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VOLCANISM AND SEDIMENTATION: ORIGINS OF THE SMOOTHEST PLAINS ON MARS
Analysis of MGS data has led to a reinterpretation of Amazonis Planitia (AP), a plain located west of Olympus Mons containing some of the smoothest surfaces on Mars. MOC and MOLA data reveal: 1) evidence of previously undetected lava flows from the Tharsis Montes and from Olympus Mons; 2) structures indicative of ice-lava interactions, suggesting that subsurface ice was present in the distant geologic past (Late Hesperian) and also very recently (1-10 Mya); 3) evidence for at least two distinct episodes of lava flow through Marte Vallis; and 4) evidence that the current surface consists of several meters of material that appears to result from aqueous flooding.
AP is a topographically low region dominated by a large smooth unit (~ 1.2 x 106 km2). The extreme smoothness of this unit has resulted from repeated resurfacing, both fluvial and volcanic. The region was first resurfaced by regional emplacement of early Hesperian wrinkle-ridged plains, then by a series of events that buried the wrinkle ridges to a depth of approximately 75 m. Sources for the mantling material include a lava flow unit from southern Tharsis, at least two stratigraphically distinct lava flow events from Elysium Planitia via Marte Vallis, and aqueous sedimentation from the south and west. Pseudocrater-like structures suggest that the fluvial and volcanic episodes were closely temporally related.
On the basis of these observations, we propose the following geological history for Amazonis Planitia: 1) Early Hesperian volcanic resurfacing; 2) wrinkle ridge formation; 3) sedimentation through Mangala Valles outflow channels; 4) phreatic saturation and ground ice formation; 5) emplacement of flows from a pre-aureole, proto-Olympus Mons volcano; 6) emplacement of the Olympus Mons aureole; 7) emplacement of flows from southern Tharsis; 8) emplacement of flows through Marte Vallis; 9) sedimentation through Marte Vallis outflow channels; 10) emplacement of very young flows through Marte Vallis.