A Major KREEP-Basalt—Mare Basalt unconformity on the Moon

The lunar regolith records both the geological history and processes of the Moon and also that of particles from the Sun and galaxy. Such a record extends back to the age of the rock unit upon which the regolith is developed, but complex impact gardening and processing makes this record difficult to recover and interpret. Ancient rock units, such as lava flows, begin to have regolith developed upon them immediately after em-placement. If covered or buried by another unit, this ancient regolith can preserve conditions as they were on the Moon billions of years ago. Such a paleorego-lith, sandwiched between two lava flows, should be common on the Moon, but may be largely inaccessible. However, in places where natural outcrops occur, such as the walls of large impact craters or the interiors of rilles and graben, such units may be exposed [1,2]. Station 2, Apollo 15 landing site. The Apollo 15 landing site in the Hadley-Apennine region contains rock units that span over a billion years of lunar history [3]. The maria here consist of at least two distinct lava flows and a pyroclastic deposit erupted around 3.3 Ga ago [3]. A non-mare volcanic KREEP basalt unit, the Apennine Bench Fm. [4] forms the 3.84 Ga basement upon which these lavas were erupted. The walls of Hadley Rille expose the mare section in this area; potentially , this section could include a 500 Ma unconformity. Where is it? The Apennine Mt. range (Imbrium basin ejecta) was a major target for the Apollo 15 crew [5]. A traverse to the front was planned for the first EVA, to St. George crater on the Hadley Delta massif to sample highlands material. The planned station also happened to be on the edge of Hadley Rille, upslope from a small, very fresh 50 m diameter crater (Fig. 1). The crew decided to stop and sample a 1 m boulder on the slope of Hadley Delta. This glass-coated block was angular and apparently recently emplaced. The block and soil from beneath it was sampled and analyzed; information from cosmic-ray exposure ages indicate that it was emplaced < 100,000 years ago [6]. The Field Geology team associated this Station 2 block with the very fresh impact crater, located about 300 m north of the LRV (Fig. 1). Samples 15205 and 15206 (Fig. 2) were collected from the Station 2 boulder. Both are classified as " re-golith …