Redox changes in the deep shelf of the East Baltic Basin in the Aeronian and early Telychian (early Silurian)

Aeronian black shales and overlying Telychian greenish-grey and red claystones of the deep shelf of the East Baltic Basin indicate different synsedimentary redox conditions of the bottom water of the sea. In the Aeronian, the primary bioproductivity rise caused accumulation of organicrich black shale in deep shelf and formation of microcrystalline limestone with chertification, chert nodules, and barite in shoreward areas. In the early Telychian, the bioproductivity decreased, as concluded from the absence of indicators of high primary bioproductivity in the Aeronian. The suggested mechanism regulating primary bioproductivity and oxygen content of bottom waters of deep shelf was a change from wind-induced upwelling in the Aeronian to downwelling in the Telychian. The sedimentation rate did not influence the change in the redox regime of the deep shelf sediment, as it was low for both the Aeronian and Telychian.