Differentiating between tectonic and eustatic signals in ...3.2 Climatic control on sedimentation The regularity of the layering is suggestive of Milan - kovitch forcing on sedimentation

1. Introduction

The outcrops of the Boom Clay along the Rupel riverin Belgium make up the historical stratotype of theLower Oligocene Rupelian stage (Van Simaeys andVandenberghe 2006). The deposits occur over a largearea in outcrop and extend in the subsurface of NorthBelgium, The Netherlands and Germany grading intothe Oligocene clays of the North Sea (Fig. 1).

Detailed studies on the sedimentology, stratigraphy,geochemistry, mineralogy, and geophysical well re-sponses have led to a substantial database on the Ru-

pelian in its type area (Vandenberghe 1978, Vanden-berghe et al. 1997, 2001, 2004, Van Simaeys and Van-denberghe 2006, Abels et al. 2007).

During the Rupelian global sea level was stronglyinfluenced by the waxing and waning of ice caps on Antarctica (Abreu and Anderson 1998, Wade andPälike 2004, Pälike et al. 2006). Also the end of theEocene and the Oligocene are times of important tec-tonic rearrangements in West and Northwest Europemainly due to the Alpine tectonic evolution (Sissingh2003); the most relevant rearrangements for the studyregion are the Pyrenean uplift and tilting, differential

Newsletters on Stratigraphy, Vol. 46/3 (2013), 319–337 ArticleStuttgart, October 2013

Differentiating between tectonic and eustaticsignals in the Rupelian Boom Clay cycles (Lower Oligocene, Southern North Sea Basin)

Noël Vandenberghe1 and Jeroen Mertens2

With 14 figures

Abstract. The Rupelian Boom Clay in Belgium is a marine sedimentary deposit with an extensive data set.Astronomical control on high-frequency cyclicity has been proven before, and sedimentological analyseshave shown climate-driven cycles caused by sea-level fluctuations. A long cycle in grain-size and bed thick-ness, involving the entire Boom Clay section, is related to tectonism. Shorter-duration low-frequency cycles,attributed in the past also to climate-driven eustasy, show a relationship with sediment supply expressed by bed thickness but related to tectonism. This apparent contradiction is studied by measuring the thicknessdifferences of the individual Milankovitch-related (astronomical) beds in several wells with the thickness ofthe same bed in a reference well in the area. Such an approach eliminates eustasy as a controlling factor inthe observed cyclicity patterns. Cumulative differential evolution maps of the basin are provided, and the evo-lution of the relative subsidence in all individual wells through time is visualised as cumulative-differencecurves. Both approaches demonstrate that the levels considered in previous studies as controlled by eustasyin fact reflect tectonic history.

Key words. cyclicity, eustasy, tectonics, Rupelian, Boom Clay Formation, Belgium

© 2013 Gebrüder Borntraeger, Stuttgart, GermanyDOI: 10.1127/0078-0421/2013/0034

www.borntraeger-cramer.de0078-0421/2013/0034 $ 4.75

Authorsʼ addresses:1 Laboratory Applied Geology and Mineralogy, Celestijnenlaan 200E, B-3001 Leuven-Heverlee, Belgium. E-Mail: [email protected] (corresponding author)2 NIRAS-ONDRAF, Brussels, Belgium, presently at FANC-AFCN, Ravensteinstraat 36, 1000 Brussels, Belgium. E-Mail:[email protected]

http://www.borntraeger-cramer.de

vertical movements between old Caledonian andVariscan blocks like the London-Brabant Massif, theArdennes and the Campine Basin, the reactivation ofthe Lower Rhine graben, and the uplift of Scandinavia.A time scale of the Boom Clay depositional history,spanning almost the full Rupelian, was tentativelyconstructed by Abels et al. (2007) based on the avail-able biostratigraphy, magnetostratigraphy and astro-cyclicity data. Astronomical control on high-frequen-cy cycles in the Boom Clay is demonstrated by the re-sults of spectral analysis (Van Echelpoel and Weedon1990, Schwarzacher 1993, Abels et al. 2007), but theorigin of the observed lower-frequency cycles (group-ing more than 10 to 20 high-frequency cycles) has re-mained debatable, although a eustatic origin was com-monly favoured (Vandenberghe and Van Echelpoel1987, Stover and Hardenbol 1993).

The present study reports on the results of detailedthickness measurements on high-frequency cycles inseveral boreholes in North Belgium and their meaning

for distinguishing a eustatic from a local or regionaltectonic origin for low-frequency cycles. Eliminatingeustasy as a possible genetic cause is achieved by com-paring differences between thicknesses of the layersand the same layers in a reference well in the studyarea. This procedure might be applicable more gener-ally whenever several wells with geophysical logsshowing a cyclic arrangement are available in an area.

In the first part of the paper, the necessary back-ground on the observed cyclicity of different orders andtheir origin in the Boom Clay is summarised. In the sec-ond part, the methodology of comparing layer thick-nesses with a reference well is explained and applied.

