- Email: [email protected]
Texture, composition and stratigraphy of volcanic ash beds in lower Palaeocene chalk from the North Sea Central Graben area
ARTICLE IN PRESS
Marine and Petroleum Geology 23 (2006) 767–776 www.elsevier.com/locate/marpetgeo
Texture, composition and stratigraphy of volcanic ash beds in lower Palaeocene chalk from the North Sea Central Graben area Lars Simonsen, Jørgen Toft Mærsk Olie og Gas AS, Esplanaden 50, DK-1263 Copenhagen K, Denmark Received 6 October 2004; received in revised form 30 September 2005; accepted 14 June 2006
Abstract A large number of volcanic ash beds have been identified in Danian chalk from the Danish North Sea sector Central Graben area. The volcanic input was prominent during deposition of the lower part of the Danian chalk, whereas it was much less during deposition of the upper part of the Danian chalk. The non-carbonate fraction of the ash beds predominantly consists of smectite with some quartz, illite and minor amounts of kaolinite/chlorite and pyrite and trace amounts of zircon. Fragments of volcanic glass are common in the ash beds. The mineralogical composition of the ash beds varies from bed to bed, which makes ash bed correlation a promising high-precision correlation tool. Volcanic provinces that were active in connection with the opening of the North Atlantic Ocean are possible sources of the volcanic ash. The different composition of the ash layers could indicate that they originate from more than one source area. r 2006 Elsevier Ltd. All rights reserved. Keywords: Volcanic ash; Danian; Chalk; Central Graben; Smectite; Zircon; Glass; Correlation
1. Introduction
2. Ash beds
Volcanic ash layers are well established components of the late Palaeocene–early Eocene sediments in Denmark, e.g. in the Ølst, Røsnæs Clay and Lillebælt Clay Formations where ash beds occur interbedded with marine clays (Huggett, 1993; Knox, 1997). Similarly, volcanic ash is well known in the Mo-clay diatomite where they occur as prominent layers encountered in outcrops in Northern Jutland. Analyses of core material from wells drilled in the Central Graben area of the Danish North Sea sector (Fig. 1) show that numerous layers of volcanic ash are also present in the chalk from the Lower Palaeocene Danian stage. The ash beds are believed to originate from the volcanism associated with the initial phases of continental break up, which eventually, during the earliest Eocene, lead to the formation of the northern North Atlantic Ocean.
2.1. General stratigraphy and depositional setting
Corresponding author. Tel.: +45 33 36 37 79; fax: +45 33 36 37 83.
E-mail address: [email protected] (L. Simonsen). 0264-8172/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.06.003
In the Danish North Sea sector, the Danian chalk, which is comparable to the Ekofisk Formation in the northern North Sea, is divided into two units. An upper sub-unit, the D1 (Svendsen, 1979), which predominantly consists of relatively clean chalk, and a lower sub-unit, the D2 (Svendsen, 1979), which mainly consist of chalk with a higher content of argillaceous and siliceous material. Deposition of the Danian chalk occurred in an open marine environment, dominated by continuous slow deposition of coccolith debris and some pelagic foraminifers. Except for siliceous sponges, which appear to have been relatively abundant in some areas and intervals, macrofossils are rare. Common smectite rich clay beds attest to the occurrence of multiple events of short lasting deposition of wind borne volcanic ash, which, mixed with chalk particles, was deposited as thin distinct beds of a regional extent in the otherwise homogeneous chalk. The amount of ash beds identified in individual wells varies significantly. This,
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
768
E-5
G-1X
SIF-1X
G-2X
S.E.IGOR-1X
NANA-1XP MFA-4
Fig. 1. Study well location map of the Danish North Sea Central Graben area.
however, is likely to largely be related to variations in local overprints, such as small-scale erosion, reworking or bioturbation, all of which could presumably quickly remove the outline of a thin ash bed. The vast majority of ash beds are located in the D2, but ash beds are encountered in five well-defined stratigraphic intervals of the D1. The total number of ash beds in the Danian chalk section may well be over a hundred, but the largest amount of discernible ash beds so far encountered is 86 in the Sif-1X well. Ash beds have been encountered in all wells included in the study.
Fig. 2. Ash beds (A) in Danian chalk from the S.E. Igor well. Most of the ash beds in this interval are quite well defined. Most of the distinct thin dark streaks within the chalk intervals are Zoophycos isp. burrows.
The core chips consisted of chalk with variable contents of ash. The XRD samples were acquired from intervals with the highest concentration of ash.
2.2. Analytical methods
2.3. Textural aspects of ash beds at core scale
The results of this study are based on analysis of core material and evaluated petrophysical logs, acquired from seven exploration and appraisal wells drilled in the southern part of the Danish North Sea sector (Fig. 1). Core material from the seven wells has been analysed at various scales, including thin-section and visual core examination. Mini-permeameter measurements were performed on two slabbed core samples. The mineralogy of 20 clay rich core chips from four wells was analysed by X-ray diffraction (XRD). The following XRD analyses were performed:
In cores the ash beds appear as intervals with increased clay mineral content. The clay concentration of the ash layers varies significantly. In some cases, the ash beds are well defined and form true horizons (Fig. 2), whereas other ash beds appear as subtle discolorations of the chalk. It is impossible to conclusively demarcate grey chalk from very diluted ash beds. In many cases, the mixing effect of bioturbation is evident. Figs. 3–5 display examples of chalk–ash re-distribution by bioturbation. Only rarely are there indications of hardground formation below the clay beds, rather, different evidence suggests that in general the chalk was soft and un-compacted at the time of volcanic ash deposition. Often the base of ash beds display clear signs of being mixed with the chalk below it, indicating that the chalk was soft when the volcanic ash was emplaced. In other cases, the base of ash beds is sharp,
Quantitative XRD-mineralogy of total rock samples (bulk mineralogy). Quantitative XRD-clay mineral analyses of o2 mm fractions.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
769
Fig. 3. Mixture of Danian chalk and volcanic ash. Thallasinoides isp. burrow in ash bed, which has been filled with relatively pure chalk and later deformed during compaction and diagenesis of the sediment. When viewed on a slabbed core face (B) such burrows may easily be mistaken for embedded chalk clasts.
