Walker J. Facies models, Canada, 1992

Подождите немного. Документ загружается.

thin (less than 100 m) glacial deposits

of the last glacial cycle (younger than

125,000 years) resting on a regional

erosion surface cut across Mesozoic

and older strata. Without subsidence,

no older glacial record has been pre-

served. In contrast, along the central

graben of the North Sea Basin, where

subsidence is active, over 1 km of

Quaternary sediments are preserved.

Glacio-eustatic sea level changes

are most likely to be recorded in slowly

subsiding, nonglaciated basins far

removed from glaciated areas. In North

America, glacio-eustatic changes dur-

ing the Late Paleozoic glaciation of

Gondwanaland (about 250-350

m.y.

ago) are recorded by well-developed

coal-bearing depositional successions

5. GLACIAL MODELS 97

(cyclothems; Veevers and Powell,

1987). The recognition of possible gla-

cio-eustatic sea level change in the

Mesozoic (Chapter

12)

is one argu-

ment for the existence of ice sheets at

this time, even though no direct evi-

dence of glacial sedimentation has

been found (Chapter 2).

APPLIED GEOLOGY OF

GLACIAL DEPOSITS

Two recent developments in the fields

of petroleum geology and environ-

mental geology are having a major

effect on the future direction of studies

in glacial sedimentology.

Oil,

gas,

and

coal

Until recently, glacial strata have been

overlooked by petroleum geologists.

However, in the southern Gondwana

continents there is now active explo-

ration for hydrocarbon resources in

Permo-Carboniferous glacial strata.

The largest field discovered so far is in

Oman, where the

A1 Khlata Formation

contains about

5.6

x 1

m3 (3.5 billion

barrels) of oil

(Levell

a/.,

1988).

Reservoirs occur in ice-contact deltaic

deposits around the basin edge, and

glaciolacustrine silts and clays form

stratigraphic seals on top of the reser-

voirs (Figs. 21, 22). In Bolivia, the

Bermejo, Palmer and Santa Cruz

fields are important producers. Cor-

relative strata in Argentina host the

Duran, lcua and Madrejones fields,

and in Brazil the ltarare Group has sig-

Figure

Computer drawn three-dimensional block diagram showing bedrock topography of southern Ontario. Contour intervals are

and represent bedrock elevations above sea level. Note prominent bedrock channel (Laurentian Channel) linking Lake Huron with Lake

Ontario. This channel focusses regional groundwater flow towards the south. GTA

Greater Toronto Area. Data compiled from water well

records.

98 EYLES, EYLES

nificant but as yet subcommercial gas

shows within the Parana Basin

(Franca

and Potter, 1991). Within these fields,

the principal reservoirs occur in thick,

sandy turbidites deposited around the

basin margin. Rain-out and debris flow

diamictites are the stratigraphic seals.

In northern Saudi Arabia and Jordan,

hydrocarbons occur in Lower Silurian

glacial

outwash channels. In Australia,

cold climate, wind-blown sands are ex-

ploration targets in

Permo-Carboni-

ferous glacial deposits of the Cooper

Basin. As well as oil and gas, coal de-

posits are a major resource in many

Permo-Carboniferous cold climate

basins of the Gondwana continents

(e.g., Martini and Glooschenko, 1985).

Environmental geology

Recent concerns with environmental

geology have led to much new research

on Quaternary glacial depositional

systems. Urbanization and increasing

demand on groundwater resources is a

stimulus to detailed subsurface investi-

gations. Southern Ontario, which sup-

ports 35 per cent of the Canadian

population, provides a good example of

an area where industry, agriculture

and urban development are having a

major impact on groundwater supply

and quality. Particular problems in-

clude waste management, the proper

design of landfills, and the tracing of

subsurface contaminant plumes

(Rowe and Booker, 1990). Solving

these problems requires a detailed un-

derstanding of the three-dimensional

geometry of glacial facies and their

hy-

drogeology. For example, more than

1200 active and inactive landfill sites

occur in southern Ontario (Fig.

36).

Most of these are located in aban-

doned

glaciofluvial sand and gravel

quarries and are associated with sub-

surface plumes of chemically contami-

nated water (leachate). Fine-grained,

relatively impermeable glaciolacustrine

sediments afford some protection from

leachate migration into underlying

groundwaters. However, many clay

deposits are fractured and afford little

protection. Silt- and sand-rich tills

dominate the surficial geology of

southern Ontario, but despite their

massive, dense character, they have

significant bulk permeabilities and con-

tain intraformational coarser-grained

sediments.

In southern Ontario, the regional

ground water flow in Quaternary glacial

sediments is controlled by the slope of

the underlying bedrock surface. The

bedrock surface is characterized by a

major south-draining paleochannel

(Laurentian Channel; Fig.

37), in which

glacial sediments reach thickness of

over 100 m. Drilling and

downhole

geophysical logging (Chapter 3) allow

regional correlation of the major glacial

stratigraphic units (Figs.

21,

22). The

more permeable units (aquifers)

include the Scarborough Sands, and

the impermeable units (aquitards)

include the glaciolacustrine sediments

that overlie these sands. Groundwater

flow is generally within the permeable

stratigraphic units, but fracturing and

changes in the local facies geometry

allows flow between separate aquifers

(e.g., Howard and Beck, 1986; Eyles

and Howard, 1988). Thus an under-

standing of the original glacial deposi-

tional system is fundamental to pre-

diction of local facies geometries, the

delineation of aquifers and aquitards,

and hence the movement of natural

and contaminated groundwater.

Close spacing of boreholes allows

local facies geometries and ground-

water flow paths to be established at

various scales, some as fine as indi-

vidual landfill sites (about the size of

several football fields). Techniques

such as drilling, digging test pits, shal-

low seismic reflection, and the use of

ground-penetrating radar are used to

establish the detailed site stratigraphy.

