Regular orientation of joints in horizontally bedded sedimentary rocks of the East European platform

摘要

The paper summarizes long-term investigations of the jointing of sedimentary rocks of the East European platform. The conclusion on the existence of two joint systems striking NE and NW that are preserved in rocks of various ages of the platform sedimentary cover is drawn from the study of the jointing orientation in Paleozoic and Mesozoic rocks and processing of measurement results with the use of an original procedure reliably detecting jointing systems. In their nature, the joints are synsedimentary. It is shown that a sediment is lithified under the influence of heterogeneities at the boundary between the underlying rock (substratum) and the sediment. Cracks start to nucleate in the process of diagenesis (sediment-to-rock transformation) at the base of the newly forming bed due to the motion of a pore fluid upward, orthogonally to the sediment-substratum boundary. Azimuthal anisotropy of the pore fluid motion is predetermined by the substratum anisotropy, giving rise to the formation of extended master cracks, which control the orientation of second-order cracks developing later. The described mechanism of inherited development of cracks is capable of explaining why fixed directions of joints persist in rocks of different ages. During the subsequent deformations accommodating the development of local structural forms, either primary (lithogenetic) cracks change their bedding elements (if they are diagonal with respect to the strike of folds) or conjugate shear cracks develop and form their own, essentially tectonic jointing.