Climatic Influence on Sedimentology and Geomorphology of the Rio Ramis Valley, Peru

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.

Late Quaternary climate variability and terrestrial carbon cycling in tropical South America

“…Increased precipitation and lower temperatures caused snowlines in the Titicaca catchment to descend to elevations of ~4400-4600 m (Klein et al, 1999), resulting in significant glacial expansion. The presence of glaciers and generally wet conditions also resulted in a long period of sediment aggradation in Titicaca watershed rivers during the LGM and late glacial period, as evidenced by high terraces (~20-40 m, ~10-20 m above modern river level, respectively) in both Ramis and Ilave river valleys thought to be deposited during this time (Farabaugh, 2005;Rigsby et al, 2003).…”

“…The early-mid Holocene was marked by the driest conditions of the last 20 kyr and a ~100 m drop in lake level from ~8-4 ka . Reduced precipitation and lower lake level also contributed to erosion of fluvial deposits and episodic downcutting throughout the early-mid Holocene in the Ramis and Ilave river valleys (Farabaugh, 2005;Rigsby et al, 2003).…”

“…With wetter conditions, new terrace development suggests rivers switched from a largely erosional phase to an aggradation phase in the late Holocene (~3 ka to present) (Farabaugh, 2005;Rigsby et al, 2003).…”

“…During the last glacial period, generally wet conditions as well as the presence of well developed glaciers within the catchment led to high erosion rates and sediment accumulation on river floodplains (Farabaugh, 2005;Rigsby et al, 2003). Colder glacial temperatures also reduced the density of vegetation cover in the catchment .…”

“…Lower base level and reduced flow led to river sediment instability, leading to downcutting episodes and sustained erosion of floodplain sediment deposits during the mid-Holocene (Farabaugh, 2005;Rigsby et al, 2003). The low F 14 R wax values in mid-Holocene horizons thus likely reflect contributions of very old waxes from glacial-age river sediment deposits, exposed lacustrine sediments, and/or soil mobilized by erosion events.…”