but generally the interface is smooth with little relief and without evidence of hardground formation. Fig. 6 displays two examples of different basal ash to chalk interfaces. The thickness of the ash beds varies from a few millimetres to at least 20 cm, the average thickness being around 2 cm. Presumably the thicker ash beds may comprise several individual ash beds. The ash bed thickness is, however, only weakly related to the original depositional thickness, as the majority of the ash beds have experienced some mixing with the surrounding chalk. The chalk immediately above and below ash beds generally has lower porosity than typical Danian chalk from the area. 2.4. Textural aspects of ash beds at thin section scale In thin sections, the ash beds typically appear as clayrich laminated bioclastic packstone. The lamination is generally caused by some compactional realignment of clay particles and elongated clasts. Cemented bed-parallel micro-fractures are moderately common. In some ash layers, a high content of spar-filled bioclasts masks the overall impression of a laminated rock. None of the larger clasts exhibit signs of rounding prior to deposition. Fig. 7 displays photomicrograph examples of the ash bed texture. Preserved glass fragments are common and show large variations in shape and sizes with maximum lengths of some 200 mm, but with significantly smaller average grain size (see Fig. 8). The glass fragments are translucent, and the colour ranges from almost colourless over pale brown to orange. The glass fragments are isotropic or display slight birefringence, with undulating extinction. Some glass
Fig. 4. Mixture of Danian chalk and volcanic ash. Zoophycos isp. burrow in chalk and Ash bed. It is unclear to which degree the coloration of the Zoophycos isp. burrows is primary, and to which it results from incorporation of ash. Generally the Zoophycos isp. seem to have avoided the ash beds as they are rarely encountered inside those.
fragments exhibit clear signs of alteration along the periphery. An example of this is displayed in Fig. 8. In general, however, the outlines of devitrified glass fragments have been obscured during diagenesis. An analogue preservation of glass fragments have been reported from the Palaeocene and Eocene ash beds in the Ølst Formation (Huggett, 1993). A large proportion of the calcitic tests of planktonic foraminifers inside the ash beds have been severely corroded or completely dissolved prior to being filled with calcitic spar, which in some instances appear to be ferrous. This indicates that volcanic ash probably had a significant impact on the stability of carbonate particles in its proximity. In thin sections, the visible effect of bioturbation of the ash layers is variable. In some samples, rare Chondrites isp. penetrate the uppermost millimetres of the ash layers, whereas burrows are rare in the deeper parts of the layers. In other samples, bioturbation is moderately common
ARTICLE IN PRESS 770
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Fig. 6. Clay beds in core. The base of the lower clay bed is clearly mixed with the chalk below it, indicating that the chalk was soft when the clay, or its precursor, was emplaced. The base of the upper clay bed is more sharp, but without evidence of hardground formation.
Fig. 5. Example of mixture of Danian chalk and volcanic ash. Diverse bioturbation in chalk has dispersed ash into the chalk, thereby causing the grey coloration which is typical of D2.
throughout the ash layers. The fill in burrows within the ash beds is commonly chalk. 2.5. Ash bed mineralogy In general, the ash bed samples look very clay-rich; however, the whole rock XRD analyses reveal that calcite is the dominating mineral even in the most clay rich samples, constituting about 75% of the bulk volume. The non-carbonate fractions of the ash bed samples predominantly consists of smectite with some quartz and illite and minor amounts of kaolinite/chlorite and pyrite. Trace amounts of zircon are encountered. The results of the XRD analyses of the non-carbonate fraction are displayed in Table 1. The mineralogic composition of the volcanic ash beds of this study compare well to the compositions reported from late Palaeocene to early
Eocene volcanic ash beds from mainland Denmark (e.g. Huggett, 1993). The smectite, primarily montmorillonite, is by far the most abundant clay mineral. The montmorillonite c-axes dimension swells to 17 A˚ on glycollation, and the peaks have an unusually large height/width ratio compared to sedimentary smectites. This indicates that the volcanic material was transformed in situ, or at least that the authigenic smectites derived from volcanic material has only experienced very minor transport (Ole Bjørslev Nielsen, Written communication). Similarly, Elliott et al. (1989) interpreted high-Mg smectite from the Danian outcrop in Limhamn and the Cretaceous-Tertiary boundary at Stevns Klint to be the result of alteration of pyroclastic basaltic ash. 2.5.1. Natural gamma ray spectrometry Natural gamma ray spectrometry logs indicate that the D2 has a distinctly higher thorium content than the underlying Maastrichtian chalk and the D1 above. Thorium is an insoluble trace element which is not associated with biogenic carbonates. Consequently, the higher concentration of Thorium in the D2 relative to the chalk intervals above and below reflects the relative higher abundance of non-carbonate material in this interval.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
771
Fig. 7. Typical ash bed lithologies. In both samples glass chards (brown grains) are dispersed in the matrix. In picture B, the clay minerals are concentrated in dark bands defining a distinct lamination of the rock. Photomicrographs in plane polarised light. Scale bars ¼ 100 mm.
Fig. 8. Picture A and B: partly dissolved glass particle. The blue arrow marks the approximate boundary of the original glass particle. The green arrow marks the boundary of the remaining glass fragment. Pictures C–F: glass fragments from volcanic ash beds. Scale bars ¼ 50 mm.
