Abstract

Fluctuations in regional precipitation and in base-level (the level of Lake Titicaca) trigger changes in fluvial erosion and deposition in the largest tributary valley of Lake Titicaca, the Rio Ramis, Peru. Relationships between fluvial sedimentation and terrace formation demonstrate that large-scale aggradational and downcutting events are associated with specific regional climatic events that have been previously documented in the Lake Titicaca basin and elsewhere on the Altiplano. Five laterally extensive terrace tracts (A through E) are present within the valley. Downcutting of the Ramis valley is correlative with base-level (Lake Titicaca level) fall caused by a decrease in precipitation. Aggradation is correlative with base-level rise during increasing precipitation. Sedimentary facies underlying the terraces record deposition in a large paleolake followed by deposition of both meandering and braided fluvial sediments. The highest terrace in the valley—E—is underlain by a thick sequence of lacustrine strata that was deposited during an extremely wet period in the Lake Titicaca region that terminated at about 40,000 cal yr BP. These lacustrine strata were subsequently downcut, likely at a time of large drop in lake-level, resulting in a deeply incised valley. The main phase of fluvial aggradation in the valley (sediments underlying the D through B terraces) was slow, spanning about 25,000 years. It occurred before, during, and after the Last Glacial Maximum (from before 35,140 ± 790 cal yr BP until about 10,421 ± 181 cal yr BP), a time period that was mostly characterized in the Lake Titicaca basin by high precipitation rates and high base-levels. The aggradation resulted in almost 40 m of sediment accumulation in the valley and was followed by rapid (about 2,000 years), but episodic, incision into the underlying fluvial sediments to form the C and B terraces, as well as the erosional surface beneath the modern fill terrace (A). This erosional phase corresponded to several periods of local base-level (Lake Titicaca) lowering between 10,241 ± 181 and 1,668 ± 76 cal yr BP. The modern fill terrace has aggraded about 2 m above modern river level, likely within the last 1,500 years.

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