Geological Survey of Denmark and Greenland Bulletin 35, 2016, 51-54


51© 2016 GEUS. Geological Survey of Denmark and Greenland Bulletin 35, 51–54. Open access:  www.geus.dk/publications/bull

The chemical composition of chalk and marl reflects the 

mixture of carbonate particles and clastic input deposited 

on the seabed together with growth of authigenic minerals 

and diagenesis. The Rørdal quarry in Jylland (Fig. 1) is 

known for its alternating chalk–marl succession (Surlyk et 

al. 2010) and the aim of this article is to investigate how 

this cyclicity is reflected in the geochemical signature of 

the sequence and test if this has implications for the in-

terpretation of the depositional environment as well as the 

chemostratigraphy in the chalk. 

 The observed variation in the benthic fauna and the cy-

clic character of the chalk–marl succession may reflect an 

environmental response to orbital forcing. The benthic fauna 

shows higher species diversity and density in the chalk than 

the marl layers, suggesting more favourable living conditions 

in the former (Lauridsen & Surlyk 2008), whereas no differ-

ences in environmental stress between the two environments 

could be derived from a study of the trace fossils (Lauridsen 

et al. 2011). 

 The c. 10 m thick Rørdal Member was established as 

a lithostratigraphic unit by Surlyk et al. (2010) and recog-

nised as an expression of a late Maastrichtian cooling event. 

X-ray diffraction (XRD) analysis indicates that the marly 

layers represent a relative increase in smectite clays and il-

lite together with some quartz and analcime. The inorganic 

geochemical evolution in Maastrichtian chalk was investi-

gated by Jørgensen (1986) who noted changes in Sr/Ca and 

Mn/Ca ratios towards the end of the Maastrichtian in the 

North Sea Central Graben, leading to the conclusion that 

geochemistry is a “conceivable tool in basin analysis in the 

lithologically rather monotonous chalk sequence” (p. 267). 

However, a systematic geochemical analysis of the chalk 

has never been undertaken and this work is an attempt to 

look into the potential of understanding the geology of the 

chalk by analysing a wide range of elements in chalk and marl. 

Chemical variability related to clay content
Sixty-three samples from the Rørdal quarry were ground, 

digested in aqua regia and analysed for 66 elements using 

the Elan 6100 Quadrupole ICP-MS at the Geological Sur-

vey of Denmark and Greenland (GEUS). The data reduc-

tion was based on TotalQuant software with emphasis on 

the trace element analysis. The analytical procedure fol-

lows Larsen et al. (2009). 

 The cyclicity and the alternation between chalk and 

marl are easily recognised in the geochemical profile (Fig. 

2) with an increase in the concentration of major and minor 

elements such as Al, Si, Fe, Mg and K in the marly layers 

caused by the presence of clay. The eight peaks in Fig. 2 can 

be correlated to the eight peaks in the marly layers of the 

Rørdal Member which can be identified in the gamma log 

in a nearby well (Surlyk et al. 2010). The gamma-radiation 

in the well log can be explained by the increased content of 

K in the marl (Fig. 2A). Iron is the element after Al with 

the highest increase in concentration in the marly layers 

indicating that the clay minerals are rich in Fe. Magnesium 

also shows an increase in the marly layers (Fig. 2A), and the 

combined clay minerals in the marl – which according to 

Surlyk et al. (2010) consist of smectite and illite – must be 

rich in Fe, K and Mg. The content of Al, Fe and K is about 

three times higher in the marl than in the chalk. 

