The formation of the caldera coincides with the eruption of the Portezuelo Ignimbrite/Portezuelo Formation and took place between 168,000 ± 4,000 and 82,000 ± 2,000 years ago. This ignimbrite formation where it is not buried by younger eruption products spreads radially around the caldera and reaches a maximum exposed thickness of ; it covers an area of about on the northern and southern sides of Payún Matrú, and its volume is estimated to be about . The event was probably precipitated by the entry of mafic magma in the magma chamber and its incomplete mixing with pre-existent magma chamber melts, or by tectonic processes; the resulting Plinian eruption generated an eruption column, which collapsed, producing the ignimbrites. Different layers of magma in the magma chamber were erupted during the course of the eruption and eventually the summit of the volcano collapsed as well, forming the caldera; activity continued and emplaced lava domes and lava flows in the caldera area. These post-caldera volcanic formations are subdivided into three separate lithofacies.