Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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CLACIAI SEDIMFNTS: PROCESSES, ENVIRONMENTS AND EACIES

325

Figure G14 Tidewater f^laciL'i^ in

larriman Eiord, southern Alaska. Glaciers such as this deliver laif>e .imiainls

scdimenl, t'S|)ft iaily

sLihj;kK iai streams to the marine environment, and sediment accumulalion rates may be tens of meters per year.

remains stable long enough for sediment to build up to ihe

surface of the tiord. Where a glacier becomes disconnected

from the water body, alluvial sedimeiils may prograde to form

fliiviodeltaic comple.xes. again dominated by sand and gravel.

In addition to these large-scale forms (measured in IDOs of

meters), there are small-scale features (measured in

meters or less), found particularly on beaches, such as iceberg

tool-marks, ridges, depressions Irom melting of buried ice.

bounce-marks, chattcrmarks and roll-marks. Icebergs can also

churn up submarine sediment, particularly on shoals, produ-

cing iceberg turbales.

Depositional forms on continental shelves are best known

IVom a combination of deep drilling and seismic profiling.

Some, as in Alaska, are simply larger scale analogues of liordal

features, such as delta-fan complexes, but others form under

floating tongues that are typical of the colder iee of the polar

regions. These include subglaeial deltas, till tongues, diamicton

aprons and the immense (up to 400 km wide) trough-mouth

fans.

Other features are similar to those on land, including

shelf moraines, tlutes and transverse ridges.

Terminology and classification of

glaeigenic sediments

The terminology used to describe glacigenie sediment has

always been in a state of flux, and the evolution of terms may

be linked to progressive improvement of understanding glaeial

processes. For an objective study of sedimentary lacies. a non-

genetic classilication of poorly sorted sediments is required,

before process-related terms such as "tiU'" are used. The terms

diamicton (unlilhilied). diamictite (litliified) and diamict (both)

are now well established for "a non-sorted or poorly sorted

terrigenous sediment that contains a wide range of particle

sizes"

(Flint etal.. I960). However, this definition hides a vast

range of textures, and the literature abounds with eonflicting

ideas about what constitutes a "diamicton". A textural

classification, based on the relative proportions of sand and

gravel, has subsequently been developed (Hambrey. l*-)94)

(Table GIO).

The genetic terminology is perhaps even more confused.

First, we need to defme some widely used general terms:

Glaeigenic

sediment: "of glacial origin": the term is used in

the broad sense to embrace sediments that have been

influenced by glacier ice.

Glaeial debris: material being transported by a glacier and

thus in contact with glacier ice.

Glactaldrift: all rock material in transport by glacier ice. all

deposits released by glaciers, and all deposits predominantly of

glacia! origin deposited in the sea by icebergs, or from glacial

meltwater.

For a more detailed process-based classilication of glaci-

genic sediments, the recommendations of a commission of the

International Quaternary Association are followed (Dreimanis,

1989;

Hambrey 1994). Although somewhat out-of-date, this is

the only scheme that has achieved a wide measure of consensus

(Table Gil), in this genetic classilteation. the key term is ////

[tillile where lithitied). Primarytil! is delined as 'an unsorted

sediment with a wide range of grain sizes deposited directly

from glacier ice. without subsequent disaggregation and

flow". Secondary tills are the products of resedimentation of

glaeial debris that has already been deposited by the glacier,

with little or no sorting by water. Within these categories

several varieties of till have been documented, although

these represent end members in a continuous spectrum of

depositional types (Dreimanis, 1989). Benn and Fvans (1998:

386 399) have provided a concise review of the characteristics

of tills, and ihe main types (with their lextural characteristics)

are:

Meltout

till:

deposited by slow release of glacial debris from

ice that is not moving. Typically, meltout tills inherit the

326 GEAGAE SEDIMENTS: PROCESSES, ENVIRONMENTS

AND

EACIES

Table

GIO

Textural classification

poorly sorted sediments (from EHambrey,

1994;

modified from Moncrieff, 1989).

order

derive

sedinieni name, percent

gravel

first estimated (horizontal axis)

and

Ihen proportion

percent

sand

in the

matrix (vertical axis)

Mud

<0.06 mm

0.11

Ssnd/mud

ratio

of 1

matrix

Sand

2.0-0.06

Percent gravel (<2mm}

whole sediment

Trace (<0.01)

MUD(STONE)

Sandy

MUD(STONE)

Muddy

SAND(STONE)

<1%

MUD(STONE) with

dispersed clasts

Sandy

MUD(STONE)

with dispersed

clasts

Muddy

SAND(STONE)

with dispersed

c lasts

SANDjSTONE] with

dispersed clasts

1-5%

clast-poor muddy

DIAM1CT(ON/1TE)

clast-poor

50 intermediale

DIAMICT(ON/ITE)

ciast-poor

sandy

DIAMICT(ON/ITE)

5-50%

clast-rich muddy

DIAMICT(ON/ITE)

clast-rich

intermediate

DIAMICT(ON/ITE}

ciast-rich

sandy

DIAMICT(ON/ITE)

Gravelly SAND(STONE)

50-95%

Muddy GBAVEUBRECCIA/

CONGLOMERATE

GRA

BREC

CONGLO

Sandy GRAVELBRECCIA/

CONGLOMERATE

>95%

VEU

:c\A/

MERATE

Percent

sand

matrix

50%

gravel

100

Table

Gil

Genetic classification

terrestrial glaeigenic sediments lad<i|ile{l from Dreimanis, 1989).

The

vertical columns

are

independent

each olher

and no

correlation horizonlally

implied.

