THE STRATIGRAPHIC RECORD EXPOSED AT THE RIM AND WALLS OF IAZU CRATER, MARS

Iazu Crater is a late-Noachian- or Hesperian-aged 6.8 km-diameter crater, located ~25 km south of Endeavour Crater in Meridiani Planum. The crater morphology is consistent with formation by a low angle (20-30°) impact, as evidenced by an up-range forbidden zone present in the western ejecta deposit and an asymmetrical crater profile. A wide saddle-shaped erosional gap in the crater rim on the west side has allowed dark sands to be transported into the interior by aeolian processes. Along-track oversampled CRISM observations show a strong absorption feature near 1 μm in these sands consistent with the presence of hematite, likely similar to hematitic concretions known from Opportunity rover observations to have been concentrated as lag deposits on the Meridiani plains. CRISM observations also show that the crater walls expose a thick section of Burns formation polyhydrated sulfate deposits, with dispersed gypsum occurrences, unconformably overlying Noachian-aged basalts that have been slightly altered to Fe-Mg smectites. After identifying and accounting for the eject overturn hinge zone within these layers, we estimate that the original Burns formation section was ~250 m thick. The Noachian section exhibits several repeated deposits in which resistant dark layers with smectite signatures are overlain by banded bright and dark layers. These banded layers become dominant above the main contact with the Burns formation. We interpret the pre-existing Noachian section as documenting the transition from regional-scale fluvial erosion to deposition, culminating in the Burns formation evaporite deposits. Within the sulfate-bearing section, individual layers are traceable for tens to hundreds of meters laterally until faults interrupt continuity. Alternating bright and dark layers repeat with relatively consistent thicknesses, suggesting a rhythmic process of sedimentary deposition.