Alluvium of antiquity: Polycyclic terraces in a confined bedrock valley
Author(s)
Kermode, S.
Cohen, T.
Reinfelds, I. V.
Nanson, G.
Pietsch, Tim
Griffith University Author(s)
Year published
2012
Metadata
Show full item recordAbstract
Confined river valleys are not the localities where long term preservation of alluvium would be expected. The 25 km long low gradient (0.0014 m.m− 1) confined valley setting of the Shoalhaven River has archived alluvium of middle Pleistocene age to maintain a relatively uniform channel as an efficient conduit for a wide range of flows in a confined bedrock valley of variable morphology. Single-grain optically stimulated luminescence (OSL) dating has identified polycyclic terraces up to 193 ka in age (marine isotope stage [MIS] 7) with lower terrace remnants dating from 173–140 ka (MIS 6) and 106 ka (MIS 5). Holocene alluvium ...
View more >Confined river valleys are not the localities where long term preservation of alluvium would be expected. The 25 km long low gradient (0.0014 m.m− 1) confined valley setting of the Shoalhaven River has archived alluvium of middle Pleistocene age to maintain a relatively uniform channel as an efficient conduit for a wide range of flows in a confined bedrock valley of variable morphology. Single-grain optically stimulated luminescence (OSL) dating has identified polycyclic terraces up to 193 ka in age (marine isotope stage [MIS] 7) with lower terrace remnants dating from 173–140 ka (MIS 6) and 106 ka (MIS 5). Holocene alluvium 2–3.5 ka in age caps these old Pleistocene units and a well-constrained combination of one-dimensional and two-dimensional hydraulic modelling demonstrates that these polycyclic terraces are clearly within reach of the modern hydrological regime. The 106 ka terrace at 17 m above low flow is inundated by floods recurring on average every ~ 20 years, and the 140–193 ka terraces at 20–22 m are overtopped every 50–100 years. These ancient diachronous landforms exhibit complex depositional histories and are on-lapped by longitudinal benches of modern sand and gravel. Their polycyclic nature appears to be a response to flow reduction, using alluvium to adjust the boundary of the otherwise inflexible morphology of a bedrock gorge.
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View more >Confined river valleys are not the localities where long term preservation of alluvium would be expected. The 25 km long low gradient (0.0014 m.m− 1) confined valley setting of the Shoalhaven River has archived alluvium of middle Pleistocene age to maintain a relatively uniform channel as an efficient conduit for a wide range of flows in a confined bedrock valley of variable morphology. Single-grain optically stimulated luminescence (OSL) dating has identified polycyclic terraces up to 193 ka in age (marine isotope stage [MIS] 7) with lower terrace remnants dating from 173–140 ka (MIS 6) and 106 ka (MIS 5). Holocene alluvium 2–3.5 ka in age caps these old Pleistocene units and a well-constrained combination of one-dimensional and two-dimensional hydraulic modelling demonstrates that these polycyclic terraces are clearly within reach of the modern hydrological regime. The 106 ka terrace at 17 m above low flow is inundated by floods recurring on average every ~ 20 years, and the 140–193 ka terraces at 20–22 m are overtopped every 50–100 years. These ancient diachronous landforms exhibit complex depositional histories and are on-lapped by longitudinal benches of modern sand and gravel. Their polycyclic nature appears to be a response to flow reduction, using alluvium to adjust the boundary of the otherwise inflexible morphology of a bedrock gorge.
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Journal Title
Geomorphology
Volume
139-140
Subject
Geomorphology and Regolith and Landscape Evolution
Geology
Physical Geography and Environmental Geoscience