Layered Deposits and Pitted Terrain in the Circum Hellas Region

Introduction: Much of the southern highlands has been mantled since the Noachian, including a general blanket of possibly airfall-derived sediment that softens the landscape [1,2,3,4], the Electris mantle [5] including knobby 'chaos' in several basins [6], and a variety of deposits that are the subject of this study that share the common characteristics of being generally confined to basins and crater floors and that manifest irregular interior depressions. Many of these features occur in a zone surrounding Hellas (Fig. 1). These deposits share the general characteristics of having fairly smooth, nearly planar surfaces and abrupt scarps bordering interior and marginal depressions. Despite these common characteristics, a wide range of morphologies occurs. Several end-members are discussed below. End-Member Landforms: Pitted crater floors in the Hellas rim zone At least 33 craters surrounding the Hellas basin exhibit irregular pitting (Fig. 1), most of which occur near the center of an otherwise smooth crater floor (e.g., Fig. 2, P in Fig. 1). The abrupt scarps bordering the pits generally surround a nearly flat-floored inner basin. Apparent layering is exposed on some scarps (although not in Fig. 2). Pitted craters west of Hellas are generally associated with smooth, undulate, and sparsely channeled crater rims. The 'softened' crater rims may have been formed either by selective airfall deposition or by non-linear creep processes [2]. The temporal and process relationship between the pitting and the smoothed, channeled crater walls is uncertain but under investigation. Layered Deposits of Terby and Other Craters A thick sequence of layered deposits occurs on the northeast margin of Terby Crater (T in Fig. 1). Apparent remnants of the top of the deposit slope southward from the northern rim of Terby as broad plateaus, but much of the deposit has been eroded by deep troughs incised up to 2.5 km below the plateau surfaces. The most complete erosion has occurred at the southern portion of the deposit, creating a moat-like depression. Similar, but thinner deposits occur in a number of large crater basins north and east of Hellas, including Millochau Crater, as discussed by [7] (M in Fig. 1). Gorgonum Chaos Platforms Flat-surface benches ring a central depression about 100-150 m lower in the center of the Gorgonum Chaos Basin. The bench planforms are very irregular, with long, branching reentrants, enclosed depressions, flat-topped guyot-like mesas within the central depression, and rings around older mesas of the Gorgonum 'Chaos' [8]. The platform edges …