Lithology and evolution of Devonian carbonate and carbonate-cemented rocks in Estonia

The Devonian sedimentary sequence of Estonia is mainly composed of terrigenous rocks, but includes also carbonate and mixed carbonate-siliciclastic rocks. The accumulation of siliciclastic sediments has recurrently alternated with carbonate sedimentation. Carbonate rocks are best represented in the Narva Regional Stage where they are fully dolomitized. In addition to pure dolostones, dolomitic marlstones, and siliciclastic sediments, mixed carbonate-siliciclastic rocks were recorded in six studied drill cores (Ruhnu (500), Taagepera, Tartu (453), Valga (10), Võru, and Värska (6)). Mixed rocks were deposited in transitional tidal flat conditions of shallow epicontinental sea during simultaneous accumulation of carbonate and siliciclastic material. They include also terrigenous rocks cemented by dolomite during diagenesis. According to mineralogical, petrographical, and geochemical data, early diagenetic dolostones and dolomitic marlstones were deposited in the shallow nearshore tidal flat environment in the sea water saturated with Mg, but changed in the next stages of diagenesis. Siliciclastic rocks were cemented during middle and late diagenesis by Mg- and Mn-enriched fluids. Sometimes cement of siliciclastic and mixed rocks is represented by gypsum and late diagenetic calcite. The porosity of Devonian rocks increases with increasing clay content, and decreases with mechanical compaction, dolomitization, and carbonate cementation. The sedimentation and diagenetic history are reflected in gamma-ray logs.