Detecting the Late Frasnian semichatovae event on the deep shelf of north Gondwana

The semichatovae event is a mid-late Frasnian (Late Devonian) episode of a major marine transgression that coincides with the spread of the conodont Palmatolepis semichatovae. The occurrence of this species and the associated event were therefore proposed to define the Middle-Late Frasnian boundary. As this species is not found on the north Gondwana platforms, this event is only documented on the Laurussia marine outer-shelves. As such, it is not known in the Frasnian reference section within the Upper Quarry Coumiac, located in the Montagne Noire (southern France). In this contribution, we have used a multidisciplinary approach that combines sedimentology, conodont assemblages, magnetic susceptibility, and concentrations of certain elements that are indicative of detrital components to precisely date this event in the absence of Palmatolepis semichatovae by means of global scale correlations. We found that a significant part of the environmental signal has been preserved in the strata, and we could locate the semichatovae transgressive event in the cross-section in the lower part of Zone FZ 11. During this period of recurrent environmental perturbations, conodonts responded to sea-level changes only above a certain depth threshold which is reached only with the semichatovae event.