Upper Berriasian chemostratigraphy, clay minerals and calcareous nannofossils of the Barlya section (Western Balkan, Bulgaria): Implications for palaeoclimate and productivity changes, and stratigraphic correlations across the Alpine Tethys

Abstract

Upper Berriasian chemostratigraphic, clay mineral and calcareous nanofossil data are presented from a precisely dated hemipelagic section of Barlya (Western Balkan, Bulgaria). The section covers an interval from the upper part of the lower Berriasian (Calpionella elliptica Subzone, magnetozone M17r) to the lowermost Valanginian (Calpionellites darderi Subzone, magnetozone M14r). The study aims to reconstruct the major palaeoenvironmental changes (variations in lithogenic input, palaeoredox and palaeoproductivity) and their relation to palaeoclimate and regional tectonic regime, as well as their application to stratigraphic correlations with the Vocontian Basin and Jura Mts. A long-term increase in terrigenous input during the late Berriasian was controlled mostly by the orogenic activity in the NeoTethyan Collision Zone and to a lesser degree by climate humidification, as revealed by variations in kaolinite content and in lithogenic proxies (Ti/K, Th/K, Ti/Al and Zr/Rb ratios). A good correlation is observed between geochemical palaeoproductivity proxies (sedimentation rates of non-detrital (excess or authigenic) portions of P, Zn and Cd) and nannofossil fluxes, determined as the total abundance and species richness. Major calcareous nannofossil peaks, represented by high-diversity and high-abundance nannofossil assemblages, fall within the low-productivity intervals. A smaller peak formed by a low-diversity and high-abundance assemblage, dominated by Watznaueria barnesiae/fossacincta, coincides with the variable, but mostly high-productivity interval, which indicates high plasticity of Watznaueria concerning to environmental conditions. Additionally, trophic changes seem to correspond to bulk rock carbon-isotopic composition, with rising δ13Ccarb values in more oligotrophic intervals. This offers perspectives for long-distance chemostratigraphic correlations between pelagic and platform sections, supplementing traditional schemes based on bio- and sequence stratigraphy. A holostratigraphic correlation is proposed between the Western Balkan (Barlya section), Vocontian Basin (Berrias and Monclus sections) and Jura Mts (La Chambotte section) based on bio-, magnetic and carbon-isotope stratigraphy, as well as climatic and palaeoproductivity proxies.