Carbon and oxygen isotope composition of Cambra-Silurian limestone and anthraconite from Bornholm: Evidence for deep burial diagenesis.

Abstract

The Cambro-Silurian elastic deposits of the Island of Bornholm in the Baltic Sea include carbonate rocks developed as primary limestone, early diagenetic concretions (mainly anthraconite) and late diagenetic vein and cavity fillings. Oxygen isotope compositions of primary limestone and concretions are lithologi- cally and stratigraphically homogenized to values significantly different from those of diagenetically unaf­fected marine carbonate. Instead, the composition is explained as a result of burial diagenesis at tempera­tures in excess of 90"C. On the other hand, the same rocks have retained their original carbon isotope compositions. The values reflect differences between carbonate precipitated in equilibrium with marine bicarbonate and carbonate formed in the sulphate reduction zone of the bottom sediments. The vein- and cavity fillings have isotopic compositions which are deviating markedly from the other material. Depen­ding on choice of geothermal gradient and primary, depositional parameters the observed thermal impact can be explained from burial of the Lower Palaeozoic sequence to depths greater than 2 km. It is sug­gested that this subsidence took place in Late Silurian to Earliest Devonian time in relation to the major Caledonian phase.