Noi .ill

comhinations are feasible

Release

glacial debris

and its

deposition

I Environmcni

PosiiiLin

Ice-marginal

Frontal

Lateral

Glacioterrestria! Supragiacial

Subglacial

Substratum

or redcposition

III Process

Primary

Mtrlting

out

Lodgement

Sublimation

Squeeze-flow

Subsote drag

B Secondarv

Gravity flow

Slum pine

Sliding

and

rolling

Depositional genetic

environment

Terrestrial

non-aquatic till

varieties

till

position

Ice-marginal till

Supragiacial till

Subglacial till

process

Primary till

Meltout till

Lodgement till

Siiblimaiion till

Deformation till

Squcez-flow till

Secondary

nil

Flow till

granulomctry

and

particle-size distribution

the parent ice.

may

massive

layered, commonly with

strong ciast

orientation parallel

flow.

Lodgement till: deposited

"plastering"

glacial debris

from

the

sliding base

of a

moving glaeier

pressure melting

or other mechanical processes. This process also normally

produces

massive diamicton. although sometimes with

anastomosing planar discontinuities resulting from shear

(Figure G15(A)).

consistent flow-parallel clast orienttition

is evideni

lodgement lills.

Basal

litl:

embraces both

the

above.

Siihlimaiion

nil: is

released

sublimation direct from

debris-rich

iee in

sub-zero temperatures. Te.xturally. this

is a

diamicton.

but is a

rare deposit, only found

in a few

arid parts

of Antarctica.

Deformation till: comprises weak rock

unconsolidated

sediment that

has

been detached

by the

glaeier

at its

source

and

subsequently deformed

disaggregated,

and

some foreign

material admi.xed. Tcxturally. this

variable, often breccia-

like:

rafts

rock

sediment

may be

many meters

length,

Ftowtill. till which

has

been remobilized

gravity llowage.

although better known

as a

glaeigenicsedimcnf

flow.

degree

of sorting into gravel-rich

and

gravel-poor components

accompanies this process,

and the

clast fabrie

weak

and

highly variable.

The above discussion refers primarily

terrestrial glacial

sedimenls. There

is no

agreed genetic classilication

for

glaciomarine sediments.

The

following terms, however,

are

eommonly used:

Ice-pioximaliilactomaiinesediment: sediment that

released

either froin floating basal glacier

ice or by

continuous rain-out

from icebergs, without subsequent winnowing

currents

and

waves; this embraces

ihe

term waterlain till which

is no

longer

favored. Typically, this

is a

diamicton. either massive where

GLACIAL SEDIMENTS: PROCESSES, ENVIRONMENTS AND FACIFS

Figure C15 Some typit<il lithulaties lr{jm glaciiil environments: (A) Clast-rich massive diamictile mlerprfltn! as lud^etiifiil

lill,

overlain

sLKxessivety by siratitiecl diamitlile of glatiolatuslrine origin, and more massive diamictile, Siriub Croup (Neogene), Shdtkleton Glacier,

Trans.intarttic Mountains. (8) CList-pnor weakly slratilled diamictile, interpreted as prf)ximal glaciomarine sediment, Wilsnnbreen Formation

(Neoproferozoicl, Backlundtoppen, Spitsbergen. (Cl Dropstone-laminile, interbedded with diamictite from Paleoprolerozoic Gowganda

Formation,

Whitefish Ealls, Ontario; inlerproted as cyclopsams and cyclopels didal), and subaquatic debris flows. {D) Thin section of laminated

clay idarkl and sand/sill (light) with dropstone, Neoproteruzoic Pctrovbreen Member, NE Spitsbergen; these couplets are typical of

glaciolacustrine sediments, each pair formed in one year la varvjte).

328

GLACIAL SEDIMENTS: PROCESSES, ENVIRONMENTS AND FACIES

iceberg rainout is dominant and continuoLis. or has

faint wispy

stratilication (Figure G15(

B)).

Clast fabrics are weak or random.

ke-distal glaciomarine sediment: sediment that is released

from icebergs, but subject lo winnowing, and admixed with other

marine sediment, including biogenic components. Texturally,

this facies is sandy mud or muddy sand with dispersed stones,

and ill modern environments often rich in diatoms.

Iceberg tiirbate: sediment deposited on the sealloor that is

subsequently leworkcd by grounded icebergs. The end product

is commonly a massive diamicton with a heavily grooved

upper surface,

Cyclopels (silt/mud couplets) and cyclopsams (graded sand/

mud couplets): rhythtnically laminated sediments derived from

turbid overtlow plumes from subglacial discharge, especially in

fiords, typically producing two couplets a day according to

the tidal cycle. If deposition is accompanied by ice-rafting,

dropstone structures may be evident (F'igure G15(C)).

Glaciolacustrine facies are similar to those in glaciomarine

environments. In addition, lakes commonly have rhythmically

laminated sands and muds called

varve.s

(or varvites if lithified),

similar to cyclopsams and cyclopels, but with each couplet

forming over one year. Sand laminae represent input to the

lake during summer, and silt/clay represent settling out from

suspension in winter. Iceberg debris, in the form of dropstone

structures, may disrupt the laminae (Figure G15(D)). Other

laminated sediments in lakes or the sea may be deposited from

turbidity currents and also include dropstone structures.

Differentiation of the processes responsible for the formation

of laminated facies is challenging.

Facies analysis of glacigenic sediments

The facies approach

Compared with other sedimentologists, glacial geologists have

been rather slow in adopting {he facies approach. However, it

is now recognized that for objective treatment this approach

must be used.

The first step is to describe the lithofacies found in a vertical

section or core, using a wide range of criteria (texture,

sedimentary structures, bed geometry and boundary relations,

clast characteristics), and avoiding genetic terms such as

"tiU"".

Some authors use formal lithofacies codes (Eyles etal.. 1983).

but this is thought by others

(e.g.,

Hambrey, 1994: Miller,

1996) to be too inflexible and include interpretative elements.

