The cyclic Rørdal Member – a new lithostratigraphic unit of chronostratigraphic and palaeoclimatic importance in the upper Maastrichtian of Denmark

Abstract

The Maastrichtian chalk of the Danish Basin has been referred to the Tor Formation of the North Sea, but this may not be tenable because this formation in its type area shows a much higher degree of redeposition than the Maastrichtian chalk of the Danish Basin. The onshore succession has not been lithostratigraphically subdivided due to its rather monotonous nature and the widely scattered outcrops. An exception is the uppermost Maastrichtian exposed at Stevns Klint which is been referred to the Sigerslev Member, comprising rather benthos-poor, deep-water pure chalk, and the overlying mound-bedded, bryozoan-rich chalk which is placed in the Højerup Member. In addition, a thin marly chalk bed, the Kjølby Gaard Marl Member, containing Tethyan planktonic foraminifers is known from localities in northern Jylland and from water wells around Køge, eastern Sjælland. The new Rørdal Member is a cyclic chalk-marl unit, about 10 m thick, sandwiched between pure white chalks. It is well exposed in the large Rørdal quarry in Aalborg, and is recognised in boreholes south of Aalborg and in the Stevns-1 and Karlslunde-1 boreholes south of Copenhagen. Coccolith and brachiopod data show that it belongs to the UC20b-cBP nannofossil zone of the North Sea scheme for the Upper Cretaceous Boreal province, and the semiglobularis-humboldt iibrachiopod zone, both indicating the lower upper Maastrichtian. Isotope data show that it represents a distinct early late Maastrichtian cooling event. The member thus has a basinwide distribution and is an important isochronous marker.

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

“…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.…”

“…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.…”

“…Accordingly, the Ce anomaly reflects the combined effect of the composition 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 eruptions. Such clay, derived from alteration of volcanic glass, is likely to have an overall resemblance to the REE distribution in the source.…”

“…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.…”

“…The c. 10 m thick Rørdal Member was established as a lithostratigraphic unit by Surlyk et al (2010) and recognised 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 illite together with some quartz and analcime.…”