Terraces and Gilbert-type deltas in crater lakes in Ismenius Lacus and Memnonia (Mars)

Paleolacustrine environments have been widely recognized from landform associations on the surface of Mars. In contrast to paleolakes in more open areas, putative crater lakes commonly display well‐developed sedimentary landforms, especially wave‐formed terraces and Gilbert‐type deltas. This study focuses on terraces and deltas of Western Memnonia and Ismenius Lacus. As on Earth, these landforms provide important indications of water dynamics and sedimentary processes. The putative Martian paleolakes must have been active for thousands of years to have permitted the water movement and wave formation necessary to form one or more terraces. Fan‐shaped landforms at the mouths of channels entering crater‐lake areas may indicate various channel water interaction. Of these, Gilbert‐type deltas are particularly informative. Their presence indicates multiple flooding events whereby the inlet channels conveyed coarse‐grained sediment to the crater lake. Both terraces and deltaic progradation indicate that the crater lakes of Memnonia and Ismenius Lacus were active for long time periods, at least for thousands of years, during the post heavy bombardment period of the later Martian history. The unequivocal presence of standing bodies of water of relatively long durations indicates that climatic settings very different from those prevailing today must have occurred, at least locally and temporarily, during parts of the Martian planetary history.