re: Recent localized aqueous weathering still possible.
One of the locations where OMEGA identified sulfates were north polar dunes. This area has long been known to be geologically young:
FORMATION OF THE MARTIAN NORTH POLAR GYPSUM DEPOSIT DURING THE AMAZONIAN.
K. E. Fishbaugh1, F. Poulet2, Y. Langevin2, V. Chevrier3, and J-P. Bibring2. 1International Space Science Institute (ISSI), Hallerstrasse 6, Bern CH-3012 Switzerland, ****@issi.unibe.ch. 2Institut d'Astrophysique Spatiale (IAS), Bâtiment 121, 91405 Orsay Campus, France. 3CEREGE, Europóle de l’Arbois, BP80, 13545 Aix-en-Provence Cedex 04, France.
Fourth Mars Polar Science Conference (2006)
http://www.lpi.usra.edu/meetings/polar2006/pdf/8041.pdf
Bob Clark