However, an understanding of glacial

depositional models is the key to the

prediction of lateral variability in facies

characteristics and hydrogeological

properties. Water origin and flow in dif-

ferent aquifers can be traced by study

of major and minor ions, isotopes, and

by radiocarbon dating. These tech-

niques establish the overall

hydro-

stratigraphy of the site, and provide a

means of modelling groundwater flow

through the various glacial facies.

Examples of such studies are provided

by Desaulniers et

a/.

(1981), Rowe

and Booker (1990) and Eyles et

a/.

(in

press, a). Determining fluid flow paths

in Quaternary and pre-Quaternary

glaciated basins not only provides fun-

damental information about the geom-

etry of permeable and impermeable

facies, but also the detailed disposition

of facies in the glacial depositional

system. In this way, data from individ-

ual case studies are fed back into, and

help refine, general models.

GLACIAL FACIES MODELLING

The development of comprehensive

glacial facies models is still at an early

stage as a result of relatively recent

sedimento~o~ical interest in glacial de-

posits. A high priority for future work

must be integration of different data

sets from Proterozoic and Phanerozoic

glaciated basins. Most ancient glacial

strata are still poorly known. The mod-

els presented in this chapter have con-

siderable value in applied projects,

particularly in the prediction of facies

geometries and associations in the

subsurface. In particular, these models

can be used to predict the extent and

form of permeable and impermeable

facies in the subsurface or the three-

dimensional variability of

geotechni-

cal properties. This is extremely impor-

tant in the search for hydrocarbons in

glaciated basins and in

hydrogeolog-

ical assessments of Quaternary glacial

deposits, where the relationship of

reservoirs (aquifers) and seals

(aqui-

tards) must be evaluated.

REFERENCES

Basic sources of information

Anderson, J.W. and Ashley, G.M., eds.,

1991,

Glacial marine sedimentation;

paleoclimatic significance: Geological

Society

America, Special Paper

261,

232

Contains good descriptions of

Quaternary and pre-Quaternary

glacio-

marine strata deposited in polar and

temperate glacial settings. See also

Powell and Elverhoi

(1989), and Dow-

deswell and Scourse (1990).

Boulton, G.S.,

1972,

Modern arctic glaciers

as depositional models

for former ice

sheets: Journal of the Geological

Society of London, v.

127,

361-393.

classic paper on the importance of

understanding modern ice marginal

sedimentation as a basis for work in

ancient basins.

Dowdeswell, J.D. and Scourse, J.D., eds.,

1990,

Glaciomarine environments: pro-

cess and sediments: Geological Society

London, Special Publication

53,423

Drewry, D.,

1986,

Glacial geologic pro-

cesses: Baltimore, Edward Arnold,

276

Excellent overall summary of physical

sedimentation processes in glacial envi-

ronments.

Eyles, N., Eyles, C.H. and Miall, A.D.,

1983, Lithofacies types and vertical

profile analysis; an alternative approach

to the description and environmental in-

terpretation of glacial diamict and dia-

mictite sequences: Sedimentology, v.

30,

p. 393-41 0.

Describes widely-used sedimentological

approach to field description of glacial

facies.

Eyles, C.H., Eyles, N. and Miall, A.D., 1985,

Models of glaciomarine sedimentation and

their application to the ancient glacial

record: Palaeogeography, Palaeo-

climatology and Palaeoecology, v. 51,

p. 15-84.

A thorough review of glaciomarine envi-

ronments, facies types and associations

in Quaternary and pre-Quaternary set-

tings.

Powell, R.D. and Elverhoi, A., eds., 1989,

Modern glaciomarine environments:

glacial and marine controls of modern

lithofacies and biofacies: Marine

Geology, v. 85, p. 101 -41 8.

Ruddiman, W.F. and Wright, H.E., Jr.,

eds., 1987, North America and adjacent

oceans during the last glaciation:

Geological Society of America, The

Geology of North America, v.

K-3,

p. 13-37.

Contains excellent papers on glaciology

and dynamics of the last North

American ice sheet.

Wilgus, C.K., Hastings,

B.S., Posamentier,

H.W., Ross, C.A. and Kendall, C.G.St.C.,

eds., 1988, Sea level changes: an inte-

grated approach: Society of Economic

Paleontologists and Mineralogists,

Special Publication 42, 407 p.

Collection of papers emphasizing con-

ceptual basis of sequence stratigraphy

plus numerous case studies of sea level

change from the geological record.

Other references

Alley, R.B., Blankenship, D.D., Bentley,

C.R., and Rooney,

S.T, 1987, Till

beneath ice stream

B, 3. Till deforma-

tion: Evidence and implications: Journal

of Geophysical Research, v. 92,

p. 8921-8929.

Andrews, J.T., 1987, The Late Wisconsin

glaciation and deglaciation of the

Laurentide Ice Sheet, in Ruddiman, W.F.

and Wright, H.E., Jr., eds., North America

and adjacent oceans during the last

deglaciation: Geological Society of

America, The Geology of North America,

K-3,

p. 13-38.

GLACIAL

MODELS

Ashley, G.M., 1975, Rhythmic sedimentation

in glacial Lake Hitchcock, Massa-

chusetts-Connecticut, in Jopling,

A.V.

and McDonald, B.C, eds., Glaciofluvial

and glaciolacustrine sedimentation:

Society of Economic Paleontologists and

Mineralogists, Special Publication 23,

304-320.

Bell, M. and Laine, E.P., 1985, Erosion of

the Laurentide Ice Sheet by glacial and

glaciofluvial processes: Quaternary

Research, v. 23, p. 154-1 74.

Black, R.F., 1976, Periglacial features in-

dicative of permafrost; ice and soil

wedges: Quaternary Research, v.

p. 3-26.

Boulton, G.S., 1987, A theory of drumlin

formation by subglacial sediment defor-

mation, in Menzies, J. and Rose, J.,

eds., Drumlin symposium: Rotterdam,

Balkema, p. 25-80.

