Theoretical rifting models suggest that rifting events have predictable stages, time spans, and characteristics. The stratigraphy and sedimentary tectonics of northern Virginia were compared with the theoretical models for the opening of ocean basins. According to our model the rift valley stage of initial uplift and graben formation (0–10 m.y. after onset of rifting) is represented in Virginia by the Lynchburg and equivalent formations. The initiation of ocean crust formation immediately follows the rift valley stage (10–15 m.y.) and is represented in Virginia by the Catoctin Formation. Sedimentation on an unstable, thermally subsiding continental margin (youthful stage, 10–60 m.y.) is represented by the Weverton and Harpers Formations. The beginning of deposition on a stable continental margin occurs no later than 60 m.y. after onset of rifting according to the theoretical models, and in Virginia it is represented by the Antietam Formation. A biostratigraphic age of Early Cambrian (ca. 570 Ma) for the Antietam provides the datum for proposing that the rifting which opened the Proto-Atlantic in Virginia was initiated 60 m.y. before deposition of the Antietam, at most about 630 Ma.

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