A Rapid Clay-Mineral Change in the Earliest Priabonian of the North Sea Basin?

Netherlands Journal of Geosciences / Geologie en Mijnbouw 83 (3): 179-185 (2004)

A rapid clay-mineral change in the earliest Priabonian of the North Sea Basin?

R. Saeys, A. Verheyen & N. Vandenberghe1

K.U. Leuven, Historical Geology, Redingenstraat 16, B-3000 Leuven, Belgium 1 corresponding author: [email protected]

Manuscript received: February 2004; accepted: August 2004

Abstract

In the Eocene to Oligocene transitional strata in Belgium, clay mineral associations vary in response to the climatic evolution and to tectonic pulses. Decreasing smectite to illite ratios and the systematic occurrence of illite-smectite irregular interlayers are consequences of a cooling climate. A marked increase in kaolinite content occurs just after a major unconformity formed at the Bartonian/Priabonian boundary and consequently is interpreted as resulting from the breakdown of uplifted saprolites.

Keywords: clay minerals, Paleogene, North Sea Basin

Introduction Stratigraphy of the Eocene to Oligocene in the southern North Sea The International Geological Correlation Program Project No 124 integrated the Cenozoic stratigraphic The study of the cored Doel 2b well, north of data of the North Sea Basin, which were dispersed at Antwerp in Belgium, has documented the Bartonian, that time in the literature, and the traditions of the Priabonian and Early Rupelian transition layers by different countries around the North Sea (Vinken, lithostratigraphy, nannoplankton and dinoflagellate 1988). Since this study, it has become known that biostratigraphy, and by geophysical well logs broadly speaking the Eocene sediments in the basin (Vandenberghe et al., 2003). Verheyen (2003) has have a clay mineralogy dominated by smectite whilst tested the regional validity of the subdivisions in contrast the Oligocene sediments have a clay min established in the Doel 2b well by examining the geo eralogy characterized by mixtures in which illite and physical well logs, the lithology and the biostratigraphic kaolinite are prominently present (Quinif et al., 1983; data of more than fifty wells in North Belgium. Some Nielsen, 1988; Pearson & Small, 1988; Pearson, 1990). key wells, their well-log based correlation and their The precise stratigraphic position of this transition stratigraphic interpretation are represented in the however is poorly documented and also the reasons for regional model on Fig. 1. This regional correlation is this general change in clay mineral associations in the documenting the Eocene to Oligocene transition in the North Sea Basin remain debatable. The objective of type area for the southern North Sea Basin stratigraphy. the present study is to better define the timing of this A main regional stratigraphic feature in that time change in clay mineral composition and to discuss its interval is the unconformity underlying the Tongeren cause in the light of the present knowledge of the Group sediments in northeast Belgium. Log correlation paleoclimatic and paleodynamic evolution of the area. and biostratigraphic calibration demonstrate that this

