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Characteristic Features of Glacial Sediments

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Characteristic Features of Glacial Sediments Don J. Easterbrook Department of Geology Western Washington University Bellingham, Washington INTRODUCTION Few depositional environments are as varied as those associated with gla- ciers, resulting in deposits of widely differenting physical characteristics. Sediments deposited directly from gla- cial ice generally are poorly sorted and unstratified diamictons whereas those indirectly associated with ice via meltwater streams or lakes are sorted and stratified. Recognition of glacial clastic sediments is important because of climatic implications and rapid fa- des changes associated with glacial deposition. DIAMICTONS The term diamicton is used for poorly sorted, unstratified deposits of unspecific origin. The most common glacial diamictons are till and gla- ciomarine drift, both deposited more or less directly from ice without the winnowing effects of water. They are characterized by a heterogeneous mix- ture of sediment sizes, ranging from boulders to clay, and a lack of stratifica- tion. Particle size distribution is often bimodal with concentrates in the peb- ble-cobble and silt-clay fractions (Fig. 1). Both types of diamictons are usually massive with only minor stratified in- tercalations. Till Glacial till is deposited in direct con- tact with glacial ice. Although it does not make up substantial sediment thicknesses in the geologic record, till makes a discontinuous cover for as much as 30% of the earth's continental landmasses and forms significant de- posits in Precambrian and Permo- Carboniferous rocks of South America, Africa, and North America (Fig 2). Till consists of unsorted, unstratified 30- 20- 10 - GLACIAL TILL . 40- 20- GLACIOMARINE DRIFT 64 32 16 2 1 V 4 ' ib- ' MM 8 4 2 1Vj VA 0/ /O 30- 20- 10 20 16 8 4 2 1 Vj Va Fig. 1—Particle size distribution of till and glaciomarine drift. pebbles, cobbles, and boulders in a ma- trix of sand, silt, and clay. The coarser fraction is mostly pebble size with cob- bles and boulders scattered through- out. Many pebbles are rounded to sub- rounded, suggesting that they were in- corporated by ice riding over stream gravel; others have been faceted, stri- ated, and polished by glacial abrasion. Sand and silt in the matrix is usually angular to subangular quartz, with much of the fine silt consisting of quartz rock flour. Physical properties which permit recognition of a diamicton as a till in- clude the following: (1) poor sorting with bimodal particle size distribution; (2) lack of stratification (Figs. 2, 3); (3) Copyriyht 1 8 4 2 1 V 2 VA PARTICLE SIZE (MM) 60- 40 - 20 - 32 16 8 4 2 1 V2 V 4 faceted, polished, and striated stones (Fig. 4); (4) fabric consisting of a pre- ferred orientation of long axes of elongated particles; (5) compactness in close packing of constituent particles as a result of overriding ice, high bulk densities, and low void ratios; (6) er- ratic lithologies of stones and heavy minerals; (7) striated and polished bed- rock surface beneath the till; and (8) strongly sheared, folded, structures produced by glacial movement (Fig. 5). At least three different types of tills, lodgement, ablation, and flow have been recognized on the bases of varia- tion in physical properties and dif- fering depositional processes from gla- cial ice. © 1981 by The American Association of Petroleum Geologists.
Transcript

Characteristic Features of Glacial Sediments Don J. Easterbrook Department of Geology Western Washington University Bellingham, Washington

INTRODUCTION Few depositional environments are

as varied as those associated with gla­ciers, resulting in deposits of widely differenting physical characteristics. Sediments deposited directly from gla­cial ice generally are poorly sorted and unstratified diamictons whereas those indirectly associated with ice via meltwater streams or lakes are sorted and stratified. Recognition of glacial clastic sediments is important because of climatic implications and rapid fa­des changes associated with glacial deposition.

DIAMICTONS The term diamicton is used for

poorly sorted, unstratified deposits of unspecific origin. The most common glacial diamictons are till and gla-ciomarine drift, both deposited more or less directly from ice without the winnowing effects of water. They are characterized by a heterogeneous mix­ture of sediment sizes, ranging from boulders to clay, and a lack of stratifica­tion. Particle size distribution is often bimodal with concentrates in the peb­ble-cobble and silt-clay fractions (Fig. 1). Both types of diamictons are usually massive with only minor stratified in­tercalations.

Till

Glacial till is deposited in direct con­tact with glacial ice. Although it does not make up substantial sediment thicknesses in the geologic record, till makes a discontinuous cover for as much as 30% of the earth's continental landmasses and forms significant de­posits in Precambrian and Permo-Carboniferous rocks of South America, Africa, and North America (Fig 2).

