Timing of the Rupelian transgression into the Upper Rhine Graben and Mainz Basin: a strontium stratigraphic approach on micro- and macrofossils

The exact timing of the second Rupelian transgressions (sensu Hardenbol et al., 1998) into the Upper Rhine Graben (URG) and whether it was synchronous in the Mainz Basin (MzB) is not very precisely constrained. Biostratigraphic evidence place the beginning of the Rupel 2 transgression in the MzB into the Wallau Subformation (NP22-23) and at about the same time for the URG. Here we present the first comprehensive Sr isotope study on marine calcareous macro- and microfossils as well as a few shark teeth from various drill cores and outcrops of both settings, that ideally represent strata deposited at the base and top of the Rupel 2 transgression. Ostracods and foraminifers as well as some bivalve shells (oysters, Glycymeris sp.) and shark teeth were analysed for their Sr isotope composition. Microfossil preservation was assessed using single shell LA-ICP-MS-analysis to select only well-preserved specimens for high-precision TIMS or solution mode MC-ICP-MS Sr isotope analysis. Bivalves were screened by cathodoluminescence and LA-ICP-MS analysis for least altered areas in which intra-shell in situ LA-MC-ICP-MS Sr isotope analysis were made. The 87Sr/86Sr were compared to the global seawater Sr isotope curve of MacArthur et al. (2001) to infer the age and duration of the transgression. While 87Sr/86Sr of shark teeth and most oysters from Beuchille (except a few domains) were diagenetically altered, most ostracods and foraminifers from the URG are well-preserved, yielding a mean 87Sr/86Sr of 0.707960±0.000016 (n = 25; range: 0.707916 to 0.707985). These values enable us to infer a Sr stratigraphic age of 30.6±0.4 Ma for the late Rupelian regression and deposition of the „Cyrenenmergel“ in the URG and equivalent deposits in the Delémont Basin. Well-preserved foraminifers from the Bodenheim Formation in the MzB gave a mean 87Sr/86Sr of 0.707981±0.000012 (n = 8; range: 0.707964 to 0.707995). This yields a significantly (p<0.00002) younger Sr stratigraphic age of 30.1±0.3 Ma than for the URG deposits. Two aragonitic Glycymeris shells from the lower Alzey Formation (87Sr/86Sr = 0.707999) and Stadecken Formation (Albig Bank; 87Sr/86Sr = 0.707964) yield ages of 29.6 and 29.4 Ma, respectively. Thus the marine late Rupelian strata in the URG and MzB were probably deposited in about a million years.