Seismically induced deformation structures in Oligocene shallow-marine and aeolian coastal sands (Paris Basin)

79

Tectonophysics, 206 (1992) 79-89 Elsevier

Science

Publishers

B.V., Amsterdam

Seismically induced deformation structures in Oligocene shallow-marine and aeolian coastal sands (Paris Basin) Isabelle

Cojan and MCdard Thiry

Ecole Nationale Suphieure des Mines de Paris, Centre de Gt!ologie G&hale 77305 Fontainebleay France (Received

April 25, 1991; revised version

accepted

et Minike, 35 rue Saint-Honor;,

October

14, 1991)

ABSTRACT

Cojan, I. and Thiry, M., 1992. Seismically induced deformation sands (Paris Basin). Tectonophysics, 206: 79-89.

structures

in Oligocene

shallow-marine

and aeolian

coastal

Soft-sediment deformation structures have been identified in the Oligocene coastal sand body in the southern part of the Paris Basin (Stampian, Sables de Fontainebleau Formation). These coastal barrier sediments are characterized by pure, fine-grained sands. The deformed beds are observed over some 60 X 110 km* and are present either in foreshore sands or aeolian sand dunes. Convoluted laminations of an average amplitude around 0.70 m were identified in the foreshore or interdune sediments. These are interpreted as the result of liquefaction. Conversely, deformed sets of aeolian dune sand often involve large amounts of sediment. These large-scale, contorted laminae (several metres thick) with minor internal faulting suggest deformation by slope failure under wet conditions. Such large-scale soft-sediment deformations may have been generated by storm surges and earthquakes which have similar liquefaction potentials. We propose that the deformation structures were created by a single triggering event because of the absence of superposition of deformed layers over the study area. The earthquake-triggering hypothesis is supported by the large amount of sand involved in the slope-failure features and the persistence of the contorted laminations over a wide area.

Introduction Soft-sediment deformation structures, subcontemporaneous with sedimentation, may be explained by many different mechanisms (Dzulynski and Walton, 1965; Rigby and Hamblin, 1972; Allen, 1982; Seilacher, 1984). Failure of underlying layers as well as slope failure generate structures which are greatly influenced by sediment composition and water content. Failure of underlying sediments is found in flat-lying sediments (lacustrine, shoal). Convoluted beds are then underlain and overlain by undeformed strata. The

Correspondence

to: I. Cojan,

Mines de Paris, Centre rue Saint-Honor&

0040-1951/92/$05.00

Ecole Nationale

de Geologic

Superieure

Gin&ale

77305 Fontainebleau,

0 1992 - Elsevier

et Miniere,

France.

Science

Publishers

des 35

mechanism generating these convoluted laminations is often a function of sediment liquefaction (Dzulynski and Walton, 1965; Sims, 1975; Kokurek, 1981; Pettijohn et al., 1987) or air escape (de Boer, 1979; Kokurek, 1981; Fryberger et al., 1990). Slope-failure features have been recorded in marine deposits on the continental slope (Kelling and Stanley, 1971; Seilacher, 1984) and in aeolian dunefields (Bigarella et al., 1969; Fryberger et al., 1988). As this paper deals with coastal sands, only slope failure in the aeolian dunes is entertained as a possible mechanism. In aeolian deposits, oversteepening, overloading or undercutting of the lee-face of the dune are often considered as the major triggering mechanisms leading to slump (MC Kee et al., 1971; Fryberger et al., 1988). Wetness of the sand will then influ-

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I. COJAN

80

ence the preserved avalanche structures. Fadingout structures are present in a dry or watersaturated sand flow, while minor faulting and contorted strata develop in wet sand slump (Bigarella et al., 1969; MC Kee et al., 1971). The main purpose of this paper is to determine the mechanism responsible for generating the soft-sediment deformations observed in both shallow-marine deposits and aeolian coastal sand bodies. In all studied sections, a single large-scale contorted horizon is observed whether it is in foreshore or aeolian facies. The position of the deformed horizon in the sequential arrangement and its broad palaeogeographic distribution suggest that these soft-sediment deformations record a single event in different environments. Various styles of deformation are identified and correspond to failure of underlying beds in horizontally stratified sediments (foreshore and interdune) or to slope failure of steep-angle stratifications (aeolian dunes). A comparison with modern analogs is given in order to identify the discriminating criteria applying to the formation of such structures in any of these two environments and to investigate the possibility of a single triggering mechanism.

