how ‘hard’ are hard-rock deformations?

Earth reflections

How ‘hard’ are hard-rock deformations?

A.J. van Loon

Geocom, P.O. Box 336, 6860 AH Oosterbeek, The Netherlands

Received 2 August 2002; accepted 20 September 2002

Abstract

The study of soft-rock deformations has received increasing attention during the past two decades, and much progress has

been made in the understanding of their genesis. It is also recognized now that soft-rock deformations—which show a wide

variety in size and shape—occur frequently in sediments deposited in almost all types of environments. In spite of this,

deformations occurring in lithified rocks are still relatively rarely attributed to sedimentary or early-diagenetic processes.

Particularly faults in hard rocks are still commonly ascribed to tectonics, commonly without a discussion about a possible non-

tectonic origin at a stage that the sediments were still unlithified. Misinterpretations of both the sedimentary and the structural

history of hard-rock successions may result from the negligence of a possible soft-sediment origin of specific deformations. It is

therefore suggested that a re-evaluation of these histories, keeping the present-day knowledge about soft-sediment deformations

in mind, may give new insights into the geological history of numerous sedimentary successions in which the deformations

have not been studied from both a sedimentological and a structural point of view.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Soft-sediment deformations; Tectonics; Pseudo-tectonics; Structural history

1. Introduction

Rock successions—and particularly the succes-

sions of sedimentary rocks—have been recognized

from almost the very beginning of geological inves-

tigations to contain irregularities in the form of

deformations. It became also clear soon that many

of these deformations, particularly faults and folds,

may provide valuable data about the tectonic history

of the complex involved. Ongoing research into tec-

tonic processes—based for at least a major part on

field evidence—proved to allow reliable reconstruc-

tions of orogenesis, basin development and (at a more

recent stage) plate tectonics.

This great achievement, which made repetitions of

sedimentary successions understandable, but also

apparent hiatuses and lateral ‘inconsistencies’, made

structural geology to one of the basic disciplines in the

earth sciences. Students are now, from the very begin-

ning of their education, made aware of the importance

of rock deformations for the understanding of the

geological history, and they become used to analyse

deformations from a tectonic point of view. A possible

sedimentary origin of specific deformations gets, how-

ever, as a rule, still hardly any attention during geo-

logical training of students. The result is that a

sedimentary origin of deformations in lithified rocks

is not commonly considered, even though sedimentol-

0012-8252/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

PII: S0012 -8252 (02 )00157 -5

E-mail addresses: [email protected], [email protected]

(A.J. van Loon).

www.elsevier.com/locate/earscirev

Earth-Science Reviews 61 (2003) 181–188

ogy developed as a separate geological discipline

already in the 1950s, and has become a more mature

discipline in the 1960s of the past century.

It was, from the point of view of deformation

analysis, rather unfortunate that a prime study object

of sedimentology during the late 1950s and the early

1960s was syntectonic sedimentation. This focus on

flysch and related topics was a consequence of the

highly interesting studies carried out at the time on

turbidity currents and their deposits, turbidites (Kue-

nen and Migliorini, 1950). Fully developed turbidites

were recognized to contain a unit that sometimes is

characterized by convolute lamination, and which

thus shows intrastratal deformation. The flysch suc-

cessions in which turbidites occur abundantly tend to

be deformed so strongly also by typically tectonic

processes, however, that the tectonic deformations

commonly received much more attention—even from

sedimentologists—than did the convolutions. The

same was true for soft-sediment deformations in

otherwise deformed successions, and for other defor-

mations (Fig. 1). The result was that deformations in

hard-rock sediments did, even in the 1960s, still

receive relatively little attention from sedimentolo-

gists, apart from cases where they could help to

reconstruct the palaeogeographical development of

basins characterized by syntectonic sedimentation

(or synsedimentary tectonics).

The interpretation of sedimentary structures became

gradually more important in sedimentology, particu-

larly as a means to reconstruct depositional processes

and to explain horizontal and vertical facies transi-

tions. It appeared that many of these processes could

be reconstructed on the basis of sedimentary struc-

tures, and the analysis of such structures became a

main sedimentological objective, culminating in the

famous overview by Allen (1982) of the then state-of-

the-art with respect to sedimentary structures. By that

time, it had become clear that not all non-tectonic

structures in sediments were due to sedimentary pro-

cesses, but that numerous types had developed during

early diagenesis, i.e. after deposition of the layer(s)

involved but before lithification started (penecontem-

poraneous deformations). More important, it had also

become clear that many deformations with a ‘tectonic’

appearance could not be explained by endogenic

processes, but only by processes that affected exclu-

sively the topmost sediments. In much rarer cases, it

appeared that specific layers were deformed that had

already been buried under new sedimentary layers that

remained unaffected themselves (intrastratal deforma-

tions).