Boulton,

G.S.,1990, Sedimentary and sea-

level changes during glacial cycles and

their control on glaciomarine facies ar-

chitecture, in Dowdeswell, J.A. and

Scourse, J.D., eds., Glaciomarine envi-

ronments: processes and sediments:

Geological Society of London, Special

Publication 53, p. 15-52.

Boulton, G.S. and Hindmarsh, R.C.A., 1987,

Sediment deformation beneath glaciers:

rheology and geological consequences:

Journal of Geophysical Research, v. 42,

p. 9059-9082.

Boyce, J.I. and Eyles,

N.,

1991, Drumlins

cawed by deforming till streams below

the Laurentide Ice Sheet: Geology, v. 19,

p. 787-790.

Crowell, J.C. and Frakes, L.A., 1975, The

late Palaeozoic Glaciation, in Campbell,

K.W.W., ed., Gondwana Geology

Australian National University Press,

p. 31 3-331.

Croot, D., ed., 1988, Glaciotectonics: forms

and processes: Rotterdam, Balkema,

212 p.

Desaulniers, D.E., Cherry, J.A. and Fritz, P.,

1981, Origin, age and movement of pore-

water in argillaceous Quaternary deposits

at four sites in southwestern Ontario:

Journal of Hydrogeology, v. 50, p. 231-

257.

Deynoux, M., Kocurek, G. and Proust, J.N.,

1989, Late Proterozoic periglacial

aeolian deposits on the West African

Platform, Taoudeni Basin, Western Mali:

Sedimentology, v. 36, p. 531 -549.

DiLabio, R.N.W. and Coker, W.B., 1989,

Drift prospecting: Geological

Survey of

Canada, Paper

89-20,30 p.

Dionne, J.C., 1985, Formes, figures et

faci6s; sedimentaires glaciels des es-

trans vaseux des regions froides:

Palaeogeography, Palaeoclimatology,

Palaeoecology, v. 51, p. 41 5-451.

Domack,

E.W.,1988, Biogenic facies in the

Antarctic glaciomarine environment:

basis for a Polar glaciomarine summary:

Palaeogeography, Palaeoclimatology,

Palaeoecology, v. 63, p. 357-372.

Edwards,

M.B., 1979, Late Precambrian

glacial loessites from North Norway and

Svalbard: Journal of Sedimentary Petrol-

ogy,

49,

85-92.

Eisbacher, G.H., 1985, Late Proterozoic

rifting, glacial sedimentation, and sedi-

mentary cycles in the light of Winder-

mere deposition, Western Canada:

Palaeogeography, Palaeoclimatology,

Palaeoecology, v. 51, p. 231 -254.

Eyles, C.H., 1987, Glacially-influenced sub-

marine channel sedimentation in the

Yakataga Formation, Middleton Island,

Alaska: Journal of Sedimentary Pet-

rology, v. 57, p. 1004-1017.

Eyles, C.H., 1988, Glacially- and tidally-in-

fluenced shallow marine sedimentation

of the late Precambrian Port Askaig

Formation, Scotland: Palaeogeography,

Palaeoclimatology, Palaeoecology, v.

68,

p. 1-25.

Eyles, C.H. and Eyles,

N.,

1983,

Sedimentation in a large lake; a reinter-

pretation of the Late Pleistocene stratig-

raphy at Scarborough Bluffs, Ontario,

Canada: Geology, v. 11, p. 146-1 52.

Eyles, C.H. and Eyles, N., 1989, The Late

Cenozoic White River

"Tillites" of

southern Alaska; subaerial slope and

fan delta deposits in a strike-slip setting:

Geological Society of America, Bulletin,

101, p. 1091 -1 102.

Eyles, N., 1990, Glacially-derived, shallow

marine gold placers of the Cape

Yakataga district, Gulf of Alaska: Sedi-

mentary Geology, v. 66,

171 -1 85.

Eyles,

and Clark, B.M., 1985, Gravity

induced soft-sediment deformation

structures in glaciomarine sequences of

the Late Proterozoic Port Askaig For-

mation, Scotland: Sedimentology, v. 32,

p. 784-814.

Eyles, N. and Clark, B.M., 1988, Storm-in-

fluenced deltas and ice-scouring in a

Late Pleistocene glacial lake:

Geological Society of America, Bulletin,

100, p. 793-809.

Eyles, N. and Howard, K.W.F., 1988, A hy-

drochemical study of urban landslides

caused by heavy rain; Scarborough

Bluffs, Ontario, Canada: Canadian

Geotechnical Journal, v. 25, p. 455-466.

Eyles,

and McCabe,

A.,

1989, The Late

Devensian

(~22,000 YBP) Irish Sea

Basin: the sedimentary record of a col-

lapsed ice sheet margin: Quaternary

Science Reviews, v. 8, p. 307-351.

Eyles, N., Sladen, J. and Gilroy, S., 1982, A

depositional model for stratigraphic

complexes and facies superimposition

in lodgement tills:

Boreas, v. 11,

p. 317-333.

100

EYLES, EYLES

Eyles, N., Mullins, H. and Hine, A.C.,

1991,

The seismic stratigraphy of Okanagan

Lake, British Columbia; a record of rapid

deglaciation in a deep "fiord-lake" basin:

Sedimentary Geology, v.

73,

13-41.

Eyles, N., Boyce, J. and Hibbert, J., in

press, a, Geology of garbage in

southern Ontario: Geoscience Canada.

Eyles, N., Vossler, S. and Lagoe, M.B., in

press, b, lchnology of a

glacially-influ-

enced continental shelf and slope: the

Late Cenozoic Gulf of Alaska (Yakataga

Formation): Palaeogeography, Palaeo-

ecology, Palaeoclimatology.

Frakes, L.A. and Francis, J.E.,

1988,

guide to Phanerozoic cold polar cli-

mates from high latitude ice-rafting in

the Cretaceous: Nature, v.

333,

547-

549.

