In the recent issue of Geology, there is a report by Schon et al. on constraining the age of depositional fans by using impact crater density and cratering rays. The title of the paper is "Unique Chronostratigraphic Marker in Depositional Fan Stratigraphy on Mars: Evidence for ca. 1.25 Ma Gully Activity and Surficial Meltwater Origin"
These depositional fans have several potential mechanisms of formation. Schon et al. divide these mechanisms into 3 categories: Dry mechanisms, wet mechanisms invoking groundwater release (from a confined aquifer or similar feature), and wet mechanisms invoking surficial meltwater. However, testing these hypotheses is difficult. This is further complicated by an inability to constrain the ages of depositional events.
The authors attempt to overcome the latter problem by applying impact crater densities in conjunction with cratering rays. The concept of impact crater density is commonly used to date surfaces in planetary geology. The technique measures crater density over certain areas and uses this density as a proxy to determine how long that surface has existed. The implication is the longer a surface is exposed, the greater the proportion of impact craters compared to surface area. This method is only limited by bodies that recycle their surfaces (like Earth having active plate tectonics) and has limited utility on bodies that readily recover their surface (like Io having active volcanism). However, another problem presents itself when scientists try and constrain the age of a surface that is relatively small. This is the major obstacle to applying this technique to martian depositional fans.
Schon et al. have proposed using secondary craters and rays that can be traced back to other primary craters. These primary craters will, ideally, be located in regions of sufficient surface area that impact crater density is a viable tool for age constraint. Then by applying a little bit of super-position and, bingo, you have constrained the age of the depositional fan. In order for a secondary crater, or a crater ray, to be preserved on the surface of the depositional fan, the depositional fan had to have been present before the primary crater.
Below is the second figure from this paper, showing an image of the depositional fan being studied. The authors have outlined several depositional lobes. The clearest application of the authors method would be contrasting the crater rich area that defines lobe 1 with the other lobes (2-4) which post-date secondary cratering. Since lobe 1 is the oldest depo-center, it would have the highest crater density. Comparing this with subsequent depositional centers, it is clear that the lack of cratering indicates that the subsequent lobes formed after the intial cratering event. This constrains the formation of the younger lobes to within 1.25 Ma (the age of the primary crater).
I thought that this was a very good example of how surficial features can be applied to determining general stratigraphic patterns. This is, to my eyes, an excellent method for constraining the maximum age of deposition. However, this study did not have any method to examine depositional mechanisms. The authors argument, that this study supports an interpretation of meltwater as the depositional process, rests on the fact that there are multiple depositional events forming this fan. I don't argue this contention, I just don't think that surficial meltwater is the only possible mechanism that can display this pattern of deposition. Therefore, I don't know if there is enough evidence yet to make the assertion that liquid water is (was) present as recently as 1.25 Maa.
Schon, S., Head, J., & Fassett, C. (2009). Unique chronostratigraphic marker in depositional fan stratigraphy on Mars: Evidence for ca. 1.25 Ma gully activity and surficial meltwater origin Geology, 37 (3), 207-210 DOI: 10.1130/G25398A.1