5 Apparent sea-Ievel fluctuationsand a palaeostress field forthe lr[orth Sea region

S. Cloetingh,* K.Lambeclet and H. McQueen$

* Vening Meinesz Laboratory, Institute of Earth Sciences, University ofUtrecht, P.O. Box 80.021, Utrecht, 35B4CD, The Netherlandsf Research School of Earth Sciences, Australian National University,P.O. Box 4, Canberra, A.C.T. 2601, Australiaf Department of Oceanography, Dalhousie University, Halifax,BBH 4JI. Canada

A palaeostress curve derived from the sea-level record ofthe North Sea Basin mirrors the tectonic evolution ofthis region. Sea-level changes in the basins ofnorth-western Europe can be associated with the major rifting andcompressional episodes of Mesozoic and Tertiary ages. The major Tertiary lowerings in relative sea level at thebasin margins can be interpreted in terms of increasing horizontal compression and the major falls in sea levelcoincide with the compressive folding phases of the Alpine orogeny.

The gradual rise in sea level recorded in the north-west European basins throughout the Jurassic andCretaceous can be associated with the extensional tectonics and the periods of offiap can be associated withrelaxation of the regional stresses as would result locally when active rifting occurs. Most of the Jurassic andCretaceous North Sea offiaps do coincide with active rifting phases.

INTRODUCTION

During the last decade, major advances in quantita-tive analysis of the sea-level record in sedimentarybasins have resulted from studies by Vail and his co-workers at Exxon (Haqet ø1.,1987; Vail and Todd,1981; Vail et ø1., L977;Yail et ø1.,1984). Although theYail et al. (L977) coastal onlap/offiap curves are basedon data from various basins around the world, theyhave been heavily weighted in favour of NorthAmerica, the Gulf Coast, the northern and centralAtlantic margins and especially the North Sea. Theirresults, therefore, do not seem to be a true reflectionof global changes; this is evident from Vail et al.'s(L977) Fig. 5, which shows several regions thatconform with the 'global' pattern neither in magni-tude nor in the timing of the cycles (see also Miall,1986). At the same time there has been a continuousdebate on the mechanisms that cause Vail et al.'sthird-order cycles in sea level (e.g. Bally, 1980, 1982).Recently, Cloetingh et ø1. (7985) proposed a ne\átectonic mechanism for regional short-term sea-levelvariations (time scales of a few million years andlonger) at rates of about 1-10 cm/1000 years with a

magnitude of up to about hundred metres. Theirmodel explains these changes, provided that horizon-tal stresses of the order of a few hundred MPa exist inthe lithosphere and changes in these stress fieldsoccur on geologic time scales. The proposed model(see Fig. 1(a)) represents the interaction betweenthese stresses and the deflections of ihe lithospherecaused by sedimentary loading and thermal contrac-tion (Sleep, 1971). Apparent sea-level changes of arate and magnitude inferred from seismic stratigra-phy can be produced at the flanks of sedimentarybasins by this interaction (Fig. 1(b)). ThereforeCloetingh et ø1. (1985) pointed out that glacialfluctuations (Pitman and Golovchenko, 1983) are notthe only mechanism capable ofproducing apparentsea-level fluctuations in excess of 1 cm/1000 years aswell as magnitudes of about 100 metres. Alternati-vely, ifthe proposed mechanism is accepted then theapparent sea-level curves can be interpreted in termsof palaeostress fields (Cloetingh, 1986).

Several independent studies of lithospheric defor-mation in active continental margin and intraplatetectonic settings lead to the conclusion that horizon-tal stresses exist in the lithosphere and that these

Petroleutn Geology of North West Europe @ J. Brooks and K. Glennie, eds (Graham & Trotman, 19BT) pp. 49-57.

Cloetingh, S., Lambeck, K. and McQueen, H., 1987. Apparent sea-level fluctuations and a palaeostress field for the North Sea region. In: Petroleum Geology of North West Europe, (J. Brooks and K. Glennie, Eds), Graham & Trotman, 49-57.