Franca, A.B. and Potter, P.E.,

1991,

Stratigraphy and reservoir potential of

glacial deposits of the ltarare Group

(Carboniferous to Permian), Parana

Basin, Brazil: American Association of

Petroleum Geologists, Bulletin, v.

75,

62-85.

French, H.M.,

1976,

The periglacial envi-

ronment: London, Longmans,

309

Hambrey, M.J. and Harland, W.B., eds.,

1981,

Earth's pre-Pleistocene glacial

record: Cambridge, Cambridge Uni-

versity Press,

1004

Howard, K.W.F. and Beck, P.,

1986,

Hydrochemical interpretation of ground-

water flow systems in Quaternary sedi-

ments of southern Ontario: Canadian

Journal of Earth Sciences, v.

23,

938-

947.

Hsu, K.J. and Kelts, K.R., eds.,

1984,

Quaternary geology of Lake Zurich: an

interdisciplinary investigation by deep

lake drilling: Amsterdam, Elsevier,

Contributions to Sedimentology, v.

13,

210

Hughes, T.,

1987,

Ice dynamics and

deglaciation models when ice sheets

collapsed,

Ruddiman, W.F. and

Wright, H.E., eds., North American and

adjacent oceans during the last

degla-

ciation: Geological Society of America,

The Geology of North America,

K-3,

183-220.

Kasizcki, C.A.,

1987,

A model for glacial

and proglacial sedimentation in the

shield terrane of southern Ontario:

Canadian Journal of Earth Sciences, v.

24,

2373-2391.

Lagoe, M.B., Eyles, C.H. and Eyles, N.,

1989,

Paleoenvironmental and paleocli-

matic significance of foraminifera1

biofacies in the glaciomarine Yakataga

Formation, Middleton Island, Gulf of

Alaska: Journal of

Foraminifera1 Re-

search,

19,

194-204.

Lawson, D.E.,

1982,

Mobilization, move-

ment and deposition of active subaerial

sediment flows, Matanuska Glacier,

Alaska: Journal of Geology, v.

90,

279-300.

Levell, B.K., Braakman, J.H. and Ruttenk,

V.,

1988,

Oil-bearing sediments of

Gondwana glaciation in Oman: American

Association of Petroleum Geologists,

Bulletin, v.

72,

775-796.

Martini, I.P. and Glooschenko, W.A.,

1985,

Cold climate peat formation in Canada,

and its relevance to lower Permian coal

measures of Australia: Earth Science

Reviews, v.

22,

107-1 40.

Martinson, D.G., Pisias, N.G., Hays, J.D.,

Imbrie, J., Moore, T.C. and Shackleton,

N.J.,

1987,

Age dating and the orbital

theory of the ice ages: development of

a high-resolution

3,000,000

year

chronostratigraphy: Quaternary Re-

search, v.

27,

1-29.

Miall, A.D.,

1985,

Sedimentation on an

early Proterozoic continental margin

under glacial influence: the Gowganda

For-mation (Huronian) Elliot Lake area,

northern Ontario, Canada:

Sedimento-

logy,

32,

763-788.

Nystuen, J.P.,

1985,

Facies and presenta-

tion of glaciogenic sequences from the

Varanger ice age in Scandinavia and

other parts of the North Atlantic region:

Palaeogeography, Palaeoclimatology,

Palaeoecology, v.

51,

209-229.

Paul, M.A. and Eyles, N.,

1990,

Constraints

on the preservation of diamict facies

(melt-out tills) at the margins of stagnant

glaciers: Quaternary Science Reviews,

51-69.

Piper, D.J.W., Stow, D.A.V. and W.R.

Normark,

1985,

Laurentian Fan, Atlantic

Ocean,

Bouma, A.H., Normark, W.R.

and Barnes, N.E., eds., Submarine fans

and related turbidite systems: New

York, Springer-Verlag, p.

137-142.

Postma, G., Roep, T.R. and Ruegg, G.H.J.,

1983,

Sandy gravelly mass flow deposits

in an' ice marginal lake (Saalian,

Leuvenumsche Beek Valley, Veluwe, the

Netherlands) with emphasis on plug-flow

deposits: Sedimentary Geology, v.

34,

59-82.

Rowe, K.R. and Booker, J.R,

1990,

Contaminant migration through frac-

tured till into an underlying aquifer:

Canadian Geotechnical Journal, v.

27,

484-495.

Ryder, J.M.,

1971,

The stratigraphy and

morphology of paraglacial alluvial fans

in south-central British Columbia:

Canadian Journal of Earth Sciences,

279-298.

Sauer, E.K., Gareau, L.F. and Christiansen,

E.A.,

1990,

Softening of overconsoli-

dated Cretaceous clays by glacial

erosion: Quarterly Journal of Engi-

neering Geology, v.

23,

307-324.

Shaw, J.,

1977,

Sedimentation in an alpine

lake during deglaciation: Okanagan

Valley, British Columbia, Canada:

Geo-

grafiska Annaler, v.

59A,

221-240.

Shaw, J.,

1979,

Genesis of the Sveg tills

and Rogen moraines of central Sweden;

a model of basal melt-out:

Boreas, v.

409-426.

Shaw, J. and Sharpe, D.R.,

1987,

Drumlin

formation by subglacial meltwater

erosion: Canadian Journal of Earth

Sciences, v.

24,

2316-2322.

Syvitski, J.P.M., Burell, D.C. and Skei, J.M.,

1987,

Fjords: processes and products:

New York, Springer-Verlag,

379

Teller, J.T. and Clayton, L., eds.,

1983,

Glacier Lake Agassiz: Geological Asso-

ciation of Canada. Special Paper

26,

451

Vaslet,

D.,

1990,

Upper Ordovician glacial

deposits in Saudi Arabia: Episodes, v.

13,

147-161.

Veevers, J.J. and Powell, C. M.,

1987,

Late

Paleozoic glacial episodes in

Gond-

wanaland reflected in transgressive-re-

gressive depositional sequences in

Euramerica: Geological Society of

America, Bulletin, v.

