Could be maars.
Nice images. Thanks Marcia. Someone posted an prelanding image of Meridiani showing "craters". The suggestion was these were volcanic caldera rather than impact craters. They may well have been maars, which are round volcanic structures due to volcano-water interaction.
The many craters visible in your second image may then be analogous to maar fields:
Lake Nyos, Oku Volcanic field, Cameroon
http://volcano.und.nodak.edu/vwdocs/volc_images/africa/nyos.html
Bob Clark