Till consists of unsorted, unstratified

30-

2 0 -

10 -

GLACIAL TILL

. 4 0 -

20-

GLACIOMARINE DRIFT

64 32 16 2 1 V4 ' ib- ' MM

8 4 2 1 V j VA

0/ /O

30-

20-

10

20

16 8 4 2 1 Vj Va

Fig. 1—Particle size distribution of till and glaciomarine drift.

pebbles, cobbles, and boulders in a ma­trix of sand, silt, and clay. The coarser fraction is mostly pebble size with cob­bles and boulders scattered through­out.

Many pebbles are rounded to sub-rounded, suggesting that they were in­corporated by ice riding over stream gravel; others have been faceted, stri­ated, and polished by glacial abrasion.

Sand and silt in the matrix is usually angular to subangular quartz, with much of the fine silt consisting of quartz rock flour.

Physical properties which permit recognition of a diamicton as a till in­clude the following: (1) poor sorting with bimodal particle size distribution; (2) lack of stratification (Figs. 2, 3); (3)

Copyriyht 1

8 4 2 1 V2 VA

PARTICLE SIZE (MM)

60-

40 -

20 -

32 16 8 4 2 1 V2 V4

faceted, polished, and striated stones (Fig. 4); (4) fabric consisting of a pre­ferred orientation of long axes of elongated particles; (5) compactness in close packing of constituent particles as a result of overriding ice, high bulk densities, and low void ratios; (6) er­ratic lithologies of stones and heavy minerals; (7) striated and polished bed­rock surface beneath the till; and (8) strongly sheared, folded, structures produced by glacial movement (Fig. 5).

At least three different types of tills, lodgement, ablation, and flow have been recognized on the bases of varia­tion in physical properties and dif­fering depositional processes from gla­cial ice.

© 1981 by The American Association of Petroleum Geologists.

2 D. J. Easterbrook

Fig. 3—Till, north of Uppsala, Sweden.

Fig. 2—Precambrian Gowganda tillite scoured by Pleistocene ice sheets, near Lake Ontario, Canada,

Fig. 4—Striated stones in till, Solheimajokull, Iceland,

Lodgement till — This type is depo­sited subglacially from basal, debris-laden ice under the influence of shear stress, as the till is plastered over the ground beneath the ice (Fig. 6). Thus, lodgement till is characterized by pre­ferred fabric and a high degree of com­paction.

The long axes of rod-shaped stores are preferentially oriented with a pri­mary maximum parallel with the direc­tion of ice movement and a secondary smaller maximum at right angles. Shearing and weight of ice produce compaction of the deposit evidenced by high bulk densities and low void ratios of uncemented deposits.

Ablation till — This till is deposited from englacial and superglacial debris dumped on the land surface as the ice

Fig. 5—Deformation of glacial gravel by ice shove, near Bern, Switzerland.

melts away. In contrast to lodgement till, ablation till usually is only loosely consolidated and possesses a random fabric in the absence of strong shearing stresses.

Flow till — This deposit is formed by water-saturated debris flowing off gla­cial ice as mud flows. It hasn't the de­gree of compaction of lodgement till but elongate stones may have a pre­ferred orientation as a result of flowage (Fig. 7).

Glaciomarine Drift Deposits of glacial debris melted out

of ice floating in marine water are col­

lectively termed glaciomarine drift in recognition of both the glacial and ma­rine influence on resulting sediment. Because of their poor sorting and lack of stratification, glaciomarine drifts of­ten resemble glacial tills in general ap­pearance and have sometimes been re­ferred to as "marine tills," but the term glaciomarine drift is more appropriate because sediment is not deposited in contact with glacial ice and often in­cludes facies which are not till-like in appearance. The floating ice may con­sist of shelf ice or berg ice calving from a glacier margin (Figs. 8, 9).

Melting of floating ice releases clay,

Fig. 6—Lodgement till being deposited from the base of moving ice, Breidamerkurjokull, Iceland.

Fig. 7—Flow till with parallel orientation of stones, Kristineberg, Sweden.

ICEBERGS

GLACIOMARINE DRIFT O

o O ,

O Q

o -O TILL

Fig. 8—Depositional environments of glaciomarine drift and basal till.

Glacial Sediments 3

silt, sand, pebbles, cobbles, and boul­ders which settle to the underlying seafloor, often burying marine mol-lusks living on the bottom (Figs. 8,10).

The coarser fraction of glaciomarine deposits consists of pebbles, cobbles, and a few boulders, many of which are faceted, polished, and striated, ran­domly scattered throughout a matrix of clay, silt and sand. The pebble to silt-clay ratio varies considerably, grading from till-like diamictons to silty clay with only a few pebbles. Typical parti­cle size distributions are shown in Fig­ure 1.

