Abstract. The
grain-size distribution of ancient alluvial systems is commonly determined
from surface samples of vertically exposed sections of gravel deposits. This
method relies on the hypothesis that the grain-size distribution obtained
from a vertical cross section is equivalent to that of the riverbed. Such an
hypothesis implies first that the sediments are uniform in size in the river
bed, and second that the sampling method implemented on a vertical section
leads to a grain-size distribution equivalent to the bulk one. Here, we
report a field test of this hypothesis on granulometric samples collected in
an active, gravel-bedded, braided stream: the Urumqi River in China. We
compare data from volumetric samples of a trench excavated in an active
thread and from surface counts performed on the trench vertical faces. Based
on this data set, we show that the grain-size distributions obtained from all
the samples are similar and that the deposit is uniform at the scale of the
river active layer, a layer extending from the surface to a depth of
approximately 10 times the size of the largest clasts. As a consequence, the
grid-by-number method implemented vertically leads to a grain-size
distribution equivalent to the one obtained by a bulk volumetric sampling.
This study thus brings support to the hypothesis that vertical surface counts
provide an accurate characterization of the grain-size distribution of
paleo-braided rivers.
STUDY ON RIVER-BED VARIATION AND ITS GRAIN SIZE DISTRIBUTION IN COMPOUND STONY-BED RIVER WITH MEANDERING AND STRAIGHT CHANNEL
