Characteristics, Chronology, and Sedimentation History of Avulsion Facies in Fluvial Fan Stratigraphy from the NW Indo–Gangetic Basin

Fluvial aggradation in foreland setting is influenced by factors such as discharge variability, sediment supply and longitudinal profile. These factors are in turn governed by allogenic controls of climate change and/or tectonic activity. Multistoried channel sandbodies (MSB), a manifestation of such controls archived in fan stratigraphy, are good examples of long-term record to constrain the role of allogenic as well as autogenic controls on fan evolution. We present the results of a detailed analysis of the late Quaternary fluvial fan systems formed by the major Himalayan rivers, the Sutlej and Yamuna, in the northwestern Indo-Gangetic basin in NW India. Nine sediment cores down to ~50 m depth along two transects across the fan surface were used to reconstruct the shallow stratigraphy of the fan systems. Discontinuous MSBs are separated by floodplain fines that show evidence of pedogenesis and mark the end of episodes of channel aggradation. Within the MSB, individual channel deposits are identified by the presence of channel scour surfaces. Mapping of sand bodies coupled with chronostratigraphic data permits reconstruction of channel migration patterns and their timing across the fluvial fans. We obtained 46 optically stimulated luminescence (OSL) dates on sand samples from five cores to bracket the timing of channel-filling episodes, and their spatial distribution. Sediment aggradation is laterally continuous in the lower part of the fan as temporal overlaps are observed in depositonal ages across the transect. In the upper part of the fan, sediment aggradation is discontinous both laterally and vertically, and this is in line with 2-5 times reduction in aggradation rates as compared to the lower part of the fan systems. Our results show that much of the fan stratigraphy is characterised by channel sand bodies that show variation in age with spatial position along the transect. This suggests that the stratigraphic record of the fan was built by avulsing fluvial channels. However, in the upper part of the succession, the OSL results reveal the presence of a major incised valley fill eroded into the fan stratigraphy. Finally, the scale and timing of valley incision are suggestive of strong climatic control on fan evolution.