Stratigraphic sequence and ages of volcanic units in the Gruithuisen region of the moon

Abstract

Several domes indicative of non-mare volcanism, morphologically as well as spectrally distinct from surrounding mare and highland units, are located in the Gruithuisen region in northern Oceanus Procellarum. The determination of the nature and age of non-mare volcanism helps to put constraints on lunar crustal evolution. The ages of the domes were previously thought to be mid-Imbrian or possibly even Eratosthenian. In order to assess more closely the stratigraphic relationships in this region, we carried out geologic mapping and crater size-frequency measurements (1) on the Gruithuisen volcanic domes, (2) on the adjacent mare units, and (3) in the highland areas. (4) The nearby Iridum impact event was also dated by means of crater counts. A recently updated and improved version of the lunar production function polynomial was used to fit measured crater size-frequency distributions of geologic units, and a cratering chronology model was applied in order to assign these units to the periods and epochs of the lunar chronology system. Oceanus Procellarum mare materials in the study area were found to range in age from Late Imbrian to Eratosthenian and show peak cratering model ages at 3.55 Gyr (1 Gyr = 1 billion years), 3.2-3.3 Gyr, and about 2.4 Gyr. Some mare units show evidence for at least two events in their crater size-frequency distributions; these are most likely due to the superposition of later thin lava flows. Mare volcanism, hence, has lasted over a period of several hundred million years in the Gruithuisen region. The Gruithuisen domes show a much more restricted age range; they were found to range in age from 3.85 to 3.72 Gyr and thus were formed in the early part of the Late Imbrian, an older age than previously thought. Iridum crater, stratigraphically older than the Gruithuisen domes, was formed about 3.7-3.8 Gyr ago. The Iridum impact event and the formation of the two Gruithuisen domes cannot be well separated by means of crater counts. A few stratigraphically older highland units and some mare areas show ages of less than 2.0 Gyr, but these are less reliable due to (1) the crater distributions being close to saturation, (2) less effective crater retention on slopes, and (3) erosional removal of craters.