The Importance of Autogenic Dynamics in Multidimensional Models of Grain Size Fining

Variations in fluvial grain size have long been used to decipher past climatic and tectonic events within stratigraphy. Thus, a thorough understanding of grain size fining response to external forcing and autogenic dynamics over long timescales (Myr) has implications within the interpretation of the sedimentary record. This work presents a new method (GSFast) that generalizes the Fedele and Paola (2007) self-similar gravel grain size method into multidimensions (downstream, across the basin, and overtime/depth) using the FastScape (Braun and Willett, 2013) landscape evolution model. The self-similar model results in a fining rate that is dominantly controlled by the rate of deposition relative to flux. Previously, the Fedele and Paola (2007) grain size fining model has been applied along a single river long profile (1D) to infer a subsidence pattern from observed fining rates such as within the Eocene Montsor Fans in the Southern Pyrenees (Duller et al. 2010; Whittaker et al., 2011). Here, using GSFast, we demonstrate the role of across basin dynamics on grain size fining through a sensitivity analysis comparison with the Duller et al. (2010) single river profile approach. For this, we performed a series of simple numerical experiments to predict grain size fining rate in sedimentary systems of varying spatial extents that are fed by an orogenic source area and undergoing subsidence at a prescribed rate.

When applied in 1D or within shorter confined basins, where all the upstream catchment flux is deposited as gravel within the fan (no alluvial plain), GSFAST can capture the Duller et al. (2010) results under comparable subsidence and flux conditions. This is because both 1D systems and short basins are characterized by no or limited lateral channel mobility resembling sheet flow or incised channel flow.

Conversely, in wide or long (unconfined) basins, the rate of grain size fining predicted by GSFast deviates from the Duller et al. (2010) single profile solution. This deviation occurs due to multiple mobile drainage channels that form when the gravel flux leaving the upstream catchment is unconfined and deposited in both the alluvial fan and adjacent plain. Deviations in grain size fining trend from the Duller et al. (2010) approach correlate with channel mobility dynamics that preferentially form in wide basins or long systems that connect a fan to a large alluvial plain.

Thus, under the same tectonic and climatic boundary conditions, our more dimensionally complex model that incorporates channel mobility leads to different predictions of subsidence patterns from grain size fining curves. This is because our multidimensional approach leads to a growing disconnect between subsidence and deposition rates as channel mobility increases. It also predicts markedly different fining patterns between short systems (i.e., limited to a steep fan-like structure), and long systems (i.e., systems that also incorporate a low gradient alluvial plain).

Duller et al. (2010). JGR: ES 115(F3).

Fedele & Paola. (2007). JGR: ES 112(F2).

Whittaker et al. (2011). Bulletin, 123(7-8).