THE RELATIVE TIMING OF ALLUVIAL ACTIVITY IN GALE CRATER, MARS
Crater statistics in combination with geologic mapping reveal the oldest units in Gale date from the early-to-mid Hesperian and suggest their emplacement began shortly after formation of the crater. The older units likely include the bulk of the deposits associated with the alluvial fans. However, younger ages are also derived for some surfaces, implying a period of geomorphic activity during the later Hesperian or into the Amazonian.
Much of the later activity may relate to exhumation as the deposits comprising Mt Sharp retreated to the current, eroded form near the crater center. However, local surfaces on some fans, including portions of the Peace Vallis fan, may reflect a veneer of sediments on the broader fan form. Erosional relief (e.g., required for topographic inversion of relict distributaries) implies ~5-10 m surface lowering on these surfaces, consistent with a paucity of craters <30-40 m. Coupled with the absence of pedestal craters or other morphologies pointing to exhumation as the sole cause of late activity, these data suggest late occurring, local alluvial activity on some fans. The small areal extent of the units and differences in surface properties make age determination from small diameter craters difficult. Nevertheless, the timing of possible late alluvial activity in Gale correlates with alluvial activity in Margaritifer Terra hypothesized to result from melting snow. At Peace Vallis, any late deposition appears limited to the upper fan, though sediment contributions to the areas explored by Curiosity cannot yet be ruled out. Regardless, associated water would have drained downslope and could have contributed to development of late diagenetic features observed by Curiosity in the rocks at Yellowknife Bay.