Fig. 1. Small-scale deformations in the Late Carboniferous Westward Ho! Fm. along the coast of Westward Ho! (Great Britain). The intrastratal

character of the deformations—which underwent liquefaction, plastic deformation and brittle deformation (faulting)—makes a sedimentary

origin obvious.

A.J. van Loon / Earth-Science Reviews 61 (2003) 181–188182

2. Soft-sediment deformations under discussion

It is now recognized that soft-sediment deforma-

tions show a wide variety in shape. My experience in

both hard-rock and soft-rock areas is that soft-sedi-

ment deformations do certainly not show less variety

than tectonic deformations in hard rock; they are

sometimes even much more complex, except for some

hard-rock deformations that may be formed during

metamorphism.

In unlithified rocks, all types of folds and faults

occur that are known from tectonically affected hard-

rock units, and many more in addition. Not all soft-

sediment deformations were recognized and accepted

as such, however, until a few decades ago. Archives

of manuscript reviews can be highly interesting in this

context, particularly from the point of view of the

history of science. That deformations such as, for

instance, kink structures can occur in unlithified sedi-

ments (Fig. 2) seemed amazing, not so long ago. In

Fig. 2. Well-developed kink zone in a Pleistocene ice-pushed ridge near Balderhaar (Germany). From van Loon et al., 1984.

A.J. van Loon / Earth-Science Reviews 61 (2003) 181–188 183

fact, a manuscript describing such structures and

submitted in 1982 to the journal ‘Tectonophysics’

received originally a negative judgement from one

referee (most likely a structural geologist, considering

his comments), who stated that the interpretation of

deformations in a Pleistocene sandy ice-pushed ridge

and of comparable ones in subrecent silty/peaty

lagoonal sediments could not be correct: ‘‘. . .Afterreading the paper I am not entirely convinced that the

structures described are directly related to what are

normally called kink bands in crystals and rocks with

strong planar anisotropy. It seems to me that mechan-

ical properties of weakly cohesive water-laden sedi-

ments would in no way resemble those of materials of

great directional strength. . .’’ (anonymous referee).

This view expressed well the then common belief

that kink structures could, in a geological context, be

formed only within crystals or in materials with

comparable anisotropy. It should be emphasised, how-

ever, that the manuscript was also refereed by a

famous sedimentologist, who was less pre-occupied:

‘‘. . .This paper deals with a very exciting topic—kink

band development in soft-sediments. . .’’. Eventually,the manuscript was accepted, but it was advised, in

order to avoid too much ‘confusion’ among structural

geologists, to split up the manuscript into one dealing

with kinks in sandy sediments and one dealing with

kinks in fine-grained sediments. These contributions

were eventually published in 1984 and 1985, indicat-

ing how much time the discussion about these struc-

tures took. Similar experiences exist with respect to

other soft-sediment deformations that were formed

under conditions that were—as was considered at

the time—impossible for areas that were not affected

by severe tectonics. However ‘shocking’, these

‘impossible’ structures in unconsolidated sediments

may have been up to the middle 1980s, it is now well

known from material science that it is only logical that

such deformations occur in unlithified sediments, and

few well-educated structural geologists will nowadays

still be truly surprised by such observations.

3. Gaps in knowledge

The earlier scepsis with respect to a natural, non-

tectonic origin of soft-sediment deformation structures

is understandable in the context of the knowledge we

had a couples of decades ago. One might expect,

however, that it is now generally recognized that

deformations within sediments may have a soft-sedi-

ment origin, but this appears not to be true. This is

probably, at least partly, due to the fact that earth-

science students at most universities have to specialize

at a fairly early stage of their study. This way, they

may become petrologists with hardly any paleonto-

logical knowledge, structural geologists with only a

limited sedimentological training, or sedimentologists

almost without fundamental insight into structural

geology. This makes it difficult for geologists with

limited field experience to distinguish between tec-

tonic and sedimentary deformations; particularly in

metamorphosed terrains. It may, however, also be

extremely difficult for experienced geologists to find

out whether specific structures are due to tectonics or

that they represent soft-sediment deformations (cf.

