Valley fill in the Andean Eastern Cordillera: a response to transient incision of the Río Iruya, NW Argentina

BOVAY, C.; HARBOR, D.J.; RAHL, J.M.; STAFFO, K.; GALLI, C.I.

San Francisco, California

Congreso; 2010 Fall Meeting, AGU; 2010

AGU

Headwater valley fill in the Río Iruya chronicles a complex record of deposition andincision at the eastern flank of the Andes. Other intramontane valley fills in NW Argentinahave been interpreted to record important dynamics of climate, tectonics, and hillslopefailure; specific mechanisms of aggradation include Late Cenozoic climate change, rockuplift and defeat of the channel network, and landslide- or tributary-induced damming.The valley fill is dominated by a horizontally bedded, muddy, coarse-grained, angularconglomerate with few channel cuts, interpreted as gravel-bed, braided river deposits withsignificant tributary fan volume. The fill contains almost no fine-grained, laminated, orlacustrine deposits, and a general lack of soil horizons in the up to 300 m-thick fill likelyindicates continuous deposition. The geometry of preexisting and modern channelnetwork is largely similar. Downstream, the river is actively incising bedrock and hasremoved most of the fill. In contrast, the headwater reaches contain well-preserved andabundant fill terraces and the river is less commonly eroding bedrock in the valley bottom.The fill becomes mostly eroded downstream of several major tributaries that provided asignificant volume of sediment. Ongoing and alternating aggradation and incision areevident in both inset terraces and the modern channel. We consider the applicability ofpreviously established interpretations of valley fill generation. In addition, we evaluate thepotential for complex response to base level lowering to cause the aggradation andincision in the Río Iruya. Hypothesized drainage basin reorganization and capture wouldproduce a base level drop, resulting in enormous sediment flux. To estimate themaximum possible erosion rate in the headwaters region, we studied the valley fillsediments using differential GPS. The contact between the fill and the bedrock along thechannel was measured to determine the total volume of the fill. Accordingly, ages fromthe bottom and top of the fill constrain the beginning and ending of the aggradationperiod. These data yield both a headwaters sediment flux rate and a post-fill excavationrate, which are of particular interest as we assess the.effects of stream capture onlandscape evolution.