Geological

setting

During Tertiary times, the Paris Basin acted as a stable platform on the southern edge of the North Sea where thick deposits accumulated (Fig. 11. Several sequences of continental, lagoonal and shallow-marine deposits formed in response to sea-level changes. In most parts of the basin, thicknesses do not exceed 200 m and are less than 100 m over wide areas. The thinness of the deposits was partly a function of the progressive migration of the subsidence centre from north to south during the entire Tertiary (Cavelier and Pomerol, 1979). Prevailing evidences of tectonic activity in the Paris Basin are smooth movements recorded by sediment coarsening or deposit thinning around the major anticlines of the Mesozoic series. The main tectonic features of the Paris Basin were sealed during Cretaceous times; faults of Tertiary age are sparse and anticlines result

AND

4

I

Fig.

1. Extent

tectonic

of the Oligocene

framework and

(framed

anticlines

by a rectangle)

Basin;

50 to 300 m in thickness; matic);

zones. during

Ziegler,

graben

The

to the

elastics

in the subsiding

sands (dominant

(from

delimit

corresponds deposits

the contemporary

Europe

faults

uplifted

I= thick marine

ness accumulated marine

active

define

elastic shelf and shoreline Paris

sea and

of northwestern

1988). Most of the major tures

M. THIRY

study

the Tertiary

in the

up to 1000 m in thickNorth

3 = inversion

Sea;

2 = shallowfrom

axis; 4 = faults (sche-

5 = active deformation

Sables de Fontainebleau

area

southernmost

on the shelf) and shales ranging

more from a differential true uplift.

struc-

front.

subsidence

than from

Formation

During the Stampian stage (Late Oligocene), the most southward transgression, observed in the Paris Basin during Cenozoic times, led to the deposition of marine sands (Dollfus, 1911; Alimen, 1936; MCgnien, 1980). The sand body was widely stripped off in the northern part of the basin during Pliocene and Quaternary times. The southern part, however, is preserved and well exposed in large quarries allowing detailed observations of the sedimentary structures of these deposits (Fig. 2). The sands are fine-grained and well sorted, with a fine grain size around 0.1 mm and coarse grain size 0.13-0.20 mm (Riveline-Bauer, 1970). The clay content is low (< 10%) and pebbles are present only in the lower part of the deposit. The

SEISMICALLY

INDUCED

DEFORMATION

STRUCTURES

IN OLIGOCENE

sands are mainly reworked from former sand deposits in the basin (Lorenz et al., 1984). In cores, the sands are dark greenish-grey to black. When exposed in quarries walls excavated on the valley edges, the sands are white or coloured by iron oxides. The white facies results from an oxidation of dark sands by groundwater circulation (Thiry et al., 1989). Due to this alteration, the original content of clay and organic matter in the sediments cannot always be determined. The sedimentary structures show two successive evolutions from shoreface to foreshore facies and then to aeolian deposits (Fig. 3). At the top of the sand formation (Fig. 2), the aeolian deposits form parallel arcuate beach-ridge dunes over at least 100 km long (Dollfus, 1911; Alimen, 1936). The successive sequences are interpreted as response of the sand body to rapid deepening related to relative sea-level changes (Cojan and Thiry, 1989).

SHALLOW-MARINE

Deformation

AND AtOLlAN

C‘OASTAI.

81

SANDS

structures

In the upper part of the second sequence of this sand ridge, a large-scale, convoluted horizon was observed (Fig. 3). These soft-sediment deformations are present in either the foreshore facies or the aeolian deposits (Plaziat et al., 1989; Figs. 2 and 3). The depositional regime controls the characteristics of the contorted beds. Foreshore-facies

deformations

Convoluted beds are present in sands displaying subhorizontal planar laminations and continue laterally for some tens of metres. Heavymineral layers are commonly observed; some rare burrows and low-relief troughs are also present. All of these structures characterize foreshore to backshore facies (Reineck and Singh, 1975; Moslow, 1988).

CHARTRESO

50 km

C -4

l -5%.,-6

Fig. 2. Map of the study area showing studied

quarries.

