Abstract
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 interpretation of the depositional environment as well as the chemostratigraphy in the chalk.
Highlights
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 rare-earth elements (REE) are found in c. 1.5 times higher concentrations in the marl, and these elements are located both in the clay and the carbonate component of the succession
Summary
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 Survey of Denmark and Greenland (GEUS). The cyclicity and the alternation between chalk and marl are 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. 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 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
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