98,

475-487.

Visser, J.N.J.,

1983,

Submarine debris flow

deposits from the Upper Carboniferous

Dwyka

Tillite Formation in the Kalahari

Basin, South Africa: Sedimentology, v.

30,

51 1-523.

Visser, J.N.J.,

1989,

The Permo-Carbon-

iferous Dwyka Formation of southern

Africa: deposition by a predominantly

subpolar marine ice sheet: Palaeo-

geography, Palaeoclimatology, Palaeo-

ecology, v.

70,

377-391.

Washburn, A.L.,

1980,

Permafrost features

as evidence of climatic change: Earth

Science Reviews, v.

15,

327-402.

Williams, G.E.,

1986,

Precambrian per-

mafrost horizons as indicators of

paleo-

climate: Precambrian Research, v.

32,

233-242.

Williams, P.J. and Smith, M.W.,

1989,

The

frozen earth: fundamentals of

geocry-

ology: Cambridge, Cambridge University

Press,

305

Woodworth-Lynas, C.M.T. and

GuignB,

J.Y.,

1990,

Iceberg scours in the geo-

logical record; examples from glacial

Lake Agassiz,

Dowdeswell, J.A. and

Scourse, J.D., eds., Glaciomarine envi-

ronments: processes and sediments:

Geological Society of London, Special

Publication

53,

217-234.

Wright, H.E., Jr., Almendinger, J.C. and

Gruger, J.,

1984,

Pollen diagram from

the Nebraska Sandhills and the age of

the dunes: Quaternary Research,

24,

11 5-1 20.

INTRODUCTION

As in all other facies models, ancient

volcaniclastic rocks must be interpreted

by comparison with recent accumula-

tions. In this chapter we will present the

major characteristics of volcaniclastic

deposits.

In a general plate tectonic framework

(Fig.

I),

oceanic volcanism at midocean

ridges, seamounts, and oceanic islands

is dominantly basaltic. Due to the lower

volatile content of basaltic magmas,

volcaniclastic rocks in these regions

tend to be formed by fragmentation of

lava flows rather than by explosive

eruptions. Pillow breccias and

hyalo-

clastites are commonly formed. By

contrast, island arc environments are

dominantly basaltic to andesitic in

composition. Explosive volcanoes com-

monly produce subaerial and sub-

aqueous pyroclastic fallout and flows.

Remobilized volcaniclastic mass flows

and turbidites may be deposited in the

submarine environment. Continental

magmatic arc environments are domi-

nantly andesitic to rhyolitic in composi-

tion. Here, subaerial pyroclastic flows

and fallout deposits are common, as

are their remobilized equivalents. The

forearc environment, however, may be

partly submarine. Continental rift envi-

ronments are commonly associated

with caldera structures where acid

sub-

aerial pyroclastic deposits are found.

Basaltic cinder cones and tuff rings also

occur here, and are commonly asso-

ciated with lava flows. In summary,

oceanic extension produces effusive

volcanoes where the magma is mostly

basic, whereas subduction produces

explosive volcanoes

(e.g., Indonesia,

Japan, the Cascades, and the Lesser

Antilles), where the magma is interme-

diate to acid in composition. Thus, dif-

ferent tectonic environments have dis-

tinctive volcaniclastic facies.

The models discussed in this chapter

are best used as general frameworks

for future observations. A few models,

Volcaniclastic Rocks

Jean Lajoie and John Stix, Departement de geologie,

Universite de Montreal, Montreal, Quebec H3C 3J7

particularly for pyroclastic rocks, have

been tested on different volcanoes and

may thus be used to initiate interpreta-

tions, but more work needs to be done

before they can be considered norms

and predictors. One should keep in

mind that most pyroclastic models dis-

cussed in the present chapter were de-

veloped in subaerial deposits. We shall

see that subaqueous accumulations

may have different characteristics.

TERMINOLOGY

Volcaniclastic rocks include all frag-

mental volcanic rocks that result from

any mechanism of fragmentation.

Epi-

clastic

fragments result from the weath-

ering of volcanic rocks.

Autoclastic

fragments are formed by mechanical

breakage or gaseous explosion of lava

during mcvement.

Hyaloclastic

frag-

ments, a variety of autoclastic, are pro-

duced by quenching lava that enters

OCEAN-OCEAN .OCEANIC OCEANIC

SUBDUCTION

INTRAPLATE EXTENSION

IBASALT-ANOESITEI

VOLCANISM

IBASALTI

ISLAND SEAMOUNTS, MID.OCEAN

BACKARC ARC FOREARC

OCEAN ISLANDS RIFT ZONE

OCEAN-CONTINENT CONTINENTAL

SUBDUCTION

EXTENSION

IANOESITE-RHYOLITE) (BASALT-RHYOLITE)

MAGMATIC

ARC

BACKARC RIFT ZONE

Figure

Tectonic setting of volcaniclastic deposits.

LAJOIE, STIX

water, water-saturated fragmental de-

posits, or ice.

Pyroclastic

fragments

originate from explosive eruptions and

are projected from volcanic vents.

Showers of pyroclastic debris produce

fall deposits.

Clasts ejected from vents

may be transported

en masse

on land

or in water by

nuees ardentes.

This de-

scriptive term was proposed by Lacroix

(1904) from his studies of the 1902

eruption of Mount Pelee, to refer to hot

pyroclastic density currents. These de-

posits may thus result from more than

one transporting mechanism.

In the volcanic literature, pyroclastic

deposits originating from laminar trans-

port of hot, high-concentration mixtures

of fragments in gas are commonly re-

ferred to as

flow

deposits. Those that

accumulate from hot, relatively

low-

concentration turbulent suspensions

are termed

surge

deposits (Fisher and

Schmincke, 1984; Cas and Wright,

1987). Surges are further subdivided

into

base, ground,

and

ash-cloud

surges depending on the position of

the turbulent cloud relative to the

entire density current. This highly

genetic terminology is certainly very

useful to volcanologists working on

recent volcanoes and is well en-

trenched in the volcanic literature.

