Earthquake-induced resedimentation in the Badenian (middle Miocene) gypsum of southern Poland

In the Badenian (middle Miocene) basin of the Carpathian foreland of southern Poland, gypsum breccias occur associated with laminated gypsum deposits. These breccias consist of large clasts of gypsum, carbonates, marls and clay chips of variable size embedded in a gypsarenitic matrix. Constituent gypsum grains and clasts commonly appear to be mechanically abraded and chemically corroded crystals or fragments of selenitic, laminated and alabastrine gypsum. Gypsorudites are commonly accompanied by laminated gypsarenites and gypsolutites which show graded bedding; a vertical sequence of graded gypsum beds showing Bouma sequences may be recognized in borehole sections. Microfolding is common within the folded laminated gypsum, and is closely associated with expressions of extensional strain. Both are accompanied by pervasive microfaulting, suggesting a semi-coherent downslope mass movement. The stratiform geometry of the breccias, together with the intensity of slumping relatively independent of the palaeoslope, suggest earthquake shocks as the initial, main cause. Gypsum deposits form a constant, laterally extensive sequence of different lithofacies. The occurrence of the same lithologies and shallowing-up cycles over a wide area reflects thrusting of the Carpathians over the Carpathian foredeep. Local tectonism has also played a significant role. The tectonic framework favoured activation of dip-slip faults promoting shallow-focus earthquakes. These in turn resulted in the resedimentation of gypsum by slumps, debris flows and turbidites. A similar basinward resedimentation of clastic material by gravity flows initiated by fault-induced earthquakes could be of great importance in the foreland geological setting, and may explain some phenomena observed in other evaporite formations from different geological settings, especially of rift type.     

Palaeogeographic and burial controls on anhydrite genesis: the Badenian basin in the Carpathian Foredeep (southern Poland, western Ukraine)

The Badenian (Middle Miocene) Ca-sulphate deposits of the fore-Carpathian basin – including the shelf and adjacent salt depocentre – have undergone varying degrees of diagenetic change: they are preserved mainly as primary gypsum in the peripheral part of the platform, whereas toward the centre of the basin, where great subsidence occurred during the Miocene, they have been totally transformed into anhydrite. The facies variation and sequence of Badenian anhydrites reflect different genetic patterns of two members of the Ca-sulphate formation. In the lower member (restricted to the platform), anhydrite formed mainly by synsedimentary anhydritization (via nodule formation), whereas in the upper member (distributed throughout the platform and depocentre) the various gypsum/anhydrite lithofacies display a continuum of distinctive anhydrite type-fabrics. These fabrics are based on petrographic features and show from the centre to the margin: (1) syndepositional, interstitial growth of displacive anhydrite; (2) early diagenetic, displacive to replacive (by replacement of former gypsum) anhydrite formation near the depositional surface; (3) early diagenetic, displacive to replacive anhydrite formation during shallow burial; and (4) late-diagenetic (and only partial) replacement of gypsum at deeper burial. The cross-shelf lateral relations of anhydrite lithofacies and fabrics suggest that the diagenesis developed as a diachronous process. These fabrics of the upper member reflect both palaeogeographic (linked to different parts of the basin) and burial controls. Anhydrite growth started very early in the basin centre, presumably related to high-salinity pore fluids; anhydritization prograded updip toward the shelf (landward in a generalized cross-section through the basin). The intensity of gypsum replacement by anhydrite was progressively attenuated landward by a decrease in the salinity of the pore fluids. In each part of the basin, the anhydrite fabric was also controlled by the texture and degree of lithification of the fine-grained primary gypsum lithofacies. Recrystallization of these anhydrite fabrics during late diagenesis, linked to deeper burial conditions, is insignificant, allowing reconstruction of the original anhydritization pattern.