Thick upper Ordovician and Silurian conglomerate bodies in the northern Appalachian eugeosynclinal belt are exposed in three tectonic slices on New World Island in eastern Notre Dame Bay, north-central Newfoundland. The conglomerate is distinctive in being polymictic with many clasts of acidic plutonic rocks.

Sedimentary structures in conglomerate and interbedded finer clastic rocks indicate that many conglomerate beds are fluxoturbidites, particularly in the northern tectonic slice which adjoins the Lukes Arm fault. The conglomerate typically is thickly bedded, poorly graded, and shows channels at the base of beds. The rounded pebbles probably were shaped in a littoral environment and carried into the basin by high-density turbidity currents, slumping, and sliding. The abundance of fluxoturbidites and the presence of the coarsest pebbles in the northern slice, as well as paleoecologic data, suggest that the conglomerate units were derived from north of the Lukes Arm fault; this interpretation is supported by paleocurrent and provenance data.

The dominant igneous clasts are dacite porphyry, basaltic rocks, andesite, and tonalite; no clasts of regionally metamorphosed rock are found. Modal analyses of tonalite clasts are closely similar to those of the Twillingate batholith, which is north of the Lukes Arm fault.

In the New World Island area, plutonic rocks were exposed to erosion by late Ordovician time. The Taconian orogeny consisted of plutonism and block faulting, and possibly folding, from medial Ordovician through early Silurian time, and there was progression from flysch- to molasse-type sedimentation laterally and vertically. The history of the area is much like that of the Southern Uplands, Scotland.

Comparison of the Newfoundland conglomerate units with other eugeosynclinal plutonic-pebble conglomerate shows that plutonic pebbles first appear in the flysch stage of eugeosynclinal development. Furthermore, the tonalitic character of the oldest granitic plutons in Newfoundland and in certain eugeosynclinal belts elsewhere suggests that a simatic or semi-simatic crust might be considered as the eugeosynclinal basement, whereas, in other eugeosynclinal belts, data clearly show that plutonic pebbles are derived from uplifts of sialic basement.