2. Geological setting

The Boom Clay was deposited in the epicontinentalNorth Sea Basin (Fig. 1) between about 31 and 28 Ma(see Lagrou et al. 2004, De Man et al. 2010). The sed-

Noël Vandenberghe and Jeroen Mertens320

Fig. 1. Paleogeographical setting of the study area. The letter abbreviations are Aberdeen (A), Edingburgh (E), Dublin (D),London (L), Paris (P), Brussels (Br), Bonn (Bo), Prague (Pr), Berlin (B), Vienna (V) and Stockholm (S).

iment consists of a variable mixture of clay mineralsand coarse silt to very fine sand. The coarse fractionconsists mainly of quartz and feldspars with minormuscovite and chlorite flakes. The clay fraction con-sists dominantly of about equal amounts of discretemontmorillonite, illite, and random mixed layers of illite and smectite, with smaller amounts of kaoliniteand traces of chlorite (Zeelmaekers 2011). All sedi-ment particles are detrital. The normally grey colour of the clay can turn into pale grey by the addition offine carbonate particles and into black colour by theaddition of silt-sized plant-derived organic particles(Vandenberghe 1978). Sediment particles are partlyderived from the local shorelines, but the main sedi-ment mass is derived from the northern North Sea area by counter-clock currents (Fig. 1). This is shownby the provenance of the heavy minerals and the prop-erties of coal particles in the phytoclasts (Vanden-berghe 1976) as well as by the increased abundance oforganic molecules derived from gymnosperm resinsand kaolinite (Fig. 3) at times of the highest relativesea-level positions in the high-frequency layering(Laenen 1997, 1998).

3. Climate-driven high-frequencycyclicity

3.1 Grain-size cycles

The most striking property of the clay is its banded na-ture, as can be observed in outcrops (Fig. 2) and in thegeophysical well-log signature of the sediment (Van-denberghe and Van Echelpoel 1987, Vandenberghe etal. 2001). The individual clay layers are a few tens ofcm up to over a meter thick. The layering is mainly dueto alternations of silty clay and very pure clay. Thecoarsest percentile in both clay and silty-clay layers is about 80 μm and remains constant. The grain-sizefrequency curves show a gradual vertical evolution inskewness: from maximally skewed with highest con-tents of coarse silt and fine sand in the middle of a siltlayer to maximally skewed in the opposite direction,with highest contents of the clay fraction in the middleof the clay layers below and above the silt layer (Van-denberghe 1978). In addition to these grain-size prop-erties, the absence of erosional features at the layer con-tacts, the extreme lateral persistence of the sequence of

Differentiating between tectonic and eustatic signals in the Rupelian Boom Clay cycles 321

Fig. 2. Photograph of the high-frequency layered Boom Clay in a clay pit (Putte). The height of the excavation wall is16.5 m. The layering is due to the varying clay-silt ratio in the sediment. Black hues are due to increased organic matter andthe prominent dark layer about 6 m above the base of the pit is the DB (double layer) consisting of a very high silt/clay frac-tion ratio. Note that the 2 silt layers are stacked without clay layer in between. The boundary Terhagen-Putte Members, between grey and black clay, is at 2 m above the base of the pit.

layers along strike, and the consistent mineralogypointing to a constant provenance all have led to theconclusion that the grain-size evolution represents agradual alternation from sediments deposited belowwave base (i. e., the pure clay layers) to sediments deposited under the influence of wave turbulence andwinnowing (i. e., the silty-clay layers) (Vandenberghe1978, Vandenberghe and Van Echelpoel 1987).

3.2 Climatic control on sedimentation

The regularity of the layering is suggestive of Milan -kovitch forcing on sedimentation. The main control for

the high-frequency layering described above is the 40–41 ka obliquity. Additionally, 100 and 400 ka eccentric-ity cycles are present, grouping high-frequency layersinto lower-frequency bundles (Abels et al. 2007). Fromplanktonic and benthic foraminiferal stable isotopestratigraphy of ocean sediment cores, Oligocene climateis known to fluctuate along the beat of obliquity and eccentricity, with corresponding eustatic sea-level vari-ations of several tens of meters (Wade and Pälike 2004).A similar order of magnitude, taking backstripping intoaccount, is obtained for the Early Oligocene; it is attrib-uted to the growth and decay of Antarctic ice sheets(Miller et al. 2005). Therefore the layering in the Boom


Fig. 3. Lithological properties of a high-frequency cycle. Note the rapid change in grain size marking the boundary betweenthe clay and silty-clay layers while the grain-size maximum and minimum values (expressed as % � 32 μm) are at the centreof the layers. Note also at the base of a clay layer the occurrence of black layers of land-derived organic matter. The differ-ence between the minimal and maximal kaolinite contents is about 2 to 5%.

Clay can be explained as climate-driven eustasy cycles;during cold periods, there is more continental ice andtherefore the sea level is low and the wave turbulencecan sort the sediments at the bottom producing silty-clay layers. In warmer periods, sea level is high and relative water depth is larger preventing wave-turbu-lence to affect the sediments leading to very clay-richlayers. An average depositional depth of the Boom Clayof about 100 m is based on effective wave lengths in theepicontinental North Sea Basin during the Rupelianwhich are estimated from present-day measured wavecharacteristics in the southern North Sea (InternationalMarine and Dredging Consultants 2005). The longestyearly wavelength is 112 m, but occasionally also156 m wavelength occur. Taking into account the longertime interval to be considered over which the upperBoom Clay deposited sediment is homogeneously sort-ed by frequently occurring waves, an effective 200 mwavelength is assumed. Because turbulence depth isabout half the wavelength, 100 m is a reasonable waterdepth estimate, deeper during the deposition of the pureclay layers and shallower during the deposition of thesilty clay layers. Water depth estimates from fora mini -feral assemblages are also around 100 m (De Man andVan Simaeys 2004), whereas those from fish remainsare at least 50 m or deeper (Steurbaut and Herman1978). The most silty clays at the base of the clay are deposited at shallower depth considering the lower claycontent and the presence of rare gutter channels (Van-denberghe et al. 2002).

