I thought of hydrothermal for Meridiani, I hoped for lake-effects in Gusev, but hints of hydrothermal activity (ok, in the visual) is winning so far for me at Gusev as well, imho.

Keep in mind that the Apollinarispatera is 'nearby' to the North. Hydrothermal activity could have happened here along with volcanic ash/flow. This was the best image I found of Apollinaris on the web.

Note the difference in appearance between the pitted, weathered/eroded rocks to the 'slate'-like rocks right next to each other in this image at Gusev.

Aeolian, Loess(don't know, just had to put this one in here too - a dry to wet clay consistency... due to heavy rains and dry periods on Earth), hydrothermal.... these two types of 'rocks' pictured above were created during different times and under different circumstances (again, mho).

I was hoping we'd hear something by now... news brief...