Geochemistry of the Maastrichtian Rørdal Member, 
Jylland, Denmark: Ce anomaly as a palaeo-redox proxy

Christian Knudsen and Bodil W. Lauridsen

Denmark Sweden

50 kmGermany

Jylland

Møns Klint

Stevns Klint

Rørdal quarry
N

 

Lake

Profile

1 km

10°E

55°

57°N

A

L
o
w

e
r

U
p
p
e
r

M
aa

st
ri

c
th

ia
n

P
al

e
o
c
e
n
e

D
an

ia
n

Fiskeler Mb

Rørdal Mb

C

B

Fig. 1. A: Location of the Rørdal quarry, Stevns Klint and Møns Klint 
in Denmark. B: Map of the Rørdal quarry with the profile where the 
samples were collected. C: Stratigraphic section with Rørdal and Fiskeler 
members. 



5252

 The concentration of many trace elements, including the 

rare-earth elements (REE), is also elevated in the marly lay-

ers (Fig. 2B). If all REE were located in clay the increase 

would be three times higher, as is the case with Al, K and 

Fe, but the content of REE in the marl is ‘only’ 1.5 times 

higher than in the chalk. This suggests that although the 

REE concentration is higher in the clay than in the chalk 

a substantial fraction of the REEs is located in the carbon-

ate component of both chalk and marl. Lanthanum and 

Nd follow parallel tracks (Fig. 2B), whereas Ce crosses both 

tracks with relatively high Ce in the marl. Light REE (La, 

Ce, Pr, Nd and Sm) are more abundant than heavy REE 

(Er, Tm, Yb and Lu) in the marl compared to the chalk, 

which can also be seen as higher La/Lu ratios (Figs 3E, 4). 

 Lithophile elements such as Th, Cs, Rb, Be, Cr, Sc and 

Li increase in concentrations in the marl (of which Rb and 

Li are shown in Figure 2C) and are c. 2.5 to 3 times higher 

in the marl than in the chalk, similar to the behaviour of 

Al and K. This suggests that these elements are almost en-

tirely located in the clay. Chalcophile elements such as Pb, 

As, Zn and Cu (Fig. 2D) are c. 1.5 times higher in the marl 

than in the chalk similar to the REE mentioned above. 

 The content of most elements in the succession varies 

with the clay content. However, this is not the case for Nb, 

Zr, Mn and Ti (Fig. 3A). Mica and clay commonly contain 

some Ti, Zr and Nb, elements that also occur in mafic 

volcanic material. These elements are difficult to leach out 

of minerals and generally follow clay and mica in sedimen-

tary environments. If the clay was clastic or volcanogenic 

one would expect that the content of elements such as Ti, 

Zr and Nb would be elevated in the marl. However, this is 

not the case, suggesting that a fraction of the clay may be 

of authigenic origin. 

 Manganese is probably hosted in the carbonate and its 

concentration is relatively stable throughout the analysed 

section. There is a decrease in the Ba content in the Rørdal 

Member (Fig. 3B) relative to the underlying Hvidskud 

Member with the lowest contents in its middle part. There 

is an increase in Sr content in the overlying Sigerslev Mem-

ber. Jørgensen (1986) observed a similar increase in the Sr 

content in chalk from the North Sea Basin and this may be 

regional.

Ce anomaly as a proxy for redox potential
The REE commonly occur as cations with a valency of 

three and the different REE have similar geochemical be-

haviour in the sedimentary environment. However, one of 

the elements, Ce, can also occur as Ce4+ with a very low 

solubility in marine environments as compared to Ce3+. 

Where no fractionation of Ce relative to the other REE 

has occurred, the distribution would be seen as a straight 

line on Fig. 3D. To quantify the fractionation of Ce, the 

term Ce anomaly (Ce/Ce*) has been introduced; Ce is the 

measured cerium concentration and Ce* is what the Ce 

content would have been without fractionation, based on 

the contents of La and Nd. The Ce anomaly is calculated: 

