Fluvial activity in the Lomas de Lachay during the upper Pleistocene and Holocene

Abstract

The Lomas de Lachay are the first ridges of the western slope of the Andes, about 100 km northward from Lima. This study examines the structure and age of fluvial terraces in the valleys of different orders. Accumulation of alluvium took place during the periods ca. 57 ka, 24–15 ka and likely the Late Glacial/Early Holocene. These periods correlate with pluvial Minchin, Tauca, and probably Coipasa (when fluvial activity in some Peruvian valleys was also noted). These alluviation phases were separated by erosion and terrace formation. The progressive aridization of the climate caused LGM aeolian covers to appear (21–18 ka) for the first time. Wind-blown sands fossilized older forms of relief and covered the valley slopes in the border zone between the lomas and desert. In the Holocene period, three types of geosystems developed in line with different morphogenetic activity: Coastal Desert (typified by a prevalence of aeolian processes); Lomas (very stable, with a prevalence of chemical weathering – tafones); and Mountain Desert (typified by very active fluvial and slope systems during El Nino events). Settlement phases in Lomas de Lachay were related to a high frequency of ENSO, and intensive grazing in recent centuries caused the creation of terracettes and buried soils on the slopes.