There is conflicting evidence for an ancient ocean which occupied the
northern hemispheric basin on Mars. Along different regions of the
dichotomy boundary, sediment fans have been interpreted as either
forming into a large water body or a series of smaller paleolake basins.
Here, we investigate fluvial systems in the Memnonia Sucli region of
Mars, set along the dichotomy, which comprise erosional valley networks,
paleolake basins, inverted channel systems, and sediment fans. We focus
our analysis on the evolution of the upslope catchment and
characterizing the ancient environment of a large, downslope basin,
bound by the topographic dichotomy and the Medusae Fossae Formation. The
catchment fluvial systems comprise highly degraded valley networks and
show a complex history of incision and filling, influenced by paleolake
basin overflow, impact crater damming, aggradation, and possibly a
downstream water body. The morphology of the sediment fans is consistent
with either fluvial fans or deltas and they form at discrete elevations,
rather than a common elevation plane. Our analysis is consistent with
the sediment fans forming into a series of paleolake basins set along
the dichotomy, rather than into a large inner sea or ocean-sized water
body. The fluvial systems were likely active between the mid Noachian
and early Hesperian periods. Our results demonstrate the complex,
multi-phase evolution of fluvial systems on ancient Mars and highlight
the importance of regional and local studies when characterising ancient
regions of the dichotomy.