Clastic dikes in Middle Devonian sandstones of the Gauja Formation, southeastern Estonia

Clastic dikes of sedimentary origin are exotic, but still remarkable minor geological structures in the Lower Palaeozoic sequence of northern Baltoscandia. They often carry essential information about tqhe processes and environments that governed during sedimentary breaks. The first up to 10 cm wide clastic dikes in the Devonian sequence of Estonia were found in sandstones of the Gauja Formation in Piusa and Tabina glass sand quarries. Field observations, lithological, mineralogical, and geochemical studies of dikes revealed that these dikes were formed during a short sedimentary break at the end of the Middle Devonian. Eluvial sediments that were partly cemented in tropical arid conditions were the probable source of sandy matrix of dikes and abundant clasts embedded in it. Gravitational failure and temporary water stream processes were the ways of mass transport into the fractures. However, the amount of water was not large enough to fill the fractures and to cause any kind of stratification during the within-fracture sedimentation. Local uplift of the area and extensional deformation of rocks with the formation of open fractures in Middle Devonian layers suggest a tectonic phase in the vicinity of the regional Liepaja-Riga-Pskov zone of tectonic dislocations. This regional zone was formed due to the Late Caledonian compression and was afterwards recurrently activated. Opening of the fractures was immediately followed by filling. At that moment the levels observable in the present sandstone quarries must have been located more than 20-30 m above the groundwater level. Subsequent subsidence of the area caused preferable vertical circulation of groundwater along the clastic dikes. In comparison with the surrounding rocks goethite-rich fluids have caused stronger cementation of clastic dikes during the post-dike history. Also some other minor diagenetic alternation processes took place in dikes.