Composition of orientale basin deposits and implications for the lunar basin-forming process


  • Paul D. Spudis,

  • B. Ray Hawke,

  • Paul Lucey


The Orientale basin, on the lunar western limb, has served as the prototype lunar multiring basin for many years. In order to constrain possible models of the formation and excavation of basin ejecta, we have studied the composition of Orientale basin deposits, using both earth-based spectral reflectance and Apollo orbital geochemical data. Results indicate that Orientale basin ejecta are dominantly anorthositic with compositions ranging from pure anorthosite to noritic anorthosite. Although minor quantities of mare basalt and low-K Fra Mauro basalt are present, ultramafic and KREEP components appear to be absent. Portions of the inner rings of the basin may be composed of anorthosite whose shock levels exceed 20 Gpa (200 kbar). In conjunction with photogeologic evidence for the preservation of prebasin topography and structure within the Cordillera basin ring, we suggest that the diameter of the excavation crater of the Orientale basin is on the order of 500 to 600 km. Effective depths of excavation are probably less than about half the local crustal thickness, which in this region is on the order of 100 km. We find that the Orientale impact excavated dominantly upper crustal rocks with a depth of excavation consistent with a proportional-growth, basin-forming model.