2.5.2. Zircon Zircon is a rare accessory mineral in the ash beds. The zircon crystals are micron sized, up to around 100 mm long, but generally smaller (10–20 mm) and euhedral. Zircons have been observed both in insoluble residue and in thin section. Fig. 9 displays four high magnification thin-section photomicrographs of zircon crystals. Zircon is a common accessory mineral of acid and intermediate igneous rocks. Detrital zircons are typically worn and abraded during transport, but in general the zircons from the ash beds show no signs of abrasion. Also, it is quite unlikely that zircon and smectite were deposited in the same bed by sedimentary processes, given the highly unequal hydraulic properties of the two minerals. 2.6. Ash bed stratigraphy The distribution of ash beds displays a strong correlation to porosity, as the majority of ash beds are associated with distinct porosity reductions. Likewise, the distribution of
ash beds displays a positive correlation to Thorium content. Fig. 10 displays a wireline log and ash bed distribution plot across the Danian interval of the Nana1XP well. The lower porosity of chalk in the vicinity of ash beds indicates that the diagenetic alteration of the chalk is influenced by the presence of ash. The repeatability of log signatures between wells suggests that even in wells where a particular ash bed has not been identified, but where the associated porosity and thorium signatures are present, the ash bed was deposited and did influence diagenesis. Based on their stratigraphic position, the ash beds have been clustered into nine groups. Five groups, numbered 0 to IV, have been identified in D1, and four groups, numbered IV to VIII, have been identified in D2. Fig. 11 shows the correlation of ash beds between four wells from the study area. In D1, the individual ash beds are separated by thick intervals of clean chalk, which makes the ash bed bearing intervals quite distinct. In D2, on the other hand, ash beds
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
772
are abundant, and chalk intervals without ash beds are few. Furthermore, the general greyish appearance of the D2 chalk demonstrates that reworking, largely by bioturbation, has caused significant redistribution and mixing of ash and chalk in D2, which has commonly led to complete obliteration of thin ash beds. Table 1 XRD analyses of the non-carbonate fraction, shows that the noncarbonate fraction of the ash bed samples predominantly consist of smectite with some quartz and illite and minor amounts of kaolinite/ chlorite and pyrite XRD sample
Non-carbonate fraction Quartz (%)
Ka/Chl. (%)
Illite (%)
Smectite (%)
G-2X, 66500 1100 G-2X, 66690 800
11 31
6 7
20 16
63 46
S.E. Igor-1X, 66750 200 S.E. Igor-1X, 66780 500
10 43
8 10
17 21
65 26
12 32 56 36 72 15 14 8
5 9 8 6 4 9 5 2
22 19 12 18 5 20 18 10
61 39 25 40 19 55 63 81
12 8 10 14 12 59 7 12
4 3 5 3 3 1 2 1
19 21 17 20 18 15 7 14
65 68 68 63 67 24 84 73
Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X,
67420 000 67600 200 67660 900 67670 800 67700 100 67820 300 68310 900 68340 1000
67610 800 67770 500 67790 67800 100 67870 100 67920 500 68180 600 68190
The XRD analysis included five ash bed samples from D1 and 15 samples from D2. In D1, three samples were acquired from ash bed group II and two samples from ash bed group IV. In D2 eight samples were acquired from ash bed group V, three samples from ash bed group VII and four samples from ash bed group VIII. The results of the XRD analysis, sorted by ash bed group, are presented in Table 2. The number of samples acquired from D1 is very limited, but still, the three ash bed group II samples, which were acquired from three different wells, indicate that the postdiagenetic mineralogical composition of the same ash bed is constant throughout the study area (Table 2). Consequently, the compositional similarities of samples acquired from the same ash bed group in D1 indicate that individual ash beds can be identified by mineralogy, and based on this they can be correlated between wells. This makes ash-bed correlation a promising high-precision correlation tool, at least in the D1. The number of individual ash beds in D2 is large and it is not, based on the currently available data, possible to correlate individual ash beds within the ash bed groups. Large variations in mineralogy exists within individual D2 ash bed groups, as e.g. evidenced by the three closely spaced Sif-1X ash bed group V samples (Table 2), which exhibit distinct variations in mineralogical composition. There are, however, several prominent ash beds in the D2, which can be identified in most of the study wells. These very prominent ash beds are associated with significant drops in porosity which can be correlated from well to well. The D1 porosity pattern can be correlated between wells (Fig. 10) with only minor thickness variations suggesting continuous pelagic sedimentation of the D1. In contrast, the D2 display large differences in the thickness indicating more unstable sedimentary conditions possibly connected
Fig. 9. Zircon crystals from volcanic ash beds. All pictures show zircons from samples treated with undiluted hydrochloric acid.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Nana-1XP 0.5
ASH_BEDS 0
0
1
0
ASH_BEDS
1
XRD sample
Non-calcite
THORIUM
CORE 10
Table 2 Results of the XRD analysis, sorted by ash bed group
POROSITY
DEPTH FT
773
0
10
PPM
6950
Quartz Ka/Chl. (%) (%)
Illite (%)
Smectite (%)
Ash bed group no.
Sif-1X, 67420 000 G-2X, 66500 1100 E-5X, 67610 800
12 11 12
5 6 4
22 20 19
61 63 65
II II II
Sif-1X, 67600 200 G-2X, 66690 800
32 31
9 7
19 16
39 46
IV IV
Sif-1X, 67660 900 Sif-1X, 67670 800 Sif-1X, 67700 100 S.E. Igor-1X, 66750 200 S.E. Igor-1X, 66780 500 E-5X, 67770 500 E-5X, 67790 E-5X, 67800 100
56 36 72 10 43 8 10 14
8 6 4 8 10 3 5 3
12 18 5 17 21 21 17 20
25 40 19 65 26 68 68 63
V V V V V V V V
Sif-1X, 67820 300 E-5X, 67870 100 E-5X, 67920 500
15 12 59
9 3 1
20 18 15
55 67 24
VII VII VII
Sif-1X, 68310 900 Sif-1X, 68340 1000 E-5X, 68180 600 E-5X, 68190
14 8 7 12
5 2 2 1
18 10 7 14
63 81 84 73
VIII VIII VIII VIII
The XRD analysis included five ash bed samples from D1 (ash bed groups II and IV) and 15 samples from D2 (ash bed groups V and VIII).
to the hiatus on top of the Cretaceous-Tertiary boundary (Ekdale and Bromley, 1984). 2.7. Diagenetic alteration of ash beds
7000
Fig. 10. Petrophysical log displaying porosity, thorium content and ash bed distribution in the Nana-1XP well. Ash beds marked with a solid line are prominent, whereas ash beds marked with a dashed line are less prominent. The cored interval of the well is indicated with the solid green line in depth track.
The bulk of the original volcanic ash consisted of glass, most of which was converted to smectite and silica during diagenesis. Zircon is inert to diagenesis, and is therefore preserved as a diagnostic component in the clay beds. During diagenesis volcanic glass is transformed into montmorillonite and silica, which is precipitated in the surrounding chalk. Mini-permeameter measurements on core-pieces across ash beds indicate that the diagenetic influence of ash beds on the surrounding chalk is very local, and that precipitation of material derived from the ash bed is probably confined to a few centimetres around the ash bed. Fig. 12 shows mini-permeameter measurements on a core-piece from Sif-1X across an ash bed and the surrounding chalk. However, the regional extent of individual ash beds plus the combined effect on permeability of ash beds and re-precipitated silica causes the ash beds to obstruct vertical flow across intervals with high ash bed concentrations. The high concentration of ash beds in the D2, consequently, causes the vertical permeability of the D2 to be significantly reduced. Chlorite and some illite were probably formed by diagenetic transformation of smectite. However, with a present day burial depth of approximately 2 km, the
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
774
G-2X G-1X TVD FT
0.5
PHI v/v
TVD FT
0.5
PHI v/v
Sif-1X
0
0
TVD FT
Nana-1X 6500
TVD FT
0.5
PHI v/v
0
0.5
PHI v/v
0
6600
0 I
6500
II
D1
III IV V VI VII
D2 6600 6900
6700
VIII 6600
Fig. 11. Correlation of ash beds in four wells from the study area. The numbers in the right side of the figure displays the Ash bed group numbers.
