New isotopic ages and a fresh understanding of stratigraphic relations among siliciclastic strata in the Pensacola Mountains along the northern margin of the East Antarctic craton result in removal of some constraints for the Proterozoic break-up of Rodinia and necessitate revision of the subsequent history of the East Antarctic margin. These rocks, formerly all included in the Patuxent Formation, were thought to be of mid-Neoproterozoic age, to have formed as a consequence of Rodinia rifting, and to have been deformed during a Neoproterozoic orogenic event. Our data show, in contrast, that these siliciclastic strata were deposited in two chronologically distinct basins. The older basin, in which the Hannah Ridge Formation (new name) accumulated, received sediment that contains detrital zircons of latest Neoproterozoic or Early Cambrian age. It was deformed and its contents uplifted and eroded prior to the late Mid-Cambrian in an orogenic event that we interpret as the early stage(s) of the Ross orogeny. The second basin formed later, accumulated turbidite-rich sediments of the redefined Patuxent Formation of Mid- and probably Late Cambrian age, and was subsequently deformed, possibly in Ordovician time. Review of both biostratigraphic and isotopic ages along the length of the Transantarctic Mountains indicates that almost everywhere the main episodes of deformation predate 500 Ma and are thus older than latest Mid-Cambrian, rather than Ordovician, as they are commonly considered to be. Only in the accreted Bowers and Robertson Bay terranes of northern Victoria land, which reveal no clear record of pre-latest Mid-Cambrian or older folding, is the principal episode of Ross orogenic deformation demonstrably younger than Late Cambrian.