Glaciomarine drift is generally characterized by a lesser degree of compaction than glacial till, presum­ably because deposits were never un­der appreciable glacial loading (Fig. 11).

The glacial origin of these fossilifer-ous deposits is indicated by: (1) lack of sorting; (2) faceted, striated and pol­ished pebbles showing little or no sec­ondary rounding (Fig. 12); and (3) face­ted and polished erratics of distance provenance.

Till-like deposits (Figs. 13,14) proba­bly represent times when debris-laden floating ice was extensive, whereas pebbly silt and clay probably represent times when floating ice was less abun­dant or lacked large quantities of de­bris.

The relatively uniform distribution of the coarser fraction throughout the clay-silt matrix indicates that a nearly continuous rain of unsorted material took place during the greater part of sediment accumulation. If berg ice was the principle transport agent, calving and melting of the bergs must have oc­curred rapidly to produce the relatively uniform distribution of coarse mate­rial.

Criteria for identification of gla­ciomarine drift includes: (1) whole, un­broken shells; (2) articulated pelecy-pod valves (Fig. 15); (3) preservation of mollusks in growth position (Fig. 15); (4) barnacles and other marine mol­lusks attached to glacially faceted sur­faces of pebbles; (5) distribution of shells throughout a deposit; (6) ab­sence of underlying fossiliferous de­posits from which shells could be re­worked; (7) preservation of delicate ornamentation on shells; (8) foramini-fera and diatoms in matrix material; (9) sediment inside articulated shells and worm tubes indicating that organisms were living in the environment of depo­sition of the diamicton; (10) regional distribution of deposits; and (11) high content of Na and total exchange ca-

4 D. J. Easterbrook

"SI

Fig. 9—Debris-laden icebergs from Breidamerkurjokuil, Iceland.

Fig. 10—Fossiliferous Pleistocene glaciomarine drift, Penn Cove, Washington.

Glacial Sediments 5

.9 -

5 .5 g § .4

.3 -

.2 -

.1 -

~

_

-

-

-

"

-

"

o

I

G

O

I I

G

1

0

G

1

G

0

0 G

G Q 0 G A

A V A

V

1 1

A VASHIONTILL

V SUMASTILL

G GLACIOMARINE DRIFT

A ^

v A * * >

1 1 1 1 1 1

1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7

BULK DENSITY

Fig. 11—Void ratios and bulk densities of Pleistocene glaciomarine drift and till, Puget Lowland, Washington.

Fig. 13—Till-like texture of Pleistocene glaciomarine drift, Puget Lowland, Washington.

^-""W

Fig. 12—Faceted, polished and striated cobble from glaciomarine drift, Puget Lowland, Washington.

mm I;;

-

I

Fig. 14—Shell-bearing, till-like glaciomarine drift, Puget Lowland, Washington.

Fig. 1S—Articulated valves of pelecypods preserved in growth positions in Pleistocene glaciomarine drift, Bellingham, Washington.

tions in clay size fractions (Fig. 16). Measurement of exchangeable Na in

clay-size fractions may be used to dis­tinguish some glaciomarine sediments from similar appearing diamictons (Pe-vear and Thorsen, 1978). Sodium con­tent in expandable clay is higher in gla­ciomarine drift apparently because of enrichment from sea water (Fig. 16).

ICE-CONTACT DEPOSITS

Not all debris transported and depo­sited by glaciers consists of till. Meltwater on, under, within, or mar­ginal to glaciers produces detritus which, when deposited on, against, or beneath ice, forms deposits known col­lectively as ice-contact sediments. Be­cause most such deposits involve

6 D. J. Easterbrook

6

E H

TILL

n E -

JR3

GMD H I = unoxidized

p ILpi n , i—i £—D i 2 3 4 5 6 7

Na, meg./100 g. clay, normalized to 1 Na, K, Ca, My = 100

Na Ca

20>

5 -

TILL

I f f T I

GMD

• 01 1 2 3 4 5 6

Meg.! OOy

2 0 '

15'

1C'

5'

TILL

T — n -

GMD

15 10

5-

TILL

GMD

2 0 -15-10-

5

Mg Total Cations

TILL

GMD

Meg lOOg

0 10 20 30 4050 60 -70

MeglOOg

0 10 20 30 40 50 60 70

Meg/IOOg

20 30 40 50 60 70 80 90 -100

Meg, 100g

Fig. 16—Exchangeable sodium and other cations in glaciomarine drift and till, Puget Lowland, Washington (after D. R. Pevear, 1978).