3.3 Sedimentology of organic matter

In addition to grain-size layering, slightly thinnerblack layers also occur in the Boom clay deposit(Fig. 2). When present, they occur always out of phasewith the grain-size layering, occurring from the top ofa silty-clay layer into the base of the overlying clay-rich layer but ending below the middle of the clay-richlayer (Fig. 3). The black staining is caused by Oligo -cene land-derived organic particles (Vandenberghe1978). The climate-driven model of eustatic sea-levelchanges for the origin of the grain-size layers also ex-plains the particular position of the black layers. Aftera sea-level drop the renewed slow rise forms the upperpart of the silty-clay layer until it transgresses rapidlyover the land. The flooding of the coastal vegetationleads to the production of detrital plant particles thatare swept into the basin where they form the black lay-ers. When sea level approaches its maximal floodingposition, marked by the middle of a clay-rich layer, no

more coastal vegetation is broken down, and the blackcolour of the clay is again replaced by the usual gray(Vandenberghe et al. 1997).

4. A long-duration grain-size cycleand low-frequency cycles:climatically or tectonicallydriven?

On geophysical well logs the whole Boom Clay For-mation is part of a long-duration trend in grain sizewhilst this trend is further subdivided in several sub-cycles grouping each a number (about 10 to 20) ofhigh-frequency Milankovitch cycles (Fig. 4). Thelong-duration trend is further also named the longgrain-size cycle and the subcycles are called low-fre-quency cycles in contrast to the high-frequency Mi-lankovitch cycles.

4.1 A long grain-size cycle: tectonic forcing

The long-duration cycle in grain size observed in geo-physical well logs throughout the entire Boom Claysection and spanning the major part of the Rupelian(Early Oligocene) time (Figs. 4, 9) is considered a tec-tonically induced second-order transgressive/regres-sive cycle (Hardenbol and Vandenberghe 2003). Thehigher silt content in the basal and the upper part ofthis long-duration Boom Clay cycle points to shallow-er-water conditions in these parts of the clay. The cy-cle reflects a gradual subsidence from the base to max-imum depth in the middle of the Boom Clay, expressedby the lowest resistivity values and hence highest claycontents, followed by a renewed uplift towards the topof the Boom Clay.

The trend in cycle thickness of the high frequencycycles over the whole Boom Clay (Fig. 5) can confirmthe tectonic nature of this cycle. Indeed, as the high-frequency layers are obliquity-controlled they are ofequal time duration. Thickness differences betweencycles therefore represent differences in sediment sup-ply to the basin. Differences in sediment supply at thehigh frequency scale are associated with slow changesin erosion intensity from the headland by changing uplift rates. The regularity of the layering in the BoomClay over a very wide area and the constant grain-sizerange and mineralogy in the entire Boom Clay point tochanges in uplift of the basin rim that are slow.


The thinner cycles in the middle of the long cyclefrom the bottom to the top of the Boom Clay (Fig. 5)are the expression of the reduced tectonic uplift aroundthe basin at that time. This model relating bed thick-ness to tectonic uplift was already proposed by Van-denberghe et al. (1997), but it was only based on datain the outcrop area. In the present study, the relation-ship is further elaborated by accurate thickness meas-urements on a Fullbore Formation Microimager (FMI,Schlumberger) log of borehole Mol-1 (location inFig. 10) of all silty clay and clay layers over the wholethickness of the Boom Clay as shown in Fig. 5.

The evolution of the clay/silt thickness ratio of theMilankovitch controlled (i. e., high-frequency) cyclesis shown in Figs. 6a and 6b. In contrast to the totalthickness of the cycles, the thickness ratio between theclay and the silty-clay parts in a cycle is independentof sediment supply. Whatever the total sediment thick-ness of a cycle, the ratio clay/silt in all cycles must re-main the same. This is valid as long as the percentageof time during the cycle when wave turbulence reach-es the bottom to sort sediment remains the same (seealso Fig. 3.25 in Van Echelpoel 1991). Therefore, theobserved changes in the thickness ratio (Figs. 6a, b)


Fig. 4. The RES (electrical resistivity, SN(short normal) and LN (long normal)curves) log evolution through the BoomClay Formation in well SCK-15 (locationsee Fig. 10). The long cycle is subdividedin low-frequency cycles based on the re-sistivity trends expressed by black lines tothe left of the resistivity curves. These low-frequency cycles are bounded by specificlayers discussed in literature (a. o. Vanden-berghe and Van Echelpoel, 1987, Vanden-berghe et al. 2001) and coded according toa sequence stratigraphic interpretation:TS1 (transgressive surface), the R (red-dish) layer considered as a main floodingsurface (mfs1), the DB (double layer) ofvery silty nature and considered a sequenceboundary (SB2), the S60 (sideritic septariahorizon SID1) also considered as a mainflooding surface (mfs2), a sequenceboundary expressed by outspoken silty na-ture (SB3), a second siderite horizon SID2interpreted as a main flooding surface(mfs3), a sequence boundary at a RES maxzone (SB4) and a main flooding surface atthe base of the silty Boeretang Member(mfs4). PT is the organic matter boundarybetween the Terhagen and Putte Members.

can be interpreted as changes in sorting duration. Sort-ing duration depends on water depth. Although somevariance is present in the data, over the whole BoomClay the ratio clay/silt increases towards the middleand decreases to the top (Figs. 6a, b), meaning that wa-ter depth was shallower in the basal and upper part ofthe Boom Clay. This is compatible with the uplift his-tory derived from the thickness trend shown in Fig. 5.Therefore, the trends of both total thickness and thick-ness ratio between the clay and silt parts in high-fre-quency cycles support the tectonic origin of the longgrain-size trend over the entire Boom Clay section.

