ABSTRACT

A thin-section examination of sixteen clastic rock samples returned by the Apollo 14 mission from the Fra Mauro region of the moon suggests the presence of at least two distinctly different lithologies. Five of the samples (group I) are comparable texturally with "breccias" returned by the Apollo 11 and 12 missions. The group I rocks are characterized by an abundant glassy, matrix and glass particles (including glass spheres and ropy glass) and lesser amounts of plagioclase and pyroxene grains, and lithic clasts. Accretionary lapilli are present in almost all group I samples. The morphology of the detrital particles was determined largely by the soil forming processes (comminution and impact vitrification). Group I rocks are probably formed by sintering or welding of superficial depos ts of lunar soil by an impact-generated base surge mechanism. The impact events that generated the group I rocks were probably relatively small and seldom penetrated the soil layer. Eleven of the rocks that were studied (group II) were probably derived from local Fra Mauro Formation bedrock by the Cone Crater event. Group II samples are relatively fine grained, very poorly sorted, and consist largely of pyroxene, plagioclase, and lithic clasts set in an abundant mineralic matrix. The rocks were deposited to form the Fra Mauro Formation by a highly energetic turbulent debris flow which probably traveled in surges, alternately depositing and eroding the substrate. Abrasion appears to have been the most active process modifying grain morphology, because the mean roundness of the detrital pa ticles is relatively high. Grain size may increase towards the bottom of the Fra Mauro Formation, which may indicate a sorting process or incomplete mixing with penecontemporaneously eroded materials. In general, available data are all consistent with the Fra Mauro Formation baring been deposited by a large highly energetic debris flow or base surge that was generated by a large meteorite impact, probably the same event that excavated Mare Imbrium.

Group I and II lithologies were probably both deposited from impact generated base surges. The differences between the two lithologies stem not as much from the basic sedimentary processes as from the differences in the magnitude of the events generating the base surges and the resultant difference in available detrital materials. Base surges forming group I rocks are relatively small and probably form sedimentary deposits comparable with terrestrial volcanic base surges. The large scale base surge which formed the group II rocks had a much greater dynamic viscosity than the events which formed the group I rocks and produced textures comparable with those encountered in subaqueous mass movement deposits on the earth.