FIELD GEOLOGY ON THE MOON WITH APOLLO 17: RESULTS
Field observations at Shorty Crater in the valley of Taurus-Littrow, type location of orange and black pyroclastic ash, combined with post-mission examination and analysis show that orange and black ash layers form a small isoclinal fold, enclosed by light-gray regolith. This regolith derives from the impact pulverization of a post-pyroclastic lava flow that protected the ashes from being mixed with other regolith for about 3.5 billion years. In spite of this exposure time, a very low measured "maturity" for the light-gray protective regolith indicates micro-meteor flux probably is highly variable over Solar System history.
Observation and sampling of large boulders at the base of the 1600 to 2100m high massif walls of the valley permit tentative dating of four of the large basin-forming impact events on the Moon. A synthesis of 40-39Ar ages of impact melt-breccia matrixes from South Massif boulders at Station 2 indicate a an age of ~4.04 Ga for the Tranquillitatis event and of ~3.98 Ga for Serenitatis. A large boulder at Station 6, that rolled from a location about 400m above the base of the 1600m high North Massif, includes vesicular, light green-gray impact melt-breccias that have intruded earlier-formed dark blue-gray melt-breccias. The matrixes of both these melt-breccias have a cooling age of ~3.97 Ga, close to that found at Station 2 and supporting this as the age of Serenitatis.
A Station 7 dark blue-gray melt-breccia from a boulder that rolled from a location ~500m above the source of the Station 6 boulder and ~700m below the crest also has a matrix with a ~3.98 Ga age. The matrix of vesicular, light green-gray melt-breccia in contact with the Station 7 blue-gray melt-breccia, however, has an age of ~3.86 Ga. This latter age may date melt-breccia ejected from the Imbrium basin, the edge of which is ~770km northwest. If so, it would indicate ~700m of Imbrium ejecta exists on the North Massif at this location, about 1350km from the center of Imbrium.