SEDIMENTARY WEDGE

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stresses may reach magnitudes up to a few hundredMPa (Lambeck et u.1.,1984; McAdoo and Sandwell,1985; McQueen, 1986; Stephenson and Lambeck,1985). Of interest here is the modification of the basinshape by variations in intraplate stress fields. Figure2 schematically illustrates the relative movementbetween sea level and the crust at the edge of a basinimmediately landward of the principal sedimentIoad. When horizontal compression occurs, the per-

ipheral flexural bulge is magnified and migrates in aseaward direction, uplift of the basement occurs, anoflap develops, and an apparent fall in sea levelresults, possibly exposing the sediments to produce

an erosional or weathering horizon. For a horizontaltensional stress field, the flanks ofthe basin subsideand migrate landwards, producing an apparent risein sea Level so that renewed deposition, with acorresponding facies change, is possible. The natureof this facies change depends, inter øliø, on the natureof the sediment supply. Induced deflections in thecentre of the basin, although of the sâme mâgnitude,are small compared to the total subsidence there and

are of less stratigraphic significance. As the sign andmagnitude of the apparent sea-level change will be afunãtion of the location of the sampling point,however, this may provide a means of testing thetectonic model and, independently, of distinguishingthis mechanism for eustatic contributions' In fact,Hallam (1987) has shown that a significantnumber ofJurassic unconformities are confined to the flanks ofNorth Sea basins, consistent with the predictions ofthe tectonic mechanism. Our modelling has demon-strated (Cloetingh, 1987) that the incorporation ofintraplate stresses in models of basin evolution can,in principle, predict a succession of onlap and offiappa[tr.nt such as observed along the flanks of theUnited States passive margin.

In this chapter (for a more detailed account seeLambeck et øt.,1987) we apply the tectonic model forapparent sea-level fluctuations proposed by Cloet-ingtr e¿ al. (1985) to the sea-level record published fornorth-western Europe. We have selected this regionbecause, in the words of Ziegler (1978)' this region isthe crossroads of orogenic belts and rifts, where

0

Fig. 1. (a) Model for apparent sea-level fluctuations resulting from variations in the in-plane stress field proposed by

õiãetirrgh e¿ o¿. (1985). The vertical displacement of the lithosphãre at a passive marginevolves through timebecause of the

thermal contraction anJ strengthening of the layer, and the sedimeni loading. The latter is repre-sente¿ bY -a wedge of

sediments that grows with timä (see iñset on lefi for the position of this wedge on the-outer shelf, slope and rise)' The

iilrro.pt "r"

is mîdered mathemaìically as a uniform elasiic layer overlying a fluid asthenosphere' Differences between

continental and oceanic lithosphere aró neglected. (b) Apparent sea-level fluctuation lAWl (metres) at basin,edge (position

*""L"á ¡y rrrow in Fig. 2la)) due to superpoãition ofìáriatìotts ofregional stress field on flexure caused by sediment loading'

äã tÀwlär" plotted u" r ilí"tio" of tire aee of the underlying lithosphere and sediment loading accordingto the reference

model (after Turcotte and Ahern, L977) of Fig. 1(a). CLruã" give results for stress changes of 50, 100 and 200 MPa'

respectively.

orogenic events have alternated with periods of'rifting throughout Phanerozoic time. In conse-quence, the basin stratigraphy and apparent sea-level curves can be expected to exhibit considerablevariations associated with changes in the stressregime.

THE APPARENT SEA=I,EVEL RECORDOF NORTH-\ryESTERN EUROPEANBASINS

Figure 3 summarizes some of the sea-level indicatorsthat have been published for this region. The Terti-ary result is from Yall et aL (1977) and appears tocorrespond to the Moray Firth Basin (Fig. a of Vail elø1. 1977, Part 4). In this curve, gradual rises arefollowed by abrupt, nearly instantaneous falls, thelargest of which, the Mid-Oligocene, is purported tohave an amplitude of nearly 400 metres. Yall et ø1.(1984) and Haq et ø1. (1987) have interpreted theseonlap-oflap sequences in terms of more gradualdrops in sea level, and Fig. 3 also illustrates ourmodification of this curve following the modifica-tions made by Vail and Hardenbol (1979) of theirglobal Tertiary curve. There does not appear to bemuch support for the very large global sea-levelchange. Thorne and Bell (1983) derived a eustatic sea-level curve from histograms of North Sea subsidencewhich is consistent with lower estimates of theamplitude of the sea-level changes. Modelling subsi-dence at the United States passive margin (Watts andThorne, 1984) also has provided revised quantitativeestimates of the magnitude of the Mid-Oligocene fallin sea level, which is now estimated to be at most 5O-60 metres. If this change is of tectonic origin thenthere is no reason why it should be everywhere of thesame amplitude. Whether this particular change is ofglacial origin remains questionable. Climatologicalindicators do suggest that temperatures were suffi-ciently low for the onset ofglaciation to occur at thistime but that major ice-caps did not form (Kennett,1977); but even if the sea-level change is of glacialorigin, some regional difference in magnitude can beexpected (Nakada and Lambeck, 1987).

