The origin of the White Beds below the Cretaceous-Tertiary boundary in the Gubbio section, Italy

Abstract

Immediately below the Cretaceous-Tertiary (K-T) boundary clay layer in Umbrian pelagic limestone sections is a 0.5 m thick interval of white limestone beds. Geochemical and rock magnetic studies have been carried out to investigate the origin of these beds. Fe/Si ratios suggest that the iron abundance is a constant proportion of the detrital component in the red marlstone above the boundary and the pink limestone below the white beds. The stratigraphic variations of initial susceptibility and the IRM intensities acquired in different fields behave similarly to the Fe/Si ratio, and reflect primarily the changing concentrations of the ferromagnetic minerals caused by sedimentary fluctuations. The Fe/Si ratio, overall Fe abundance and the intensities of the magnetic parameters are all anomalously low in the white beds. The differing magnetic properties of the samples are due mainly to differing proportions and grain-size distributions of pigmentary hematite and detrital magnetite. The coercivity spectra of samples from the white beds are very similar to those of the pink beds, but are clearly distinct from those of the red beds. The white beds were probably deposited under the same conditions as the underlying pink beds. The anomalously low intensities in the white beds result from reduction of hematite in originally pink beds and subsequent removal of the Fe 2+ ions. The reduction may be a consequence of downwards infiltration of reducing waters resulting from the large quantity of organic matter produced by the extinctions at the K-T boundary.