11:00 AM - 11:15 AM
[HCG20-02] Autostratigraphic modelling of the growth of alluvial-shelf systems during base-level cycles with long-term subsidence
Autostratigraphic modelling of the growth of alluvial-shelf systems during base-level cycles with long-term subsidence
Bofu Liu1, Junhui Wang2 , Tetsuji Muto1
1Department of Environmental Science, Nagasaki University, Nagasaki 852-8521, Japan
2College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
The stratigraphic evolution of sedimentary shelves, whether continental or non-continental, is a central topic in autostratigraphy and also in the “source-to-sink” study. To explore non-equilibrium responses that the alluvial-shelf system takes through constant base level cycles with long-term constant subsidence, we conducted model experiments using tank facilities at Nagasaki University (Margi 6, 650 cm × 60 cm × depth 130 cm). The experiments brought a finding that the subsidence can inevitably give rise to an unsteady manner of shelf evolution, just like the autoretreat-autobreak process in case of a river delta growing under a single constant base-level rise.
The experiments were conducted as follows. Sediment (uniform quartz sand, 0.2 mm in diameter, 1.38 g/cm3 in bulk density) was supplied at a constant rate (qs = 0.147 cm2 s-1) with constant water discharge (qw = 4.56 cm2 s-1) from the upstream end of an acrylic open channel (length 410 cm × internal width 2 cm × height 57 cm) placed in the tank. Height of water surface in the tank (i.e., base level) was controlled by PC-connected electro-magnetic flowmeter, whereby base level cycles of an arbitrary pattern could be realized. A small amount of coal powder was added at the end of every half cycle, to produce visible time lines and make it easy to identify half-cycle depositional units. Three series (FP, SR, FreS) were designed. In the FP series, the subsidence rate was fixed through runs, whereas the half-cycle period for base level fall was changed by runs. In the SR series, the rate of the subsidence was changed by runs, whereas the half-cycle periods for each of base level rise and fall were fixed through runs. In the FreS series, the subsidence rate was fixed through runs, whereas the frequency and amplitude of base level cycles were changed by runs. The locations of moving boundaries (delta front DT, deltaic or non-deltaic shoreline S, alluvial-basement transition ABT) were measured in ImageJf rom pictures taken with interval photographing (15−60 s) during runs.
The experimental results clearly indicate that the shelf system takes a particular manner of non-equilibrium response through base level cycles, as summarized below. A lowstand shelf-edge trajectory advanced basinward in the early cycles, but turned to landward retreat as the cycles progresses, and eventually attains a critical moment after which the shelf can no longer retain its original geometry. The shelf system under long-term subsidence has a strict limit in basinward expansion.
Bofu Liu1, Junhui Wang2 , Tetsuji Muto1
1Department of Environmental Science, Nagasaki University, Nagasaki 852-8521, Japan
2College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China
The stratigraphic evolution of sedimentary shelves, whether continental or non-continental, is a central topic in autostratigraphy and also in the “source-to-sink” study. To explore non-equilibrium responses that the alluvial-shelf system takes through constant base level cycles with long-term constant subsidence, we conducted model experiments using tank facilities at Nagasaki University (Margi 6, 650 cm × 60 cm × depth 130 cm). The experiments brought a finding that the subsidence can inevitably give rise to an unsteady manner of shelf evolution, just like the autoretreat-autobreak process in case of a river delta growing under a single constant base-level rise.
The experiments were conducted as follows. Sediment (uniform quartz sand, 0.2 mm in diameter, 1.38 g/cm3 in bulk density) was supplied at a constant rate (qs = 0.147 cm2 s-1) with constant water discharge (qw = 4.56 cm2 s-1) from the upstream end of an acrylic open channel (length 410 cm × internal width 2 cm × height 57 cm) placed in the tank. Height of water surface in the tank (i.e., base level) was controlled by PC-connected electro-magnetic flowmeter, whereby base level cycles of an arbitrary pattern could be realized. A small amount of coal powder was added at the end of every half cycle, to produce visible time lines and make it easy to identify half-cycle depositional units. Three series (FP, SR, FreS) were designed. In the FP series, the subsidence rate was fixed through runs, whereas the half-cycle period for base level fall was changed by runs. In the SR series, the rate of the subsidence was changed by runs, whereas the half-cycle periods for each of base level rise and fall were fixed through runs. In the FreS series, the subsidence rate was fixed through runs, whereas the frequency and amplitude of base level cycles were changed by runs. The locations of moving boundaries (delta front DT, deltaic or non-deltaic shoreline S, alluvial-basement transition ABT) were measured in ImageJf rom pictures taken with interval photographing (15−60 s) during runs.
The experimental results clearly indicate that the shelf system takes a particular manner of non-equilibrium response through base level cycles, as summarized below. A lowstand shelf-edge trajectory advanced basinward in the early cycles, but turned to landward retreat as the cycles progresses, and eventually attains a critical moment after which the shelf can no longer retain its original geometry. The shelf system under long-term subsidence has a strict limit in basinward expansion.