Nacientes del Biobío Formation

Suárez and Emparan have identified three interdigitating members of the formation the older one being Miembro Icalma, the youngest one being Miembro Lonquimay and Miembro Lolén-Pacunto being in-between.[1] The ages of the formation as well of that of the members have been established through fossil ammonite findings.[2]

Miembro Icalma contains basaltic pillow lava and other features indicative of deep marine eruptions. Volcanic rocks, including basalt, belong to the tholeiitic and calc-alkaline series showing geochemical affinities with island arc magmas.[1] Miembro Icalma do also contains marine fossils of Harpoceratinae and Atacamiceras in its sedimentary strata that indicate a Pliensbachian to Mid-Toarcian age for the sub-unit.[1][2]

Miembro Lolén-Pacunto is made up chiefly of mudstone and sandstone. Miembro Lonquimay stands out for containing limestone and paleochannels filled with sandstone.[1]

The formation is interpreted as indicative of a Jurassic marine regression showing a successive shallowing of deep marine environments (pillow lavas, turbidites) to shallow marine environments (limestone) and culminating with an upper region displaying a continental environment (paleochannels). Tectonically, the formation is assumed to have deposited either in the westernmost fringes of Neuquén Basin or in an intra-arc basin. In both cases the formation must have been deposited very close to a volcanic arc, active in the Jurassic.[1]