Schmitt reports that a lens-shaped deposit of orange and black “soils” at Shorty Crater (Station 4) was investigated and sampled during Apollo 17 exploration of the valley of Taurus-Littrow in December 1972. Postmission examination and analysis of samples 74220 and 74001/2 disclosed that these “soils” consist of very small beads (25-40 µm) and shards of orange glass and black, partially crystallized colorless glass. The apparent black color results from dark microscopic crystallites of olivine, pyroxene and ilmenite. The spherical shapes, very small size, and associated surface volatiles of both orange and black beads are those expected of volcanic ash produced by a volatile-driven eruption of basaltic magma in extreme vacuum. The detailed structure of the ash deposit is likely that of an isoclinal anticline with its axial plane partially overturned and sub-parallel to the rim of Shorty Crater. This structure is consistent with the radial direction of impact forces that created Shorty Crater.
The exposed crest of the fold has been planed off through the top orange ash layer. A trench dug through this layer and the fold’s crest exposed light-gray regolith, statigraphically overlying the orange ash unit. Stratigrapy of Pre-Shorty Impact units. The impact that produced Shorty Crater penetrated about 14 m of roughly layered units of light mantle, ash, and various regolith-like materials as well as the upper portions of underlying basalt. The explosive forces of the impact both ejected and laterally compressed these layers away from the point of impact, creating the floor, walls, rim and surrounding ejecta blanket. Samples obtained provide information on all these local layered units with the exception of the light mantle avalanche deposit and the regolith units developed on the underlying basalt flows. Based on the track exposure
age of orange ash sample 74220 and the cosmogenic Kr exposure age of rim basalt 74255, this impact occurred 10-19 million years ago.
The inferred pre-impact stratigraphy at Shorty Crater is presented in the paper. The Type A, subfloor basalt flow at the site, possibly represented by sample 74235, has not been dated. The average of 36 dates between 3.80 and 3.54 Ga for Taurus-Littrow subfloor basalt samples is 3.74 Ga; however, it is likely that this late flow would be the product of one of the youngest Type A basalt eruptions in the valley.
Two Shorty rim samples of Type C basalt, 74275 and 74255, probably represent flows younger that the youngest Type A basalt. The lower Ba/Rb ratios of Type C basalts apparently required a period of fractional crystallization and separation of plagioclase in the subfloor basalt magma chamber. The presence of clinopyroxene phenocrysts with olivine cores in 74255 vs. olivine interpreted as zenocrysts in 74275 suggests that the latter is a sample of the older of these two flows.
Particle-size grading from coarse to fine with depth in the deepest definable unit in the double drive tube core suggests that the core penetrated a tilted axial plane of the isoclinal fold and that this unit of the ash (#4) is overturned relative to other units.
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