The Jurassic and Early Cretaceous onlap-oflapsequences have been published by Vail and Todd(1981); see also Vail eú al. (798Ð for.two areas-theInner Moray Firth and the Northern Viking Graben(FiS. 3). An independent result, based mainly onfacies analysis, has been published by Hallam (1978,1981). This is actually a'global'curve but the resultsare very much dominated by north-western Europeand the eastern margin of North America prior to theAtlantic rifting (Figs 5-9 of Hallam, 1978), and all thevariations seen in this curve are also seen in theNorth Sea data. As such, we adopt this result forcomparative purposes. The Hallam and Yail et ø1.results are in good agreement (see also Hallam, 1984,1987) although some differences in detail occur.Hallam, for instance, finds no evidence for the drop insea level in the Early Jurassic (at the end of theHettangian) and concludes that the Early Sinemur-ian is a time of sea-level rise, not fall (Fig. 3). Theexceptionally high frequency of the occurrence ofshort-term fluctuations towards the end ofthe Juras-sic occurs simultaneously with a pronounced in-crease in fault-controlled tectonic activity (Hallam,1987). Hallam (1987) recognized a number of signifi-

cant regressive events that appear to be caused byregional tectonics rather than 'global' fall in sealevel.

Vail and colleagues have not published Mid andLate Cretaceous sea-level fluctuations for north-western Europe. Instead, we adopt the results basedon facies analyses from the studies of Kauffman(7977) and Hancock and Kauffman (1978); see alsoHancock (1984). Juignet (1980) has published twosimilar curves for the western part of the Paris Basin.He identified seven transgressive phases for thisperiod, six of which can also be seen in the Kauffmanand Hancock results (Fig. 3). The two drops in sealevel in the Early Cretaceous, in the Late Aptian andLate Albian, can also be seen in the Vail and Todd(1981) global sea-level curve. Hallam suggests thatthe Jurassic rise in sea level was nearly 200 m whileJuignet (1980) suggests a Cretaceous rise in sea levelof about the same amount. As previously noted, it isimportant to recognize that if tectonic factors areresponsible for this change, considerable regionalvariation in amplitude can occur because the hori-zontal stress propagation through the lithosphereneed not be uniform and the initial perturbations willbe variable.

A composite sea-level curve from Early Jurassic tothe Late Miocene is illustrated in Fig. 3, and thisshould be viewed as indicative only of qualitativechanges in sea level. The magnitude of the sea-levelchanges is scaled such that the Oligocene fall in sealevel is 50 m. The principal first-order feature of thisapparent sea-level curve is the gradual iise in sealevel throughout Jurassic and Cretaceous time, fol-lowed by a fall in sea level during the Tertiary. Thiscycle is usually said to be of global extent evenalthough it may also be appropriate to associate itwith the break-up of Pangea along the AtlanticRidge. These changes have been attributed to anincrease in ocean-ridge volume during the Jurassicand Tertiary (Hallam, 1963; Pitman, 1978) althoughPitman's model is associated with very considerableuncertainty (Kerr, 1984; Kominz, 1984).In this con-text, it is interesting to note that high-frequencyoscillations in Cretaceous sea levels with character-istic periods of a few million years that cannot becorrelated with fluctuations in spreading rates andridge lengths (Schlanger, 1986), could be the result ofadjustment ofstresses in the process ofthe opening ofthe Atlantic. Modelling studies (Cloetingh and Wor-tel, 1986; Wortel and Cloetingh, 1983) suggest thatrapid temporal fluctuations in intraplate stress fieldsoccur, and geological evidence also points to episodictectonic events on time scales of a few million years(Letouzey, 1986; Megard. et ø1.,1984). It is importantto realize that the degree of correlation between thetiming of sea-level changes induced by fluctuationsin intraplate stress fields will depend primarily on thedimensions of the stress province. Numerical modell-ing (Cloetingh and Wortel, 1986; Wortei and Cloet-ingh; 1983) and observation of lithospheric deforma-tion (Illies et al., l98I; McAdoo and Sandwell, 1985;Zoback and Zoback, 1980) show that the stressprovinces can vary in size from that of an entirelithospheric plate to that of a small part of a plate.The regional character of the tectonic mechanismproposed by Cloetinghet al. (1985) sheds new light onsome observed deviations from the Vaii et ø1. (7977)curve in areas in both the Northern (Harcis et al.,1984) and Southern (Carter, 1985; Chaproniere, 1984)

