Evidence for an Hesperian-aged South Circum-Polar Lake Margin Environment on Mars

A broad pitted plain and an elongated low rise occur near the south pole of Mars between a region of major cavi (Cavi Angusti) and a regionally smooth and broad valley (Argentea Planum). Viking, Mars Global Surveyor (MGS), and Odyssey data reveal a densely pitted plain covering 6750 km, and containing 4300 irregularly shaped, steep-walled and flat-floored depressions with a mean diameter of 3.5 km. At the southernmost (poleward) extent of this plain are 12 north/south trending linear valleys that are characterized by theatershaped heads abutting a major cavi within Cavi Angusti. The pitted plain, which abuts Cavi Angusti to the southwest, is separated from the floor of Argentea Planum by a smooth, elongated low rise that extends parallel to the plain for 200 km. These unusual features are all found within the Hesperian-aged circumpolar Dorsa Argentea Formation, which has been interpreted by some workers to be an icerich glacier-related deposit. We interpret the pitted plain to represent the maximum northern extent of the Angusti lobe ice deposit. The pits are analogous in morphology and distribution to terrestrial kettle holes, which form from the melting of isolated ice-blocks surrounded and partly buried by sediment, to leave hollows. The linear valleys are consistent with sapping valleys formed from the release of an elevated groundwater table, fed by meltwater lakes. On the basis of these characteristics, relationships and analogs, we interpret the marginal facies to represent an ice-sheet/lake contact environment that existed during Hesperian time. r 2006 Elsevier Ltd. All rights reserved.