Michelangelo Quadrangle - Geologic History

Geologic History

The interpretable geologic history of the Michelangelo quadrangle begins with the formation of the four ancient, multiring basins. From oldest to youngest, they are: Barma-Vincente, Bartok-Ives, Hawthorne-Riemenschneider, and Eitoku-Milton. These basins presumably formed during the period of heavy bombardment inferred from lunar history (Wilhelms, in press). Contemporaneous with their formation and shortly afterward, was the deposition of the intercrater plains material. This unit has a complex history of deposition; it was reworked in place and probably includes brecciated plutonic rocks and possibly ancient volcanic flows. Deposition of the intercrater plains material was waning as the next oldest basins (Dostoevskij, Tolstoj) were formed. Partly overlapping their formation was the deposition of the intermediate plains material, probably emplaced partly as distal basin ejecta and partly as volcanic flows. Regional deformation of these plains units by compressive tectonics, forming scarps, was contemporaneous with their deposition.

The Caloris impact occurred during the time of formation of the intermediate plains material. In the map area, Caloris ejecta may be present at depth or may have been reworked locally by adjacent impacts. Two groups of Caloris secondary craters are evident. Shortly after the Caloris impact, extensive smooth plains material, probably of volcanic origin, was deposited. During this period of deposition occurred the impacts of the last of the major basins (Beethoven, Michelangelo, Valmiki, and Bach). Minor tectonic activity continued as scarps andl unar mare-type wrinkle ridges developed within the smooth plains materials.

The cratering rate declined rapidly as the c3, c4 and c5 craters were produced. Regolith production continues to the present day on all units. If the geologic history of the Moon is a guide, most of the events discussed were essentially complete within the first 1.5 to 2.0 billion years of Mercury’s history (Murray and others, 1975). A summary of global mercurian geology may be found in Guest and O’Donnell (1977) and Strom.