Netherlands Journal of Geosciences / Geologie en Mijnbouw 83(3) 2004 179

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Fig. 1. Regional geological model of the Eocene to Oligocene strata in North Belgium, based on geophysical well log correlation and biostratigraphic calibration of some key wells. The geographical location of the wells is shown in the inset map.The lithostratigraphic nomenclature used is in accordance with Laga et al. (2001). unconformity continues westward into the boundary For the purpose of this study, a simplified semi between the latest Bartonian (a3 coded Onderdijke quantitative analysis is made based on peak height Member on Fig. 1) and earliest Priabonian (S3.1 coded measurements and applying correction factors with lower part of the Bassevelde Member on Fig.l). In respect to illite (smectite 0.25, kaolinite 0.7, chlorite that area the Priabonian is mainly represented by the 0.34). Clay mineral associations in the overlying Bassevelde Sands; it was demonstrated however, that Rupelian Boom clay are known in detail from the these sands in fact consist of three sequences, respec studies of Laenen (1997) and Vandenberghe & Laenen tively belonging to the NP18 (S3.1), NP19-20 (S3.2) (1999). and NP21 (S3.3) biozones (Vandenberghe et al. 2003). The results in the different wells being similar The youngest sequence (S3.3) includes a clay layer at (Saeys, 2003), three wells are choosen for a repre the top (a4 codedWatervliet Member on Fig. 1). Only sentative composite section in Fig. 3, on which illite- this youngest sequence has a large geographical smectite irregular interlayers are included in the illite extension and the two older Bassevelde sequences group. It appears that the Late Lutetian and (S3.1 and S3.2) have a geographically limited occur Bartonian sediments are systematically dominated by rence, with S3.2 extending more southeastwards than smectite (swelling till 17 A) and have lesser quantities the S3.1 sequence (Fig. 2). The upper sequence S3.3 of illite and even less kaolinite. At the base of the in fact connects to the east with the marine part of the oldest Priabonian sequence (the S3.1 lower part of Tongeren Group (Sint-Huibrechts-Hern Formation, the Bassevelde Sands), a rapid decrease in smectite consisting of Grimmertingen and Neerrepen Mem occurs in favour of a sudden appearance of significant bers) of the same NP21 age (Fig. l).This sequence amounts of kaolinite, and without a significant extends southeastwards even over the Ardennes change in the amount of illite. Illite-smectite random (Vandenberghe et al. 1998, Vandenberghe et al., this interlayers become also systematically associated with volume) and is of earliest Oligocene age, based espe illite during the Priabonian whereas in the older cially on dinoflagellate content (Vandenberghe et al., sediments their presence seems more irregular. On 2003). Fig. 3, illite-smectite random interlayers are included The relative low sea level between this last sequence in the illite group as their content rarely exceeds 10% (Bassevelde S3.3 and marine Tongeren Group) and of the total clay minerals and therefore does not the next younger sequence (S4 and Bm on Fig.l) is influence the trends appearing in Fig. 3. Chlorite is very pronounced. Rising sea level subsequently forms present in both intervals but in small quantities. lowstand deposits (S4 coded Ruisbroek sand and con The Priabonian clay mineral association of smectite tinental Borgloon Formation of the Tongeren Group, (20-25%), kaolinite (30-35%), illite (25-30%) and both in the NP22 biozone), and transgressive Boom illite-smectite irregular interlayers (10-15%) also Clay which already contains NP23 flora at its base. compares well to the clay mineral association found in The low sea level coincides with the sudden shift to a the overlying Rupelian Boom Clay (see also Laenen, much colder oxygen isotope signature (De Man et al., 1997). this volume). Climatic and tectonic influences on the Clay mineral analysis of the Eocene to clay mineral associations Oligocene sediments in North Belgium In the oceanic domain outside the North Sea Basin, Based on the stratigraphic analysis discussed above similar changes in clay mineral composition between and shown in Fig. 1, eight cored boreholes were smectite-dominated Eocene and illite-dominated selected for clay mineral determinations (Fig. 2). More Oligocene sediments have been reported from both than 200 samples were analyzed (Saeys, 2003); the the South and the North Atlantic Ocean drilling sampling positions were chosen to have clay mineral program (Chamley, 1989; Robert, 1980). Such a successions in stratigraphic reference wells, such as general change of an Eocene smectite-dominated clay the Doel 2b and the Sint-Huibrechts-Hern wells, and mineral association to a more varied association with to confirm these successions and their position with important illite quantities around the Eocene- respect to biostratigraphic data in other wells (Fig. 2). Oligocene boundary strongly suggests that a general Clay mineral analysis has been performed using the change in weathering conditions must have taken classical XRD analysis of oriented slides after air- place. Therefore, a major contribution of altered drying, ethylene glycol saturation and heating treat volcanics (Pearson, 1990) to the North Sea Eocene ments. The clay minerals are identified using the smectite is less likely. The change in weathering clay procedures outlined inThorez (1975, 1976). products obviously can be associated with the general

