you'll find lots craters -- including such delightful examples as Becquerel -- with interior deposits of dark material that, so far as I know, has not been analysed. It is 'mysterious' -- as is the white stuff that overlies some of this dark material in some craters.

Oxia Palus from MOC map:
http://www.msss.com/mars_images/moc/moc_atlas/mc11.jpg

These spots, by the way, have long been observed telescopically.

Be that as it may, what we have here is some highly peculiar terrain. The standard interpretation is that the dark material is some kind of 'black sand' -- an erosional material, source unknown -- and the streaks tending generally southwest from the craters are thought to be 'wind streaks.'

However, close inspection shows that these streaks are actually etched into the surface. The floors of the streaks are some ten to fifty meters below the general level of the plains here. The etching is quite striking. There are some interesting formations inside some of these 'streaks' as well.

Further, it appears that the white stuff -- for example, in Becquerel -- has actually been extruded or erupted from below. As the white stuff is generally on top of the black stuff, it was likely deposited later. If the white stuff, whatever it is, came from below the craters, why couldn't the black stuff have been erupted or extruded, too?

Further, there is no sign that the dark deposits in the streaks were deposited by wind. It appears instead that the substance was once fluid and actually flowed out of the craters. (Especially notable in the area around 10-20N/10-20W) Note, also, the proximity to the various chaos and outflow channels to the west. Quite possibly all these features and factors are related.

Cheers, BFelix