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Preliminary observations on the stratigraphy of the Son river sediments: the YTT, seasonality and post-eruptive river response

Preliminary observations on the stratigraphy of the Son river sediments: the YTT, seasonality and post-eruptive river response

Emma Gatti, Cambridge Quaternary, Department of Geography, University of Cambridge, Downing Street, CAMBRIDGE CB2 3EN, England

During the 2009 field season several sites in the Son river valley were examined to investigate the stratigraphy of the underlying sediments. In particular, attention was directed at establishing the relationship, occurrence and lateral variability of the Young Toba Tuff (YTT, dated at 74 ± 2 ka BP). The aim of these investigations was to determine the impact of the ash fall on the river system sedimentation and the general environmental setting.

The modern Son river is a shallow meandering sand-bed stream with extensive floodplain surfaces and in-channel bars composed predominantly of sand. The river is confined between cliff-like banks which comprise terrace accumulations of earlier late Middle to Late Pleistocene sediments. The occurrence of the modern cliffed valley demonstrates that since the deposition of these terrace-like sediments, the river has incised its’ valley and appears to be migrating broadly southwards.

Preliminary observations on the Ghoghara sites suggest that the YTT overlies a distinctive unit of stratified sand and gravel. The ash accumulated consistently in the fluvial environment, particularly on the floodplain. The ash is, in turn, buried by a several metres of silt-dominated fine-grained sediments and capped by a palaeosol (Holocene). This overall fining-upward sequence appears to reflect a decline in the river flow energy and possible infill of an early course of the river valley floor. The re-equilibration of the river following the ash deposition might have taken place in a relatively short time. The impact of the ash on the system is also connected to the season during which the eruption occurred. The study of the sedimentary structures implies that the ash deposition possibly took place during the dry season, and that the monsoon rains might have contributed to the deposition of the subsequently reworked ash noted in several sections.

Further work will be directed towards elucidating these hypotheses concerning the responses of the river and contemporaneous changes to the surrounding environment, as well as establishing a chronology for these and related events. To this end, it will be necessary to investigate the detailed sedimentology of the pre- and post-depositional characteristics of the sediments, together with the geomorphology of the area.

Publications

  • Gatti, E. 2010 Preliminary observations on the stratigraphy of the Son river sediments: the YTT, seasonality and post-eruptive river response. In: The Toba super-eruption: a critical moment in human evolution? 20-21 February 2010, Centre for Archaeology, Art and Culture : University of Oxford.