DISTAL OROGENIC EFFECTS ON PERIPHERAL BULGE SEDIMENTATION - MIDDLE AND UPPER ORDOVICIAN OF THE NASHVILLE DOME

A major switch in depositional style in the Ordovician carbonates of t he Nashville Dome corresponds closely with the onset of the late Middl e Ordovician Taconic orogeny. This time marks a shift from tropical-ty pe to temperate-type carbonates, the initiation of widespread major ph osphate deposition, the introduction of large amounts of terrigenous s ilt and clay, the occurrence of widespread seismically induced soft-se diment deformation, and a change from a low energy flat-topped carbona te shelf to a high-energy doubly dipping carbonate ramp. Soft sediment deformation and the introduction of siliciclastics are direct effects of the Taconic orogeny; the switch from tropical-type to temperate-ty pe carbonates, the initiation of phosphate deposition, and the switch in carbonate ramp are largely oceanographic effects triggered by the o rogeny. In particular, phosphate deposition and the switch to temperat e type limestones appears to have been driven by upwelling along the e astern side of the Nashville Dome within the newly deepened Taconic fo reland basin. A fourfold decrease in the rate of relative sea-level ri se occurred on the Nashville Dome nearly 3 m.y. following the onset of thrusting and foreland basin initiation. Subsidence rates were consta nt before and after this decrease, and no evidence of a change in subs idence rates is seen to coincide with the onset of thrusting. The slow ing of subsidence may reflect viscoelastic uplift of the Nashville Dom e, but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models.