Ancient Appalachian passive margin subsidence

The restored upper Precambrian-Middle Ordovician passive margin in the central Appalachians forms an eastward-thickening sedimentary wedge, 1.0-4.5 km thick and 380 km long, across depositional strike. The ancient margin sequence unconformably overlies Grenville-age (1.1 Ga) basement and includes upper precambrian clastics and volcanics, Eocambrian and Lower Cambrian clastics with local evaporites, and Cambrian-Ordovician carbonates. The breakup unconformity is marked by a regionally extensive, transgressive marine sandstone (Lower Cambrian Antietam Formation). The 3-km thickness of peritidal carbonate sediments deposited in 90 m.y. cannot be accounted for by sedimentary loading alone. Using a backstripping method modified for lithified strata that quantitatively accounts for the effects of sediment and water loads through time, the authors constructed tectonic subsidence curves at four positions across the strike of the ancient margin.