TERRAIN-CONFORMING LIGHT-TONED MANTLE IN THE HELLAS BASIN REGION, MARS

Discontinuous exposures of layered strata are abundant on the northern slopes of Hellas Basin. Many of these exposures involve light-toned layering that can reach up to 2 km in aggregate thickness as at Terby crater. The origin of this layering is uncertain; lacustrine, airfall, and deltaic origins have been suggested. Exposures of light-toned deposits often form resistant scarps on crater floors and are locally exposed in eroded convolute patterns on high-relief terrain. Diagnostic exposures of light-toned deposits occur south of Nako crater at 30.7S and 82.9E indicate that deposition occurred as a draping mantle over pre-existing terrain with local relief of at least 800 m. The mantle was deposited on a fluvially-eroded landscape and deposition may have been contemporaneous with fluvial activity. The light-toned deposit was subsequently covered with darker deposits. Subsequent aeolian differential deflation has resulted in intricate exposures of the light-toned deposits. Light-toned deposits are common in the nearby craters Batson, Nako and Majuro and their outcrops over ~4 km of relief suggest a widespread event of airfall deposition in the late Noachian or Hesperian, which might have been sourced from explosive volcanic eruption, a large crater impact, or an epoch of intense dust storms. Such light-toned mantles are important regional stratigraphic markers.