sediments have only just entered the illite-forming mesodiagenetic realm. Consequently, the depth is not necessarily great enough to facilitate effective illitization. Absence of potassium might also have been a limiting factor, as potassium is necessary for illite formation. 2.8. Possible provenance of the volcanic ash Several volcanic provinces in the North Atlantic area were active during the lower Palaeocene and could constitute the source of the ash beds in the Danian chalk (e.g. Holm et al., 1992; Chalmers et al., 1993; Chalmers et al., 1995; Clift et al., 1998; Dam and Nøhr-Hansen, 2001). Sea-floor spreading in the northern Atlantic ocean between the British Isles and Greenland, which commenced at the Palaeocene–Eocene boundary (at 53 Ma), was accompanied by intense volcanism in West Scotland and Northern Ireland (Thulean volcanics). The culmination of the Thulean extrusions occurred some 9 million years after
the termination of the Danian stage. However, volcanism was widespread during much of Palaeocene time (Chalmers et al., 1995). The earliest reported volcanics in western Scotland are dated to 62.8 Ma. The East Greenland volcanic province from Kangerlussuaq to Scoreby Sund culminated at around 58 Ma (Holm et al., 1992) but basalts from the East Greenland margin have been dated to 62.6 Ma (Clift et al., 1998) correlating with the Danian age of the ash layers in the North Sea. 3. Conclusion At least 86 layers of volcanic ash are present in the Lower Palaeocene Danian chalk in the Central Graben area. The D2 holds the majority of the ash beds, but ash beds are encountered in five well-defined stratigraphic intervals of the D1. Ash beds thickness varies from a few millimetres to at least 20 cm, the average thickness being around 2 cm.
ARTICLE IN PRESS 775
Ash bed
Ash bed
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Fig. 12. Mini-permeameter measurements in mD on a core-piece from Sif-1X across an ash bed and the surrounding chalk illustrate how the impact of silica released during the diagenetic alteration of volcanic glass on permeability is reduced significantly a few centimetres away from the ash beds. Core piece is 12 cm across.
Calcite constitutes about 75% of the bulk volume of most ash beds. The non-carbonate fraction of the ash beds predominantly consist of smectite (montmorillonite) with some glass, quartz and illite and minor amounts of kaolinite/chlorite and pyrite. Trace amounts of zircon are encountered. The distribution of ash beds displays a strong correlation to porosity, and to thorium content, which indicates that
the presence of ash beds influences the diagenetic alteration of the chalk. A total of nine ash bed groups, five in D1 and four in D2, have been identified by mineralogical composition and stratigraphic position. This makes the ash beds a promising correlation tool. The bulk of the original volcanic ash consisted of glass, most of which, during diagenesis, was converted to
ARTICLE IN PRESS 776
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
smectite and silica. The diagenetic influence of ash beds on the surrounding chalk is very local. However, vertical fluid flow is significantly reduced across intervals with high ash bed concentration. Several volcanic provinces in the North Atlantic area were active in the Palaeocene and could be the source of the ash beds in the North Sea Danian chalk. Acknowledgements We wish to thank Mærsk Olie og Gas AS for permission to publish the work presented in this paper. Furthermore, we wish to thank Ole Bjørslev Nielsen, University of Aarhus, for XRD analyses and discussions of the results. References Chalmers, J.A., Pulvertaft, T.C.R., Christiansen, F.G., Larsen, H.C., Laursen, K.H., Ottesen, T.G., 1993. The southern West Greenland continental margin: rifting history, basin development, and petroleum potential. In: Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe: Proceedings of the Fourth Conference. The Geological Society, London, pp. 915–931. Chalmers, J.A., Larsen, L.M., Pedersen, A.K., 1995. Widespread Palaeocene volcanism around the northern North Atlantic and Labrador Sea: evidence for a large, hot, early plume head. Journal of the Geological Society, London 152, 965–969.
Clift, P.D., Carter, A., Hurford, A.J., 1998. The erosional and uplift history of NE Atlantic passive margins: constraints on a passing plume. Journal of the Geological Society, London 155, 787–800. Dam, G., Nøhr-Hansen, H., 2001. Mantle plumes and sequence stratigraphy; Late Maastrichtian-Early Paleocene of West Greenland. Bulletin of the Geological Society of Denmark 48, 189–207. Ekdale, A.A., Bromley, R.G., 1984. Sedimentology and Ichnology of the Cretaceous-Tertiary Boundary in Denmark: implications for the causes of the terminal Cretaceous extinction. Journal of Sedimentary Petrology 54 (3), 681–703. Elliott, W. Crawford, Aronson, J.L., Millard, H.T., Gierlowski-Kordesch, E., 1989. The origin of the clay minerals at the Cretaceous/Tertiary boundary in Denmark. Geological Society of America Bulletin 101, 702–710. Holm, P.M., Hald, N., Nielsen, T.F.D., 1992. Contrasts in composition and evolution of Tertiary CFBs between West and East Greenland and their relations to the establishment of the Icelandic mantle plume. In: Storey, B.C., et al. (Eds.), Magmatism and the Causes of Continental Break-up. Geological Society Special Publication No. 68, pp. 349–362. Huggett, J., 1993. Petrology and diagenesis of Paleogene clays from Ølst and A˚lbækhoved, Denmark. Bulletin of the Geological Society of Denmark 40, 256–271. Knox, R.W.O’B., 1997. The late Palaeocene to early Eocene Ash Layers of the Danish Mo-clay (Fur Formation): stratigraphic and tectonic significance. In: Thomsen, E., Pedersen, S.A.S. (Eds.), Geology and Palaeontology of the Mo-clay, Aarhus. Aarhus Geoscience 6, 7–11. Svendsen, N., 1979. The Tertiary/Cretaceous chalk in the Dan Field of the Danish North Sea. In: Birkelund, T., Bromley, R.G. (Eds.), Cretaceous–Tertiary Boundary Events Symposium, vol. II. University of Copenhagen.