Netherlands Journal of Geosciences / Geologie en Mijnbouw 83(3) 2004 181

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I Q I Quarternary IBml Boom Formation Borgloon Formation Zelzate Formation ran Onderdijke Member | Ge | - Gelinden Member - Putte Member (Pu) lO-Bl - Oude Biezen Member | S4 I - Ruisbroek Member Buisputten Member - Temagen Member (Te) E§] -Henis Member ^g . watervliet Member ri2i Zomergem Member - Belsele-Waas Member (B-W) rsn Onderdale Member Sint-Huibrechts-Hern Formation I S3 I - Basseveide Member Ursel Member (a1A) Bilzen Formation [Ne~l - Neerrepen Member S3.3-S3.2-S3.1 sequences rrn Asse Member (a1 B) I Be I - Berg Member |Gr [ - Grimmertingen Member rwii Wemmel Member a" o Fig. 2. The stratigraphic interpretation and correlation of 8 wells at the Eocene-Oligocene transition in % North Belgium, used for clay mineral analysis. The positions of the samples analyzed are indicated at the left side of each column. The lithostratigraphic nomenclature used is in accordance with Laga et al. (2001).

o o change from a humid and warm Eocene climate to a the cooler Early Oligocene, the weathering does not cold Oligocene climate as is demonstrated by the lead any longer to mainly smectite; instead a less oxygen isotope data (Zachos et al., 1994). Since about mature weathering mixture is produced of smectite the latest Ypresian, a gradual cooling trend started and illite-smectite irregular interlayers, explaining the and a sudden cooling occurred in the earliest Oligo systematic occurrence of illite-smectite random cene. Paleobotany data in the area investigated show interlayers since the Priabonian in our sections. a clear seasonality in the latest Ypresian and a still Obviously, the general appearance of kaolinite in warm but dryer climate in the beginning of the the Oligocene does not fit the weaker chemical Lutetian (Quinif et al., 1983). Superposed on the weathering in a colder climate. Therefore, the jump to general cooling trend, a marked warming pulse occurs higher kaolinite content in the Early Priabonian lower at the Lutetian-Bartonian transition, and also in the Bassevelde sand (S3.1) can not be related to the short Early Priabonian a renewed warming occurs (Boharty warming pulse at that time, before the onset of major & Zachos, 2003). cooling above the top of the Bassevelde sands (S3.3). Illite and small amounts of chlorite are consistently The kaolinite jump just above a major unconformity present in the Eocene-Oligocene transitional interval. however, suggests that kaolinite is delivered to the They point to a continuous contribution of almost basin by uplifting older kaolinite-rich saprolites that unaltered particles derived from the older late-dia- had developed in earlier Paleogene and probably genetic and metamorphic rocks and slate belts Mesozoic time. Such deeply weathered kaolinite around the basin (e.g. from the Ardennes). Reduced bearing saprolites have developed on top of the weathering intensity in the cooler climate is probably Ardennes (Yans et al., 2003). the main reason. The apparent increase in illite to Laenen (1997, 1998) and Vandenberghe & Laenen smectite ratio (illite-smectite random interlayers (1999), however, have demonstrated that the main content remains rather constant) in the latest amount of kaolinite in the Early Rupelian of the Bartonian, Priabonian and earliest Rupelian (Fig.3) is southern North Sea was derived from the northern probably also related to the Pyrenean uplift (uncon North Sea. In that northern area, especially Fenno- formity at the Bartonian-Priabonian boundary on scandinavia, major uplift is known to have started at Fig. 1) and also to the uplift and relative low sea level, the beginning of the Oligocene (Faleide et al., 2002, with consequent increase of erosion, at the base p. 251; Huuse, 2002) and continued as a major uplift of the sequence S4-Bm on Fig. 1 (for details see during the late Oligocene to Pliocene time (Dore et Vandenberghe et al., 2002). al., 1998). Quinif et al. (1983) also see the origin of During the Eocene humid and warm climate the kaolinite in the breakdown of older ferralithic soils mainly smectitic clays are produced in the soils, but these authors suggested that the climate degra subsequently reflected in the Eocene marine detrital dation was itself responsible for the breakdown of the sediments. When weathering becomes less intense in vegetation cover initiating the erosion of the soils.

Fig. 3. Composite section of the clay mineral composition of the successive lithostratigraphic units from Lutetian till Rupelian in the Sint-Kathelijne-Waver, Stekene and Doel 2b wells; the data are representative for the clay mineral data obtained in the 8 wells examined and 40 50 60 shown in Fig. 2. The symbols of the litho % Clay Mineral stratigraphic units are the same as in n ar Kaolinite • Smectite H Illite j |"{fflff •Chlorite Figs. 1 & 2.

