Lower Devonian conodonts from the Karlík Valley and Na Branzovech sections in the Barrandian area, Czech Republic, and their significance for Pragian conodont zonation

Abstract

Two Early Devonian sections used for this study display a markedly different facies development and lie on opposite limbs of the central Barrandian synform, in the stratotype area of the Pragian Stage. The present biostratigraphical information based on 18 species with a total of 350 conodont specimens is mostly related to the late Lochkovian and early Pragian, whilst the remaining parts of the Pragian involve mostly long-range conodont species. The late Lochkovian conodonts correspond to two zones eurekaensis and delta. The latest Lochkovian zone pesavis was found in the northwestern limb, but not on the opposite side. The absence of pesavis Zone in the south (Koneprusy) and, as it has been newly shown, in the south-east of the central Barrandian synform (Karlík Valley) suggests that a prominent lowstand sedimentary starvation affected much larger areas than only the tops of elevated ridges. The lower two thirds of the Pragian sequences can be zoned using two newly suggested conodont zones steinachensis and serratus, respectively, that are defined by the first and last occurrences of these taxa. The occurrence of  Latericriodus steinachensis (AL-RAWI) eta morph. marks the lower third and the entry of Pelekysgnathus serratus serratus JENTZSCH corresponds to the base of the middle third of the Pragian. The newly suggested stratigraphical applicability of these two species can effectively substitute the pre-existing sulcatus and kindlei subdivisions that have no zonal species representation in the stratotype area. The first occurrence of Latericriodus bilatericrescens bilatericrescens (ZIEGLER) seems to indicate the base of the Emsian in the Na Branzovech section. The entry of this taxon is connected with the black-shale Graptolite Event within the latest Dvorce-Prokop Limestone that is several beds higher than the levels usually considered for entry of a debatable taxon Polygnatus pireneae. The base of the Graptolite Event gives better chance for the correlation of the Pragian-Emsian boundary than any of the underlying levels. Evolution of the Barrandian conodont assemblages shows many parallels with those of Morocco and Carnic Alps.