Andrew Moodie, Timothy Goudge

Interpreting the structures, morphology, and chemistry of the exposed stratigraphic record on Mars is complicated by ancient surface processes that have variably removed parts of the record. Previous research has used the lack of smaller craters (<50 m diameter) interbedded with fluvial deposits to constrain atmospheric pressure when rivers were active on Mars; the notion being that higher atmospheric pressure would prevent smaller craters from forming. We hypothesize that contemporaneous channel lateral migration and avulsion could have reworked sedimentary deposits and eliminated craters from the stratigraphic record, thereby undermining atmospheric paleo-pressure interpretations. To test this hypothesis, we simulated coeval river-delta development and crater production, and quantified crater preservation in resulting stratigraphy. We document widespread crater rim degradation (~67% of craters <50 m are at least partially eroded), and observe a marked increase in preservation with increasing crater diameter. That is to say, fluvial reworking preferentially removes smaller craters from the stratigraphic record. However, synthetic crater-diameter distributions incorporating fluvial reworking effects do not reproduce observations on Mars, because many smaller craters generated remain preserved in the simulated stratigraphy. We find that, although river channels are sometimes in the right place to eliminate crater deposits from the stratigraphic record, production of smaller craters outpaces fluvial reworking under all modeled circumstances, and that a higher pressure ancient atmosphere is necessary to reproduce observations (i.e., consistent with existing interpretations of interbedded crater records). Our findings therefore bolster studies that assert fluvial reworking is not a primary control on smaller interbedded crater counts on Mars.