Changes in paleoenvironmental conditions during the Late Jurassic of the western Neo-Tethys: Calcareous nannofossils and geochemistry

Abstract

Five cores from an Upper Jurassic carbonate mudstone succession deposited within a Middle East intra-shelf sub-basin were investigated to evaluate the vertical and lateral distribution of calcareous nannofossils. Coupled with lithologic elemental compositions, the results were interpreted in terms of paleoenvironmental and stratigraphic analysis.Six calcareous nannofossil species were identified from approximately 100 samples. Most of the species identified are dissolution-resistant taxa that belong to two clusters: oligotrophic and eutrophic surface waters. Poorly preserved foraminiferal remains were also found within the studied interval. Most of the identified foraminiferal genera are typical of a shallow marine environment, implying that they had been transported into the basin.Elemental analysis results from non-destructive micro-XRF analyses indicate a high percentage of calcium (Ca), moderate percentages of terrigenous-associated elements including silicon (Si), aluminum (Al), titanium (Ti), potassium (K), iron (Fe), and fluctuations in redox-sensitive trace elements (Mo, Ni, Cu, V) across the basin (proximal versus distal). Six geochemical clusters were identified using principal component analysis (PCA) and hierarchical clustering of principal components (HCPC) based on representative major and trace elements as input variables. Strong positive correlation was observed between terrigenous-associated elements and calcareous nannofossil relative abundance (RA), suggesting an increase in nutrient supply coupled with an increase in terrigenous-sourced materials and better oxygenation condition. Conversely, strong negative correlation was observed between calcareous nannofossil RA and redox-sensitive trace elements. In low-oxygen settings, environmental conditions were not favorable for the organisms to live, as shown by the decrease and absence of calcareous nannofossils and foraminiferal faunas. Variations in calcareous nannofossils and elemental compositions were likely controlled by sea-level fluctuations and other factors that resulted in stratification of the water column.