Environmental changes close to the Lower-Middle Devonian boundary; the Basal Chotec Event in the Prague Basin (Czech Republic)

The Basal Chote or jugleri Event, close above the Lower-Middle Devonian boundary, has been regarded as a minor but important eustatic transgressive event, which is characterized by significant environmental changes, faunal extinction, appearance of new forms, and maximum radiation, particularly among the goniatites. This study contributes to a more precise stratigraphic allocation of the event, and provides a reconstruction of paleoenvironmental settings in the type area of the event, the Prague Basin (Czech Republic). The beginning of a transgression is recorded already in the TA (TM) ebotov Limestone (partitus Zone, Eifelian, early Middle Devonian). The basin-wide change in the sedimentation pattern (onset of peloidal and crinoidal grainstones (calciturbidites) of the Chote Formation) corresponding to the uppermost partitus and costatus conodont zones, base of Nowakia (Dmitriella) sulcata sulcata dacryoconarid Zone, and Pinacites jugleri goniatite Zone is interpreted here to be linked to a maximum flooding of the basin. A hypothesis of enhanced nutrient load during sedimentation of the Chote Formation is suggested here as a triggering mechanism for intense micritization and peloid formation and prasinophyte blooms, which could be, along with a greater depositional depth, responsible for oxygen deficiency and consequent reduction of diversity and habitat tracking among benthic invertebrates.