which she studied. The hardness of

minerals is usually quoted in terms of Moh's scale (Tabor, 1954), but this can be

converted to indentation hardness using the expression

log H = M. log 1.64 + log 25 37 (8)

(Riley, 1982). Quartz has a Moh's hardness of 7, equivalent to an indentation hardness of 671 kg mm 2, and it follows that It can only be abraded by minerals

with a hardness in excess of 778 kg mm -2 (Moh's hardness 7 3) Haldorsen (1983)

states that the ancillary minerals In the sandstones used in her experiments include

mlcrocllne ( M = 6 6.5), alblte, muscovite ( M = 2 . 5 - 3 ) , calcite ( M = 3 ) , biotite (M = 2.5-3), chlorite (M = 2-3) and sericlte, all of which have hardnesses signifi-

cantly less than that of quartz. One would not therefore expect to observe apprecia-

ble abrasive comminution of quartz in Haldorsen's experiments, and since relatively

few commonly occurring minerals have hardnesses in excess of 7.3 (examples include garnet, zircon, staurohte and sillimanlte), abrasive wear of quartz will probably

occur relatively infrequently in the glacial environment Quartz is much more likely

to act as the tool which produces comminutlon of other mineral species.

Although abrasive wear ceases when H a < 1.16H m, other forms of wear such as

brittle fracture and adhesive wear do still occur, and one might expect to find that

the effects of these processes would dominate the surface textures of quartz sand grains. The "edge abrasion" reported by Whalley (1978) and Whalley and Langway (1980) from subglacial and experimentally ground quartz grains is not true abrasion,

but brittle fracture localised on grain edges (cf. Fig. 1F). It probably reflects the

collapse of these edges in contact situations charactensed by a high angle, 0,

42

between the two faces def ining the grain edge and the contac ted surface, or a small

hardness dif ferent ia l between that face and the gram This process is most l ikely to

occur in envi ronments character ised by frequent par t i c le -bed or in ter-par t ic le con-

tacts, and its effects should therefore be more c o m m o n in subglaclal ly t r anspor ted

debr is than in supraglacla l ly t ranspor ted debris, as in fact appears to be the case

(Whalley, 1978, Dowdeswell , 1982).

4 Wh~ ts abras ive wear ¢oncen t ra ted on boulders' a n d ~ohbles '~

Reference to eqn (5) shows that the only variable inf luencing abras ive wear

which is dependen t upon part ic le size is o, the appl ied normal load. Accord ing to

Hailer (1979, 1981), this can be visuahsed as an effective contac t force, F, which

arises from the buoyan t weight of the par t ic le and f rom the pseudo-viscous drag

exer ted on the par t ic le by the componen t of ice flow directed towards the glacier

bed Hence.

F = 4 / 3 rr R ~ ( 0r - P, )g cos 0 + (4~r~IR-~/R. + R 2 )V~, (9)

where R IS the radius of a spherical part icle, 0r ~s the densi ty of rock, p, is the

densi ty of ice, g is the gravi ta t ional accelerat ion, 0 is the local average downglac le r

bed slope, ~ is the viscosity of l inear wscous ice, 1,], is the velocity componen t of the

ice normal to the bed, and R , is a cri t ical radms at which the drag per unit

cross-sect ion area is maxLmlsed It ~s clear from this equat ion that both the buoya n t

weight of the par t ic les and the drag exerted on them by the Ice will increase with

par t ic le size, and it therefore follows that the rate at which part icles are ab raded will

s imilar ly increase with par t ic le size, as was clearly shown by Hal le t (1979, 1981)

CHARACTERISTICS OF FRACTURE SURFACES

The above discussion suggests that Ha ldorsen ' s results can be logically expla ined

in terms of the mechanics of br i t t le fracture and abrasive wear, and we therefore

argue that the br i t t le fracture of gravel-sized clasts is the pr incipal mechan i sm by

which quar tz sand grains are p roduced in the subglaclal envi ronment Subsequent

modi f ica t ion of these grains is l imited to the locahsed col lapse of sharp grain edges

as a result of stresses imposed by inter-par t ic le and par t ic le-bed contacts . This view

can be tested by compar i son of the surface textures repor ted from subglacmlly

der ived quar tz sand grains with those p roduced dur ing exper iments on the bri t t le

fracture of quartz.

