re: Sedimentary rock?them thar hills
The maar explanation for the hills would also provide an origin for the basaltic lava seen in Gusev:
Juicy evidence hidden under Martian lava.
"To their great disappointment, Spirit has found little evidence of that water, instead finding that a thick layer of basalt and other volcanic rock flowed over much of the crater, burying the juiciest evidence. It is not clear where the lava would have flowed from."
http://msnbc.msn.com/id/5612594/
It is known olivine containing basaltic lava can emanate from maars.
Bob Clark