Refined model age for Orientale Basin derived from zonal crater dating of its ejecta

The Orientale basin is the most well-preserved multi-ring impact basin on the Moon and it has been used to define the end of the Early Imbrian Epoch. Thus a precise determination of the absolute model age (AMA) of Orientale Basin is important for lunar geology study. The method of crater size-frequency distribution (CSFD) has been used to date Orientale Baisn in previous studies, and the resultant AMA ranges from 3.68 Ga to 3.8 Ga. The inconsistence may be attributed to the choice of counting area and identified superposed craters. In this research we have mapped 27,093 craters larger than 0.7 km in diameter on the entire ejecta of the Orientale Basin, and derived that the AMAs of the Orientale Basin and the pre-existing luanr suface are 3.80(-0.0079)(+0.0074) Ga and similar to 4.14(-0.022)(+0.019)Ga, respectively. And the knee point of the CSFD of the entire counting area is similar to 10 km, corresponding to similar to 2 km of the average thickness of the Orientale Basin ejecta. To further analyze the variation of the ejecta thickness of Orientale Basin, the counting area was divided into eight 45-degree subsections. The analysis of the knee points of the CSFD in the subdivided counting areas demonstrates that Orientale Basin was formed by an oblique impact from the south-west, and the impact incidence was between 45 degrees and 20 degrees.