Geologic Map of the Tolstoj (h-8) Quadrangle of Mercury

PLAINS MATERIALS pvs VERY SMOOTH PLAINS MATERIAL—Occurs as smooth planar surfaces only in floors of c 4 and c 5 craters (FDS 529126). Very few superposed craters at Mariner 10 resolutions. Interpretation: Probably shock-melt and fallback associated with formation of individual craters ps SMOOTH PLAINS MATERIAL—Forms flat to gently rolling plains in extremely large annulus surrounding Caloris Basin, and in southeastern part of quadrangle in smaller patches associated with basin (for example, Tolstoj; FDS 166912) and crater (c 1 to c 3) floors (FDS 223). Also occurs in southeast in vague topographic lows of uncertain origin. Characterized by abundant small bright-halo craters (FDS 214, 221) and sharp overlapping contacts with older terrain. Sinuous to lobate ridges are abundant. Interpretation: Uncertain; some areas may consist of fluidized shock-melt materials created by Caloris impact. Alternatively, may be post-Caloris volcanic flows. Unequivocal volcanic vents and lava-flow fronts not observed. Unit appears to be equivalent to and younger than a late c 3 crater age, as indicated by relatively few superposed c 3 craters >30 km in diameter psi INTERMEDIATE PLAINS MATERIAL—Forms flat to rolling plains in c 1 to c 3 craters and in topographic depressions. Contains fewer superposed craters than intercrater plains but may form more convex, overlapping contacts with older terrain than do smooth plains (see craters Goya and Rublev: FDS 166907, 246, 223). Interpretation: Uncertain; may consist of shock-melt and breccia materials moved into topographic depressions by mass-wasting processes; may also consist in part of volcanic materials. Age relations to intercrater and smooth plains uncertain, primarily because of insufficient resolution. May have origin similar to smooth plains but deposited over a considerably longer period. Possibly analogous to widespread light lunar plains pi INTERCRATER PLAINS MATERIAL—Forms very extensive, gently rolling ground between large c 1 and c 2 craters, primarily in southeast half of quadrangle (FDS 124). Overlaps parts of Tolstoj Basin. Unit has high density of superposed 5-to 10-km-diameter craters, many of which are elongate and shallow or open on one side, features suggesting secondary-crater morphology (FDS 223). Surface nearly saturated with visually indistinct but topographically distinct circular depressions but essentially free of well-defined craters larger than about 50 km in diameter. Interpretation: A generally planar surface consisting of highly eroded remnants of large craters and basins that are only very shallow circular depressions (Malin, 1976). May consist of mixture of basin-related ejecta (breccia) and volcanic deposits emplaced during later …