δ13C records across the late Silurian Lau event: New data from middle palaeo-latitudes of northern peri-Gondwana (Prague Basin, Czech Republic)

Abstract

During the late Silurian the Prague Basin was located in middle southern latitudes. In contrast to palaeocontinents positioned in tropical and subtropical latitudes like Baltica, no reefs are developed, which is in accordance with the predicted cooler water. The Prague Basin represents a relatively restricted and shallow rift basin with a complex tectonic history. Sections in different palaeoenvironments have been studied to document the most prominent Silurian stable carbon isotope excursion recorded during the late Silurian (Ludfordian) Lau Event from this part of peri-Gondwana. Deeper water deposits of the Kopanina Formation investigated in the present study were deposited on the slope-to-basin transition near the Kosov volcanic centre in the western part of Prague Basin. The sediments are developed as an alternation of dark, partly laminated limestones and marls with an increase of the limestone–marl ratio in the upper part of the succession. A pronounced positive carbon isotope excursion starts in the Neocullograptus kozlowskii graptolite and in the upper Polygnathoides siluricus conodont zone. The maximum of the shift is observed in the lower part of an interval characterised by the Ananaspis fecunda– Cyrthia postera community. The maximum values scatter around 8‰, which represent the highest values reported hitherto from the Prague Basin. In low latitudes, often a decrease of δ 13C values towards deeper water settings is reported. In contrast, in the present study the δ 13C values of about 8‰ are much higher than those recorded from the contemporaneous shallow-water sections studied in the classical Mušlovka and Požáry quarries. The most reasonable explanation is the presence of stratigraphical gaps in the shallow parts of the basin. As indicated by karstification these gaps were caused by a sea-level drop. Another effect of this sea-level fall was a strongly reduced sedimentation of the cephalopod limestone facies around volcanic and tectonic elevations.