High-resolution integrated stratigraphy of the Hauterivian in Umbria-Marche (Central Italy)

Lower Cretaceous Hauterivian sediments are found in Central Italy as part of the Maiolica Fm. These sediments present strongly contrasting lithologies. From the original radiolarian calcareous ooze that constituted the bulk of the initial deposits, biogenic silica exclusively migrated in cherts, while calcium carbonate reprecipitated into the limestones. Moreover, thin black shale levels and stylolites are enriched in insoluble elements and minerals, which contrast with the nearly pure limestone and chert (> 90 % CaCO3 and SiO2 respectively, mainly negative magnetic susceptibility for both).
In this context, as part of the CRASH (Checking the Reproducibility of Astrochronology in the Hauterivian) project, we are testing whether a Milankovitch imprint is preserved through these lithologies, despite the diagenetic processes implied by their contrasting nature.
High-resolution sampling was performed for cyclo- and magnetostratigraphic purposes in order to avoid the accumulation of positioning uncertainty coming from compiling heterogeneous data sets. To deal with the data in a comprehensive way we developed the StratigrapheR package, available in the free software environment R (https://CRAN.R-project.org/package=StratigrapheR). It is designed to generate lithologs and to process stratigraphical information for cyclostratigraphy. For the investigated sections it allowed to create high-resolution logs that can be queried in the R environment. As the high-resolution lithologs show all the features observed in the field they can be used to reduce the positioning uncertainties for future field missions as much as possible. Additionally they can be used to compare overlapping parts of different sections.
Our preliminary work allows to correlate bundles of thin black shale sequences between two sections spanning overlapping deposition intervals, using magnetostratigraphy as a stratigraphic reference. This is further confirmed by organic matter (OM) analyses, showing similar OM preservation trends between the sections. On the other hand chert layers patterns are more difficult to match between sections. Similarities can be found using the black shale levels as reference, but discrepancies are equally frequent. The ubiquity and regularity of chert layering make it generally difficult to identify any specific pattern.
Furthermore we document the "Weissert" and Faraoni events in our sections. Both present a decrease of the natural remanent magnetisation intensity, which would imply an effect of these events on magnetite. Magnetite is indeed the main contributor for the primary component of magnetisation in the Maiolica Fm. (Channell et al., 1995). The Faraoni level also marks a decrease in the concentration of lithogenic elements and in magnetic susceptibility.

References

Channell J. E. T., Cecca F. & Erba E. 1995. Correlations of Hauterivian and Barremian (Early Cretaceous) Stage Boundaries to Polarity Chrons. Earth Planet. Sci. Lett, 134, 125–40.