Deep vs. shallow – two contrasting theories? A tectonically activated Late Cretaceous deltaic system in the axial part of the Mid-Polish Trough: a case study from southeast Poland

Abstract

Abstract. The Polish Basin is a part of a trans-European sedimentary basin comprising the Central European Basin System and stretching from Denmark through Germany to southeastern Poland, and even further to the south into Ukraine. It experienced uplift during the Late Cretaceous, which consequently resulted in the inversion of its axial part (i.e., the Mid-Polish Trough) and development into the Mid-Polish Anticlinorium. In many existing paleotectonic interpretations, southeast Poland – i.e., the subsurface San Anticlinorium and the present-day Roztocze Hills – was included in the Mid-Polish Trough during the Late Cretaceous, representing its axial, most subsiding part. This paleotectonic model was the basis for facies and bathymetric interpretations that assumed Upper Cretaceous sediments, deposited close to the axial part of the Mid-Polish Trough (e.g., in the Roztocze Hills), constituted the deepest facies. In recent years, several studies have contradicted this notion. A growing body of data suggests that this area (southeast Poland) was already a landmass by the Coniacian and Santonian – and certainly in the Campanian and Maastrichtian – rather than forming the deepest part of the Polish Basin. The shallow marginal marine, cyclic middle Campanian, marly to sandy deposits, recently discovered in the Roztocze Hills, were interpreted to be of deltaic origin. This has led to the adoption of new facies and bathymetric models, which – notably – are in stark opposition to most previous interpretations. Notably, the new interpretation implies the presence of a landmass in the place where, in prior frameworks, the deepest and most subsiding part of the Polish Basin (i.e., Mid-Polish Trough) was located during the Cretaceous. Here, we document this Late Cretaceous depositional system – the Szozdy delta developed in the axial part of the Mid-Polish Trough. These middle Campanian deposits crop out extensively in the Roztocze Hills region, close to the village of Szozdy, and exhibit coarsening-upward tripartite cyclothems. Three facies associations have been distinguished: (1) dark grey calcareous mudstones, (2) a yellow calcareous sandstone, and (3) calcareous gaize lithofacies. The sequence, as a whole, was accumulated via the repeated progradation and abandonment of deltaic complexes on the delta front platform setting, with the primary transport direction originating from the axis of the inverting Mid-Polish Trough (thus from the subsurface San Anticlinorium) toward the northeast. This interpretation is supported by a suite of sedimentological, palynofacies, and heavy mineral data. The development of the Szozdy delta system is framed in the context of the dynamic tectonic processes operating contemporaneously in southeast Poland: that is, tectonic inversion (uplift) on one hand, and the generation of new accommodation space via enhanced subsidence on the other. This discovery sheds new light on our understanding of Late Cretaceous facies distribution, bathymetry, and paleogeography and might potentially suggest a different burial history than assumed so far.