We present detailed stratigraphic and spectral analyses that focus on a region in
northern Sinus Meridiani located between 1°N to 5°N latitude and 3°W to 1°E longitude.
Several stratigraphically distinct units are defined and mapped using morphologic
expression, spectral properties, and superposition relationships. Previously unreported
exposures of hydrated sulfates and Fe/Mg smectites are identified using MRO CRISM and
MEX OMEGA near‐infrared (1.0 to 2.5 µm) spectral reflectance observations. Layered
deposits with monohydrated and polyhydrated sulfate spectral signatures that occur in
association with a northeast‐southwest trending valley are reexamined using highresolution
CRISM, HiRISE, and CTX images. Layers that are spectrally dominated by
monohydrated and polyhydrated sulfates are intercalated. The observed compositional
layering implies that multiple wetting events, brine recharge, or fluctuations in evaporation
rate occurred. We infer that these hydrated sulfate‐bearing layers were unconformably
deposited following the extensive erosion of preexisting layered sedimentary rocks and
may postdate the formation of the sulfate‐ and hematite‐bearing unit analyzed by the MER
Opportunity rover. Therefore, at least two episodes of deposition separated by an
unconformity occurred. Fe/Mg phyllosilicates are detected in units that predate the sulfateand
hematite‐bearing unit. The presence of Fe/Mg smectite in older units indicates that the
relatively low pH formation conditions inferred for the younger sulfate‐ and hematitebearing
unit are not representative of the aqueous geochemical environment that prevailed
during the formation and alteration of earlier materials. Sedimentary deposits indicative of
a complex aqueous history that evolved over time are preserved in Sinus Meridiani, Mars