Uplift and subsidence activity during the Oligocenein the area can be expected. Notwithstanding that during the Rupelian large ice caps started to grow onAntarctica and consequently the global sea level musthave dropped, a major deepening of the sedimentarybasin occurred in northwest Europe during the Ru-pelian (see maps in Dercourt et al. 2000, and Popov etal. 2009). However compressional deformation in theAlps during the Rupelian, with the installation of thePennine nappes, certainly has triggered differentialvertical epeirogenetic movements in Western Europe.For example, towards the end of the Rupelian theBoom Clay has been uplifted in the Antwerp area com-

pared to the Campine area in the east at only 50 km distance (see Fig. 10 for locations), resulting in 80 m ofclay erosion in the west during the latest Rupelian andearliest Chattian (De Man et al. 2010). At the begin-ning of the Late Oligocene, just to the east of the studyarea, the Roer Graben actively renewed its subsidence.At the same time, the Paleozoic Ardennes block to thesoutheast also started to become uplifted (see Fig. 1 forlocations). The subsidence of the Roer Graben waspreceded by a short uplift during the last part of theRupelian as shown by the presence of more fine sandyfacies during the later Rupelian (Eigenbilzen Forma-tion) (Vandenberghe et al. 2001). In light of this re-gional geological context, the presence of a long-termgrain-size cycle related to the uplift history of the basinrims is possible.

4.2 Low-frequency cycles

The tectonically-driven long-duration resistivity orgrain-size cycle throughout the entire Boom Clay sec-tion can be subdivided into several subcycles (Fig. 4).It has long been recognised that a low-frequency cycleenvelope is superposed on the grain-size data of thehigh-frequency cycles in the outcrop area, represent-


Fig. 5. Evolution of the total cycle length, silty-clay plus clay thickness, throughout the Boom Clay using a moving averageof nine cycles. Each cycle is measured from the middle of the silt layer underlying a clay layer till halfway the silt layeroverlying the clay layer. The layer codes are the same as described in Fig. 4.

ing only the lower part of the entire Boom Clay Formation (Figs. 4, 7), (Vandenberghe 1978, Vanden-berghe et al. 2001). This envelope is also expressed onthe resistivity logs as illustrated in Fig. 4.

The interpretation of the origin of these low-fre-quency cycles in the envelope has been based on thesimilarity of their grain-size evolution with the high-

frequency cycles. Therefore, this low-frequency enve-lope has also been interpreted as climate-driven eusta-tic sea-level variations (Vandenberghe et al. 1997); thetwo cycles observed in the field area (Fig. 7) were correlated with the Rupelian third-order sequences ofHaq et al. (1987) (Vandenberghe and Van Echelpoel1987, Stover and Hardenbol 1993).


Fig. 6. a Evolution of the ratio clay-to-silt-layer thickness in each cycle over the full Boom Clay section. b Evolution overthe full Boom Clay section of the ratio clay-to-silt-layer thickness in each cycle as in Fig. 6a, but raw data smoothed by asliding average over three cycles.

However, no hard data prove this inferred relation-ship with eustasy, which is only based on the similari-ty of the grain-size evolution in the low-frequency cy-cles and in the high-frequency cycles. In order to testa potential tectonic influence, we attempted to identi-fy similar low-frequency cycles on the long trend inhigh-frequency cycle thickness (Fig. 5) and in clay/silt

thickness ratio (Fig. 6a). These two parameters wereselected because they show a relationship with the tectonic evolution over the entire Boom Clay sectionas discussed above. The identification is attempted byanalysing different smoothing runs over the parameterdata using different moving average lengths. Thesmoothed evolution of the full-high-frequency-cycle


Fig. 7. Low-frequency grain-size envelope in the outcrop part of the Boom Clay Formation. The envelope is obtained byaveraging over 15 grain-size data representing five high-frequency cycles. The codes for the specific horizons TS1, R(mfs1),DB(SB2) and S60 (mfs2) are the same as in the other figures (see Fig. 4). The right column represents the original thicknessmeasurements of the high-frequency cycles in the outcrop area alone.

lengths (Fig. 5) does not show clear superposed low-frequency cycles. However, the clay/silt thickness ratio evolution over the full Boom Clay section,smoothed with a moving average of three cycles(Fig. 6b), reveals clear low-frequency cycles of whichat least some are related to the grain-size low-frequen-cy cycles identified and traditionally interpreted as eustasy-related (compare Fig. 6b to Figs. 4, 7). Theanalysis of the thickness evolution of the singled outclay parts of the high-frequency cycles (Fig. 8) showssimilar low-frequency cycles. Clay parts in the high-frequency cycles are more representative of the origi-nal sediment supply than the silty parts of the cycles.

More attempts have been published to group high-frequency cycles into lower-frequency cycles. Har -den bol and Vandenberghe (2003) have suggested thepresence of long eccentricity cycles based on thegrouping of successive packages of coarser and finerlayers (red and blue bundles in Fig. 9). Moreover, geo-chemical cycles with a duration between 0.5 and 1 Mahave been described by Laenen (1997, 1998) and wereinterpreted as obscured signals of longer-duration Mi-lankovitch-related climate-driven cycles. Also, in thecyclostratigraphic analysis of Abels et al. (2007) sev-eral 405-ka-long eccentricity cycles were interpretedbased on the analysis of the resistivity log of the fullBoom Clay section; the lowest of this long eccentrici-ty cycles coincides with the lowermost low-frequency

grain-size cycle observed in the outcrop area as shownon Fig. 7.

It can be concluded from the previous discussionthat analyses of the low-frequency cycles remain in-decisive with respect to a tectonic and eustatic origin.In order to further explore the possibility of distin-guishing eustatic and tectonic signals in the sedimen-tation of the Boom Clay Formation, a new approachand new data set has been developed that is discussedbelow.