1980, atlas) performed horizon

palaeogeographic

and by dune

crest alignments

in quarries

Fontainebleau

which

Formation

has been observed

Localities:

the present

A broad

B = Butteaux;

trend

(from Alimen,

are indicated

by stars

below the lacustrine in (4) foreshore

location

sediments

Maisse;

deposits

by the southern

1936). Detailed

and dots.

limestone

BM = Bourron-Marlotte;

of Stampian

is indicated

plateaus;

N = Epernon;

observations

I = Sables

deposits:

Breuil-en-Vexin; SY = Saint-Yon:

in the Paris Basin and the names transgression

of the sedimentary

de Fontainebleau

3 = dune crests alignments;

and in (5) aeolian BV=

(Oligocene)

limit of the Stampian

6 = southern

E = Etampes; V = Villejust.

Formation

structures outcrops;

4, 5 = quarries

of

(from Megnien, can only be 2 = Sables

de

where the deformed

limit of the Stampian

transgression.

LG = Larchant-Gondonnieres;

M =

I. COJAN

82 ENVIRONMENT .dlmth

LmiOLOGY

m

shell lag ICIW angle troughs

trans fessive lag sub. ifarizontal sets bidire~ion~ ripples

storm deposits

Fig. 3. Typical vertical sequence of the Sables de Fontainebleau Formation. A description of the primary sedimental structures is given on the right, as well as the location of the deformed horizon (dot and star refer to Fig. 2). Environment of deposition is shown in the left column: two sequences are recognized, each of them being characterized by an overall coarsening-up in grain size in correspondence to the evolution of a prograding barrier.

From observations made in several quarries, the deformed horizon is unique in the sequence and can be traced for tens of metres. It is underand overlain by undisturbed sediments. Convolute lamination is the prevalent mode of deforma-

AND

M. THlRY

and is easily recognizable by grain-size contrasts or heavy-mineral layers (Fig. 4a). Deformation structures in this type of sediment correspond to folding with some minor internal faulting (Fig. 4b): (a) Folding is characterized by broad synclinal structures and sharply crested anticlinal ones. Flame structures are sometimes present in the anticlines (Fig. 5). These folds die out downward as well as upward. Distances between anticlines differ considerably (Figs. 4 and 5). Two sizes are recognized, one with wave lengths around 0.30 m and one around 1.3 m. The convoluted layer varies from 0.15 to 1.5 m in thickness, and averages around 0.70 m. (b) Faulting is also observed within the convoluted beds, but displacements do not exceed a few centimetres. This faulting only affects some of the laminations and, therefore, probably expresses the differential mechanical behaviour of some layers during deformation (Fig. 4b).

tion

Dunefield deformations

The aeolian deposits are characterized by high-angle sets of strata and well sorted sands. Dune morphologies can be traced by root horizons and interdune deposits such as lacustrine limestones. Most of the interdune deposits, however, consist of nearly parallel stratified sands

Fig. 4. Small-scale soft-sediment deformations in the nearly parallel-laminated foreshore facies (Epernon). (a) Regularly spaced convoluted laminae marked by dark layers of heavy minerals. Thickness of the convoluted bed is around 0.15 m. (bi Small-scale faults with dispIacement of about 2 cm affecting parai~el-laminated sands (bar scaie in cm).

SEISMICALLY

INDUCED

Fig. 5. Large-scale Convoluted

soft-sediment

laminae

the anticline

DEFORMATION

showing

(Villejust).

STRUCTURES

deformation

broad

in foreshore

synclinal

(b) Deformed

structures

horizon,

parts display flame-like

IN OLIGOCENE

and minor faulting

0

Fig. 6. Deformed

interdune

horizon

by parallel-laminated

overlain

interpretation horizon

sediments

is that liquefaction

is overlain

by prograding

under-

AEOLIAN

accumulation

Flame-like

and over-lain

COASTAL

by undeformed

with displacements

and over-lain

of wind-blown

sand). (a)

are often present

sediments.

in

The anticlinal

of 0.20 m (Epernon).

