Basin Research | 2019
Impact of recycling and lateral sediment input on grain size fining trends—Implications for reconstructing tectonic and climate forcings in ancient sedimentary systems
Abstract
1 2 Grain size trends in basin stratigraphy are thought to preserve a rich record of the climatic and tectonic 3 controls on landscape evolution. Stratigraphic models assume that over geological timescales, the 4 downstream profile of sediment deposition is in dynamic equilibrium with the spatial distribution of 5 tectonic subsidence in the basin, sea level and the flux and calibre of sediment supplied from mountain 6 catchments. Here we demonstrate that this approach to modelling stratigraphic responses to 7 environmental change is missing a key ingredient: the dynamic geomorphology of the sediment 8 routing system. For three large alluvial fans in the Iglesia basin, Argentine Andes we measured the 9 grain size of modern river sediment from fan apex to toe and characterise the spatial distribution of 10 differential subsidence for each fan by constructing a 3D model of basin stratigraphy from seismic 11 data. We find, using a self-similar grain size fining model, that the profile of grain size fining on all 12 three fans cannot be reproduced given the subsidence profile measured and for any sediment supply 13 scenario. However, by adapting the self-similar model, we demonstrate that the grain size trends on 14 each fan can be effectively reproduced when sediment is not only sourced from a single catchment at 15 the apex of the system, but also laterally, from tributary catchments and through fan surface recycling. 16 Without constraint on the dynamic geomorphology of these large alluvial systems, signals of tectonic 17 and climate forcing in grain size data are masked and would be indecipherable in the geological record. 18 This has significant implications for our ability to make sensitive, quantitative reconstructions of 19 external boundary conditions from the sedimentary record. 20