Origin and evolution of heavy mineral assemblages of the Late Cretaceous Szozdy Delta System (Polish Cretaceous Basin): insights into burial history, weathering processes and parent rocks

Abstract

During the Late Cretaceous, inversion tectonics affected basins across Europe; in Poland, the axial part of the Polish Basin – the Mid-Polish Trough – was uplifted and transformed into one of the most prominent Mesozoic structures of Europe – the Mid-Polish Anticlinorium, with the Lower San Anticlinorium forming its supposed southeastern extension. For decades, the axial part of the Polish Basin was considered to represent the deepest, most rapidly subsiding part of the basin during the Mesozoic times, simultaneously being filled by deposits of that age. The recently defined Łysogóry-Dobrogea Land in the heart of the then-inverting axial part of the Polish Cretaceous Basin (the present-day subsurface Lower San Anticlinorium) overturned this framework. Moreover, the discovery of the Late Cretaceous Szozdy Delta System on the northern peripheries of Łysogóry-Dobrogea Land shed new light on the palaeogeography and tectonic evolution of the southeastern part of the Polish Basin – a part of the trans-European Basin System that developed over a major part of epicratonic Europe. It clearly appears that, during the Late Cretaceous, this area should be considered as an emergent landmass supplying the Szozdy Delta System, instead of as the deepest part of the basin. Although the prominent quartz sand input into the Cretaceous deposits of  the Roztocze Hills is a well-known phenomenon, it has never been explored to reveal the potential parent rocks and possible degree of burial. To fill this gap, we have analysed the heavy mineral assemblages from the Szozdy Delta System, demonstrating their truly polycyclic nature. High ZTR (zircon, tourmaline, rutile) index values, the high degree of roundness, and the presence of some grain surface textures reflecting abrasion indicates multiple rounds of redeposition. However, the assemblages also include predominantly angular grains of minerals which are less resistant to erosion, which is inconsistent with repeated redeposition. Accordingly, the data suggest at least two distinct sources were involved. The first comprised multi-recycled mineral phases dominated by minerals of ZTR group, for which the source might be metapelites and/or metapsammites. The second “fresh” source delivered angular garnet, kyanite and staurolite grains, for which metamorphosed rocks were the main source. Such characteristics would be difficult to obtain from the suggested Mesozoic sedimentary cover of the San Anticlinorium (if a Mesozoic cover was even present), necessitating a search for other sources. Thus, these heavy mineral assemblages suggest a different burial history than previously assumed.