New U-Pb ages for detrital and igneous zircons constrain the depositional age and sedimentary provenance of the Beardmore Group, a siliciclastic succession that records transformation of the East Antarctic margin during Rodinia breakup and subsequent Gondwana amalgamation. We divide rocks previously mapped as the Beardmore Group into (1) an inboard late Neoproterozoic assemblage (probably ≤670 Ma) and (2) a volumetrically dominant, outboard assemblage that is latest Early Cambrian or younger (≤520 Ma). The inboard assemblage contains mature, multicycle sediment derived from mixed cratonic sources dominated by 2.8- and 1.9-1.4-Ga components. It was deposited in a platformal-to-shoreline setting along an existing rifted margin. A new zircon age of 668 ± 1 Ma for mafic igneous rocks within this assemblage is younger than previously reported, indicating deposition in the late Neoproterozoic and raising questions as to the age of rifting. The outboard assemblage contains first-cycle sediment with dramatically different provenance, including fresh, young (580-520 Ma), locally derived igneous material and contributions from ~1400-, 1100-940-, and ~825-Ma sources. The youngest zircon ages (525-522 Ma) are consistent with newly discovered Cambrian-aspect trace fossils. Therefore, these outboard rocks are best considered as siliciclastic units of the upper Byrd Group. The detrital age patterns suggest a change from passive-margin sedimentation derived from the adjacent craton to a younger succession receiving detritus from an active-margin igneous source. Unique ~1.4-Ga age components, unknown in Antarctic and Australian cratons, coupled with eastward paleocurrents in the outboard assemblage, indicate that the ~1.4-Ga Laurentian anorogenic igneous province may extend beneath the polar ice cap in Antarctica. Together, the new age data support a Rodinia fit between Antarctica and Laurentia and suggest that sedimentation across the rifted margin was substantially younger than previously inferred.