However, it is ambiguous because

both pyroclastic

flows

and

surges

are

flows

sensu stricto.

Furthermore, there

is no consensus among workers as to

what are

flow

and

surge deposits

(Lipman and Mullineaux, 1981), raising

few questions as to the applicability

of this nomenclature to ancient de-

posits. Cas and Wright (1987, p.

355)

note that "it is a brave person who

walks to an outcrop (of pyroclastic de-

posits) and applies a genetic classifi-

cation or terminology". In the present

chapter we shall therefore favour the

more descriptive terminology when-

ever possible.

Pyroclastic fragments are called

primary

if ejected from a vent, and

secondary

if they are recycled from

unconsolidated

primary deposits.

Lahar, a term first used at Kelud volca-

no on Java, refers to coarse

remobi-

lized primary pyroclasts where the

interstitial fluid is water. In ancient de-

posits it may not always be possible to

distinguish between primary and sec-

ondary pyroclastic debris.

Table

Differences between pyroclastic flow and epiclastic deposits.

Epiclastic Pyroclastlc

Texture Rounded grains of volcanic Angular to subrounded pumice,

rocks and crystals. Depending scoria, shards, accretionary and

on environment of deposition,

armoured

lapilli that may be de-

sorting may be excellent to formed or welded. Sorting is poor

poor. to very poor.

Composition Volcanic rock fragments which Mostly acid to intemdiate

frag-

tend to be polygenetic, with

ments which tend to be

mono-

common incompatible phases genetic.

such as quartz and basalt.

Structure Normal

sedimentary.struc- Common presence of normal or

tures. reverse grading, of thick diffuse

strata, low angle cross laminae,

and bomb sags.

BLOCKS

AND

BOMBS

BRECCIA

Figure

Pyroclastic grain-size classifi-

cation with rock nomenclature. Modified

from Fisher and Schmincke

(1984,

92).

Isolated-pillow

breccia

Carbonaceous

cherty limestone

Figure

Typical pillow breccia-hyalo-

clastite sequence in the Triassic of Quadra

Island, British Columbia. From

Carlisle

(1 963).

SURFACE BRECCIA

FINELY VESICULAR PUMICE

OBSIDIAN

COARSELY VESICULAR

PUMICE

BASAL BRECCIA

TEPHRA

Pyroclastic fragments coarser than

Figure

Hyaloclastite transitional with Figure

Schematic cross section of

rhy-

64 mm (-6@) are called

bombs

pillow lava, Rouyn-Noranda region, Quebec. olitic obsidian flow. After Fink

(1980).

blocks, lapilli if grain size ranges from

populations are classified in Figure

64 to

mm (-6 to I$), and ash if finer

Volcaniclastic sedimentation is a

than

mm (-14). Ashes are further

vast subject and has so many vari-

subdivided into coarse and fine at 1/16 ables that it will not be possible to treat

mm (44). Pyroclastic rocks with mix- all of its aspects. Except for

composi-

tures of these fundamental size sub- tion, epiclastic rocks do not differ from

RISING GAS

BUBBLES

OlRECTlON OF

FLOW MOVEMENT

Figure

Evolutionary model of the two types of acid hyaloclastites and lobes. From

Furnes

a/.

(1980).

other siliciclastic rocks. They thus will

not be treated here except to summa-

rize the petrographic differences with

pyroclastic fragments (Table

1).

These

differences are in part due to the dif-

ferent fragmentation processes, and in

part to the temperature of accumula-

tion. We will describe briefly the char-

acteristics of autoclastic and

pyro-

clastic rocks, and discuss their ob-

served and possible variations in time

and space (facies). The reader will find

good descriptions of the morphology

and petrography of volcaniclastic

fragments in Fisher and Schmincke

984), and Heiken and Wohletz (1 985).

AUTOCLASTIC ROCKS

Flow breccias and hyaloclastites are

common in basaltic sequences and un-

common in acid sequences. This is be-

cause basic lavas flow more readily

than acid ones, due to their lower vis-

cosity. In these two types of autoclastic

rocks, the clasts are generally

mono-

genetic, with most fragments being

derived from the same parent liquid.

Lava may erode vent walls or rip up

fragments as it flows, but such exotic

fragments are rarely reported in des-

criptions of flow breccias. In basaltic

sequences, the breccias commonly

consist of complete pillows

and/or

pillow fragments set in a matrix of de-

vitrified glass shards and lumps. Acid

flow breccias are made up of abundant

angular blocks with coarse and fine

sand size fragments set in a glassy

matrix.

Facies

flow breccias and

hyaloclastites

Due to their mode of origin, in situ flow

breccias and hyaloclastites should

show little or no systematic lateral var-

iations in clast content, size, and com-

position. Autoclastic associations, how-

ever, are much more complex.

The vertical variations in a typical

flow breccia of basaltic composition are

summarized in Figures

and 4. The

pillowed lava grades upward into an

isolated-pillow breccia that is overlain

and transitional with a broken-pillow

breccia. In the

Archean of the Noranda

region, similar sequences are occa-

sionally overlain by fine-grained

hyalo-

clastites (Fig. 4). In this succession,

clast size decreases from base to top.

This grain size variation cannot prop-

erly be called graded bedding because

LAJOIE, STIX

there is no bedding to begin with. The

clasts are formed in situ rather than

being transported and deposited. The

distinctive characteristics of this type of

breccia are the monogenetic composi-

tion of the clasts and the transitional

contact with the underlying lava. Since

the fragmentation is formed by quench-

ing, the shards are not welded.

Subaerial lava flows of acid composi-

tion commonly have flow breccias

which underlie

andlor overlie the lavas.

