Stratigraphy of the descartes region (Apollo 16): implications for the origin of samples

Analysis of terrain in the Apollo 16 Descartes landing region shows a series of features that form a stratigraphic sequence which dominates the history and petrogenesis at the site. An ancient 150 km diam crater centered on the Apollo 16 site is one of the earliest recognizable major structures. Nectaris ejecta was concentrated in a regional low at the base of the back slope of the Nectaris basin to form the Descartes Mountains. Subsequently, a 60 km diam crater formed in the Descartes Mountains centered about 25 km to the west of the site. This crater dominates the geology and petrogenetic history of the site. Stone and Smoky Mountains represent the degraded terraced crater walls, and the dark matrix breccias and metaclastic rocks derived from North and South Ray craters represent floor fallback breccias from this cratering event. Subsequent major cratering occurred in the region (Dollond B, etc.) prior to the Imbrium and Orientale basin-forming events but had minor effect on the site. Based on this interpretation, contributions from Imbrium at the Apollo 16 site are minor and those from Orientale negligible. The petrology of the Apollo 16 rocks supports this stratigraphic and process model of a local crater-dominated history for this region.