Tectonic evolution of the late Cretaceous Nysa Kłodzka Graben, Sudetes, SW Poland

Abstract

The Nysa Kłodzka Graben, located in the Sudetes of SW Poland, developed as a result of Coniacian (middle Upper Cretaceous) N-trending faulting of the Variscan crystalline basement rocks that comprise the crest of the Orlica-Śnieżnik Dome. The graben was transgressed by a late Cretaceous sea that encroached during the Cenomanian from the northwest. Up to 700 m of Coniacian shales, sandstones and conglomerates were deposited in the graben, with shales (the ~500 m thick Idzików 'clays') dominating the graben's central section. On the western side of the graben, shales grade upwards to greywackes in a style that resembles a turbidite sequence; on the eastern side, shales are overlain by sandstones and conglomerates (the Idzików conglomerates) that represent extensive late Cretaceous fan deltas. These within-graben fan deltas date the onset of fault-block movements that uplifted the Sudetes region during the late Cretaceous-Cenozoic. By the end of the Cretaceous, both the sedimentary infill and the underlying Cenomanian and Turonian strata were steepened at the graben margins and were gently folded, the fold axes paralleling the graben's marginal faults. Subsequent Cretaceous-Paleocene ('Laramian') deformations resulted in NW-trending reverse faulting, which restructured the earlier N-S template of the graben, and in transcurrent faults, which cut the N-trending folds, modified the north and south ends of the graben and strongly affected the graben's western walls. The total thickness of the Upper Cretaceous strata of the Nysa Kłodzka Graben is 3 times that of the Intra-Sudetic Synclinorium, implying that the two units developed independently.