U-Pb direct dating on calcite paleosol nodules: first absolute age constraints on the Miocene continental succession of the Paris Basin

Abstract. Continental sedimentary successions are typically less complete and more poorly preserved than the marine record, leading to limited correlations between basins. Traditionally, intra-basin correlation employs radiometric dating of volcanic markers or relative dating based on the fossil record. However, volcanic markers may not always be present, and biostratigraphy relies on index fossils which are often sparse to absent in continental successions. Recent progress in carbonate U-Pb dating can improve correlations between continental successions by providing absolute age constraints on carbonate deposition and/or on syn- to post-depositional processes such as pedogenesis.

In this study, we analysed pedogenic calcite nodules within a continental Miocene succession in the southwestern Paris Basin (the important paleontological site at Mauvières quarry, France). Following multimethod petrographic characterisation of the samples, LA-ICP-MS U-Pb dating was employed to obtain formation ages on the pedogenic calcite nodules. The Tera-Wasserburg intercept ages from five nodules from the same horizon (19.3±1.3/1.4 Ma, 18.0±3.2/3.2 Ma, 19.11±0.84/0.94 Ma, 19.0±2.3/2.3 Ma and 19.4±2.7/2.7 Ma) are in excellent agreement with previous biostratigraphic constraints on the sequence. Petrographic evidence points to a single crystallisation event, and we conclude that the formation of the calcite nodules occurred at 19.34 ± 0.58/0.73 Ma (central age from a radial plot of the five Tera-Wasserburg intercept ages). This calcite formation age is regarded to represent a minimum depositional age of the strata hosting the root nodules. It provides the first direct age for the continental Miocene succession (and Neogene mammal zone MN3) of the Paris Basin and allows correlation with other continental basins independent of their fossil assemblages or when the fossil content is missing.