Illitization of the lower Cambrian (Terreneuvian) Blue Clay in the northern Baltic Palaeobasin

The clay mineral composition of the lower Cambrian (Terreneuvian) Blue Clay (BC) in the northern Baltic Palaeobasin was studied. The proportion of illite in mixed-layer illite-smectite in the BC increases gradually from ca 85% in northern Estonia to ca 92% in central Latvia with the present burial depth increasing from a few hundred metres to ca 1000 m. The high level of illitization suggests a mature diagenetic grade of the sediments, which is typically achieved with burial at several kilometres. However, uncompact nature and thermally immature organic material suggest only shallow burial and maximum palaeotemperatures not exceeding 50 °C. The smectite-to-illite transformation in the BC was described using a kinetic modelling to assess the constraints on burial-driven illitization. Modelling results show that the present illitization level is possible to achieve by assuming burial during the Devonian to Permian prior to the erosion in the Mesozoic. The thickness of eroded sediments in the northern part of the basin was in this case only about 400–800 m. The smectite-to-illite transformation process in the BC in the northern Baltic Palaeobasin was controlled rather by time than by temperature.