Here,

we advoeate that workers devise their own scheme to

suit the distinetiveness of their sections. The next step is to

group the lithofacies into facies associations that contain a set

of attributes that will allow the depositional setting to be

determined, such as glacioterrestrial or glaciomarine. On a

regional scale, given sufficient three-dimensional exposure on

land,

or extensive seismic and borehole stratigraphy offshore,

derivation of ihc

Jciciesarchitecture

is a desirable aim, allowing

one to address larger issues, such as basin evolution, ice sheet-

scale fluctuations and sea-tevel changes. Representative facies

associations from terrestrial, lacustrine, and glaciomarine

settings are illustrated in Figure G16.

Glacial sediment/Iandform associations

The concept of glacial sediment/landform associations (the

'''landsystems" approach) is relatively new to glacial geology.

The approach rests on the assumption that at the large-scale it

is possible to identify areas of land with common attributes,

distinct from the surrounding areas, that can be related to the

processes involved in their development (Benn and Evans,

199S). The tandsystems approach was pioneered tor glaciated

terrain by Boulton and Paul (1976) in an attempt to identify

tracts of land created by similar till-forming processes and

therefore of similar geotechnical properties. The glacial land-

system concept was then applied to former iee-sheet beds.

Originally, three distinct landsystems were defined:

The suhglacial landsystem, in which the dominant glacial

sediment/landform associations are formed at the glacier bed.

The

supraglacial

latulsystem.

in which the dominant glacial

sediment/landtbrm associations are largely composed of a

drape of supraglacial debris.

The glaciated valley landsystem. in which the dominant

glacial sediment/landform association is that formed by

mountain glaciation.

As the concept has developed, other landsystems have been

added to this simple list. These include landsystems formed in

proglacial and glaciomarine environments, as well as those

tbund in settings where surge-type glaciers are common. It is

also possible to relate landsytems to glacier thermal regime.

Thus we can identify landsystems related to warm-based

("temperate"), polythermal and cold-based ("polar") glacier

margins. A criticizm of the landsystems approach is that it

takes no account of spatial patterns of ice-sheet erosion and

deposition, and how these vary during ice-sheet growth and

decay. Elucidating these links remains a challenge for glacial

geologists.

Glacial sediments

in the

geological record

Identification

glaciation

in the

geological record

The recognition of glacigenic sediments in the rock record is of

fundamental importance to paleoclimatological and paleo-

environmental reconstruction. Fven where exposure is good,

evidence for glaciation may be equivocal as, for example, in

alpine regions where mass-movement and fluvial processes

may overprint direct evidence for glacial deposition, or in

those areas where tectonic deformation has overprinted

depositional features. However, detailed analysis may yield

sufficient criteria that, taken together, could form the basis of a

glacial interpretation. Table G12 lists the principal criteria for

establishing Ihe glacial origin of a diamict-bearing succession.

Preservation potential

glacigenic successions

Tectonic setting is the principal control on whether a glacial

succession is preserved. Eyles (1993) has provided a detailed

review of the wide range of tectonic settings suitable for

preservation. Intracratonic basins, rifts, forearc and backarc

basins, and subsiding continental shelves are particularly

suitable receptacles lor preservation of a glacial record,

especially if those sites are first buried and later uplifted and

exposed.

Except for deposits of Quaternary age. land areas are

less likely to preserve glacial sediment, as continued erosion

tends to remove traces of glaciation. There are, however.

notable exceptions in the geological record.

GLACIAL SEDIMENTS: PROCESSES, ENVIRONMENTS AND FACIFS

329

iA> CIROS-1 McMurdo Sound

Age

mbsf

gram sue

120

140

160

180

200

220-

240-

260-

280-

300

320-

340-

360

••

.*-«-r?^

-j^^^iT^

366rn u/c

-^^

(B!

Sirius Group, Shackleton Glacier

to Sinus Group, ShacKleton

Glacier

Predominantly glacio-terrestrial:

stacked lodgement tills, except

where indicated

Shallow shelf/ramp dominated

by glaciomarine sedimentation,

reworking by slumping & debris

flowage and occasional

ice-grounding

Expanded section o( (B)

Grounding-line fan complex in

ice-cohtact lake; gravity-flow

varve deposition and ice-ratting

dominant; bounded by

lodgement tills

LITHOFACIES

•*«j»^

conglomerate

breccia

I mudstone

I conglomerate/breccia

L* l'»| diamictite

sandstone

rhythmite

lonestones

INTERPRETATION

glacicfluvial (B), subaquatic discharge of

subglacial streams (C), debris-flows (A)

slumped supraglacial debris (B only)

subaquatic debris-flow with rip-up clasts

proximal glaciomarine (A), lodgement

till (A,B), debris-flows (C)

marine (possibly subglacial

stream-sourced) (A), glaciofluvial (B,C)

marine {sedinient released

from suspension)

varvites (C) with ice-rafting

ice-rafted debris, commonly exottc

dropstone

structures

contorted

bedding

small-scale

cross-bedding

load structures

burrowing

large-scale

cross-bedding

•^ intraclasts

—, horizontal

burrows

-A-

flame structures

Figure G16 Ch,iracteristic: faties associations from; 1A| Ule Oligocene-Miocene proximal continental shell' facies assuLiation, upper part of

CIRt3S-l drill-core, McMurdo Sound, Antarctica, illustrating deposition in a subsiding rit't-basin, with occasional ice-grounding evenis idata from

Barren,

1989). (B) A terrestrial setting dominated by repeated lodgement till deposition, represented by stacked diamictites; Sirius Group

iNeogene), Shackleton CiLuier, Transantarctic Mountains (cf. Figure

G1S(A)1.

(Ci Glaciolacustrine association in middle of section shown in B,

illustrating ice-proximal deposition dominated by slumping and debris-flowage of rain-out diamicts and varve-sedimentation.

