Year

2011

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Terraced alluvial deposits in the Middle Son Valley, Madhya Pradesh, India contain Youngest Toba Tuff (YTT) deposits and an archaeological record that spans the Acheulean to the Neolithic. For the past three decades, this region has been the focus of geological and archaeological investigations that aim to understand the impact of the ~74 thousand year (ka) Toba volcanic super-eruption on the environment and human populations in India. The research presented in this study is focussed on two main themes: 1) developing and applying luminescence dating techniques to alluvial sediments in the Middle Son Valley to assess the reliability of the YTT ash as a reliable chronostratigraphic marker in palaeoenvironmental investigations; and 2) to test a previously published model of alluvial deposition for the Middle Son Valley near the confluence of the Rehi and Son Rivers. The luminescence dating potential of potassium feldspar (KF) was explored at both the single aliquot and the single grain level for sediments in the Middle Son Valley. In this study, KF grains are shown to be suited to single-aliquot regenerative-dose measurement procedures and individual KF grains exhibit fading rates ranging from 0 to more than 20 %/decade. Post-infrared infrared signals (pIRIR) are shown to be less susceptible to anomalous fading, as expected, but evidence presented here suggests that pIRIR ages for alluvial sediments in the Middle Son Valley are less reliable than IRSL ages, because the source traps for these signals are less likely to be completely emptied by sun exposure during transport and deposition in the Son River. IRSL ages from KF and OSL ages from quartz presented in this study suggest that the final deposition of the sediments above and below the YTT ash deposits in the Middle Son Valley occurred sometime (possibly up to a few tens of thousands of years) after the Toba volcanic super-eruption. This chronology suggests that: 1) the YTT ash has been reworked by fluvial processes and cannot be considered a reliable chronostratigraphic marker as was previously thought; or 2) the YTT ash was deposited soon after the volcanic event ~74 ka ago, but the underlying sediments have since been eroded and replaced by younger, inset fluvial sediments. In both cases, the temporal framework presented here calls into question the validity of previous palaeoenvironmental interpretations and hypotheses that were based on pedogenic carbonates sampled above, below and within the YTT ash. A previously published model of alluvial deposition for the Middle Son Valley subdivides the preserved alluvium within the valley system into five stratigraphic formations. These formations include (from oldest to youngest) the Sihawal, Khunteli, Patpara, Baghor and Khetaunhi Formations. The accuracy of this model was tested near the Rehi-Son confluence using cross-valley topographic profiles, field observations, and IRSL age estimates from terraced alluvial sediments. IRSL age estimates for the highest terrace mark the beginning of incision of the Middle Son Valley alluvium at ~16 ka (as predicted by the model) and the termination of deposition of the Baghor Formation fine member silts between ~16 and ~21 ka ago. Maximum IRSL age estimates of ~1.9–2.7 ka, ~10 m above river level, mark the termination of deposition of the Khetaunhi Formation silts and the sands on the lowest terrace. These ages are only slightly younger than previously reported radiocarbon age estimates of ~3–5 ka for this formation. The age estimates for coarse sands and gravels that lie at intermediate elevations (~20 m above river level) between these two terraces contradict what is predicted by the model. According to the model, these sediments should be between ~40 and 58 ka in age and form part of the Patpara Formation, which has been exposed by fluvial erosion of the overlying Baghor Formation. By contrast, maximum IRSL age estimates presented here suggest that these deposits are only up to ~5–7 ka in age and form inset sediments that were deposited during a brief aggradational phase, sometime after incision of the highest alluvial surface ~16 ka ago. Incision of the Middle Son Valley alluvium across from the Rehi-Son confluence began shortly after ~16–21 ka, probably as a result of SW monsoon intensification. The inset coarse sand and gravel at ~20 m above river level likely aggraded under wet conditions in the early Holocene, and the lowest (~10 m high) inset terrace probably aggraded under more arid conditions during the late Holocene. Both the Middle Son Valley, and its neighbour, the Belan Valley, record major phases in regional climate and human occupation of northern India. Improved chronological control on the stratigraphies of these valleys therefore has implications for enhancing our understanding the history of climate change and human occupation in India.

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