Geomorphic and stratigraphic analysis of Crater Terby and layered deposits north of Hellas basin, Mars

Abstract

The geologic history of Crater Terby is developed through geomorphic and stratigraphic analyses within the regional context of the Hellas basin. Terby exhibits ~2-km-thick sequences of layers that consist of repetitive, subhorizontal and laterally continuous beds. The layers are predominantly fine-grained as indicated by their ease of aeolian erosion, although a few consolidated layers weather to form rubbly talus. The grain size or composition of the deposited materials fluctuated, producing layering, but the overall properties of the deposits are similar throughout the sequence and are comparable to layered deposits in other crater basins around Hellas. The original depositional geometry, physical and geological characteristics of the layers in Terby and the other basins lead us to favor a lacustrine origin, but a loess-like origin cannot be ruled out. The formation of the layers corresponds to a period when the circum-Hellas region may have been occupied by a lake(s) up to 3.6 km deep. Once the lake in Hellas decreased, the layers in Terby were incised by troughs and a moat-like depression. We attribute this erosion to scour beneath an ice cover due to a lack of integrated fluvial drainage or large aeolian deflation features. The presence of viscous flow features in a crater on Terby's northwestern rim and lobate features on Terby's crater floor are also indicative of ice. The lack of depositional features associated with the postulated glacial activity suggests there was a contemporaneous shallow (ice-covered?) lake covering the floor of Terby that transported material into the greater Hellas basin.