Quar tz is general ly regarded as a highly bri t t le mater ia l with no p ronounced

crys ta l lographic anIsot ropy Al though the s t ructural a r rangement of the S I - O bonds

does not appear to facil i tate cleavage (cf Hoffer , 1961) it ma.~, however, favour

cleavage in some direct ions more than others (Fa l rba l rn , 1939, Bloss and Gibbs ,

1963, Krlnsley and Smalley, 1973, Wel lendor f and Krlnsley, 1980) In o rder of

43

perfection, Fairbalrn (1939) observed that cleavage should be best developed on the following planes, r (1011), z (01]1), m (1010), c (0011), a (1111), s (1111) and x

(5111) Quartz might therefore be said to exhibit an imperfect cleavage, and

although crack paths would not be expected to follow partmular crystallographic

planes, intersections with these planes will probably influence crack paths on the

local scale (cf. Bloss, 1957) Hence steps on fracture surfaces in quartz (eg Fig. 1D) appear to reflect crack growth in sawtooth fashion as a series of mlcrocracks opening

alternately along rhombohedral r and z cleavage planes, and they are not directly related to crack branching (Martin and Durham, 1975; Norton and Atklnson, 1981).

As a result of the influence of crystallographic planes on the local crack path, the

detailed pattern of surface markings on the fracture surface will change as the orientation of the fracture surface changes relative to the crystallographic orientation of the particle. Fairbalrn (1939) found that conchoidal fractures (e.g. Fig 1C) were best developed normal to m (1010), while Martin and Durham (1975) observed that

fracture surfaces parallel to the basal plane of quartz crystals were marked by straight steps defined by the r and z planes, whereas surfaces normal to the basal

plane were very smooth Norton and Atklnson (1981) found that on surfaces perpendicular to the c-ax~s of crystals the imtlatmg flaw was surrounded by an area of relatively large steps approximately parallel to the flaw boundary. This zone was succeeded outwards by one of much finer steps and then by a second set of large steps defined by the r and z planes, and which lndmated increasing crystallographic control on the fracture prior to crack branching On surfaces parallel to m (1010) the mmat ing flaw was surrounded by a perfectly planar area which passed into a region of steps approximately parallel to the r and z planes The size of these steps increased along the fracture until the point where the crack branched

The relative size of the textural zones on the fracture surface depends on the fracture stress, or, and the specimen size (Norton and Atklnson, 1981) The distance, r*, from the imtiating flaw to the point where rhombohedral steps are initiated is

given by

A * = o f r *1 /2 (10)

where A* is constant for a given quartz type. Hence the larger the fracture stress, the closer to the lnitmtlng flaw the rhombohedral steps will begin. Norton and Atklnson (1981) found that for glass specimens less than 5 mm in diameter A* increased with increasing specimen size, and a similar relationship may hold for quartz. Fracture stress and spemmen size will therefore determine the extent to whxch pamcula r surface textures appear on any given fracture surface, but since quartz sand grams are defined by intersecting fracture surfaces, the extent to which complete sets of textures are preserved wall depend upon the extent to which mdlwdual surfaces are truncated by others.

