Large-diameter visible and buried impact basins, seen as "quasi-circular depressions" (QCDs) in MOLA gridded data, provide a self-consistent chronology for major events on early Mars in terms of N(200) crater retention ages. On the basis of a conversion to model absolute ages, this chronology extends back hundreds of millions of years into a previously unknown "pre-Noachian" epoch during which a now buried highlands surface was established, in which several very large impact basins formed while the global magnetic field was still present. A cluster of very large "lowland-making" basins occurred at a model age of about 4.13 GY (or earlier), forming the fundamental topography of the Mars crustal dichotomy at a time prior to the age of the oldest visible highland crust. This early event in Martian history marks the transition from the "pre-Noachian" to the Early Noachian when the well-preserved Hellas, Argyre, and Isidis basins formed, all after the global magnetic field died.