Ce/Ce* = 3CeN/(2LaN + NdN) where REEN is the measured 

concentration normalised relative to chondrite (Boynton 

1984). In this paper a Ce anomaly is referred to as nega-

tive when Ce is depleted relative to the other REE and 

as positive if Ce is enriched relative to the other REE. In 

0

1
0
0
0

2
0
0
0

3
0
0
0

4
0
0
0

5
0
0
0

K M
g

F
e

A
l

S
i

A

ppm

1
.0 1
0

B

Y
b

D
y

G
d

S
m

N
d

P
r

C
e

L
a

0
.1

1
.0 1
0

C
R

b

G
a

C
r

VS
c

L
i

0
.1 1
0

1
.0

D

P
b

A
s

C
u

N
i

C
o

Z
n

0

5

10

15

m
 (

h
e
ig

h
t)

U
p
p
e
r 

M
aa

st
ri

c
h
ti
an

R
ø
rd

al
 M

b
S
ig

e
rs

le
v
 M

b
H

v
id

sk
u
d
 M

b
Fig. 2. Chemical profiles through the Rørdal 

Member including the uppermost part of the 

Hvidskud Member and the lowermost part 

of the Sigerslev Member. Marly layers are 

shown as grey shaded bands.  A: Major and 
minor elements. B: REE; logarithmic scale. C: 
Lithophile trace elements; logarithmic scale. 

D: Chalcophile elements with Ni and Co; 
logarithmic scale. Stratigraphy from Surlyk et 

al. (2013).



53

the modern marine environment, a negative Ce anomaly is 

indicative of oxic conditions (German & Elderfield 1990) 

which, in turn, is caused by the low solubility of Ce4+ and 

low availability of Ce. This feature offers the possibility to 

look into variations in the redox conditions during deposi-

tion of the chalk (Jeans et al. 2015). In Fig. 4 it can be seen 

that there is a negative Ce anomaly both in the chalk and 

in the marl in the Rørdal quarry as well as in the chalk and 

the fish-clay at Stevns. There is also a negative Eu anomaly 

which is probably caused by the source of the REE being 

slightly depleted in Eu (Frei & Frei 2002). The negative Ce 

anomaly in the chalk at Stevns is more pronounced than 

in the chalk at Rørdal (Fig. 4) suggesting that the environ-

ment during deposition of the chalk at Stevns was more 

oxic than at Rørdal. The chalk at Stevns has a higher strati-

graphic position (in the Sigerslev Member) and the change 

in the Ce anomaly with time could suggest that the oxygen 

availability in the chalk sea increased with time during the 

late Maastrichtian. Alternatively, this could suggest that 

the chalk at Stevns was deposited in a shallower sea. 

 The Ce concentration at Rørdal changes with lithology 

and stratigraphic position in the section (Fig. 3D); it is low-

er in the chalk than in the marl, indicating that the envi-

ronment was more oxic in the chalk than in the marl. The 

REE (including Ce) are located in the carbonate compo-

nent as well as in the clay as mentioned above. Accordingly, 

the Ce anomaly reflects the combined effect of the compo-

sition of the seawater where the coccoliths and where the 

clay were formed. Surlyk et al. (2010) suggested that the 

clay in the Rørdal Member is derived from volcanic erup-

tions. Such clay, derived from alteration of volcanic glass, is 

likely to have an overall resemblance to the REE distribu-

tion in the source. However, the transformation from glass 

to clay in the marine environment could be the cause of the 

negative Ce anomaly found in the marl. The fish-clay at 

Stevns also has this negative Ce anomaly which is indica-

tive of formation of clay in a marine environment (Kastner 

et al. 1984; Frei & Frei 2002). The question is then whether 

the clay was (trans)formed from volcanic glass in the free 

water masses or in the seabed. The seabed is likely to have 

been less oxic than the free water mass, and formation of 

the clay in the seabed would explain the difference in the 

redox potential compared to the chalk formed in the free 

water mass. Finally, the composition of the clay minerals 

could have been modfied during diagenesis in the presence 

of organic material affecting the Ce anomaly.