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Petroleum Geology of North West Europe

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DISTANCE (km)

Fig. 2. (a) Effect of variations in intraplate stress fields on the deflection of a plate witÞ a thickness corresponding to a

iiiño"pn"íi" age of B0 Ma. Differential subsidence or uplift (metres)from the deflection in the absence of an intraplate stress

fie¡l is given For intraplate stress fields of 100 MPa compresslon (dashed curve) and 100 MPa tension (dotted curve). (b)

i¿erliruä stratigraphyãt the edge of a basin underlain by ôs Ma-old lithosphere predicted on the basis ofthe calculations

shown in Fig. 2(a).

hemispheres. While such deviations from a globalpattern are a natural consequence ofthe character ofthe tectonic mechanism, they do not preclude thepresence of global events in the stratigraphic record.These are to be expected when major reorganizationsin lithospheric stress fields occur simultaneously inmore than one plate, as conjectured for the earlyCenozoic global plate reorganization (Rona andRichardson, 1978), or when glacial eustatic changesdominate.

A PALAEOSTRESS CURVE FORNORTH-WESTERN EUROPE

If Cloetingh et q,l.'s (1985) model is assumed to bevalid for the basins of north-western Europe, then itbecomes possible to interpret the relative sea-levelcurve of the previous section in terms of changes inthe regional stress state. There are severalcomplications that call for caution in interpretingany results of such an inversion. First, in order toobtain quantitative estimates of sttess, themechanism of basin formation must be known asmust the relevant lithospheric parameters. Differentsubsidence mechanisms may have operated within agiven area at different times, and it would beimportant to examine relative onlap-offiapsequences from individual basins in the region andeven from different locations within the same basin,rather than examine the regional curves. Inparticular, much of the subsidence may be faultcontrolled. Secondly, the relative sea-level change islikely to be a function of both tectonic and eustaticprocesses (Hallam, 198?). If both contribute withinthe same time interval then the interpretation of the

sea-level curve becomes considerably more complex

and not all of the fluctuations correspond to stresschanges.

The tectonics of north-western Europe have been

described in detail by Ziegler (1982). During the

Mesozoic, the tectonics can be characterized as apredominantly horizontal tensional and stretching

iegime with the development of numerous grabens,

,rttlil r few major grabens developed upon which

much of the subsequent deformation was focused.

The predominant mechanism of basin formation is

therefore one of crustal stretching and subsidence(e.g. Barton and Wood, 1984; Sclater and Christie,

1980). During the Tertiary, the region w?s

tectonically one of quiescence in which basin

evolution was controlled mainly by thermal

subsidence following upon lithospheric cooling. The

region, however, was also affected by the

compressional events of the Alpine Orogeny,producing some inversion of basins up to 1000 km

north of the Alpine collision front (Ziegler, 1982)'

Ziegler (1982) and Sclater and Christie (1980) have

pointed out the existence of a correlation betweenlectonic phases in the North Sea Basin and changesin the history of opening of the Atlantic and Tethyal

oceans (see also Schwãn, 1985, for a more general

discussion). Changes in sþreading rates in the

Atlantic and Tethyan regions are probably caused

by, or associated with, changes in plate-tectonic

fórces. Both modelling of lithospheric stress fields

(Ctoetingh and Wortel, 1985; Richardson et øl',7979;

Wortel ãnd Cloetineh, 1983) and measurement of

directions of present-day stress fields (Illies et al',

1981; Ktein uttd. B"tt, 1986) and palaeostress frelds(Letouzey, 1986; Letouzey and Tremolieres, 1980)demonstrate that the induced stress changes arepropagated over great distances from the plate

boundaries into the north-western European basins,which provides a dynamic explanation for the

observed correlations. Tectonic lineation rosettes,

Appørent Seø-Ieuel Fluctuøtions and ø pølaeostress Field.

