Being the closest planet to the Sun as well as the smallest planet, Mercury is very difficult to observe.
The next geologic era is Calorian, named after Mercury's largest crater, Caloris Basin.
It covers ~3.9 until 3.5-3.0 Gya (the boundary with Mansurian is not well-determined).
It lasts from 3.5-3.0 Gya (the boundary with Calorian is not well-determined) to ~1.0 Gya.
This epoch represents light cratering and a continued decrease in planetary activity and increase in cooling.
The first is simply the Pre-Tolstojan, which covers the planet's formation ~4.5 Gya (billion years ago) until ~4.0 Gya.