EVOLUTION OF THE NASIELSK–DĘBE GRABEN (EASTERN CENTRAL POLAND) DURING CRETACEOUS TIMES

Abstract

The paper portrays the geological evolution of the Nasielsk–Dębe tectonic graben during Cretaceous times. The analysis is based on data from the Dębe 1, 2, 5, 6, and 7 and Nasielsk 1 and 2 boreholes. Only well logs, archived borehole materials (drill core description) and published data have been used because the drill cores are no longer available. The graben’s Cretaceous evolutionary stages have been identified based on thickness analysis, and correlated to depositional cycles. The analysis shows that greater thickness gradients are observed at the north-eastern graben-bounding fault, which indicates its higher activity compared to the south-western fault throughout nearly the entire Cretaceous. The upper part of the Cretaceous succession reveals the presence of at least two discontinuity surfaces (possibly even hardgrounds): approximately at the Campanian/Maastrichtian boundary, and near the lower Mastrichtian/upper Maastrichtian boundary. It is suggested to distinguish two separate lowest-order cycles within the lower-order cycle K4-IV: cycle K4-IVa correlated approximately with the late early Maastrichtian, and cycle K4-IVb corresponding roughly to the early late Maastrichtian.