330

GLACIAL SEDIMENTS: PROCESSFS, FNVIRONMFNTS AND FACIFS

Table G12 Criteria for esr.iblishing a glacial origin ol diamict-beiiring

successions for different environments {from Hambrey, 1999)

(a) Evidence for terrestrial glaciation

A hrtitk'el s u

rfiic

Grooved striated and/or polished surriices

Crescentic gouges, friction cracks, chattermarks

Striated boulder pavements

Roches moutonnecs

Nyc channels, "p-forms"

Diamicl

(inil

inmhly hoiihley ^ni\cl hcds with:

Little or no stratification

Irregular thickness, typically several m (but may be up to 10s of m)

Preferred clast orienUitioii

lenses of sand/gravel (glaciofluvial)

Shear structures parallel lo depositional surface

Dt'jiDsiiitinaUauiljonus

For example, moraines, eskers. but rarely recognisable in

pre-Quaternaiy successions

Associiiliiin

wilh

cxioisiw sheels ojsainl and snivel—iuferred

(b) Evidence of glaciomarine/glaciolacustrine deposition

Massive or stratified diamicts. often tens to hundreds o( m ihick.

with giadational boundaries

Dropstones in stratified units

Random clast fabric

Slighl sorting or winnowing at top of beds, to give lag deposits

Association with iiisilii fossils

Association with rhytlimites (varves in lakes: tidal rliythmilcs in Ijords:

turbidiles in botli)

Association with rescdimcntcd dcposit.s (siibiit|uaiic gravity flows)

Range in si/^c up to a few meters

Range in lilhologies. reflecting terrain over which ice Howed

Constant mix of ciasts over wide area common

Range in shape tVom very angular to roinided (e.g.. subangular to

siibrounded predominant in subgiaciai debris), sometimes influenced

by prior transport history

Surface features inchide facets and striations from transpor! wilhin the

basal ice zone, and flal-iron or bullet-nosed shapes from persistent

gtaeial transport

Fragile clasts survive uell

Calcareous crusts in some deposits

Grains

Surface of quart? grains show typical fracture characteristics

Surface of quartz grains show chattermarks

Unstable (fL-rromagnesianl minerals remain relatively unweathered

(d) Other evidence of eold climate

Ice wedge casls

Sorted stone circles, polygons and stripes

Solilluction lobes

Association with litliilied loess (loessite)

Earth's glacial record

Far more is known abottt the Qtiafertiary ghtcialions than all

previous glaciations put together, even though some of those

earlier events were equally dramatic in terms of

scale.

Since the

International Geological Cortx'lalion Programme compiled an

inventory oi" all known pre-Quatemary glacigenic sequences

(Hambrey and Harland. 19SI), several detailed syntheses have

been undertaken (e.g., Eyies, 1993: Deynoiix ci a!., 1994:

Crowell. 1999). For the Quaternary Period there is a vast

literature to chose from.

The oldest known glacigenic sediments are ot^ late Archean

age (2,600 3,100 Ma) iVom South AlVica. E.xtensive Paleopro-

terozoic (2.5(10 2.000 Ma) tillites are known from South

Africa, Australia and Finland. The most prolonged and

globally extensive glacial era took place in Neoproterozoic

time (1,000 550 Ma). Evidence for glaciation occurs on all

continents, leading to the "snowball earth"" hypothesis thai

envisages the Earth being totally ice-covered. Sporadic Early

Protcrozoic glacial events are best represented by the late

Ordovician to early Silurian deposits and erosional features of

Gondwana. notably in Africa. The most prolonged and

extensive phase of glaciation during the Phanerozoic Eon

spanned about 90 Ma of the Carboniferous and Permian

Periods, affecting all Gondwana continents. Finally, after a

phase of global warrnth. with little evidence of iee on Earth,

the Cenozoic glaciations began in Antarctica at the Eocene/

Oligocene transition (35 Ma), Northerti Hemisphere glaciation

followed, with minor ice-rafting events recorded in the North

Atlantic Irom late Miocene titne, until full-scale glaciations

began in late Pliocene lime (2.4 Ma).

State-of-the-art and future prospects

Considerable strides have been made in the last three decades

in understanding glacial processes, which, when combined

wi!h rigorous facies analysis, have ted to well-founded

reconstructions of past glacial environments (see, e.g, compila-

tions and reviews by Dowdeswell and Scourse, 1990; Hambrey,

1994:

Menzies. 1995. 1996; Bennett and Glasser. 1996; Ben'n

and Evans, 1998). Major advances in recent years have

included: (i) elucidation of oceanographic and sedimentary

processes in the glaciotiiarine environtnent: (ii) the recognition

o\'

the imporiance for ice dynatnics of soft, deformable beds

(particularly beneath large ice streams in the Antarctic); (iii) the

role played by glacier surges in terrestrial glacier sedimenta-

tion: (iv) linking deformation in glacier ice. for example,

folding and thrusting, to facies and landforms in terrestrial

settings: and (v) establishing the relationship between glaeier

flucuiations and climatic change. These advances have

provided ihe tneans for enhanced evaluation of Quaternary

and ancient glacial sequences, but there remain considerable

opportunities to undertake such work. Several important

questions and controversies remain: what determines the

italure of the glacier bed: how do conditions at the bed

influence long-term dynamics and stability of ice sheets: what

is the role of glaciotectonics in the generation of landforms;

how may proximal glaciomarine sequences, that may not

provide clear indications of water-depth, be reconciled uith

sequence stratigraphy: and how do sediment facies associa-

tions vary according lo climatic and topographic regimes?

Michael J. Hambrey and Neil F, Glasser

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J., 1960.

Diamictite:

substitute term

for

symmictite.

Geological

Society of America Bulle-

tin,

71: 1809-1810.

Hambrey,

M.J.,

1994.

Glacial

Environments. Vancouver: University

British Columbia Press,

and

London: LICL Press.

Hambrey,

M.J.,

1999.

