The Crisium basin (Figure 5.1) was recognized as a multi-ring structure by Baldwin (1949, 1963) and Hartmann and Kuiper (1962), who were struck by its remarkable elliptical appearance. Although similar to other basins in the morphologic elements of ring massifs, ejecta, and secondary craters, Crisium displays several features that suggest it may have undergone a distinctly different style of post-impact modification. I will describe the regional and basin geology of the Crisium area and address the nature and causes of these morphological differences.

Regional geological setting

The Crisium basin (Figure 1.1) is on the eastern edge of the near side of the Moon, north of Mare Fecunditatis and southeast of Mare Serenitatis. The basin appears to have formed within a zone of typical highlands crust and mare volcanism was active in this region prior to the basin impact (Schultz and Spudis, 1979, 1983). The average thickness of the crust here is about 60 km (Bills and Ferrari, 1976). The interior of the Crisium basin is completely mare-flooded, which has obscured the relations of basin materials; this obscuration has resulted in controversy regarding the true topographic rim of the basin (Howard et al., 1974; Wilhelms, 1980b, 1987; Croft, 1981b), as discussed below.

The Crisium basin (Figure 5.1) appears to have had minimal interaction with older basin structures. The nearest pre-Crisium basin is the Fecunditatis basin, whose center is located approximately 700 km to the south of Crisium. This basin is tangential to the outermost Crisium ring of about 1000 km diameter mapped by Wilhelms and McCauley (1971) and Fecunditatis effects on the generation of Crisium topography have probably been relatively minor.