Marine and Petroleum Geology 23 (2006) 767–776 www.elsevier.com/locate/marpetgeo
Texture, composition and stratigraphy of volcanic ash beds in lower Palaeocene chalk from the North Sea Central Graben area Lars Simonsen, Jørgen Toft Mærsk Olie og Gas AS, Esplanaden 50, DK-1263 Copenhagen K, Denmark Received 6 October 2004; received in revised form 30 September 2005; accepted 14 June 2006
Abstract A large number of volcanic ash beds have been identified in Danian chalk from the Danish North Sea sector Central Graben area. The volcanic input was prominent during deposition of the lower part of the Danian chalk, whereas it was much less during deposition of the upper part of the Danian chalk. The non-carbonate fraction of the ash beds predominantly consists of smectite with some quartz, illite and minor amounts of kaolinite/chlorite and pyrite and trace amounts of zircon. Fragments of volcanic glass are common in the ash beds. The mineralogical composition of the ash beds varies from bed to bed, which makes ash bed correlation a promising high-precision correlation tool. Volcanic provinces that were active in connection with the opening of the North Atlantic Ocean are possible sources of the volcanic ash. The different composition of the ash layers could indicate that they originate from more than one source area. r 2006 Elsevier Ltd. All rights reserved. Keywords: Volcanic ash; Danian; Chalk; Central Graben; Smectite; Zircon; Glass; Correlation
1. Introduction
2. Ash beds
Volcanic ash layers are well established components of the late Palaeocene–early Eocene sediments in Denmark, e.g. in the Ølst, Røsnæs Clay and Lillebælt Clay Formations where ash beds occur interbedded with marine clays (Huggett, 1993; Knox, 1997). Similarly, volcanic ash is well known in the Mo-clay diatomite where they occur as prominent layers encountered in outcrops in Northern Jutland. Analyses of core material from wells drilled in the Central Graben area of the Danish North Sea sector (Fig. 1) show that numerous layers of volcanic ash are also present in the chalk from the Lower Palaeocene Danian stage. The ash beds are believed to originate from the volcanism associated with the initial phases of continental break up, which eventually, during the earliest Eocene, lead to the formation of the northern North Atlantic Ocean.
2.1. General stratigraphy and depositional setting
Corresponding author. Tel.: +45 33 36 37 79; fax: +45 33 36 37 83.
E-mail address: [email protected] (L. Simonsen). 0264-8172/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.06.003
In the Danish North Sea sector, the Danian chalk, which is comparable to the Ekofisk Formation in the northern North Sea, is divided into two units. An upper sub-unit, the D1 (Svendsen, 1979), which predominantly consists of relatively clean chalk, and a lower sub-unit, the D2 (Svendsen, 1979), which mainly consist of chalk with a higher content of argillaceous and siliceous material. Deposition of the Danian chalk occurred in an open marine environment, dominated by continuous slow deposition of coccolith debris and some pelagic foraminifers. Except for siliceous sponges, which appear to have been relatively abundant in some areas and intervals, macrofossils are rare. Common smectite rich clay beds attest to the occurrence of multiple events of short lasting deposition of wind borne volcanic ash, which, mixed with chalk particles, was deposited as thin distinct beds of a regional extent in the otherwise homogeneous chalk. The amount of ash beds identified in individual wells varies significantly. This,
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
768
E-5
G-1X
SIF-1X
G-2X
S.E.IGOR-1X
NANA-1XP MFA-4
Fig. 1. Study well location map of the Danish North Sea Central Graben area.
however, is likely to largely be related to variations in local overprints, such as small-scale erosion, reworking or bioturbation, all of which could presumably quickly remove the outline of a thin ash bed. The vast majority of ash beds are located in the D2, but ash beds are encountered in five well-defined stratigraphic intervals of the D1. The total number of ash beds in the Danian chalk section may well be over a hundred, but the largest amount of discernible ash beds so far encountered is 86 in the Sif-1X well. Ash beds have been encountered in all wells included in the study.
Fig. 2. Ash beds (A) in Danian chalk from the S.E. Igor well. Most of the ash beds in this interval are quite well defined. Most of the distinct thin dark streaks within the chalk intervals are Zoophycos isp. burrows.
The core chips consisted of chalk with variable contents of ash. The XRD samples were acquired from intervals with the highest concentration of ash.
2.2. Analytical methods
2.3. Textural aspects of ash beds at core scale
The results of this study are based on analysis of core material and evaluated petrophysical logs, acquired from seven exploration and appraisal wells drilled in the southern part of the Danish North Sea sector (Fig. 1). Core material from the seven wells has been analysed at various scales, including thin-section and visual core examination. Mini-permeameter measurements were performed on two slabbed core samples. The mineralogy of 20 clay rich core chips from four wells was analysed by X-ray diffraction (XRD). The following XRD analyses were performed:
In cores the ash beds appear as intervals with increased clay mineral content. The clay concentration of the ash layers varies significantly. In some cases, the ash beds are well defined and form true horizons (Fig. 2), whereas other ash beds appear as subtle discolorations of the chalk. It is impossible to conclusively demarcate grey chalk from very diluted ash beds. In many cases, the mixing effect of bioturbation is evident. Figs. 3–5 display examples of chalk–ash re-distribution by bioturbation. Only rarely are there indications of hardground formation below the clay beds, rather, different evidence suggests that in general the chalk was soft and un-compacted at the time of volcanic ash deposition. Often the base of ash beds display clear signs of being mixed with the chalk below it, indicating that the chalk was soft when the volcanic ash was emplaced. In other cases, the base of ash beds is sharp,
Quantitative XRD-mineralogy of total rock samples (bulk mineralogy). Quantitative XRD-clay mineral analyses of o2 mm fractions.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
769
Fig. 3. Mixture of Danian chalk and volcanic ash. Thallasinoides isp. burrow in ash bed, which has been filled with relatively pure chalk and later deformed during compaction and diagenesis of the sediment. When viewed on a slabbed core face (B) such burrows may easily be mistaken for embedded chalk clasts.
