Complex evolution of paleolacustrine systems on Mars: an example from the Holden Crater.

The ˜150 km wide Holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water-related environments. We undertook geological mapping and a stratigraphic survey in order to probe the evolution of water-related landforms and their paleoenvironmental implications. Our investigations lead us to propose that the Holden area was subjected to a ‘‘wet’’ lacustrine phase of Hesperian age and an ‘‘icy’’ phase during the Amazonian. Deltaic, coastal, and lacustrine environments occurred during the ‘‘wet’’ phase, some displaying a cyclic depositional pattern presumably related to autogenic processes. Water was delivered to the basin by the Uzboi Vallis and by surface runoff channels from a series of drainage basins along the crater walls. Fan delta geometries and coastal onlap enabled estimation of major water levels. Two levels of major stand of the water have been recognized, possibly reflecting allogenic controls. Geologic units related to this ‘‘wet’’ lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of Amazonian age, defining an ‘‘icy’’ phase. These features are consistent with a warm-based glacier entering the Holden crater through the wide Uzboi Vallis to form a proglacial lake in the central part of the crater. Changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation.