ABSTRACT

Early Paleozoic histories of the volcanic geosynclinal (eugeosynclinal) belts of Nevada and Newfoundland are compared. The rocks of the two regions are in different structural settings. The central parts of the Cordilleran volcanic belt are concealed, but the counterparts in Newfoundland are exposed. Two main paleotectonic belts in the Cordillera are an eastern, non-volcanic (miogeosynclinal) belt 300 miles wide, dominantly of carbonates overlying Lower Cambrian quartzites laid on a basement of subsiding swales and swells, and a western belt in which intermediate stratigraphic sequences are preserved in lower thrust sheets as a parautochthon, and volcanic rock-bearing sequences are in upper thrust sheets as an allochthon. Depositional patterns were first strongly affected in the Late Devonian (Antlerian orogeny), when thrust sheets glided from above a welt of Lower Cambrian quartzite in west-central Nevada.

Pre-Carboniferous rocks in the western eugeosynclinal belt are argillites, quartzites, cherts, lavas, and fragmental volcanic rocks. Graywackes of flysch type, graded bedding, and sole-marks are rarely seen. Cambrian is little exposed, and Silurian and Devonian are not widespread. The Ordovician is best known, and well time-controlled because of the prevalence of graptolitic shales. Plutonism, which seems independent of orogenic times, did not occur until mid-Mesozoic.

Record of early Paleozoic deformation within the Precambrian crystalline-floored miogeosynclinal belt of Cambrian and Ordovician quartzites and carbonate rocks, exists in a 60-mile broad area along the west coast of Newfoundland and in adjacent Labrador. The allochthon of this coastal belt includes thrust sheets having rocks of intermediate facies corresponding to the parautochthon of the west, as well as thrust sheets having volcanic rocks and ultrabasic intrusions; it lacks post-Ordovician rocks, and is unconformably overlain by later Ordovician and younger sediments. The thrust sheets may have glided from a rising anticlinorium in the western Burlington Peninsula area like the thrust sheets of the Taconian orogeny that glided from the Green Mountain region of Vermont and Quebec. The uplift in Newfoundland may have been a great tilt block.

Ordovician and Silurian in Newfoundland are widely exposed in the eastern eugeosynclinal belt. The paleogeography can be well understood southward as far as southern New England. Rocks similar to those in Nevada prevail over much of the northern Appalachian eugeosynclinal belt, though graywackes are more common and cherts relatively rare. Fossiliferous shelly and shaly facies in both Nevada and Newfoundland in some instances can be interrelated. In north-eastern Newfoundland, the Silurian has thousands of feet of conglomerates in some sections, with volcanic and plutonic clasts that may have come from fault-bounded blocks within the volcanic geosynclinal belt: the broad, deeply subsiding troughs were separated by tectonic welts, some of which seem to have synkinematic intrusions. The plutonic rocks in this belt are principally Devonian (Acadian), but some may be synchronous with the Ordovician Taconian orogeny.

Southeast of the lower Paleozoic eugeosynclinal belt in Newfoundland is the Paleozoic Avalon Platform, a eugeosynclinal belt in Precambrian time that was deformed and bevelled prior to the Cambrian. It was a platform for Cambrian

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and early Ordovician sedimentation, subsiding only a few thousand feet prior to being further folded and faulted.

The structural history of the miogeosynclinal belt and the relationships to the eugeosynclinal border are best exhibited in the west; the history of the volcanic belt is better revealed in the east. Each belt shows appreciable variation from place to place and through time, so that conventional portrayals of geosynclinal belts are generalizations having limited application to specific regions.