The geochemistry of Lower Paleozoic shales in the Oslo Region (Norway) : a reconnaissance study

Abstract

This study includes the compilation and modification of a substantial whole-rock geochemical dataset containing Lower Paleozoic shale samples from northern Europe, including Norway, Sweden, Estonia, and Russia. The main purpose of this dataset is to (1) support future shale studies for northern Europe by making the compilation publicly available here, and for this study to (2) enhance current knowledge of the geological conditions under which the Lower Paleozoic shale was formed in the Oslo Region. These geological conditions were studied based on the Norwegian sample’s geochemical composition, provenance, paleoweathering, paleosalinity, paleoproductivity, and paleoredox conditions. Lower Paleozoic black shale formations in Norway can have a detrimental impact on the natural environment because of their acid rock drainage (ARD) potential. The results of this study may be useful in obtaining a better understanding of this issue.

An extensive quality assessment was done for all compiled data, which determined that only the Oslo Region whole-rock data was credible enough for further analysis in this study. Key findings from the selected paleoenvironmental analyses include the following: (1) No significant paleoweathering changes were detected, suggesting that the regions paleoclimate remained relatively stable from Late Cambrian to Middle Ordovician. (2) The youngest analyzed shale formations (Early Ordovician, Middle Ordovician) showed signs of increased paleosalinity compared to the older formations (Late Cambrian, Early Ordovician). This is suspected to have affected the mobility and distribution of specific trace elements. (3) In this study, sedimentary mass accumulation rates for Ba, P and total organic carbon (TOC) were used as paleoproductivity proxies. Although the results from the three proxies are somewhat contradictory, a general trend of decreasing paleoproductivity from the Late Cambrian to the Early Ordovician is observed. (4) Paleoredox conditions were analyzed with Total Degree of Pyritization (DOPT) and TOC vs. organic P ratios. The results indicate that the older formations were deposited under more reducing conditions, and that pyritization, particularly for the Early Ordovician, might be influenced by other factors (e.g. hydrothermal activity). (5) None of the analyzed paleoenvironmental conditions showed a direct correlation with the ARD potentials associated with the individual Norwegian shale formations (Appendix B4). This indicates that the development of severe ARD potential is influenced by a combination of paleoenvironmental conditions or by later processes, such as hydrothermal activity.