Struktury hydratacyjne i deformacyjne w skałach czapy gipsowej wysadu solnego Dębiny w rowie Kleszczowa

Abstract

Deformation and hydrational structures in cap rocks of the Dêbina Salt Dome, the Kleszczów Graben, central Poland.A b s t r a c t.The Dębina salt dome is situated in central part of the Tertiary Kleszczów Graben, central Poland. Cap rocks of the dome were characterized on the basis of analysis of core material from a well drilled in the northern part of that structure. The cap rock sequence may be subdivided into two parts. Its lower part with breciated structure represents residuum from dissolution of saline series whereas the upper comprises rocks developed by coalescence of shale cover with gypsum. Gypsum rocks of the cap form 6 lithotypes. In these lithotypes it is possible to distinguish: 1) recrystallizational structures developed during diagenesis and multiple hydration transformations, 2) structures developed in a local stress field related to displacements evoked by dissolution of the cap rocks and fluid circulation. Structures of the first type include microcrystalline gypsum with anhydrite relics and carbonate nodules, lenticular and acicular gypsum, porphyroblastic and even-grained gypsum as well as vein gypsum and first generation of stylolites. The other type of structures is represented by: gypsum showing shape fabrics and associated gypsum nodules, gypsum grains with geometry of -clasts, fibrous gypsum in pressure shadows, structures of the core-and mantle type, subgrains developing in bigger crystals and second generation of stylolites and slickolites. Five sets of fractures were recorded. These fractures developed in the cape rocks under stress field with the vertical maximum stress axis and active shear planes. The normal faults of B, C and D sets could be formed during upward motion of the dome, its stagnation, or during gravitational collapse triggered by karst processes. Oblique striations on microfault B surfaces point to later reactivation under local normal-slip regime caused by ongoing subsidence of the cap rocks, or alternatively — a reactivation that occurred in response to external stress field which controlled evolution of the extensional Kleszczów Graben.