FLUVIAL ACTIVITY AND LAKE FORMATION IN A NOACHIAN CLOSED-SOURCE DRAINAGE BASIN CRATER

We report on the geology of a 54-km diameter Noachian-aged crater in Terra Sabaea (20.3˚S, 42.6˚E) that preserves a broad system of inverted fluvial channels and lacustrine deposits in its interior, but is notably lacking in external drainage sources. This lies in contrast to previously described closed-basin crater lakes (e.g. Gale), which are distinguished by one or more inlet channels that provide a source of external fluvial drainage, and open-basin lakes (e.g. Jezero), which have one or more outlet channels in addition to an inlet channel. The crater we observe in Terra Sabaea has neither inlet nor outlet channels; we therefore term this new type of crater a “closed-source drainage basin” (CSDB).

Most open- and closed-basin lakes are thought to have formed in a warm and wet climate through breaching of crater rims by channels sourced from regional catchments. The drainage pattern within the CSDB crater in Terra Sabaea suggests that fluvial activity was highly localized and was not sourced from an external drainage network. Groundwater sapping has been invoked to explain crater basin lakes with no apparent external drainage sources; however, most of these basins have an implied groundwater table of ~–4 km, nearly 6 km lower than the elevation of the CSDB crater. Depositional landforms observed in both externally drained and groundwater-fed crater basin lakes on Mars are also not found in the CSDB crater. A series of arcuate, upslope-facing scarps at the base of the CSDB crater wall appear analogous to those formed by retreating cold-based glacial fronts in Amazonian-aged mid-latitude craters, which can also lead to proglacial outwash and channel incision. If the fluvial and lacustrine features in the CSDB crater formed via cold-based glacial processes, this would provide novel evidence for a cold and icy early Mars climate hypothesis.

We plan to continue our investigations of the CSDB crater in Terra Sabaea in order to determine the likely formation mechanisms for the fluvial and lacustrine features in the crater, and how these may differ from open- and closed-basin crater lakes on Mars. The recognition and documentation of these features provides specific criteria to search for other examples of localized crater degradation in the southern highlands in order to further test hypotheses of Mars climate evolution.