In-situ U-Pb dating of Ries Crater lacustrine carbonates (Miocene, South-West Germany) : implications for continental carbonate chronostratigraphy

The Nördlinger Ries Crater lacustrine basin (South-West Germany), formed by a meteorite impact in the Miocene (Langhian; ∼14.9Ma), offers a well-established geological framework to understand the strengths and limitations of U-Pb LA-ICPMS (in situLaser Ablation-Inductively Coupled Plasma Mass Spectrometry) geochronology as chronostratigraphic tool for lacustrine (and more broadly continental) carbonates. The post-impact deposits include siliciclastic basinal facies at the lake centre and carbonate facies at the lake margins, coevally deposited in a time window of >1.2 and <2Ma. Depositional and diagenetic carbonate phases (micrites and calcite cements) were investigated from three marginal carbonate facies (Hainsfarth bioherm, Adlersberg bioherm and Wallerstein mound). Petrography combined with C and O stable isotope analyses indicate that most depositional and early diagenetic carbonates preserved pristine geochemical compositions and thus the U-Pb system should reflect the timing of original precipitation. In total, 22 U-Pb ages were obtained on 10 different carbonate phases from five samples. The reproducibility and accuracy of the U-Pb (LA-ICPMS) method were estimated to be down to 1.5% based on repeated analyses of a secondary standard (speleothem calcite ASH-15d) and propagated to the obtained ages. Micrites from the Hainsfarth, Adlersberg and Wallerstein facies yielded ages of 13.90 ±0.25, 14.14 ±0.20 and 14.33 ±0.27Ma, respectively, which overlap within uncertainties, and are consistent with the weighted average age of 14.30 ±0.20Ma obtained from all the preserved depositional and early diagenetic phases. Data indicate that sedimentation started shortly after the impact and persisted for >1.2 and <2Ma, in agreement with previous constraints from literature, therefore validating the accuracy of the applied method. Later calcite cements were dated at 13.2 ±1.1 (nw=2), 10.2 ±2.7 and 9.51 ±0.77Ma, implying multiple post-depositional fluid events. This study demonstrates the great potential of the U-Pb method for chronostratigraphy in continental systems, where correlations between time-equivalent lateral facies are often out of reach. In Miocene deposits the method yields a time resolution within the 3rd order depositional sequences (0.5–5Ma).