less than a metre in thickness, often forms lenses of some tens of metres long. Its relationship to the deformed dune sediments will be considered later. Dune sands with cross-stratification of 15” to 35” show more complex and less regularly organized styles of deformation (Fig. 7); but again the contorted layers are over- and under-lain by regularly stratified dune or interdune deposits (Fig. 7a,b). Deformed sediment bodies may be several metres thick (Fig. 7a,b) and show different styles of deformation: (a) Folding is of different amplitudes and irregular wave lengths. Laterally, deformation is not evenly distributed; next to highly contorted laminations, stretches of several metres of sediments may show gentle wavy undulation. Anticlinal and synclinal forms are very irregular and show flame structures and highly contorted strata, some of which are isolated. (b) Faulting is mainly subvertical. Offsets do

0 I

by undeformed

is interpreted

subhorizontal

as an interdune

in layers below groundwater

dune sets and the favoured

83

SANDS

deformations

lm

sediments

occurred

AND

area = modern anticlines.

3 m thick, is under-

which are difficult to distinguish from the upper beach deposits (Kokurek, 1981; Fryberger et al., 1990). In this type of sediment, convolute lamination is sometimes difficult to identify because of the low grain-size contrast of aeolian deposits, and because there are no heavy-mineral layers. Thicknesses of the contorted layer are highly variable and range from some decimetres to several metres. Where complete observation of the contorted horizon was possible, it had a lens shape and was under- and overlain by undisturbed sediment. Two styles of deformation have been identified: the first one resembles the type observed in the foreshore facies and corresponds to interdunes (Fig. 6); the second one is linked with the dunes and involves large masses of folded and wrinkled sand (Fig. 7). Interdune convolutions have broad synclinal and slightly pinched anticlinal structures that die out vertically (Fig. 6). The convoluted horizon,

a

(dotted

and fairly pinched

around

structures

deposits

SHALLOW-MARINE

laminated

deposit

level (Bourron-Marlotte).

interpretation

level or below the toe of the prograding

sediments.

deformed

(a) The convoluted

by liquefaction.

(h) The large-scale

in this case is that liquefaction dune (Maisse).

5m I

occurred

Favoured convoluted near ground

1. C‘OJAN

84

while others show compressive or extensive structures, depending on their reIative positions on the lee-side of the dune (BigareIla et al., 1969).

not exceed a few centimetres and fault planes die out in about a metre (Fig. 7c,d). Some of the faultings seem related to slump scars (Fig. 7d)

Fig. 7. Contorted showing deformed surface

sets in aeolian

water-escape-like horizon interpreted

temporary)

dunes

structures.

displaying as related

of the sand leading

sand with a 4-cm displacement

irregular to slump

due to slope failure (b) Roughly slump

parallel

structures

to the formation of the sand

(Butteaux). horizontal

of 2-3

scar. The presence

(a) Regular

aeolian

laminated

interdune

m amplitude

of root traces

(hammer

(e) Detail

of a typical

structures.

wrinkled

strata

overlaying

sediments

length

in some dunes

of such crests (scale in cm). (d) Internal

laminae.

AND M THIRY

horizon

a 5-m-thick

0.30 m). (c) Faulting

of a crusted

testifies minor

a contorted

underlying

to a certain

wetness

(even if

fault in a large mass of deformed

mass of sand showing

numerous

flame-like

SEISMICALLY

INDUCED

DEFORMATION

STRUCTURES

IN OLIGOCENE

(c) Wrinkled masses of sand correspond to convoluted horizons with poorly developed or continuous structures (Fig. 7e). In some quarries, convoluted horizons in the interdune sediments as well as contorted masses of sand related to dune failure were observed (Fig. 8). On a vertical axis, they are spatially close to each other; laterally, overlapping of convoluted interdune strata by contorted dune sediments has been observed (Etampes, Butteaux). Possible

deformation

mechanisms

In order to understand possible mechanisms generating such soft-sediment deformations, comparisons have been made with recent sediments. The aim is also to test the hypothesis that a single event might have triggered these deformations in shallow-marine as well as aeolian coastal sands. Mechanisms in the two environments will be considered separately, even though there are some clear connections between the marine influence (tides and storms) and the dunefield deposits. Foreshore sands

