Borealis Quadrangle - Stratigraphy

Stratigraphy

Within the Borealis region, three widespread plains units are recognized largely by their obvious differences in crater density, which is closely related to relative age (Soderblom and Boyce, 1972). From most heavily cratered (oldest) to least cratered (youngest), these units are intercrater plains material, intermediate plains material, and smooth plains material. Visual identification is confirmed and refined by actual crater counts. If one uses the lunar surface as a frame of reference, the crater density of Mercurian plains in the Borealis region is bracketed by that of the lunar uplands, the most heavily cratered lunar surface, and that of Oceanus Procellarum, a moderately cratered lunar mare surface. The curve for the lunar uplands was derived from crater counts in the region northwest of crater Tsiolkovskiy, between crater Mendeleev and Mare Smithii. The curve for the southeastern part of Oceanus Procellarum was obtained in an area centered near lat 2°00' N. and long 31°00'W., south of the crater Kunowsky. Ocean Procellarum has long been considered close to the “average lunar mare” (Hartmann, 1966, 1967); its crater density is intermediate between those of the heavily cratered Mare Tranquillitatis and the lightly cratered Mare Serenitatis.

Material of Borealis Planitia was not included in the smooth plains count because images of the area were blurred by spacecraft motion, and so reliable crater counts could not be obtained. However, smooth plains south of lat 65° N. in the Shakespeare quadrangle, in the crater Strindberg and in Suisei Planitia, are included in these counts. The plains materials that lie outside Borealis Planitia are distributed in irregular belts, which are subparallel to the terminator and to one another. Eastward from long 190° W., the following belt pattern is observed: intercrater plains material, intermediate plains material, and again intercrater plains material. All three belts extend southward into the Shakespeare quadrangle (Guest and Greeley, 1983).

Distinguishing one type of plains material from another by variations in roughness and crater density is highly dependent on the resolution and lighting conditions of individual Mariner frames (Schaber and McCauley, 1980). This constraint is well documented for the Moon (Masursky and others, 1978, p. 80–81) and for Mars (Boyce and others, 1976). In the Borealis region, where intercrater and intermediate plains materials were imaged at an increasingly low sun angle close to the terminator, the number of observable small craters increase with decreasing distance from the terminator and concomitantly decreasing sun angle. This discrepancy in the apparent abundance of craters occurs only for craters that have small diameters and can be obviated by counting only craters larger than 3 km (1.9 mi) in diameter.