Inverted depositional features on Mars

FYI - this post is not about brand new discoveries -- you can find information about these features on Mars Global Surveyor website and also check out a Science paper from 2003 by Malin et al. that goes over this stuff in more detail...I just felt like posting about it.

I was living in Colorado some years back when I went on a trip to look at the local geology of the Front Range and had one of those great field moments when the trip leader pointed out some inverted topography. In this case, a relatively small volcanic eruption produced lava flows that were isolated in a mountain valley. Since that time, the surrounding much softer sediments and sedimentary rocks have eroded away leaving lava-capped mesas that are now segmented by the modern river. The image below is of Golden showing the modern Clear Creek cutting the lava-capped mesa in two (yes, that complex of structures right in the valley is Coors brewery). Image courtesey of www.tablemountains.org.

These modern positive topographic features are representing an ancient valley! Very cool.

So, while wasting time on the internet - in 7-minute increments - I came across some old bookmarks about some inverted topographic features on Mars. The group of features, discovered in November 2003 by Mars Global Surveyor (MGS), have been dubbed the Eberswalde Delta.

The image above is Fig. 2 straight from Malin et al. (2003) showing the exhumed fluvial distributary fan (click on all images for larger view). The conspicuous curvilinear features at closer examination are actually positive topography. Inset of box A below clearly shows the deposits of a migrating meandering stream standing as a plateau above the surrounding area.

This is a striking example of inverted topography. In this case, the channel deposits contain coarser material (likely sand/gravel) and therefore are more resistant than the out-of-channel material (likely mud/silt). No data from the planet surface exists for this location but these are reasonable guesses. The cross-cutting relationships of this feature also nicely reveal that the wide meander swing was abandonded for a shorter, more direct route at some point.

The image below (inset box B) shows more cross-cutting ridges representing channel-fill deposits. I would sure like to walk up the edges of those ridges and plateaus to check out the cross-section of that fill. Another thing to notice is the apparent spreading out of coarser-grained material from left to right. Malin et al. interpret this as the lobe deposits at the end of these channel features....which seems pretty reasonable to me.


The implications of all this, of course, are further evidence of flowing water on Mars. The big problem we are faced with at this point is not so much if water flowed on Mars, but when. The next generation of rovers are gonna half to brush up on their geochronology.