Stratigraphy of Ejecta from the Lunar Crater Aristarchus
Ejecta from lunar crater Aristarchus consist of mappable units that have different surface characteristics, lithologies, and geneses. Photogeologic mapping demonstrates that these units can be ordered into a stratigraphic succession representing stages in the emplacement of ejecta during a single impact event. Four main ejecta units are recognized: (a) a highly fractured rim unit consisting of an overturned flap of country rock, (b) a continuous ejecta blanket, (c) a zone of bright discontinous ejecta outside the continuous ejecta blanket associated with numerous secondary impact craters, and (d) a group of ejecta deposits on the rim which are genetically related to each other and include hummocky material (the hummocky rim unit), blocky lobes, and smooth flows. Other units, such as the ridged and leveed flows and the dark “lakes” or “playas,” are relatively younger than the ejecta and are only briefly discussed. The continuous ejecta blanket lies stratigraphically above the rim unit. To the east and south, the ejecta blanket has been stripped off the overturned rim unit by outward flowage from the crater during ejecta production, leaving parts of the rim eroded bare. Fall of large missiles to form the secondary craters and bright ejecta preceded the emplacement of the continuous ejecta. The general asymmetry of the continuous ejecta-blanket distribution appears to be related to the Aristarchus Plateau boundary fault, which may have controlled the way material was excavated. The hummocky rim unit only occurs on the northern and western rim and may be an overturned flap of premare Aristarchus Plateau bedrock not present near the surface on the southern and eastern rim of the crater. Blocky lobes and smooth flows mainly associated with the hummocky rim unit are interbedded, arguing that the smooth flows are part of the sequence of ejected rocks and are not younger volcanic flows.