but generally the interface is smooth with little relief and without evidence of hardground formation. Fig. 6 displays two examples of different basal ash to chalk interfaces. The thickness of the ash beds varies from a few millimetres to at least 20 cm, the average thickness being around 2 cm. Presumably the thicker ash beds may comprise several individual ash beds. The ash bed thickness is, however, only weakly related to the original depositional thickness, as the majority of the ash beds have experienced some mixing with the surrounding chalk. The chalk immediately above and below ash beds generally has lower porosity than typical Danian chalk from the area. 2.4. Textural aspects of ash beds at thin section scale In thin sections, the ash beds typically appear as clayrich laminated bioclastic packstone. The lamination is generally caused by some compactional realignment of clay particles and elongated clasts. Cemented bed-parallel micro-fractures are moderately common. In some ash layers, a high content of spar-filled bioclasts masks the overall impression of a laminated rock. None of the larger clasts exhibit signs of rounding prior to deposition. Fig. 7 displays photomicrograph examples of the ash bed texture. Preserved glass fragments are common and show large variations in shape and sizes with maximum lengths of some 200 mm, but with significantly smaller average grain size (see Fig. 8). The glass fragments are translucent, and the colour ranges from almost colourless over pale brown to orange. The glass fragments are isotropic or display slight birefringence, with undulating extinction. Some glass
Fig. 4. Mixture of Danian chalk and volcanic ash. Zoophycos isp. burrow in chalk and Ash bed. It is unclear to which degree the coloration of the Zoophycos isp. burrows is primary, and to which it results from incorporation of ash. Generally the Zoophycos isp. seem to have avoided the ash beds as they are rarely encountered inside those.
fragments exhibit clear signs of alteration along the periphery. An example of this is displayed in Fig. 8. In general, however, the outlines of devitrified glass fragments have been obscured during diagenesis. An analogue preservation of glass fragments have been reported from the Palaeocene and Eocene ash beds in the Ølst Formation (Huggett, 1993). A large proportion of the calcitic tests of planktonic foraminifers inside the ash beds have been severely corroded or completely dissolved prior to being filled with calcitic spar, which in some instances appear to be ferrous. This indicates that volcanic ash probably had a significant impact on the stability of carbonate particles in its proximity. In thin sections, the visible effect of bioturbation of the ash layers is variable. In some samples, rare Chondrites isp. penetrate the uppermost millimetres of the ash layers, whereas burrows are rare in the deeper parts of the layers. In other samples, bioturbation is moderately common
ARTICLE IN PRESS 770
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Fig. 6. Clay beds in core. The base of the lower clay bed is clearly mixed with the chalk below it, indicating that the chalk was soft when the clay, or its precursor, was emplaced. The base of the upper clay bed is more sharp, but without evidence of hardground formation.
Fig. 5. Example of mixture of Danian chalk and volcanic ash. Diverse bioturbation in chalk has dispersed ash into the chalk, thereby causing the grey coloration which is typical of D2.
throughout the ash layers. The fill in burrows within the ash beds is commonly chalk. 2.5. Ash bed mineralogy In general, the ash bed samples look very clay-rich; however, the whole rock XRD analyses reveal that calcite is the dominating mineral even in the most clay rich samples, constituting about 75% of the bulk volume. The non-carbonate fractions of the ash bed samples predominantly consists of smectite with some quartz and illite and minor amounts of kaolinite/chlorite and pyrite. Trace amounts of zircon are encountered. The results of the XRD analyses of the non-carbonate fraction are displayed in Table 1. The mineralogic composition of the volcanic ash beds of this study compare well to the compositions reported from late Palaeocene to early
Eocene volcanic ash beds from mainland Denmark (e.g. Huggett, 1993). The smectite, primarily montmorillonite, is by far the most abundant clay mineral. The montmorillonite c-axes dimension swells to 17 A˚ on glycollation, and the peaks have an unusually large height/width ratio compared to sedimentary smectites. This indicates that the volcanic material was transformed in situ, or at least that the authigenic smectites derived from volcanic material has only experienced very minor transport (Ole Bjørslev Nielsen, Written communication). Similarly, Elliott et al. (1989) interpreted high-Mg smectite from the Danian outcrop in Limhamn and the Cretaceous-Tertiary boundary at Stevns Klint to be the result of alteration of pyroclastic basaltic ash. 2.5.1. Natural gamma ray spectrometry Natural gamma ray spectrometry logs indicate that the D2 has a distinctly higher thorium content than the underlying Maastrichtian chalk and the D1 above. Thorium is an insoluble trace element which is not associated with biogenic carbonates. Consequently, the higher concentration of Thorium in the D2 relative to the chalk intervals above and below reflects the relative higher abundance of non-carbonate material in this interval.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
771
Fig. 7. Typical ash bed lithologies. In both samples glass chards (brown grains) are dispersed in the matrix. In picture B, the clay minerals are concentrated in dark bands defining a distinct lamination of the rock. Photomicrographs in plane polarised light. Scale bars ¼ 100 mm.
Fig. 8. Picture A and B: partly dissolved glass particle. The blue arrow marks the approximate boundary of the original glass particle. The green arrow marks the boundary of the remaining glass fragment. Pictures C–F: glass fragments from volcanic ash beds. Scale bars ¼ 50 mm.
