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Outcrop, Mine and Borehole Studies |
odawa salt diapir, central Poland
aw Burliga
Institute of Geology, Adam Mickiewicz University, ul. Maków Polnych 16, 61-686 Pozna
Poland
The K
odawa salt diapir consists of deformed Zechstein rock salts, with subordinate anhydrite, dolomite and shales. Upward movement of these rocks and the structural evolution of the diapir started in the Triassic and continued at least until the Tertiary. Lithological, and thus rheological variation resulted in the development of a variety of tectonic structures on different scales, which can be observed in mine galleries intersecting the diapir at several levels. The rocks are folded, faulted, fractured and boudinaged, and their behaviour is strictly related to the competence contrasts between layers of different lithology. There is a range of competence from dolomite through anhydrite, shale, clayey salt and rock salt, to the least competent, potassium salt. Kinematic indications at level 600 are provided by boudins, book-shelf structures, asymmetric folds, small-scale faults, tension gashes and veins. These were produced during different stages in the evolution of the diapir and provide evidence for various mechanisms of deformation, ranging from ductile flow to brittle fracturing. The small-scale structures allow the visualization and identification of larger folds and the depiction of their development. They record changes of shear directions along the fold limbs, and also indicate a change in folding mechanism from flexuralslip folding to shear folding. High strain is localized in bedding-parallel zones and within areas dominated by low competence rocks. These zones usually form discrete horizons separating domains of more competent rocks, which show little evidence of deformation and, occasionally, undeformed synsedimentary structures (dessication polygons). The displacements were zonal, accommodated by low competence rocks, with more competent rocks passively transported as rafts within the former. The zones of intense transport are localized within beds of rock salt and potassium salt.
