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Silurian strata in Gaspe Peninsula comprise mainly greenish gray, massive and thin-bedded, calcareous siltstone. Volcanic rocks, chiefly interbedded andesitic flows and volcanic conglomerate, lie in a belt extending into central Gaspe from the southwest; conglomerate and reefal limestone interfinger with the volcanics in places on the margins of the belt. A relatively thin zone of gray, fossiliferous limestone with associated quartz arenites is present near the base of the Silurian in most outcrop regions, but appears to be absent within the volcanic belt.
The Silurian System rests on Upper Ordovician and older Paleozoic rocks; angular unconformity with the Middle Ordovician and older series is well documented and angular unconformity with the Upper Ordovician appears probable. Devonian rocks, where present, appear to overlie the Silurian conformably; elsewhere the Silurian is unconformably overlain by Carboniferous rocks. Direct correlation of the Gaspe Silurian with the Llandovery, Wenlock, and Ludlow Series of England is provided by graptolites, principally of the genus Monograptus. Monograptus ultimus occurs a few hundred feet below the top of the St. Leon Formation which, for purposes of this study, is considered to be the upper limit of the Silurian System.
The restored Silurian isopach map reveals three principal tectonic elements: Central Gaspe trough, St. John arch, and Southern Gaspe Volcanic trough. Thicknesses range from 500 feet at Forillon Peninsula in the east to at least 10,000 and possibly 20,000 feet in southwestern Gaspe. Eastward convergence of these troughs, regional eastward thinning, and eastward overlap on the sub-Silurian unconformity, suggest that the Gaspe region during Silurian time formed an eastern, spoon-shape terminus of rapid subsidence within the Appalachian geosynclinal complex.
Four new stratigraphic names are proposed: Laforce Formation, Cedar Barn Volcanic Member, Baldwin Volcanic Member, and Black Cape Formation. Several formation boundaries are adjusted, notably those near the base of the Chaleurs Group, to eliminate overlapping terminology. New classification and nomenclature are proposed for Silurian strata in northeast Gaspe, wherein rocks previously assigned to the St. Alban Formation of McGerrigle (1950) are subdivided, in ascending order, into the Griffon Cove River, Roncelles, and St. Leon Formations. The St. Leon was traced from its type area in western Gaspe and recognized throughout the northern half of the peninsula. The name St. Alban is not used, but it remains available as a possible group name. The "Mont Joli Formation" of the Perce area i abandoned.
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