K-Ar age determinations on tertiary volcanic rocks: 5. Siebengebirge, Siebengebirge-Graben

Sanidines from 16 tuffs of the first volcanic phase in the Siebengebirge and the Siebengebirge-Graben were dated by the K/Ar-method. Ages range from 24.1 (Nachtigallental) to 22.9 M.y. (core Rott). Three mineral ages from surface outcrops in the Central Siebengebirge (23.9 ± 0.5 M.y.) are statistically indistinguishable from those of the sub-surface samples Stieldorf-1, Stieldorf-2, and Rott (23.0 ± 0.5 M.y.). These ages confirm the Uppermost Oligocene biostratigraphic age of the 'Blatterkohle Rott' which is interlayered with the trachyte tuff. Sanidine and biotite mineral ages from five trachyte samples give similar apparent ages from 26.4 to 24.6 M.y. These results show the trachytes to be older than the tuffs, in disagreement with the geologically established sequence. The sequence of eruptions in the Siebengebirge area however (trachyte-tuff, trachyte, latite and alkaline-basalt) was confirmed wherever outcrops allow observations. This discrepancy is discussed. The apparent ages of three dated latites are in the same range (26.2 to 25.1 M.y.), in agreement with the geological sequence. The latite ages show that the time span between the eruptions of the trachytes and the latites must have been very small. Isolated basalts belonging to a fourth phase yield K/Ar ages from 25 to 19 M.y. Pliocene volcanism could not be verified in the Siebengebirge. In addition, stratigraphically well-dated glauconites from the Tertiary of the Niederrhein area were used to correlate our isotopic age date with the Cenozoic time scale.           ARK: https://n2t.net/ark:/88439/y070559 Permalink: https://geophysicsjournal.com/article/259 &nbsp