Fink (1980) presented a schematic

cross section of subaerial rhyolitic ob-

sidian flows based on studies from

California, New Mexico, and Lipari in

the Eolian Islands (Fig.

5).

The approxi-

mately 30 m-thick lava flow has a two

m-thick breccia at the base and a one

m-thick flow-top breccia. The texture

and composition of the fragments are

similar to those of the associated

parent lava flow. Compared to flow

breccias of basaltic composition, how-

ever, the breccias of acid flows are

thinner; the transition with the parent

lava is more abrupt, and may even be

sharp; and the fragments are large and

chaotically organized.

There are few modern examples of

submarine acid lava flows, and they

are rare in the rock record. Subglacial

acid hyaloclastites from Iceland show

a greater lithological diversity than

basaltic hyaloclastites (Furnes

a/.,

1980). They are made of structureless

to strongly flow-banded fragments of

pumice and obsidian, commonly associ-

ated with large (long axes average 7 m)

irregular to subspherical bodies (lobes)

of vesicular to nonvesicular rhyolite.

Two genetically different types of hyalo-

clastites are described (Fig. 6). Type

hyaloclastites, said to be by far the most

common, consist entirely of pumiceous

fragments that vary considerably in size

and shape, and result from phreatic ex-

plosive events. Type 2 hyaloclastites

consist of fragments of obsidian, flow-

banded or flow-folded pumices, and

lithic rhyolite invariably associated with

ellipsoidal to irregularly shaped bodies

(lobes) that may commonly reach 70 m

in length. These lobes, which are min-

eralogically and chemically similar to

the pumice, consist of an outer shell of

obsidian adjacent to a zone of flow-

banded and flow-folded vesiculated and

glassy rhyolite. Type

hyaloclastites

are derived from the lobes during the

fragmentation of lava flows or magma

bodies (Fig. 6) that have intruded

water-saturated Type

hyaloclastites.

These Type 2 hyaloclastites are closely

related, are in sharp contact with the

parental lava, and have no internal

structures indicative of transport.

Autoclastic fragments may be re-

worked by bottom currents or resedi-

mented from density currents. In certain

cases the deposit may show some of

the characteristics of pyroclastic rocks.

These secondary hyaloclastites may be

difficult to distinguish from pyroclastic

rocks, because both types of fragmen-

tation may form similar textures. Pu-

mice and scoria should, however, be

more common in pyroclastic deposits.

Honnorez and Kirst (1975) have shown

that the percentages of concave, con-

vex, and planar grain boundaries can

be used to distinguish a hyaloclastic

from a pyroclastic fragment.

The model proposed for basaltic flow

breccias and hyaloclastites is suffi-

ciently well established to be used as a

norm.

PYROCLASTIC ROCKS

Pyroclastic debris consists of pumice,

scoria, glass shards, crystals, and ac-

cessory lithic fragments. All of this ma-

terial is explosively ejected from vents

and then falls or flows in air or water

under the influence of gravity. There is

consensus among workers that gra-

vity plays a major role in the en masse

ASH FALL

0.5

\;ISPERSED

ASH

PYROCLASTIC

GRAVITY FLOW

DEPOSITS

transport of pyroclasts. Settling veloci-

ties of pyroclasts are proportional to

fragment size, shape, and density.

Therefore, the principles that govern

the sedimentation of pyroclasts are

similar to those controlling the deposi-

tion of other

clastic debris.

The lateral extent and geometry of

pyroclastic deposits are influenced in

part by magma composition and in part

by the environment in which the erup-

tion takes place. Basaltic subaerial

eruptions generally produce cones of

scoria and ash of limited areal extent

around or downwind of the cones. Ba-

saltic eruptions that take place in

shallow water, or where water has ac-

cess to the vent, are more strongly ex-

plosive and produce ash rings and ash

layers that may have considerable la-

teral extent. Eruptions of acid and in-

termediate compositions are generally

explosive due to the volatile content of

the magma. These eruptions may pro-

ject very large volumes of pyroclastic

debris to heights well in excess of 25

km and may produce thick fall and

flow deposits.

The influence of the environment is

il-

lustrated by the asymmetric distribution

of pyroclastic deposits east and west of

the Lesser Antilles arc (Fig.

7).

On the

west side of the arc,

nubes ardentes

entering the Caribbean Sea descend on

steep slopes and are transported into

deep water where their characteristics

PYROCLASTIC

GRAVITY FLOW

DEPOSITS

DISPERSED

ASH

CARIBBEAN

WESTERLJES

ATLANTIC

-k

w;-

VOLCANIC

'*i

Figure

Asymmetric distribution of pyroclastic deposits east and west

the Lesser

Antilles arc. Volcanic islands are roughly

km apart. From Sigurdsson

eta/.

980).

are preserved, and they form various

types of density-current deposits. Those

entering the east side of the arc, in rela-

tively shallow water, are reworked and

lose their original characteristics, thus

becoming epiclastic deposits. Also, due

to the prevailing winds, most of the

fallout deposits are found on the Atlantic

side of the arc.

Facies

pyroclastic fall deposits

Pyroclastic fallout deposits range from

small volume, localized scoria and

cinder cone accumulations to large

volume sequences that mantle topog-

raphy over large distances. The depos-

its are the products of ballistic fallout

and turbulent convection in the eruption

column. Large volume fallout deposits

commonly are associated with flow de-

posits from the same eruption. The

change from fallout to flow is caused by

an increase in the magma discharge

rate and/or a decrease in the magmatic

volatile content. Two classifications of

fallout deposits are presented (Fig. 8).

DISPERSAL

INDEX

(km2)

I-

50-

THICKNESS HALF-DISTANCE

(km)

SURTSEYAN PHREATOPLINIAN ,O1cL;RA-

PLINIAN

ct\F

PLINIAN/I'

MlCROPLlNlAN

(STROMBOLIAN)

,,0'

Figure

Two classifications of pyroclastic fall deposits. A) Classification in terms of dis-

persal

(area enclosed by the 0.01TmaX isopach, where T,,,,, is the maximum thickness of

the deposit) and degree of magma fragmentation F (percentage of a deposit finer than

mrn

at the intersection of the dispersal axis and the

O.lTm,, isopach). Modified from Walker

973), Self and Sparks

978), Walker

980), and Francis et

a/.

