Can we use only Grain Size Data for Paleo-Flow Reconstructions?

Abstract:

Paleo-flow and paleo-environmental reconstruction from ancient deposits is a critical task for earth surface scientists interested in the sedimentary record. Forming processes are commonly interpreted from the architectural characteristics of sedimentary deposits using quantitative relationships derived from experiments or geomorphic studies. However, very little attention has been paid to the equivalent problem at the scale of micro-facies: can we interpret the conditions at the time of sediment accumulation from grain size information in a small sample? Here we investigate the use of grain size distributions alone to reconstruct the flow conditions based on a set of experiments conducted in a 2D flume tank in the Eurotank facilities at Utrecht University.

The experiments are designed for the examination of grain size distributions within sediments which were deposited by flows with known conditions (e.g. velocity, turbulence, shear velocity, concentration). By changing the slope of the flume tank we were able to create a range of flows from strongly depositional (depletive) to bypassing. Inspired by Eastwood et al (2012), we propose a working methodology to link the grain size distribution of the deposit to flow conditions. Our method utilizes the following empiric relations: i) the finer fraction of the deposit was deposited while most of that range surpass the Bagnold (1966)’s suspension threshold (shear velocity υ_* approx. 3 times the settling velocity ω_s); ii) the mean fraction was at incipient suspension stage (υ_* ~ ω_s); iii) the finer portion of the coarser grains were at bedload/saltation stage, where u_* is approx. υ_*c (critical shear velocity for initiation of motion); and iv) the coarser portion of the coarser grains were at creep-bedload stage, where υ_* is approx. 0.7υ_*c . We test whether this set of rules can be applied to the probability distribution function of deposit grainsize in an inversion that converges on a single value for the shear velocity, and whether this inverted shear velocity is in accordance with the experimental flow conditions. Preliminary application of this method to outcrop work will be given.

Bagnold, R. (1966), U. S. Geol. Surv. Prof. Pap., 422-I, 37 pp.

Eastwood, E. N., G. Kocurek, D. Mohrig, and T. Swanson (2012), J. Geophys. Res., 117, F03035, doi:10.1029/2012JF002368.