From the program of the September, 2005 Division of Planetary Sciences of the AAS meeting:

http://www.aas.org/publications/baas/v37n3/dps2005/dps2005block.html

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[32.12] Outcrops of clay rich crust in Mawrth Vallis, Mars, revealed by OMEGA and HRSC/MEX
D. Loizeau, N. Mangold (IDES-Orsay), F. Poulet, J.-P. Bibring, A. Gendrin, C. Gomez, Y. Langevin, B. Gondet (IAS-Orsay), V. Ansan, P. Masson (IDES-Orsay), G. Neukum (FU-Berlin), OMEGA Team, HRSC Team

The hyperspectral imager OMEGA aboard Mars Express found hydrated minerals in the region of Mawrth Vallis, Mars, by the detection of the 1.9 μm hydration absorption band, caused by the H2O molecule inside the mineral structure. For these hydrated minerals, the combination bands due to the Fe-OH bond at 2.3 μm and the Al-OH bond at 2.2 μm reveal the presence of clay minerals: ferric smectites and montmorillonites (Poulet et al., this conference). HRSC images indicate that the clays correspond to bright outcrops on the plateaus each side of Mawrth Vallis. These plateaus are part of highly cratered Noachian terrain (> 3.7 Gy). On these bright clay rich outcrops, MOC images show light-toned layered deposits, as seen by Malin and Edgett (Science, 2000). The intense wind erosion of these outcrops implies that clays are not only surfacial, but that the bright sedimentary rock itself is made of clays. The observation of such a large amount of clays in this region implies extensive alteration of igneous rocks by water, and the subsequent deposition of clays.

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http://www.aas.org/publications/baas/v37n3/dps2005/575.htm

It is interesting these clays are part of the Noachian terrain on Mars, the earliest epoch in Mars geological history, earlier than about 3.5 to 3.8 billion years ago.
This is the period when it was hypothesized that an ocean could have existed on Mars. The recent announcement from Mars meteorite analysis that Mars has been cold for billions of years might not be in conflict with this if this ocean disappeared subsequent to the Noachian age.

Bob Clark