In addition to crystallographmally controlled planes of weakness, quartz grams may also contain structural defects which were introduced by the considerable

44

stresses generated during rock formation (Smalley and Krlnsley, 1974). These defects

may take the form of mlcrofractures which are typically spaced at intervals of 1 10

/~m (Moss and Green, 1975) Mlcrofractures may be generated by the stresses

associated with the high low quartz transmon (Smalley, 1966, 1974) or by deforma-

tion under pressure and temperature condmons at which quartz is essentially brittle

but at whxch associated minerals remain relatively ducule (Moss, 1966, Moss et al,

1974). The influence of microfractures on the fragmentation and resultant mor-

phology of individual quartz sand grains has been clearly demonstrated (Moss, 1966,

1972, 1973; Moss et al_, 1973, 1974, 1981, Moss and Green, 1975) Where both

mlcrofractures and crystallographically controlled planes of weakness are present, a

crack may segment on a number of planes in order to maximlse ~ts surface area

parallel to the preferred fracture direction (Craggs, 1960)_ Structural considerations

may therefore influence the development of "mlcroblocks" (e g F~g. IE), a texture

reported from subglaclally transported grams by Eyles (1978), on the fracture

surface, and discontinuities in step geometry such as "cleavage plates" may result

from the partial overlap of crack fronts (Lawn and Wllshaw, 1975)

It is therefore apparent that the characteristic effects of brittle fracture on quartz

(Fig 1C, D and E) are (conchoMal) fracture surfaces and their associated irregular-

ities (steps, microblocks, cleavage plates), which reflect energy dissipation during

crack growth These irregularities may be locahsed by the intersection of the crack

front with internal structural defects, and they may be the dominant surface textures

of quartz sand grains ff fracture stresses are h~gh and defects occur at high densities

Smooth fracture surfaces probably reflect some combination of 1o~ fracture stresses,

low defect densities and fracture orientations normal to the basal plane of quartz

crystals_ InspecUon of Table 1 makes ~t clear that the surface textures commonly

observed on quartz sand grains taken from (sub)glacial environments are exactly those textures which are produced by brittle fracture. Edge rounding appears to be

the only effect which may distinguish subglacial from supraglacial environments and

~t probably reflects the frequency of contacts in this environment and the relatively great hardness of quartz which protects It from significant modification b~, abrasive w e a r

C O N C L U S I O N S

Consideration of the mechanics of the commlnution processes active in the

subglaclal environment leads us to the conclusion that monommeral quartz sand

grains are mostly produced by the crushing of larger particles Although abrasive wear is undoubtedly a very important process in the basal traction zone of glaciers it does not s~gnlflcantly affect quartz particles because of their relatively great hard- ness. Quartz particles most commonly act as tools which produce comminutJon of

softer minerals, and the surface textures of subglaclally derived quartz grams will be dominated by the effects of brittle fracture

45

Since sand grains produced by the mechanical weathering of bedrock which are passwely transported m englacial and supraglaoal envxronments may also show dominant surface textures produced by brittle fracture it may be difficult to dlstlngmsh between subglacially and supraglacmlly transported debris using surface texture analysis (as indicated by Whalley and Krinsley, 1974). When quartz sand grains come into contact with particles of s~mllar hardness, however (an event which may be more common in subglacml environments than In englacial or supraglaclal ones), the most common response ~s the fracture of sharp grain edges to produce edge rounding. This may be more easily detected using closed form Fourier analysis of gram outhnes than using surface texture analysis (cf Dowdeswell, 1982) Al- though the effects of abrasion in the subglacial environment may be more easily discerned from study of the surface textures of large clasts (Krinsley and Donahue, 1968; Boulton, 1978) or of non-quartz sand grams, detailed study of quartz sand-gram surface textures might be able to provide valuable reformation about the energy levels characteristic of particular glacml sub-environments and their relationship to the lnltmt~on and intensity of fracturing

A C K N O W L E D G E M E N T S

We wish to acknowledge support from the BGRG Research and Pubhcatlons Fund and a Royal Society Scientific Investigations Grant to BG. Work was carried out while MS held a Jumor Research Fellowship at Merton College, Oxford. Figure 1F is reproduced by permission of the Editor, Geografiska Annaler Series A We thank J.A. Dowdeswell for his critical review of an earlier draft of the paper.

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