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

Paleolacustrine environments have been widely recognized from landform asso ciations on the surface of Mars. In contrast to paleolakes in more open are as, putative crater lakes commonly display well-developed sedimentary landf orms, especially wave-formed terraces and Gilbert-type deltas. This study f ocuses 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 acti ve for thousands of years to have permitted the water movement and wave for mation 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 informati ve. Their presence indicates multiple flooding events whereby the inlet cha nnels conveyed coarse-grained sediment to the crater lake. Both terraces an d deltaic progradation indicate that the crater lakes of Memnonia and Ismen ius Lacus were active for long time periods, at least for thousands of year s, during the post heavy bombardment period of the later Martian history. T he unequivocal presence of standing bodies of water of relatively long dura tions indicates that climatic settings very different from those prevailing today must have occurred, at least locally and temporarily, during parts o f the Martian planetary history.