5. Comparing layer thicknesses of the high-frequency obliquitycycles between boreholes

5.1 Methodology

To differentiate between a tectonic or eustatic origin of the low-frequency cycles, the exact thickness of allindividual high-frequency layers has been measuredon geophysical logs. Subsequently, the thicknesses forthe same high-frequency layer have been compared to the thickness of that particular layer in an internalreference well, and finally in each well the differenceswith regard to the reference well have been cumulatedto the scale of the debated lower-frequency cycles.


Fig. 8. Thickness evolution of individual clay layers (y-axis) over the Boom Clay section (x-axis) using a 3 cycle movingaverage. The layer codes are the same as in Fig. 6.

Comparing layer thickness differences with layersin a reference well rather than comparing absolutethicknesses between wells has the crucial advantagethat after doing so eustasy can be excluded as a possi-ble genetic cause for the differences. This is becausean eustatic component in the thickness of a layerwould have the same value in all wells and disappearswhen subtracting layer thicknesses between wells.

This approach demands the recognition of all indi-vidual high-frequency cycles. This is possible as de-tailed aspects of the individual layers in the clay haveallowed to establish a composite micro-stratigraphyacross the entire Boom Clay section in the subsurface(Vandenberghe et al. 2001). Therefore it is possible to identify each particular layer in all boreholes, tomeasure its thickness and to compare the thicknessesof equivalent layers between different boreholes. Thewell logs used in this study are the resistivity logs asthese are commonly available and resistivity is an ex-cellent proxy for the grain-size properties that definethe cycles. As discussed above, the ratio of the relativeproportion of coarse grains to fine clay minerals variesgradually through the layers, with a maximum ofcoarse grains in the middle of a silt layer and the maximum of clay minerals in the middle of a clay layer. Therefore, the variations in grain-size propertiesare sinusoidal; however, the change from silt- to clay-dominated and vice versa occurs very rapidly, allow-ing to define fairly exactly the boundary between asilty layer and a clayey layer (Fig. 3). In earlier stud-ies, this property has allowed Walsh step function-analysis of the periodicity in the thickness of the cycles(Van Echelpoel and Weedon 1990).

The thicknesses of all wiggles, clays and silty-clayparts in the cycle separately have been measured inhigh-quality geophysical logs of 17 boreholes of theCampine area in North Belgium (Fig. 10). As the sameclay-type lithology is involved for all measurementsand no significant burial depth differences exist for theboreholes, no corrections for compaction have beencarried out. The thickness differences of the individualhigh-frequency layers with the thickness of the samelayer in the Mol well, taken as the reference well, havebeen further used in this study to analyse lateral andtemporal changes in sedimentation rate without influ-ence of eustasy.

5.2 Lateral changes in sedimentation rate

As the differences for individual layers with the layersin the reference borehole are small, thickness differ-


Fig. 9. Electrical resistivity (RES) log of the Boom Clay inthe Weelde well (location Fig. 10) with red and blue bundlesdelineated based on the alternating dominant grain-size:clayey figured blue, silty figured red. Hardenbol and Van-denberghe (2003) suggested these bundles show long-ec-centricity cyclicity. The positions of characteristic levelsused as reference in the other figures (see Fig. 4) are plottedto the right of the RES (electrical resistivity) curve.

ences of successive layers are integrated from the bottom of the clay upwards in all 17 boreholes; the differences are contoured into regional maps. A seriesof such maps is shown in Fig. 11A to F, representingthe evolution from the earliest depositional phase ofthe Boom Clay to the top of the clay in the outcroparea, i. e., to about halfway the thickest total BoomClay section in the boreholes in the Campine area. Themapped part of the clay is therefore present in all bore-holes studied. Cumulative differential thicknesses canalso be calculated and compared between particularstratigraphic levels in the section. This can help to document the sedimentation evolution during particu-lar time slices of the Rupelian. This is illustrated inFig. 11G and H where the differential evolution ismapped between the DB (see Fig. 4) and two overly-ing levels demonstrating the evolution in that time in-terval from localised subsidence to a general subsi-dence in the north of the Campine Basin.

That this procedure produces coherent regional pat-terns (Fig. 11) indicates that the measurements are accurately enough to record a meaningful signal. Substantial noise could have been produced because ofthe small differences to be measured on the logs, butthis is overcome also by cumulating the differences

as demonstrated in Fig. 12 rather than comparing indi-vidual thicknesses.

As the high-frequency layers are orbitally con-trolled, the formation time of a particular layer is thesame in the entire study area, and therefore the thick-ness differences of that particular layer between wellsrepresent different sediment accumulation rates. Thecumulative difference maps (Fig. 11) show that theCampine area was subdivided into several small com-partments that collected different amounts of sedimentsduring successive time slices. It can also be observedthat the sedimentation rates in the compartmentschanged relative to each other with time. In principle,sedimentation rates can vary and be modified by vari-able amounts of sediment reaching the basin compart-ment due to the existing current system, for example.However, it is difficult to imagine that sediment supplyrates were different in the different basin compartmentsgiven the extreme similarity of the very fine shelfal sus-pended sediments over the whole Campine area (Van-denberghe et al. 2001). Also, the pattern of compart-mental thickness distribution and evolution shows nei-ther a relation with distance to a coastline in the southnor a linear trend with respect to the transport directionof the main sediment mass from the northwest (Fig. 1).


Fig. 10. Location map of the Campine in North Belgium with borehole names and locations studied.