(o)

km

85 Mo

8O Mo

75 Mo

70 Mo

(b)

85 Mo

8O Mo

75 Mo70 Mo

(c)

80 Mo

75 Mo

determined from spectral analysis of North Seatectonic subsidence, show a rotation from aprominent E-'W trend in the Triassic to N_S in theLate Cretaceous (Thorne, 1g86), a trend. also reflectedil^tl" measured paleostress orientations (Letouzey,1986). The influence of the Alpine Orogeny isreflected in differences in orientation betweenPliocene and other Cenozoic directions (Thorne,1986). As noted by Letouzey (1986), Late Cretaceousto present compressive pulses along the Alpine beltand in foreland regions are rélated io plateconvergence and collision. Furthermore. a causalrelationship might exist between these stresschanges and the timing of salt diapirism in the NorthSea Basin (Thorne, 1g86).

The specific model of Cloetingh et ø1. (1935) is notappropriate for this region during the Mesozoic.Nevertheless, the concept of a mecñanism of steadvsubsidence modulated by changes in the magnitudäof the horizontal regional tensional force can beapplied for this time interval. Without specificallyd_eveloping such a model it is not possible tä quantifythe stress field. The first-order cycle of Mesoãoic sea_level rise and Tertiary fall is consistent with ahorizontal stress regime that is predominantly one oftension during the Mesozoic (Fig. 4), resuiting insubsidence of the basin floor eitfr", by gribenformation and subsidence of the graben Ruit. tysediment loading (Beaumont, 19Zg), or througircrustal stretching (McKenzie, lg7g). The laftãrmechanism requires tensional stresses of the ord.er ofa few hundred MPa (Cloetingh and Nieuwland, 19g4;Houseman and England, 1986).

By the Late Cretaceous, a reduction in tension. oreven a change to overall compression, produced. areversal of this trend. Superimposed upon-these long_term cycles were numerous rises and falls in app"""ñtsea level, many of which can be associated withspecific tectonic phases within north-western Europe(Ziegler,1982). The Early Jurassic to Early TertiaryLaramide_ tectonic phases (Fig. 4), appear to havebeen predominantly tensional eventJ ihat affectednorth-western Europe as a whole and which are alsoseen beyond this region. These events appeâr asrenewed phases of rifting associated with thefoundering ofPangea, and this part ofthe sea-levelcurve can be interpreted in terms of periods ofgtadual stretching and subsidence, foilowed bvperiods of lithospheric relaxation, or failure anärifting, when this stress is partially released and. sealevels at the edges of the basins uppeu, to fall. Thecycle -the-n repeats itself. Within- lhis framework,periods ofgradual increase in sea level are associated.with times of more gradual build-up of tension andstretching, while the lowering of sea level isassociated with discrete rifting episodes. Most of the

Fie. 2b. (aþ-Onlap associated with cooling of the litho-sphere in the absence of an intraplate strãss fietd. Thesubsidence curve is illustrated on the left. (b)-A transition(in an interval Â?) to ã0 Mpa compression at ô0 Mainduces a short-time phase of offiap ai that time and anapparent fall in sea level superimposed on the thermalcooling subsidence curve. (c)-A transition to 50 Mpatension produces an additional short-term phase of onlapat ô0 Ma; that is, the subsidence will

"pp""i to be greater

tha¡ predicted by the thermal-cooling subsidenceLodel,and this may lead to a facies change in the sedimentation.The figures on the left represent thè incremental change inthe onlap and in apparent sea level.

