10:45 AM - 12:15 PM
[MIS20-P02] Reconstructing past sea-level change on Mars using delta morphology
Keywords:Mars, delta geomorphology, flume-tank experiment
Present-day Mars is extremely cold and dry environment with no liquid water on surface. However, it is thought that ancient Mars had an ocean in the lowlands of the northern hemisphere about 4.0 to 3.8 billion years ago. Evidence for this is the presence of fluvial landforms such as outflow channels, and deltaic landforms formed at the river mouths similar to those on Earth. There have been several previous studies on Martian delta such as Di Achille and Hynek (2010), which examined the distribution of deltas connected to inflow and outflow rivers. However, there have been few studies examining the relationship between the global distribution of delta geomorphology and their formational mechanisms based on the experimental studies. In this study, we explored the distributions of Martian deltas and classified them by morphology. In addition, we compared the morphology of Martian deltas with the experimental results which investigated variations in delta shape based on changes in sea-levels and sediment discharges, and considered under what conditions each morphology of deltas was formed.
We explored the delta geomorphology around the dichotomy boundary which are interpreted as the coastlines of ancient Mars, using CTX, THEMIS, and HiRISE images by ArcGIS software. The deltas were classified into five types based on its morphology, such as bird-foot, fan, stepped, and lobe, and the distribution of each types were plotted. The altitude profiles of each deltas were created from CTX DEM. The delta morphologies were then compared with the experimental results, and the formational mechanisms of each delta were discussed. Deltas were widely found around dichotomy boundary, and the five deltas were distributed at slightly different elevations. On the other hand, delta was not found around dichotomy boundary at 80°W to 140°W and in the area where is located more north of dichotomy boundary. The former one is in the Tharsis volcanic zone, and the latter one is an area of Amazonian sediments, where the delta cannot be formed.
Next, we compared the delta morphology with experimental results. We found that the stepped deltas were similar to those of an experiment simulating sea-level rise (Muto and Steel, 2001, Geology), suggesting that they were formed by transgression. Some of the stepped deltas also have channels near the delta distal part, similar to the results of the sea-level fall experiment (Muto and Steel, 2004, Geology). Therefore, it is interpreted that the proximal part of the delta was formed by sea-level rise, then regression happened under non-sediment-supplied environment. Thereafter, the distal part of the delta with channels may be formed due to the regression under a sediment-supplied conditions. On the other hand, the lobe delta is similar to the experimental results of sea-level fall (Muto et al., 2016, Sedimentology), suggesting that it was formed by regression. This is consistent with the interpretation based on stratigraphic analysis from a previous study (Fawdon et al., 2018, EPSL) on the same deltaic landform.
We also note that the stepped deltas, which are thought to have formed during the transgressive process, are distributed at higher elevations than the lobe and bird-foot deltas, which are thought to have formed during the regression process. The latter one can be formed during the large-scale regression process during the Late Hesperian period when Martian surface water disappeared. We are now conducting to quantitatively evaluate sea-level change on ancient Mars by more detailed topographic analysis and comparing with the sea-level changes in the experimental setting, taking into account the scaling law factors.
We explored the delta geomorphology around the dichotomy boundary which are interpreted as the coastlines of ancient Mars, using CTX, THEMIS, and HiRISE images by ArcGIS software. The deltas were classified into five types based on its morphology, such as bird-foot, fan, stepped, and lobe, and the distribution of each types were plotted. The altitude profiles of each deltas were created from CTX DEM. The delta morphologies were then compared with the experimental results, and the formational mechanisms of each delta were discussed. Deltas were widely found around dichotomy boundary, and the five deltas were distributed at slightly different elevations. On the other hand, delta was not found around dichotomy boundary at 80°W to 140°W and in the area where is located more north of dichotomy boundary. The former one is in the Tharsis volcanic zone, and the latter one is an area of Amazonian sediments, where the delta cannot be formed.
Next, we compared the delta morphology with experimental results. We found that the stepped deltas were similar to those of an experiment simulating sea-level rise (Muto and Steel, 2001, Geology), suggesting that they were formed by transgression. Some of the stepped deltas also have channels near the delta distal part, similar to the results of the sea-level fall experiment (Muto and Steel, 2004, Geology). Therefore, it is interpreted that the proximal part of the delta was formed by sea-level rise, then regression happened under non-sediment-supplied environment. Thereafter, the distal part of the delta with channels may be formed due to the regression under a sediment-supplied conditions. On the other hand, the lobe delta is similar to the experimental results of sea-level fall (Muto et al., 2016, Sedimentology), suggesting that it was formed by regression. This is consistent with the interpretation based on stratigraphic analysis from a previous study (Fawdon et al., 2018, EPSL) on the same deltaic landform.
We also note that the stepped deltas, which are thought to have formed during the transgressive process, are distributed at higher elevations than the lobe and bird-foot deltas, which are thought to have formed during the regression process. The latter one can be formed during the large-scale regression process during the Late Hesperian period when Martian surface water disappeared. We are now conducting to quantitatively evaluate sea-level change on ancient Mars by more detailed topographic analysis and comparing with the sea-level changes in the experimental setting, taking into account the scaling law factors.