DEVELOPMENT OF FRACTURES WITHIN THE LOWER PALEOZOIC SHALE FORMATIONS FROM THE NORTHERN PART OF THE CALEDONIAN FORELAND BASIN (POLAND)

Cores of Ordovician and Silurian shale formations from the northern part of the Caledonian Foreland Basin (e.g. Poprawa et al., 1999) expose a network of natural fractures related to the development of detachment folds. Microstructural analysis of fractures has revealed two modes of failure: extensional representing mode I and shear-related representing mode II. The fractures occur in two groups in relation to bedding: 1) parallel and 2) sub-perpendicular or slightly oblique.

The bedding-parallel fractures are covered by fibrous textures resulting from the antitaxial continuous growth of calcite. We interpret them as an effect of horizontal shortening during the initial stage of rock buckling.

The sub-perpendicular or slightly oblique fractures (2) cut the bedding-parallel fractures (1). These fractures (2) are sealed by blocky and elongated crystals resulting from syntaxial and stretched growth of calcite related to a subsequent crack-seal process. The position and textures of the fractures (2) suggest their development during the buckling stage of deformation. The presence of hydrocarbons within fluid inclusions indicate that these fractures (2) were pathways for hydrocarbon brines in the rock sequences. The cross-cutting relationships of the fractures confirm the hypothesis of their subsequent formation during folding.

The research was conducted in the frame of the ShaleMech project funded by the Polish National Centre for Research and Development (NCBiR), grant no. BG2/SHALEMECH/14.

The core samples were provided by the Polish Oil and Gas Company.

REFERENCES

Poprawa P., Šliaupa S., Stephenson R.A. & Lazauskiene J., 1999. Late Vendian-Early Palaeozoic tectonic evolution of the Baltic basin: regional implications from subsidence analysis. Tectonophysics, 314, 219-239.