The

record

earth's glacial history during

the

last 3000 Ma.

Barrett,

P.J., and

Orombelli,

(eds.). Geological

Records of

Global

and Planetary Changes. Terra Antartica Reports,

pp.

73-107.

Hambrey, M.J.,

and

Harland, W.B., 1981. Earth'sPre-PleistoceneGla-

cial

Record.

Cambridge: Cambridge University Press.

Hambrey, M.J., Huddart,

D.,

Bennett, M.R.,

and

Glasser, N.F.,

1997.

Dynamic

and

climatic significance

"hummocky moraines":

evidence from Svalbard

and

Britain. Journal

of the

Geological

Society of London, 154: 623-632.

Hambrey,

M.J.,

Bennett,

M.R.,

Dowdeswell,

J.A.,

Glasser, N.F.,

and

Huddart,

D.,

1999. Debris entrainment and transfer

polythermal

valley glaciers. Journal of Glaciology, 45: 69-86.

Knight,

P.G., 1997. The

basal

ice

layer

glaciers

and ice

sheets.

Quaternary Science Reviews, 16: 975-993.

Maltman,

A.J.,

Hubbard,

B., and

Hambrey,

M.J.

(cds.), 2000. Defor-

mation of

Glacial

Materials. Geological Society

London, Special

Publication,

176.

Menzies,

(ed.),

1995.

Modern Glacial Environments: Processes,

Dynamics and Sediments. Oxford: Butterworth-Heinemann.

Menzies,

(ed.),

1996.

Past Glacial Environments: Sediments, Eorms

and

Teehniques.

Oxford: Butterworth-Heinemann.

Miller, J.M.G., 1996. Glacial sediments.

Reading,

H.G.

(ed.). Sedi-

mentary Environments:

Processes,

Eacies

and Stratigraphy,

3rd edn.,

Oxford: Blackwell Science, Chapter 11,

pp.

454-484.

Monereiff,

A.CM.,

1989.

Classification

poorly sorted sediments.

Sedimentary Geology, 65:

191

-194.

Paterson, W.S.B., 1994.

Physics

of Glaciers. Oxford: Pergamon.

Powell,

R.D., and

Alley,

R.B., 1997.

Grounding-line systems:

processes, glaciological inference

and the

stratigraphic reeord.

Barker, P.F.,

and

Cooper,

A.K.

(eds.).

Geology

and Seismic Strati-

graphy of the Antarctic

Margin,

Antarctie Research Series, Volume

71,

pp.

169-187. Washington DC: American Geophysical Union.

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IAHS (ICSI)-UNEP-UNESCO.

Cross-references

Climatic Control

Sedimentation

Debris Flow

Floods

and

Other Catastrophic Events

Lacustrine Sedimentation

Sediments Produced

Impact

Tills

and

Tilhtes

Varves

GLAUCONY AND VERDINE

Glaucony

Glaucony, a general term introduced by Odin and Letolle

(1980) to represent a depositional facies, corresponding to

sand-sized green glauconitic grains regardless of their min-

eralogical structure, consists of 2 : t layered, potassium and

iron-rich, dioctahedral minerals with a high Fe^''"/Fe^''' ratio.

Glauconitic grains generally occur as light to dark green

pellets.

Glauconitization is characterized by the formation of a

glauconitic precursor, corresponding to a K-poor and Fe-rich

smectite. Maturation includes progressive incorporation of

potassium and iron and change of mineralogical structure

toward an end member constituted by a non-expandable, K.-

rich glauconitic mica (Odin and Matter, 1981; Odin and

Fullagar, 1988), namely glaueonite (Burst, 1958; Hower, 1961).

Common substrates of glauconitization include microfossil

tests,

carbonate grains, fecal pellets, and rock fragments.

Several types of glaucony can be identified based upon

mineralogical structure and chemical composition. X-ray dif-

fraction patterns from well-ordered, highly-evolved glaueonite

reveal sharp and narrow (001) reflections at lOA (illite-type

clay mineral). A predominantly ordered structure is also

indicated by well shaped (111-) and (021) reflections, and high-

intensity (112) and (112-) reflections of comparable size to the

(003) peak. Poorly evolved glaucony is structurally disordered,

with broad peaks between 12A and 14A, a small number of

diffractions and low-intensity (112) and (112-) peaks.

Four stages of evolution of glaucony (nascent, slightly

evolved, evolved and highly evolved), each reflecting specific

morphological habits, can be differentiated on the basis of

potassium content of the green grains (Odin and Matter, 1981).

K2O generally shows a direct correlation with other attributes,

such as the color of grains. K-poor, nascent (K2O = 2 percent

to 4 percent) and slightly evolved (K2O = 4 percent to

6 percent) glaucony generally has a pale to light green color,

with a few exceptions, and generally shows obvious traces of

the primary substrate of glauconitization. In contrast, evolved

(K2O = 6 percent to 8 percent) and highly evolved (K2O

> 8 percent) glaucony, where substrate has been completely

dissolved, is green to dark green and display characteristic

fractures (craquelures of French geologists) at grain surfaces.

Incorporation of iron mostly predates any potassium uptake

into the structure of glauconitic minerals, as shown by high

(>15 percent) Fe2O3 values also in the least evolved, K-poor

samples. However, the direct correlation between K2O and

Fe2O3 observed within evolved and highly evolved glaucony,

combined with the characteristic higher paramagnetic suscept-

ibility of the most evolved samples, indicate that additional

332 CLAUCONY AND VERDINE

Fe2O3 can be incorporated as potassium begins to enter the

smectite structure of glaucony.

