Late Palaeozoic-Mesozoic development of the Skrzynno Fault (northeastern border of the Holy Cross Mts.)

Abstract

The prominent regional Skrzynno Fault, running NW-SE, crosses the northeastern periphery of the Holy Cross Mts. On either side of this fault two deep boreholes were drilled 5 km apart (Ostalów 1 and Ostalów PIG 2). A comparison of the stratigraphical successions in these boreholes enabled a reconstruction of the fault's evolution. This is a normal fault with a downthrown northeastern wall. At the sub-Cenozoic surface close to the fault plane, the lower Middle Jurassic is in contact with lowermost Lower Jurassic. The base of the Jurassic is thrown by 810 m, and the base of the Permian by 1010 m. These values suggest that the Skrzynno Fault was a synsedimentary fault, active from the Permian until the Jurassic. On the upthrown side, the Permian is underlain by the Middle Devonian, but on the downthrown side, by the Lower Carboniferous. Estimating the thickness of deposits removed from the upthrown side shows that the pre-Permian fault throw could have amounted to about 1300 m. On the downthrown side thin Rotliegend deposits are preserved. A local saline basin was formed during deposition of the Zechstein PZ1 cycle. The southwestern fault wall was subsiding during sedimentation of the upper Zechstein and in the Early Triassic -- the difference in thicknesses of the Buntsandstein is nearly 200 m. Another stage of fault activity and the lowering of its northeastern wall occurred during Keuper (upper part of the Triassic) sedimentation. These deposits are thicker by approximately 300 m here, as compared with the southwestern side. Later evolution of the fault cannot be precisely reconstructed due to erosion of post-Hettangian sediments of the upthrown side. General knowledge of the geology of the region indicates that the Skrzynno Fault was strongly active in particular around the Jurassic/Cretaceous boundary and during the Paleogene tectonic inversion.