Dasgupta, 2002; Gairola and Srivastava, 2002).

Even if no metamorphism has occurred but if the

area has undergone one or more folding phases, dis-

tinction between purely tectonic and tectonically

deformed sedimentary deformations can be compli-

cated. Both sedimentologists and structural geologists

should be aware that lithified and tectonically affected

rock successions may contain soft-sediment deforma-

tions (interesting examples are provided by Ghosh et

al., 2002). This implies that, at the one hand, sedi-

mentologists should—when reconstructing the paleo-

slope of a basin by measuring the fold axes of slump

folds—correct for tectonic tilt. It implies also, on the

other hand, that structural geologists should be aware

that faults may have a soft-sediment origin; in such a

case, the stress directions deduced from these struc-

tures cannot be used to reconstruct the stress systems

that played a role during the orogenesis that deformed

the entire succession. How difficult it can be to

distinguish between tectonic and sedimentary struc-

tures, particularly if an orogenic belt has been meta-

morphosed, is well described by Bradley and Hanson

(2002). In addition, it can be difficult to distinguish

clastic sills and intrastratal deformation layers from the

sediments in between which they are positioned,

because there often is no obvious lithological differ-

ence (Kawakami and Kawamura, 2002).

This difficulty has the consequence that lack of

distinction between non-tectonic and tectonic defor-

mations has frequently led to incorrect reconstruc-


tions. Unfortunately, a similar lack of understanding

still can play a role nowadays. In my opinion, this can

be overcome only by stopping the early specialization

in geological education: earth scientists should truly

be earth scientists, rather than structural geologists,

sedimentologists or specialists in any other earth-

science discipline. Only after a firm geological

basis—with significant field experience as a conditio

sine qua non—has been established, the specialization

that is, understandably, required nowadays should be

started. Such a broadly oriented educational program

would result in less gaps in knowledge and, conse-

quently, in less misinterpretations of field observa-

tions.

4. The need for re-interpretations

It must be recognized that previously collected

earth-scientific data, however well interpreted in the

past on the basis of the then state-of-the-art knowl-

edge, now often need re-interpretation. This holds, for

instance, for sedimentology, in which discipline so

much more is known now about the various deposi-

tional environments than only half a century ago that

new analyses commonly provide an entirely new

picture of dynamic environments. The dynamics result

in shifting facies, whereas other dynamics result in

orbital-forced sequences, and in global processes such

as changing oceanic circulation patterns. Similar

adaptations of previous ideas are common in stratig-

raphy, in which discipline it has been recognized that

diachronic lithological changes are the rule rather than

the exception. This insight has made it necessary for

instance to change the interpreted age of rocks in even

classical areas such as the Ardennes in Belgium,

where rocks that were originally mapped as belonging

to the uppermost part of the Devonian (Frasnien 2d)

are now considered to belong to the Carboniferous

(Tournaisian), but—for reasons of consistency—are

still commonly mapped with the Fa2d code.

It has also been recognized in stratigraphy that

many important stratigraphic boundaries have been

established on the basis of hiatuses (so that new

chronostratigraphic units had to be introduced, with

the Cantabrian as an example: Wagner, 1966),

whereas other chronostratigraphic units had to be

deleted (or are being discussed) because they were

introduced on the basis of regionally diverging litho-

and biofacies rather than on a previously truly ‘miss-

ing’ time interval (Montien).

In structural geology, insights have been greatly

changed after the concept of plate tectonics had been

introduced. Classical ideas about geosynclines and

related topics had to be abandoned or at least funda-

mentally revised. Many ideas related to the assumed

fairly frequent occurrence of nappes also had to be

changed drastically. The huge amount of field data, in

the form of, among other data, bedding-plane dip

values, directions of fold axes and the orientation of

minor fault planes, has rarely been considered—and

still is rarely considered—as needing re-interpretation.

This is the more astonishing as sedimentary defor-

mations occur most frequently under conditions of

rapid sedimentation and unstable depositional surfa-

ces. Such conditions are very common during oro-

genesis, so that sedimentary deformations occur

frequently in flysch and molasse deposits. The inter-

relationship between tectonics and sedimentation is

commonly so close under these circumstances that it

is difficult to decide whether syntectonic sedimenta-

tion takes place, or rather synsedimentary tectonics.

