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)

The Basal Chotec Event, slightly above the Lower-Middle Devonian boundary in the Prague Synform, is significant not only for its faunal and lithological changes but also for its distinct and widely correlatable magnetic susceptibility (MS), gamma spectrometric (GRS), and geochemical patterns. The upper part of the Trebotov Limestone was composed of calcisiltic material deposited from distal storm or turbidite currents, which alternate with slowly deposited and condensed hemipelagic material. The lower part of the overlying Chotec Limestone was also composed of turbidite deposits. A more proximal depositional environment is recorded in the Na Skrabku Quarry whereas the Prastav Quarry represents a more distal environment. A decrease in the proportion of pelagic components was observed toward the event datum and an increase in the amount of recycled lithoclastic-skeletal detritus carbonate material. A similar trend is displayed by the shallow-water stratigraphic equivalents in the Cerveny Quarry. The MS record across the studied interval produces a smooth curve below the first event-related beds (Trebotov and Suchomasty Limestone). a drop in MS values at the event base (base of the Chotec and Acanthopyge Limestone), and a rapid increase in MS values with high-magnitude and high-amplitude oscillations above the first event-related dark beds. The GRS record shows a reversal in the Th/U ratio at the event base from Th/U >> 1 to Th/U<1. The upper part of the event-related interval is characterized by a GRS-U peak. REE distributions show very uniform patterns both in the deeper-water facies and shallow-water open ocean facies, indicative of their origin by aeolian atmospheric deposition. Light and heavy mineral assemblages are dominated by paramagnetic (kaolinite, muscovite, chlorite, feldspars, pyroxene and amphiboles, apatite, and barite) and ferromagnetic minerals (mostly Fe oxides or oxyhydroxides). The MS and GRS records, together with the lithological character of the sediments, suggest the sea-level rise at the Basal Chotet event interval. (C) 2010 Elsevier B.V. All rights reserved.