2.5.2. Zircon Zircon is a rare accessory mineral in the ash beds. The zircon crystals are micron sized, up to around 100 mm long, but generally smaller (10–20 mm) and euhedral. Zircons have been observed both in insoluble residue and in thin section. Fig. 9 displays four high magnification thin-section photomicrographs of zircon crystals. Zircon is a common accessory mineral of acid and intermediate igneous rocks. Detrital zircons are typically worn and abraded during transport, but in general the zircons from the ash beds show no signs of abrasion. Also, it is quite unlikely that zircon and smectite were deposited in the same bed by sedimentary processes, given the highly unequal hydraulic properties of the two minerals. 2.6. Ash bed stratigraphy The distribution of ash beds displays a strong correlation to porosity, as the majority of ash beds are associated with distinct porosity reductions. Likewise, the distribution of
ash beds displays a positive correlation to Thorium content. Fig. 10 displays a wireline log and ash bed distribution plot across the Danian interval of the Nana1XP well. The lower porosity of chalk in the vicinity of ash beds indicates that the diagenetic alteration of the chalk is influenced by the presence of ash. The repeatability of log signatures between wells suggests that even in wells where a particular ash bed has not been identified, but where the associated porosity and thorium signatures are present, the ash bed was deposited and did influence diagenesis. Based on their stratigraphic position, the ash beds have been clustered into nine groups. Five groups, numbered 0 to IV, have been identified in D1, and four groups, numbered IV to VIII, have been identified in D2. Fig. 11 shows the correlation of ash beds between four wells from the study area. In D1, the individual ash beds are separated by thick intervals of clean chalk, which makes the ash bed bearing intervals quite distinct. In D2, on the other hand, ash beds
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
772
are abundant, and chalk intervals without ash beds are few. Furthermore, the general greyish appearance of the D2 chalk demonstrates that reworking, largely by bioturbation, has caused significant redistribution and mixing of ash and chalk in D2, which has commonly led to complete obliteration of thin ash beds. Table 1 XRD analyses of the non-carbonate fraction, shows that the noncarbonate fraction of the ash bed samples predominantly consist of smectite with some quartz and illite and minor amounts of kaolinite/ chlorite and pyrite XRD sample
Non-carbonate fraction Quartz (%)
Ka/Chl. (%)
Illite (%)
Smectite (%)
G-2X, 66500 1100 G-2X, 66690 800
11 31
6 7
20 16
63 46
S.E. Igor-1X, 66750 200 S.E. Igor-1X, 66780 500
10 43
8 10
17 21
65 26
12 32 56 36 72 15 14 8
5 9 8 6 4 9 5 2
22 19 12 18 5 20 18 10
61 39 25 40 19 55 63 81
12 8 10 14 12 59 7 12
4 3 5 3 3 1 2 1
19 21 17 20 18 15 7 14
65 68 68 63 67 24 84 73
Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, Sif-1X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X, E-5X,
67420 000 67600 200 67660 900 67670 800 67700 100 67820 300 68310 900 68340 1000
67610 800 67770 500 67790 67800 100 67870 100 67920 500 68180 600 68190
The XRD analysis included five ash bed samples from D1 and 15 samples from D2. In D1, three samples were acquired from ash bed group II and two samples from ash bed group IV. In D2 eight samples were acquired from ash bed group V, three samples from ash bed group VII and four samples from ash bed group VIII. The results of the XRD analysis, sorted by ash bed group, are presented in Table 2. The number of samples acquired from D1 is very limited, but still, the three ash bed group II samples, which were acquired from three different wells, indicate that the postdiagenetic mineralogical composition of the same ash bed is constant throughout the study area (Table 2). Consequently, the compositional similarities of samples acquired from the same ash bed group in D1 indicate that individual ash beds can be identified by mineralogy, and based on this they can be correlated between wells. This makes ash-bed correlation a promising high-precision correlation tool, at least in the D1. The number of individual ash beds in D2 is large and it is not, based on the currently available data, possible to correlate individual ash beds within the ash bed groups. Large variations in mineralogy exists within individual D2 ash bed groups, as e.g. evidenced by the three closely spaced Sif-1X ash bed group V samples (Table 2), which exhibit distinct variations in mineralogical composition. There are, however, several prominent ash beds in the D2, which can be identified in most of the study wells. These very prominent ash beds are associated with significant drops in porosity which can be correlated from well to well. The D1 porosity pattern can be correlated between wells (Fig. 10) with only minor thickness variations suggesting continuous pelagic sedimentation of the D1. In contrast, the D2 display large differences in the thickness indicating more unstable sedimentary conditions possibly connected
Fig. 9. Zircon crystals from volcanic ash beds. All pictures show zircons from samples treated with undiluted hydrochloric acid.
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Nana-1XP 0.5
ASH_BEDS 0
0
1
0
ASH_BEDS
1
XRD sample
Non-calcite
THORIUM
CORE 10
Table 2 Results of the XRD analysis, sorted by ash bed group
POROSITY
DEPTH FT
773
0
10
PPM
6950
Quartz Ka/Chl. (%) (%)
Illite (%)
Smectite (%)
Ash bed group no.
Sif-1X, 67420 000 G-2X, 66500 1100 E-5X, 67610 800
12 11 12
5 6 4
22 20 19
61 63 65
II II II
Sif-1X, 67600 200 G-2X, 66690 800
32 31
9 7
19 16
39 46
IV IV
Sif-1X, 67660 900 Sif-1X, 67670 800 Sif-1X, 67700 100 S.E. Igor-1X, 66750 200 S.E. Igor-1X, 66780 500 E-5X, 67770 500 E-5X, 67790 E-5X, 67800 100
56 36 72 10 43 8 10 14
8 6 4 8 10 3 5 3
12 18 5 17 21 21 17 20
25 40 19 65 26 68 68 63
V V V V V V V V
Sif-1X, 67820 300 E-5X, 67870 100 E-5X, 67920 500
15 12 59
9 3 1
20 18 15
55 67 24
VII VII VII
Sif-1X, 68310 900 Sif-1X, 68340 1000 E-5X, 68180 600 E-5X, 68190
14 8 7 12
5 2 2 1
18 10 7 14
63 81 84 73
VIII VIII VIII VIII
The XRD analysis included five ash bed samples from D1 (ash bed groups II and IV) and 15 samples from D2 (ash bed groups V and VIII).
to the hiatus on top of the Cretaceous-Tertiary boundary (Ekdale and Bromley, 1984). 2.7. Diagenetic alteration of ash beds
7000
Fig. 10. Petrophysical log displaying porosity, thorium content and ash bed distribution in the Nana-1XP well. Ash beds marked with a solid line are prominent, whereas ash beds marked with a dashed line are less prominent. The cored interval of the well is indicated with the solid green line in depth track.
The bulk of the original volcanic ash consisted of glass, most of which was converted to smectite and silica during diagenesis. Zircon is inert to diagenesis, and is therefore preserved as a diagnostic component in the clay beds. During diagenesis volcanic glass is transformed into montmorillonite and silica, which is precipitated in the surrounding chalk. Mini-permeameter measurements on core-pieces across ash beds indicate that the diagenetic influence of ash beds on the surrounding chalk is very local, and that precipitation of material derived from the ash bed is probably confined to a few centimetres around the ash bed. Fig. 12 shows mini-permeameter measurements on a core-piece from Sif-1X across an ash bed and the surrounding chalk. However, the regional extent of individual ash beds plus the combined effect on permeability of ash beds and re-precipitated silica causes the ash beds to obstruct vertical flow across intervals with high ash bed concentrations. The high concentration of ash beds in the D2, consequently, causes the vertical permeability of the D2 to be significantly reduced. Chlorite and some illite were probably formed by diagenetic transformation of smectite. However, with a present day burial depth of approximately 2 km, the
ARTICLE IN PRESS L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
774
G-2X G-1X TVD FT
0.5
PHI v/v
TVD FT
0.5
PHI v/v
Sif-1X
0
0
TVD FT
Nana-1X 6500
TVD FT
0.5
PHI v/v
0
0.5
PHI v/v
0
6600
0 I
6500
II
D1
III IV V VI VII
D2 6600 6900
6700
VIII 6600
Fig. 11. Correlation of ash beds in four wells from the study area. The numbers in the right side of the figure displays the Ash bed group numbers.