(1 990).

Classification in

terms

thickness half-distance b, (distance over which the thickness of a deposit halves)

and half-distance ratio

bJb,, where b, is the maximum clast half-distance (distance across

which maximum clast size halves).

Curves show eruption column heights

(H,)

for different

values of

b,. As with (A) above, fragmentation increases toward the top, and dispersal in-

creases from left to right. Modified from Pyle (1989).

HAWAIIAN

7ZsPLINIAN

10 100

(1000// 10000 100 000

0.05 5 500 50 000

The deposits are grouped according to

their dispersal and degree of particle

fragmentation. Small magnitude erup-

tions form

hawaiian, microplinian

(strombolian), and surtseyan fallout

units. Larger magnitude eruptions

deposit

subplinian, plinian, ultraplinian,

and phreatoplinian fallout sheets. Surt-

seyan and phreatoplinian deposits are

characteristically fine grained (-1 to

Q),

even close to the source vent, in-

dicating efficient fragmentation by in-

teraction of magma and external

water. Another type of fall deposit,

termed

co-ignimbrite fallout ash, may

be formed during the emplacement of

a pumiceous pyroclastic flow

(ign-

imbrite) by the preferential loss of vitric

material from the flow to the atmo-

sphere. Co-ignimbrite ashes also are

fine grained and cover large areas.

Basaltic pyroclastic eruptions fre-

quently build scoria and cinder cones.

The resulting fallout deposits are termed

hawaiian, strombolian, or microplinian

(Fig.

8A). The deposits dip both away

from and into the cone, generally have

small volumes

1 km3), and are local-

ized in areal extent (Fig.

9A). The juve-

nile material has a range of

vesicu-

larities and grain sizes, the fragments

being crudely bedded and framework

supported. The deposits are variably

sorted because of

fragments with dif-

ferent vesicularities and densities,

particle agglomeration in fine-grained

layers,

mixing of turbulently sus-

pended material and ballistic blocks

and bombs, and

welded spatter that

results from high eruption rates. Fallout

material that accumulates on the steep

slopes of the cone may be mobilized

downslope by grain flows, resulting in

beds that are inversely graded. Scoria

and cinder cone sequences may have

rapid vertical and lateral transitions

among scoria and cinder layers, welded

agglutinated units, and fine-grained

beds, reflecting changes in eruptive

conditions, distance from the vent, and

phreatomagmatic influences.

Widespread fallout tephra sheets of

large volume

(10-lo3 km3) are usually

silicic in composition, although basaltic

examples do exist. Large explosive

eruptions frequently show a progres-

sion from fallout to pyroclastic flows

during the course of the eruption. The

fallout deposits consist primarily of

framework-supported pumice or scoria

clasts with subordinate crystal and lithic

106 LAJOIE, STIX

fragments. Pumices are commonly

angular and fractured from impact.

Pumices near the vent can have pink

oxidized interiors, indicating hot em-

placement. As a rough guide, in these

deposits lithic fragments are half as

large as pumices, depending on the

differences in density.

Fallout deposits can be massive

and/or stratified. Massive sequences

suggest sustained eruptions, while

stratified parts may indicate fluctuations

in eruption intensity or wind direction

(Fig. 10). In fallout deposits it may not

be easy to distinguish between primary

stratification due to factors stated

above and stratification due to rework-

ing after deposition where sorting im-

proves, and pumices commonly are

subrounded. Mantle bedding, in which

thickness over a limited area is uni-

form, is common except in topographic

depressions where thicker deposits

can accumulate due to secondary

movement of material. In this case,

however, the deposits are no longer

primary. Fallout deposits are

com-

monly better sorted than flow deposits,

but fine-grained ash that has agglom-

erated in clusters can result in poorly

sorted fall deposits. Ash clusters have

been documented in the deposits from

the May

18th, 1980 eruption of Mt. St.

Helens and the 1982 El Chichon erup-

tions. On the other hand, low-density

ash clusters can transport higher-

density crystals

(e.g., magnetite, py-

roxene) to greater distances from the

volcano than would normally be the

case if the grains fell individually (see

Kittleman, 1973; Fisher and Schmincke,

1984, p. 159-1 60). Fallout deposits may

be ungraded, normally graded, inverse-

ly graded, or a combination of the

above (Figs.

11, 12). Normally graded

sequences indicate that the eruption

was most intense during its initial

phases. By contrast, inversely graded

fallout deposits can be overlain directly

by flow deposits from the same erup-

tion, suggesting that eruption intensity

increased during the eruption. Beds

that exhibit both normal and inverse

grading imply that the intensity varied

during the course of the eruption. Some

fallout deposits have a fine-grained

base with accretionary

lapilli, suggesting

that the magma interacted with meteoric

water during eruption,

popsibly through

a crater or caldera lake. Such deposits

CARVAO C.

S. MIGUEL (AZORES)

FOGO,

1563,

SAO MIGUEL (AZORES)

CHICHON

APR.

1982

(MEX

Figure

lsopach maps for four fallout deposits. All thicknesses in mm.

Map of the Carvao C deposit, Azores. Note the limited areal

extent of the pyroclasts. From Booth

a/.

(1978). B) lsopach map of the El Chichon

deposit, Mexico. Note the relatively concentric

isopachs. From Carey and Sigurdsson (1986). C) lsopach map of the Fogo 1563 deposit, Azores. The deposit is strongly asymmetric. From

Walker and Croasdale (1971).

lsopach map of the 18 May 1980 Mt.

St.

Helens deposit. Secondary thickening occurs 300

downwind of

the volcano. From

Sarna-Wojcicki

eta/.

(1981).