Abstract

Differences in the modal composition of samples from an Apollo 11 core tube (10005) suggest the presence of at least three depositional units. The double-drive-tube core sample from the Apollo 12 site shows evidence of at least 16 depositional episodes. The earliest recognizable event at the Apollo 12 site carried light-colored soil rich in feldspathic-rock fragments into the area and may be related to ejection of ray material from Copernicus. Prior to the formation of Surveyor Crater at least 11 depositional episodes took place. Unit 7-8, the thickest unit intersected by the core tube, is tentatively identified as the Surveyor Crater ejecta blanket. Secondary ray material from an unknown source was ejected onto the Surveyor Crater ejecta blanket. Finally two thin depositional units were deposited probably by the events which formed the unnamed 10 m. diameter crater next to Halo Crater and Halo Crater respectively. Most of the soil at the Apollo 11 and 12 sites was probably generated locally with a smaller contribution coming from a more distant source possibly in the lunar highlands. Over geologic time the soil evolved to its present form due to impact vitrification and comminution of the detrital material. Initially comminution was probably the dominant process modifying the grain-size distribution of the lunar soil, but beyond a point in time its effect was neutralized by impact vitrification which removed finer-grained materials from the soil both by fusion and agglutination. The overall effect with time was to stabilize the median grain size of the soil and produce an increasingly more negatively skewed and more-poorly-sorted grain-size distribution. Since glass production is energy dependent the glass content of the lunar soil is a measure of the total energy involved in its sedimentation and can thus be used as a measure of maturity. The Apollo 11 samples are more mature than those from the Apollo 12 site which is consistent with the age difference between the two sites.

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