sediments have only just entered the illite-forming mesodiagenetic realm. Consequently, the depth is not necessarily great enough to facilitate effective illitization. Absence of potassium might also have been a limiting factor, as potassium is necessary for illite formation. 2.8. Possible provenance of the volcanic ash Several volcanic provinces in the North Atlantic area were active during the lower Palaeocene and could constitute the source of the ash beds in the Danian chalk (e.g. Holm et al., 1992; Chalmers et al., 1993; Chalmers et al., 1995; Clift et al., 1998; Dam and Nøhr-Hansen, 2001). Sea-floor spreading in the northern Atlantic ocean between the British Isles and Greenland, which commenced at the Palaeocene–Eocene boundary (at 53 Ma), was accompanied by intense volcanism in West Scotland and Northern Ireland (Thulean volcanics). The culmination of the Thulean extrusions occurred some 9 million years after
the termination of the Danian stage. However, volcanism was widespread during much of Palaeocene time (Chalmers et al., 1995). The earliest reported volcanics in western Scotland are dated to 62.8 Ma. The East Greenland volcanic province from Kangerlussuaq to Scoreby Sund culminated at around 58 Ma (Holm et al., 1992) but basalts from the East Greenland margin have been dated to 62.6 Ma (Clift et al., 1998) correlating with the Danian age of the ash layers in the North Sea. 3. Conclusion At least 86 layers of volcanic ash are present in the Lower Palaeocene Danian chalk in the Central Graben area. The D2 holds the majority of the ash beds, but ash beds are encountered in five well-defined stratigraphic intervals of the D1. Ash beds thickness varies from a few millimetres to at least 20 cm, the average thickness being around 2 cm.
ARTICLE IN PRESS 775
Ash bed
Ash bed
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
Fig. 12. Mini-permeameter measurements in mD on a core-piece from Sif-1X across an ash bed and the surrounding chalk illustrate how the impact of silica released during the diagenetic alteration of volcanic glass on permeability is reduced significantly a few centimetres away from the ash beds. Core piece is 12 cm across.
Calcite constitutes about 75% of the bulk volume of most ash beds. The non-carbonate fraction of the ash beds predominantly consist of smectite (montmorillonite) with some glass, quartz and illite and minor amounts of kaolinite/chlorite and pyrite. Trace amounts of zircon are encountered. The distribution of ash beds displays a strong correlation to porosity, and to thorium content, which indicates that
the presence of ash beds influences the diagenetic alteration of the chalk. A total of nine ash bed groups, five in D1 and four in D2, have been identified by mineralogical composition and stratigraphic position. This makes the ash beds a promising correlation tool. The bulk of the original volcanic ash consisted of glass, most of which, during diagenesis, was converted to
ARTICLE IN PRESS 776
L. Simonsen, J. Toft / Marine and Petroleum Geology 23 (2006) 767–776
smectite and silica. The diagenetic influence of ash beds on the surrounding chalk is very local. However, vertical fluid flow is significantly reduced across intervals with high ash bed concentration. Several volcanic provinces in the North Atlantic area were active in the Palaeocene and could be the source of the ash beds in the North Sea Danian chalk. Acknowledgements We wish to thank Mærsk Olie og Gas AS for permission to publish the work presented in this paper. Furthermore, we wish to thank Ole Bjørslev Nielsen, University of Aarhus, for XRD analyses and discussions of the results. References Chalmers, J.A., Pulvertaft, T.C.R., Christiansen, F.G., Larsen, H.C., Laursen, K.H., Ottesen, T.G., 1993. The southern West Greenland continental margin: rifting history, basin development, and petroleum potential. In: Parker, J.R. (Ed.), Petroleum Geology of Northwest Europe: Proceedings of the Fourth Conference. The Geological Society, London, pp. 915–931. Chalmers, J.A., Larsen, L.M., Pedersen, A.K., 1995. Widespread Palaeocene volcanism around the northern North Atlantic and Labrador Sea: evidence for a large, hot, early plume head. Journal of the Geological Society, London 152, 965–969.
Clift, P.D., Carter, A., Hurford, A.J., 1998. The erosional and uplift history of NE Atlantic passive margins: constraints on a passing plume. Journal of the Geological Society, London 155, 787–800. Dam, G., Nøhr-Hansen, H., 2001. Mantle plumes and sequence stratigraphy; Late Maastrichtian-Early Paleocene of West Greenland. Bulletin of the Geological Society of Denmark 48, 189–207. Ekdale, A.A., Bromley, R.G., 1984. Sedimentology and Ichnology of the Cretaceous-Tertiary Boundary in Denmark: implications for the causes of the terminal Cretaceous extinction. Journal of Sedimentary Petrology 54 (3), 681–703. Elliott, W. Crawford, Aronson, J.L., Millard, H.T., Gierlowski-Kordesch, E., 1989. The origin of the clay minerals at the Cretaceous/Tertiary boundary in Denmark. Geological Society of America Bulletin 101, 702–710. Holm, P.M., Hald, N., Nielsen, T.F.D., 1992. Contrasts in composition and evolution of Tertiary CFBs between West and East Greenland and their relations to the establishment of the Icelandic mantle plume. In: Storey, B.C., et al. (Eds.), Magmatism and the Causes of Continental Break-up. Geological Society Special Publication No. 68, pp. 349–362. Huggett, J., 1993. Petrology and diagenesis of Paleogene clays from Ølst and A˚lbækhoved, Denmark. Bulletin of the Geological Society of Denmark 40, 256–271. Knox, R.W.O’B., 1997. The late Palaeocene to early Eocene Ash Layers of the Danish Mo-clay (Fur Formation): stratigraphic and tectonic significance. In: Thomsen, E., Pedersen, S.A.S. (Eds.), Geology and Palaeontology of the Mo-clay, Aarhus. Aarhus Geoscience 6, 7–11. Svendsen, N., 1979. The Tertiary/Cretaceous chalk in the Dan Field of the Danish North Sea. In: Birkelund, T., Bromley, R.G. (Eds.), Cretaceous–Tertiary Boundary Events Symposium, vol. II. University of Copenhagen.