0
.0

1

1
.0

1
0
0

A

N
b

Z
r

R
b

M
n

T
i

ppm

0

2
0

4
0

B

B
a

0

5
0
0

1
0
0
0

1
5
0
0

C

S
r

0

0
.2

0
.4

0
.6

D

Ce anomaly

4
0

8
0

1
2
0

E

L
a/

L
u

0

10

15

m
 (

h
e
ig

h
t)

5
U

p
p
e
r 

M
aa

st
ri

c
h
ti
an

R
ø
rd

al
 M

b
S
ig

e
rs

le
v
 M

b
H

v
id

sk
u
d
 M

b

Fig. 3. Chemical profiles in the Rørdal Mem-

ber. Same stratigraphy as Fig. 1. Marly layers 

are shown as grey shaded bands. A: Nb, Zr, 
Rb, Mn and Ti; logarithmic scale. B: Ba. C: 
Sr. D: Ce anomaly. E: La/Lu ratio; logarithmic 
scale.

Fish Clay, Stevns

Chalk, Stevns

Marl, Rørdal

Chalk, Rørdal

La

Ce Nd Sm Gd Dy Er Yb

LuPr Pm Eu Tb Ho Tm

1
0

S
am

p
le

/R
E
E
 c

h
o
n
d
ri

te
1
0
0

Fig. 4. Chondrite-normalised REE distribution patterns for the Rørdal 

samples compared with REE data from Stevns Klint (Frei & Frei 2002). 



5454

Summary and outlook
The chemical composition of the chalk–marl sequence at 

Rørdal in Jylland reflects the varying proportions of carbon-

ate and clay. The content of major and minor elements such 

as Si, Al, Fe and K is proportional to the clay content, with 

a threefold increase in the marly layers relative to the chalk 

layers. It is suggested that these elements are located in clay 

minerals together with lithophile trace elements such as Li, 

Ga, Rb, Cs and Th which likewise increase threefold in the 

clay relative to the chalk. Other elements such as Pb, As, Zn 

and Cu and the REE are found in c. 1.5 times higher con-

centrations in the marl, and these elements are located both 

in the clay and the carbonate component of the succession. 

The clay has a light REE enrichment compared to the chalk. 

 Formation of the clay as authigenic minerals may explain 

why large ion lithophile elements such as Zr and Nb, as well 

as Ti, are not incorporated in the clay. The content of Ba in 

the Rørdal Member is low relative to the under- and over-

lying strata, and the deposition of Ba could be tested as a 

chemo-stratigraphic marker and environmental proxy.

 The REE distribution shows a negative Ce anomaly 

both in the chalk and in the marl, suggesting that the en-

vironment was oxic throughout the deposition, and as the 

Ce anomaly is more pronounced in the chalk, the environ-

ment was more oxic where the carbonate was formed than 

in the clay. This observation matches the observation that 

there is higher species diversity and density in the chalk 

than in the marl (Lauridsen & Surlyk 2008). If the clay 

and the carbonate were formed in the same water mass, 

the observed alternating redox level would be indicative of 

changing conditions in the sea. However, if the main part 

of the clay was formed in the seabed as authigenic minerals 

or was affected by diagenesis, then the alternating redox 

levels indicated by the Ce anomaly reflects changes in redox 

conditions at or in the seabed. The Ce anomaly in chalk 

from Stevns is larger than in chalk from Rørdal, suggesting 

that the environment was more oxic in the chalk sea to-

wards the end of the Maastrichtian. These differences could 

be related to changes in temperature over time. A recent 

study by Thibault et al. (2016) suggests a global cooling 

with superimposed cool/warm fluctuations in the last 8 Ma 

of the Maastrichtian including e.g. the Rørdal and Sigerslev 

Members. It is suggested that the Ce anomaly can be used 

as a palaeo-redox indicator or proxy to compare environ-

ments across chalk basins. In this context it is interesting to 

analyse both carbonate and clay components from the marl. 

Acknowledgement
We wish to thank Lars Stemmerik for constructive comments on earlier 

versions of the manuscript. 

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Authors’ address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark, E-mail: ckn@geus.dk