Netherlands Journal of Geosciences / Geologie en Mijnbouw 83(3) 2004 183

Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.42, on 27 Sep 2021 at 23:49:40, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016774600020242 They see an argument in the smaller quantities of References kaolinite appearing already in the Lutetian. However, Blondeau, A., Cavelier, C, Feugueur, L. & Pomerol, C, 1965. at the end of the Ypresian and the beginning of the Stratigraphie du Paleogene du Bassin de Paris en relation avec Lutetian starts also an important uplift of the Artois- les bassins avoisinants. Bulletin Societe Geologique de France, Brabant-Ardennes axis (Blondeau et al., 1965; 7 ieme serie 7: 200-221. Pomerol, 1975), probably initiating the breakdown of Bohaty, M. S. & Zachos, J.C., 2003. Significant Southern Ocean the older kaolinite bearing ferralithic soils leading to warming event in the late middle Eocene. Geology 31: 1017-1020. the admixture of kaolinite in the marine sediments Chamley, H., 1989. Clay Sedimentology. Springer-Verlag (Berlin): 623pp. along the coasts bordering the uplifted blocs. De Man, E., Ivany, L. & Vandenberghe, N., (this volume). Stable oxygen isotope record of the Eocene-Oligocene transition in NE Conclusions Belgium. Netherlands Journal of Geosciences / Geologie en Mijnbouw. A detailed regional geological study has demon Dore, A.G., Lundin, E.R., Jensen, L.N., Birkeland, 0., Eliasen, 0. & Fichler, C. 1998. Principal tectonic events in the evolution of strated an important tectonic event at the Bartonian- the northwest European Atlantic margin. In: Fleet, A.J. & Boldy, Priabonian boundary. During the Priabonian two S.A.R. (eds): Petroleum Geology of Northwest Europe. sandy sequences (two lower Bassevelde sand units) Geological Society London: 41-61. with a limited extension have been deposited in North Faleide, J.I., Kyrkjebe R., Kjennerud, X, Gabrielsen R.H., Jordt, Belgium. In the earliest Oligocene a very widespread H., Fanavoll, S. & Bjerke, M.D., 2002. Tectonic impact on transgression occurred documented by marine sedimentary processes during Cenozoic evolution of the northern North Sea and surrounding areas. In: Dore, A.G., Tongeren Group sediments, a top part of the Basse Cartwright, J.A., Stoker, M.S., Turner, J.P. & White, N. (eds): velde sands and theWatervliet clay (NP 21 sequence). Exhumation of the North Atlantic Margin: Timing, Mechanisms Between this earliest Oligocene sequence and the and Implications for Petroleum Exploration. Geological Society, next sequence occurs a major sea-level fall coinciding London, Special Publication 196: 235-269. with a major cooling demonstrated by oxygen isotopes. Huuse, M., 2002. Cenozoic uplift and denudation of southern Norway: insights from the North Sea Basin. In: Dore, A.G., Clay mineral analysis shows that a sharp change Cartwright, J.A., Stoker, M.S., Turner, J.P. & White, N. (eds): occurs at the base of the lowest Priabonian sequence. Exhumation of the North Atlantic Margin: Timing, Mechanisms A smectite and illite association below is replaced and Implications for Petroleum Exploration. Geological Society above by an association of smectite, kaolinite, illite London, Special Publication 196: 209-233. and illite-smectite irregular interstratifications. Laenen, B., 1997. The Geochemical Signature of Relative Sea-level It is argued that the changing ratio of smectite to Cycles Recognized in the Boom Clay. Doctoral thesis K.U. 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Essai de thanked for their constructive remarks which have reconstitution geodynamique du Paleogene du bassin beige a improved the original manuscript. partir des donnees de la mineralogie des argiles, de la geochimie

des radioelements (U, Th, K20) et de la palynologie. Comptes Rendus de 1' Academie des Sciences (Paris) II 296: 1621-1624. Robert, C, 1980. Climats et courants Cenozoi'ques dans l'Atlantique Sud d'apres l'etude des mineraux argileux (legs 3,39 et 40 DSDP). Oceanologica Acta 3 (3): 369-376.

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