Paper No. 273-4
Presentation Time: 8:45 AM
CHRONOLOGICAL CONSTRAINTS ON FLUVIAL FAN STRATIGRAPHY IN THE NW GANGES BASIN, INDIA, REVEAL AUTOGENIC AND CLIMATIC CONTROLS ON FAN EVOLUTION
The sedimentary evolution of large fluvial fan systems is not well constrained and as a result the role of autogenic and allogenic controls on fan evolution remains poorly understood. A significant issue has been the absence of well-constrained chronologies for fan stratigraphy with which to evaluate competing models. Here, we report 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. We map the spatial distribution of channel sand bodies deposited by these rivers and develop a chronostratigraphic model for the fluvial succession in a depositional dip perpendicular transect. Sediment cores up to ~50 m deep along two transects were used to reconstruct the shallow stratigraphy of the fan systems. Discontinuous channel sand bodies are separated by floodplain fines that show evidence of pedogenesis and mark the end of episodes of channel aggradation. We obtained 46 optically stimulated luminescence (OSL) dates through the fan stratigraphy to bracket the timing of channel-filling episodes, and their spatial distribution. Mapping of sand bodies coupled with chronostratigraphic constraints permits reconstruction of channel migration patterns and their timing across parts of the Sutlej-Yamuna fans. Chronostratigraphy permits temporal correlation with published measures of monsoon variability. 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 suggesting 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. The scale and timing of valley incision are suggestive of climatic control on fan evolution.