Stratigraphic analysis of the Eberswalde sedimentary fan using orbital imagery and comparison with the Jezero delta, Mars

Introduction

The Jezero fan delta has been well studied ever since the Mars 2020’s Perseverance rover landed in the crater in February 2021. There is now a comprehensive understanding of the architecture of the delta thanks to both in situ and orbital imagery; the fan formed in a closed lake as a Gilbert-type delta, showcasing the distinctive topset-foreset-bottomset tripartite [1].

Now that Jezero western fan is better understood, we would like to know if the Jezero delta architecture is relevant elsewhere on Mars: can we find similar geometries? Is Jezero the rule or the exception? To begin to try to answer this question, we focused on another martian sedimentary structure of similar size and volume: the Eberswalde sedimentary fan.

The Eberswalde crater (24.3° S, 326.5°E), located immediately northeast of the Holden crater, along the fluvial network connecting Argyre Planitia to Ares Vallis  displays a spectacular fan shaped feature (~115 km²) with meandering channels in its westernmost part, that was most likely emplaced during the Late Noachian [2] in a closed-basin lake [3]. The volume of the fan is similar to that of Jezero’s western delta, the thickness of sediment is about 50 to 100 m in both cases [2], which makes it an interesting starting point for the comparison of Jezero to other martian sedimentary fans.

Methods

This study focuses on the geomorphological and stratigraphic analysis of the Eberswalde fan using orbital imagery. The HiRISE coverage of the Eberswalde crater is really good, due to the crater being previously considered among the potential landing sites of MSL and Mars 2020 missions. Thanks to HiRISE data, we have access to images with a resolution of up to 0.25 m/pix on the whole fan area.

The HiRISE orthoimages were taken as stereo pairs, which allowed creating stereo-derived DTM, giving access to topographic information. Contour lines and topographic profiles can therefore be extracted on the QGIS software. The dip angles of the outcropping layers can also be measured using the DTM on QGIS, thanks to the Three-Points-Method plugin, which extracts the elevation data from three points placed along the layer and calculates the dip angle and strike of said layer. Dipping layers were already observed by Goudge et al. (2018) [4] before rover data, showing that it is possible to observe them from orbit.

Results

On orbital imagery of the Eberswalde sedimentary fan, extensive meandering inverted channels are visible (Figure 1). It is possible to distinguish five lobes that were emplaced at different periods in time (Figure 1) [2]. The shape of the river channels on the Eberswalde fan differs from that of the channels on the Jezero delta, where they are rectilinear [5]. This indicates a difference in the fluvial regime responsible for the emplacement of these channels.

Figure 1: Map of the meandering inverted channels on the Eberswalde sedimentary fan and their stratigraphic relationships, from oldest to youngest lobes (coloured scale) [2]. White box represents the studied area of Figure 2. (HiRISE – MRO).

The beds outcropping at the front of the Eberswalde fan have dips mostly around 0° to 5°, occasionally reaching up to 8°, and almost always follow the contour lines (Figure 2a). These outcropping strata are therefore mostly subhorizontal, there are no steeply inclined layers at the front of the fan that would correspond to Gilbert-type foreset strata (Figure 2) as was observed in Jezero delta [1].

Two main phases can be distinguished from the orbital data: a lacustrine phase, with the subhorizontal beds outcropping at the front, and the fluvial (meandering) phase occurring afterwards.

Figure 2: (a) Close-up of an area on the Eberswalde sedimentary fan. Contour lines at 5 m are extracted from the stereo-derived HiRISE DTM (HiRISE – MRO). Red line: log line AA’ of the cross-section in (b).
(b) Interpreted cross-section of log AA’ with dip angles measured using the DTM reported on the section. Grey zone represents the lacustrine beds, blue the fluvial beds (recognizable also on the orbital imagery), and brown zone is the dust-covered areas. The red dashed line represents the possible discontinuity contact between the fluvial and lacustrine phases.

Preliminary conclusions

The stratigraphic and geomorphologic data of the present study indicate that the Eberswalde sedimentary fan did not emplace as a Gilbert-type delta, as is the case for the Jezero delta. It is therefore necessary to understand how the Eberswalde fan formed. Two hypotheses can be proposed:

• The fan emplaced in two separate phases, a lacustrine phase first, and a late-stage fluvial phase, emplacing the meandering rivers, with a gap between these two stages.
• Eberswalde is a shoal-water delta, that emplaced in a shallow lake, possibly as an aggradational delta [6], contrary to the progradational settings of the Jezero delta.

Those preliminary conclusions and hypotheses highlight the fact that the processes leading to the formation of the Eberswalde and Jezero fans are different, and the scenario of the Jezero delta should not be taken as the rule on Mars, although we have not explored more fans yet. The in-depth study of other martian sedimentary fans is fundamental for a better understanding of the availability of liquid water at the time they formed.

References

[1]          Mangold N. et al. (2021) Science, 374, 711-717, doi: 10.1126/science.abl4051.

[2]          Pondrelli M. et al. (2008) Icarus, 197, 429-451, doi: 10.1016/j.icarus.2008.05.018.

[3]          Malin M. C. & Edgett K. S. (2003) Science, 302, 1931-1934, doi: 10.1126/science.1090544.

[4]          Goudge T. A. et al. (2017) Earth and Planetary Science Letters, 458, 357-365, doi: 10.1016/j.epsl.2016.10.056.

[5]          Kronyak R. E. et al. (2023) 54^th LPSC, Abstract #2806

[6]          Lewis K. W. & Aharonson O. (2006) J. Geophys. Res., 111, doi: 10.1029/2005JE002558.