The stratigraphy, geochronology and paleophysiography of a Miocene fresh-water interarc basin, southern Chile

During Miocene time, the Andean region between 38° and 39°S was an area where active volcanoes, lakes and rivers formed under a temperate to cold humid climate. The volcanic products and sedimentary deposits which accumulated in lakes and rivers adjacent to the volcanoes constitute the Cura-Mallin Formation. The deep roots of the volcanic chain are represented by middle to late Miocene granitoids which formed a north-south trending belt that separated a marine fore-arc basin (Temuco basin) to the west from the continental sedimentary deposits of the Cura-Mallin Formation to the east. Twenty-three KAr dates ranging between 20 and 11 Ma, constrain the age of the Cura-Mallin Formation to the early to middle Miocene. The alternation of pyroclastic strata and lava flows in the Guapitrio Member of the Cura-Mallin Formation suggests the former existence of strato-volcanoes, whilst the presence of ignimbrites suggests caldera formation. A major lacustrine basin ( >100 km long) represented by some facies of the Rio Pedregoso Member of the Cura-Mallin Formation existed from at least 17.5 ± 0.6 to 13 ± 1.6 Ma. Lacustrine accumulations terminated during the progradation of deltas and infilling of volcanic material. The Cura-Mallin Formation lakes could have been formed, in part, by intra-arc extension, by the damming of rivers caused by volcanic products and/or by the filling of calderas. The occurrence of Gilbert-type delta complexes in the Rio Pedregoso Member may indicate a steep basin margin generated by faulting and/or volcanic accretion.