Fig. 17—Sediment accumulating in a moulin, Breidamerkurjokuil, Iceland.

Fig. 18—Deformation of ice-contact glaciofluvial deposit by collapse of supporting ice. Ostersund, Sweden.

Glacial Sediments 7

cransportation by meltwater streams, they exhibit better sorting and stratifi­cation than sediments laid down di­rectly from ice. The finer fraction is winnowed from the coarser detritus so particle size distribution curves lose most or all traces of bimodal distribu­tion typical of glacial till. Pebbles and cobbles are quickly rounded after only short distances of transport.

Accumulation of glaciofluvial sand and gravel in englacial or superglacial cavities of a glacier (Fig. 17) leads to deposition of irregular bodies of sedi­ment exhibiting ice-slump deformation when supporting ice melts away (Fig. 18,19). Post-depositional melting of ice can produce extensive internal defor­mation of sediments. Deformation in deposits may include a variety of col­lapse features, such as tilting, faulting, and folding. Often, stratified sediments and flow tills or ablation tills are in-

terbedded(Fig. 19).

GLACIOFLUVIAL DEPOSITS

The character of sediments deposi­ted from glacial meltwater often bears the imprint of glacial environments, Al­though mechanisms of transportation and deposition show similarities to other fluvial environments, large fluc­tuations in discharge on a daily, sea­sonal, or long-term basis produce abrupt particle size changes and sedi­mentary structures reflecting the fluc­tuating discharges and proximity to glaciers.

Although measurements of many glacial sediment characteristics have been made, recognition of a specific environment is often difficult because variation in rock types and recycling of detritus through several environments produce changes in pebble sizes and shapes making the influence of the fi­

nal depositional environment obscure. Price (1973) found no significant differ­ences in roundness values between moraines and eskers at Breidamerkur-jokull, Iceland, presumably because many of the clasts in the moraines were formed by reworking of glaciofluvial sediments.

Glaciofluvial deposits grade into normal fluvial downstream as the gla­cial influence diminishes. Near the gla­cial margin, clasts may exhibit a lower degree of rounding than nonglacial flu­vial sediments. Rounding increases abruptly, however, in transport down­stream, with recycling of previously overriden material diminishing the value of rounding in distinguishing gla­cial and nonglacial deposits.

Pebble and cobble clasts in outwash deposits are sometimes well imbrica­ted. Upstream dips are considered good indicators of current direction. Long axes of larger elongate clasts are often oriented transverse to main cur­rent flow.

Gravel bars on proximal portions of outwash fans consist of poorly sorted, imbricated, stratified gravel. Down­stream migration of megaripples produces large-scale festoon cross-bedding, with migration of longitudinal and linguoid bars resulting in cross-bedding.

Near the glacier margin, outwash may be deposited on stagnant ice (Fig. 22). Melting of the buried ice produces significant disruption of both morphol­ogy and internal structure of the de­posit (Fig. 23). Downstream, such sedi­ments grade into kettled but relatively undisturbed glaciofluvial sand and gravel, eventually forming fluvial de­posits lacking evidence of glacial ori­gin.

Fig. 19—Interstratified ice-contact stratified sediments anddiamictons, Sveg, Sweden.

Fig. 20—Glacial outwash channel and terrace, Breidamerkurjokull, Iceland. Fig. 21—Outwash gravel in meltwater channel, Breidamerkurjokull, Iceland.

8 D. J. Easterbrook

Fig. 22—Outwash sediment being deposited on stagnant ice, Breidamerkurjokull, Iceland.

Fiq. 23—Collapsed outwash deposit deformed by melting of buried ice, Breidamerkurjokull, Iceland, DELTAS AND

GLACIOLACUSTRINE DEPOSITS

Where meltwater streams discharge into lakes or the sea, deltas are formed. If the glacial margin is close by, an ice-contact delta is produced (Fig. 24) and the resulting deposit will show various slump deformation structures made by melting of supporting ice in addition to normal topset, foreset, and bottomset stratification. Deltas made by glacial outwash distant from the ice terminus (Fig. 25) do not exhibit ice collapse

structures, but are still subject to wide­ly fluctuating discharges typical of meltwater streams. These result in con­siderable variation in sediment dis­charge and particle size distribution in sediments. Foreset bedding often con­sists of relatively parallel beds of sand and gravel inclined up to 30° (Fig. 26).

