DISTRIBUTION OF MATERIALS EXCAVATED BY THE LUNAR CRATER BULLIALDUS AND IMPLICATIONS FOR THE GEOLOGIC HISTORY OF THE NUBIUM REGION

Previous spectroscopic studies of the lunar crater Bullialdus, located in the Nubium Basin, indicated an unusual stratigraphy of two gabbroi c layers overlying a noritic unit. The possible existence of a layered mafic pluton at Bullialdus was suggested. To investigate the geologic context with more detailed spatial information, charge-coupled device (CCD) images of Bullialdus were obtained using eight filters (0.40, 0 .56, 0.73, 0.91, 0.95, 0.98, 0.99, and 1.02 mum). A linear mixing mode l was used to investigate the fractional abundances of spectral end-me mbers chosen from within the multispectral image. Since the reflectanc e properties of lunar materials over this wavelength range are sensiti ve to variations in composition and soil maturity, fractional abundanc e images were used to create a new geologic map of the crater. The spa tial relationships of the surface materials confirm the previously inf erred stratigraphy, and further reveal the central peaks to exhibit tw o distinct compositional units: noritic anorthosite and anorthositic n orite. Three models for the origin of the observed stratigraphy are co nsidered: Bullialdus has excavated stratigraphic units containing (1) early mare basalt overlying anorthositic-noritic crustal material, (2) part of a layered mafic pluton, and/or (3) part of an impact melt she et formed by the Nubium Basin impact event. (C) 1994 Academic Press. I nc.