SEDIMENTOLOGY AND GEOMORPHOLOGY OF THE RIO RAMIS VALLEY (PERU)

Quaternary fluvial and lacustrine strata underlying five terrace tracts (T1-T5) in the Rio Ramis valley, Peru, record large-scale aggradation and downcutting events in this Lake Titicaca (LT) tributary that are likely to be correlative with basin-wide climate changes and local complex response mechanisms (changes in sediment source, topographic variability, etc.). The terraces occur as both paired and unpaired tracts or, in areas constricted by bedrock, as straths. In unconstricted reaches, well-developed, broad, flat terraces range in height from 1.7 to 52.7 m above river level and extend laterally up to 9500 m across the valley. T1, the youngest surface, is consistently ~2 m above river level. The height of T2 generally decreases downstream (from 12.3 to 5.5 m). The T3 surface (8.7 to 19.6 m above river level) is highest within very broad embayments and upstream of bedrock constrictions. The T4 and T5 terrace surfaces are controlled by the erosional topography of the underlying strata.

The terraces are underlain by meandering- and braided-river sands and gravels and by lacustrine muds. The oldest lacustrine strata are separated from the overlying fluvial strata by an erosional unconformity that marks a period of downcutting. Subsequent aggradation, punctuated by equilibrium periods of soil formation, resulted in the deposition of the fluvial strata that underlie the T2 through T4 terraces. The aggradation was followed by episodic downcutting that formed the T2 and T3 surfaces. A final aggradational phase created the T1 fill-terrace.

The history of river valleys in the LT region is generally correlative with regional climatic variations. Typically, downcutting occurs when base level (LT level) is low and precipitation is decreased; aggradation occurs when base level is high and precipitation is increased. Radiocarbon dates (in progress) will allow us to compare the timing of fluvial events in the Rio Ramis to the timing of climatic events recorded in LT and elsewhere on the Peruvian and Bolivian Altiplano and to better understand the climatic effects on both fluvial landscapes and regional cultural evolution.