Roundness and surface features of quartz grains in Middle Devonian deposits of the East Baltic and their palaeogeographical implications

The roundness and surface texture of quartz grains were studied in Devonian sediments of 32 drill cores. Roundness was estimated in more than 800 samples and surface textures were examined under scanning electron microscope on 70 grains. The grains were mostly subrounded to subangular. The concentration of rounded grains in some beds of the Pärnu Formation (Fm.), the basal beds of the Leivu Fm., and the upper part of the investigated sequence indicates zones of sediment recycling and reworking. Based on the distribution of roundness degrees of quartz grains, some possible ancient coastlines in Pärnu time were defined.Variations in quartz surface textures were detected at different stratigraphical levels, whereas the texture created by mechanical processes was predominating. Abundant abrasion features like V-shaped pits, and linear and curved grooves usually give evidence of transport in a fluvial medium. On levels with intensive reworking and redeposition (Burtnieki and Gauja formations and on some levels of the Pärnu Fm.) preliminary relief of grains was smoothed and obliterated. The diagenetic dissolution features were more frequent in the Pärnu Fm., Kernave Fm., and lower part of the Aruküla Fm., where intercalation of dolomitic and siliciclastic rocks is common. Possible chemical etching in Devonian soil profiles was observed in the upper part of the section. Data on the roundness of particles and surface textures of quartz grains, and their horizontal and vertical variability help to clarify the history of basin evolution and to distinguish stratigraphical discontinuites.