Facies and geochemical aspects of the Dolomite-Anhydrite Transition Zone (Zechstein 1-2) in the Batum-13 well, northern Jutland, Denmark: a key to the evolution of the Norwegian-Danish Basin

The Dolomite-Anhydrite Transition Zone is an important marker horizon separating the thick successions of grey halite of the Zechstein 1 and 2 cycles in the central parts of the Norwegian-Danish Basin. Within this unit, six main facies have been recognised in the Batum-13 well in northern Jutland. These can be grouped into three facies associations which, due to their physical log characteristics can be traced over large parts of the basin. The three facies associations reflect major events in the evolution of the basin, which are closely related to the evolution seen in the southern part of the Zechstein Basin as reflected by the Stassfurt carbonates (Ca2) and the Basal Anhydrite (A2). Hence, facies association 1 consisting of bedded massive anhydrite and carbonate conglomerates, records the transgression heralding the Zechstein 2 cycle, whereby the brines of the basin went through a recessive phase (Alr). Facies association 2 is dominated by dark bituminous carbonate rhythmites with nodules of anhydrite and marks the beginning of the Zechstein 2 cycle; it reflects the maximum transgression of the Zechstein sea (Ca2). Continued rising salinities probably led to the formation of a sulphate platform as reflected by the resedimented sulphates in the bottom of facies association 3 (A2). Sulphates, now seen as distorted bedded mosaic anhydrite in the upper part of facies association 3, were subsequently precipitated over the entire basin. Ultimately, halite precipitation (Na2) returned to the basin.