Fig. 11. Maps of areal distribution of differential sedimentation rates. The maps are produced by contouring the cumula-tive thickness differences of the individual layers with the Mol well as reference. Maps A to F show the differences from the base of the clay till the level 2(A), 8(B), 16(C), 32(D), 40(E) and 55(F); maps G and H show the differential evolutionbetween the DB level and level 45(G) and DB and level 55(H). For situation of levels with respect to reference horizons (R, DB, (SID, . . .)) see Figs. 5, 6, 13 and 14. The colour scale is expressed in meter; blue indicates larger and red less thick-ness in the reference well.

The pattern of the different thickness compartmentsis, however, related to the tectonic evolution of theCampine Basin. Indeed, the southwestern and easternparts, which accumulated less sediments (Fig. 11A–F), are areas of known relative uplift. The southwestlost about 80 m of Boom Clay by erosion at the veryend of the Rupelian (De Man et al. 2010), and the eastis the border area of the Roer Graben, separated fromthe West Campine by the Mol-Rauw fault (Fig. 1). Renewed subsidence took place in the Roer Grabenfrom the Chattian (Late Oligocene) on, but just before,during the later part of the Rupelian, a slight uplift ofthat block occurred, expressed by the development ofmore sandy sediments making up the Eigenbilzen For-mation (Vandenberghe et al. 2001). The gradual shiftwith time to more pronounced subsidence in the northof the Campine that can be observed on the cumulativedifferential thickness maps (Fig. 11F–H) is also in ac-cordance with the final total clay thickness distributionand reflects the main regional basin subsidence patternduring Rupelian times.

Although this relationship of sediment accumulationrate with tectonic blocks experiencing variable subsi-dence is obvious from the maps, the process however by which the suspended sediment can be preferentiallyattracted to a particular more subsiding part of the seafloor is not evident. The most logic explanation is thatthe suspension sediment arriving equally all over the

Campine area winnows out from the shallower parts to the deeper parts; indeed the water-depth-dependentturbulence during an obliquity cycle takes longer in theshallower parts of the basin. This is expressed also by the relationship between the ratio of thicknesses be-tween clay and silty-clay parts in individual cycles whengoing from shallower to deeper water and back to shal-lower water over the whole Boom Clay section as shownin Fig. 6a and discussed above. In fact, this can also beobserved in the field measurements of Van Echelpoel(1991) (see also Vandenberghe et al. 2001) showing theproportion of clay-sediment volume with respect to siltvolume to systematically increase to the deeper waterarea in northern direction. This is expressed in Fig. 2showing in a more coastal direction (Putte clay pit) the stacking of the silty layers in the Double Layer. Theshallowest part of the clay deposit at its base, the Belse-le-Waas Clay, is also composed of unusually thick siltlayers, manifesting a stack of individual silt layers as aresult of prolonged sorting. In the shallower southernarea it can even be observed that the silty clay grades into coastal sands (Berg Sand) (Fig. 4 in Vandenbergheet al. 2001). These observations make a transport of finesto the deeper water a reasonable explanation for the re-lationship between subsidence and thicker suspensiondeposits as shown in the maps of Fig. 11.

On the other hand, it cannot be excluded that alsoeven a slight dip of the sea bottom contributed to the


Fig. 12. Technique for establishing cumulative thicknessdifference curves of individual boreholes.

attraction of fine mud to subsiding parts in the basin.A plausible mechanism is that the settling silt and claysuspensions get sufficiently concentrated just abovethe sea floor in order to become subject to gravity.Such a fluid mud layer is a transient dense suspensionzone forming directly above the sea bottom before becoming transformed into cohesive mud at the seabottom and having enough density to be attracted bygravity down slope (Wright and Friedrichs 2006).

In conclusion, it can be confirmed that the method-ology using the thickness differences with a referencewell leads to the expected isolation of the tectoniccomponent in the cyclic Boom Clay section.

5.3 Temporal changes in sedimentationrate at individual localities

The same thickness difference data as discussed abovecan also be represented for each single well. As longas the relative differences in thickness with the refer-ence well have the same sign, the cumulative thicknessdifference curve will have a similar slope, when thesign reverses the slope will change and eventually be-come opposite as demonstrated in Fig. 12. Changing to opposite slope means that the basin at the boreholelocation in comparison to the basin at the referencewell changes from subsiding more rapidly and catch-ing more sediment to more slowly and catching lesssediment, or vice versa.

Figure 13 shows the results for all studied wells.Changes are seen either as a sudden change to anotherslope value or as the transition to an opposite slope ofthe curve. Although not all wells behave exactly in thesame way and the thickness differences become moreimportant if wells are farther removed from the refer-ence well, an overall similar pattern can be observedwith breaks and slope changes clearly positioned at thesame stratigraphic layer in all wells. Even the Desselwell, very close to the reference well, shows a dampedbut similar pattern as the very well expressed patternin the Weelde well in the north of the Campine Basin(Figs. 13, 14)

Remarkably, the positions of these changes in rela-tive accumulation rate in the pattern of Fig. 13 are pre-cisely the turning points in the grain-size evolution di-agrams of the low-frequency cycles, characterised byspecial lithological properties that also have been in-terpreted as low-frequency relative sea-level indicatorsas discussed above (compare Figs. 4, 13): marineplanktonic mollusc influx at the R-level, the most in-tense sorting in two consecutive silty layers or the DB

double layer (Fig. 2), the presence of siderite as indica-tor of very slow sedimentation (Laenen and De Craen2004) at high sea-level position (S60) and at the levelmarked Sid2, and the coarse grain-size level markedSB3. Because the data are collected as differentialthicknesses, eustasy cannot be involved and therefore it is logical to explain the low-frequency relative sea-level changes by the relative tectonic subsidence evo-lution of the basin, contrary to previous suggestions for climate-driven sea-level changes. However, as dis-cussed above at least some parts of this low-frequencyevolution, and hence the particular lithological refer-ence layers such as R, DB, S60, seem also linked to cli-mate-driven long eccentricity cycles (see, e. g., Abels etal. 2007), a coincidence not understood at the moment.