Whatever term is preferred, it is obvious that many of

the sedimentary deformations in such successions are

not representing the then tectonic stress fields; it is

equally obvious that many of such deformations have

been used for structural analyses.

This implies that not all structural research con-

cerning mountainous (and other tectonically affected)

areas is based on reliable data. The question is thus:

how ‘hard’ are ‘hard-rock deformations’? It is easy to

believe that deformations originating from the time

that lithification had not yet taken place are of minor

importance in tectonically affected areas. Such a belief

is, however, unjustified because research in nowadays

tectonically active areas shows that sedimentary

deformations in the uppermost layers—in the form

of seismites (Mohindra and Thakur, 1998) as well as

in the form of gravity-induced mass-movement pro-

cesses on an unstable slope—are fairly common (cf.

Rossetti, 2002). If, for instance, material eroded from

an area that is being uplifted is deposited in large

quantities on the slope of a subsiding basin in front of

the rising hinterland (a common situation near island

arcs), numerous slump masses may flow down; these

will—because the dip of the slope will remain more or


less constant for a relatively long time—show fold

axes that are all oriented in the same direction. This

may easily give the impression of a tectonics-related

stress system, but it is not (Fig. 3). One should, in

addition, keep in mind that mass flows may involve

huge amounts of sediment: it was found during ODP

Leg 157 that a huge mass failure of El Hierro (one of

the volcanic Canary Islands) resulted in a thick

deposit on the ocean floor probably some 6000 years

ago, as stated in a presentation at the recent IAS

conference (Jarvis and Weaver, 2002). Even more

recent (November 2000) was a mass failure of the

Fig. 3. Deformations due to gravity-induced plastic mass flow. (A) Modern rock glacier (in bird’s eye view). (B) Cross-section of a slump in the

Cretaceous Poumanous Fm. near Pobla de Segur (Spain). The folds in these tilted sediments are not related to the tectonics that affected the area,

but must be attributed to a soft-sediment deformation pattern as shown in (A).


Kilauea volcano (Fig. 4), reaching the sea, involving

some 2000 km3 that slip-slided away without, how-

ever, resulting in vast mass-transported deposits at the

foot of the volcano on the ocean floor (Cervelli et al.,

2002; Ward, 2002). Even larger masses have been

found on the bottom of the Mediterranean. If such

deposits, with thicknesses that may reach several

dozens of meters, would be encountered in the field

in the form of lithified rock, it would most probably

not be easy to find out the sedimentary origin of large-

scale deformations resulting from plastic behaviour

during transport. Another example is the Neoproter-

ozoic ‘Great Breccia’ in Scotland (Fig. 5), which

reaches a thickness of 50 m and was originally erro-

neously interpreted as a subglacial deposit (Spencer,

1971), but recently turned out to be a three-unit

complex of mass-flow deposits; this sedimentological

re-interpretation, partly based on the recognition of

Fig. 4. Part of the Kilauea flank material that slip-slided towards the ocean (November 2000). Photograph kindly provided by Peter Cervelli

(Stanford University).

Fig. 5. Large dolomite block with recumbent folds, constituting a megaclast in the Great Breccia (Port Askaich Fm., Neoproterozoic) near

Elieach an Noaimh, Scotland. The cliff is approximately 35 m high. Photograph kindly provided by Emmanuelle Arnaud (University of

Guelph).


soft-sediment deformations, has important paleoclima-

tological implications, because the original ‘proof’ of

glacial conditions is no longer valid (Arnaud and

Eyles, 2002).

It may be true that Earth offers less and less areas

that have not been geologically mapped, and of which

the structural history has not yet been analysed. It

seems, however, that the insufficiently hard character

of many structural data—being partly based on soft-

sediment deformations—offers the challenge of re-

analysing all tectonically affected areas where the

structural history has been written without present-

day insight into the importance of soft-sediment

deformations. An instructive example is the study by

Kusky and De Paor (1991), who were able to simplify

the structural history of a metamorphic unit in Canada

dramatically on the basis of re-interpretation of defor-

mation structures, which they found to be of sedi-

mentary origin. Earth apparently waits for a new

episode of structural mapping.

