Stratigraphy, sedimentology, and geothermal reservoir potential of the volcaniclastic Cura-Mallín succession at Lonquimay, Chile

Abstract

The Tolhuaca Volcano near Lonquimay in south-central Chile has been the subject of several studies due to its geothermal manifestations, but little is known about the stratigraphy and reservoir potential of the Cura-Mallin Formation forming its basement. Field work and U-Pb dating of detrital zircons allow us to redefine this succession as the Cura-Mallin Group, consisting of the volcano-sedimentary Guapitrio Formation, sedimentary Rio Pedregoso Formation, and volcano-sedimentary Mitrauquen Formation. The Rio Pedregoso Formation can be subdivided into three formal units, namely the Quilmahue Member, Rucananco Member, and Bio-Bio Member. The base of the Quilmahue Member interfingers laterally with the base of the Guapitrio Formation, for which a previous K/Ar date of 22.0 +/- 0.9 Ma was apparently discarded by the original authors. However, this date is consistent with the stratigraphic position of the Quilmahue Member and new zircon dates from the overlying units, also coinciding with the initiation of an extensional phase in the Biobio-Alumine Basin. Deposition of the Quilmahue Member continued throughout the early Miocene, as confirmed by dates of 17.5 Ma reported by previous authors and 16.5 Ma obtained in this study. The Rucananco Member was deposited during the Serravalian around 12.6 Ma, whereas the Bio-Bio Member was dated at the Serravalian-Tortonian limit (11.6 Ma). Although all three members were deposited in a fluvio-lacustrine environment, they were dominated respectively by flood plains with crevasse splays, lake margins with distributary mouth bars and Gilbert-type deltas, and distal braided and meandering rivers. Whereas the Quilmahue Member was deposited during basin extension, the Rucananco Member was formed during a period of basin inversion and compression. Temporary tectonic quiescence during deposition of the Bio-Bio Member allowed denudation of the landscape, but around 9.5 Ma tectonism was renewed again during deposition of the Mitrauquen Formation. From a geothermal point of view, the Guapitrio Formation has a low potential to host significant reservoirs due to extensive hydrothermal alteration that produced secondary minerals clogging pore spaces and fractures. In the Rio Pedregoso Formation, on the other hand, the Rucananco Member seems to have the best reservoir potential, as it has relatively thick, semi-permeable sandstones and conglomerates deposited in a lake-margin environment.