The Pre-Imbrium is the earliest lunar geological era. It began with the formation of a primitive igneous crust between 4.6 and 4.5 billion years ago and ended, together with the initial phase of heavy cratering, 3.9 billion years ago.
The Humorum basin, and several other lunar basins, were formed in a period which ended around 4.1 billion years ago. It did not fill with mare material until the later Eratosthenian era, between 3.9 and 3.2 billion years ago.
Photo: Richard Evans. The full size original of this and other lunar images, with additional narratives, may be seen at his Lunar Homepage.