Siliciclastic grain size composition of the Upper Ordovician rocks of the Blidene Formation in western Latvia

New data on the siliciclastic grain size composition of the Upper Ordovician Blidene Formation in its stratotype region in western Latvia are presented. The formation represents a marly interval at the transition from Keila to Oandu stages, where the succession of deposits has been influenced by essential changes in the depositional regime in the Baltic palaeobasin. The Blidene Formation overlies the organodetrital limestones of the Adze Formation and is succeeded by the organic-rich black shales of the Mossen Formation. It is characterized by a high content (50-85%) of siliciclastic material, increasing upwards. The siliciclastic material consists mainly of clay fraction (<2 µm), decreasing amounts of fine, medium, and coarse silt, and sporadic sand grains (>63 µm), forming less than 5%. The data obtained on the Blidene Formation in western Latvia are compared with earlier data from southern Estonia. The increase in the siliciclastic component of the Ordovician carbonate sequence in Keila time is considered to be related with climatic and sea level changes, and with the changes in the sea bottom topography due to tectonic movements.