Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Hladíková, Hladil, Zusková

Discrimination between facies and global controls in isotope composition of carbonates: Carbon and oxygen isotopes at the Devonian reef margin in Moravia (HV-105 Křtiny borehole)

Journal of the Czech Geological Society, volume 42 (1997), issue 1-2, 1 - 16

Relations among the isotope ratios, chemical composition, sedimentological features, eustatic fluctuations and diagenetical history have been documented from the reef/basin transition in the Moravian Karst (Křtiny HV-105 borehole). The δ¹³C and δ¹⁸O values of the Late Devonian limestones are within the ranges for the Late Devonian marine sedimentary environment. Nevertheless, the originally diverse isotopic compositions of the fossils and other rock components were changed in closed marine pore-water/rock systems, under conditions of rapidly decreased permeability. In the studied sequence δ¹³C values of 2 to 2.5 ‰ are characteristic for bioherms, parts of the fore-reef to off-reef slope, and off-shore lagoons. The δ¹³C values close to 0 ‰ are characteristic for shallow back-reef. The low δ¹³C values of reef margin developed during the occasional emergence of this facies. A significant anomaly in δ¹³C values (up to +5.5 ‰) has been documented at the transition between the proximal and distal forereef in the Pa. transitans Zone of the Early Frasnian, just before the maximum sea-level rise. This unusual positive excursion of the ¹³C values does not correspond to the global-event anomalies. The existence of this anomaly on the slope of the Moravian Karst is tentatively explained by a local III^rd-category upwelling, a result of the diversion of the deeper contour and shallower wind-driven currents away from the shore. This anomaly corroborates the hypothesis of a strong facies control of the δ¹³C content in carbonates.