The Caviahue–Copahue volcanic complex in the Southern Volcanic Zone of the Andes is composed of the Pliocene–Pleistocene Caviahue depression and the active Copahue volcano. This volcanic complex is located in a zone of profuse volcanic and tectonic activity, and thus, the origin of the depression includes both tectonic and volcanic hypothesis (e.g., formation of a collapse caldera and a pull-apart basin). According to current knowledge, the oldest rocks filling the depression are lavas and ignimbrites of the Pleistocene Las Mellizas Formation. However, clastic sedimentary deposits underlying this unit have been recognized. Here, we document the characteristics of these deposits at the south shore of the Caviahue lake using field data and digital outcrop models (DOM). We have concluded that the deposits are deltaic and alluvial in origin and made up by a prodelta/distal deltaic front, delta front deposits, delta plain sediments and alluvial deposits. An unconformity separates the deltaic deposits from overlying alluvial deposits, indicating the decrease in the stratigraphic base level probably associated with a sudden coarse-grained clastic input and a drop in the lake level. Our study has implications on the timing and evolution of the Caviahue depression and indicates that the depression precedes the Las Mellizas Formation. Thus, Las Mellizas ignimbrite should not be interpreted as the main unit related to the Caviahue caldera as it has been proposed by some authors. If the depression had indeed been formed by a collapse caldera, this event would be previous to the described sediments.

安第斯山脉南部火山区的Caviahue-Copahue火山群由上新世-更新世Caviahue凹陷和活跃的Copahue火山组成。该火山复合体位于火山活动和构造活动丰富的区域，因此，洼地的起源既包括构造假设又包括火山假说（例如，坍塌火山口和拉脱盆地的形成）。根据目前的知识，填充凹陷的最古老的岩石是更新世拉斯梅利萨斯组的熔岩和火成岩。但是，已经认识到该单元下面的碎屑沉积物。在这里，我们使用现场数据和数字露头模型（DOM）记录了Caviahue湖南岸这些矿床的特征。我们得出的结论是，这些矿床起源于三角洲和冲积层，由前三角洲/远三角洲锋，三角洲锋沉积，三角洲平原沉积物和冲积层组成。不整合使三角洲沉积物与上冲积物沉积物分离，表明地层基础水平的下降可能与突然的粗粒碎屑输入和湖泊水位下降有关。我们的研究对Caviahue凹陷的时间和演化有影响，并表明该凹陷先于Las Mellizas组。因此，Las Mellizas的火成岩不应被解释为与Caviahue破火山口有关的主要单位，正如一些作者所提出的那样。如果凹陷确实是由坍塌的火山口形成的，那么该事件将早于所描述的沉积物。