Glauconitization typically occurs in submarine, low-energy

conditions, under slow sedimentation rates, within confined

microenvironments at the interface between oxidizing sea-

water and slightly reducing interstitial water. Typical depths

for glauconitization are comprised between

m and 500 m,

with temperatures below 15°C. In modern environments,

authigenic glaucony typically develops on the outer margins

of continental shelves and adjacent slope areas. Rapid burial

limits sediment residence time in the appropriate sub-oxic

redox regime, preventing glaucony evolution. The maturity of

glaucony thus reflects mostly the duration of nondeposition

before burial. Present-day forming glaucony is a poorly evolved

(nascent to slightly evolved) glauconitic smectite. Glaucony

can undergo more or less pronounced modifications due to

changes in local subsidence, terrigenous supply, and other

factors, such as availability and abundance of iron, pH/Eh,

and size and composition of substrates.

Despite its importance, the origin of glaucony is not fully

understood and considerable debate exists about both the

nature of the chemical reactions involved and the parameters

that control its development. The major models of glauconi-

tization include the layer lattice theory of Burst (1958) and

Hower (1961), the verdissement theory of Odin and Matter

(1981) and the two-stage evolutionary model of Clauer etal.

(1992).

All these models involve derivation of the constituents

ions from both parent sediment and seawater.

In common usage glaucony is depicted as an autochthonous

constituent of marine sediments. Since the first half of this

century, glaucony has been regarded as one of the most

reliable indicators of low sedimentation rate in marine settings,

and glaucony-bearing horizons have been considered as

diagnostic of transgressions, because of their presence in lower

parts of transgressive/regressive cycles. Glaucony has been

commonly reported from condensed sections (Loutit et ai,

1988),

in association with phosphate grains and abundant

fossils. Glaucony may also occur as a coating and incrusting

film

facies,

associated with hardgrounds or burrowed omission

surfaces.

Despite the widely held concept that the accumulation of

glaucony generally takes place in open-marine environments

and far from zones of active sedimentation, preferably during

long periods of sediment starvation due to relative sea-level

rise (Odin and FuUagar, 1988), glaucony is commonly

encountered within a variety of deposits in the rock record.

Penecontemporaneous remobilization of the green grains by

storms, tidal currents and waves, or reworking caused by

subaerial shelf exposure during relative sea-level fall are likely

to lead to important concentrations of allochthonous glaucony

in a variety of environments poorly suited to glauconitization,

such as nearshore, lagoonal, estuarine, incised-valley and

turbidite systems, and even alluvial plains.

A reliable interpretation of glaucony-bearing deposits thus

requires that spatial distribution, maturity, and genetic

attributes of glaucony be determined. This implies the

evaluation of autochthonous versus allochthonous glaucony,

with a further division of allochthonous glaucony to para-

utochthonous (intrasequential) versus detrital (extrasequen-

tial).

The integration of sedimentological, petrographic and

mineralogical studies provides a comprehensive framework to

differentiate autochthonous from allochthonous glaucony

(Amorosi, 1997). Criteria that might be useful in recognizing

an allochthonous origin of glaucony include: (i) association

with nonmarine deposits; (ii) selective spatial distribution of

grains; (iii) high degree of sorting and roundness; (iv) absence

of fractures in the most evolved samples, since such features

represent zones of weakness that are likely to facilitate

mechanical breakdown of grains in the case of prolonged

transport or reworking.

The distinction between parautochthonous and detrital

glaucony can be sometimes performed by radiometric dating.

More commonly, it requires that compositional attributes of

glaucony be matched against those of putative sources. The

combination of factors that primarily control glaucony

development is unique during each transgressive-regressive

cycle, and glauconies from distinct parent rocks generally have

different maturity and carry unique geochemical and miner-

alogical fingerprints.

Glaucony is a valid tool for the understanding of deposi-

tional cyclicity in the sedimentary record (Amorosi, 1995).

Stratigraphically condensed intervals, interpreted to have

formed during prolonged breaks in sediment accumulation at

sequence scale, may include up to 90 percent autochthonous

glaucony. Within these condensed sections, glaucony abun-

dance may show rhythmic fluctuations, with maximum

concentration near the base of each cycle and a systematic

upward decrease. In these instances, the burrowed, glaucony-

rich cycle boundaries correspond to marine flooding surfaces,

and the vertically stacked cycles reflect short term sea-level

fluctuations, at the scale of parasequences or parasequence sets

(Amorosi and Centineo, 2000). Although autochthonous

glaucony is most commonly associated with condensed

sections and marks the major marine flooding surfaces within

the transgressive systems tract, the green grains may be present

at any site of the third-order depositional sequence. Para-

utochthonous glaucony can be widespread throughout the

sequence, generally showing lower concentration and maturity

than its autochthonous counterpart. Detrital glaucony is

present mainly within falling-stage and lowstand deposits, its

concentration and composition depending on the character-

istics of the source horizon.

Radiometric dates of K-rich glauconies have been widely

used to determine the depositional age of sedimentary rocks,

and have provided numerous age constraints for the calibra-

tion of the relative time scale in strata lacking reliable high-

temperature chronometers (Odin, 1982). Although glaucony

supplies 40 percent of the absolute-age database for the

geological time scale of the last 250 million years, radiometric

dating of glaucony often presents large practical limitations,

because glaucony can be affected by tectonics, alteration and

diagenetic effects, and K-poor glauconies and (to some extent)

evolved glaucony are likely to provide apparent ages that are

significantly different from the actual age of glaucony.

Verdine

Verdine is a 1 : 1 layered, trioctahedral, 7A iron-rich silicate,

known for long under the name berthierine, but also referred

to as phyllite V or odinite. In terms of color and morphology,

the verdine facies is similar to and often visually indistinguish-

able from the glaucony facies. Verdine minerals, however,

differ from glauconitic minerals in their mineralogical struc-

ture (major peaks at 7.2A and 14A) and chemical composition

(contain significantly less K2O than glaucony).

