Integrated subsurface characterisation of the Röt succession of the Bolesławiec Syncline (SW Poland): stratigraphic and tectonic constraints of the geological record

Abstract

This study presents the first comprehensive geological analysis of the Triassic Röt sedimentary succession in the Bolesławiec Syncline (BS), North-Sudetic Synclinorium (NSS), SW Poland, and fills information gaps regarding the local lithology, stratigraphy, extent and organisation of this succession. During Triassic time, the BS was located on the southern periphery of the epicontinental Germanic (Central European) Basin. The mixed siliciclastic-carbonate Röt succession transgressively overlies the continental siliciclastics of the Buntsandstein (Bunter), locally with the occurrence of a stratigraphic gap. The Röt succession displays a complex vertical organization with basic, about 1-m-scale, fine siliciclastic-carbonate couplets, and larger-scale sequences. The well-log profiles allow identification and provisional subdivision of the succession into several 1D sequences, 10–25 m thick, which can be correlated across the study area. The collected palynological material indicated the latest Olenekian to early Anisian age of the local Röt succession. A surprisingly strong thickness diversification of the succession, as well as its frequent incompleteness, evidenced by both palynology and correlation of well-log profiles, is interpreted in terms of local erosion, non-deposition, synsedimentary tectonic activity, and later tectonic deformation. The authors conclude that synsedimentary tectonics resulted in faulting of the area and was the principal factor responsible for differential subsidence and variable sediment accumulation rates. The lower stratigraphic interval of the Röt succession is defined explicitly by the presence of numerous and diversified small-scale soft-sediment deformational structures. The coincidental occurrence of the first marine deposits in the BS area, together with the intense soft-sediment deformation appear to indicate that the end of Early Triassic transgression was associated with intensified regional seismic/tectonic activity. The Röt sedimentary succession of the BS area also was affected by younger tectonic deformation. The pre-Cenomanian events brought about at least local uplift and limited erosion of the Röt succession. Late Cretaceous–Palaeogene inversion was responsible for the final development of the present-day structure of the NSS.