6. Conclusions

The Rupelian Boom Clay in Belgium is a sedimentarydeposit for which an extensive collection exists of sedimentological, tectonical, mineralogical and strati-graphical properties obtained from outcrops, core andwell log data.

In this context, new insights are gained from a newdata set of measurements, namely the cumulative dif-ference between climate driven high-frequency cyclethicknesses measured in wells with the thickness of the same layers in a reference well in the area.

The basis for the interpretation of these differentialthickness measurements is the realisation that using dif-ferential rather than absolute thickness measurementsallows to eliminate eustasy as a possible factor. As thesediment type and the cyclostratigraphy of the BoomClay is very regular, changes in sediment provenancecan reasonably be excluded, and tectonic influenceseems the sole logical explanation for the observed vari-ations in cumulative differential thicknesses.

Maps of the cumulative differential evolution in thebasin, considering the data in all the wells at the samebed level, show a compartmentalisation of the basin inzones with more or less subsidence. This observationimplies that the fine suspension sediment reacts to subtle depth differences of the basin floor probablythrough winnowing from shallower water part of thebasin to the deeper part, although a contribution fromfluid mud gravitational movement cannot be excluded.

The cumulative evolution with time of the differen-tial thicknesses of the layers in the individual wells hasshown that the levels considered in earlier work to becontrolled by eustasy reflect in fact a tectonic history.



Fig

.13.

Res

ults

of

thic

knes

s di

ffer

ence

mea

sure

men

ts (

mov

ing

aver

age

of 3

) w

ith a

ref

eren

ce w

ell o

f in

divi

dual

bor

ehol

es lo

cate

d on

Fig

.10,

bas

ed o

n th

e m

etho

dolo

gyou

tline

d in

Fig

.12.


Fig

.14.

Com

pari

son

of c

umul

ativ

e th

ickn

ess

diff

eren

ce m

easu

rem

ents

, det

aile

d fr

om F

ig.1

3, s

how

ing

the

sim

ilari

ty in

the

evol

utio

n pa

ttern

of w

ells

ver

y cl

ose

to th

e re

f-er

ence

wel

l (D

esse

l) a

nd f

urth

er a

way

(W

eeld

e, P

oppe

l) (

see

loca

tion

of w

ells

in F

ig.1

0).

Acknowledgements. The authors sincerely thank NI-RAS-ONDRAF for the access to the borehole data and thepermission to publish this work. Greet Willems is thankedfor the considerable help with the figures. The reviewers arethanked for their detailed comments that have considerablyimproved the paper.

References

Abels, H. A., Van Simaeys, S., Hilgen, F. J., De Man, E.,Vandenberghe, N., 2007. Obliquity dominated glacio-eu-static sea-level change in the early Oligocene: evidencefrom the shallow marine siliciclastic Rupelian stratotype(Boom Formation, Belgium). Terra Nova 19, 65–73.

Abreu, V. S., Anderson, J. B., 1998. Glacial Eustasy duringthe Cenozoic: Sequence Stratigraphic Implications. Bul-letin Association American Petroleum Geologists 82(7),1385–1400.

De Man, E., Van Simaeys, S., 2004. Late Oligocene Warm-ing Event in the southern North Sea Basin: benthicforaminifera as paleotemperature proxies. NetherlandsJournal of Geosciences/Geologie en Mijnbouw 83(3),227–239.

De Man, E., Van Simaeys, S., Vandenberghe, N., Harris,W. B., Wampler, J. M., 2010. On the nature and chrono -stratigraphic position of the Rupelian and Chattian stra-totypes in the southern North Sea Basin. Episodes 33(1),3–14.

Dercourt, J., Gaetani, M., Vrielynck, B., Barreir, E., Biju-Duval, B., Brunet, M. F., Cadet, J. P., Crasquin, S., Sand-ulescu, M. (Eds.), 2000. Atlas Peri-Tethys, Paleogeo-graphical maps. Commission de la Carte Géologique du Monde-Commission for the Geological Map of theWorld, Paris.

Haq, B. U., Hardenbol, J., Vail, P. R., 1987. Chronology offluctuating sea levels since the Triassic. Science 235,1156–1167.

Hardenbol, J., Vandenberghe, N., Van Simaeys, S., De Man,E., 2003. Sequence Stratigraphy of the Boom Clay in Bel-gium. Field Guide to the Rupelian Stratotype, Addendum.Symposium on the Paleogene, Leuven, Belgium, 5 p.

International Marine and Dredging Consultants, 2005. Hy-draulisch randvoorwaardenboek Vlaamse kust. Ministe -rie van de Vlaamse Gemeenschap, Afdeling WaterwegenKust, Oostende, Belgium, 49 p.

Laenen, B., 1997. The Geochemical Signature of RelativeSea-Level Cycles Recognized in the Boom Clay. Vol-ume 3 The Organic Matter of the Boom Clay. Doctoralthesis, Geology K. U. Leuven, 164 p.

Laenen, B., 1998. The geochemical signature of relative sea-level cycles recognised in the Boom Clay. AardkundigeMededelingen University Press Leuven 9, 61–82

Laenen, B., De Craen, M., 2004. Eogenetic siderite as an indicator for fluctuations in sedimentation rate in theOligocene Boom Clay formation (Belgium). Sedimenta-ry Geology 163, 165–174.