GRADING, GRADED BEDDING

333

As with glauconitic minerals, minerals

of the

verdine facies

form

marine settings near

the

sediment-water interface,

under conditions

of low

sedimentation rates

and

iron

availability (they contain iron

both

its

reduced

and

oxidized

states).

The

environment

verdine formation, however,

different than that

for

glaucony, corresponding

wanner,

tropical (low-latitude) settings, with water temperature near

greater than 25°. Verdine

abundant

shallower depths than

glaucony (15 m

m),

and has

been observed

replace

glaucony

present continental shelves

about 60 m depth

(Thamban

and Rao,

2000). Verdine generally occurs

proximity

river mouths, which largely contribute

the

iron

needed

for

verdine formation (Odin

and Sen

Gupta, 1988).

Verdine

is a

newly distinguished facies that

has

been

reported from less than

locations

in the the

world

(Rao

etal., 1995). However, most

the occurrences

are

restricted

Holocene deposits from tropical oceans,

and

verdine

very

rarely present

in the

rock record. Similarly

glaucony,

verdine

abundant

coincidence with condensation inter-

vals.

date,

the

only sequence stratigraphic interpretation

for

verdine deposits

is the one by

Kronen

and

Glenn (2000). More

work thus

needed

constrain

the

range

occurrence

verdine

in the

stratigraphic record.

Summary

Glaucony

is one of the

most reliable indicators

of low

sedimentation rate

marine settings

and is a

potentially

powerful tool

for

basin analysis

and

stratigraphic correlations,

because

of its

occurrence within stratigraphically condensed

intervals formed during major transgressive events. However,

the presence

glaucony

in the

record

is not

diagnostic perse

of sediment starvation

and

transgression.

Glaucony

can

provide useful information only

if the

variability

of its

physicochemical properties

and

spatial/

temporal characteristics

are

taken into account. This approach

requires that

the

following types

grains

differentiated:

(i) low-maturity (K-poor) versus mature (K-rich) glaucony;

(ii) autochthonous versus allocthonous glaucony;

(iii)

intra-

sequential versus extrasequential glaucony. Glaucony char-

acterization should

performed

integrated mineralogical

and geochemical investigations. Caution should

taken when

interpreting glaucony-rich deposits based upon visual esti-

mates

of the

green grains.

Verdine

visually indistinguishable from glaucony,

but can

be easily identified

by its

peculiar mineralogical structure

and

chemical composition.

with glaucony, verdine

concen-

trated

relatively condensed sections, although

appears

form

shallower depths (<50m) than glaucony (50 m

500

m),

under strongly river-influenced conditions.

The

geolo-

gical significance

the verdine facies

is not

fully understood,

due

to its

extremely

low

abundance

in the

rock record.

Alessandro Amorosi

Amorosi,

A., and

Centineo,

M.C.,

2000. Anatomy

of a

condensed

section;

the

lower cenomanian glaucony-rich deposits

of Cap

Blanc-Nez (Boulonnais, Northern France).

Marine Authigenesis:

From Globial

Microbial. SEPM (Society

for

Sedimentary

Geology), Special Publication,

66, pp.

405-413.

Burst,

J.F., 1958.

"Glaueonite" pellets; their mineral nature

and

applications

stratigraphic interpretations. American Association

of Petroleum

Geologists

Bulletin,

42;

310-327.

Clauer,

N.,

Keppens,

E., and

Stille,

P.,

1992.

isotopic constraints

the process

glauconitization. Geology,

20;

133-136.

Hower,

J., 1961.

Some factors concerning

the

nature

and

origin

glaueonite. American Mineralogist,

46;

313-334.

Kronen, J.D.

Jr., and

Glenn,

C.R.,

2000. Pristine

reworked verdine;

keys

sequence stratigraphy

mixed carbonate-siliciclastic

forereef sediments (Great Barrier Reef).

Marine Authigenesis:

Erom Globial

Microbial. SEPM (Society

for

Sedimentary

Geology), Special Publication,

66, pp.

387-403.

Loutit,

T.S.,

Hardenbol,

J.,

Vail,

P.R., and

Baum,

G.R., 1988.

Condensed sections;

the key to age

determination

and

correlation

of continental margin sequences,

tn Sea

Level Changes:

Integrated Approach. SEPM (Society

for

Sedimentary Geology),

42,

pp.

183-213.

Odin,

G.S., 1982.

Numerical Dating

Stratigraphy. Chichester;

J. Wiley.

Odin,

G.S., and

Letolle,

R.,

1980. Glauconitization

and

phosphatiza-

tion environments;

tentative comparison.

Marine Phosphorites.

SEPM (Society

for

Sedimentary Geology), Special Publication,

29,

pp.

227-237.

Odin,

G.S., and

Matter,

A., 1981. De

glauconiarum origine.

Sedimentology,

28;

611-641.

Odin,

G.S., and

Fullagar,

P.D., 1988.

Geological significance

of the

glaucony facies.

Odin,

G.S.

(ed.). Green Marine Clays.

Amsterdam; Elsevier,

pp.

295-332.

Odin,

G.S., and Sen

Gupta,

B.K., 1988.

Geological significance

the verdine facies.

Odin,

G.S.

(ed.). Green Marine Clays.

Amsterdam; Elsevier,

pp.

205-219.

Rao,

V.P.,

Thamban,

M., and

Lamboy,

M., 1995.

Verdine

and

glaucony facies from surficial sediments

of the

eastern continental

margin

India. Marine

Geology,

127; 105-113.

Thamban,

N., and Rao, V.P.,

2000. Distribution

and

composition

verdine

and

glaucony facies from

the

sediments

of the

western

continental margin

India.

Marine Authigenesis: Erom Globiat

to Microbial. SEPM (Society

for

Sedimentary Geology), Special

Publication,

66, pp.

233-244.

