Stepped Fans and Phyllosilicates on Mars

Post by Peter Grindrod, Natural History Museum, London, UK.

A number of different studies have catalogued features on Mars that could be given the general heading of sedimentary fans [e.g. Irwin et al., 2005; Kraal et al. 2008]. These features occur whenever the velocity of a river or stream decreases, and the water no longer has enough energy to carry its sediment, and thus begins to deposit its load. This drop in energy often occurs when the water flows into flatter and wider regions. The distribution of these fans on Mars is important because it shows the location of past water flows, and the amount of material that has been transported (which can be used as a proxy for flow duration).

However, one of the fundamental problems when looking at these features with orbital data alone, is that it is difficult to determine whether the river flowed into a standing body of water (for example a lake) or just an empty canyon or crater. Of course, the implications of this problem are important if we want to understand the volume and distribution of past water on Mars, which in themselves feed into understanding the past climate and even habitability of Mars.

Figure 1

Image 1: Location of the two fans in Coprates Catena, SE Valles Marineris. MOLA elevation overlain on THEMIS daytime image.

We recently [Grindrod et al., 2018] looked at a couple of interesting sedimentary fans on Mars, which have distinctive steps present (Images 1-3). These two fans had been looked at before in two separate studies that came to opposite conclusions [Di Achille et al., 2006; Weitz et al., 2006], and we wanted to use the most recent data to see if we could help solve the problem of how they formed.

Figure 2

Image 2: 3D perspective of Fan 1, made with a CTX stereo Digital Terrain Model (DTM). The fan is approximately 8 km across at its widest point near its base, and 1 km in height from top to bottom. Total fan volume is about 6.1 km3. No vertical exaggeration. DTM made from images P05_003157_1650 and P08_003658_1658.

Both fans are in relatively small, closed canyons in the South East of Valles Marineris. In one case there is an obvious channel that has fed the fan, but tectonism has probably removed any channel at the other site. In addition to the steps, one of the most distinctive features about these fans is that they are very close to some light-toned deposits, which we and others had previously studied and concluded were rich in phyllosilicate and sulfate minerals that were evidence of past water action [Grindrod et al., 2012; Weitz and Bishop, 2016]. Moreover, the layering in some of these light-toned deposits matched that we’d expect if they were laid down in a standing body of water.

Figure 3

Image 3: 3D perspective of Fan 2, made with a HiRISE stereo DTM. Fan 2 is approximately 3.5 km across at its widest point near its base, and 0.5 km in height from top to bottom. Total fan volume is about 0.8 km3. Vertical exaggeration is x2. DTM made with images ESP_016857_1645 and ESP_017569_1645.

By making high-resolution Digital Terrain Models (DTMs), we looked at the detailed geomorphology and sedimentary structure of the fans. Overall, despite their different sizes, the fans have some important similarities. Both fans can be split into lower ridged deposits (with no steps) that interfinger with the light-toned deposits, and stepped deposits that occur in the upper part of the fan. And importantly, both fans show evidence for late-stage sub-aerial fluvial action through small channels and associated small fans on the upper surface of the main fans.

We interpreted this general structure to be the result of a two-stage process (Image 4): (1) deposition in the troughs was at first into relatively shallow standing water, forming fluvial or alluvial fans that terminated in delta deposits and interfingered with lake deposits, followed by (2) a later period of deposition under sub-aerial conditions, forming alluvial, sheet-like, fan deposits.

The lower deltaic deposits underwent relief inversion of channel-like coarser-grained units during aeolian erosion, leaving ridged digitate features, before the upper deposits were laid down. So there was likely a gap in the fluvial activity, during which time the shallow water in the troughs disappeared. In the upper stepped fan deposits, this aeolian erosion preferentially removed the underlying fines from beneath a single layer, causing undercutting and backward erosion of the coarse-grained material at the surface.

We therefore suggest that the distinctive stepped appearance of these fans is the result of this aeolian erosion, and is not a primary depositional feature, unlike in previous studies. The period of standing water coincides with the timing of phyllosilicate formation in these troughs, and although it is not possible to determine whether these clays are detrital or formed in situ, we have found evidence in both troughs that could support both diagenetic or detrital processes.

Figure 4

Image 4: Animation of schematic formation – not to scale.

This combined formation framework for stepped fans and phyllosilicates can also explain other similar features on Mars, and adds to the growing evidence of fluvial activity in the equatorial region of Mars during the Hesperian and Early Amazonian.

The definite identification of equivalent facies in the fans and phyllosilicates is rare on Mars, and so if anybody is thinking of future work, then it would be great if there was a systematic study of these stepped fans, looking to see if they all have a similar architecture. And especially important would be looking for channels carved into the upper surfaces (steps) that reflect the changing depositional environment, and the final alluvial processes.

References

Di Achille, G., G. G. Ori, D. Reiss, E. Hauber, K. Gwinner, G. Michael, and G. Neukum (2006), A steep fan at Coprates Catena, Valles Marineris, Mars, as seen by HRSC data, Geophys. Res. Lett., 33, L07204, doi:10.1029/2005GL025435.

Grindrod, P. M., M. West, N. H. Warner, and S. Gupta (2012), Formation of an Hesperian-aged sedimentary basin containing phyllosilicates in Coprates Catena, Mars, Icarus, 218, 178-195, doi:10.1016/j.icarus.2011.11.027.

Grindrod, P.M., N.H. Warner, D.E.J. Hobley, C. Schwartz, and S. Gupta (2018), Stepped fans and facies-equivalent phyllosilicates in Coprates Catena, Mars, Icarus, doi:10.1016/j.icarus.2017.10.030.

Irwin, R. P. A. D. Howard, R. A. Craddock, and J. M. Moore (2005), An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolakes development, J. Geophys. Res., 110, E12S15, doi:10.1029/2005JE002460.

Kraal, E. R., M. van Dijk, G. Postma, and M. G. Kleinhans (2008), Martian stepped-delta formation by rapid water release, Nature, 421, 973-976, doi:10.1038/nature06615.

Weitz, C. M., R. P. Irwin, F. C. Chuang, M. C. Bourke, and D. A. Crown (2006), Formation of a terraced fan deposit in Coprates Catena, Mars, Icarus, 184, 436-451, doi:10.1016/j.icarus.2006.05.024.

Weitz, C. M. and J. L. Bishop (2016), Stratigraphy and formation of clays, sulfates, and hydrated silica within a depression in Coprates Catena, Mars, J. Geophys. Res., 121, 805–835, doi:10.1002/2015JE004954.

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