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Autogenic response of fluvial deltas to steady sea-level fall: Implications from flume-tank experiments

¹ Faculty of Environmental Studies, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan
² Department of Geological Sciences, University of Texas, 1 University Station, C-1100, Austin, Texas 78712, USA

Flume-tank experiments on fluvial deltas, conducted with constant fall of relative sea level (rate A < 0), constant sediment discharge (q_S), and constant water discharge (q_W), reveal a built-in geomorphic process that inevitably causes valley incision ("autoincision") after the beginning of the sea-level fall and forms paired stream terraces on the abandoned alluvial slope. Despite steady sea-level fall, deltas aggrade without major incision until the autoincision threshold is attained, at which time the aggradational regime is replaced by a degradational one. Results of the experiments imply that multiple valley incision and terrace formation can occur as an autogenic response of the depositional system to steady forcing by constant sea-level fall. Changes of A, q_S, and q_W, or changes in river energy, are not required to account for these geomorphic events. The understanding of the autogenic response, when combined with the theory of shoreline autoretreat, provides an alternative view of the geomorphic development of fluviodeltaic systems during base-level fall.

Key Words: deltas • flume studies • regression • sea-level changes • terraces

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