关键词: 古洪水沉积物, 汉江上游, 空间变化, 粒度

Abstract: Through the stratigraphy and geomorphology investigation along the upper reach of the Han River, a loess-soil profile recorded palaeoflood slack-water deposits (SWD) was found at the riverbank near the Yanjiapeng site. This profile (YJP) was examined in great details in the field. The palaeoflood slack-water deposits samples were collected systematically and the grain size of the samples was analyzed in the laboratory. Results indicate that the grain size of the palaeoflood slack-water deposits shows a significant change in space. In the horizontal direction, grain size distribution, cumulative probability distribution and average grain size are the same in the same layer. In the top samples, the content of clay (<2 μm) is between 2.46%-2.97%, the content of silt (2-63 μm) is between 33.80%-44.50%, and the content of sand-sized particles (>63 μm) is between 52.66%-63.64%. The grain size distribution curves of these deposits are consistent in the same layer. In the vertical direction, however, grain size distribution, cumulative probability distribution and average grain size vary, and the grain size becomes smaller down from the top—the grain size distribution curves shift to the left gradually. The particle size of the peak position shows the following feature: top samples > middle samples > bottom samples. Some previous studies indicate that in the vertical direction the grain size of palaeoflood slack-water deposits becomes larger down from the top in the Wei River and Jing River. The results of this study at the YJP site in the upper reach of the Han River proved that the spatial variation of palaeoflood slack-water deposits has different forms. Similar grain size in the same layer indicates that the hydrodynamic force was basically the same in the horizontal direction and this result is good for sample collection in the field. The change of grain size in the vertical direction indicates that the hydrodynamic force was different and the reasons for this change may be related to the temporary river damming during the floods or the gradual built-up and quick withdrawal cycles of the palaeofloods. Samples should be collected layer by layer in the field study of grain size of palaeoflood slack-water deposits. If the palaeoflood slack-water deposits pinch out point cannot be identified in the field, grain size of palaeoflood slack-water deposits pinch out point of the same layer at close proximity can be used instead.

Key words: grain size, palaeoflood SWD, spatial variation, upper Han River