Application of the authigenic?¹⁰Be/⁹Be dating to constrain the age of a long-lived lake and its regression in an isolated intermontane basin: The case of Late Miocene Lake Turiec, Western Carpathians

The depositional record of intermontane basins provides a valuable archive of the temporal evolution of orogenic belts; their common isolated nature may, however, hinder the efficient usage of standard approaches to constrain the age of a basin fill. In this paper the authigenic Be-10/Be-9 dating method is employed and tested to construct an age model of the existence and regression of Lake Turiec, which appeared during the Late Miocene in the intermontane Turiec Basin, Central Western Carpathians (Central Europe). The initial authigenic Be-10/Be-9 ratio, essential for the dating method, was analyzed on five sites of the Pleistocene-Holocene age, and back-calculation of the paleo-initial ratio was performed using lacustrine succession dated by magnetostratigraphy. Noteworthy, the initial ratio appears to be affected mainly by (post)depositional settings, and not by different provenance of the analyzed sediment. The sedimentological analysis of the Upper Miocene outcrops allowed the definition of the facies associations of an alluvial fan, a braided river, a fan delta and an open lake. The distribution of facies associations in the basin fill implies an increase in sediment supply to accommodation rate ratio due to the replacement of the open lake environment by the interconnected alluvial fan-braided river-fan delta system. Authigenic Be-10/Be-9 dating yielded ages of the lacustrine succession reaching & SIM;10.7-7.8 Ma, while the age of the fan delta succession, considered as the timing of the regression, was established to & SIM;6.7 Ma. The deposition of alluvial fans dominated the basin after the regression of the lake, possibly up to & SIM;4.0 Ma. The abrupt increase in sediment supply to the accommodation ratio at & SIM;6.7 Ma was caused by the uplift of the Mala Fatra Mts. at the western margin of the Turiec Basin, which had previously formed a moderate topography. The timing of this process generally coincides with the overall decrease in accommodation rates in the region due to the inversion of the Pannonian Basin System. The endorheic character of the basin before & SIM;6.7 Ma prevented the transport of the sediment denudated from the recently most uplifted parts of the Western Carpathians and carried towards the Pannonian Basin System, while the present-day source-to-sink system could only be formed later. This implies a low sediment supply and restricted denudation rate not only from the morphostructures surrounding the basin, but also from the Tatra Mts. and Nizke Tatry Mts. during the period & SIM;10.7-6.7 Ma. The research supports the suitability of intermontane basins for authigenic Be-10/Be-9 dating in a setting of high accommodation rates and limited redeposition of older muddy sediment. However, a special attention has to be paid to the depositional settings during determination of initial Be-10/Be-9 ratios.