A Holocene "frozen accident": sediments of extreme paleofloods and fires in the bedrock-confined upper Huis River, Western Cape, South Africa

Bordy, E. M., Bowen, D. A., Moore, J., Garnett, M. H. and Tsikos, H. (2018) A Holocene "frozen accident": sediments of extreme paleofloods and fires in the bedrock-confined upper Huis River, Western Cape, South Africa. Journal of Sedimentary Research, 88(6), pp. 696-716. (doi:10.2110/jsr.2018.29)

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Abstract

Wildfires and flooding events are common and are forceful intrinsic controls over landscape evolution, biodiversity, and preserved sediment architecture in dryland environments. Charcoal-bearing Holocene flood sediments of the upper Huis River provide a rare perspective on the powerful and episodic sedimentary processes in a bedrock-confined fluvial setting in the tectonically stable SW Cape Fold Belt in South Africa. The sediments described in this paper are associated with high-magnitude, debris-flow-dominated paleofloods, and their charcoal content is linked to a series of radiocarbon-dated Holocene paleofires that occurred from ∼ 2165 ± 37 BP to ∼ 653 ± 35 BP. The five sedimentary facies associations are documented as products of: a) noncohesive pseudoplastic debris flows; b) transitional, high-matrix-strength debris flows with heterogeneous fluid content and flow behavior; c) low-cohesion debris flows; d) hyperconcentrated flows; and e) fluvial channel flow in the upper Huis River. The last is interpreted mainly from massive, subrounded to subangular boulder bars, which provide key evidence for the dramatic scouring of the upper Huis valley. The paleofloods, which not only filled the valley with debris-flow sediments up to 12 m thick, but also subsequently flushed it out nearly to the bedrock, had estimated extreme discharges of few thousands of m3/s. In summary, the upper Huis River sediments are exceptional because they preserve the geological record of recurring fires, and at least three extreme paleofloods (i.e., massive sedimentation events) over a period of ∼ 1500 years in an area typified by the fire-prone and fire-dependent Fynbos Biome. Furthermore, this study provides insights into what the gaps in the commonly fragmented bedrock-confined alluvial stratigraphic record would be like, should there be “more record than gap.”

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Garnett, Dr Mark
Authors: Bordy, E. M., Bowen, D. A., Moore, J., Garnett, M. H., and Tsikos, H.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Journal of Sedimentary Research
Publisher:Society for Sedimentary Geology
ISSN:1527-1404
ISSN (Online):1938-3681
Published Online:05 June 2018
Copyright Holders:Copyright © 2018 SEPM (Society for Sedimentary Geology)
First Published:First published in Journal of Sedimentary Research 88(6):696-716
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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