Timing, Emplacement, and Distribution of Mare-Fill Units in Oceanus Procellarum, a Large Nearside Lunar Basin

William A. Ambrose
Bureau of Economic Geology, Austin, TX

Although only 17% of the entire lunar surface is covered by basalt and underlying associated magma cooling units, ~60% of the western hemisphere on the lunar nearside contains magmatic complexes emplaced in numerous episodes ranging from approximately 3.75 Ga (billion years ago) to possibly as recently as 0.9 Ga, inferred from crater counts and overlapping relationships between lava-flow units and bright rays associated with Copernican-age craters. Oceanus Procellarum contains the largest continuous extent of lunar basalts on the Moon and its upper fill is a complex of at least four different flow units, recognized on the basis of albedo and spectral reflectivity. Individually, these flow units are only a few hundreds of meters thick, but may be underlain by 2-4 km thick basin-filling units. Oceanus Procellarum has been interpreted by some authors as the western part of the 2,400-km-wide Gargantuan Basin, inferred to have formed from a giant impact ~4.3 Ga. Gargantuan Basin lacks a surrounding mountain rim and underlying mascon, features commonly associated with other nearside lunar basins such as Mare Tranquillitatis, Serenitatis, and Crisium. However, the absence of these features may be due to the Gargantuan Basin having formed so early that the lunar crust may have not been sufficiently rigid to support rim material and excess masses of thick basin-filling units.

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