Precise position of the Basal Chotec event and evolution of sedimentary environments near the Lower-Middle Devonian boundary: The magnetic susceptibility, gamma-ray spectrometric, lithological, and geochemical record of the Prague Synform (Czech Republic)
Abstract
Basal Choteč Event (close the Lower-Middle Devonian boundary, Prague Synform) has distinct correlatable magnetic susceptibility (MS), gamma spectrometric (GRS), and geochemical patterns. The MS record produces a smooth curve below the first event-related beds and a rapid increase in MS values with oscillations above.
The GRS record shows a reversal in the Th to U ratio at the event base from values above 1 to below 1. REE distributions show very uniform patterns indicative of their aeolian origin.
A similar trend is displayed both in the deeper- and shallow-water environments. Light and heavy mineral assemblages are dominated by dia-, paramagnetic (kaolinite, chlorite, pyroxene/amphiboles, apatite, barite, feldspars, muscovite) and ferromagnetic minerals(Fe oxides-oxyhydroxides).
The MS and GRS data, together with the lithology(calcisiltic hemipelagic material below the event base and recycled skeletal detritus from turbidite currents above),suggest the sea-level rise at event interval.