The significance of Upper Cretaceous hardgrounds and other discontinuity surfaces for basin-wide correlations, based on drillcore data from boreholes in northern Poland

The paper presents the hardgrounds and some other correlative discontinuity surfaces found in Upper Cretaceous (Cenomanian–Maastrichtian) borehole sections of northern Poland. They were briefly described, and depositional environment was identified for both the underlying deposit (UD) and the overlying deposit (OD). The significance of these features for both basin-wide correlations and broadly understood sequence stratigraphic techniques is highlighted. They were correlated with other major discontinuity surfaces identified in boreholes of northern Poland in both drill cores and well logs if such identification was reliable based on geophysical borehole data. All these discontinuity surfaces were referred to the boundaries of the individual depositional cycles determined within the Upper Cretaceous succession: K3-II/K3-III, K3-III/K3-IV, K3-IV/K4-I, K3-IV/K4-II, K4-I/K4-II (or II or IV or V), K4-III/K4-IV (or V), K4-IVa/K4-IVb, probably K4-IVb/K4-V, and K4-V/Pc-I. The discontinuity surfaces can be related to both sea level fluctuations and tectonic activity during the Subhercynian phases. Most of them developed in relatively calm sedimentary conditions, outside the central part of the Mid-Polish Trough. The exceptions are three hardgrounds from the Człopa–Szamotuły Zone, which seem to be associated mainly with the Late Cretaceous tectonic (including salt tectonics) activity of this zone. The most common UD/OD configuration of sedimentary environments is the open-marine carbonate shelf both beneath and above the discontinuity surface. The second most common situation is the open-marine carbonate shelf beneath and the open-marine shelf with carbonate-siliceous sedimentation above. The gaps at the surfaces span variously long intervals, ranging from short periods (spanning a fraction of a depositional cycle) to long periods (comprising one or more depositional cycles). The position of the discontinuity surfaces facilitates searching for any possible hiatuses in the sections of other boreholes, and paying attention to local changes in sediment distribution patterns, and local tectonic activity.