53

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Iig. ¡. Mesozoic-Cenozoic onlap/offiap curves and apparent sea-level variations. Columns from left to right give the Vail et al. (Ig77)Tertiary North Sea curve, its modification based on the Vail and Hardenbol (1979) revised 'global' curve, the Jurassic onlap/offiapgurYes given by Vail and Todd (1981) for the Northern Viking Graben and the Inner Moray Firth, the Jurassic sea-level curve byHallam (1981), the Cretaceous North Sea curve from Hancock (1984) and the Cretaceous sea-level curves for the Paris Basin byJuignet (1980). The last column is an average Mesozoic-Cenozoic sea-level curve for the north-western European basins based on theinformation given in the columns to its left. To establish a scale for the sea-level change we adopt a magnitude of 50 m for theOligocene lowering in sea level, a further 150 m rise in the Cretaceous and a 150 m rise in the Jurassic (after Lambeck et øl,1gB7).

NW EUROPECRETACEOUSseolevel curve

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NW EUROPEJURASSIC

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Fig' 4. Synthetic palaeostress curve derived from the generalized sea-level curve for the north-western European basinsgiven in Fig' 3' Stresses are plotted relative to an arbitrãry present-day.stress level. co*p""i.o" with columns ón the right-hand side with the timing of tectonic events in north-w-esiuï" P"rop" ( after ziegleir, rsgãi.ilo*. correlation with tectonicphases in the Atlantic, the North-western European rifb system ".rd

th" Alpineäomain. it ict and thin wavy lines denotemajor and minor rifting phases, respectively. MaJor and minor folding ptt".uå ""e

i;didàïtìhick and thin .^*iåátt rirr"r;stars denote phases ofanorogenic volcanism (after Lambect< et at, itigl\.

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Jurassic and Cretaceous North Sea rifting episodesdo, in fact, correlate well with these apparentloweiiings in sea level, as has already been noted byZiegler (1978).

The sea-Ievel falls in the Mid-Cretaceous(Cenomanian and Turonian) do not appear tocorrespond to North Sea rifting episodes, but theyoccur at a time of active development of the RockallTrough and the opening of the Bay of Biscay (Ziegler,1982). The Laramide tectonic phase is considered tobe a mild rifting phase, and its association with asubstantial drop in sea level would appear to beinappropriate here. The major Tertiary lowerings ofsea level occur at times of the various folding phasesof the Alpine Orogeny. In particular, the Mid-Oliogocene change in sea level corresponds to one of

. the main compressive phases of the orogeny, whilethe Mid-Palaeocene drop may be more appropriatelyassociated with the first of the three main orogenicphases than with the mild rifting episode seen atabout the same time in the North Sea. From Fig. 4,the change in stress required to produce the Mid-Oligocene drop in sea level of about 50 m is about200-300 MPa. This is very much an upper limit; stressrelaxation and a depth-dependent rheology, aneffectively young crust, episodes of rapidsedimentation and movements on existing faults canall lead to substantially larger deflections of thelithosphere than is predicted by the elastic modelupon which Fig. 1(b) is based. The majority of theother apparent sea-level changes_ require smallervariations in stress, and are well within the range ofhorizontal stress differences observed in thelithosphere.

Distinguishing regional events in the sea-levelrecord from eustatic signals is usually a subtlematter, especially if biostratigraphic correlation isimprecise (Hallam, 1984, 1987). In this chapter, apalaeostress curve for the North Sea region has beenderived from the sea-level record on the assumptionthat the apparent sea levels are controlled byregional tectonics. A further and more detailedexamination of the stratigraphic record of individualbasins in the North Sea area, in connection withindependent numerical modelling of palaeostressesin north-western Europe using techniques developedby Wortel and Cloetingh (1981, 1983) and Cloetinghand Wortel (1985, L986), provides an additional andnew approach for separating eustatic and tectoniccomponents in the sea-level record of this region.

CONCLUSIONS

The North Sea sea levels for the Jurassic to Tertiaryexhibit numerous fluctuations on a time scale of a fewmillion years and longer, superimposed upon long-term (time scales of the order of 100 Ma) fluctuations.The latter can be attributed to the change in thepredominant horizontal stress state of thelithosphere from one of significant tension andcrustal thinning to one of less tension or evencompression. The short-period fluctuations can beinterpreted as the irregular response ofthe crust inthe form of rifting episodes, or of compressionalpulses to the more regional stress state. Quantitativeestimates for the changes in stress field required areof the order of about 200 MPa for Tertiary times but,as previously emphasized, these are upper limits.

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