Though it seems that soft-sediment deformations are not too frequent in foreshore sediments, most of the observations on recent sediments have been made on intertidal shoals. Beside trapped air, which may create a reverse density stratification and soft-sediment deformations (de

interdune liquefaction

Fig. 8. Profile

along an excavation

of interdune

sediments

convoluted

in thickness

of the contorted

by liquefaction

aeolian

sediments

AND

AEOLIAN

exaggeration).

and contorted is clearly

85

SANDS

Dunefields

In coastal dunefields, not all deformations are related to the avalanching of the dune lee-face. Some are linked with interdune sediments below the water table or submitted to sudden submersion. In aeolian deposits with a high angle of deposition, penecontemporaneous deformation is one of the characteristic features of the upper slipface strata (Bigarella et al., 1969). The degree of sand moisture greatly affects the type of contorted structures (MC Kee, 1979). Sand avalanching with fading-out laminae commonly develops in dry or saturated sands that flow easily. Wet sands or sands with a crust, as often found in coastal sand

Length aeolian

of the outcrop

dune deposits

I

permitted

generated

seen at this scale of observation

represent

COASTAL

Boer, 1979), these deformations result most often from liquefaction of fine-grained sands with low clay content. Tidal current drag, liquefaction by waves or dewatering of sands have been considered as possible causes; however, they do not exert sufficient shear to initiate liquefaction (Allen and Banks, 1972; Reineck and Singh, 1975; Dalrymple, 1979). The liquefaction potential of breaking waves and storm surges is far higher. It can easily be compared to that associated with earthquake vibrations (Dalrymple, 1979; LindstrBm, 1979) and is likely to generate soft-sediment deformations (no more than some 0.70 m) which may occur in beach deposits, even above the normal reach of spring tides.

dune slope failure

I

wall (note vertical

SHALLOW-MARINE

silcrete.

m

the simultaneous

by slope failure.

(Bourron-Marlotte).

observation

A lateral

change

Hatched

zones

86

dunes in areas with a certain amount of rainfall, have a higher degree of cohesion and deform by faulting and folding during slumping (MC Kee et al., 1971). Contortion is frequently caused by the slipping of laminae sets along the lee-side due to high angle of deposition or overloading. Slumping may also be caused by dune base erosion from tidal currents (spring tides, storms) invading the interdune domain (Fryberger et al., 1988). Interdune areas, characterized by relatively parallel laminations, can easily be invaded during high tides or, on a less regular basis, during spring tides or storms. Fluid escape structures may result from these flooding episodes when air is displaced by downward seepage of the seawater (Fryberger et al., 1990). Similar deformations are induced by liquefaction of interdune deposits when dunes prograde and load the watersaturated interdune deposits. Such a mechanism requires pronounced textural differences (grain size, algal mats) between the overlying dune and the saturated sediments (Kokurek, 1981; Fryberger et al., 1988). Whatever the mechanisms for such deformations may be, the scale of the resulting structures is always small. Slump marks, contorted laminae and air escape structures average a few centimetres and do not exceed 0.2 m in thickness and 1 m in length (Gripp, 1961; Bigarella et al., 1969; Glennie, 1970; MC Kee, 1979; Kokurek, 1981; Fryberger et al., 1988). Large-scale structures have also been described in other ancient aeolian deposits (Peacock, 1966; Steidtmann, 1974; Horowitz, 1982). These structures have been interpreted as caused by earthquakes liquefying interdune deposits near or below the water table in low topographic areas and inducing a lee-face collapse of the aeolian dune (Horowitz, 1982). It has also been proposed that these large-scale structures developed from slumps or undercutting of slipfaces (Peacock, 1966; Steidtmann, 1974). When comparing the triggering mechanisms that have been hypothesized, a few apply to sediment deformation in both environments. Fluid escape structures resulting from displacement of trapped air can be generated in foreshore and interdune domains by tidal currents (regular inva-

I. COJAN

AND

M. THIRY

sions or catastrophic ones during spring tides or storms). These structures can be interpreted as resulting from sand liquefaction caused by a storm (associated or not with a spring tide) or by a seismic shock. Slope-failure deformations involving small amounts of sand are a normal feature of dune avalanching. When large amounts of sand are displaced, earthquakes or large development of slump marks have to be considered as the cause. Discussion

