Quantifying sediment dynamics on alluvial fans, Iglesia basin, south Central Argentine Andes

LINDA KIRSTEIN; HARRIES REBEKAH; PERALTA , SILVIO H.; MIKAEL ATTAL; ALEX WHITTAKER

Basilea

Conferencia; EGU General Assembly 2017; 2017

EGU General Assembly 2017-Geophysical Research Earth Surface

Qualitative interpretations of environmental change drawn from alluvial fan stratigraphy typically tie the depositionof greater volumes of coarser sediment to wetter climatic periods. For example, step changes in sediment fluxand discharge associated with glacial-interglacial cycles are often linked to the progradation and back steppingof a fan?s toe (Harvey et al, 2002). Indeed, more recent quantitative stratigraphic models demonstrate changes inthe volume and calibre of sediment fluxed from an uplifted catchment can produce predictable shifts in the rate atwhich fluvial deposits fine downstream (Duller et al. 2010, Armitage et al. 2011).These interpretations, however, make three important assumptions: 1) the volume and calibre of the sedimenttransferred from an eroding mountain belt to a depositional basin is directly related to climate through somevalue of time-averaged discharge or catchment wetness; 2) lateral sources of sediment, such as tributaries, do notsignificantly influence the pattern of deposition in a basin and, similarly, 3) the reworking of older fan surfaces isminimal and does not impact the depositional pattern of younger deposits.Here we demonstrate each of these assumptions underestimates the importance of variance in transportablegrain sizes in influencing the local and basin-wide deposited grain size trends. Using the Iglesia basin in theArgentine south Central Andes as a natural laboratory, we compare three large, adjacent, alluvial fan systemswhose catchments experience the same background tectonic and climatic forcing.We find regional climate forcingis not expressed uniformly in the downstream grain size fining rates of their modern systems. Furthermore,we observe the variance in transportable grain sizes supplied from each primary catchment and the varianceof material introduced by tributaries and fan surfaces downstream can act as first order controls on the rate ofdownstream fining. We also raise the importance of considering factors such as climate storminess and degree ofglacial cover in having a dominant control on the variance of sediment released.These findings have significant implications for our ability to invert the fluvial stratigraphy for climaticallydriven changes in discharge and highlight a need to quantify the impact of sediment dynamics on modern systemsso that we may better understand the limitations in applying quantitative models to ancient stratigraphy.