17:15 〜 18:45
[HQR04-P07] Alluvial-deltaic response to steady cycles of sea-level rise and fall: An insight from physical experiments
キーワード：埋積、オート層序学、削剥、非平衡応答、海水準変動、下刻
A conventional framework of alluvial-deltaic stratigraphy is based on the hypothesis that the stratigraphic response of the depositional system is determined by the phase of the sea-level cycle on a reasonable scale, and that the system repeats a particular response when the sea level cycles to the same phase. In this scheme, it is often explained that the depositional system tends to undergo degradation, typically in the form of valley/channel incision, during the falling stage, especially when the sea level falls rapidly with a large amplitude. Here, we present a totally different view that the alluvial-deltaic response to a steady sea level forcing is generally unsteady (i.e., non-equilibrium response), because the progressive expansion of the system makes it impossible to maintain the same specific response. Thus, the effect of system growth is an essential factor when attempting to resolve stratigraphic responses to known or unknown sea-level changes from the geological record.
We focus here on the falling stage of steady and symmetric sea-level cycles, where rise and fall occur at the same rate and amplitude. Our 2D and 3D physical experiments show that the geomorphic and stratigraphic processes in the alluvial-deltaic system during each cycle change as the number of cycles progresses. What happens in the next cycle is not simply a repeat of what happened in the previous cycle, and the non-equilibrium response itself also changes as the cycles progress. For example, even if there is a deep valley incision during the falling stage of an early cycle, in subsequent cycles there will be an aggradation-sustainable phase where no system-wide degradation occurs at any time. The timing, or the number of cycles elapsed Ncycle_AS, of the transition from the degradation-inclusive phase to the aggradation-sustainable phase is proportional to the cycle amplitude Arsl*, which is made dimensionless with the autostratigraphic length scale (Ncycle_AS ∝ Arsl*). This suggests that no matter how fast the sea level falls with any large amplitude, the transition from the degradation-inclusive phase to the aggradation-sustainable phase will be realized as long as the same pattern of sea level cycles of rise and fall are repeated. If the stratigraphic record of an alluvial-deltaic system obtained from outcrops, borehole samples, seismic profiles, etc., shows evidence of significant valley incision, it may be interpreted as a “snapshot“ of the transition process that will eventually lead the system into the aggradation-sustainable phase. It would thus make sense to consider Quaternary alluvial-deltaic stratigraphy in terms of the successful and unsuccessful attainment of the aggradation-sustainable phase.
We focus here on the falling stage of steady and symmetric sea-level cycles, where rise and fall occur at the same rate and amplitude. Our 2D and 3D physical experiments show that the geomorphic and stratigraphic processes in the alluvial-deltaic system during each cycle change as the number of cycles progresses. What happens in the next cycle is not simply a repeat of what happened in the previous cycle, and the non-equilibrium response itself also changes as the cycles progress. For example, even if there is a deep valley incision during the falling stage of an early cycle, in subsequent cycles there will be an aggradation-sustainable phase where no system-wide degradation occurs at any time. The timing, or the number of cycles elapsed Ncycle_AS, of the transition from the degradation-inclusive phase to the aggradation-sustainable phase is proportional to the cycle amplitude Arsl*, which is made dimensionless with the autostratigraphic length scale (Ncycle_AS ∝ Arsl*). This suggests that no matter how fast the sea level falls with any large amplitude, the transition from the degradation-inclusive phase to the aggradation-sustainable phase will be realized as long as the same pattern of sea level cycles of rise and fall are repeated. If the stratigraphic record of an alluvial-deltaic system obtained from outcrops, borehole samples, seismic profiles, etc., shows evidence of significant valley incision, it may be interpreted as a “snapshot“ of the transition process that will eventually lead the system into the aggradation-sustainable phase. It would thus make sense to consider Quaternary alluvial-deltaic stratigraphy in terms of the successful and unsuccessful attainment of the aggradation-sustainable phase.