Cross-references

Authigenesis

Berthierine

Clay Mineralogy

Cyclic Sedimentation

Illite Group Clay Minerals

Phosphorites

Provenance

Sediment Fluxes

and

Rates

Sedimentation

Smectite Group

Substrate-Controlled Ichnofacies

GRADING, GRADED BEDDING

Bibliography

Amorosi,

A.,

1995. Glaucony

and

sequence stratigraphy;

conceptual

framework

distribution

silieielastic sequences. Journal

Sedimentary Research, B65; 419-425.

Amorosi,

A., 1997.

Detecting compositional, spatial,

and

temporal

attributes

glaucony;

tool

for

provenance research. Sedimentary

Geology,

109;

135-153.

Graded bedding (Bailey, 1930) characterizes

clastic sedimen-

tary deposit

there

is a

progressive upward change

in the

mean, maximum,

modal grain size.

If the

particle-size

variation

repetitive rather than progressive, then

the

deposit

is stratified

scale)

laminated

{< 1

scale),

but not

necessarily graded. Stratified

laminated beds

may

also

334

C;RADINCi, GRADED BEDDING

graded if the average particle size changes progressively

upward at a scale exceeding that of single laminae; this is the

case in graded turbidites containing Bouma.sequences. Graded

beds range in thickness from ^Icm to many meters. The

thickest graded beds are so-ealled "megaturbidites" which may

be thicker than lOOm (Labautne et tt!., 1985). Thick graded

beds may be composed of gravel and sand or predominantly

mud (Weaver and Rothwell, 1987). The term "grading'" should

not be used to describe gradual changes in rock type across-

bedding planes. Instead, it is better in such cases to refer to

grudatiotial bed

coiUacts.

Graded bedding may be normal., with an upward decline in

particle size, or

feyer.se

(sometimes called

inver.'ic')

if there is an

upward increase in particle size. In some conglomerates,

reverse grading at the base of a bed passes upward into normal

grading (Walker. 1975). Graded bedding must be distinguished

from so-called upnitnlftnin^ or itpuardcoar.senitti; successions,

which involve many beds and generally more than one facies

(Nichols, 1999, pp. 42-43). Because normal grading is much

more common than reverse grading, its presence is often used

for way-up deterruittalion.

Graded beds are commonly the result of deposition from

short-lived cyents that inlroduce sediment by downslope tran-

sport, offshore transport, or flooding. Examples are turbidity

(7/frt'/j/,s, storms on shallow-marine shelves, and overbank flood-

ing of rivers. If the energy available for transport increases

during the depositional event, then reverse grading may result.

Alternatively, reverse grading may result from ^rainimcradion

(i.e.,

collisions and near-collisions) in a basal grain-rich layer

sheared along by the overriding ctirrent (Bagnold, 1956). or by

downward percolation of fmer particles through coarser

fractions during near-bed shearing lAllen. 1984, p. 157).

The normal grading that characterizes most turbidites

(Allen, 1984. p. 404) has been interpreted to result frotn a

progressive decline in the carrying capacity of tlie current for

all size fractions initially suspended in the flow (Hiscott. 1994).

This decline in capacity occurs at eaeh point along the

transport path, and is ascribed to the temporal deceleration

of a waning How (Kneller, 1995). Alternatively, depositing

turbidity currents can produce normally graded beds as they

pass over a li.xed point if there is a lateral size grading from the

front to the rear of the llow (Walker, 1965). The lateral grading

of the suspended load develops because the body of uirbidity

cnrrents moves faster than the head of the flow on basin slopes.

The head does not grow unchecked as suspension ft"om the

body of the current overtakes it. Instead, the rate of body flow

into the head region is balanced by loss of suspension from the

wake at the back of the head. This ejected material settles back

into the How lop according to fall velocity, with the coarsest

grains settling into the body nearest the head and the finest

grains returning far behind the head, leading to lateral size

grading in the flow.

Kneller (1995) modeled the development of grading in

turbidites depending on whether flow velocity at a single site

decreases, remains steady, or increases with time (temporal

deceleration and acceleration): and whether velocity decreases,

remains uniform, or increases along the transport path (spatial

deceleration and acceleration). Five combinations of spatial

and temporal acceleration and deceleration are predicted to

generate deposits with distinctive grading profiles (Figure G17).

This theoretical treatment applies whenever transport veloeity

changes systematically through time or along the transport

path, and is not limited to turbidites.

WANING STEADY WAXING

ACCUMU-

LATIVE

UNIFORM

DEPLETIVE

erosion/

non-

deposition

+ve

zero

-ve

+ve

Figure G17 Acceleration matrix where u/t is temporal acceleration at

a fixed point, and

u-du/c^x

is sp.i(ial acceler.ition in the downtlow

direction.

Three combinations of these variables produce normal

grading,

and one produces reverse grading. Arrows point dovvnflow.

Reproduced by permission ot" Ben Kneller (Kneller, 1993).

DtSTRIBUTIOS

GRADING

COARSE-TAIL

GRADING

Geam size (log

Grain si;e |k)g scale)

Figure G18 Vertical changes in grain-size distributions tor beds

showing distribution

gr<iding

ilefti and coarsc-Uil grading irighl). The

dashed lines indicate the mean size. An alternative type of coarse-tail

gliding would have a symmetrical (bell-shapodi basal sample with no

tail of coarse material, and overlying size distributions Iruncited at

their coarse end.

Nonnal grading can involve the entire size population

{distributioiigritding) or only the coarsest

percent to 5 percent

of the population (coarse-tailgraditig: Middleton, 1967). In

distribution grading, there is a progressive shift to finer sizes,

so that central measures like mean and modal size clearly

reflect the grading (Figure GI8). In coarse-tail grading, the

percentage of the coarsest grains, and the maximum size, both

decrease upward. However, the mean size decreases only very

slightly and the mode remains unchanged. Coarse-tail grading

is best developed in the deposits of high-concentration

turbidity currents.