U-Pb zircon geochronology of the Siebengebirge Volcanic Field – Implications for the dynamics of the Lower Rhine Basin (Germany)

The Oligocene Siebengebirge Volcanic Field (SVF) is located at the south-eastern termination of the extensional Lower Rhine Basin (LRB). Tuffitic deposits, repeatedly embedded within the siliciclastic basin sediments, provide a largely untapped record on the evolution of intracontinental volcanism and basin dynamics. With the aim to provide radiometric age constraints, we applied laser-ablation U-Pb zircon geochronology on trachytic rocks from Siebengebirge surface outcrops, and tuffitic layers within drill cores of the Köln Formation.

While the lithostratigraphic sequence of the LRB is perfectly established [1], previous attempts using K-Ar on trachyte tuff sandine returned conflicting results, possibly due to diagenetic alteration [2]. Sampled tuffitic layers within the Oligocene of the Köln Formation (archived drill cores Haus Ölgarten and Niederpleis, GD NRW) returned U-Pb zircon populations from the Horizons 05/06 transition, as well as from Horizon 08 to Horizon 1, that are identical to those from the Siebengebirge trachytic outcrops.

In accord with previous results from Ar-Ar sanidine [3], U-Pb zircon geochronology confirms that differentiated volcanism in the SVF is restricted to a single, relatively short time period. First results for the Köln Formation suggest that the trachytic volcanism should indeed be placed at the base of Horizon 06 within the lithological standard profile, whereas similar material in younger horizons had probably been reworked. As a consequence, the voluminous trachytic SVF volcanism seems to coincide with accelerated subsidence of the rift basin.

[1] Schäfer et al. (2004) Newsl Stratigr; [2] Todt & Lippolt (1980) J Geophys; [3] Przybyla et al. (2018) IJES.