ALBERT

Description: We present here a reconstruction of the post late Miocene landscape evolution of the western slope of the Andean Cordillera Principal near 34°20’ S. We base our analysis on the available geological information, a morphological characterization of the landform assemblages in the Cachapoal and southern Maipo catchments, and the first 10Be exposure ages for moraines in this area. The Cachapoal drainage basin is characterized by a variety of morphological features, like an elevated low-relief surface, volcanoes and lava flows on valley slopes, U-shaped valley sections, roches moutonnées, and large glaciated areas. Different kinds of deposits have been included in the study, such as moraines, lacustrine and landslide deposits, and a well-developed system of fluvial terraces in the more distal part of the Cachapoal catchment. Landslides are mostly developed on rocks of the late Eocene-early Miocene Abanico Formation. and are less frequent in outcrops of the overlying, early to middle Miocene Farellones Formation. We estimate that the lowest end moraine in the Cachapoal catchment is located next to the locality Bocatoma Chacayes (~950 m altitude), though covered by a major landslide. No evidence exist for glacial deposits further down stream in this region. Lateral moraine ridges of the Cachapoal Glacier at Los Cerrillos yielded 10Be exposure ages of 20.3±2.9 and 21.9±5.3 ka that indicate they are associated with the Last Glacial Maximum (LGM). Holocene moraines exist next to all glacier tongues. Of particular interest in this region is the 12 km-long debris-covered Cachapoal Glacier, the longest valley glacier in the central Chilean Andes, and its distal and proximal moraine deposits. Two lateral moraines adjacent to the present-day Cachapoal Glacier yielded exposure average ages of 13.5±2.4 ka for the external ridge, indicating the Younger Dryas, and 3.8±0.8 ka for the internal ridge, an age that coincides with the 4.2 ka global climatic event that marks the beginning of the Meghalayan Age, at the end of the Holocene. The large size of this moraine on both sides of the ice tongue indicates the great development of the glacier at that time. Some of these ages coincide with ages obtained further north in the Maipo drainage basin, at the latitude of Santiago, and in the eastern flank of the cordillera, however, no pre-LGM deposits were found here, unlike the other mentioned regions. This difference together with the much lower altitude of the LGM moraine deposits in the study region suggests that the Cachapoal catchment is a transition zone to a more humid region further south, and indicates the great need for further reconnaissance and dating of glacial deposits in this Andean region. Our analysis of the geomorphological evolution is consistent with incision start for the Cachapoal Valley in latest Miocene. In this process, glacier incision was apparently not much effective until mid-Pleistocene time, when volcanism was active in the higher regions of the mountain range covering areas not yet incised, whereas in the western Principal Cordillera lavas flowed in deeply incised valleys. Pleistocene glaciers deepened and shaped the already incised valleys, which are presently mostly occupied by rivers.

Description: The article presents a conceptual approach for the spatiotemporal distribution pattern of principal lake types in the context of the glaciation history in the Cordillera Blanca. The tropical mountain range hosts one of the main concentrations of proglacial lakes in high-mountain settings worldwide, which have formed as a result of the dominant trend of modern glacier retreat. In the 20th century, glacial lake outbursts have severely affected large settlement areas in the Rio Santa Basin. Additionally to the striking newly emerged lakes, geomorphological evidence of paleolakes is found throughout the middle and lower valley sections. Based on empirical data from field research in over 20 valleys and the analysis of air and satellite images, the study provides a genetic classification of major lake types and a generalized model for the distribution of the present lakes and paleolakes. The origin of the lakes and their recurrent distribution pattern are associated with the individual stages of the Pleistocene to modern glaciation and their corresponding geomorphological landforms. Apart from the individual lake, the focus is put on the spatial arrangement of the lakes to each other based on a holistic landscape assessment. Implications are drawn for the hazard potential, in particular in terms of outburst cascades involving two or more lakes. On a supraregional scale, a clustering of certain lake types occurs in different mountain ranges of the Andes according to their specific topographical and glaciological settings. Even though the glaciated areas have all been subject to major ice losses, only some mountain regions are prone to form moraine-dammed lakes such as in the Cordillera Blanca. The key controlling factors for their formation are highlighted from a glacial-geomorphological point of view. The distribution of principal types of glacial lakes is outlined in a N–S profile along the Andes.