Strontium isotope composition of Middle Miocene primary gypsum (Badenian of the Polish Carpathian Foredeep Basin): evidence for continual non-marine inflow of radiogenic strontium into evaporite basin

摘要

Strontium isotope compositions of ancient sulphate deposits not only provide chemostratigraphic information but also offer insight into the system in which the evaporites precipitated. Primary gypsum from two Middle Miocene (Badenian) sections in southern Poland shows steadily higher Sr-87/Sr-86 ratios than those expected from a marine-derived formation. The ratios are interpreted as the result of increasing inflow into the basin at the time of gypsum precipitation. Palaeogeographic reconstructions suggest that riverine runoff sources were situated in the West and East European platforms (to the north and east, respectively) and the Carpathians (to the south), which are mostly composed of Mesozoic sedimentary rocks; their dissolution cannot be responsible for the higher Sr-87/Sr-86 ratios recorded. We conclude that Archaean and Palaeoproterozoic igneous and supracrustal rocks of the Ukrainian Shield were the source of the higher Sr-87/Sr-86 ratios recorded in the Badenian primary gypsum. A distinctive decreasing trend of Sr-87/Sr-86 ratios from western Ukraine to southern Poland is explained by a consistent direction of brine inflow during gypsum crystallization (typical cyclonic circulation controlled by the Coriolis effect).