[H-CG21] Earth surface processes related to deposition, erosion and sediment transport

[HCG21-07] Stratigraphic convergence with channel stabilization of the downstream alluvial system under steady sea-level cycles

*Tetsuji Muto¹, Keigo Motomiya², Yasutaka Iijima², Kenya Ono², Miya Shairah Soriano Pamutan¹, Junhui Wang³ (1.Department of Environmental Science, Nagasaki University, 2.INPEX, 3.China University of Petroleum (Beijing))

Keywords:aggradation, autostratigraphy, degradation, non-equilibrium response, regression, transgression

Conventional stratigraphy assumes that the stratigraphic response of the downstream alluvial system is specifically determined by the phase of the eustatic or relative sea-level cycle on a reasonable scale, and that the system repeats a particular response when the sea level cycles into the same phase. Recent advances in experimental stratigraphy and autostratigraphy have led to a fundamentally different view of this question, which can be summarized as follows. (1) In principle, the alluvial response to steady sea-level cycles changes drastically as the system grows because its progressive expansion makes it impossible to maintain the same specific response over time. (2) Eventually, the mature system acquires a particular stratigraphic architecture, after which there is no further change in the sediment stacking pattern, although it is quite different from that as assumed in conventional models. The process towards this final stable state is referred to here as ‘stratigraphic convergence.’ Our physical experiments using the 3D Margi-5 tank at Nagasaki University support these findings. The specific results of the experiments are reviewed and discussed. During the rising stage in early cycles, the downstream alluvial system may experience deltaic regression, possibly followed by deltaic transgression and non-deltaic transgression. As the system continues to expand as the cycle progresses, only non-deltaic transgression can occur. The latter transition is reached when the alluvial system has over-expanded beyond the critical spatial size. In late cycles, when alluvial systems are highly over-expanded, the alluvial channels are intensely stabilized with no avulsion or lateral migration during non-deltaic transgression. In contrast, in earlier cycles, when alluvial systems are less overexpanded, the channels are unstable and frequently change paths. During the falling stage in early cycles, the downstream alluvial system is usually subject to a deep valley incision, whereas in later cycles, there can be an aggradation-sustainable phase in which no system-wide degradation occurs at any time. Regardless of how fast the sea level falls at 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 is repeated. After this autogenic transition in the sedimentary regime, the alluvial channels are virtually aggradational but close to a graded state where they tend to simply extend downstream with little or no avulsion and lateral migration. Recognition of the effects of system growth and stratigraphic convergence is critical to understanding the stratigraphic response of downstream alluvial systems to known or unknown sea-level changes from the geologic record. Although this discussion focuses on alluvial systems growing under steady, symmetric sea-level cycles, it may be applicable to those growing under unsteady and asymmetric cycles, and it is useful to examine the possibility of stratigraphic convergence under each of these conditions. At the very least, the idea that the same stratigraphic response is repeated each time the sea-level reaches the same phase is not generally true, and should be kept in mind when identifying the dominant sea-level forcing and regional correlations.