Defomation

structures

In the coastal sands studied in this paper, soft-sediment deformations identified in foreshore and dunefield deposits have the following characteristics in common. (a> A single deformed horizon has been identified in all quarries. In the aeolian dunefields, two neighbouring horizons were sometimes observed and interpreted as a differential response to the same mechanism: liquefaction in interdune layers and slope failure of dune deposits collapsing on top of them. (b) This deformed layer is always situated in the upper part of the second sequence of sand ridges. (c) The deformed layer has a lateral extension of at least tens of metres and can be followed over the whole length of the quarry wall (around 200 m in average). In the aeolian deposits, structures generated by slope failure are large-scale and the thickness of the contorted sediments is highly variable, generally greater than 3 m. (d) The deformed horizon has been traced over a large area (60 x 110 km*) and geographic distribution in relation with the coastline has to be considered. The coastline, as inferred from the orientation of the dune ridges (Alimen, 1936), ran roughly west-northwest to east-southeast. Perpendicular to this direction, convoluted beds were observed in the foreshore-backshore sediments over 25 km and are restricted to the northern part of the study area, while contorted sediments in the aeolian sediments were characteristic in the southern part, as far as 25 km inland (Fig. 2).

SEISMICALLY INDUCED DEFORMATION

STRUCTURES

IN OLIGOCENE SHALLOW-MARINE

The dunefield and the foreshore facies will be considered separately and a possible correlation between the mechanisms that generated the defo~ations in each of these environments will be discussed in the interpretation. It is hypothesized that the contorted beds in the dunefield were generated by slope failure. Observed structures correlate well with those generated by avalanches in wet coastal sand dunes (Bigarella et al., 1969; MC Kee, 1979). The largescale deformations in the Sables de Fontainebleau dunes (several metres in height and tens of metres laterally) is not comparable to the relatively small-scale features common in modern aeolian sands (few decimetres in height and some metres laterally). Therefore, these large-scale deformations cannot be considered as part of the normal avalanching process of dune lee-sides. A storm, accompanied by seawater invasions of the dunefield, couId have created them by dune base erosion, but the modern examples involve only restricted amounts of sand (Fryberger et al., 1988). In ancient deposits, large-scale structures have been interpreted as initiated by earthquakes or by undercutting of slipfaces. In the foreshore facies, as in the interdune deposits, the large-scale convolutes, are more likely to have been initiated by sand liquefaction than by air escape. The fine grain size of the Fontainebleau Sands fits well with known cases of liquefaction in modern environments (Seed and Idriss, 1967). When soft-sediment deformations were identified in the foreshore facies, the primary sedimentary structures corresponded more to an upper beach than to an intertidal zone. Therefore, in these environments, potential mechanisms able to generate convolution by Iiquefaction are likely to be earthquakes, stormwave action or sand-charged surges. Interpretation

In this coastal environment of the Fontainebleau Sands, foreshore and dune facies are part of the same depositional system. Through time, the sand dunes migrated seaward as the barrier

AND AEOLIAN COASTAL SANDS

87

prograded. Unfortunately, as hardly no fossits are preserved, the ridge migration cannot be delineated by time-lines; however, synchronous occurrence of dune and foreshore environments has to be kept in mind. Comparison with soft-sediment defo~ations in modern deposits indicates that the two likely triggering mechanisms are storms (wave action or sand-charged surges) and earthquakes (Iiquefaction, undercutting of slipface). The influence of each of these mechanisms is considered over the coastal-sand system. Convoluted beds, that are created by stormgenerated processes in the foreshore facies, diminish as the water surges invade the dunefield. Contorted beds, associated with dune base erosion, are present inland over a restricted area, largely in connection with inlets. In the Sables de Fontainebleau Formation, because the intensity of deformations does not decrease in a direction perpendicular to the coastline, the slope-failure deformations may be the result of several storms, occurring over a period of time. Since superposition of deformed horizons has never been observed in the foreshore, the successive favourable environments for preservation of contorted beds never coincided spatially. Such a balance between the occurrence of storms and the rate of barrier progradation sounds unlikely to have happened. So, the storm hypothesis as a triggering mechanism and the multiple events are not entirely satisfactory. The degree of seismic wave deformation is variable according to the earthquake intensity. Liquefaction of sediments requires magnitudes of at least 4 to 5. The regional extent of the deformation can be explained by either multiple or single events. Several events with an intensity of 4-5 which occurred at varied intervals of time and would generate convoluted layers of rather limited extent. A single earthquake of a magnitude of 6, probably 7-8 (Kuribayashi and Tatsuoaka, 1975; Youd, 1977), would generate softsediment defo~ations over a large area (some 60 X 110 km21 with no fading intensity relative to the coastline. As with the storms, the operation of a number of triggering events has to be such (epicentre migration, intensi~) that no superposi-

