Unusual features of the Middle Devonian Narva Formation covering the oil shale bearing rocks in Estonia

Mineral composition and properties of the Devonian carbonate rocks of Narva Regional Stage of NE Estonia, unconformably overlying the oil-shale bearing rocks of the Ordovician Kukruse Regional Stage, are discussed. Kerogen particles disintegrated from the kukersite-bearing Ordovician Viivikonna Formation due to erosion during the Late Silurian - Middle Devonian break in sedimentation and redeposited in lowermost layers of the Middle Devonian Vadja Formation. This allothigenic organic component specifically influenced the further diagenetic processes in the sediments. Star-like dolomite splays and needle-like crystals, chalcedony nodules, siderite and sulphide minerals of authigenic dolomite were formed. The presence of organic material and sulphur favoured for accumulation of total iron including FeO. When compared with coeval rocks from South Estonia, these rocks differ also by decrease in grain and bulk density.