A new biozone for the late Viséan miospores from the Cortaderas Formation in western Argentina is proposed. It is the Reticulatisporites magnidictyus-Verrucosisporites quasigobbettii (MQ) Interval Biozone. The base of the biozone is defined by the range bases of Apiculiretusispora microseta, Dibolisporites disfacies, Reticulatisporites magnidictyus, Rugospora australiensis, and Verrucosisporites quasigobbettii. The upper limit of the MQ Interval Biozone is marked by the appearance of monosaccate pollen. Two distinct associations are recognized; Association A is characterized by the range bases of the diagnostic species, and the presence of a group of long-ranging species. Association B is characterized by the range bases of Anapiculatisporites austrinus, Anapiculatisporites kekiktukensis, Apiculatisporis variornatus, Crassispora kosankei, Cristatisporites menendezii, Kraeuselisporites tendatus, Kraeuselisporites volkheimerii, Raistrickia paganciana, Raistrickia rotunda, and Spinozonotriletes hirsutus, and is represented in the upper part of the Cortaderas Formation.