Glacially-related deltas exhibit un­usually rapid sedimentation. A delta in Malaspina Lake studied by Gustavson, Ashley and Boothroyd (1975) contains approximately 9 million cu m of sedi­

ment deposited in less than 10 years. The Chicopee delta of glacial lake Hitch­cock in Massachusetts was deposi­ted at a rate of 2.2 million cu m per year. Meltwater streams carry large amounts of glacially ground rock flour which is discharged as suspended load into lakes or the seas along with bedload material. The sand-silt-clay rock flour deposits from suspension, often as par­allel laminae which mimic ripples and other bedforms. Such structures are termed draped laminations by Gustav­son, Ashley and Boothroyd (1975). Other common sedimentary features include graded bedding from turbidity current deposition, flow rolls, varves, ripples, load casts, "flame" structures, and involutions (Fig. 27).

REFERENCES CITED Aario, R., 1972, Associations of bed forms

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Allen,J.R.,1970, Physical processes of sedi­mentation: Allen and Unwill, London, 248 P-

Allen, E, 1975, Ordovician glacials of the central Sahara, in Ice Ages: Ancient and Modern: Spec. Issue Geol. Jour., no. 6, p. 275-286.

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Banerjee, I., and B. C. McDonald, 1975, Na­ture of esker sedimentation, in Glacioflu­vial and Glaciolacustrine Sedimentation: SEPM Spec. Pub. 23, p. 132-154.

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Glacial Sediments 9

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—•—-and J. M. Ryder, 1972, Paraglacial

Fig. 24—!ce contact delta, Crillon glacier, Alaska (photo by Coastal Research Center, Univ. Mass.).

Fig. 25—Outwash delta, Baffin Island, Canada.

sedimentation: a consideration of fluvial processes conditioned by glaciation: Geol. Soc. America Bull., v. 83, p. 3059-3072.

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Engeln, O, D. von, 1912, Phenomena asso­ciated with glacier drainage and wastage, with special reference to observations in the Yakutat Bay region, Alaska: Zeitschr. Gletscherkunde u. Glazialgeologic v. 6, p. 104-150.

Evenson, E. B., 1977, Subaquatic flow tills: a new interpretation for the genesis of some laminated till deposits: Boreas, v. 6, p. 116-133.

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Francis, E. A., 1975, Glacial sediments: a se­lective review, in Ice Ages: Ancient and Modern: Spec. Issue, Geol. Jour. no. 6, p. 43-68.

Fig. 26—Foreset bedding in glacial delta, Lake Cavanaugh, Washington.

10 D. J. Easterbrook

Goldthwait, R. P., 1971, Introduction to till, today: in Till—A Symposium: Ohio State Univ. Press, Columbus, Ohio, p. 3-26.

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Hamilton, W., and D. H. Krinsley, 1967, Up­per Paleozoic glacial deposits of South Africa and southern Australia: Geol. Soc. America Bull., v. 78. p. 783-799.

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1960, Evolution of tillstore shapes, central New York: Geol. Soc. America Bull, v. 71, p. 1645-1660.

Howarth, P J , 1971, Investigation of two eskers at eastern Breidamerkurjokull, Iceland: Arctic and Alpine Research, v. 3, p. 305-318.

King, C. A. M, and J. T. Buckley, 1968, The analysis of stone size and shape in arctic environments: Jour. Sed. Petrology, v. 38, p. 200-214.

Krumbein, W. C, 1939, Preferred orienta­tion of pebbles in sedimentary deposits: Jour. Geology, v. 47, p. 673-706.

Ostrem, G., 1975, Sediment transport in gla­cial meltwater streams: in Glaciofluvial and Glaciolacustrine Sedimentation: SEPM Spc. Pub. 23, p. 101-122.

Price, R. J., 1969, Moraines, sandar, kames and eskers near Breidamerkujokull, Ice­land: Inst. British Geographers Trans, v. 46, p. 17-43.

1970, Moraines at Fjallsjokull, Ice­land: Arctic and Alpine Research, v. 2, p. 27-42.

1971, The development and destruc­tion of a sandur, Breidamerkurjokull, Ice­land: Arctic and Alpine Research, v. 3, p. 225-237.

1973, Glacial and fluvioglacial land-forms: Oliver and Boyd, Edinburgh, 242 p.

R J. Howarth, 1970, The evolution of the drainage system (1904-1965), in front of Breidamerkurjokull, Iceland: Jokull, v. 20, p. 27-37.

Rust, B. R, 1975, Fabric and structure in glaciofluvial gravels: i n Glaciofluvial and Glaciolacustrine Sedimentation: SEPM Spec. Pub. 23, p. 238-248.

Saunderson, H. C, 1975, Sedimentology of the Brampton esker and its associated deposits: an empirical test of theory: in Glaciofluvial and Glaciolacustrine Sedi­mentation: SEPM Spec. Pub. 23, p. 155-176.


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