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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 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
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.2 -
.1 -
~
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-
-
-
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-
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I
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1
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A V A
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A VASHIONTILL
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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
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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
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TILL
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Mg Total Cations
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GMD
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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).
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Allen,J.R.,1970, Physical processes of sedimentation: Allen and Unwill, London, 248 P-
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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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Fig. 26—Foreset bedding in glacial delta, Lake Cavanaugh, Washington.
10 D. J. Easterbrook
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