GEOLOGY AND CHRONOLOGY OF HADRIACUS PALUS, MARS

Topographic and sedimentary basins in the Martian cratered highlands are defined by both impact craters and the inter-crater plains (palus, pl. paludes). Though past mapping suggests that the latter contain volcanic, sedimentary, and impact-related rocks and sediments, basin sequences are rarely exposed in section and are thus poorly understood. We are conducting a multi-scale geologic mapping investigation of layered exposures in Hadriacus Palus to determine the range of depositional environments preserved in highland inter-crater plains. Hadriacus Palus (HP; 27.4°S, 78.2°E) is located immediately north of Hellas Planitia and west of Hadriaca Patera and is defined by Noachian- to Hesperian-age terrains. The arc-like margins of HP and the alignment of bounding massifs imply HP evolved from overlapping, ancient impact craters that post-date Hellas basin. Runanga (41 km diam.) and Jörn (20 km) craters dominate the NW portion of HP and ejecta from Jörn obscures part of the basin’s uppermost surface. The floor of HP ranges from -2450 to -2700 m, with a very slight west-southwest slope (<0.1°). The westernmost margin of the basin is un-dissected except for a single groove-like channel that debouches into Terby crater (-28.0°S, 74.1°E). No channel connects the inflow channels in the north and east with the channel into Terby, suggesting HP includes some lacustrine sediments. The lower standing southern margin, relative to the northern and eastern margin, of HP basin deposits has ~1 km of relief with layered deposits forming plateaus, buttes, scarps, and scalloped depressions adjacent to several high-relief massifs. Blocky layers and columnar joints interspersed with finer-grained layers indicate that several volcanic episodes resulted in the infilling of the basin. Exposed channel cross-sections, and inverted-relief and sinuous channels provide evidence of temporally interspersed fluvial environments, which played a role in shaping the current local landforms. We used crater size frequency distributions on representative surfaces to model ages in advance of more detailed stratigraphic studies. These ages provide brackets for the deposition and erosion of the deposits in the region. The crater statistics indicate that the basin was active from the Early Noachian through the Late Hesperian/Early Amazonian.