Impact structure in Meridiani Planum.
Kye Goodwin on the Mark Carey Mars forum, http://www.markcarey.com/mars/discuss-21735-help.html, posted a link to a report given at the 2003 Lunar and Planetary Science Conference showing that Meridiani is the site of an 800 km wide impact basin:
Evidence for an 800KM diameter impact structure in Meridiani Planum and associate channels and basins.
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1414.pdf
This counters the argument that Meridiani could not have been a long lasting lacustrine or oceanic environment because of the lack of a basin there.
Bob Clark