Lagrou, D., Vandenberghe, N., Van Simaeys, S., Hus, J., 2004.Magnetostratigraphy and rock magnetism of the BoomClay (Rupelian stratotype) in Belgium. Netherlands Journalof Geosciences/Geologie en Mijnbouw 83(3), 209–225.

Miller, K. G., Kominz, M. A., Browning, J. V., Wright, J. D.,Mountain, G. S., Katz, M. E., Sugarman, P. J., Cramer, B.,Christie-Blick, N., Pekar, S. F., 2005. The PhanerozoicRecord of Global Sea-Level Change. Science 310, 1293–1298.

Pälike, H., Norris, R. D., Herrie, J. O., Wilson, P. A., Coxall,H. K., Lear, C. H., Shackleton, N. J., Tripati, A. K., Wade,B. S., 2006. The Heartbeat of the Oligocene Climate sys-tem. Science 414, 1894–1898.

Popov, S. V., Akhmetiev, M. A., Lopatin, A. V., Bugrova,E. M., Sytchevskaya, E. K., Scherba, I. G., Andreyeva-Grigorovich, A. S., Zaporozhec, N. I., Nikolaeva, I. A.,Kopp, M. L., 2009. Paleogeography and Biogeography ofParatethys Basins. Part 1 late Eocene–Early Miocene,Moscow Scientific World, 200 p. (in Russian).

Schwarzacher, W., 1993. Cyclostratigraphy and the Milan -kovitch theory. Developments in Sedimentology 52. El-sevier, 225 p.

Sissingh, W., 2003. Tertiary paleogeographic and tectono -stratigraphic evolution of the Rhenish Triple Junction.Paleogeography, Paleoclimatology, Palaeoecology 196,229–263.

Steurbaut, E., Herman, J., 1978. Biostratigraphie et poissonsfossiles de la formation de l’argile de Boom (OligocèneMoyen du Bassin Belge). Géobios 11(3), 297–325.

Stover, L. E., Hardenbol, J., 1993. Dinoflagellates and de-positional sequences in the Lower Oligocene (Rupelian)Boom Clay Formation, Belgium. Bulletin BelgischeVereniging voor Geologie 102, 5–77.

Vandenberghe, N., 1978. Sedimentology of the Boom Clay(Rupelian) in Belgium. Verhandelingen Koninklijke Aca -demie voor Wetenschappen, Letteren en Schone Kunstenvan België, Klasse der Wetenschappen 147, 137 p.

Vandenberghe, N., Van Echelpoel, E., 1987. Field guide tothe Rupelian stratotype. Bulletin Société belge de Géolo-gie 96(4), 325–337.

Vandenberghe, N., Herman, J., Steurbaut, E., 2002. DetailedAnalysis of the Rupelian Ru-1 Transgressive Surface inthe Type Area (Belgium). Proceedings 8th Biannual Meet-ing Regional Committees Northern Paleogene and Neo-gene Stratigraphy on Northern European CenozoicStratigraphy, Flintbek, Germany, p. 67–81.

Vandenberghe, N., Laenen, B., Van Echelpoel, E., Lagrou,D., 1997. Cyclostratigraphy and climatic eustasy. Exam-ple of the Rupelian stratotype. Comptes rendus de l’Aca -démie des Sciences de la Terre et des Planètes 325, 305–315.

Vandenberghe, N., Hager, H., van den Bosch, M., Verstrae-len, A., Leroi, S., Steurbaut, E., Prüfert, J., Laga, P., 2001.Stratigraphic Correlation by calibrated well logs in theRupel Group between North Belgium, the Lower-Rhinearea in Germany and Southern Limburg and the Achter-hoek in The Netherlands. Aardkundige MededelingenUniversity Press Leuven 11, 69–84.


Vandenberghe, N., Van Simaeys, S., Steurbaut, E., Jagt,J. W. M., Felder, P. J., 2004. Stratigraphic architecture ofthe Upper Cretaceous and Cenozoic along the southernborder of the North Sea Basin in Belgium. NetherlandsJournal of Geosciences/Geologie en Mijnbouw 83(3),155–171.

Van Echelpoel, E., 1991. Kwantitatieve cyclostratigrafie vande Formatie van Boom (Rupeliaan, België). De method-o logie van het onderzoek van sedimentaire cycli via Walsh analyse. Doctoral thesis, Geology K. U. Leuven,164 p. (in Dutch).

Van Echelpoel, E., Weedon, G. P., 1990. Milankovitchcyclicity and the Boom Clay Formation: an Oligocenesiliciclastic shelf sequence in Belgium. Geological Mag-azine 127(6), 599–604.

Van Simaeys, S., Vandenberghe, N., 2006. Rupelian. Geo-logica Belgica 9, 95–101.

Wade, B., Pälike, H., 2004. Oligocene Climate dynamics. Pa-leoceanography 19 PA4019 doi:10.1029/2004PA001042.

Wright, L. D., Friedrichs, C. T., 2006. Gravity-driven sedi-ment transport on continental shelves: a status report.Continental Shelf Research 26, 2092–2107.

Zeelmaekers, E., 2011. Computerized qualitative and quan-titative clay mineralogy. Introduction and application to known geological cases. Doctoral thesis, GeologyK. U. Leuven, 397 p.

Manuscript received: December 11, 2012; rev. version ac-cepted: June 24, 2013.


http://dx.doi.org/10.1029/2004PA001042

Documents

Differentiating between tectonic and eustatic signals in ...3.2 Climatic control on sedimentation The regularity of the layering is suggestive of Milan - kovitch forcing on sedimentation