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Ancient Appalachian Passive Margin Subsidence

Robert K. Goldhammer, Frederick A. Diegel, Lawrence A. Hardie

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, we constructed tectonic subsidence curves at four positions across the strike of the ancient margin. Because compaction features are abundant in the section, corrections for compaction are included using experimental porosity-depth curves for each lithologic type. The reconstructed (decompacted) stratigraphy gives the depth to basement through time and is unloaded in stages using a one-dimensional Airy isostatic model to calculate tectonic subsidence for each time interval. Rift-related subsidence rates (absolute = 25 cm/1,000 years; basement = 11 cm/1,000 years) and drift-related subsidence rates (absolute = 1.3-6.8 cm/1,000 years; basement = 0.7-3.5 cm/1,000 years) are comparable to present-day margins. Departure from the time-d pth exponential behavior predicted by thermal models reflects the transition from a divergent to a convergent margin associated with the Middle Ordovician Taconic orogeny.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.