Acknowledgements

I want to express my thanks to Dwight Bradley

(USGS, Anchorage), who provided me with some

well-chosen studies supporting the view that re-

mapping—taking soft-sediment deformations into

account—can change the reconstruction of structural

histories considerably. I am indebted to Peter Cervelli

(Stanford University) for providing Fig. 4, and to

Emmanuelle Arnaud (University of Guelph, Ontario)

for providing Fig. 5.

References

Allen, J.R.L., 1982. Sedimentary structures—their character and

physical basis. Developments in Sedimentology, vol. 30 (2 vol-

umes). Elsevier, Amsterdam. 593 + 663 pp.

Arnaud, E., Eyles, C.H., 2002. Catastrophic mass failure of a Neo-

proterozoic glacially influenced continental margin, the Great

Breccia, Port Askaig Formation, Scotland. Sedimentary Geol-

ogy 151, 313–333.

Bradley, D.C., Hanson, L.S., 2002. Paleocurrent analysis of a de-

formed Devonian foreland basion in the Northern Appalachians,

Maine, USA. Sedimentary Geology 148, 425–447.

Cervelli, P., Segall, P., Johnson, K., Lisowski, M., Miklius, A.,

2002. Sudden aseismic fault slip on the south flank of the Ki-

lauea volcano. Nature 415, 1014–1017.

Dasgupta, P.K., 2002. Preservation of primary sedimentary features

in high-grade paragneissic complex from some parts of Eastern

Ghats and adjoining areas, Peninsular India and its significance

in basinal evolution. 16th International Sedimentological Con-

gress (Johannesburg, 2002) Abstracts Volume, pp. 74–75.

Gairola, V.K., Srivastava, V., 2002. Preserved sedimentary attrib-

utes, structures and metamorphic history of the Chhotanagpur

Granite Gneiss Complex exposed to the east of Renukoot, Dis-

trict Sonbhadra, U.P., India. 16th International Sedimentological

Congress (Johannesburg, 2002) Abstracts Volume, p. 109.

Ghosh, S.K., Sengupta, S., Dasgupta, S., 2002. Tectonic deforma-

tion of soft-sediment convolute folds. Journal of Structural

Geology 24, 913–923.

Jarvis, I., Weaver, Ph., 2002. Submarine and subaerial growth

phases of volcanic oceanic islands as seen in turbidite sequen-

ces: Miocene–Holocene of the Canary Basin, NE Atlantic. 16th

International Sedimentological Congress (Johannesburg, 2002)

Abstracts Volume, p. 175.

Kawakami, G., Kawamura, M., 2002. Sediment flow and deforma-

tion (SFD) layers: evidence for intrastratal flow in laminated

muddy sediments of the Triassic Osawa Formation, northeast

Japan. Journal of Sedimentary Research 72, 171–181.

Kuenen, Ph.H., Migliorini, C.I., 1950. Turbidity currents as a cause

of graded bedding. Journal of Geology 58, 91–127.

Kusky, T.M., De Paor, D.G., 1991. Deformed sedimentary fabrics

in metamorphic rocks; evidence from the Point Lake area, Slave

Province, Northwest Territories. Geological Society of America

Bulletin 103, 486–503.

Mohindra, R., Thakur, V.C., 1998. Historic large earthquake-in-

duced soft sediment deformation features in the Sub-Himalayan

Doon valley. Geological Magazine 135, 269–281.

Rossetti, D.F., 2002. Events of sediment deformation and mass

failure in Upper Cretaceous vestuarine deposits (Cameta Basin,

northern Brazil) as evidence for seismic activity. 16th Interna-

tional Sedimentological Congress (Johannesburg, 2002) Ab-

stracts Volume, pp. 312–313.

Spencer, A.M., 1971. Late Precambrian glaciation in Scotland.

Memoirs of the Geological Society of London 6, 1–100.

van Loon, A.J., Brodzikowski, K., Gotowala, R., 1984. Structural

analysis of kink bands in unconsolidated sands. Tectonophysics

104, 351–374.

Wagner, R.H., 1966. Sur l’existence, dans la Cordillere Cantabri-

que, de series de passage entre Westphalien et Stephanien: la

limite inferieure de ces formations Cantabriennes. Comptes

Rendues de l’Academie des Sciences de Paris 262 (Serie D),

1337–1340.

Ward, S.N., 2002. Slip-sliding away. Nature 415, 973–974.


how ‘hard’ are hard-rock deformations?

Documents