88

tion of deformations are recorded. A single event is also in agreement with the regional distribution of the contorted beds in dune and shoreface facies. Despite the fact that no precise contemporaneity of the deformations can be verified on a stratigraphic base, the large amount of sediment involved in the slope failure deformed horizons and the absence of superposed deformed layers favour the hypothesis of a single seismic event. The proposed synchronous formation of the deformed beds by a seismic wave suggests that the contorted layer can be used as a regional time marker. Tectonic setting Overall, the Oligocene is a period of high tectonic activity in western Europe. It is the period of the main Alpine front thrusting and the development of the north-south graben systems (Mainz-Rhine, Hesse, Alsace, SaGne, Limagnes and RhBne, Fig. 1). In the Paris Basin, the deformation structures in the Sables de Fontainebleau Formation are observed over an area which comprises several anticlines and faults (Bray, Seine and Remarde) of a Hercynian trend (northwestsoutheast) (Megnien, 1980). Earthquakes recorded by the soft-sediment deformations may then be interpreted as the manifestation of localized tectonic stresses along these Hercynian structures during the Late Oligocene. The consistency of this contorted horizon can be explained by the fact that these coastal sands were deposited in an area which was not in a very active seismic region. Earthquakes with sufficient intensity to cause this deformation, were probably rare events.

I. COJAN

gle seismic event. This type of deformation has been observed in two environments (foreshore and aeolian dunefield) as a response to local stress, but caused by different deformational mechanisms. To interpret the presence of earthquake recurrence intervals in lacustrine environments, certain criteria must be met (Sims 1973, 19751. These include liquefaction-induced structures that extend over a large area, are confined to a single horizon and have not been influenced by slope failure. Sims’ criteria for earthquake triggering mechanisms can be easily transposed to foreshore sediments, where convoluted deformation was generated by liquefaction. In the aeolian dunefield, simultaneous recording of slope failure and liquefaction-induced structures corresponded to a high water table, close to ground level. The amount of sand involved in the avalanche process may be considered as a distinctive criterion between a non-tectonic process (avalanche failure generated by overloading, oversteepening, etc.) and a seismically induced deformation which will generate larger contorted sand masses. However, more observations are needed to validate criteria defining past seismic activity in aeolian dunefields. Acknowledgements

We appreciate the helpful and thoughtful comments on the manuscript by A. Seilacher, M. Friedman and an anonymous reviewer. References Alimen,

H., 1936. Etude

Mtm.

Conclusions

Allen,

J.R.L.,

J.R.L.

and

Structures:

Developments

Amsterdam,

analysis tology,

Basis.

Their

Character 30,

663 pp.

Banks,

N.L.,

of recumbent-folded

1972. An interpretation deformed-bedding.

and

Sedimen-

19: 2.57-283. J.J., Becker,

R.D. and Duarte,

dunes

from Parana

(Brazil).

tion

de Paris.

in Sedimentology,

Bigarella, Cavelier,

du Bassin

pp.

1982. Sedimentary

and Physical Allen,

sur le Stampien

Sot. geol. Fr., 31, 304

Elsevier,

Aeolian dunefields belong to a depositional environment where seismic events cannot be easily recognized because other mechanisms such as slope failure and liquefaction which produce similar deformations, are common features. We interpret the Stampian, coastal-sand softsediment deformations, to be generated by a sin-

AND M. THIRY

C. and Pomerol, des

evenements

G.M.,

Mar. Geol.,

C., 1979. Chronologie tectoniques

Bassin de Paris. Bull. Sot. gtol.

1969. Coastal

7: S-55. et interpreta-

cenozor’ques

dans

Fr., 7, XXI, 1: 33-48.

le

SEISMICALLY

INDUCED

DEFOKMATION

STRUCTURES

IN OLfCiOCENE

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SANDS

89

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