We used the sensitive high-resolution ion microprobe (SHRIMP) to determine the U/Pb ages of zircons within clasts from quartz-pebble conglomerates in two Appalachian clastic wedges in order to further constrain the provenance and tectonic architecture of the central Appalachian basin at key times in its development. The ages of 64 zircons from samples of quartz pebbles in the Lower Silurian Shawangunk Formation and the Middle Pennsylvanian Pottsville Formation were determined using the SHRIMP. Zircons from Shawangunk quartz pebbles are no younger than 900 Ma, and most yield ages of 950--1200 Ma, coincident with the Grenville orogeny. Because multicycle sedimentary sources for the Shawangunk pebbles are not obvious, we conclude that a Grenville terrane, apparently unaffected by significant late Precambrian or early Paleozoic metamorphism, was uplifted, unroofed, and incised during the Taconic orogeny. Zircons from quartz clasts in the Pottsville Formation range in age from 380 Ma to 2100 Ma and also contain in a major Grenville component. Some clasts contain zircons in the age range 380--640 Ma, including a significant grouping between about 400 and 450 Ma. These zircons may have been derived from internal terranes affected by the waning stages of the Taconic orogeny. Clasts containing zircons as much as 2.1 Ga are problematic. They may have been derived from the distant interior craton (e.g., Superior Province) or were perhaps derived from the adjacent West African craton during the Pennsylvanian.¿ 1997 American Geophysical Union