The Middle-Upper Cambrian sedimentary succession of the southern Montagne Noire consists of two mixed (siliciclastic-carbonate) intervals bounded by a middle coarse-grained, siliciclastic one. Five formations are distinguished, from bottom to top, the La Tanque, Coulouma, Ferrais, La Gardie and Val d’Homs Formations. A three-fold chronostratigraphic subdivision is reported here for the Middle Cambrian, comprising the Leonian, Caesaraugustian and Languedocian stages, a chart used in the entire Mediterranean area. The Lower-Middle Cambrian boundary is not recognized in the southern Montagne Noire : the first Leonian trilobites occur across the La Tanque-Coulouma transition. A late Leonian-Caesaraugustian trilobite radiation is recognized coinciding with a broad transgression, indicated by the stepwise immigration and evolution of relative cosmopolitan trilobites, such as the paradoxidids, conocoryphids and solenopleurids. A substantial decline in trilobite diversity is recorded across the latest Caesaraugustian-mid Languedocian interval. By contrast, echinoderms were not greatly affected : they increased in diversity across the lower Languedocian and disappeared at the middle middle Languedocian. Finally, an abrupt increase in trilobite diversity, related to transgressive pulses and immigration of previously known families and new Asiatic invaders, occurs throughout late Languedocian and Late Cambrian times. The Upper Cambrian echinoderm assemblages represent a diversification event pre-dating the Ordovician radiation, and comprise mitrates, glyptocystitid cystoids, and edrioasteroids.
ÁLVARO et al. (Mon,) studied this question.